Symposium Organizers
John D. Baniecki Fujitsu Laboratories Ltd.
Jonathan A. Malen Carnegie Mellon University
G. Jeffrey Snyder California Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
DD1: Nanostructures/Nanocomposites/Nanowires
Session Chairs
John Baniecki
David Cahill
Monday PM, April 05, 2010
Room 2002 (Moscone West)
9:00 AM - **DD1.1
Nanostructures and the Lattice Thermal Conductivity of Thermoelectric Materials.
David Cahill 1
1 Materials Research Laboratory, U. Illinois, Urbana, Illinois, United States
Show AbstractEfforts aimed at improving the efficiency of thermoelectric materials often involve the introduction of some kind of nanostructure to reduce the thermal conductivity of the lattice. Nanostructures are intended to scatter the relatively long-wavelength acoustic phonons that carry a significant fraction of heat in semiconductor alloys. In collaboration with colleagues at the U. of Oregon, Lincoln Labs, UC Santa Cruz, UC Berkeley, and UC Santa Barbara, we have investigated the thermal conductivity of a wide variety of nanostructured thermoelectric materials (e.g., Bi2Te3, PbTe, and InGaAs) by time-domain thermoreflectance (TDTR). Multilayer films of Bi2Te3 incorporating turbostratic TiTe2 enable studies of the effective thermal conductivity of Bi2Te3 layers as thin as 2 nm. In contradiction with previous reports, the lattice thermal conductivity of PbTe/PbSe nanodots superlattices does not fall significantly below 1 W/m-K. For InGaAs:ErAs, measurements of the thermal conductivity as a function of the thermal penetration depth used in the TDTR experiment support the idea that ErAs nanodots effectively scatter low frequency phonons that contribute significantly to the thermal conductivity of the pure InGaAs alloy. The dependence of the thermal conductivity on the phonon boundary-scattering length-scale in these nanostructured thermoelectrics is in good agreement with simple Debye-Callaway models.
9:30 AM - DD1.2
Thermoelectric Materials With Embedded Nanoparticles- An Effective Medium Approach.
Mona Zebarjadi 1 , Keivan Esfarjani 1 , Zhixi Bian 1 , Ali Shakouri 1
1 , UCSC, Santa Cruz, California, United States
Show AbstractThe effect of adding spherical nano-particles with a size distribution inside a host matrix is investigated using the coherent potential approximation. A parabolic band structure is assumed for the host matrix and it is shown that for a volume fraction of about 10%, depending on their barrier height, nano-particles can increase the effective mass by up to 20% or more. The effective band-structure can be fitted by the standard non-parabolic relation, resulting in a negative non-parabolic coefficient in the case of barrier type nano-particles. Interesting peaks have been observed in the group velocity curve versus energy when the nano-particles are deep wells and their volume fraction is more than few percent. This feature can strongly affect the Seebeck coefficient and thus the power factor of the samples.
9:45 AM - DD1.3
MOCVD Growth of Erbium Antimonide Nanoparticles Embedded in Indium Gallium Antimonide Matrix.
Takehiro Onishi 1 , Tela Favaloro 1 , Nobuhiko Kobayashi 1 , Elane Coleman 2 , Gary Tompa 2
1 Electrical Engineering, University of California, Santa Cruz, Moffett Field, California, United States, 2 , Structred Materials Industries Inc., Piscataway, New Jersey, United States
Show Abstract In the continuing quest for ever more efficient and environmentally friendly energy generation, thermoelectric are one of the “new” frontiers where applications of nanotechnology promise to generate dramatic performance improvements to a well established technology. One of the most promising avenues of development are rare earth nanocomposites because they can maintain high electrical conductivity, be made p- and n-type and embedded nanopartilces can be used to scatter phonons generating a low thermal conductivity – the net result is a High Zt material system. We herein report on our recent efforts to develop rare earth nanocomposites consisting of ErSb nanopartilces in InSb alloys using the production scalable thin film production technology - Metal Organic Chemical Vapor Depositions (MOCVD). This is the first report of the growth of ErSb by MOCVD. Our MOCVD process is carried out at low pressure, using metalorganic sources. We have successfully grown In(Er,Ga)Sb:Zn matrix and deposited ErSb on the n type InSb substrate. SEM images show smooth surfaces, XRD on the matrix indicates good crystal structures. XRF and EDX confirm the composition of the In(Ga,Er)Sb films. FTIR measurements also confirm film properties. In addition to fundamental properties, temperature dependencies of Seebeck and electric conductivity measurements will be reported.Acknowledgement:DARPA/DSO (Program Manager: Khine Latt)References:[1] H. Choi, C. A. Wnag, G. W. Turner, M. J. Manfra, D. L. Spears, G. W. Charache, L. R. Danielson, D. M. Depoy, Appl. Phys. Lett. 71, 3758 (1997). [2] V. B. Khalfin, D. Z. Garbuzov, H. Lee, G. C. Taylor, N. Morries, R. U. Martinelli, J. C. Connolly, AIP Conf. Procj. 460, 247 (1998). [3] Z. A. Shellenbarger, G. C. Taylor, R. K. Smeltzer, J. Li, R. U. Martinelli, K. Palit, AIP OCnf. Proc. 653, 314 (2003). [4] Y. Z. Yu, AIP Conf. Proc. 653, 335 (2003). 17C. A. Wang, H. K. Choi, D. C. Oakley, G. W. Charache, AIP Conf. Proc. 460, 256 (1)98).[5] J. M. Zide et al. Appl. Phys. Lett. 87, p. 112102, 2005.
10:00 AM - DD1.4
Fabrication of InGaAs/GaAs Nanocomposites Using Ion Implantation.
Michael Warren 1 , J. Schrauwen 2 , Rachel Goldman 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Information Technology, Ghent University, Ghent Belgium
Show AbstractThe controlled formation of semiconductor nanocomposites offers a unique opportunity to tailor functional materials with a variety of novel properties. In particular, nanocomposites consisting of InAs nanostructures embedded in GaAs have been proposed for high efficiency photovoltaics and high figure-of-merit thermoelectrics. A promising approach to nanocomposite synthesis is matrix-seeded growth, which involves ion-beam-amorphization of a semiconductor film, followed by nanoscale re-crystallization via annealing [1]. In earlier studies of In+ implantation into GaAs, it has been suggested that In aggregates upon annealing [2-4], suggesting the possibility of selective formation of InAs-rich nanocrystals in a GaAs matrix. However, calculations have shown that the In concentration is limited by the steady-state sputtering condition before a sufficient concentration of In has been implanted, preventing the formation of InAs nanocrystals. To counteract this, we have developed a sputter-mask method, where a thin, sacrificial layer of a material with a lower sputter yield than GaAs is used as a cap to prevent the implanted In from being sputtered out. For this work we have implanted 100kV In+ ions into GaAs with 100 nm sputter-masks consisting of AlAs and SiO2. Following implantation, the films were annealed between 700 and 800°C for 30s to 1 minute. We will discuss the influence of In+ dose, annealing time, and sputter-mask material on the nucleation and growth of InAs, as well as the thermoelectric properties of these structures. This work is supported in part by the GAANN Fellowship and AFOSR-MURI.[1] X. Weng, W. Ye, S. Clarke, A. Daniel, V. Rotberg, R. Clarke, and R.S. Goldman, J. Appl. Phys. 97, 064301 (2005).[2] M.V. Ardyshev and V.F. Pichugin, Russian Physics Journal 47, 175 (2004).[3] M. Kulik, F.F. Komarov, and D.D. Maczka, Vacuum 63, 755 (2001).[4] M. Kulik, A.P. Kobzev, D. Jaworska, J. Zuk, and J. Filiks, Vacuum 81, 1124 (2007).
10:15 AM - DD1.5
Thermoelectric Power Factors of the n-type and p-type InGa(Al)As Semiconductors With Epitaxially Embedded ErAs Nanoparticles.
Je-Hyeong Bahk 1 , Gehong Zeng 1 , Ashok Ramu 1 , John Bowers 1 , Hong Lu 2 , Peter Burke 2 , Arthur Gossard 2 , Tela Favaloro 3 , Zhixi Bian 3 , Ali Shakouri 3
1 ECE, UCSB, Santa Barbara, California, United States, 2 Materials, UCSB, Santa Barbara, California, United States, 3 EE, UCSC, Santa Cruz, California, United States
Show AbstractWe present the thermoelectric power factors of the InGa(Al)As semiconductors with epitaxially embedded ErAs nanoparticles grown by molecular beam epitaxy. Various compositions of the semiconductor matrices and various Er concentrations are investigated both experimentally and theoretically to find the optimal power factors and thermoelectric figure of merits for efficient thermoelectric power generation. The n-type ErAs:InGa(Al)As materials are only doped with Er, and show increasing carrier density with increasing temperature, which allows for the carrier density close to the optimal levels in a wide temperature range, and thus makes the power factor increase with temperature. A theoretical model also shows that the effective energy-dependent electron scattering by ErAs nanoparticles can enhance the Seebeck coefficient at high temperatures. At 600K, a power factor of 4.5 x 10^(-3) W/mK^2 is measured for the n-type 0.08% ErAs:InGaAs, which is about 80% higher than that of the bulk PbTe alloy in this temperature range. The p-type ErAs:InGaAs materials are grown with Be or C dopants along with Er, and their power factors are comparable with their n-type counterparts at room temperature, and remains almost constant at high temperatures due to the decreasing electrical conductivity and increasing Seebeck with temperature.
10:30 AM - DD1.6
Nanostructured Silicon-based Composites for High Temperature Thermoelectric Applications.
Sabah Bux 1 , Richard Kaner 1 , Jean-Pierre Fleurial 2
1 Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California, United States, 2 Power and Sensor Systems, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California, United States
Show AbstractRecently nanostructured bulk silicon and silicon-germanium have achieved large increases in the thermoelectric figure of merit (ZT). The ZT enhancement is attributed to a significant reduction in the lattice thermal conductivity while maintaining relatively high carrier mobility. Silicon-based thermoelectric devices are attractive due to their low-toxicity, thermal stability, low density, relative abundance and low cost of production. Although significant enhancements in ZT have been achieved using the nanostructuring route, additional decoupling of the thermal and electric transport terms is still necessary in order for silicon-based materials to be a viable for thermoelectric applications such as waste heat recovery or radioisotope thermoelectric generators. It is theorized that further increases in ZT could be achieved by forming composites with nanostructured inert inclusions to further scatter the heat carrying phonons. Here we present the impact of insulating and conductive nanoparticle composites on ZT. The nanostructured composites are formed via ball milling and high pressure sintering of the nanoparticles. The thermoelectric properties and microstructure of the silicon-based composites will be presented and discussed.
10:45 AM - DD1.7
Performance Expectations for Self Assembled Thermoelectric Coolers.
Gary Hendrick 1 , G. Nolas 2 , Nathan Crane 1
1 Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 2 Physics, University of South Florida, Tampa, Florida, United States
Show AbstractApplication requirements and performance goals push thermoelectric coolers to progressively smaller sizes. Many applications can achieve peak performance with element thicknesses in the 100-400 micron range. However, devices are not easily assembled in these sizes with current methods. Recently, self assembly has been proposed as an alternative approach. However, this introduces new system challenges. In particular, self-assembled systems are likely to contain random errors. This paper examines the potential impact of these random errors on the heat pumping capacity of the thermoelectric coolers. Using a 1D approximation of the device performance, the impact of element redundancy, number of elements, and error rates are examined. The models show that in many cases, high performance levels can be achieved even with part error rates over 20%. At higher rates of assembly errors, a simple process strategy is proposed to still permit the assembly of high performance devices.
11:15 AM - DD1.8
Non-thermal Plasma Created Silicon and Germanium Nanocrystals for Thermoelectric Nanocomposites.
Ariel Chatman 1 , Amanda Dillman 3 , Yves Adjallah 2 , Lee Wienkes 2 , Uwe Kortshagen 1 , Mark Zimmerman 3 , David Kohlstedt 3 , James Kakalios 2
1 Department of Mechanical Engineering, University of Minnesota, Minneapolis, Minnesota, United States, 3 Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota, United States, 2 School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractNanocomposite materials, consisting of densely compressed mixtures of nanocrystals, are of increasing interest in thermoelectric materials research. Interface scattering of phonons in nanocomposites reduces the thermal conductivity below the alloy limit while it may be possible to improve the electrical conductivity and Seebeck coefficient through quantum confinement effects. In addition, as nanocomposites are larger scale, dense materials, they can easily be integrated into practical device designs. To better explore the advantages of nanocomposite thermoelectrics, a plasma based, bottom-up synthesis method for silicon and germanium nanocrystals is employed. Coupled with hot-pressing to densify the composite, this method yields a controlled design of materials with smaller and more uniform grain sizes for study. A radio frequency plasma system synthesizes silicon or germanium nanocrystals in an aerosolized flow at a rate of approximately 5 mg/min. Nanocrystal size is characterized by TEM to be monodisperse with an average size controllable down to 2-3 nm. A sufficient sample for compression and testing of thermoelectric properties can be prepared in one hour; a significant advantage over ball milling methods that can take up to 60 hours. This system allows the synthesis of n-type or p-type doped nanocrystallites. The collected particles are hot-pressed in a graphite die into cylindrical samples 6 mm in diameter. The density and final grain size of Si nanocomposites are studied as a function of the pressure and duration of the hot press using geometric and mass measurements and wide angle x-ray diffraction. Final grain sizes are as small as ~10 nm. Measurements of the thermal conductivity down to 1.2 W/(m*K) are obtained using a modified Raman spectroscopy method. Measurements of the Seebeck coefficient and electrical conductivity are made in a heated vacuum system over a range of ΔT values and average temperatures. The hot-pressing of nanocomposites including both Si and Ge nanocrystals as well as the effects of dopants will be described.This work was supported partially by the MRSEC Program of the National Science Foundation under Award Number DMR-0819885, NSF grants DMR-0705675, the NINN Characterization Facility and Nanofabrication Center, NSF Graduate Research Fellowship Program and the University of Minnesota.
11:30 AM - DD1.9
Influence of Spark Plasma Sintering Parameters on the Thermoelectric Performance of Nanocrystalline Silicon.
Gabi Schierning 1 2 , Niklas Stein 1 2 , Tania Claudio 3 , Raphael Hermann 3 , Nils Petermann 4 2 , Hartmut Wiggers 4 2 , Tim Huelser 5 , Ralf Theissmann 1 2 , Roland Schmechel 1 2
1 Faculty of Engineering, University of Duisburg-Essen, Duisburg Germany, 2 Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Duisburg Germany, 3 Institut für Festkoerperforschung, Forschungszentrum Juelich, Juelich Germany, 4 Institute for Combustion and Gas Dynamics, University of Duisburg-Essen, Duisburg Germany, 5 , Institute of Energy and Environmental Technology (IUTA), Duisburg Germany
Show AbstractNanocrystalline bulk silicon samples are being explored regarding their thermoelectric properties. A gas phase process based on a microwave plasma reactor was used to synthesize silicon nanoparticles. By variation of microwave power, chamber pressure, and concentration of the precursor gas silane (SiH4) and the plasma gases Ar and H2, crystalline particles of predefined dimension in a narrow size distribution were obtained. N-type doping was realized by adding phosphine (PH3) to the precursor gas in a nominal concentration of 5 x 1020 cm-3.The compaction of the nanoparticles was done by a spark-plasma sintering process. Several grams of the nanoparticles were pre-compacted and subsequently sintered to dense pellets. Sintering temperature was varied between 850 °C and 1150 °C in order to optimize the thermoelectric performance of the samples. Hold time at that temperature was 3 minutes.Structural characterization of raw powder and sintered pellets was performed by transmission electron microscopy and x-ray diffraction combined with Rietveld-analysis. Samples are homogenous, nanocrystalline, and no texture was found. Several methods were utilized for transport characterization: The Seebeck-coefficient α was measured using a reference method, while the electrical conductivity σ was characterized in van-der-Pauw geometry and the thermal conductivity κ was obtained by a Laserflash method. Additionally, a physical property measurement system was used to obtain all transport coefficients at the same time in the same transport direction.Especially the electrical conductivity of the nanocrystalline silicon can be tuned sensitively by variation of sintering parameters and oxygen content within the nanoparticles. Room temperature values for the electrical conductivity were found between σ = 10 S cm-1 (sintering temperature TS = 850 °C) and σ = 900 S cm-1 (TS = 1150 °C), while the thermal conductivity varied between approximately κ = 6 W m-1 K-1 (TS = 850 °C) and κ = 30 W m-1 K-1 (TS = 1150 °C). With a Seebeck-coefficient around α = - 150 µV/K, the range of ZT values for this series of samples is between 0.001 and 0.02.
11:45 AM - DD1.10
Improved Thermoelectric Performance of P-type Nanostructured Bismuth Antimony Telluride Bulk Alloys Prepared by Hot Forging.
Junjie Shen 1 , Xinbing Zhao 1 , Tiejun Zhu 1 , Zhenzhong Yin 1
1 Department of Materials Science and Engineering, State key laboratory of Silicon Materials, Hangzhou, Zhejiang, China
Show AbstractHigh performance Bi0.5Sb1.5Te3 thermoelectric materials have been prepared through a relatively simple route, using a hot forging process to adjusting microstructures of the hot pressed Bi0.5Sb1.5Te3 alloys. Transport properties have been measured, which indicated that the hot forged samples revealed high Seebeck coefficients and low thermal conductivities, and the electrical conductivities became lower at room temperature. The maximum ZT value of 1.32 was obtained at room temperature for the sample which was hot forged in the same direction. Compared with the one without hot forging, the ZT value was increased by almost 50% at the same temperature. Similar samples have been re-prepared twice using the commercial ingots and the same technology, and the highly repeatable nice ZT value of ~1.4 were also revealed. The related TE properties and Transmission electron microscopy investigations suggested the enhancement in thermoelectric properties of hot forged Bi0.5Sb1.5Te3 alloys should be attributed to a complex reason with some structural and compositional modulations. It is believed that this hot forging technique is extendable to improve the thermoelectric properties of other bulk alloys, and is also more suitably and environmentally friendly to prepare large bulk thermoelectric materials with nice properties for industrial production.
12:00 PM - DD1.11
Thermal Conductivity of Bulk Silicon With Nanoscale Grains made by Spark Plasma Sintering.
Zhaojie Wang 1 , Joseph Alaniz 1 , Wanyoung Jang 1 , Javier Garay 1 , Chris Dames 1
1 Mechanical Engr., UC Riverside, Riverside, California, United States
Show AbstractEngineering bulk materials with nano-sized grains is an effective strategy for reducing the thermal conductivity, due to the increased phonon scattering at the grain boundaries[1]. It has recently been shown that it is also possible to achieve an increased power factor in the same material, leading to a significant increase in the thermoelectric figure of merit[2,3,4,5,6,7,8]. To better understand the fundamental mechanisms of the thermal conductivity reduction, here we focus on the effects of grain size and temperature on the thermal conductivity of silicon with grain sizes down to 50 nm. We use a Spark Plasma Sintering (SPS) technique to consolidate the fully densed samples from nanopowders at specified pressure and high temperature over a short time scale [9]. The thermal conductivity is measured by a 3 omega method. At room temperature the thermal conductivity of a Si sample with 50 nm grains is 5 times smaller than that of bulk silicon, with much larger reductions seen at lower temperatures (e.g. 60x reduction at 80 K). These results are explained by a thermal model that accounts for the additional phonon scattering at grain boundaries. References: [1] Bed Poudel, Qing Hao, et al., Science, 320, 634, (2008)[2] Sabah K. Bux, Richard G. Blair, et al., Adv. Funct. Mater. 19, 2445-2452 (2009)[3] A. J. Minnich, M. S. Dresselhaus, Z. F. Ren and G. Chen, Energy Environ. Sci. 2, 466 (2009)[4] M. S. Dresselhaus, G. Chen et al., Adv. Mater. 19, 1043-1053 (2007)[5] Deyu Li, A. Majumdar, APL 83, 14 (2003)[6] G. H. Zhu, H. Lee, et al., PRL 102, 196803 (2009)[7] G. Joshi, H. Lee, et al., Nano Letters 8, 12, 4670 (2008)[8] X. W. Wang, H. Lee, et al., Appl. Phys. Lett. 93, 193121 (2008)[9] U. Anselmi-Tamburinia, b, S. Gennarib, J.E. Garaya and Z.A. Munir, Materials Science and Engineering A, 394, 1-2, 139-148 (2005).
12:15 PM - DD1.12
Effect of Electric Current Stressing on Thermal/Electrical Transport Properties of Sputtered Bi-Sb-Te Thin Films.
Xiao-Wei Su 1 , Chien-Neng Liao 1
1 Materials Science and Engineering, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractThermal conductivity of polycrystalline thin films usually decreases with reducing grain size due to increasing phonon scattering at grain boundary. However, an extensive post-deposition annealing mostly for defect elimination at elevated temperature may enhance electrical conductivity but cause marked grain growth and rising lattice thermal conductivity of thin films. It has been reported that electric current assisted thermal treatment can improve the thermoelectric properties of the sputtered Bi-Sb-Te based thin films at lower annealing temperatures within a short period of time. The electrically stressed Bi-Sb-Te thin films show a much higher carrier mobility and moderately lower carrier concentration than those thermally annealed at the same temperature. Besides, some Sb-rich precipitates distributing at grain boundary and surface have been observed by TEM and SEM micrographs. In general, lattice thermal conductivity may be enhanced from reducing defects or grain growth after annealing. But precipitates emergence at grain boundary can also promote more phonons-grain boundary scattering. We expect that there is a variation of lattice thermal conductivity along with defects elimination and precipitates emergence during thermal treatment. The conventional 3ω method will be used for thermal conductivity measurement. The effective thermal boundary resistance at grain boundary will also be determined from lattice thermal conductivity and grain size variation. Both electrical and thermal transport properties of the electrically stressed Bi-Sb-Te thin films will be presented with microstructure analysis.
12:30 PM - DD1.13
Synthesis of Thermoelectric CoSb3 Nanowires by Electrochemical Methods.
Dat Quach 1 , Ruxandra Vidu 1 , Joanna Groza 1 , Pieter Stroeve 1
1 Chemical Engineering & Materials Science, University of California, Davis, Davis, California, United States
Show AbstractNanostructured systems display unusual size-dependent properties and play important roles in material development. Thermoelectric materials are clean energy conversion materials that show higher figures of merit when the dimensionality is reduced from bulk to ultra thin films and nanowires. Electrochemical deposition is an inexpensive method that has great potential in producing such low-dimension structures. Bulk, partially or fully filled CoSb3 and other skutterudites have shown promising thermoelectric properties but little research has been done on the electrochemical deposition of CoSb3 on nanostructures. Here we report the electrochemical deposition of nanowires of CoSb3 skutterudites using template synthesis in nanoporous membranes. Composition and growth mechanism of CoSb3 nanowires are discussed in relation to the substrate structures and deposition conditions.
12:45 PM - DD1.14
Enhanced Thermoelectric Power of Individual Single-crystalline Bi Nanowires Grown by On-film Formation of Nanowires (OFF-ON) Method.
Seunghyun Lee 1 , Jinhee Ham 1 , Jin-Seo Noh 1 , Wooyoung Lee 1
1 , Yonsei University , Seoul Korea (the Republic of)
Show AbstractWe have investigated the thermoelectric power (TEP) of an individual single-crystalline Bi nanowire with d = 107 nm grown by On-film Formation of Nanowires (OFF-ON) Method. A micro-heater was fabricated adjacent to the end of a Bi nanowire for measurements of TEP. When a bias voltage is applied to the heater, temperature of both electrodes increases, which is monitored by resistance changes in respective electrodes. Since heat conduction through a substrate is prohibited, nanowires are an only medium to carry heat. The resulting temperature gradient across a nanowire was 0.1 – 0.5 K/μm, and the typical power consumption in the micro-heater was small enough (< 100 μW). The thermoelectric voltage across the nanowire can be readily measured using a lock-in amplifier. The Seebeck coefficient (S) for the Bi nanowire was measured to be –70 μV/K at 300 K, which is larger than those of bulk Bi (–45 μV/K), sputtered polycrystalline Bi nanowires (–25 μV/K), and pressure injected Bi nanowires array (–55 μV/K) from previous studies. Our results indicate that the enhanced S of the Bi nanowire is attributed to the modification of density of states in one-dimensional structures due to quantum confinement effect. The modulation of TEP of individual single-crystalline Bi nanowires by electric-field effect is also presented.
DD2: Thermoelectric Applications
Session Chairs
Ryoji Funahashi
Shuichi Funahashi
Monday PM, April 05, 2010
Room 2002 (Moscone West)
2:30 PM - **DD2.1
Power Generation of Cascaded Thermoelectric Arrays.
Ryoji Funahashi 1 , Saori Urata 1 , Tomoyuki Urata 1 , Yoko Matsumura 1 , Kanako Iwasaki 1 , Atsuko Kosuga 1
1 , AIST, Ikeda, Osaka, Japan
Show AbstractOxide thermoelectric materials are considered to be promising ones because of their durability against high temperature, cost, no content of toxic elements, and so on. We have produced many types of modules using p-type Ca3Co4O9 (Co-349) and n-type CaMnO3 (Mn-113) or LaNiO3. The Co-349/Mn-113 modules show power density against heated surface area higher than 4kW/m2. However, the conversion efficiency of thermoelectric power generation is evaluated as low as 3% from properties of Co-349 and Mn-113 devices. In order to enhance power generation and conversion efficiency, cascaded modules consisting of oxide and Bi2Te3 modules.Thermoelectric arrays have been produced by stacking a heat collection fin, oxide and Bi2Te3 based thermoelectric modules, and water jacket for cooling. Each array includes 4 pieces of oxide/Bi2Te3 cascaded modules. The surface area array is 140mm x 140mm. Thermoelectric power generation test was carried out using a kerosene burner. Temperature around the heat collection fin and at the hot side of the oxide module reached about 1373 K and 943 K, respectively. Out put power from one array is 50 W.
3:00 PM - DD2.2
Skutterudite-based Thermoelectric Generators.
Jeff Sakamoto 1 , Harold Schock 2 , Thierry Caillat 3 , Jean-Pierre Fleurial 3 , Ryan Maloney 1 , Trevor Ruckle 2 , Ed Timm 2
1 CHEMS, MSU, East Lansing, Michigan, United States, 2 ME, MSU, East Lansing, Michigan, United States, 3 Materials and Device Technology, JPL, Pasadena, California, United States
Show AbstractMSU and JPL are engaged in a collaborative effort to develop the next generation thermoelectric power generators for terrestrial waste heat recovery. This project is sponsored by the DOE EERE Program to improve fuel efficiency of commercial and passenger vehicles. The emphasis has primarily been on thermoelectric technology employing Skutterudite-based technology, which also includes segmenting with heritage materials such as TAGS and Bi2Te3. This work entails scaled-synthesis of n and p type Skutterudite powders, mass-production of metallized legs, module-bonding technology and thermoelectric subassembly integration into prototypical generators. Two thermoelectric generators are under consideration. First, a traditional design is under consideration whereby heat is extracted from the periphery of an exhaust component. The second design is enabled by aerogel-based thermal insulation, which integrates the thermoelectric elements inside an exhaust component giving direct access to high temperature exhaust gas. Finally, preliminary power output validation for > 50Watt Skutterudite subassemblies will be presented.
3:15 PM - DD2.3
Integrating New High Temperature Thermoelectric Materials into Power Generating Thermocouples.
Erik Brandon 1 , Thierry Caillat 1 , Su Chi 1 , Samad Firdosy 1 , Jean-Pierre Fleurial 1 , Chen-Kuo Huang 1 , Billy Li 1 , George Nakatsukasa 1 , Bill Nesmith 1 , Jong-Ah Paik 1 , Vilupanur Ravi 1 2 , William Determan 3 , Dan Matejczyk 3 , Karl Wefers 3 , Sherwin Yang 3
1 Materials and Device Technologies Group, Jet Propulsion Laboratory, Pasadena, California, United States, 2 Department of Chemical and Materials Engineering, California State Polytechnic University, Pomona, California, United States, 3 , Pratt and Whitney Rocketdyne, Canoga Park, California, United States
Show AbstractRadioisotope thermoelectric generators (RTGs) have provided power for a wide range of robotic space exploration missions over the last 40 years. RTGs employ thermoelectric devices to generate power for spacecraft from the decay of plutonium fuel, where solar insolation is insufficient or when long mission lifetimes are required. Devices used in previous RTGs have been fabricated using well-known thermoelectric materials such as silicon germanium and telluride based systems. The required power levels needed to execute future challenging exploration missions, combined with concerns over plutonium fuel availability has motivated recent efforts to incorporate new high temperature thermoelectric materials (such as Yb14MnSb11 and nano-structured SiGe) into power generating devices. Devices made from these materials can provide a pathway to higher system efficiencies and specific power levels, relative to currently available RTGs systems. Despite the promising thermoelectric properties of these new materials, significant challenges remain in their integration into power generating devices. Ongoing efforts to address these challenges will be reviewed in this talk.
3:30 PM - DD2.4
Portable Power Thermoelectric Generator – Design and Characterization.
Ramesh Koripella 1 , Lon Bell 1 , Doug Crane 1
1 , BSST, LLC, Irwindale, California, United States
Show AbstractHigh energy density and power density are two important considerations in the design of a portable power source. High efficiency of energy conversion is one of the important factors in achieving high energy density. We report on the design, fabrication and characterization of a high efficiency and power density thermoelectric generator. Segmented thermoelectric elements with a high temperature-weighted average zT are used in the fabrication of the generator to achieve high energy conversion efficiency. A comprehensive model of the generator has been developed that includes temperature dependent thermoelectric properties for the segmented thermoelectric element design, and system level thermal and electrical losses. These models are used to guide the design of a high efficiency thermoelectric generator. A 14W size portable thermoelectric generator was built and its performance for converting thermal energy into electrical energy was characterized and compared to the model predictions. The magnitude of individual loss mechanisms and their effect on overall system efficiency are presented. The evolution of the device design and critical design parameters that have been incorporated or modified to minimize performance losses and improved device robustness are discussed. Important future design optimizations that can further reduce losses for improved performance were identified.
3:45 PM - DD2.5
Thermoreflectance and Transient Characterization of 400 Element Thin Film Thermoelectric Module.
Philip Jackson 1 , Gehong Zeng 2 , Zhixi Bian 1 , James Christofferson 1 , John Bowers 2 , Joshua Zide 3 , Arthur Gossard 3 , Ali Shakouri 1
1 Electrical Engineering Department, UC Santa Cruz, Santa Cruz, California, United States, 2 Electrical and Computer Engineering Department, UC Santa Barbara, Santa Barbara, California, United States, 3 Materials Department, UC Santa Barbara, Santa Barbara, California, United States
Show AbstractThe performance of a thermoelectric power generator is determined by the thermoelectric figure-of-merits of n and p semiconductor elements, its geometry, and parasitic thermal and electrical losses. There are also reliability issues due to nonuniform heat load, thermo-mechanical stress and device defects. In this work, we study the parasitic losses and inspect defects in a module by using IR imaging and transient techniques. Thermoreflectance measurements exploit the change of a material’s reflection coefficient with temperature. Typically, the thermoreflectance coefficient, which describes the normalized change in the reflection coefficient with temperature, is on the order of 10-5 per degree Kelvin. We use a Dalsa Pantera 1M60 12 bit CCD camera with 1024x1024 pixels and dynamic range greater than 66 dB to perform through-substrate thermoreflectance imaging and to determine the temperature profile of individual thermoelectric elements inside the module. The thermoelectric module consists of 400 elements of ErAs:InGaAlAs material 20 μm thick, and AlN substrates on top and bottom, which are semi-transparent to white light. The thermal imaging shows that the thermoelectric cooling/heating is in general uniform when a 300 mA current is applied. We also found a hot spot (defect) at a corner of the module indicating a possible processing defect e.g. a solder residue.
4:00 PM - DD2: Application
BREAK
4:15 PM - DD2.6
Thermoelectric System for Waste Heat Recovery in Cars.
Matthias Trottmann 1 , Urs Cabalzar 2 , Anke Weidenkaff 1 , Sascha Populoh 1 , Simon Tischhauser 1 , Christian Bach 1
1 , EMPA, Duebendorf Switzerland, 2 , Fachhochschule, Bern Switzerland
Show AbstractDue to rising fuel prices, resource shortages and the increasing awareness regarding climate changes, the energy efficiency of automobiles gains in importance. Apart from efforts toward the optimization of the engine’s internal efficiency the use of the enormous waste heat losses seems meaningful. The thermoelectric device is a good approach for the realization of such a recuperation system. So-called thermoelectric generators allow the direct transformation of thermal into electrical energy.Based on the study of existing systems and project-relevant theories as well as different tests to evaluate the potential of waste heat recovery, a thermoelectric system was developed. The characterization models, based on thermodynamic laws, were compiled to describe the operational behavior of the generator under simplified conditions. In this regard, the electrical power generation dependence on the exhaust mass flow and the exhaust gas temperature is the major interest of our studies. To verify the theoretical values and to understand the behavior of such a recovery system a prototype based on the muffler of a VW Touran was built and tested [1]. Furthermore, based on the first results, the system could be optimized and a twenty-leg oxide [2] thermoelectric module could be installed.
4:30 PM - **DD2.7
Novel Thermoelectric Devices Based on Multi-layer Ceramic Capacitor Technology.
Shuichi Funahashi 1 , Hayashi Sachiko 1 , Takanori Nakamura 1 , Keisuke Kageyama 1
1 , Murata Manufacturing Company, Ltd., Yasu-shi Japan
Show AbstractWe prepared three materials, p- type- (La1.97,Sr0.03)CuO4, n- type- (Nd1.97,Ce0.03)CuO4 and an insulator with a mixture of Mg2SiO4 and glass. We adjusted the shrinkage of green sheets for these materials and fabricated monolithic thermoelectric devices with Multi-Layer Ceramic Capacitor (MLCC) technology. The dimensions of the fabricated thermoelectric device were 9.6× 6.9× 5.0 mm, and the thermoelectric layer thickness was 0.12 mm. The occupation ratio of the thermoelectric materials was calculated as being about 89%. The device consisted of 25 P/I/N - pairs and generated 26 mW (40 mW/cm2 at calculated values) of electric power with 360K of temperature gap. We found that the generated electric power was smaller than the theoretical value because of electric resistance and thermal losses in the P/N- junction. We found that a more appropriate adjustment in the fabrication process such as the densities of green tape, laminated density, and sintering condition improves the performance of thermoelectric MLCCs. We’ll report progress to improve the device of these materials and another.
5:00 PM - DD2.8
Trends in Mechanical Properties of Thermoelectric Materials.
Eldon Case 1
1 Chem. Eng and Materials Science, Michigan State University, East Lansing, Michigan, United States
Show AbstractThe harvesting of waste heat subjects thermoelectric materials to mechanical stresses induced by thermal gradients and temperature transients. Understanding a thermoelectric material's response to such stresses requires the knowledge of mechanical properties such as the elastic moduli, fracture strength and fracture toughness. For the limited mechanical property database that is available in the literature, the mechanical properties of key thermoelectric materials for waste heat recovery applications will be discussed, including the functional dependence of the mechanical properties on temperature and microstructural parameters such as grain size and porosity.
5:15 PM - DD2.9
Performance Optimization of a TE Generator Element With Linear Material Profiles in a 1D Scheme.
Knud Zabrocki 1 2 , Eckhard Mueller 1 , Wolfgang Seifert 2
1 Institute of Materials Research, Thermoelectric Functional Materials, German Aerospace Center, Köln-Porz Germany, 2 Institute of Physics, Martin Luther University Halle-Wittenberg, Halle (Saale) Germany
Show AbstractGraded and segmented thermoelectric (TE) elements have been studied for long, aiming at improving the performance of TE generators (TEG) which are exposed to a large temperature differ¬ence. However, it has been shown that simply adjusting maximum ZT in each segment of a stacked or graded TE element is not a sufficient rule to maximize TE device performance. From a macroscopic point of view the temperature profile T(x) throughout a TE element can be determined by using the Onsager theory. Under isotropic and steady state conditions, the equations for energy and charge conservation can be combined to obtain a thermal energy balance equation containing T(x) as target function. Besides the temperature the balance equation contains material properties represented by the Seebeck coefficient S, the electrical and thermal conductivities σ and κ, respectively. For the sake of simplicity, a 1D scheme has been chosen for the analytical and numerical treatment. Performance investigations can be done within the framework of the Constant Properties Model (CPM) or based on temperature dependent material properties. In the 1D steady state, there is an alternative approach available based on spatial material profiles. Following the approach by Müller et al., the temperature profile T(x) can be calculated within a model-free setup directly from the 1D thermal energy balance, e.g., based on continuous monotonous gradient functions for all material profiles, and independent and free variability of the material parameters S(x), σ(x), and κ(x) is assumed initially. Doing so, the optimum current density can be determined from the maximum of the global performance parameter (power output P or efficiency η). This has been carried out up to now by means of numerical procedures such as a 1D TE FEM code or the algorithm of multi-segmented elements.An analytical solution of the 1D thermal energy balance based on Bessel functions is available assuming constant material gradients. Similar solutions as ours had been discussed in the mid-1960’s by Ybarrondo et al. (for a TE cooler), but the impact on the performance couldn’t be determined quantitatively due to the lack of suitable computation tools. Therefore we revisit the corresponding solutions in particular to gain information about the influence of the slope of the material parameters on the performance, and we calculate the corresponding optimal values. For a constant electrical conductivity but linear profiles S(x) and κ(x), the authors present detailed results for P and η of a TEG element. An optimization strategy based on the analytical solution can be used as a helpful instrument for finding efficient segmentation schemes: the continuous material profiles can be transferred into segments by means of the algorithm for multiple-segmented TE elements. Each segment applies the CPM based on local averages of the linear material profiles. Exemplarily, a three stage segmented generator will be discussed.
5:30 PM - DD2.10
Hybrid Thermal Behavior from Thermoelectrics to Heat Sinking of a Thin Si Membrane With Stretched Ge Quantum Dots.
Jean-Numa Gillet 1
1 Physics, University of Lille 1 and IEMN, Villeneuve d'Ascq France
Show AbstractA membranous nanomaterial showing a hybrid thermal behavior between insulating and dissipative regimes is proposed for the first time (to my knowledge) with applications in both thermoelectrics and heat sinking. The nanomaterial is made up of a thin Si suspended membrane covered by self-assembled Ge quantum dots (QDs) with epitaxial facets. A membrane plane is designed from the orthogonal [100] and [001] directions of the diamond-cubic group that are named x and z, respectively. The Ge QDs are voluntarily stretched in [001]. The broken symmetry induced in both direct and reciprocal spaces leads to an exalted phonon wave-guiding in [001]. When hot and cold junctions are connected to the membrane following the stretching direction [001], the throughput thermal conductivity shows a significant exaltation with respect to the in-plane orthogonal direction [100] where QD constriction appears. In my theoretical model, a deflection angle β is taken in a membrane plane from the axis x to that z. The anisotropic thermal conductivity is analyzed as a function of the latter angle. An example hybrid device is designed from the superposition of molecular-scale supercell slabs containing a total of 4348 atoms. Among them, 868 Ge atoms and 3480 Si atoms are used to model, with 3D lattice dynamics, the Ge QDs and Si membrane with the respective thicknesses of 4 nm and 8 nm in the epitaxial-growth direction [010] or y. The thermal conductivity shows a huge 12-fold exaltation from 1 to 12 W/m/K when β is increased from 0° (axis x) to 90° (axis z), respectively. Other simulations will be presented to show the effects of the size parameters and density of states. The broken-symmetry nanomaterial presents a changing behavior from heat insulation to dissipation when phonon propagation is modified from the direction [100] to that [001]. As a result, it can be used for the design of nanoscale sinkers to remove heat in only one main direction when junctions are connected following [001]. Indeed, low leakage heat currents are obtained in other directions so that they cannot affect thermal budget in other parts of a device to be cooled as a silicon chip. Moreover, since heat insulation appears when junctions are linked following [100], unidirectional thermoelectric properties as Seebeck generation or Peltier cooling can be as well envisioned with the proposed nanomaterial. The author received the Outstanding Scientific Paper Award of the 2009 International Conference on Thermoelectrics (ICT 2009).
5:45 PM - DD2.11
Design and Fabrication of Thermoelectric Devices from One and Two Dimensional Nanomaterials Composed of Bismuth, Antimony, and Tellurium.
Derrick Mott 1 , Nguyen Thuy 1 , Nguyen Mai 1 , Go Nakamoto 1 , Mikio Koyano 1 , Shinya Maenosono 1
1 School of Materials Chemistry, Japan Advanced Institute of Science and Technology, Nomi, Ishikawa, Japan
Show AbstractWith the event of nanotechnology, the field of thermoelectric (TE) materials has been re-invigorated with many recent advances towards materials with high thermoelectric efficiency (dimensionless figure of merit, ZT). The realization of such materials opens up new avenues to the creation of devices that can be used in freon-less refrigeration, micro-electronic cooling, or for harnessing lost heat energy from sources such as car engines. In our own research work, we have successfully synthesized thermoelectric nanoscale materials composed of bismuth, antimony, and tellurium. By using a wet chemical thermal reduction procedure, we were able to create bismuth, antimony, and tellurium composite particles. What’s more, by employing different molecular encapsulating agents in the synthesis, we were able to control the resulting shapes of the nanomaterials, resulting in both one and two dimensional bismuth, antimony, and tellurium nanoparticles. The one dimensional nanowires exhibit a micron scale length and ~50nm diameter, while the two dimensional nanodiscs exhibit a diameter of ~75nm and a thickness of ~25nm. The unique morphology of these materials made them ideal candidates for processing into functional thermoelectric devices. This presentation focuses on our recent study of both the synthesis and processing of bismuth, antimony, and tellurium composite nanomaterials of a nanowire and nanodisc morphology into functional thermoelectric devices with enhanced activity. The resulting materials and devices are characterized using techniques such as HR-TEM, EDS, XPS, XRD, and SEM, as well as many others.
Symposium Organizers
John D. Baniecki Fujitsu Laboratories Ltd.
Jonathan A. Malen Carnegie Mellon University
G. Jeffrey Snyder California Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
DD3: Bulk Thermoelectrics - Not Oxides
Session Chairs
Tuesday AM, April 06, 2010
Room 2002 (Moscone West)
9:00 AM - **DD3.1
Recent Progress in Skutterudite-based Thermoelectrics.
Ctirad Uher 1
1 Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThermoelectricity offers many attractive features (environmentally green, no moving parts, exceptional reliability) as a fully solid-state energy conversion process and in several niche areas (e.g., deep space explorations) has proved to be a pivotal enabling technology. Unfortunately, it also suffers from a rather low conversion efficiency that reflects poorly performing current state-of-the-art thermoelectric materials that has hampered broader industrial appeal. In recent years, a worldwide research effort has focused on identifying and developing a new generation of more efficient thermoelectric materials. Among several prospective candidates, skutterudites have attracted much attention as one of the most promising novel thermoelectrics for power conversion and waste heat recovery applications in the intermediate range of temperatures between 300oC and 600oC. The cubic crystal lattice of binary skutterudites such as CoSb3 contains structural voids that can be filled by a variety of foreign species (rare earths, alkaline earths, actinides, and even elements) resulting in the so-called filled skutterudites. Filling is an efficient n-type doping process that controls the carrier density. Moreover, the fillers also act as strong scatterers of phonons leading to a dramatic reduction in the lattice thermal conductivity, the essential feature that makes filled skutterudites of interest for thermoelectricity. Exploring a variety of fillers and using innovative synthesis routes, an impressive progress has been made in enhancing the thermoelectric figure of merit of n-type skutterudites with the latest results approaching ZT=1.5. P-type forms of skutterudites, obtained by replacing some of the Co atoms by Fe, have been much less explored and their ZT’s have, so far, not exceeded the value of unity. Much effort needs to focus on improving ZT of p-type skutterudites and making them comparable to that of n-type forms of the material. I review the recent progress in the design and synthesis of efficient skutterudite-based thermoelectrics and I also discuss the current efforts to use modules fabricated with skutterudites in power generation applications such as harvesting of waste industrial heat.Supported by the U.S. Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center at U of Michigan.
9:30 AM - DD3.2
High-temperature Thermoelectric Properties of Thallium-filled Skutterudites: TlxCo4Sb12.
Adul Harnwunggmoung 1 2 , Ken Kurosaki 1 , Hiroaki Muta 1 , Shinsuke Yamanaka 1 3
1 , Osaka University, Osaka, Suita, Japan, 2 , Rajamangala University of Technology Suvarnabhumi, Huntra , Phranakhon Si Ayutthaya , Thailand, 3 , University of Fukui, Bunkyo, Fukui, Japan
Show AbstractFilled skutterudite antimonides are known as excellent thermoelectric (TE) materials. It has been reported that the voids in skutterudites such as Co4Sb12 can be filled or partially filled with a variety of different atoms, including La, Ce, Pr, Nd, Sm, Eu, Yb, Ba, Sr, and Th. However, as for Tl-filled skutterudites, there exists only one report [1], in which the TE properties below room temperature have been systematically investigated. In the present study, therefore, in order to clarify the high-temperature TE properties of Tl-filled skutterudites, we prepared polycrystalline samples of TlxCo4Sb12 (x = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, and 0.35) and examined their TE properties from room temperature to 750 K. The single phase samples having the skutterudite structure were obtained in the composition up to x = 0.25. All the samples indicated negative values of the Seebeck coefficient. Both the electrical resistivity and the absolute values of the Seebeck coefficient decreased with increasing the Tl-filling ratio. The lattice thermal conductivity rapidly decreased with increasing the Tl-filling ratio. Tl0.25Co4Sb12 exhibited the best ZT values; the maximum ZT was 0.9 obtained at 600 K.[1] B.C. Sales, B.C. Chakoumakos, and D. Mandrus, Phys. Rev. B 61, 2475-2481(2000)
9:45 AM - DD3.3
Thermoelectric Properties of the Non-equilibrium Synthesized Filled Skutterudites.
Qing Jie 1 , Juan Zhou 1 , Ivo Dimitrov 1 , Jorge Camacho 1 , Qiang Li 1 , Chang-Peng Li 2 , Ctirad Uher 2
1 , Brookhaven National Lab, Upton, New York, United States, 2 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractFilled skutterudites are very promising thermoelectric materials for intermediate temperature power generation. A non-equilibrium synthesis method was developed to further improve the energy conversion efficiency. In this paper, we report the non-equilibrium synthesis and thermoelectric property characterization of filled skutterudites. This non-equilibrium synthesis combines rapid solidification, such as melt spinning, with short time sintering under pressure, using either hot press or spark plasma sintering techniques. As processed materials are generally very dense, and have small grains in the range of a few nanometers to a few hundreds of nanometers. Measurements of thermoelectric properties, i.e. thermal conductivity, thermopower and electrical resistivity, were carried out from liquid helium temperature up to 800K. For comparison, another set of samples with the same chemical composition were also prepared by using conventional equilibrium method (long-term solid state reaction and annealing), and characterized. Several non-equilibrium synthesized p-type filled skutterudites exhibited both higher electrical conductivity and lower lattice thermal conductivity than samples prepared by the conventional equilibrium method. We will discuss the relationship between the thermoelectric properties and the structure of these materials, determined from high-resolution transmission electron microscope (HRTEM) observation and neutron scattering experiments.
10:00 AM - DD3.4
Mechanical Properties and Thermal Expansion of Skutterudites.
Gerda Rogl 1 2 3 , Peter Rogl 1 , Andriy Grytsiv 1 , Daniel Rajs 2 , Martin Kriegisch 2 , Ernst Bauer 2 , Johannes Koppensteiner 3 , Wilfried Schranz 3 , Long Zhang 1 2 , Stephan Puchegger 3 , Michael Zehetbauer 3
1 Physical Chemistry, University Vienna, Vienna Austria, 2 Solid State Physics, Vienna Universty of Technology, Vienna Austria, 3 Physics Group, Universty of Vienna, Vienna Austria
Show AbstractWith a worldwide energy shortage and decreasing supplies of primary energy thermoelectric devices are becoming more and more important. For a thermoelectric device thermocouples, consisting of a p- and an n-leg as well as of contacting electrodes are needed. The material for both legs should not only fulfil high thermoelectric standards (high ZT, preferable the same in both legs) but for long term stability under application a TE-device needs to exhibit a minimum mechanical stability and minimized thermal expansion mismatch in legs and couplings. Time-of-flight and resonant ultrasound spectroscopy techniques were used to determine Young`s modulus and Poisson ratio. Ball milled and hot-pressed didymium and mischmetal samples illustrate an obvious improvement of mechanical properties in the case of hot pressed samples compared with those produced from hand ground powders. Vickers hardness is strengthened by Co or Ni substitution and demonstrates a linear dependence on density, Youngs modulus and shear modulus. We made attempts to investigate engineering stress and strain. We determined and compared thermal expansion for Sb-, Ge-, As- and P-based skutterudites (capacitance method for low, zero force method for high temperatures or measurement of the lattice parameter at different temperatures) to calculate the thermal expansion coefficient a. For a series of high ZT didymium, mischmetal (DD,Mm)y(Co,Ni)xFe4-xSb12 and triple filled skutterudites we found a dependency of the thermal expansion coefficient on the filling level, a difference between p- and n-type alloys, but practically no difference between micro- and nano-structured alloys. The semiclassical model of Mukherjee has been successfully used to quantitatively describe a in terms of Debye and Einstein temperatures, which compare well with corresponding results from specific heat, resistivity or temperature dependent x-ray measurements.
10:15 AM - DD3.5
Synthesis and Thermoelectric Properties of Doped Yb14MnSb11-xBix and Yb14MnSb11-xAsx Zintls.
Kurt Star 1 , Alexandra Zevalkink 2 , Chen-Kuo Huang 3 , Bruce Dunn 1 , Jean-Pierre Fleurial 3
1 Department of Material Science and Engineering, University of California Los Angeles, Los Angeles, California, United States, 2 Materials Science Department, California Institute of Technology, Pasadena, California, United States, 3 Power and Sensor Systems Group, Jet Propulsion Laboratory/California Institute of Technology, Pasadena, California, United States
Show AbstractYb14MnSb11 is a very promising thermoelectric material for high temperature applications. This compound is a member of a large family of Zintl phases with a “14-1-11” A14MPn11 stoichiometry (Pn = P, As, Sb, Bi; A = Ca, Ba, La, Sr, Yb, Eu; Mn = Mn, Al, Cd, Ga, In, Nb, Zn). Yb14MnSb11 exhibits low lattice thermal conductivity values and a p-type semimetallic behavior with values of the non-dimensional figure of merit ZT peaking at 1.4 above 1200 K. There is a significant interest in investigating how substitutions on any of the atomic sites impact the charge carrier concentration, band gap and lattice thermal conductivity. Recent reports have studied substitutions on the Yb and Mn sites with the goal of reducing hole carrier concentration and improving carrier mobility values. High energy ball milling has been shown to be a convenient method of synthesis to prepare Yb14MnSb11 and it has been used here to explore the solid solution systems derived from this compound by substituting Sb with Bi or As. High energy ball milling is a non-equilibrium process and not all of the 14-1-11 compounds are easily formed with this method. Characterization of the synthesized compositions was done by X-ray diffraction, electron microprobe, and high temperature measurements of the electrical and thermal transport properties up to 1275 K. The experimental results on undoped and doped solid solution samples are compared to that of pure Yb14MnSb11 samples prepared by the same high energy ball milling technique.
10:30 AM - DD3: Bulk TE
BREAK
11:00 AM - **DD3.6
Transport in Complex Zintl Antimonides.
Eric Toberer 1 , Andrew May 2 , Jeffrey Snyder 1
1 Materials, California Institute of Technology, Pasadena, California, United States, 2 Chemical Engineering, California Institute of Technology, Pasadena, California, United States
Show AbstractZintl phases and related compounds are promising thermoelectric materials, for instance high zT has been found in Yb14MnSb11, clathrates and the filled skutterudites. The rich solid-state chemistry of Zintl phases enables numerous possibilities for chemical substitutions and structural modifications that allow the fundamental transport parameters (carrier concentration, mobility, effective mass, and lattice thermal conductivity) to be modified for improved thermoelectric performance. For example, free carrier concentration is determined by the valence imbalance using Zintl chemistry, thereby enabling the rational optimization of zT. The low thermal conductivity values obtained in Zintl thermoelectrics arise from a diverse range of sources, including point defect scattering and the low velocity of optical phonon modes. Despite their complex structures and chemistry, the transport properties of many modern thermoelectrics can be understood using traditional models for heavily doped semiconductors.
11:30 AM - DD3.7
Novel Thermoelectric Materials in the Ternary System Zn-Sb-In.
Yang Wu 1 , Andreas Tenga 1 2 , Sven Lidin 2 , Nathan Newman 3 , Ulrich Haussermann 1
1 Chemistry and Biochemistry, Arizona State University, Tempe, Arizona, United States, 2 Inorganic Chemistry, Stockholm University, Stockholm Sweden, 3 School of Materials, Arizona State University, Tempe, Arizona, United States
Show AbstractAttempts to synthesize derivatives of the state-of-the-art thermoelectric material beta-Zn4Sb3 resulted in the discovery of the new ternary intermetallic compounds Zn5Sb4In2-x and Zn9Sb6In2. Millimeter-sized crystals can be grown from molten metal fluxes, where indium was employed as both flux medium and reactant. The phases crystallize in new structure types featuring 32434 nets formed by Sb atoms, which are stacked in an antiposition fashion. This yields an arrangement of square antiprisms intervened by tetrahedra. One portion of Zn atoms distributes in tetrahedral interstices to give a Zn-Sb framework (with a composition “ZnSb”) common to all structures. The remaining Zn atoms and In atoms distribute in channels provided by the square antiprisms. Here different patterns are realized, resulting in a unique structure for Zn5Sb4In2-x and two structures for Zn9Sb6In2. Transport property measurements show that the Zn-In-Sb phases are – similar to beta-Zn4Sb3 - p-type conductors. Also, the peculiarly low thermal conductivity (1 W/mK) of beta-Zn4Sb3 is preserved in the ternary compounds. In the investigated temperature range 10 to 350 K Zn5Sb4In2-x displays higher thermoelectric figure of merits than Zn4Sb3.
12:00 PM - DD3.9
Thermoelectric Properties of Yb0.75Eu0.25Zn2Sb2 and Yb0.5Eu0.5Zn2Sb2.
Hui Zhang 1 2 , Mei-Bo Tang 1 , Ulrich Burkhardt 2 , Horst Borrmann 2 , Gudrun Auffermann 2 , Stefan Hoffmann 2 , Michael Baitinger 2 , Zhen-Yong Man 1 , Hao-Hong Chen 1 , Xin-Xin Yang 1 , Yuri Grin 2 , Jing-Tai Zhao 1
1 , Laboratory of Transparent Opto-Functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai China, 2 , Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Str. 40, Dresden Germany
Show AbstractThe thermoelectric performance of CaAl2Si2 type [1, 2] Zintl-phases could be improved by substitution elements both in cation vacancy and ion network [3, 4]. Eu substituted Yb solid solution samples of Yb0.75Eu0.25Zn2Sb2 and Yb0.5Eu0.5Zn2Sb2 were prepared from high pure elements through melting and annealing. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. The maximal ZT values of 0.84 and 0.97 at 800 K were obtained for Yb0.75Eu0.25Zn2Sb2 and Yb0.5Eu0.5Zn2Sb2, respectively. Yb0.5Eu0.5Zn2Sb2 had low electrical resistivity (1.56 mohmcm), high Seebeck coefficient (175 μV/K), low thermoconductivity (1.66 W/mK) and high power factor (19.5 μW/cmK2) at 800 K. Hall effects showed Yb0.5Eu0.5Zn2Sb2 is a p-type semiconductor with positive Hall coefficient (+0.0993 cm3C-1), high carrier concentration (6.29×1019 cm-3) and high carrier mobility (240 cm2V-1s-1) at 300 K. Magnetic measurement showed Yb0.5Eu0.5Zn2Sb2 an anti-ferromagnetic with Neel temperature 4.5 K and an effect magnetic moment 3.10 μB/f.u. Heat capacity and electrical conductivity confirmed the anti-ferromagnetic anomaly at 6 K and 7 K, respectively. Literature: [1] C. Zheng, R. Hoffmann, R. Nesper, and H. G. von Schnering, J. Am. Chem. Soc. 1986, 108, 1876.[2] H. Zhang, J. T. Zhao, Yu. Grin, X. Wang, M. Tang, Z. Man, H. Chen, X. Yang, J. Chem. Phys. 2008, 129, 164713.[3] F. Gascoin, S. Ottensmann, D. Stark, S. M. Haile, G. J. Snyder, Adv. Funct. Mater. 2005, 15, 1860.[4] X. J. Wang, M. B. Tang, J. T. Zhao, H. H. Chen, X. X. Yang. Appl. Phys. Lett. 2007, 90, 232107.
12:15 PM - DD3.10
High Temperature Transport in the Zintl Compounds Ca3AlSb3 and Ca5Al2Sb6.
Alexandra Zevalkink 1 , Nicole Crisosto 2 , Eric Toberer 1 , Jeff Snyder 1
1 Materias Science, Caltech, Pasadena, California, United States, 2 Physics, Harvey Mudd, Pasadena, California, United States
Show AbstractComplex Zintl compounds are a promising class of materials for thermoelectrics, with high thermoelectric efficiency found in AZn2Sb2, Yb14MnSb11, filled skutterudites, and the clathrate compounds. Such materials bridge between the high electronic mobility of covalent compounds and the high effective mass of ionic materials. When structurally complex, Zintl compounds exhibits some of the lowest lattice thermal conductivities known. Understanding the relationships between structure and intrinsic transport properties in Zintl compounds is thus vital for the development of advanced thermoelectric materials.Here, the high temperature electronic and thermal transport properties of the Zintl compounds Ca3AlSb3 and Ca5Al2Sb6 are investigated. If high thermoelectric efficiency is obtained in these materials through suitable doping, the low cost and non-toxicity of the constituent elements is attractive. Samples were synthesized by melting stoichiometric quantities of the pure elements, followed by milling and hot pressing. Both Ca3AlSb3 and Ca5Al2Sb6 have known structures, which consist of anionic chains of corner-sharing AlSb4 tetrahedrons, surrounded by Ca cations. The two structures are similar except for the Sb-Sb bonds which are present only in the Ca5Al2Sb6 compound. XRD was used to confirm the crystal structure of each sample and SEM/EDS was used to estimate grain size and check for impurity phases. Both compounds were predicted to be charge balanced intrinsic semiconductors using the Zintl formalism. This prediction was confirmed in the Ca5Al2Sb6 compound by transport measurements up to 500°C; the electrical resistivity was found to be high at room temperature, but decreased with increasing temperature due to carrier activation. The band gap was calculated to be approximately 40 eV. The Seebeck coefficients of both materials were found to peak at 300μV/K and 350μV/K for Ca3AlSb3 and Ca5Al2Sb6 respectively, indicating that both compounds are p-type. The thermal conductivity was found to be as low as 0.7 W/mK for both Ca3AlSb3 and Ca5Al2Sb6.
12:30 PM - DD3.11
Ab-initio Determination of the Zn4Sb3 Phase Diagram.
Gregory Pomrehn 1 , Axel van de Walle 1 , G. Snyder 1
1 Materials Science, California Institude of Techlology, Pasadena, California, United States
Show AbstractA phase diagram of Zn4Sb3 has been determined through ab initio calculations and statistical thermodynamics. This work expands on previous computational investigation by considering numerous possible defect structures around the ground state. A cluster expansion is utilized to account for the energetics of non-dilute defects. The grand canonical partition function is generated under the independent cells approximation and includes phonon contributions to the free energy. The phase boundaries are computed with respect to Zn and ZnSb end members. The results agree with previous findings that the ground state stoichiometry is Zn13Sb10, which has a positive formation energy with respect to Zn and ZnSb. The Zn4Sb3 phase is shown to be entropically stabilized.
12:45 PM - DD3.12
Thermoelectric Performance and Mechancial Properties of Nanostructured ZrNiSn Compounds Synthesized by Mechanical Alloying.
Jeffrey Germond 1 , Paul Schilling 1 , Nathan Takas 2 3 , P. Ferdinand Poudeu 2 3
1 Mechanical Engineering, University of New Orleans, New Orleans, Louisiana, United States, 2 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States, 3 Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractSamples with a composition ZrNiSn were synthesized by a combination of mechanical alloying (MA) and consolidation by either Spark Plasma Sintering (SPS) or hot pressing (HP). Appropriate stoichiometric ratios of the starting materials were milled under an inert atmosphere in a high energy ball mill for various times. A ZrNiSn half-Heusler compound was successfully synthesized the early stages of milling. Subsequent milling caused a reduction in grain size of the half-Heusler phase, reaching < 25 nm after 40 hours, based on analysis of peak broadening in the X-ray diffraction data. Samples prepared at selected milling times were consolidated using the SPS and HP methods for comparison. Thermal conductivity, electrical conductivity and Seebeck coefficient were measured as a function of temperature in the range 300 K to 800 K and compared with measurements reported for high temperature solid state reaction synthesis of this compound. HP samples, compared to SPS samples, demonstrate increased grain growth due to longer heating times. The MA/SPS samples demonstrate a reduced thermal conductivity compared to samples prepared by high temperature synthesis. This is attributed to the reduced grain size achieved by MA and SPS, which causes increased phonon scattering at grain boundaries. The samples were also characterized using microindentation and depth-sensing nanoindentation to obtain hardness and elastic modulus values.
DD4: Novel Thermoelectric Materials and Resonant States
Session Chairs
Tuesday PM, April 06, 2010
Room 2002 (Moscone West)
2:30 PM - **DD4.1
Functions of Key Structural Unit and Performance Optimization in Novel Thermoelectric Materials.
Lidong Chen 1 , Wenqing Zhang 1
1 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai China
Show AbstractThe recent progresses on enhancing conversion efficiency of thermoelectric materials will be highlighted putting the emphasis on the multi-level structure control. For CoSb3-based filled skutterudite system, after understanding the filling fraction limits for both single- and multiple- filling, we are stepping forward to maximize their TE performance through optimization of filling approach. By joining the efforts from theoretical calculations, HRTEM, inelastic neutron scattering, and property measurements, we tried to reveal some special properties due to the intrinsic cages in CoSb3 skutterudites, including the random distribution of fillers, incoherent localized rattling behavior, rattling-frequency-dependent resonant phonon scattering, and the limited but universal range of the optimum electron concentration for n-type filled CoSb3. With that, we pushed the lattice thermal conductivities of the partially filled skutterudites to their glass limit and maintained a good electrical transport, and therefore realized a very high ZT. The approach to tune TE transport by “engineering” the key structure unit have also been applied to Cu2ZnSnSe4–based compounds, which has a diamond-based structure with a key structural unit of tetrahedral blocks. The Cu2ZnSnSe4-based compounds show very low thermal conductivity primarily due to the large deformation of the tetrahedrons and randomness of multi atoms, while the perfect or undeformed compounds such as diamond and silicon always show high lattice thermal conductivities. Our work also hinted that there might exist an electron transport network, and it should not be disturbed for obtaining good electrical transport properties. By following this general understanding, we experimentally realized relatively high TE performance for a few tetrahedrally-bonded semiconductors with relatively wide gaps. Further efforts have also been made in further enhancing performance by introducing nano structure in these novel thermoelectric materials.
3:00 PM - DD4.2
Optimization of the Thermoelectric Properties of AyMo3Sb7-xTex.
Tim Holgate 1 , Terry Tritt 1 , Hong Xu 2 , Holger Kleinke 2
1 Department of Physics and Astronomy, Clemson University, Clemson, South Carolina, United States, 2 Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractModifying Mo3Sb7 via partial Sb/Te substitution led to a drastic improvement of its thermoelectric properties.[1, 2] These properties may be altered by adding small metal atoms such as A = Mg or Cu into its cubic voids, which occurs with changes in the charge carrier concentration as well as in the size of the band gap.[3] Recently we reported on our optimization attempts via variation of the Sb/Te ratio and addition of A = Ni, which culminated in ZT = 0.93 for Ni0.06Mo3Sb5.4Te6.[4] While this material exhibits a large power factor, its thermal conductivity of 5 Wm-1K-1 at 300 K is rather high for a thermoelectric material.
Therefore, we are trying to lower the thermal conductivity by varying the synthesis and the consolidation procedures. For example, using spark-plasma-sintering instead of hot-pressing led to a decrease in the total thermal conductivity by 10-20%.
[1]E. Dashjav, A. Szczepenowska, H. Kleinke, J. Mater. Chem. 2002, 12, 345.
[2]F. Gascoin, J. Rasmussen, G. J. Snyder, J. Alloys Compd. 2007, 427, 324.
[3]N. Soheilnia, E. Dashjav, H. Kleinke, Can. J. Chem. 2003, 81, 1157.
[4]H. Xu, K. M. Kleinke, T. Holgate, H. Zhang, Z. Su, T. M. Tritt, H. Kleinke, J. Appl. Phys. 2009, 105, 053703.
3:15 PM - DD4.3
Seeking Resonance Levels: Alkali Metal Doping Studies of PbTe.
Ioannis Androulakis 1 , Iliya Todorov 2 , Duck-Young Chung 2 , Sedat Ballikaya 3 , Guoyu Wang 3 , Ctirad Uher 3 , Mercouri Kanatzidis 1 2
1 Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Materials Science Division, Argonne National Lab, Argonne, Illinois, United States, 3 Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractRecently, the concept of lattice nanostructuring, as a means to effectively reduce lattice thermal conductivity, has led to considerable advancements in the field of thermoelectrics, establishing a ZT of ~1.6 in both n and p type materials based on PbTe. It is intriguing, therefore, to explore ways of pushing the PbTe power factor higher and thus achieve greater ZT enhancements. To this end the idea of resonance scattering is both attractive and promising. Guided by theoretical calculations on deep defect states in PbTe, we examined the effect of doping of alkali metals on the transport properties of PbTe, placing particular emphasis on potassium doping. We combined electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect measurements as a function of temperature and doping level to examine scattering mechanisms. Interestingly, a strong scattering mechanism, which cannot be accounted for by the effect of the heavy hole band of PbTe, was revealed. We discuss our results in the light of theoretical predictions for resonance levels in PbTe.
3:30 PM - DD4.4
Low Phonon Thermal Conductivity of the Layered Bi-Te Intermetallic Alloys.
Peter Sharma 1 , Alf Morales 2 , Ana Lima Sharma 3 , Monica Barney 2 , Fivos Drymiotis 4 , Jian He 4 , Terry Tritt 4 , James Turner 4
1 Materials Physics, Sandia National Laboratories, Livermore, California, United States, 2 Materials Chemistry, Sandia National Laboratories, Livermore, California, United States, 3 Physics, San Jose State University, San Jose, California, United States, 4 Physics, Clemson University, Clemson, South Carolina, United States
Show AbstractGood thermoelectric materials should have as low a thermal conductivity as possible in order to increase the figure of merit zT=TS^2/ρκ. The thermal conductivity of a material is often tuned using microstructure, chemistry, and/or crystal structure. For example, alloy disorder combined with high atomic weights plausibly lead to the low thermal conductivity of commercial Bi2Te3-based thermoelectrics. The crystal structure of the Bi-Te intermetallic phases is composed of layers of Bi and Bi2Te3 structural units, which can be varied nearly continuously by adjusting the Bi/Te ratio. Compared to both Bi2Te3 and elemental Bi, we found that these materials have a strongly reduced thermal conductivity below room temperature. However, this reduction did not depend on the Bi/Te ratio. The Debye-Calloway model was used to explore the origin of the low thermal conductivity in these materials.
3:45 PM - DD4.5
Multicomponent Clathrates.
Peter Rogl 1
1 Institute of Physical Chemistry, University of Vienna, Wien Austria
Show AbstractAmong the manifold of “intermetallic” clathrates, hitherto two series of clathrate type I compounds have shown interesting TE properties: EA8M16Ge30 and EA8MxGe46-x-yy (EA=earth alkaline metal). The paper focuses on a systematic study of clathrate formation, clathrate structures, bonding and structure-property relation in novel multicomponent clathrate type I materials EA8(M,M')x[Si,Ge]46-x-yy where M, M' are predominantly 3d, 4d, 5d elements. Physical as well as mechanical properties have been evaluated particularly with respect to thermoelectric applications. The present analysis shows the difficulties in preparation and design of clathrate compounds at a given electron/atom concentration. Validity and shortcomings of the Zintl concept for clathrates will be outlined. The correlations obtained may provide useful in defining compositional regions of interest for further search for novel clathrate materials with interesting thermoelectric properties.
4:00 PM - DD4: transport
BREAK
4:15 PM - **DD4.6
Antimony Based Materials for Low (FeSb2), Intermediate (Zn4Sb3) and High (Yb14MnSb11) Temperature Thermoelectric Applications.
Bo Brummerstedt Iverson 1
1 Centre for Energy Materials, Centre for Materials Crystallography and iNANO, Department of Chemistry, Aarhus University, Aarhus Denmark
Show AbstractSome of the most promising thermoelectric materials contain antimony, which is an extremely flexible element with respect to chemistry and properties. In the present talk recent results on FeSb2, Zn4Sb3 and Yb14MnSb11 materials will be discussed. FeSb2 exhibits the highest power factor ever seen and the material has great potential for low temperature (< 77 K) cooling applications [1]. The challenge is to lower the thermal conductivity, and this can e. g. be done by synthesizing thin films [2]. Intensive research has resulted in complex materials with superior thermoelectric performance to Zn4Sb3 in the intermediate temperature range (200-600 oC), but the fundamental attractiveness of Zn4Sb3 remains unchallenged. Thus, Zn4Sb3 is one of the cheapest thermoelectric materials known, and it is made of non-toxic elements [3]. The major problem is the high temperature thermal stability, and significant efforts have been devoted to understanding this aspect and reducing the degradation. Finally, the Yb14MnSb11 phases have shown great promise for high temperature energy conversion. The structure is very complex and we will discuss recent structural studies on the compound.[1] (a) A. Bentien et al., Eur. Phys. Lett. 2007, 80, 17008-17012. (b) P. Sun et al, J. Chem. Soc. Dalton Trans. 2010, 39, 1012-1019[2] Y. Sun et al., J. Appl. Phys. 2009, 106, 033710[3] (a) G. J. Snyder et al., Nature Materials 2004, 3, 458-463. (b) B. L. Pedersen et al., Appl. Phys. Lett. 2008, 92, 161907; Appl. Phys. Lett. 2006, 89, 242108; Chem. Mater. 2010, in press
4:45 PM - DD4.7
NMR and Transport Study of Ba8Ga16Sn30 Clathrates.
Sergio Rodriguez 1 , Xiang Zheng 1 , Joseph Ross 1
1 Department of Physics, Texas A&M University, College Station, Texas, United States
Show AbstractClathrates contain four-fold bonded network cages of Si, Ge or Sn that can encapsulate guest atoms inside the cages. Some of the clathrates have shown “phonon glass electron crystal” behavior, and their properties make them good candidates for possible thermoelectric applications. For this study we considered Ba8Ga16Sn30 which shows a structural dimorphism because it can crystallize as a type-VIII (α phase) or type-I (β phase), depending on the annealing conditions. Type I Ba8Ga16Sn30 has large cages allowing particularly large displacements of the cage-center atoms, and a very low thermal conductivity has been previously reported. Samples for this work were prepared by arc melting, and then sealed in quartz for further solid state reaction and annealing. Several samples of both types were prepared with different amounts of Sn and Ga excess, with structures verified by powder x ray diffraction. 71Ga NMR measurements were used to characterize the dynamical and electronic properties of the two phases. At high temperatures for type VIII the NMR shifts are constant and the T1 agrees approximately with the expectation for the Korringa law indicating that charge carriers dominate. The shift decreases at temperatures near 4 K, characteristic of a partial freezing out of carriers, as seen previously in other clathrate materials. We have also used the WIEN2k package to model the electronic behavior, comparing computed Knight shifts for different framework occupations with the measured results, giving a measure of the distribution of local electronic behavior vs. structural configuration. For type I, which exhibits the more pronounced rattling behavior, we found larger changes in T1 vs. temperature, coupled with changes in the resonance linewidth, indicative of the influence of atomic motion on the NMR. In T1 measurements for type I, we found a low-temperature peak in relaxation rate, which we attribute to the slowing down of atomic rattlers, indicating that this peak might be due to the coupling to a strongly anharmonic phonon mode of the guest atom similar to the behavior recently reported for the pyrochlore superconductor KOs2O6. We will further report the results of transport and magnetization measurements on these samples. This work was supported by the Robert A. Welch Foundation, Grant No. A-1526.
5:00 PM - DD4.8
Effect of Electric Current Stressing on Thermoelectric Properties of Hot-pressed Bi-Se-Te Nanostructured Compounds.
Sin-Shien Lin 1 , Chien-Neng Liao 1
1 Materials Science and Engineering, National Tsing-Hua University, Hsinchu Taiwan
Show AbstractThermoelectric generation has long been recognized as a prospective solution for thermal energy harvesting. Despite its plausible potential, low conversion efficiency of thermoelectric devices still remains to be improved. In recent years, bulk nanostructured materials has shown remarkable enhancement in thermoelectric properties; furthermore, due to the ease of large quantity production, bulk nanostuctured materials is promising for commercial usage. Bi-Se-Te based compounds have been widely used in commercial thermoelectric devices due to their superior thermoelectric figure-of-merit at room temperature regime. In addition to conventional hot pressing and ingot-extrusion approaches, a novel spark plasma sintering technique has been employed in preparing polycrystalline bismuth telluride based thermoelectric materials with good thermoelectric properties and mechanical strength as well as reduced process time. Nevertheless, the detailed mechanism of electric current interacting with sintered materials is not fully understood. In this study, Bi-Se-Te based nano-sized powders prepared by ball milling are hot-pressed and sintered at elevated temperatures. A high density of electric current of ~150 A/cm2 was introduced through the sintered sample during subsequent thermal annealing process. The effect of annealing time and electric current stressing on the microstructure and thermoelectric properties of the sintered Bi-Se-Te materials are investigated. The preliminary studies on cold-pressed material show that the density, carrier mobility and in turn electrical conductivity of the sintered samples increases with increasing annealing time. The thermoelectric power factor of the as-cold-pressed Bi-Se-Te material is 0.089 mW/mK2 which change to 0.119 mW/mK2 after thermally annealed at 200oC for 30min, but increases to 0.326 mW/mK2, which is nearly four times that of the as-cold-pressed, after electrically stressed at 200oC with an electric current density of 100 A/cm2 for 30min. Obviously, the electric current should not simply play the role of sample heating only. Further studies show that the power factor of hot-pressed material can even be enhanced to 1.128 mW/mK2 while the thermal conductivity is still relatively low due to the nano-grained structure of the material. The results suggest that we may improve the thermoelectric properties of the Bi-Se-Te based compounds by electric current stressing. The evolution of microstructure and thermoelectric properties of Bi-Se-Te compounds induced by electric current stressing will be discussed in the study.
5:15 PM - DD4.9
Investigation into Mechanisms for Increasing the zT of [Bi,Sb]Te Alloys.
Christopher Jaworski 1 , Joseph Heremans 1 2
1 Mechanical Engineering, Ohio State University, Columbus, Ohio, United States, 2 Physics, Ohio State University, Columbus, Ohio, United States
Show AbstractTin is a resonant level in the valence band of Bi2 Te31 that creates a nearly constant Seebeck coefficient over a narrow carrier concentration range. We report here on our continued investigation into the role that Sn plays in BiSbTe alloys, specifically our efforts to introduce the resonant Sn energy level into the ternary alloy and maintain the appropriate doping concentration. We also add a systematic investigation of metal-rich p-type Bi2+δTe3 material. Measurements are taken on single crystals grown through the Bridgeman method and on isotropic pressed and sintered ingots. We report on thermopower, electrical resistivity, Hall and Nernst Coefficients, and from these calculate the scattering parameter, effective mass, and Fermi levels. 1.Phys. Rev. B, Submitted. MRS Spring 2009 N6.8
5:30 PM - DD4.10
Thermoelectric Properties of (TlSbTe2)x(Tl0.02Pb0.98Te)1-x With Increased Seebeck Coefficient and Reduced Thermal Conductivity.
Heng Wang 1 , Ken Kurosaki 2 , Shinsuke Yamanaka 2 , Jeffrey Snyder 1
1 materials science, california institute of technology, Pasadena, California, United States, 2 Graduate School of Engineering, Osaka University, Osaka Japan
Show AbstractMuch progress has been made recently in the field of thermoelectric research. One of them is the Seebeck enhancement in PbTe:Tl. This is due to the resonant Tl states near the valence band edge and thus higher density of states at the Fermi level. In the meantime, the thermal conductivity of this system still has room for optimization. In this work we tried to combine the advantage of power factor enhancement with the promising strategy of reducing the thermal conductivity via nanocomposite formation. A series of (TlSbTe2)x(Tl0.02Pb0.98Te)1-x compounds were made using the same methodology as for PbTe:Tl. The samples are dense (>98%) and single phase. Transport property test revealed Seebeck enhancement similar to those observed in PbTe:Tl. Moreover, we observed a 25% reduction of thermal conductivity compared with PbTe:Tl at room temperature, possibily due to additional scattering centers. Based on this result it is very promising to achieve higher ZT values in a wider temperature range when alloying PbTe:Tl with TlSbTe2.
5:45 PM - DD4.11
Effect of (Pb,Ge)Te Addition on the Phase Stability and the Thermoelectric Properties of AgSbTe2.
Aikebaier Yusufu 1 , Ken Kurosaki 1 , Hiroaki Muta 1 , Yamanaka Shinsuke 1 2
1 , Osaka University, Suita, Osaka, Japan, 2 , University of Fukui, Fukui Japan
Show AbstractAlthough AgSbTe2 exhibits excellent thermoelectric properties, the single phase is unstable and second phases such as Ag2Te and Sb2Te3 are easy to precipitate. In the present study, we tried to stabilize the AgSbTe2 phase by adding a small amount of another phase. Here we chose Pb0.16Ge0.84Te as the adding phase because the crystal structure and the lattice parameter were completely the same as those of AgSbTe2. The polycrystalline samples of (AgSbTe2)1-x(Pb0.16Ge0.84Te)x (x = 0, 0.01, 0.02, 0.05, and 0.1) were prepared and the phase stability and the thermoelectric properties were examined. The AgSbTe2 phase was stabilized in the samples of x = 0.01 and 0.02, whereas Ag8(Ge,Pb)Te6 phase precipitated in the samples of x = 0.05 and 0.1. All the samples exhibited large thermoelectric figure of merit. The effects of (Pb,Ge)Te addition on the phase stability and the thermoelectric properties of AgSbTe2 will be discussed.
DD5: Poster Session I
Session Chairs
Tuesday PM, April 06, 2010
Exhibition Hall (Moscone West)
6:00 PM - DD5.1
On the Formation and Properties of Si/Ge.
Nadine Geyer 1 , Johannes de Boor 1 , Alexander Tonkikh 1 , Peter Werner 1 , Ulrich Goesele 1
1 1, Max Planck Institute of Microstructure Physics Halle (Saale), Halle (Saale) Germany
Show AbstractSi nanowires and Si nanowires containing a Si/Ge superlattice are expected to have superior thermoelectric properties (figure of merit ZT). In this poster, we report on the fabrication of Si and Si/Ge superlattice nanowires by metal-assisted chemical wet etching. This top-down approach uses different methods like colloidal lithography or reactive ion etching through anodized aluminum oxide membranes to cover the substrate with a patterned metal film. With this method, arrays of nanowires with hexagonal symmetry and adjustable diameters ranging from about 10 nm to several microns can be fabricated. High area densities of 1010 wires/cm2, control of diameter, length and position of the nanowires are possible. The morphology, the inner structure and the chemical composition are investigated by SEM, TEM and EDX. We report on the investigation of the thermoelectric properties (thermoelectric and electric conductivity) of these etched Si and Si nanowires containing a Si/Ge superlattice.
6:00 PM - DD5.10
Thermoelectric Figure of Merit Calculations for Nanowires –The Moderate Confinement Regime.
Jane Cornett 1 , Oded Rabin 1 2
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 IREAP, University of Maryland, College Park, Maryland, United States
Show AbstractBy confining a material to a one-dimensional nanowire, the resulting quantization of the electronic density of states function leads to an increase in the thermoelectric figure of merit (ZT). Increased phonon scattering as a result of the confinement decreases the lattice contribution to thermal conductivity, further increasing ZT. In 1993, Hicks and Dresselhaus reported ZT calculations for one-dimensional Bi2Te3 assuming a one-subband model [1]. Here, we have applied this method to more realistic multiple-subband systems. We report power factor and ZT calculations for silicon, indium antimonide and zirconium pentatelluride nanowires of varying radii at room temperature. Interestingly, some systems deviate significantly from the anticipated trend. The InSb results are particularly remarkable due to the non-monotonic relationship seen between n-type ZT and wire radius; where typically we expect to see only a decrease with increasing radius, for InSb ZT increases between 20 and 100nm wire radii. This is thought to be due to the high level of degeneracy of subbands for larger wire radii. These results indicate that the monotonic relationship between ZT and wire radius observed under strong confinement conditions cannot be assumed, but must be tested on a case-by-case basis for each materials system. [1] Hicks, L.D. and Dresselhaus, M.S., Thermoelectric figure of merit of a one-dimensional conductor. Physical Review B, 1993. 47(24): p. 16631-16634.
6:00 PM - DD5.11
Electronic Structure and Transport Coefficients of Binary Skutterudite Antimonide.
Ziyu Wang 1 , Wei Wei 1
1 , School of physics, Wuhan , Hubei, China
Show Abstract As a new source for energy utilization , thermoelectric materials have attracted much attention during the last few decades. The performance of thermoelectric materials is quantified by a figure of merit ZT = S2σT /(κe + κl). Here S is the Seebeck coefficient, σ is the electrical conductivity and T is the absolute temperature. κe and κl are the electrical and lattice part of the thermal conductivity, respectively. In order to get higher ZT value, many efforts have been devoted to increasing the power factor (PF = S2σ), while at the same time to decreasing the thermal conductivity κ (κ = κe + κl). As these transport coefficients are coupled with each other and closely related to the electronic and crystal structure, it is quite difficult to find a new compound with a large ZT value. For example, a typical insulator has a large S and a small κ, but the corresponding σ is quite small. A metal has a large σ; however, the S is small while the κ is large. It seems that the ideal candidates for high efficiency thermoelectric materials are semiconductors which have relatively larger S and σ and moderate κ. Recently, CoSb3-based skutterudite compounds have been extensively studied owing to their potential applications for thermoelectric power generation. It is found that appropriate doping can be used to enhance the ZT value of skutterudite compounds. Experimentally, both the n-type and p-type doping have been widely investigated. Many atoms (such as Ni, Te, Ba, etc) were introduced into the system to replace the Co (site A), Sb (site B), and to fill the intrinsic voids (site D). It should be noted that the transport properties and optimal doping level have seldom been reported in recent theoretical investigations. Meanwhile, most of the experimental works are carried out by random selection of doping atoms and doping concentration, and it is still unclear which elements are the best choices. In this work, we aim to give a general map by which one can dope appropriate elements with appropriate concentration and thus maximizethe electronic transport properties of CoSb3 compound. Using first-principles calculations, we investigate the electronic structure of CoSb3 compound by considering the spin–orbit interaction. Within the framework of Boltzmann theory, the transport coefficient (power factor) is evaluated as a function of chemical potential assuming a rigid-band picture and constant relaxation time. It is found that appropriate n-type doping in the compound may be better than p-type doping to enhance the power factor. Our theoretical calculations give a plausible guide on how to optimize the thermoelectric performance of this compound, and the upper limit of its ZT value is estimated.
6:00 PM - DD5.12
Effect of Partial Filling of the Structural Vacant Sublattice on the Thermoelectric Properties of Zr 0.25Hf0.75NiSn half-Heusler Alloy.
Julien Makongo Mangan 1 2 , Dinesh Misra 1 2 , Nathan Takas 1 2 , Kevin Stokes 1 2 , Heike Gabrisch 1 2 , Pierre Poudeu* 1 2
1 Advanced Materials Research Institute , University of New Orleans, New Orleans, Louisiana, United States, 2 Department of Chemistry , University of New Orleans, New Orleans, Louisiana, United States
Show AbstractIn order to improve the thermoelectric properties of the half-Heusler alloy Zr
0.25Hf
0.75NiSn, we have studied the effect of partial filling of its structural vacant sublattice on the thermoelectric properties. Ni
1-xSn
x (0 ≤ x ≤ 0.07) solid solutions, Ni, Co or a mixture of Ni and Co were mixed to Zr
0.25Hf
0.75NiSn matrix with concentrations up to 2 wt.%. X-ray powder diffraction patterns revealed the presence of the half-Heusler matrix together with small amount of the Heusler and two additional minor binary phases. Electrical conductivities, thermal conductivities and thermopowers of hot pressed specimens of the as synthesized composite materials will be compared with corresponding properties of nanostructured powders of the composite materials obtained by ball milling and pressed under identical conditions. Microstructural parameters such as homogeneity and grain size of a solid solution matrix, and size, shape and distribution of second phase particles will be correlated to the observed thermoelectric properties.*Corresponding author:
[email protected] (P.F.P. Poudeu)
6:00 PM - DD5.13
Thermoelectric Generator from SiO2/SiO2+CoSb Multi-layer Nanolayered Films Modified by MeV Si Ions Bombardment.
Satilmis Budak 1 , C. Smith 2 , J. Chacha 1 , M. Pugh 1 , H. Martin 1 , K. Heidary 1 , R. Johnson 3 , C. Muntele 2 , D. Ila 2
1 Electrical Engineering, Alabama A.&M. University, NORMAL, Alabama, United States, 2 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States, 3 Physics, Alabama A&M University, Normal, Alabama, United States
Show AbstractThe performance of the thermoelectric devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ or decreasing K. We have prepared the thermoelectric generator device from SiO2/SiO2+CoSb nanolayered films using the ion beam assisted deposition (IBAD). To determine the stoichiometry of the elements of Si, Co and Sb in the multilayer films and the thickness of the grown multi-layer films Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package have been used. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the nanolayered superlattice films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. We have characterized the thermoelectric generator devices before and after Si ion bombardments as we measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences. In addition to thermoelectric properties, the some optical properties of the SiO2/SiO2+CoSb multi-layer films have been characterized. Our findings will be shown during the meeting.*Corresponding author: S. Budak; Tel.: 256-372-5894; Fax: 256-372-5855; Email:
[email protected] Acknowledgement Research sponsored by the Center for Irradiation of Materials (CIM), Alabama A&M University (AAMU) and by the AAMURI, by National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, NSF-MRSEC Grant# DMR0820382, and by Department of Electrical Engineering under Nanotechnology Infrastructure Development for Education and Research with the proposal number: 54478-RT-ISP.
6:00 PM - DD5.14
Fabrication and Characterization of Thermoelectric Generator from Si/Si+Ge Multi-layer Superlattice Nanolayered Films Effected by MeV Si Ions Bombardment.
M. Pugh 1 , Satilmis Budak 1 , C. Smith 2 , J. Chacha 1 , H. Martin 1 , K. Heidary 1 , R. Johnson 3 , C. Muntele 2 , D. Ila 2
1 Electrical Engineering, Alabama A.&M. University, NORMAL, Alabama, United States, 2 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States, 3 Physics, Alabama A&M University, Normal, Alabama, United States
Show AbstractEffective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. The performance of the thermoelectric devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ or decreasing K. MeV ion bombardment caused defects and disorder in the film and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We have prepared the thermoelectric generator device from Si/Si+Ge nanolayered superlattice films using the ion beam assisted deposition (IBAD). To determine the stoichiometry of the elements of Si, Ge in the grown multilayer films and the thickness of the grown multi-layer films Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package have been used. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the nanolayered superlattice films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. We have characterized the thermoelectric generator devices before and after Si ion bombardments as we measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences. In addition to thermoelectric properties, the some optical properties of the Si/Si+Ge multi-layer superlattice films have been characterized. Our findings will be shown during the meeting.*Corresponding author: S. Budak; Tel.: 256-372-5894; Fax: 256-372-5855; Email:
[email protected] Acknowledgement Research sponsored by the Center for Irradiation of Materials (CIM), Alabama A&M University (AAMU) and by the AAMURI, by National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, NSF-MRSEC Grant# DMR0820382, and by Department of Electrical Engineering under Nanotechnology Infrastructure Development for Education and Research with the proposal number: 54478-RT-ISP.
6:00 PM - DD5.15
Thermoelectric Generator from SiO2/SiO2+Cu NanolayeredMultilayer Films Effected by MeV Si Ions Bombardment.
John Chacha 1 , Satilmis Budak 1 , Cydale Smith 2 , Marcus Pugh 1 , Hervie Martin 1 , Kaveh Heidary 1 , Claudiu Muntele 2 , R. Johnson 3 , Daryush Ila 2
1 Electrical Engineering, Alabama A&M Univeristy, Normal, Alabama, United States, 2 Center for Irradiation of Material, Alabama A&M Univeristy, Normal, Alabama, United States, 3 Physics, Alabama A&M University, Normal, Alabama, United States
Show AbstractThis efficiency of the thermoelectric devices is limited by the properties of n- and p-type semiconductors. Effective thermoelectric materials have a low thermal conductivity and a high electrical conductivity. The performance of the thermoelectric materials and devices is shown by a dimensionless figure of merit, ZT = S2σT/K, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature and K is the thermal conductivity. ZT can be increased by increasing S, increasing σ, or decreasing K. The defects and disorder in the film caused by MeV ions bombardment and the grain boundaries of these nanoscale clusters increase phonon scattering and increase the chance of an inelastic interaction and phonon annihilation. We have prepared the thermoelectric generator device from SiO2/SiO2+Cu multi-nano layered superlattice films using the ion beam assisted deposition (IBAD). Rutherford Backscattering Spectrometry (RBS) and RUMP simulation software package have been used to determine the stoichiometry of the elements of SiO2, Cu in the multilayer films and the thickness of the grown multi-layer films. The 5 MeV Si ions bombardment has been performed using the AAMU Pelletron ion beam accelerator to make quantum clusters in the multi-layer superlattice thin films to decrease the cross plane thermal conductivity, increase the cross plane Seebeck coefficient and cross plane electrical conductivity. To characterize the thermoelectric generator devices before and after Si ion bombardments we have measured the cross-plane Seebeck coefficient, the cross-plane electrical conductivity, and the cross-plane thermal conductivity for different fluences. In addition to thermoelectric properties, the some optical properties of the SiO2/SiO2+Cu multi-layer superlattice films have been characterized. Our findings will be shown during the meeting.*Corresponding author: S. Budak; Tel.: 256-372-5894; Fax: 256-372-5855; Email:
[email protected] Acknowledgement Research sponsored by the Center for Irradiation of Materials (CIM), Alabama A&M University (AAMU) and by the AAMURI, by National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, NSF-MRSEC Grant# DMR0820382, and by Department of Electrical Engineering under Nanotechnology Infrastructure Development for Education and Research with the proposal number: 54478-RT-ISP.
6:00 PM - DD5.16
Thermoelectric Properties of Thermoelectric Generator from SiO2/SiO2+Ag Nanolayered Multilayer Films Effected by MeV Si Ions.
Satilmis Budak 1 , Hervie Martin 1 , Cydale Smith 2 , John Chacha 1 , Marcus Pugh 1 , Kaveh Heidary 1 , R. Johnson 3 , Claudiu Muntele 2 , Daryush Ila 2
1 Electrical Engineering, Alabama A&M University, Harvest, Alabama, United States, 2 Center for Irradiation of Materials, Alabama A&M University, Normal, Alabama, United States, 3 Physics, Alabama A&M University, Normal, Alabama, United States
Show AbstractWe prepared 50 periodic nano-layers of SiO2/AgxSiO2(1-x) with Au layer deposited on both sides as metal contacts. The deposited multi-layer films have a periodic structure consisting of alternating layers where each layer is 10 nm thick. The purpose of this research is to generate nanolayers of nanocrystals of Ag with SiO2 as host and as buffer layer using a combination of co-deposition and MeV ion bombardment taking advantage of energy deposited in the MeV ion track to nucleate nanoclusters. Our previous work showed that these nanoclusters have crystallinity similar to the bulk material. Nanocrystals of Ag in silica produce an optical absorption band at about 420 nm. Due to the interaction of nanocrystals between sequential nanolayers there is widening of the absorption band. The electrical and thermal properties of the layered structures were studied before and after 5 MeV Si ions bombardment at various fluences to form nanocrystals in layers of SiO2 containing few percent of Ag. Rutherford Backscattering Spectrometry (RBS) was used to monitor the stoichiometry before and after MeV bombardments. In addition to thermoelectric properties, the some optical properties of the SiO2/SiO2+Ag multi-layer superlattice films have been characterized. Our findings will be shown during the meeting.Acknowledgement:Research sponsored by the Center for Irradiation of Materials (CIM), Alabama A&M University (AAMU) and by the AAMURI, by National Science Foundation under NSF-EPSCOR R-II-3 Grant No. EPS-0814103, NSF-MRSEC Grant# DMR0820382, and by Department of Electrical Engineering under Nanotechnology Infrastructure Development for Education and Research with the proposal number: 54478-RT-ISP.
6:00 PM - DD5.17
Transport Properties of a New Rare-earth Phosphide Zintl Phase: Eu3Ga2P4.
Tanghong Yi 1 , Naohito Tsujii 1 2 , Catherine Cox 1 , Peter Klavins 3 , Alex Williams 4 , G. Jeffrey Snyder 4 , Susan Kauzlarich 1
1 Department of Chemistry, University of California Davis, Davis, California, United States, 2 , National Institutute for Materials Science, Tsukuba Japan, 3 Department of Physics, University of California Davis, Davis, California, United States, 4 Materials Science Department, California Institute of Technology, Pasadena, California, United States
Show AbstractA new rare-earth phosphide Zintl phase Eu3Ga2P4 was synthesized via both a self-flux (Ga) and a solid state method. Single crystal X-ray diffraction shows that it is isostructural with Ca3Ga2N4 (Inorg Chem. 1997, 36, 1143), crystallizing in the monoclinic crystal system with the space group, C2/c. A pure phase can be obtained from solid state reaction of Eu3Ga2 with phosphorus and the phase purity was confirmed with powder X-ray diffraction. Eu is present as Eu2+ as indicated by temperature dependent susceptibility. A dense pellet was formed with spark plasma sintering and the thermoelectric properties from room temperature to 1200 K were measured. The thermal diffusivity measurements indicate extremely low thermal conductivity of Eu3Ga2P4, as low as 0.4 W/m K. The low thermal conductivity makes this new compound particularly interesting since phosphides normally have relatively high thermal conductivity. The low carrier concentration in this semiconducting Zintl compound results in a high electric resistivity. The low thermal conductivity and high Seebeck coefficient provide an indication that Eu3Ga2P4 may have potential as a high thermoelectric efficiency material if the charge carrier concentration can be controlled. The synthesis, structure, and thermoelectric properties will be presented and discussed relative to other phosphide compounds.
6:00 PM - DD5.18
The Role of Nanoparticle Inclusions on High-performance half-Heusler Thermoelectric Materials.
Rumana Yaqub 1 , Pranati Sahoo 1 , Nathaniel Henderson 1 , Nathan Takas 1 , Pierre F P.Poudeu 1 , Kevin Stokes 1
1 AMRI, University of New Orlean, New Orleans, Louisiana, United States
Show AbstractNanophase inclusions have been shown to improve the figure of merit of thermoelectric materials. Typically, this improvement is the result of a reduction in lattice thermal conductivity due scattering of phonons from the inclusions and grain boundaries. Here report the effect of adding oxide (NiO) nanoparticles on the transport properties of the half-Heusler alloys, Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 (x=0, 0.2 and 0.3). The half-Heusler matrix materials are prepared by traditional powder metallurgy methods. Stoichiometric amounts of the elemental constituents are reacted at 900°C under vacuum. The resulting bulk matrix is mixed with different volume fractions of nanosized NiO particles, previously synthesized by solution-phase chemical methods. The resulting mixture is densified under 100 MPa uniaxial pressure to form a nanocomposite. Under some reaction conditions, the resulting material contains both half-Heusler and full-Heusler phases. At low temperatures (<200°C), the half-Heusler/NiO nanocomposite has lower thermal conductivity than that of the pure half-Heusler material, but at higher temperatures (200-700°C), the thermal conductivity of the nanocomposite is higher than that of the pure matrix material. The effect of nanoparticle inclusions on the electrical transport properties (Seebeck coefficient and electrical conductivity) is also investigated.
6:00 PM - DD5.19
Impact of Dopants on the PbTe Thermoelectric Efficiency.
Ka Xiong 1 , Rahul Gupta 1 , John White 3 , Bruce Gnade 1 , Kyeongjae Cho 1 2
1 Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States, 3 , Marlow Industries, Dallas, Texas, United States, 2 Physics, University of Texas at Dallas, Dallas, Texas, United States
Show AbstractThe rapid increase of the world's energy demand and the environmental impact of fossil fuels on global climate change have driven intensive research on seeking new sustainable energy technology. Thermoelectric materials have attracted a lot of attention because of its capability of converting heat into electricity or vice versa. Lead chalcogenide PbTe is one of the best thermoelectric materials at mid-temperature range (400-800K) [1]. During the past decades, considerable efforts have been dedicated on improving its thermoelectric efficiency figure of merit (ZT). Recent experimental work showed that doping Tl into bulk PbTe could double ZT as compared to that of the undoped PbTe [2]. This increase is because the distortion of the electronic density of states would enhance the Seebeck coefficient of PbTe and hence enhance its ZT value. This finding motivates us to explore other possible dopants that could enhance the ZT of PbTe. For this purpose, we use first principles calculations to investigate the electronic structures and stability of dopants (e.g. Al, In, Ga, Tl, P, As, Sb, Bi) in PbTe with various charge states. These methods have been previously applied to investigate the impact of defects in high dielectric constant oxides [3]. In addition, transport coefficients are evaluated in order to give a direct comparison of the thermoelectric performance of these doped PbTe compounds. This study will help us to gain insights on the behavior of these dopants in PbTe and the mechanisms that cause the modulation of the thermoelectric efficiency. Our preliminary results on doping PbTe by group III elements (Al, Ga, In, Tl) reveal that Tl is the best candidate for giving an improved ZT, consistent with the experimental data. We found that Al prefers to act as a donor in PbTe, while Ga and Tl prefer to act as acceptors. In contrast, In is an amphoteric impurity in PbTe. This work is supported by the II-VI Foundation, a private foundation. References[1] G. J. Snyder and E. S. Toberer, Nature Mater. 7, 105 (2008).[2] J. P. Heremans et al, Science 321, 554 (1999).[3] K. Xiong et al, Appl. Phys. Lett. 87, 183505 (2005).
6:00 PM - DD5.2
Thermoelectric Behavior of Polymer Nanocomposites With a Segregated Network of Carbon Nanotubes.
Jaime Grunlan 1 2 , Choongho Yu 1
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States, 2 Materials Science and Engineering Program, Texas A&M University, College Station, Texas, United States
Show AbstractPolymers are intrinsically poor thermal conductors, which are ideal for thermoelectrics, but low electrical conductivity and thermopower have excluded them as feasible candidates for thermoelectric applications. By adding single-walled carbon nanotubes to a polymer emulsion, we demonstrate that polymer nanocomposites can exhibit true thermoelectric behavior (i.e., generate electricity via a thermal gradient). As the polymer emulsion is drying, the relatively large polymer particles (100 – 1000+ nm) force nanotubes to reside in the interstitial space between them. This creates a segregated network of carbon nanotubes and results in high electrical conductivity at low nanotube concentration. This high electrical conductivity can be obtained while maintaining low thermal conductivity, which is very close to the intrinsic thermal conductivity of the polymer (~ 0.3 W/m-K). When a nanotube-filled composite with an electrical conductivity of 100 S/cm is produced, a thermoelectric figure of merit (ZT) greater than 0.01 can be obtained at room temperature.
6:00 PM - DD5.20
Contact Resistance Lowering and Its Impact on Thermoelectric Cooler Efficiency.
Rahul Gupta 1 , Ka Xiong 1 , John White 2 , Kyeongjae Cho 1 , Bruce Gnade 1
1 Department of Material Science & Engineering, University of Texas at Dallas, Richardson, Texas, United States, 2 , Marlow Industries Inc., a subsidiary of II-VI Incorporation, Dallas, Texas, United States
Show AbstractContact resistance becomes a serious limitation to efficiency of thermoelectric (TE) material based solid-state coolers with thermoelement leg lengths < 100 µm [1]. From a device point of view, although a high Z material can be achieved, the device coefficient of performance (COP) can still be low due to the degradation of Z due to the contact resistivity [2]. For thin TE materials, the losses become even more extreme and low electrical contact resistivity of <10-7 Ω-cm2 is needed to minimize the impact of contact resistance on COP. Bi2Te3 is a small band gap semiconductor with a band gap of 0.16 eV. Therefore it is theoretically possible to obtain very low contact resistance, < 10-7 Ω-cm2, for an ideal metal-semiconductor contact; however, real metal-semiconductor interfaces are far more complex. Extrinsic factors such as unwanted impurities and structural imperfections tend to accumulate at the interface and dominate the behavior of the metal-semiconductor interface. Optimization of processing methods such as surface preparation, cleaning, metal deposition, and alloying for fabrication of practical metal-semiconductor contacts becomes important to achieve a near ideal contact [3]. A detailed study of the effect of surface preparation and heat treatment on contact resistance for sputtered Ni contacts to thin film Bi2Te3 is presented. The contact resistance values obtained using the transmission line method (TLM) for Ni is compared to Co as a potential contact metal to Bi2Te3. Results from our previous work where first principle calculations were performed to study the stability of the interfaces, show the Co/Bi2Te3 interfaces be more thermodynamically stable than Ni/Bi2Te3 interfaces [4]. This improved metal-TE material interface technology will be transferred to TE cooler device and tested for its impact on device figure-of-merit, ZM. This work is supported by the II-VI Foundation, a private foundation.References[1] L. Rushing, A. Shakouri, P. Abraham, and J.E. Bowers, Proc 16th International Conference on Thermoelectrics, pp. 646, Dresden, GERMANY (1997).[2] Ka Xiong, W. Wang, H. N. Alshareef, R.P. Gupta, J. B. White, B. E.Gnade and Kyeongjae Cho, Mater. Res. Soc. Symp. Proc. Vol. 1166, pp. 1166-N05-01 (2009)[3] G.Y. Robinson, Thin Solid Films, 72, pp. 129 (1980).[4] R.P. Gupta, O.D. Iyore, K. Xiong, J.B. White, K. Cho, H.N. Alshareef, and B.E. Gnade, Electrochemical and Solid- state Letters 12 (10), H395 (2009).
6:00 PM - DD5.21
Structure of Bismuth Telluride Nanowire Arrays Fabricated by Electrodeposition in Porous Anodic Alumina Templates.
Kim Ha-Yeong 1 , Kim Sung-Jin 1
1 Chemistry and Nano Science, Ewha Womans University, Seoul Korea (the Republic of)
Show AbstractWe synthesized bismuth telluride (Bi2Te3) nanowires in the present of Bi(NO3)35H2O and TeO2 in 1.0 M HNO3 using porous anodic alumina template. Bi2Te3 nanowires were deposited electrochemically by pulsed-potential deposition method at room temperature. Pulsed electrodeposition is that the pulse potentials for Bi2Te3 nanowires were selected more negative potential than reduction potential observed from cyclic voltammograms (CVs) and a delay potential was used in a region of the CVs where there is no current between reduction and oxidation potentials of the compound. To obtain dense, highly oriented and single-crystalline Bi2Te3 nanowires, the control of the growth rate is critical factor. In this work, the regulation of time plays a key role to growth the uniformed size and high-density nanowires. The microstructure and morphologies of Bi2Te3 nanowires were investigated by Powder X-ray diffraction and FE-SEM, HR-TEM. Thermoelectric properties (ZT values) of Bi2Te3 nanowires are under investigation.
6:00 PM - DD5.23
Indentation Testing of Bulk and Nanostructured Zr0.5Hf0.5Co1-xIrxSb0.99Sn0.01 Half-Heusler Alloys.
Melody Verges 1 , Paul Schilling 1 , Jeffrey Germond 1 , Puja Upadhyay 1 , William Miller 1 , Nathan Takas 2 3 , P. Ferdinand Poudeu 2 3
1 Mechanical Engineering, University of New Orleans, New Orleans, Louisiana, United States, 2 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States, 3 Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractIndentation tests were performed to assess the influence of compositional changes on the mechanical properties of several half-Heusler compounds with the general composition Zr0.5Hf0.5Co1-xIrxSb0.99Sn0.01 (x = 0.0-0.7). The samples were synthesized by high temperature solid-state reaction and consolidated by hot-pressing. In addition, nanostructured samples were prepared by the introduction of metal oxide nanoparticles prior to the hot pressing. Indentation measurements were obtained using both microhardness testing (Vickers, Knoop) and depth-sensing nanoindentation. For the bulk Zr0.5Hf0.5Co1-xIrxSb0.99Sn0.01 (x = 0.0-0.7) half-Heusler alloys, values for hardness were within the range of 10 - 13 GPa, and for elastic modulus within the range of 200 - 260 GPa. The effects of Co/Ir ratio and metal oxide nanoparticle volume fraction on the mechanical properties were examined. Possible implications of the half-Heusler mechanical properties with regard to processing parameters are discussed. Mechanical properties of the bulk and nanostructured samples are analyzed along with thermoelectric properties to assess the potential performance and reliability of these materials for thermoelectric applications.
6:00 PM - DD5.25
Microstructure Investigation of Filled Skutterudite Compound CeFe4Sb12 Prepared by Melt Spinning and Spark Plasma Sintering.
Juan Zhou 1 , Qing Jie 1 , Lijun Wu 1 , Qiang Li 1
1 , Brookhaven National Lab, Upton, New York, United States
Show AbstractRecently, rapid solidification and fast sintering techniques have been employed in bulk thermoelectric materials synthesis to achieve improved thermoelectric properties. In our group, filled skutterudite compound CeFe4Sb12 has been prepared by combining melt spinning and spark plasma sintering (SPS) process. Thermoelectric properties measurements showed that as prepared samples had much lower lattice thermal conductivity, while maintaining higher electrical conductivity, in comparison with the samples made by the conventional solid state reaction and long term annealing method. To understand the origin of such behaviors, we performed detailed TEM (transmission electron microscopy) investigations. A large variety of nanometer sized precipitates were observed in the melt spinning plus SPS processed samples and they located either inside grains or on grain boundaries. Both coherent and incoherent interfaces were found between the nanoprecipitates and the matrix grains. TEM studies of the melt spun ribbons prior to the SPS revealed the formation of nanoprecipitates even during the rapid solidification. Comprehensive structure, chemical composition and strain of these nanoprecipitates are analyzed in attempt to understand their impacts on thermoelectric properties.
6:00 PM - DD5.26
Thermal and Electrical Transport Properties of Silicon Germanium Alloy Nanowires.
Liang Yin 1 , Choongho Yu 1
1 MEEN, Texas A&M University, College station, Texas, United States
Show AbstractOne-dimensional nanostructures are very attractive for thermoelectric energy conversion due to low thermal conductivity. This is because a measure of efficiency called as thermoelectric figure of merit, ZT is described as S2σT/k, where S, σ, T, and k are the Seebeck coefficient, electrical conductivity, temperature and thermal conductivity respectively. Strong correlations between these physical properties make it extremely difficult to obtain a high ZT value. Nevertheless, size effects have proven to overcome the limit by reducing thermal conductivity without significantly changing electrical transport properties. In this work, silicon germanium alloys were made into nanowire shapes and their thermoelectric transport properties were characterized as a function of temperature by using micro devices that consist of two suspended membranes and temperature sensors/heaters. The results show that silicon germanium can be competitive in efficiency even at relatively low temperatures compared to those of bulk silicon germanium alloys.
6:00 PM - DD5.3
Enhanced Thermoelectric Properties of Nanostructured Bismuth Telluride Materials Prepared from Star-shaped Nanoparticles.
KyungTae Kim 1 , HyeMoon Lee 1 , GilGeun Lee 2 , GookHyun Ha 1
1 Functional Materials Division, Korea Institute of Materials Science, Changwon, Gyeongnam, Korea (the Republic of), 2 , Pukyong National University, Busan Korea (the Republic of)
Show AbstractBismuth telluride is the most famous thermoelectric materials mainly used for cooling applications at a room-temperature. The figure-of-merit, ZT, of the currently available Bi-Te thermoelectric materials is around 1.0 at 300oC and it is widely known that there is a possibility increasing ZT by nanostructuring the material grain boundaries and reducing more the thermal conductivity. In this study, the star-shaped Bismuth Telluride nanoparticles with sub micron size were prepared via chemical route, firstly. The interfacial structure and shape control mechanism of the unique thermoelectric nanoparticles was analyzed by high resolution transmission electron microscopy. The effect of star-shaped nanoparticles on thermoelectric properties was investigated by sintering the powders into the nanostructured Bismuth telluride in a bulk. The sintered materials show extremely reduced thermal conductivity compared to that of conventional Bi-Te obtained from ingot materials due to the effective phonon scattering effect at nano-sized grains. As a result, it is suggested that the extraordinary shaped Bismuth Telluride nanoparticles produce nano-pored and nanograined materials that lead enhanced ZT values as well as reduced thermal conductivities.
6:00 PM - DD5.33
Temperature and Amorphousness Effects on the Extremely Low Thermal Conductivity of Self-assembled Germanium Quantum-dot Supercrystals in Silicon.
Jean-Numa Gillet 1
1 Physics, University of Lille 1 and IEMN, Villeneuve d'Ascq France
Show AbstractDesign of semiconducting nanomaterials with an indirect electronic bandgap is currently one of the major areas of research to obtain a high thermoelectric yield by lowering their lattice thermal conductivity. Intensive investigations on superlattices were performed to achieve this goal. However, like one-dimensional nanowires, they decrease heat transport in only one main propagation direction of the phonons. Moreover, they often show dislocations since they are composed of layered materials with a lattice mismatch. Design of superlattices with a thermoelectric figure of merit ZT higher than unity is therefore hazardous. Self-assembly of epitaxial layers on silicon has been used for bottom-up synthesis of three-dimensional (3D) Ge quantum-dot (QD) arrays in diamond-cubic Si for quantum-device and solar-energy applications. Using the model of the atomic-scale 3D phononic crystal, I predict that high-density 3D arrays of self-assembled (SA) Ge QDs in Si can also show an extreme reduction of the thermal transport. 3D supercrystals of Ge QDs in Si present a thermal conductivity that can be as low as that of air (0.024 W/m/K at room temperature). These extremely low values of the thermal conductivity are computed for a number of Ge filling ratios and size parameters of the 3D supercrystal that are in the range of current fabrication technologies [J.-N. Gillet, Outstanding Scientific Paper Award, 28th International Conference on Thermoelectrics (ICT 2009)]. Owing to incoherent phonon scattering with predominant near-field effects, the same conclusion holds for supercrystals with moderate QD disordering. As a result, the design of highly-efficient CMOS-compatible thermoelectric devices with ZT possibly much higher than unity might be possible with these nanomaterials. Temperature effects on the size parameters and Ge filling ratios to obtain such an extreme reduction of the thermal conductivity will be analyzed in this theoretical study. Second, the degree of amorphousness and possible absence of a number of covalent bonds in the Ge QDs will be modeled to observe their influence on the thermal conductivity in the framework of the Stillinger-Weber potential and 3D lattice dynamics. This analysis will be compared to experimental measurements obtained for two-dimensional disordered layered W/WSe2 crystals [C. Chiritescu et al., Science 315, 351-353 (2007)].
6:00 PM - DD5.4
Computational Design of Fractionally Co-filled Binary Skutterudites for Thermoelectric Applications.
Maria Stoica 1 , Cynthia Lo 1
1 Energy, Environmental, and Chemical Engineering, Washington University in St Louis, St Louis, Missouri, United States
Show AbstractIn the past few years, the search for efficient and renewable sources of energy has spurred interest in the development of advanced thermoelectric materials (TEs) with higher heat to energy conversions and, correspondingly, higher figures of merit. While currently reported experimental values for the figure of merit are low, the theoretical limit is much higher. The research presented in this work focuses on improving the figure of merit in skutterudites, which are a class of thermoelectric materials with high conductivities and the advantageous property of natural voids in their crystal lattice. Previous research has shown that the filling of these voids with heavy element atoms, such as La, leads to a considerable reduction in the thermal conductivity of the lattice. Surprisingly, more recent studies have indicated that the filling of the lattice voids with smaller atoms with more metallic properties, such as In, leads to an even greater increase in the figure of merit compared to that obtained from filling with La, by increasing the power factor more than the thermal conductivity. Ag particles, in particular, show promise because they are electrically conductive, and as in the case of In, it is hypothesized that these particles will also have the effect of increasing the power factor at a greater rate than thermal conductivity, thus effectively raising ZT. Although Wolf et al (2004) have previously presented on a systematic method of scanning skutterudites filled with atoms from the La group as potential thermoelectric materials, no one has yet reported a thorough theoretical study of the thermoelectric properties of skutterudites with Ag void-filling atoms. Using both density functional theory (DFT) to calculate the band structure and density of states of the bulk structures and MD simulations, the Seebeck coefficient, thermal conductivity, and electrical conductivity of the materials can be estimated using first principles calculations. Preliminary DFT calculations of the Seebeck coefficient for Ag-filled skutterudites indicate promising improvements in ZT. A model for predicting the figure of merit as a function of fractionally filling bulk skutterudite with Ag will determine the optimum Ag filling fraction. Next, a model for the ZT value as a function of the filling element, in particular, co-filling of the voids with Ag and La, will be presented. It is expected that the tradeoff between the ability of the heavy atoms to scatter phonons and of the Ag atoms to increase electrical as well as thermal conductivity will result in an optimum filling ratio for maximizing the figure of merit. Finally, we present recommendations for future directions in improving the thermoelectric figure of merit in these materials.
6:00 PM - DD5.6
Effect of β-Co(OH)2 Reactive Template Size on Textured Structure and Thermoelectric Properties.
Myung-Hyun Lee 1 , Ho-Yoon Choi 1 2 , Seung-Hwan Na 1 , Young Soo Lim 1 , Soonmok Choi 1 , Won-Seon Seo 1 , Hong-Lim Lee 2
1 Energy Materials Center, Green Ceramic Division, KICET, Seoul Korea (the Republic of), 2 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractIn the era of green technology, saving resources by energy recovering and reducing CO2 are one of the most highlighted topics. Recently, cobalites with layer structure were found to show high thermoelectric performance for energy recovery from waste heat. Since these cobalites are chemically stable and do not include hazardous elements, the cobalites are potential materials as thermoelectric devices to be used in air atmosphere at high temperature. Thermoelectric properties of single crystalline particles were reported to be highly anistropic. Thus the fabrication of textured bulk ceramics is required for practical application of the materials. Reactive template grain growth (RTGG) method using platelike β-Co(OH)2 particles as reactive templates that provides the textured structure of layered cobalites has been applied for high temperature thermoelectric material and tape casting process was introduced and several kinds of post-processing such as hot press, HIP and SPS for increment of orientation. A few study related with size control of template particle and its effect on TE property has been reported. In this report, we evaluate the effect of factors in synthesizing process on the template particle size and the effect of particle size on physical property of cast tape and thermoelectric property of laminated bulk materials. There are various factors, such as ion concentration of reactant solution, temperature, time and feeding speed, affecting on the size of template particle. The β-Co(OH)2 particles size was most affected by ion concentration [Co+3]/ [OH-] and temperature. The size of β-Co(OH)2 template particles was varied from 0.3㎛ to 3.1㎛ and from 0.3㎛ to 1.0㎛, by changing ion concentration and temperature, respectively. To prepare slurry for tape casting, the precipitated β-Co(OH)2 as template was ball-milled with commercial CaCO3, binder, plasticizer and solvent. Prepared slurry was cast to thick tape using doctor blade. When the size of template was 0.6㎛, the lotgering factor of green tape was f=0.483. The larger the size of template was, the higher lotgering factor was. The lotgering factor was increased to f=0.718 when the green tape was made of two times larger particles. It may be caused by decreasing of complexity as a result of higher restriction on rotating and tilting of particle in slurry having pseudo-plastic behavior after being gotten high shear force between the bottom of blade and carrier film. The influence of the spark plasma sintering (SPS) procedure was also studied with respect to the template particle size and its effect. Power factor of sample sintered by SPS was increased to 10 times larger than sample sintered without SPS. It was elucidated that orientation of textured structure and thermoelectric property in RTGG method using tape casting are augmented by increasing template particle size and applying SPS process.
6:00 PM - DD5.7
Thermoelectric Properties of Polymer Blends.
Jia Sun 1 , Joe Feser 2 , Ming-Ling Yeh 1 , Dann Martin 1 , Arun Majumdar 2 , Katz Howard 1
1 Material Science and Engineering, Johns Hopkins University, Baltiomore, Maryland, United States, 2 Mechanical Engineering, University of California, Berkeley, California, United States
Show AbstractThermoelectric Properties of Polymer BlendsJ. Sun*, J. Feser#, M-L. Yeh*, D. Martin*, A. Majumdar#, and H. E. Katz* * Department of Material Science & Engineering, Johns Hopkins University# Department of Mechanical Engineering University of California Berkeley Organic and polymeric semiconductors (OSCs) have been considered as potentially high-ZT materials because of their acknowledged low value of κ, 2-3 orders of magnitude below inorganic semiconductors and metals, though the σ/κ ratio may not be particularly high in OSCs. The quantity S in doped OSCs is on the order of tens of μV/K, well below inorganics, though pure (and low-conductivity) organics can have S as high as inorganics. Low values of S in doped organics may be due to unfavorable energy and spatial distribution of the density of states relative to the ground state charge carrier energy. In various theoretical models for S, the increase in number of states with respect to energy at or just above the Fermi level is the major determining factor. In this manuscript, we report our initial results on semiconductor blends in which the major component, “bulk” molecular subunits, are designed to have carrier energies just above orbital energies of a minor component “additive”. The Fermi level is established by the additive, while the current from injected charge is carried predominantly in the higher energy orbitals of the bulk. The additive is not a dopant; in fact, a dopant could be added as a third component to increase conductivity. Doping would not greatly alter the Fermi level, in contrast to previously described systems considered for thermoelectric applications. This situation is effectively equivalent to having a large derivative of density of states with respect to energy. While all of these organic materials were studied as mixtures with dopants, in none of these cases were additional compounds introduced to alter the level at which charge carriers were induced by the dopants, and no effort was made to control or maximize charge carrier mobility above the Fermi level as dopants were added. Here, Semiconductor blends are designed in which the major components (e.g. poly-3-hexylthiophene, P3HT) have carrier energies just above orbital energies of a minor component “additive”(here, poly-3-hexylthiothiophene, P3HTT). The Fermi level is established by the additive, while the current from injected charge carriers is predominantly in the higher energy states of the bulk. The significant new result reported is that unlike in any previous studies, a regime is identified in which Seebeck coefficient and electrical conductivity both increase as a result of doping. This phenomenon is not observed in controls without the additive in which charge carriers are more stable.
6:00 PM - DD5.8
Enhanced Thermoelectric Properties of Solution Processed Conducting Polymer/Nanocrystalline Composites.
Kevin See 1 2 , Joseph Feser 3 , Rachel Segalman 2 1 , Jeffrey Urban 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Chemical Engineering, University of California - Berkeley, Berkeley, California, United States, 3 Mechanical Engineering, University of California - Berkeley, Berkeley, California, United States
Show AbstractIn order to increase the broad applicability of thermoelectrics for cooling as well as energy conversion, both efficiency and materials cost must be reduced. This work addresses both of these challenges with the synthesis and characterization of novel solution processed conducting polymer/nanocrystalline composites with thermoelectric properties enhanced beyond that of either component alone. Soluble conducting polymers are an attractive material due to their relatively low cost and ease of processing, however most highly conductive systems suffer from low thermopowers. Here, we have integrated the conducting organic moiety with inorganic nanostructures, resulting in solution-processable composites with both high thermopower and conductivity.Previous work has shown the potential for nanostructured systems to overcome the performance limitations of bulk materials, enabling improvements in the thermoelectric figure of merit, ZT. In addition to increasing the amount of phonon scattering, multicomponent systems have been predicted theoretically to show improvements in ZT via manipulation of carrier energy distributions. While solution processed nanoparticles show enhancements in thermopower as a result of quantum confinement, these systems are hampered by low electrical conductivities due to the uncontrolled organic/inorganic interface. We have addressed this by utilizing a highly conductive polymer system in concert with nanostructures to yield novel systems with thermoelectric properties that can exceed that of any known stable solution based approach (i.e. polymer or nanoparticle only). As an initial demonstration, we have synthesized Tellurium (Te) nanorods in the presence of PEDOT:PSS, a highly conducting polymer with reported thin film conductivities greater than 500 S/cm. Transmission electron microscopy (TEM) indicates that the synthesized nanorods are passivated by a thin layer of the conducting polymer that stabilizes the rods in aqueous solution. Additionally, X-ray diffraction indicates the rods are highly crystalline with hexagonal crystal structures. The resulting composite can be cast from water into smooth, uniform films allowing for further thermoelectric characterization. Using well-known solvent treatments for increasing the conductivities of PEDOT:PSS thin films, 3-4 fold increases in the conductivity of the composite were obtained without proportionate suppression of the thermopower, thus leading to enhancements in power factor. The cast films have thermoelectric power factors exceeding 50 μW/mK^2 at room temperature, surpassing recent reports for various solution based techniques. In addition, the room temperature thermal conductivities of films were measured using the 3ω technique, giving values typical for polymer systems near 0.2 W/mK. This combination of tunable power factor and low-κ provide a platform for developing all-solution processed high-ZT materials.
6:00 PM - DD5.9
Thermoelectric Power of Polymer Bound Carbon Nanotube Mats.
Corey Hewitt 1 , Jung Ho Park 1 , Nicholas Lepley 1 , David Carroll 1
1 , Wake Forest University, Winston-Salem, North Carolina, United States
Show AbstractThermoelectric power generation is the formation of a potential difference across a material that is exposed to a temperature gradient. Specifically, the thermopower of carbon nanotubes was explored in this study due to their favorable electronic properties. The thermopower of a material is described by the Seebeck coefficient and defined as potential difference divided by temperature difference. Carbon nanotube mats were created using several binding materials including PVDF and PMMA binders containing carbon nanotubes in varying weight percents. These mats were placed in a thermopower measuring device consisting of a two Kelvin temperature gradient, and tested from 300 K to 20 K. For comparison, a pure carbon nanotube mat was also tested. Results show that the mats containing PVDF had the highest Seebeck coefficient of 35 mV/K at 250 K, while the PMMA and pure mats were similar with 10 mV/K at 300 K.
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Symposium Organizers
John D. Baniecki Fujitsu Laboratories Ltd.
Jonathan A. Malen Carnegie Mellon University
G. Jeffrey Snyder California Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
DD6: Thermoelectrics Nano Bulk I
Session Chairs
Teruyuki Ikeda
Sang Mock Lee
Wednesday AM, April 07, 2010
Room 2002 (Moscone West)
9:00 AM - **DD6.1
Strategies for the High Performance Thermoelectric Materials - Lattice Distortion and Nanocomposite.
Sang Mock Lee 1 , Jong Soo Rhyee 1 , Kyu Hyoung Lee 1 , Sang Il Kim 1 , Hyun Sik Kim 1 , Eun Sung Lee 1 , Eun Seog Cho 1
1 , Samsung Advanced Institue of Technology, Yongin Korea (the Republic of)
Show AbstractHere we present new concepts for improving thermoelectric(TE) figure-of-merit. One includes the localized f-band tuning by orbital hybridization and Peierls distortion in Ce-Te and In-Se systems. Another is nano-metal decoration for lowering the lattice thermal conductivity in Bi-Te system. We demonstrate that lattice distortion is responsible for the unexpectedly low lattice thermal conductivity. And we also find that metal nanoparticles introduced in Bi-Te alloy play a role as phonon scattering centers. Our results suggest new directions in the search for high performance nanostructured bulk materials induced by lattice distortion(intrinsic) as well as nanoparticle decoration(extrinsic).
9:30 AM - DD6.2
Nanostructuring in PbTe-based Materials and Thermoelectric Energy Conversion.
Mercouri Kanatzidis 1 2
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractThe thermoelectric figure of merit has improved substantially in the past five years thanks to the successful application of new concepts in materials synthesis. New fundamental scientific insights regarding the influence of nanostructures on thermoelectric properties have emerged and have changed how we think about thermoelectric materials design. For example, coherent nanometer sized inclusions in a PbTe and SnTe matrix can serve as sites for scattering of acoustic phonons to lower the thermal conductivity. Innovative, convenient synthetic approaches have been used to create specific crystal structures and nanostructures to meet this challenge. Several novel nanostructured PbTe materials are under investigation and are promising for enhanced power factor and greatly reduced thermal conductivity. We will present several such strategies in preparing bulk materials containing nanometer-sized coherent and non-coherent inclusions and a detailed analysis of inclusion-matrix interactions.
9:45 AM - DD6.3
Reduction of the Lattice Thermal Conductivity in Immiscible PbS-PbTe Systems.
Simon Johnsen 1 , Mercouri Kanatzidis 1
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractCurrent state of the art thermoelectric materials are often tellurium based compounds. The scarcity of the latter can be problematic if thermoelectric modules reach mass markets. Cheaper alternatives containing abundant elements are therefore sought. PbS is a narrow band gap semiconductor (0.42 eV at 300 K) with fair thermoelectric properties. A significant lattice thermal conductivity (3.2 W/Km) limits the thermoelectric performance [1]. Recently we showed how nucleation and growth and spinodal decomposition in the PbTe rich part of the pseudo-binary phase diagram of PbTe-PbS leads to bulk nanocomposite materials with low thermal conductivities and enhanced thermoelectric performance relative to that of PbTe [1]. In this presentation the thermoelectric properties of the PbS-rich part of the pseudo-binary PbS-PbTe phase diagram are investigated. The diagram is dominated by an immiscibility dome which inherently leads to a two phase system. For appropriate cooling rates this leads to bulk nanocomposites known to be effective phonon scatterers. Special focus is given to find a minimum in the lattice thermal conductivity for varying x in PbS1-xTex.[1] Androulakis, J. et al. J. Am. Chem. Soc. 129(31), 9780.
10:00 AM - DD6.4
Improvements of Thermoelectric Performances in AgSbTe2 System With in-situ Ag2Te Nano-lamellae Precipitations.
Shengnan Zhang 1 , Tiejun Zhu 1 , Shenghui Yang 1 , Guangyu Jiang 1 , Xinbing Zhao 1
1 Materials Science and Engineering, Zhejiang University, Hangzhou , Zhejiang, China
Show AbstractAbstract: AgSbTe2 is the critical component in both LAST-m and TAGS-x system, which are two state-of-the-art mid-temperature thermoelectric bulk nanocomposites. In our study, the single phased AgSbTe2 was obtained with the x value from 0.78 to 0.81 (x as in (Ag2Te)x(Sb2Te3)1-x), which is consistent of the previous results on the phase diagram of (Ag2Te)x(Sb2Te3)1-x system. By adjusting the Ag2Te/Sb2Te3 ratio, the precipitations of nanosized lamella Ag2Te embedded in the AgSbTe2 matrix have been obtained and the thermoelectric properties of AgSbTe2 nanocomposites have been improved. It has been reported that the microstructures in both LAST-m and TAGS-x system can greatly contribute to their thermoelectric properties. But the microstructures in AgSbTe2 matrix have rarely been studied before. In this manuscript, the nanoscale microstructures in AgSbTe2 matrix were studied by high-resolution transmission electron microscope systematically. AgSbTe2 nanodomains with different lattice structures (such as Pm-3m and P4mmm) were observed coherently embedded in the Fm-3m matrix due to the cation ordering. Various crystallographic defects in different scales, such as dislocations and stacking faults, have also been observed. All these microstructures contributed to the very low thermal conductivity of AgSbTe2. And we prove that the Ag2Te/Sb2Te3 ratio did play an important role in the formation of microstructures in AgSbTe2 system. Also it is observed that the thermoelectric properties of AgSbTe2 system are very sensitive to the composition. The relationship among the composition, microstructure and thermoelectric properties is studied in this work. The maximum figure of merit ZT value of 1.53 was obtained at 500 K for Ag0.84Sb1.16Te2.16, implying the potential of AgSbTe2 as thermoelectric materials.
10:15 AM - DD6.5
Investigation of the Thermoelectric Properties of the PbTe-SrTe System.
Kanishka Biswas 1 , Qichun Zhang 1 , Jiaqing He 1 2 , Vinayak Dravid 2 , Mercouri Kanatzidis 1 2 3
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 3 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractPbTe-based materials are promising for efficient heat energy to electricity conversion. We present studies of the thermoelectric properties of the PbTe-SrTe system. X-ray diffraction patterns reveal that all the samples crystallize in the NaCl-type structure without noticeable secondary phase. The issue of incorporation of Sr into the PbTe lattice was addressed with transmission electron microscopy analysis and X-ray diffraction studies. Na2Te doping of the PbTe-SrTe materials resulting in a positive sign of the Seebeck coefficients indicating p-type conduction and a high power factor. Thermal conductivity results as a function of SrTe concentration will be presented.
10:30 AM - DD6.6
An Experimental Investigation into Single Phase [Ag,Sb]Te2 Thermoelectric Material.
Michele Nielsen 1 , Joseph Heremans 1 2
1 Mechanical Engineering, Ohio State University, Columbus, Ohio, United States, 2 Physics, Ohio State University, Columbus, Ohio, United States
Show AbstractAgSbTe2 has been reported to have an intrinsically low lattice thermal conductivity of ~ 0.6 W/mK1. Additionally, a zT= 1.2 at 420K was reported in AgSbTe2 doped with NaSe22. The effective working temperature range of stoichiometric AgSbTe2 material is limited, because of the presence of a minority Ag2Te phase which has a phase transition just above 420K. To avoid this, we create single-phase compounds of AgxSbyTez, varying x and y to experimentally determine the most stable regime in the phase diagram of AgSbTe2. We report on latent heat traces that indicate no presence of the Ag2Te phase as well as thermopower and electrical and thermal conductivities of these compounds. The most promising compound is doped, and its zT is reported here. 1 D. T. Morelli, V. Jovovic and J. P. Heremans, Phys. Rev. Lett. 101, 035901 (2008)2. V. Jovovic and J. P. Heremans. Journal of Electronic Materials. 38, 7 (2009)
10:45 AM - DD6: NanoBulk
BREAK
11:00 AM - **DD6.7
Nanostructure Formation in bulk PbTe-base Compounds via Phase Transformation.
Teruyuki Ikeda 1 2 , Nathan Marolf 2 , Marcus Toussaint 3 , Nick Heinz 2 , Vilupanur Ravi 4 , G. Jeffrey Snyder 2
1 PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan, 2 , California Institute of Technology, Pasadena, California, United States, 3 , Louisiana State University, Baton Rouge, Louisiana, United States, 4 , California State Polytechnic University, Pomona, Pomona, California, United States
Show AbstractThe lattice thermal conductivity of thermoelectric materials could be significantly reduced by nanostructures due to phonon scattering at phase boundaries. Nanostructures can be formed in bulk thermoelectric materials utilizing solid-state phase transformations, e.g. precipitation, eutectoid reaction, etc. In this paper, we discuss systematic ways to introduce nanostructures with heterogeneous phases in thermoelectric materials by way of phase transformations. Using phase diagrams as a basis, the pseudo-binary PbTe-Sb2Te3 system will be focused on as a model case. For the compound with the nanolamellae obtained by the eutectoid reaction, a reduction of the lattice thermal conductivity has been observed. In the PbTe-rich region, Widmanstätten plates of Sb2Te3 with tens of nanometers thickness are formed by precipitation. The size scales of both the nanolamellar and Widmanstätten structures can be controlled by heat treatments on the basis of classical theories on phase transformations. New data of thermoelectric properties on PbTe with Widmanstätten precipitates will also be presented.
11:30 AM - DD6.8
Polycrystalline Super Lattices by Self Organization – Nano-alloyed Thermoelectric Materials for High Performance Device Fabrication.
Dirk Ebling 1 , Markus Winkler 1 , Kilian Bartholome 1 , Harald Boettner 1
1 Thermoelectric and Integrated Sensor Systems , Fraunhofer Institute for Physical Measuremnt Technique, Freiburg Germany
Show AbstractSince the early nineties theoretical concepts were reported, to improve the thermoelectric figure of merit ZT by more than a factor of 2 due to nanoscale layered structures. Experimentally it has been demonstrated by Venkatasubramanian [1] for heterogeneous single crystalline nanoscale layers of the V-VI-systems. ZT-values of 2.4 and 1.6 were reported for the p-type and n-type material, respectively, due to the direct influence of the superlattice structure on the thermal conductivity. However, for device fabrication the applied VPE-technique is quite costly and other techniques like sputtering would be more valuable. By Johnson et al. it was demonstrated, that misfit layered compounds like WSe2 thin films can exhibit ultralow thermal conductivity [2]. In this work nanoscale layers of the V-VI-system were fabricated by sputtering elemental layers of few nanometer thicknesses and a subsequent interdiffusion combined with a solid state reaction to form the final superlattices on non crystalline substrates. Composition, thickness and interface properties of the super lattice were controlled by the thickness of the sputtered layers and their sequence. The electrical and thermoelectric properties are analyzed in dependence on the subsequent annealing of the layers. The thermal stability of the materials allows annealing times and temperatures of more than 2 hours and 200° C to be used without destroying the layered structure. Holding the sample at constant temperature while changing the reservoir's annealing temperature provides an independent means to control chalcogen partial pressure and hence carrier concentrations.It can be demonstrated, that the thermoelectric performance of the ordered compounds is most likely influenced by the thickness and the sequence as well as post growth treatment of the sputtered layers. 1.R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature 413 (2001), p. 5972.Seongwon Kim, Jian Min Zuo, Ngoc T. Nguyen, David C. Johnson, and David G. Cahill, “The Structure of Layered WSe2 Thin Films with Ultralow Thermal Conductivity”, Journal of Materials Research, 2008, 23, 1064-1067
11:45 AM - DD6.9
Thermoelectric Properties of Composite PbTe – PbSnS2 Materials.
Steven Girard 1 , Jiaqing He 2 , Vinayak Dravid 2 , Mercouri Kanatzidis 1
1 Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractNanostructured layered materials, superlattices, and multilayers have been shown to exhibit extremely low values of lattice thermal conductivity, which could be utilized in thermoelectric composites to enhance figure of merit. Through the high-temperature synthesis of an appropriate combination of PbTe, SnTe, and PbS, we present here a high-ZT thermoelectric composite of PbTe – PbSnS2. PbSnS2 (mineralogical name teallite) is a layered material (space group Pnma) comprised of Sn-Pb bilayers approximately 0.6 nm in thickness which form a superstructure along the a axis. High resolution transmission electron microscopy reveals the PbSnS2 segregates into fully coherent lamellar structures 50 – 100 nm in thickness that extend 100 nm – 15 μm in length. We find that incorporation of PbSnS2 in PbTe results in a significant reduction in lattice thermal conductivity to between 0.3 – 0.6 W/mK at room temperature, a reduction of almost 70% over bulk PbTe. While it appears that PbSnS2 incorporation effectively scatters phonons, a concurrent decrease in electron mobility is observed for increasing PbSnS2 concentration. An optimal ZT of 1.3 is obtained for the PbTe – PbSnS2 3% composition. Detailed microscopy analysis will be presented along with electrical conductivity, Seebeck coefficient, thermal conductivity, and Hall effect measurements.
12:00 PM - DD6.10
The Interface Structure and Composition of Nanoscale Ag2Te Precipitates Embedded in PbTe.
Jessica Lensch-Falk 1 , Michelle Hekmaty 1 , Joshua Sugar 1 , Douglas Medlin 1
1 Materials Physics, Sandia National Laboratories, Livermore, California, United States
Show AbstractThe introduction of embedded interfaces or precipitates into bulk materials by solid-state precipitation provides a way to reduce the thermal conductivity and improve the performance of thermoelectric materials. An understanding of the precipitation mechanisms and phase stability associated with this nanostructuring is crucial to engineering systems with optimal thermoelectric performance. In this presentation we discuss our investigations of the morphology, composition, and interfacial structure of Ag2Te precipitates in PbTe using transmission electron microscopy (TEM) and atom probe tomography (APT). Annealing in the region of two phase equilibrium between Ag2Te and PbTe results in the formation of Ag2Te precipitates which take the monoclinic β-Ag2Te structure at room temperature as determined by x-ray and electron diffraction studies. These precipitates form with an orientation relationship that aligns the Te sub-lattices in the monoclinic and rock salt structures. This relationship is the same as we have reported earlier for β-Ag2Te precipitates in rock salt AgSbTe2. Observations using TEM and APT suggest that the β-Ag2Te precipitates initially form as coherent spherical precipitates which upon coarsening become semi-coherent and flatten along the <100>PbTe directions which is consistent with theoretical predictions for elastically strained precipitates in a matrix. Our HRTEM observations show that sufficiently small precipitates are coherently embedded, while larger precipitates form with misfit dislocations and multiple variants to relax the elastic strain. Analysis of the composition of both precipitate groups using APT indicates that the larger precipitates exhibit compositions close to equilibrium while the smaller nanoscale precipitates exhibit enhanced Pb compositions. This detailed analysis of the interface structure and composition of embedded Ag2Te precipitates may be helpful in understanding the precipitation mechanisms and microstructure of related thermoelectric materials, such as LAST.
12:15 PM - DD6.11
The Formation of Sb2Te3 Precipitates in a Rocksalt Matrix Through the Intermediate Phase Sb3Te4.
Joshua Sugar 1 , Peter Sharma 1 , Douglas Medlin 1
1 Materials Physics Department, Sandia National Laboratories, Livermore, California, United States
Show AbstractControl of solid-state phase transformations provides one way to fabricate nanostructured bulk materials for thermoelectric applications. In order to tune a material’s microstructure with the desired nanoscale features, it is necessary to understand the microstructural evolution that occurs during its phase transformation. AgSbTe2 is one high-ZT thermoelectric material in which, depending on the composition, it is possible to precipitate second phases of either Ag2Te or Sb2Te3. When rocksalt-structured AgSbTe2 is Sb-enriched, the precipitation of tetradymite-structured Sb2Te3 closely parallels the phase transformations that occur in other multiphase thermoelectric systems, e.g. the PbTe-Sb2Te3 system. In this study, we show that the formation of Sb2Te3 proceeds by a complicated series of transformations in which stacking faults serve as a precursor to the formation of ~100-nm thick plates of an intermediate Sb-rich metastable compound, Sb3Te4. Eventually, the Sb3Te4 phase coarsens, agglomerates, and undergoes a compositional change to produce large, µm-wide Sb2Te3 plates consistent with a Widmanstätten pattern. The seven-layer Sb3Te4 structure is verified with simulations of electron diffraction patterns and high-resolution TEM images. Based on effective medium theory calculations and the precipitate morphology, the thermoelectric properties of this system behaved in a manner consistent with parallel conduction between the precipitate and the matrix phases. Understanding this transformation mechanism helps resolve much of the uncertainty surrounding the phase stability of Sb-enriched AgSbTe2 at intermediate temperatures (300-500°C).
12:30 PM - DD6.12
Transmission Electron Microscopy Investigation of Half-Heusler ZrCoSb and ZrNiSn-based Bulk Nano-composite Thermoelectric Materials.
Dinesh Misra 1 , Nathan Takas 1 , Rumana Yakub 1 , Pranati Sahoo 1 , Pierre Ferdinand Poudeu Poudeu 1 , Kevin Stokes 1 , Heike Gabrisch 1
1 AMRI, UNO, New Orleans, Louisiana, United States
Show AbstractHalf-Heusler compounds are considered to be a prospective material for thermoelectric conversion [1-4]. The efficiency of thermoelectric devices is determined by the material’s dimensionless figure of merit, defined as ZT= (S2σ/k) T, where S, σ, k and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively [5-7]. The thermoelectric properties of ZrNiSn and ZrCoSb based nanocomposite materials strongly depend on microstructure parameters, such as homogeneity and grain size of a solid solution matrix, and size, shape and distribution of second phase particles inside the matrix. In this work, we present the microstructural characterization of ZrNiSn and ZrCoSb based nanocomposites formed by dispersing metal oxide nanoparticles such as NiO and HfO2 into the matrices. Based on microstructural details, a reasonable explanation for the observed thermal conductivity of the nanocomposites compared to the thermal conductivity of the corresponding Zr0.50Hf0.50Co1-xIrxSb0.99Sn0.01 (with x=0.0 to 0.7) and Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 (with x = 0.2 and 0.4) matrices will be discussed. Additions of small amounts of nanoinclusions of NiO and HfO2, which serves as additional phonon-scattering centers, leads to significant alterations of the lattice thermal conductivity which in turn enhances the figure of merit (ZT).References: 1. C. Uher, J. Yang, S. Hu, D. T. Morelli, and G. P. Meisner, Phys. Rev. B 59, 8615 (1999).2. H. Hohl, A. P. Ramirez, C. Goldmann, G. Ernst, B. Wolfing, and E. Buche, J. Phys.: Condens. Matter 11, 1697 (1999).3. K. Mastronardi, D. Young, C. C. Wang, P. Khalifah, R. J. Cava, and A. P. Ramirez, Appl. Phys. Lett. 74, 1415 (1999).4. Q. Shen, L. Chen, T. Goto, T. Hirai, J. Yang, G. P. Meisner, and C. Uher, Appl. Phys. Lett. 79, 4165 (2001).5. D. M. Rowe, Ed. CRC Handbook of Thermoelectrics (CRC, Boca Raton, FL, 1995).6. H. J. Goldsmid, Thermoelectric Refrigeration (Plenum, NewYork, 1964). 7. T. M. Tritt, Ed. Semiconductors and Semimetals, Recent Trends in Thermoelectric Materials Research: Part One to Three (Academic, San Diego, vol. 69 to 71, CA, 2001). *Corresponding author:
[email protected]12:45 PM - DD6.13
Effect of Valence State of Tl on the Lattice Thermal Conductivities of Thallium Tellurides.
Ken Kurosaki 1 , Hiroaki Muta 1 , Shinsuke Yamanaka 1
1 , Osaka University, Suita, Osaka Japan
Show AbstractA lot of ternary thallium tellurides exhibit extremely low lattice thermal conductivity (κlat), and for that reason, some of them exhibit large thermoelectric figure of merit, e.g. ZT =1.23 is obtained at 700 K for Ag9TlTe5 [1]. In our previous study [2], TlMTe2 (M = Ga, In, or Tl) were prepared and characterized, in order to discuss the role of Tl in lowering the κlat. We predicted that TlTe exhibits the lowest κlat, but the κlatof TlTe was unexpectedly high (~2.7 Wm-1K-1), while those of TlGaTe2 and TlInTe2 were extremely low (~0.5 Wm-1K-1). TlTe is well known as a mixed valence compound containing monovalent and trivalent Tl, i.e. (Tl1+)(Tl3+)(Te2-)2, while TlGaTe2 and TlInTe2 include only monovalent Tl, i.e. (Tl1+)(Ga3+)(Te2-)2 and (Tl1+)(In3+)(Te2-)2. Therefore, we consider that there are some relations between the unexpectedly high κlat of TlTe and the valence state of Tl. In the present study, Tl2Te, Tl5Te3, TlTe, and Tl2Te3 were prepared and characterized. In Tl2Te and Tl2Te3, Tl would take only monovalent or trivalent, i.e. (Tl1+)2(Te2-) and (Tl3+)2(Te2-)3, whereas in Tl5Te3 and TlTe, Tl would take mixed valence states, i.e. (Tl1+)9(Tl3+)(Te2-)6 and (Tl1+)(Tl3+)(Te2-)2. We will discuss the relationship between the valence state of Tl and the κlat of these thallium tellurides. [1] K. Kurosaki et al., Appl. Phys. Lett. 87, 061919 (2005). [2] H. Matsumoto et al., J. Appl. Phys. 104, 073705 (2008).
DD7: Thermoelectrics Nano Bulk II
Session Chairs
Monika Backhaus
Lidong Chen
Wednesday PM, April 07, 2010
Room 2002 (Moscone West)
2:30 PM - DD7.1
High Figure of Merit Nanostructured Bulk Thermoelectrics from Doped Pnictogen Chalcogenide Nanoplate Crystals.
Rutvik Mehta 1 2 , C. Karthik 1 2 , Binay Singh 1 , Yanliang Zhang 2 3 , Eduardo Castillo 2 3 , Damien West 4 , Yiyang Sun 4 , N. Ravishankar 1 2 , Shengbai Zhang 4 , Theodorian Borca-Tasciuc 2 3 , Ganpati Ramanath 1 2
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 New York State Center for Future Energy Systems, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Mechanical Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 4 Physics, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractNanostructured forms of V-VI semiconductors based on bismuth telluride alloys are attractive for realizing high thermoelectric figure of merit (ZT) materials for solid-state refrigeration and efficient harvesting of electrical power from waste heat. We report single-component bulk assemblies of sulfur-doped nanostructured pnictogen chalcogenides (e.g., Bi2Te3, Bi2Se3 and Sb2Te3) with 25% to 250% enhancement in the room temperature ZT, compared with their respective non-nanostructured bulk counterparts, and for the first time, a bulk n-type material with a room temperature ZT > 1. We synthesized ~5- to 20-nm-thick single-crystal hexagonal sulfur-doped nanoplates of the pnictogen chalcogenides by a rapid (<~ 60 seconds), scalable surfactant-assisted microwave synthesis approach, followed by compaction and sintering to obtain bulk nanostructured pellets. We show that <1% sulfur doping from thioglycolic acid, used as a nanoplate-sculpting and surface-passivating agent, not only enhances the electrical conductivity σ and Seebeck coefficient α, but also reverses the majority carrier type in the bismuth chalcogenides. In particular, the α values are negative for the bismuth chalcogenides and range between - 235 < α < - 90 μV/K for Bi2Te3, - 80 < α < - 40 μV/K for Bi2Se3 and positive 105 < α < 125 μV/K for the Sb2Te3, while a high electrical conductivity ranges between 3 x 104 ≤ σ ≤ 2.5 × 105 Ω-1m-1. Electron spectroscopy and density functional theory calculations provide insights into the doping and majority carrier reversal mechanisms. The single-component nanostructured pellets exhibit 50% lower thermal conductivity κ and 5% higher power factor (α2σ) when compared with that of the state-of-the-art alloys. Our electron microscopy analyses reveal that measured κ values as low as 0.7 – 1.4 W/mK are due to 50-100 nm grains with intragrain structural modulations with characteristic wavelengths between 3-10 nm. Such low κ values obtained without alloying provide an attractive means to preserve high intrinsic α and σ, which collectively enables large increases in ZT. Our findings open up completely new possibilities for realizing novel high ZT thermoelectric materials through the assembly of doped single-crystal nanostructures.
2:45 PM - DD7.2
Defect Structure at Rocksalt/Tetradymite-Telluride Interfaces.
Douglas Medlin 1 , J. Sugar 1 , N. Heinz 2 , T. Ikeda 2 , G. Snyder 2
1 Materials Physics, Sandia National Laboratories, Livermore, California, United States, 2 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractUnderstanding the mechanisms that govern the formation, morphology, and stability of interfaces is important in developing high performance thermoelectric nanocomposites. Interfacial line defects, such as dislocations and steps, play key roles in these processes and may also provide insight the interfacial mechanisms affecting thermal and electronic transport. Here, we discuss our analysis of line-defect structure at rocksalt/tetradymite telluride interfaces. We begin with a discussion of interfacial coherency and its special aspects at interfaces in telluride compounds based on the rocksalt and tetradymite structures. We compare perfectly coherent interfaces, such as the Bi2Te3 (0001) twin, with semi-coherent, misfitting interfaces. We next discuss the formal crystallographic analysis of interfacial defects in these systems and then apply this methodology to high resolution transmission electron microscopy (HRTEM) observations of interfaces in the AgSbTe2/Sb2Te3 and PbTe/Sb2Te3 systems, focusing on interfaces vicinal to {111}/{0001}. These two systems pose an interesting comparison because they have very different interfacial misfits (0.8% for AgSbTe2/Sb2Te3 versus 7.1% for PbTe/Sb2Te3). Through this analysis, we identify a defect that can accomplish the rocksalt-to-tetradymite phase transformation through diffusive-glide motion along the interface. We establish this mechanism by determining the geometric properties of the defect--namely its step-height and Burgers vector, which has components both perpendicular and parallel to the interface. Climb of the perpendicular dislocation component removes a metal plane from the rocksalt phase, forming the tellurium double-layer, while glide of the parallel component places the close-packed planes into the correct tetradymite stacking sequence. The defect properties also give the atomic flux requirements for defect motion, which we analyze for different compositions of the two phases. We also discuss the defect mechanisms of interfacial strain accommodation. In addition to interfacial lattice dislocations, which have no step component, we also identify defects with large steps, several planes high, which also may play a role in the misfit accommodation. Interestingly, because the step-defects also have a Burgers vector component perpendicular to the interface, which results from a mismatch in step-heights in the two adjacent crystals, such defects also result in local lattice bending and rotation.
3:00 PM - DD7.3
Thermal Conductivity Reduction in Bulk Nano-composites With Randomly Oriented Superlattice Grains.
Fan Yang 1 , Chris Dames 1
1 Mechanical Engineering, University of California at Riverside, Riverside, California, United States
Show AbstractBulk nano-composite materials, such as the PbTe / Sb2Te3 lamellar system described by Ikeda et al. [1], are more practical than epitaxially-grown superlattices for most thermoelectric applications. Such bulk nano-composites are made of numerous superlattice grains, each oriented randomly. Here we describe the first theoretical model of the effective (i.e., macroscopic) phonon thermal conductivity of these materials. An analytical averaging rule, verified by finite element methods (FEM), gives the effective thermal conductivity as a function of the anisotropic thermal conductivity tensor of a single grain. This tensor, in turn, depends on the in-plane and cross-plane thermal conductivities of the superlattice within each grain. These superlattice conductivities are calculated from the Boltzmann transport equation (BTE), explicitly accounting for the frequency dependence of the phonon properties. The model reveals that the effective thermal conductivity is dominated by the in-plane thermal conductivity of each superlattice grain, and remains finite even if the cross-plane thermal conductivity of each grain becomes zero. The dependence of period, specularity, and temperature are also investigated, and the modeling results compared with the experimental data of Ikeda et al. [2].[1] T. Ikeda et al., Chemistry of Materials 19, 763-767 (2007).[2] T. Ikeda et al., International Conference on Thermoelectrics (Jeju Island, Korea, 2007).
3:15 PM - DD7.4
Thermoelectric Study of Bismuth Antimony Telluride (BiSbTe) Nanocomposite System at Low Temperature.
Ming Tang 1 , Bed Poudel 5 , Xiao Yan 4 , Bo Yu 4 , Gang Chen 3 , Zhifeng Ren 4 , Cyril Opeil 4 , Mildred Dresselhaus 1 2
1 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 5 , GMZ Energy, Inc., Waltham, Massachusetts, United States, 4 Physics, Boston College, Chestnut Hill, Massachusetts, United States, 3 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Physics, MIT, Cambridge, Massachusetts, United States
Show AbstractThermoelectrics have always been attractive for applications in power generation and refrigeration because of their reliability and quietness. However, they remain non-competitive due to their low efficiency. Recently, major advancement in thermoelectric performance using nanoengineering approaches has been reported by numerous research groups. Nanocomposites are among the many successful nanoengineering approaches. In the p-type Bismuth Antimony Telluride (BiSbTe) material system, a more than 30% performance improvement has been achieved. Although the cause of the improvement is known to come from the significant decrease in the thermal conductivity, much underlying physics of the nanocomposite system is still unknown. To gain further understanding of the nanocomposite system, we have investigated one of the successful materials systems at low temperature, namely, the BiSbTe nanocomposite system. Thermal conductivity, Seebeck coefficient, electrical conductivity, and Hall results are presented. A theoretical model is developed to interpret the results. Comparison between this model and the experimental results is also discussed.
3:45 PM - DD7.5
Interfacial Defect Structure of Sb2Te3 Widmanstaetten Precipitates in Thermoelectric PbTe.
Nicholas Heinz 1 , Doug Medlin 2 , Teruyuki Ikeda 1 3 , G. Jeff Snyder 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Materials Physiscs Department, Sandia National Laboratories, Livermore, California, United States, 3 PRESTO, Japan Science and Technology Agency, Kawaguchi, Saitama, Japan
Show AbstractThe reduction of lattice thermal conductivity in thermoelectric materials can be achieved by phonon scattering at the interfaces of nanostructures. The effectiveness of this scattering is determined by the roughness of the interface. In this paper we begin to examine the interfacial roughness of Sb2Te3 Widmanstätten precipitates in PbTe by analyzing their defect structure. Simulated and experimental electron diffraction is used to corroborate earlier macro scale orientation measurements using EBSD, proving that the orientation is indeed {111}PbTe//{0001} Sb2Te3. This also confirms the orientation aligns the close packed planes and directions of the precipitates to the matrix. Moreover, it is shown that this orientation holds even when the interface plane is far from the {111}//{0001} orientation. Macroscopically, the precipitates form a lenticular shape that is roughly parallel to the stated orientation, however, at higher magnification it is clear that the precipitates are terraced to {111}//{0001}. We apply crystallographic analysis methodology to high resolution transmission electron microscopy (HRTEM) observations of the interfaces vicinal to the {111}//{0001} planes in order to determine the defect mechanisms for misfit accommodation. Also, we observe a periodic strain contrast at the interfaces and we will discuss this in relationship to the strain accommodation and morphology of the precipitates.
4:00 PM - DD7.6
Thermoelectric Properties of Chemically Synthesized Bi2-xSbxTe3 Nanocrystalline Materials.
Jeffrey Dyck 1 , Brett Hernandez 1 , Yixin Zhao 2 , Clemens Burda 2
1 Physics, John Carroll University, University Heights, Ohio, United States, 2 Chemistry, Case Western Reserve University, Cleveland, Ohio, United States
Show AbstractConsiderable research effort has gone into improving the performance of traditional thermoelectric (TE) materials such as Bi2-xSbxTe3through a variety of nanostructuring approaches. One such approach is to start with chemically synthesized nanocrystalline powers and then form bulk specimens by compacting and densifying the powder. While chemical approaches have the potential of producing very small (< 50 nm) diameters with narrow size distributions and controlled shape, the TE performance of these materials has not been as good as for top down approaches such as careful ball milling of bulk-grown ingots. For this study, nanocrystals of Bi2Te3 and Bi0.5Sb1.5Te3 were synthesized using chemical methods, and characterized by XRD, SEM, and TEM. Thermoelectric transport properties of pressed pellets were measured as a function of temperature. We find that careful annealing of the powders before pressing results in a dramatic improvement of the TE performance through an increase in both electrical conductivity and Seebeck coefficient, while maintaining a low thermal conductivity due to the nanoscale grain structure. This annealing enhancement is likely due to removal of the capping layers associated with surfactant used in the synthesis. In addition, we present results on further optimizing the electronic properties by changing the ratio of Bi to Sb, and in turn the carrier concentration, in the synthetic process.
4:15 PM - DD7.7
Spinodal Decomposition in Off-stoichiometric Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler Phases.
Nathan Takas 1 , Dinesh Misra 1 , Heike Gabrisch 1 , P. Ferdinand Poudeu 1
1 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractThe formation of coherent nanostructures within the matrix of half-Heusler thermoelectric materials can be produced by spinodal decomposition of off-stoichiometric compositions. CoSb is insoluble at high temperatures in Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler phases. This phase can be solubilized into the half-Heusler matrix by the use of high energy ball milling at room temperature as the synthetic method of choice. The metastable half-Heusler material decomposes in-situ while hot-pressing the powder sample into a compact pellet. Despite the fact that the thermal conductivity of the inclusion material, CoSb, is very large, (>35W/m*K), we observed significant reduction in the thermal conductivity of the composite material. Furthermore, the electrical resistivity of the specimen was also reduced due to the metallic nature of the CoSb inclusion phase. Addition of a large fraction of the metallic inclusion leads to a percolation network of the metallic phase, thus reducing the Seebeck coefficient of the composites. High resolution transmission electron microscopy (HRTEM) is carried out in order to examine boundaries between the two phases and also the coherency of the minor phase with the half-Heusler matrix. Changes in the thermoelectric properties of Zr0.5Hf0.5Co1-yIrySb1-zSnz half-Heusler matrix with increasing volume percent of CoSb inclusion will be discussed.
4:30 PM - DD7.8
An Examination of the Thermoelectric Properties of Nanostructured Cubic SnTe Combined With Orthorhombic SnS (SnTe1-xSx).
Christopher Stender 1 , Mercouri Kanatzidis 1
1 Chemistry, Northwestern University, Evanston, Illinois, United States
Show AbstractMuch effort has been put into reducing the thermal conductivity of thermoelectric materials using various methods. We have investigated the potential of SnTe-based thermal electric materials. Use of SnTe presents several challenges, namely a relatively high thermal conductivity and a large charge carrier concentration. Here we describe the SnTe1-xSx system and the effects of nanostructure precipitates of SnS on the thermal conductivity and use of other dopants to control charge carrier concentrations. Nanostructuring of bulk thermal electric materials has thus far exhibited great promise in PbTe and LAST systems by decreasing the thermal conduction due to phonon scattering with relatively little effect on electron mobilities. Utilizing the inherent crystal structure mismatch from the cubic parent compound (SnTe) and the minority orthorhombic compound (SnS), interesting nanostructuring should occur yielding enhanced scattering of both high and low frequency phonons. SnTe-SnS (53/47%) eutectic has poor thermoelectric properties owing to the decrease in charge carrier mobility and limited effect on the thermal conductivity compared to lower SnS concentrations (~4%). Additionally, we have explored the problem of charge carrier concentration in SnTe, owing to a large amount of naturally occurring Sn vacancies, by exploring a wide variety of dopants. By judicious additions of various dopants we have eliminated some of the Sn vacancies in the lattice limiting the charge carrier concentration and increasing the Seebeck coefficient.
4:45 PM - DD7.9
Synthesis and Thermoelectric Properties of Carbon Nanotube Embedded Bi-Te Nanopowders.
KyungTae Kim 1 , DongWon Kim 1 , HyeMoon Lee 1 , GilGeun Lee 2 , GookHyun Ha 1
1 , Korea Institute of Materials Science, Changwon, Gyeongnam, Korea (the Republic of), 2 , Pukyong National University, Busan Korea (the Republic of)
Show AbstractBismuth telluride is one of the most attractive thermoelectric materials for the application at room temperature. In general, the figure-of-merit (ZT) value increases as both thermal conductivity and electrical resistivity decreases. There have been several trials to enhance the thermoelectric performance of the figure-of-merit (ZT) by nanostructuring of materials to achieve these properties. Recently, the maximum ZT value has been reached to 1.2 at 300oC by the effect of thermal conductivity reduction. In order to enhance the thermoelectric performance more, nanomaterials embedded thermoelectric nanopowders are suggested in this study. Multiwalled carbon nanotubes (MWCNTs)are dispersed in the solutions and Bi and Te salt are added to react each other. After the chemical reaction, carbon-nanotubes-embedded Bismuth telluride nanopowders are synthesized in a submicrometer size. The nano compositepowders were sintered by spark plasma sintering process and then characterized the thermoelectric properties. The results show 1.5times enhanced ZT values of carbon nanotube embedded materials compared to those of Bi-Te without MWCNTs. It is concluded that carbon nanotubes could be used for the promising candidates to improve thermoelectric performance of Bi-Te materials due to composite effect.
5:00 PM - DD7.10
High Thermoelectric Figure of Merit in Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 Half-Heusler Composites With WO3 Inclusions.
Pierre Poudeu Poudeu 1 2 , Julien Makongo 1 , Rumana Yaqub 1 , Nathan Takas 1 , Dinesh Misra 1 , Kevin Stokes 1 , Heike Gabrisch 1 2
1 Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States, 2 Department of Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractSeveral compositions of the n-type Zr0.5Hf0.5Ni1-xPdx Sn0.99Sb0.01 (x = 0.0, 0.2, 0.3, 0.4 and 0.5) half-Heusler phases were prepared in 20 g scale using traditional high temperature solid state reaction and high quality specimen (with approximately 98% of the theoretical density) for properties measurements were fabricated using uniaxial hot pressing and spark plasma sintering techniques. The isoelectronic substitution of Ni by Pd in Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 substantially reduced the lattice thermal conductivity but also drastically reduced the electrical conductivity. The maximum thermoelectric figure of merit, ZT ~0.6 at 750K was obtained for compositions with x = 0.2 and 0.3. Half-Heusler composites were produced by mechanical alloying mixtures of Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 bulk matrices with various concentration (y wt% = 1, 2, 4, 6) of the n-type WO3 nanopowder followed by hot-pressing or spark plasma sintering. Embedding WO3 nanopowder in Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 matrices can improve the overall thermoelectric performance of the composite materials due to its low thermal conductivity. The effects of WO3 inclusions on the thermoelectric properties of Zr0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 materials will be discussed.
5:15 PM - DD7.11
Metal Nanoinclusions in Bi2Te3 and Bi0.5Sb1.5Te3 for Enhanced Thermoelectric Applications.
Sumithra Santhanam 1 , Nathan Takas 1 , Nathaniel Henderson 1 , Dinesh Misra 1 , Pierre Poudeu 1 , Kevin Stokes 1
1 AMRI, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractMetal nanoinclusions in a thermoelectric matrix are expected to significantly enhance the thermoelectric figure of merit (ZT) due to the combined effect of increased power factor and reduction in lattice thermal conductivity. Appropriate nanoparticles in the matrix phase can lead to grain boundary and electronic band structure modifications which help to independently tune the seebeck coefficient, electrical conductivity and lattice component of thermal conductivity. Recent reports have theoretically shown an enhancement in power factor with metal nanoinclusions [1,2] depending on the nanoparticle volume fraction and also on the interface potential. We adopt this nanocomposite approach for further enhancement of the figure of merit: in Bi2Te3 and Bi0.5Sb1.5Te3 matrix phases. The nanostructured thermoelectric matrix of Bi2Te3 and Bi0.5Sb1.5Te3 is synthesized by ball milling elemental Bi, Sb and Te metal shots. Metal nanoinclusions (like Bi and Ag) synthesized by low temperature solution methods are incorporated into the Bi2Te3 and Bi0.5Sb1.5Te3 thermoelectric matrix at volume fractions of 0 to 15%. The nanocomposites with nanoinclusions are uniaxially hot pressed for densification. The nanocomposites are characterized for the biphasic nature by X-ray diffraction and electron microscopy. The thermoelectric transport properties: Seebeck co-efficient, electrical conductivity and thermal conductivity from room temperature to 2000C will be presented. The effect of nanoparticle inclusions on the carrier concentration, mobility and effective mass are deduced from Hall effect measurements. We find that the presence of nanoparticles can increase the electrical conductivity of the material, while the Seebeck coefficient and thermal conductivity show very little change resulting in an increase of the power factor and figure of merit for some composites. References:1) M.Zebarjadi, K. Esfarjani, A. Shakouri, J.H. Bahk, Z. Bian, G. Zeng, J. Bowers, H. Lu, J. Zide, and A. Gossard., Journal of Electronic Materials, Vol.38, No.7, 2009.2) M.Zebarjadi, K. Esfarjani, A. Shakouri, Z.Bian, J.H. Bahk, G. Zeng, J. Bowers, H. Lu, J. Zide, and A. Gossard., Applied Physics Letters, Vol. 94, 202105, 2009.
5:30 PM - DD7.12
Synthesis and Thermoelectric Properties of PbTe With Ag2Te Nano-inclusion.
Yanzhong Pei 1 , Jeffrey Snyder 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractBased on equilibrium phase diagram design, bulk PbTe thermoelectric material with homogenous nano-sized Ag2Te precipitates are obtained by melting, quenching, annealing and hot-pressing processes. Electrical and thermal transport properties measurements as well as microstructure characterization are carried out. With the effective phonon scattering by the nano-sized particles, the lowest lattice thermal conductivity of ~0.3 W/m-K obtained in the nano-composites is approaching the amorphous limit of bulk PbTe. Combination of carrier concentration optimization by La doping finally results in a high thermoelectric figure of merit, ZT. Therefore, obtaining nano-precipitates embedded in bulk materials by phase diagram design is believed to be an effective approach to reduce the lattice thermal conductivity and thus to enhance the thermoelectric performance for potential use as high efficient thermoelectric power generation from heat sources.
5:45 PM - DD7.13
Fulleride Salts: Decoupling Thermal and Electrical Conductivity for Thermoelectric Applications.
Daniel Bates 1 , C. Michael Elliott 1 , Amy Prieto 1
1 Chemistry, Colorado State University, Fort Collins, Colorado, United States
Show AbstractDevices made from thermoelectric (TE) materials can convert thermal energy from a temperature gradient to electrical energy (for power generation), or electrical energy to a thermal gradient (for refrigeration). These devices are promising candidates for more efficient and environmentally friendly energy technologies. The advantages of devices based on thermoelectric materials are that they have no moving parts, and so are vibrationless, quiet and non-emissive. One easily can imagine focusing sunlight onto one face of a thermoelectric device and converting that heat into electricity with high efficiency and no generation of waste. Current TE materials operate at fairly low efficiencies, and hence are not suitable for such an application. We are proposing to evaluate the TE properties of a new class of semiconducting materials, fulleride salts of the general form: [MLnp+]a(C60a-)p where M is Fe, Cr or Ru, L is a bipyridine, terpyridine or phenanthroline (substituted or unsubstituted); p+ = 1+, 2+ or 3+ ; and a- = 1-, 2- or 3-. We have synthesized over thirty members of this family, and will show Seebeck coefficient and electrical transport measurements. These materials differ fundamentally from other traditional electron-hopping conductors in that both the cation and anion are redox active. As a consequence both ion-types participate in electron transport conduction within the material. Moreover, the redox properties, size, and shape of the cation are exquisitely sensitive to ligand substitution. We expect to control the thermal conductivity of our materials independently of the electrical conductivity by tuning the electrostatic interactions between the ions (via changing the charge and size). The synthetic flexibility inherent to these materials is in stark contrast to the control over typical extended solids, and will afford a new route toward decoupling the thermal and electrical properties and allow us to optimize them independently.
Symposium Organizers
John D. Baniecki Fujitsu Laboratories Ltd.
Jonathan A. Malen Carnegie Mellon University
G. Jeffrey Snyder California Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
DD8: Superlattices, Nanowires, Nanoparticles I
Session Chairs
Alan McGaughey
Jeff Urban
Thursday AM, April 08, 2010
Room 2002 (Moscone West)
9:00 AM - **DD8.1
Thermal Transport Across Semiconductor Interfaces and Thin Films.
Alan McGaughey 1 , Eric Landry 1
1 Mechanical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States
Show AbstractThe ability to accurately predict the thermal boundary resistance of semiconductor interfaces will allow for improvements in the design of nanostructured materials (e.g., superlattices) with high thermoelectric figure of merit. Current models for predicting thermal boundary resistance (e.g., the acoustic and diffuse mismatch models) are inaccurate at typical application temperatures due to assumptions. For example, both models neglect the atomic-level detail of the interface and are usually applied under the Debye approximation. In this work, molecular dynamics (MD) simulations and lattice dynamics (LD) calculations are used to study thermal transport across systems containing one or two interfaces (i.e., a thin film). We begin by assessing the accuracies of two theoretical expressions for thermal boundary resistance (TBR) by comparing their predictions to independent predictions from MD simulations. In one expression (R_E), the phonon distributions on either side of the interface are assumed to follow the equilibrium distribution, while in the other expression (R_NE), the phonons are assumed to have nonequilibrium, but bulk-like distributions. The phonon properties are obtained using LD calculations, which assume that the phonon interface scattering is elastic. We consider a Si/Ge interface and a series of Si/heavy-Si interfaces, where heavy-Si differs from Si only in mass. The MD-predicted Si/Ge TBR is temperature-independent below a temperature of 500 K, indicating that the phonon scattering is elastic. For the Si/Ge interface and the Si/heavy-Si interfaces with mass ratios greater than two, R_E is in good agreement with the corresponding MD-predicted values. When applied to a system containing no interface, R_E is erroneously nonzero. While R_E is zero for a system containing no interface, it is 40-60% less than the corresponding MD-predicted values for the Si/Ge interface and the Si/heavy-Si interfaces. This inaccuracy is attributed to the assumption of bulk-like phonon distributions on either side of the interface.We then examine the thickness-dependence of the thermal resistance of semiconductor thin films at a temperature of 500 K. We consider Si and Ge films that are confined between larger extents of the other species (i.e., Ge/Si/Ge and Si/Ge/Si structures). For structures with thicknesses (L) less than 2 nm, the thermal resistance increases rapidly with increasing film thickness. This trend is attributed to changes in the allowed vibrational states in the film. When 2 nm < L < 30 nm, the MD-predicted thermal resistances are thickness-independent for the Ge/Si/Ge structures and increase with increasing thickness for the Si/Ge/Si structures. We attribute these results to phonon transport that is ballistic in the Ge/Si/Ge structures and more diffusive in the Si/Ge/Si structures based on comparisons to the theoretical calculations, which assume ballistic transport.
9:30 AM - DD8.2
Thermoelectric Properties of BiTe/BiSbTe Superlattice Nanostructures.
Raja Mannam 1 , Mangilal Agarwal 2 , Kody Varahramyan 2 , Despina Davis 1
1 Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana, United States, 2 , Indiana University–Purdue University Indianapolis, Indianapolis, Indiana, United States
Show AbstractNanostructure materials, using the quantum effects, and superlattice thin film structures, exploiting the reduced thermal conductivities, show new directions for thermoelectric research to obtain higher ZT values. In this work we explore the thermoelectric properties of V-VI nanostructure with focus on BiTe/BiSbTe superlattice nanowires. The nanostructures are fabricated by electrochemical deposition process from tartaric-nitric acid based solutions. Potentiostatic deposition is used to deposit BiTe, BiSbTe nanowires and a pulsed-potential technique is used to deposit superlattice BiTe/BiSbTe nanostructures. Seebeck coefficient measurements of the BiTe nanowires showed a maximum n-type of -318.7 µV/K and a p-type of 117 µV/K at room temperature. For BiSbTe nanowires a maximum Seebeck coefficient of -500 µV/K was obtained. The power factor (S2σ) of this nanowire sample was calculated to be 5.6x10-3 W/mK2. To the best knowledge of the author, these are the highest reported values for electrodeposited BiSbTe nanowires. Detailed analysis of thermoelectric properties of BiSbTe nanowires, BiTe/BiSbTe superlattice structures and their dependence on the alloy layers thickness and nanowires diameter will be discussed.
9:45 AM - DD8.3
Atomically Thin Films of Bismuth Telluride: Mechanical Exfoliation and Prospects of Thermoelectric Applications.
Desalegne Teweldebrhan 1 , Vivek Goyal 1 , Muhammad Rahman 1 , Alexander Balandin 1
1 Nano-Device Laboratory, Department of Electrical Engineering and Materials Science and Engineering Program, Bourns College of Engineering, University of California - Riverside, Riverside, California, United States
Show AbstractIt follows from theoretical predictions of Dresselhaus et al. [1] that a drastic improvement in ZT can be achieved in low-dimensional structures where electrons and holes are strongly confined in one or two dimensions. The latter would require carrier confinement in a quantum well with a width on the order of ~1 nm and very high potential barriers. Balandin and Wang [2] proposed another strategy for increasing ZT in low-dimensional structures by reducing its thermal conductivity via spatial confinement of acoustic phonons. The improvement of thermoelectric properties via quantum confinement [1] or phonon engineering [3] can be achieved only in thin films or nanowires with the thickness of just few atomic layers and high quality of interfaces. Conventional chemical vapor deposition, electrochemical or other techniques are not capable of producing such crystalline structures. In this talk, we show that separate Bi-Te atomic layers can be mechanically exfoliated from bulk bismuth telluride crystal following a procedure similar to the exfoliation of graphene developed by the Manchester, UK – Chernogolovka, Russia team [4]. The presence of the van der Waals bonds between -[Te-Bi-Te-Bi-Te]- five-fold layers allowed us to disassemble bismuth telluride crystals into films with the thickness of five atomic layers. Moreover, our microscopy data indicate that after special procedure these five-fold layers can be broken further leading to quasi-2-D crystals of Bi-Te and Te-Bi-Te. To the best of our knowledge, this is the first successful exfoliation of the large-area few-atom-thick films of crystalline thermoelectric material. The resulting quasi-2-D crystals retain their good electrical conduction and poor thermal conduction properties important for thermoelectric applications. The exfoliation procedure can become a new enabling technology for producing low-dimensional thermoelectric materials with complete quantum confinement of charge carriers and acoustic phonons. The obtained results may pave the way for producing crystalline bismuth telluride quantum wells with the strong quantum confinement of charge carriers, which were theoretically predicted to have an order-of-magnitude higher thermoelectric figure of merit. The work in Balandin group was supported, in part, by DARPA – SRC Center on Functional Engineered Nano Architectonics (FENA) and Interconnect Focus Center (IFC). More details can be found at group’s web-site at: http://ndl.ee.ucr.edu .[1] M.S. Dresselhaus, G. Dresselhaus, et al. “Low-dimensional thermoelectric materials,” Physics of the Solid State, 41, 679 (1999).[2] A. Balandin and K.L. Wang, “Effect of phonon confinement on the thermoelectric figure of merit of quantum wells,” J. Applied Physics, 84, 6149 (1998).[3] A.A. Balandin, “Nanophononics: Phonon engineering in nanostructures and nanodevices,” J. Nanoscience and Nanotechnology, 5, 7 (2005).[4] K.S. Novoselov, A.K. Geim, S.V. Morozov, et al., Science, 306, 666 (2004).
10:00 AM - DD8.4
Seebeck Enhancement via Schottky Barrier Electron Filtering.
Alejandro Levander 1 2 , Jinbo Cao 1 2 , Zelin Yang 1 , Junqiao Wu 1
1 Materials Science and Engineering, UC-Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Lab, Berkeley, California, United States
Show AbstractMany of the recent advancements in thermoelectrics over the last decade have been attributed to nanostructuring of materials to reduce lattice thermal conductivity. However, there is an alternative route to maximizing figure of merit (zT): enhancing the Seebeck coefficient (S). Conventional Seebeck engineering uses degenerately doped semiconductors to maximize the energy derivative of the electronic density of states at the Fermi energy, but theoretical predictions have presented the attractive option of combining diffusive thermoelectric effects with thermionic effects to increase overall S and zT. Previous results have demonstrated this combined Seebeck enhancement effect using expensive molecular beam epitaxy growth techniques to fabricate heterostructure superlattices. We have designed and successfully fabricated a test structure of n-type silicon/nickel Schottky junction lateral superlattice on the device layer of a silicon-on-insulator wafer. The interface between these two materials forms a Schottky barrier, capable of selectively conducting only hot electrons, thereby dramatically increasing the asymmetry of the differential conductivity about the Fermi energy and correspondingly increasing the Seebeck coefficient. We expect the Seebeck coefficient of the system to be a linear combination of the silicon and nickel contributions, with an added junction contribution from the n-Si/Ni interface. The system will be tested at both low and high temperatures, with an expected higher junction contribution at higher temperature, when the Fermi window is wider. The proposed system has the additional benefit of potentially reducing thermal conductivity due to phonon scattering at the interfaces, further enhancing zT.
10:15 AM - DD8.5
Thin-film 2-di Superlattices, Nano-dot Materials and Hybrid Nanostructures for Elevated Temperatures.
Rama Venkatasubramanian 1 , Gary Bulman 1 , Judith Stuart 1 , Phil Barletta 1 , Jayesh Bharatan 1 , Tom Colpitts 1
1 , RTI International, Research Triangle Park, North Carolina, United States
Show AbstractNanoscale materials – superlattices, nano dots and bulk with second phase nano-inclusions – have become the dominant approach to enhancing the figure of merit (ZT) in thermoelectric materials. The primary mechanism for ZT improvement has been the significant reduction in lattice thermal conductivity through phonon scattering processes in nanoscale materials, without affecting the electron/hole transport. There has been considerable progress in ZT near 300K in p-type Bi2Te3/Sb2Te3 superlattices and more recently, in delta-doped n-type Bi2Te3-xSex thin-films. In this presentation, we will focus on low-dimensional thin-film materials for mid- and high-temperature applications. PbTe and GeTe, with similar bandgaps but very different lattice constants and hence acoustic mismatch, offer the potential for lattice thermal conductivity reduction and at the same time for efficient cross-plane carrier transport. Transmisison electron microscopy of the PbTe/GeTe thin-films show novel hybrid 2-D and 1-D structures. For the mid-temperature applications, PbTe-GeTe superlattices have been developed with exceptionally good power factors. Thermal conductivity measurements are in progress in these hybrid nanostructures to determine ZT and the data will be presented. SiGe-nano dot sueperlattices (NDSL) material systems are being developed for higher temperatures. Thermal conductivity measurements of NDSLs indicate low values for lattice contribution, by more than a factor of two below those of alloys. More recently, power factor measurements in these Si/Ge NDSLs indicate that significant enhancement in figure-of merit can be achieved. The modeling of cross-plane carrier transport will be discussed in these two material systems as well, essential for increased cross-plane ZT.
10:30 AM - DD8.6
Nitride Metal/Semiconductor Superlattices for High-temperature Solid-state Energy Conversion.
Polina Burmistrova 1 3 , Jeremy Schroeder 2 3 , David Ewoldt 2 3 , Reja Amatya 4 , Rajeev Ram 4 , Timothy Sands 1 2 3
1 School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana, United States, 3 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana, United States, 2 School of Materials Engineering, Purdue University, West Lafayette, Indiana, United States, 4 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractNitride metal/semiconductor superlattices are novel thermoelectric “metamaterials” for power generation applications at high temperatures (up to ~1000K). Superior thermoelectric properties are expected through enhancing the power factor (S2σ) by electron energy filtering with the potential barriers and, simultaneously, suppressing the lattice contribution to the thermal conductivity via phonon scattering at the interfaces. We have previously reported the thermoelectric properties of the ZrN/ScN solid-state thermionic system at 300K. The experimental results combined with modeling suggest that ZT values exceeding 2.5 at temperatures above ~900K are possible [1], [2]. The design and fabrication of thin-film based thermoelectric generators presents new challenges due to the large temperature gradient and the parasitic electrical impedances [3]. The length of the leg is a serious limitation since the maximum thickness of a high quality nitride metal/semiconductor superlattice is in the range of several microns. To obtain a segment with a thickness greater than 50 microns, as is necessary for power density optimization, the superlattices can be removed from the associated substrates and laminated.(Hf,Zr)N/ScN superlattices with 12 nm period were grown on Si substrates with a 50nm thermal oxide by reactive DC magnetron sputtering. A 260nm (Hf,Zr)N buffer layer followed by 10.5 microns of (Hf,Zr)N/ScN superlattice were sputtered at 200W in an Ar(4 sccm)/N2(6 sccm) ambient at 5 mtorr with a Si substrate temperature of 650oC. 750 nm of Cu was deposited serve as the bonding layer. The resulting structures were bonded face-to-face with metal thermocompression bonding and the Si substrates were removed by selective etching in TMAH. Finally, the bilayers were metallized and laminated to construct a segment of a desired effective thickness. The (Hf,Zr)N/ScN superlattices on Si were (100) textured with columnar grains that exhibit lattice coherent superlattices within the grain as determined by high resolution XRD, FESEM, and TEM.The thermoelectric performance of elements with total superlattice thickness of 42 microns were evaluated using the z-meter technique [3]. Preliminary measurements have shown n-type segments with generated power up to 0.24 W/cm2 at an average temperature of 610K. Furthermore, a 21 micron superlattice segment bonded to a n-ErAs/(In,Ga)As segment grown by molecular beam epitaxy yielded 1.8 W/cm2 at an average temperature of 632K. The measured power densites, however, were limited by parasitic effects. Measurements in progress at higher temperatures and with improved load matching are expected to yield higher power densities, as modeling indicates that the optimal performance will be achieved at temperatures of ~1000K. [1] V. Rawat et al., J. Appl. Phys 105, (2009). [2] M. Zebarjadi et.al., J Elec. Matter. 38, (2009). [3] P.M. Mayer et. al, Nanoscale and Microscale Thermophys. Eng. 10, (2006).
10:45 AM - DD8.7
Thermal Transport in (Hf,Zr)N-(Sc,Y)N Superlattices.
Joseph Feser 1 , Mona Zebarjadi 3 , Jeremey Schroeder 4 , Robert Wortman 4 , Dongyan Xu 1 , Zhixi Bian 3 , Arun Majumdar 2 , Ali Shakouri 3 , Timothy Sands 4
1 Mechanical Engineering, Univ. of California, Berkeley, Berkeley, California, United States, 3 Electrical Engineering, University of California, Santa Cruz, Santa Cruz, California, United States, 4 Materials Engineering, Purdue Univerisity, West Lafayette, Indiana, United States, 2 , Advanced Research Projects Agency-Energy, Washington, DC, District of Columbia, United States
Show AbstractAlloyed metal-semiconductor superlattices composed of (Hf,Zr)N-(Sc,Y)N have been studied as candidate thermoelectric materials because their potential to simultaneously reduce the lattice thermal conductivity using phonon interface scattering and to enhance the thermoelectric power factor using energy-dependent electron filtering. Thermal conductivity measurements provide valuable insight into the phonon and electron transport processes in these materials. Temperature-dependent thermal conductivity measurements on thin films have been performed using the 3-Omega method between 30K - 800K for various alloy compositions and superlattice spacings. The results indicate a strong electronic component of thermal conductivity consistent with thermionic emission. We demonstrate that by controlling alloy composition, the barrier height of the resulting Schottky barrier can be controlled. Electronic modeling using Boltzmann Transport Theory allows for the determination of the effective barrier height and indicates that these may be highly efficient thermoelectric materials at high temperatures, with potential figure-of-merit, ZT, between 0.5 and 3 at 800K, depending on the degree of lateral momentum conservation of electrons.
11:00 AM - DD8:Nano TE II
BREAK
11:15 AM - **DD8.8
Molecular and Polymer/Inorganic Nanocomposite Thermoelectrics.
Rachel Segalman 1 2
1 Chemical Engineering, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Labs, Berkeley, California, United States
Show AbstractThermoelectric materials for energy generation have several advantages over conventional power cycles including lack of moving parts, silent operation, miniaturizability, and CO2 free conversion of heat to electricity. Historically, these materials suffered from low efficiency and have involved very expensive inorganic compounds. Molecular materials and hybrid organic-inorganics bring the promise of inexpensive, solution processible, mechanically durable devices. We demonstrate that some metal-molecule-metal junctions produce voltage when exposed to heat. This exciting initial demonstration of thermopower from molecular junctions is reinforced by the fact that relatively minor changes to chemical structure result in an unprecedented simultaneous increases in both thermopower and electrical conductance, suggesting that highly efficient devices may be designed in a manner impossible in traditional thermoelectric materials. Thermopower measurements also offer an alternative transport measurement that is used to understand the fundamental behavior of a molecule near a continuous electrode. Unlike many other experiments, molecular thermopower measurements are independent of the number of molecules in the junction and are therefore much more stable and capable of capturing detail than many other experimental methods on experimental junction. As we learn more about the nature of transport in molecular junctions, we also gain insight into the design of inexpensive, mechanically durable organic/inorganic hybrid materials for thermoelectrics. This new class of materials incorporates both the processibility, high (>100S/cm) electrical conductivity, and low thermal conductivity of conjugated polymers with the high thermopower of a nanocrystalline inorganics. In this talk, I will discuss our efforts to gain fundamental insights to molecular electronic transport as well as extensions to the design of new thermoelectric materials.
11:45 AM - DD8.9
Thermoelectric Power Factor of ErSb:InGaSb.
Zhixi Bian 1 , Mona Zebarjadi 1 , Ali Shakouri 1
1 Electrical Engineering Department, University of California Santa Cruz, Santa Cruz, California, United States
Show AbstractWe calculate the thermoelectric power factor of ErSb:InGaSb nanocomposite materials. InGaSb is a good thermoelectric candidate in that it offers large electron and hole mobilities and density of states as well. By adjusting the relative alloy composition of InSb and GaSb, one can optimize its thermoelectric power factor and reduce its thermal conductivity to pursue a large thermoelectric figure-of-merit. The narrow energy distance between Γ and L valleys plays an important role in increasing the asymmetry of density of states and enhancing the thermoelectric effect. ErSb nanoparticles can be embedded into the InGaSb matrix by adding Erbium during material growth. These nanoparticles can scatter mid-long wavelength phonons more effectively and reduce the lattice thermal conductivity further to lower than that of alloy minimum. The electron energy filtering effect of these nanoparticles is investigated in term of thermoelectric power factor optimization. It is shown that with appropriate barrier structure, thermoelectric figure-of-merit ZT>2 can be achieved. The application of ErSb:InGaSb materials in thermoelectric refrigeration and energy conversion will be discussed.
12:00 PM - DD8.10
Bismuth Compound Nanowires for Thermoelectrics.
Maria Eugenia Toimil-Molares 1 , Sven Mueller 1 , Oliver Picht 1 , Friedemann Voelklein 2 , Heiko Reith 2 , Matthias Schmitt 2 , Reinhard Neumann 1
1 Materials Research, GSI Helmholtz Center for Heavy Ion Research GmbH , Darmstadt Germany, 2 , University of Applied Sciences Wiesbaden, Rüsselsheim Germany
Show AbstractIn 1993, Hicks and Dresselhaus published theoretical calculations of the electrical conductivity (σ), the Seebeck coefficient (S), and the thermal conductivity (κ), that predicted an enhanced thermoelectrical figure of merit ZT = S2 T / κ for 2D quantum-well structures and quantum wires, establishing a new direction in the search of more efficient thermoelectric materials. Promising low-dimensional materials being investigated for thermoelectric applications include bismuth and bismuth compounds. In particular, bulk bismuth telluride based compounds are widely used for room temperature thermoelectric applications, and bismuth antimony exhibits high efficiency at liquid nitrogen temperature. The synthesis of bismuth compound nanowires with controlled size and crystallinity, and the systematic study of the thermoelectric properties of individual nanowires will contribute thus to understand the influence of mesoscopic and quantum-size effects on the thermoelectric characteristics of nanostructured materials. Here, we present the synthesis of Bi, Bi2Te3 and Bi(1-x)Sbx (0< x <1) nanowires with diameter between 20 and 300 nm and well-controlled morphological properties. The nanowires have been fabricated by electrochemical deposition in etched ion-track templates. Nanowire morphology and crystallinity are investigated by scanning electron microsocopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Energy dispersive x-ray spectrometry (EDS) indicates that the composition of these 1D thermoelectric model systems is successfully controlled by the deposition conditions. The transport properties (σ, κ and S) of single nanowires as a function of their crystallinity, diameter, chemical composition, and temperature are being measured by means of specifically designed microchips.
12:15 PM - DD8.11
Enhancing Thermoelectric Properties of Polymer Nanocomposites by Changing Concentrations and Types of Carbon Nanotubes and Stabilizers.
Kyungwho Choi 1 , Choongho Yu 1
1 Mechanical Engineering, Texas A&M University, College Station, Texas, United States
Show AbstractLow thermal conductivity of polymeric materials is promising for thermoelectric applications, but their low electrical conductivities and Seebeck coefficients (thermopowers) have excluded them as a feasible candidate in the past. Here, nanomaterial-polymer composites bring their electrical properties into degenerate-semiconductor or metallic regimes by incorporating conductive fillers into a polymer matrix. The composites were prepared by mixing poly(vinyl acetate) and carbon nanotubes (CNTs) in water and subsequent drying processes at 80°C and room temperature. Electron and thermal transport properties can be tuned by altering junctions between conductive fillers. Influence of stabilizers on thermoelectric properties was systematically investigated by changing concentration fillers and using different stabilizers. Furthermore, optimum synthesis condition including filler concentration was found to obtain highest electrical conductivity and Seebeck coefficient as well as lowest thermal conductivity. The electrical conductivity of the composites was increased over 105 S/m with a thermopower of typical carbon nanotubes and a thermal conductivity of typical polymers. The results show polymers can be viable for thermoelectric energy conversion, replacing heavy and toxic materials such as Bi-Te alloys in the future.
12:30 PM - DD8.12
Colloidal Nanostructures as Building-blocks for Macroscopic Thermoelectric Materials with Electron-crystal Phonon-glass Properties.
Marcus Scheele 1 , Niels Oeschler 2 , Katrin Meier 2 , Andreas Kornowski 1 , Christian Klinke 1 , Horst Weller 1
1 Physical Chemistry, University of Hamburg, Hamburg Germany, 2 , Max-Planck-Institute for Chemical Physics of Solids, Dresden Germany
Show AbstractDepleting thermal conductivity by the introduction of nanostructures into common thermoelectric materials has led to substantial improvements in recent thermoelectric research. Typically, these nanostructures are synthesized in a top-down-approach by ball-milling and similar techniques. In contrast to this, we have demonstrated recently that bottom-up techniques, such as nanocrystal growth in solution, are capable of producing large quantities of size-controllable, sub-10-nm nanocrystals.[1] After spark plasma sintering to macroscopic pellets, these nanostructured bulk materials display bulk-like electric conductivities in combination with drastically reduced thermal conductivities. Our contribution will cover materials such as Bi2Te3, BiSbTe3 and PbTe. As the major advantage over top-down techniques, it is demonstrated how easily morphology, size and size-distribution of the nanomaterials can be controlled by this procedure. Our presentation will include TEM, SEM and powder-XRD-studies of the constituting nanocomponents. We will explain in detail how organic ligands applied during the synthesis can be removed from the inorganic material allowing for high electric conductivities.[2] The sintered thermoelectric materials are analyzed in terms of their thermal conductivity, electric conductivity, thermopower, charge-carrier density and specific heat. For example, the thermal conductivity of Bi2Te3 nanoparticles is reduced by a factor 3 to 0.5 W m-1 K-1 at 300 K, where BiSbTe3 nanosheets show a 7-fold reduction to 0.2 W m-1 K-1. In combination with the preserved high electric conductivities, these materials are promising candidates for future thermoelectric modules with improved figure of merits. [1] M. Scheele, N. Oeschler, K. Meier, A. Kornowski, C. Klinke, and H. Weller, Adv. Funct. Mater., published online (2009).[2] M. V. Kovalenko, M. Scheele, D. V. Talapin, Science, 324 (2009), 1417.
12:45 PM - DD8.13
Semimetal-semiconductor Nanocomposites ErSb:InxGa1-xSb as a Promising p-type Thermoelectric Material.
Hong Lu 1 2 , Peter Burke 1 , Arthur Gossard 1 2 , Ashok Ramu 2 , Gehong Zeng 2 , John Bowers 2 , Dongyan Xu 3 , Arun Majumdar 3
1 Materials Department, University of California, Santa Barbara, California, United States, 2 Department of Electrical and Computer Engineering, University of California, Santa Barbara, California, United States, 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractDue to the material properties and appropriate band alignments, erbium-based antimonide nanocomposites, such as ErSb:InxGa1-xSb, are promising as p-type materials for thermoelectric applications. We started the MBE growth of these nanocomposites by homoepitaxy for better growth control and optimization of growth conditions. Epitaxial films of ErSb:GaSb and ErSb:InSb have been grown on GaSb and InSb substrates, respectively. ErSb nanoparticles are formed spontaneously during a codeposition process within the semiconductor matrix. However, these substrates are intrinsically conductive which makes it difficult to measure the electrical and thermal properties due to the substrate effect. So a lot of effort has been put into the heteroepitaxial growth on different substrates such as GaAs. Previous studies show that the ErSb nanoparticles can scatter the mid- and long- wavelength phonons, therefore reducing the thermal conductivity below the alloy limit. In addition to the ErSb nanoparticle formation, more interestingly, self-alignment of the nanoparticles is observed in the samples with high Er concentrations, as shown by TEM in Fig. 1. This alignment may be used to enhance the thermoelectric properties of the materials. More studies on these samples are underway.Series of ErSb:InxGa1-xSb samples that are undoped and doped with beryllium have been grown and characterized. The best room temperature power factor of Be doped GaSb is about 6x10-3 W/m-K2, 50% higher than the value of Bi2Te3. Indium composition has been tuned from 12% up to 53% to optimize alloy scattering and thermoelectric power factor. High temperature measurement of these samples is in progress.
DD9: Superlattices, Nanowires, Nanoparticles II
Session Chairs
Thursday PM, April 08, 2010
Room 2002 (Moscone West)
2:30 PM - **DD9.1
Phonon Transport in Silicon Nanowires for Thermoelectric Application.
Renkun Chen 1 4 , Kedar Hippalgaonkar 1 , Baoling Huang 1 , Karma Sawyer 1 , Arun Majumdar 1 3 , Jinyao Tang 2 , Hung-Ta Wang 2 , Peidong Yang 2 3
1 Mechanical Engineering, University of California, Berkeley, California, United States, 4 Mechanical and Aerospace Engineering, University of California, San Diego, California, United States, 3 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Chemistry, University of California, Berkeley, California, United States
Show AbstractSilicon nanowires have been demonstrated to be potentially efficient and inexpensive thermoelectric materials. The enhanced thermoelectric performance of Si nanowires is primarily due to the suppressed thermal conductivity that results from phonon boundary scattering. It was found that nanowire surface roughness plays an important role in decreasing the thermal conductivity below that of diffusive limit. However, the exact effect of surface roughness on phonon transport is not known. Here I present experimental studies toward understanding the phonon transport mechanism in rough silicon nanowires.
3:00 PM - DD9.2
Synthesis of Nano-sized Silicon Particles for Use as Ecological Thermoelectric Material.
Tim Huelser 1 , Gabi Schierning 2 , Roland Schmechel 2 4 , Christof Schulz 3 4 , Hartmut Wiggers 3 4
1 Nano-Energy & Nano Particle Synthesis, Institute of Energy and Environmental Technology (IUTA), Duisburg Germany, 2 Faculty of Engineering, University of Duisburg-Essen, Duisburg Germany, 4 Center for Nanointergration (CeNIDE), University of Duisburg-Essen, Duisburg Germany, 3 Institute for Combustion and Gas Dynamics (IVG), University of Duisburg-Essen, Duisburg Germany
Show AbstractToday, commercially available thermoelectric generators typically contain lead, cobalt or tellurium- compounds. These materials are toxic or their resources are limited. For a large scale production (e.g. wireless sensor networks), it is important to use unlimitedly available, non-toxic materials and implement environmentally sound production, to allow for the use as "green" technology. From these requirements we have identified nano-sized, doped silicon as a potential candidate for use in thermoelectric generators. Especially the excellent ecological and economical properties of this material enhance the chances for future applications. Therefore, we report on the gas phase synthesis of highly specific and semi-conducting silicon nanoparticles in a microwave-supported plasma reactor and on a first step to introduce this material into the field of thermoelectrics.Silicon nanocrystalline materials exhibit a very low thermal conductivity, as it is required for thermoelectric applications. May, on the other side - with skilful process management - the electrical conductivity remain constant, the total material efficiency will increase. Typically, crystallite sizes of less than 100 nm are required, while sizes below 50 nm promise for better properties. Preliminary experiments on the lab scale have shown that gas phase synthesis of doped silicon nanoparticles can be used to produce suitable material for the formation of nanocrystalline bulk samples with improved thermoelectric properties. We successfully performed the synthesis of p - and n- doped silicon nanoparticles. In the case of p-doping we decomposed silane (SiH4) and diborane (B2H6) within the plasma, while n-doping was performed using silane and phosphine (PH3).Within a first approach in the field of thermoelectric application, these p- and n-doped nanopowders were sintered under preservation of the nanostructure and the functionality was successfully demonstrated. By nanostructuring, the thermal conductivity was reduced from 150 W/(mK) for single-crystal silicon to preliminary 9.5 W/(mK) for the nanocrystalline samples. For further investigations of the thermoelectric properties, sufficient quantities of silicon nanomaterials from the gas phase are required. Up to now, nano materials with a small size and size distribution are typically available in minute quantities that are not suitable for the investigation of subsequent processing steps. Based on the lab scale experience we assembled a reactor on a pilot plant scale to synthesize nanocrystalline p- and n- doped silicon in technically relevant quantities (production rates on a scale of up to one kilogram per hour) in "solar grade" quality. Due to the now available amounts of highly-specific nano-sized material further improvements of the thermoelectric properties can be performed and investigated in detail.
3:30 PM - DD9.4
Surface Roughness, Quantum Confinement and Band Structure in Si Nanomembranes.
Feng Chen 1 2 , Edwin Ramayya 1 , Chanan Euaruksakul 1 , Franz Himpsel 1 , George Celler 3 , Bingjun Ding 2 , Irena Knezevic 1 , Max Lagally 1
1 , University of Wisconsin-Madison, Madison, Wisconsin, United States, 2 , Xi'an Jiaotong University, Xi'an, Shannxi, China, 3 , Soitec USA, Peabody, Massachusetts, United States
Show AbstractWhen a crystalline solid has dimensions smaller than the electron de Broglie wavelength, the electronic band structure becomes discrete. These discrete levers induced by the quantum size effect (QSE) will increase the figure of merit. In silicon, which has anisotropic conduction band minima (Δ), this QSE will create two sets of subband ladders, Δ2 and Δ4. In order to attempt to observe these subband ladders, we prepared ultra-thin Si nanomembranes from silicon-on-insulator (SOI), with thicknesses from 1.3 nm to 17 nm, and used x-ray absorption spectroscopy (XAS) to measure changes in the conduction band structure of these nanomembranes due to quantum confinement, using the 2p-to-Δ absorption edge. The shift of the ground state, relative to bulk, is to higher energy, as expected. However, we do not observe the higher-order subbands that should appear when the slab is thin enough (<3nm). We measured, using AFM over 5x5μm regions, the roughness of the front and back surfaces of membranes and compared to bulk Si. Because of the nature of the SOI fabrication process, the back and front surface roughnesses are not correlated. We determined that even very small rms surface roughness smears the nominally steplike features in the 2D density of states. The smearing increases as the roughness increases, and the second subband peak is totally washed out when the rms roughness reaches 0.4nm (the experimental value in our samples). The lowest-energy peak in the XAS spectra is thus made up of the optical transitions to the ground states of the Δ2 and Δ4 ladders. We obtain the splitting of these states as well as the shift of their average with quantum confinement and provide an excellent first-principles theoretical fit. We extend the calculation to the 1D density of states of Si, to demonstrate the influence of roughness appropriate for Si nanowires. Even though this DOS contains sharp peaks, moderate roughness (equivalent to one monolayer in Si) significantly washes out these peaks to a degree that will make access to all but the ground state meaningless. [unpublished] Thus earlier literature on the impact of the QSE in Si as regards the observability of sharp peaks in the DOS gives an entirely incorrect impression for “real” materials. [Research supported by DOE, AFOSR, and NSF]
3:45 PM - DD9.5
Phonon Engineering at the Nanoscale Makes Silicon a Better Thermoelectric Material.
Jen-Kan Yu 1 , Slobodan Mitrovic 1 , Douglas Tham 1 , Joseph Varghese 2 , James Heath 1
1 Chemistry, California Institute of Technology, Pasadena, California, United States, 2 Chemical Engineering, California Institute of Technology, Pasadena, California, United States
Show AbstractBulk silicon has long been considered a poor thermoelectric (TE) material with the thermoelectric figure-of-merit as low as ZT=0.01. Nevertheless, it has been demonstrated recently that silicon nanowire boasts a ZT=1, comparable to commercially available Bi2Te3 TE devices. The enhancement mainly comes from the nearly two orders of magnitude reduction in thermal conductivity, possibly due to the dimensionality crossover of the phonon modes[1] and/or the increased phonon-boundary scattering[2]. Here, we study the thermoelectric properties of a novel hole-bar-like silicon thin film (thickness ~ 25 nm). The hole-bar structure consists of a 2-D periodic array of 14 nm wide holes at a pitch of 34 nm, made by Superlattice Nanowire Pattern Transfer (SNAP) technique[3]. This phononic-crystal material exhibits thermal conductivity as low as 2 W/m-K at 300K, which is 3.5 times smaller than that of nanowires with similar dimensions (k~7W/m-K, wire diameter = 22 nm)[4]. We attribute this reduction of thermal conductivity to the modification of phonon energy bands by the periodic hole-bar structure. This is the first such demonstration of the TE enhancement mediated by a nanoscale phonon crystal[5].References:1.Boukai, A.I. et al. Silicon nanowires as efficient thermoelectric materials. Nature 451, 168-171 (2008).2.Hochbaum, A.I. et al. Enhanced thermoelectric performance of rough silicon nanowires. Nature 451, 163-167 (2008).3.Heath, J.R. Superlattice Nanowire Pattern Transfer (SNAP). Accounts of Chemical Research 41, 1609-1617 (2008).4.Li, D. et al. Thermal conductivity of individual silicon nanowires. Appl. Phys. Lett. 83, 2934-2936 (2003).5.Maldovan, M. & Thomas, E.L. Simultaneous localization of photons and phonons in two-dimensional periodic structures. Appl. Phys. Lett. 88, 251907-3 (2006).
4:00 PM - DD9.6
The Optimal Seebeck Coefficients and Minimal Length Scales, for Obtaining the Maximum Power Factor in Silicon Based Bulk and Nanostructured Thermoelectrics.
Paothep Pichanusakorn 1 , Prabhakar Bandaru 1
1 Materials Science Program, Mechanical Engineering department, UC, San Diego, La Jolla, California, United States
Show AbstractThe efficiency of thermoelectric device is measured by the Figure of merit,Z=(S2σ)/(ke+kL ), and is constituted from both electronic (the Seebeck coefficient: S, the electrical conductivity: σ, and the electronic part of the thermal conductivity: ke) and lattice properties (lattice thermal conductivity: kL) However, the electrical properties can vary by orders of magnitude and depend mainly on the carrier concentration. We show, through extensive analytical work and numerical simulations that there exists an optimum reduced Fermi level (ηopt), where the power factor: S2σ is maximized, and which is largely independent of material, device geometry, and temperature. Consequently, given the well-determined relationship between S and η, we will show that there exists an optimal Seebeck coefficient, Sopt, which can now be set as a practical and direct measure for finding the optimal carrier concentration in any material at any temperature. The relationship between the electrical properties and the electronic band structure will be intuitively explained through the Boltzmann transport equation.We also consider the influence of the characteristic scattering exponent (r) in the range of -0.5 to +1.5. For example, given a constant relaxation time (r=0), Sopt in bulk material, quantum well, and quantum wire were calculated to be approximately 130, 167 and 186 µV/K, respectively. However, if acoustic phonon scattering is dominant, then Sopt is always equal to 167 µV/K. Given the optimum Fermi level and Seebeck coefficient, we then determine the minimum quantum well/wire thickness required to achieve an enhancement in S2n over bulk values. We then show, through a critical comparison of the electron density distribution in the bulk and nanostructured forms for a variety of thermoelectrics, e.g., Bi2Te3, PbTe, SrTiO3, Si, and Si1-xGex, etc. that there exists an optimal length scale only below which the S2σ of nanostructures is enhanced over the bulk value. We calculate that the minimum required quantum well and wire thickness for n-Si are both approximately 6.5 nm. We will also discuss the issue of breaking of valley degeneracy in nanostructures, which can reduce the power factor and cause major deviation from the ideal value.It is then concluded that it is the increase in the magnitude of the integrated density of states (DOS) and not the change of shape, as is commonly believed, to be most responsible for the increase of the power factor. Our results lay the foundation for future research into the synthesis and characterization of nanostructured thermoelectric materials
4:30 PM - DD9.7
Holey Silicon Film as Efficient Thermoelectric Material.
Hung-Ta Wang 1 , Jinyao Tang 1 2 , Peidong Yang 1 2
1 Department of Chemistry, University of California, Berkeley, California, United States, 2 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractConverting thermal energy to electricity directly by solid state thermoelectric device is an attractive area since this device has great reliability and is suitable to operate at wide range of temperature and power density. These virtues of thermoelectrics make it perfect solution for harvesting waste heat. However, the current commercial thermoelectric materials are primarily based on heavy metal such as Pb, Bi, Te, which are not only environmentally hazardous but also expensive and difficult to process. Furthermore, after five decades of research, the performance of thermoelectric device is still only about 10% of Carnot efficiency and there is no clear route towards figure of merit (ZT=S2σT/k) from 1 to 3 found. As the most widely used semiconductor, Silicon has never been considered as good candidate for thermoelectric material due to its high thermal conductivity (140 W/K.m @ room temperature). Recent researches show that nanostructuring enhances the phonon-surface scattering thus lowers the thermal conductivity of rough silicon nanowires that approach the amphorous limit and dramatically enhance the ZT by almost 100 times. In this work, we extend our investigation to thin silicon film. By incorporating holes with diameter around 100nm inside the silicon film, suppression of thermal conductivity is observed. In our research, holey structured silicon films are created by nanosphere lithography. Holey silicon ribbons is formed which duplicate the shape of assembled nanobeads. These ribbons can be lifted-off and transferred to thermal measurement devices, or directly incorporated into thermal devices. The reduction of thermal conductivity measured is comparable to the best silicon nanowires in the previous research. In this presentation, we will discuss in detail about both thermal and electrical measurements. This work illustrates the importance of the phonon scattering center arrangement in thermal transport, and provides an guideline to better thermoelectric material design.
4:45 PM - DD9.8
The Effect of Cross-sectional Geometry on ZT Enhancement in Rough Silicon Nanostructures.
Jyothi Swaroop Sadhu 1 , Marc Ghossoub 1 , Sanjiv Sinha 1
1 Mechnical Science and Engg, University of Illinois Urbana Champaign, Urbana, Illinois, United States
Show AbstractThe possibility of using silicon as a thermoelectric material for waste heat recovery is technologically significant due to silicon’s economy of scale and vast processing knowhow. While the figure-of-merit ZT is too low for bulk silicon to be used as an effective thermoelectric material, it was shown that ZT is enhanced hundredfold in silicon nanowires of diameter less than 100 nm compared due to the decrease in the thermal conductivity [1]. It has been further shown that silicon nanowires with rough walls exhibit an even higher enhancement in ZT [2]. However, the mechanism for this enhancement remains unclear. We have previously shown that surface roughness leads to a conversion of propagating acoustic phonon modes to evanescent modes thus localizing phonons with localization lengths on the scale of a few micrometers [3]. This arises from multiple scattering and is distinct from the typical boundary scattering under the Fuchs-Sondheimer model [4]. Here we extend our work to investigate the effect of size and surface roughness for nanostructures of different cross-sections such as cylindrical wires, square wires and tubes. We estimate the thermal conductivities for these different cross sections using a multiple scattering theory for phonons involving many-particle effects. This work provides new insight into developing high ZT nanostructured Si.[1] A. I. Boukai et al, Nature Lett. 451, 168-171 (2008).[2] A. I. Hochbaum et al, Nature Lett. 451, 163-167 (2008)[3] S. Sinha, B. Budhaev, and A. Majumdar, Mater. Res. Soc. Symp. Proc. 1166, (2009) [4] E. H. Sondheimer, Advances in Phys. 1, 1 (1952)
5:00 PM - DD9.9
Size-dependent Thermal Conductivity in Individual Single Crystalline PbTe Nanowires.
Jong Wook Roh 1 , Seunghyun Lee 1 , So Young Jang 3 , Joohoon Kang 1 , Jin Seo Noh 2 , Jeunghee Park 3 , Wooyoung Lee 1 2
1 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 3 Department of Chemistry, Korea University, Seoul Korea (the Republic of), 2 Institute of Nano-science and Nano-technology, Yonsei University, Seoul Korea (the Republic of)
Show AbstractOver the past decade, thermoelectric devices have been of great interest because thermoelectric phenomena are expected to play an important role in meeting the energy challenge of the future. However, the thermoelectric figure of merit (ZT=S2σ/κ), which decide the performance of thermoelectric devices, has shown a limited value because the three parameter S, σ and κ for bulk materials are inter-related. With the development of nano-technology, ZT can be enhanced by classical size effect and quantum confinement effects, which enable to control S, σ and κ independently. In particular, PbTe has been focused on due to its interesting thermoelectric properties. In this work, we have investigated the thermal conductivity of individual PbTe nanowires. The single-crystalline PbTe nanowires were grown using chemical vapor transport method. The PbTe nanowires grown by this method were observed to be several hundred micrometers in length and a few tens of nanometers in diameter. A high-resolution transmission electron microscopy (HR-TEM) revealed that the PbTe nanowires are high quality single crystalline. We have used a suspended microdevice in order to measure the thermal conductivity of individual PbTe nanowires. The suspended microdevice was designed to prevent heat conduction through the substrate. As a result, a certain amount of heat generated by heat membrane is conducted to the sensing membrane only through the PbTe nanowire. The whole measurement was carried out in a cryostat under a vacuum pressure below 1 ×10-6 Torr. The thermal conductivity of an individual single crystalline PbTe nanowires with various diameter(d). The thermal conductivity(κ) of PbTe nanowires with d=182 nm is about 1.3 W/m.K at 300 K. which is half of that of bulk PbTe (κ for bulk PbTe ≈ 2.6 W/m.K at 300 K). This result clearly indicates that heat transport is prohibited in PbTe nanowires by the strong phonon boundary scattering. It was also found that the thermal conductivity is linearly decreased as the diameter of a PbTe nanowire decreases.
5:15 PM - DD9.10
Direct Growth of Semiconductor Nanowires by On-film Formation of Nanowire for High-efficiency Thermoelectric Devices: Bismuth Telluride.
Jinhee Ham 1 , Wooyoung Shim 1 3 , Do Hyun Kim 2 , Seunghyun Lee 1 , Jongwook Roh 1 , Kyu Hwan Oh 2 , Peter W. Voorhees 3 , Wooyoung Lee 1
1 Department of Materials Science and Engineering, Yonsei University, Seoul Korea (the Republic of), 3 Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Department of Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractHigh efficient thermoelectricity requires materials with a large figure of merit, ZT, defined as ZT = σS2T/κ, where σ is the electrical conductivity, S the thermoelectric power, and κ the thermal conductivity. However, due to the interdependence of σ, S, and κ, the optimization of thermoelectric properties remains challenging. It is well known that there are two approaches to enhanced ZT values. One is to utilize quantum confinement effects of nanostructures, providing an opportunity to individually control σ and S, and thus to promote ZT because of the increase of the density of states (DOS). The other is to reduce κ without having an effect on σ and S by using semiconductors of high atomic weight such as bismuth telluride (Bi2Te3) with much lower atomic vibration frequencies.In this work, we present a novel method to grow high-quality, single-crystalline Bi2Te3 nanowires, which can be used in thermoelectric devices with high thermoelectric figure-of-merit ZT values.BiTe thin films were grown on an oxidized Si substrate using a co-sputtering system with a Bi (99.999%) and a Te target (99.99%). For the growth of Bi2Te3 nanowires, the co-sputtered films were transferred to a furnace for heat treatment in the temperature 350 °C.Interestingly, uniform and straight Bi2Te3 nanowires with high aspect ratios were found to grow on the surface of the co-sputtered films after heat treatment. The growth of the Bi2Te3 nanowires is attributable to the relaxation of stress, originating from a thermal expansion mismatch between the film and the substrate. This mismatch is due to the large difference in the coefficient of thermal expansion of BiTe (~13 × 10-6/°C), SiO2 (0.5 × 10-6/°C) and Si (2.4 × 10-6/°C). It was found that grain boundary diffusion in the presence of stress gradients is responsible for the growth of Bi2Te3 nanowires. A novel growth method for Bi2Te3 nanowires is termed the on-film formation of nanowires (OFF-ON), as based on the observation of the spontaneous growth of Bi2Te3 nanowires from BiTe thin films without the use of conventional templates, catalysts, or starting materials. A HRTEM study reveals that the Bi2Te3 nanowire with d = 100 nm grown along the <001> direction is high-quality single crystalline. The diffraction pattern recorded perpendicular to the long axis of the nanowire can be indexed to the hexagonal lattice of Bi2Te3 with [001] zone axis. Four-terminal devices based on individual 77-nm-diameter Bi2Te3 nanowires were found to exhibit the lowest resistivity, which is indicative of high-quality single crystalline nanowires grown by the proposed method.Our results demonstrate that single-crystalline Bi2Te3 nanowires can be grown by the stress-induced method, providing a motivation for exploring the high-efficiency thermoelectric properties of single-crystalline Bi2Te3 nanowires. Further details on the mechanism of the single-crystalline Bi2Te3 nanowires will be presented.
5:30 PM - DD9.11
Synthesis and Thermoelectric Properties of MnSi1.75 Nanowires – Nanostructured Complex Crystals as Thermoelectric Materials.
Jeremy Higgins 1 , Arden Moore 2 , Michael Pettes 2 , Li Shi 2 3 , Song Jin 1
1 Department of Chemistry, University of Wisconsin - Madison, Madison, Wisconsin, United States, 2 Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas, United States, 3 Materials Science and Engineering Program, Texas Materials Institute, The University of Texas at Austin, Austin, Texas, United States
Show AbstractWe report the chemical synthesis, structural characterization, and thermoelectric property investigation of nanowires (NWs) and nanoribbons (NRs) of semiconducting higher manganese silicide (HMS), MnSi1.75, with the Nowotny chimney ladder structure. “MnSi1.75” is a homologous family of robust thermoelectric semiconductors with high Seebeck coefficient and relatively high figure of merit (ZT), up to 0.8 near 800 K in bulk crystals. This unusual family of complex tetragonal structures has reported c lattice parameters ranging from 1.75 nm to greater than 10 nm. Free-standing 1-D HMS nanostructures were synthesized for the first time using chemical vapor deposition (CVD) of a single source precursor. Their morphology and microstructures were characterized using scanning electron microscopy and transmission electron microscopy, while the composition and phase were determined using energy dispersive x-ray spectroscopy and electron diffraction. The thermoelectric properties of these 1-D nanostructures were characterized using suspended microdevices, allowing transport and structural characterization on the same nanostructure. Our measurements showed that the room-temperature thermal conductivity is suppressed from the already low bulk HMS value of 2-4 W/m-K even lower to 1 W/m-K in HMS NWs and NRs, approaching the amorphous limit. We attribute this phonon-glass behavior to the combination of a complex crystal structure and nanoscale effects. The large lattice parameter along the c axis results in a small Brillouin zone along the corresponding direction giving rise to numerous optical phonon modes that conduct heat poorly. This feature provides an explanation for the already low bulk thermal conductivity. In HMS NWs and NRs, surface roughness scattering reduces the mean free path of the small fraction of acoustic phonon modes further reducing the thermal conductivity.
5:45 PM - DD9.12
Thermoelectric Properties of Si and Si-Ge Nanopillars Fabricated by Metal-assisted Etching.
Nadine Geyer 2 1 , Katrin Bertram 1 , Peter Werner 2 , Bodo Fuhrmann 1 , Hartmut Leipner 1
2 , Max Planck Institut of Microstructure Physisc, Halle Germany, 1 Center of Materials Science, Martin Luther University, Halle Germany
Show AbstractThe implementation of nanotechnology concepts opens up a road for traditional silicon-based materials in thermoelectric applications. In a top-down approach, we fabricate Si and Si-Ge nanopillars by metal-assisted wet-chemical etching. The structuring is based on the pattering of the substrate with silver through colloidal lithography. With this method, arrays of nanopillars with hexagonal symmetry and adjustable diameters ranging from about 10 nm to several micrometers can be fabricated. High area densities of 10^10 cm^-2 and the control of diameter, length, and position of the pillars are possible. The morphology, the internal structure, and the chemical composition are investigated by scanning and transmission electron microscopy, as well as energy-dispersive X-ray analysis. Results of first measurements of thermoelectric transport properties, such as the Seebeck coefficient, the thermal conductivity, and the resistivity of arrays of nanopillars are presented.
DD10: Poster Session II
Session Chairs
Friday AM, April 09, 2010
Salon Level (Marriott)
9:00 PM - DD10.1
Electronic Structure and Transport Coefficients of the Thermoelectric Materials Bi2Te3 from First-principles Calculations.
Ziyu Wang 1 2 , Wei Wei 2
1 School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, United States, 2 School of physics, Wuhan University, Wuhan, Hubei, China
Show Abstract High-performance thermoelectric materials greatly affect to our society because of their applications in cooling and power generation. The efficiency of a thermoelectric device depends on its geometry and on the product of the thermoelectric figure of merit ZT = S2σT/(κe+κl), where S is the Seebeck coefficient,σ is the electrical conductivity, T is the temperature,κe and κl are the electric and lattice thermal conductivities. A higher value of ZT indicates a better thermoelectric performance for the materials. Increasing electrical conductivity or decreasing thermal conductivity are constructive in enhancing the thermoelectric performance of materials. Recently, the series of Bi2Te3 alloys have been intensively investigated because of their extremely high figure of merit,ZT. It was experimentally found that a ZT value of 1.35 can be reached for bulk Bi2Te3 at room-temperature (300 K) , which is the best ZT value of bulk materials in the room temperature range. The commercial alloys also have been prepared primarily by a traditional zone melting method and have obtained the highest dimensionless figure of merit ZT close to 1.0 over the temperature range of 200-400 K. In order to better understand how the thermoelectric materials work, many theoretical works had been done. Using pseudopotential and Korringa–Kohn–Rostoker (KKR) method, the energy band structure of bulk Bi2Te3 was calculated and an energy gap of approximately 0.11 eV was found. Scheidemantel et al. and Wang et al. also calculated the electronic structures of Bi2Te3 using the first-principles full potential linearized augmented plane-wave method. Both the theoretical calculations and experimental works are focused on enhancing the electronic properties of this compound and reducing the thermal conductivity of Bi2Te3 by using size effect. However, the doping effects few works attention in this kind of material. In this work, we calculated the electronic structure of bulk Bi2Te3 crystals usingfirst-principles method. The transport coefficients are then calculated within the Boltzmann theory, and further evaluated as a function of chemical potential assuming a rigid band picture. From this analysis, insight can be obtained into how doping can make a material exhibit high ZT value; we can find the right doping concentration to produce more efficient materials.
9:00 PM - DD10.10
Thermoelectric Properties of TiS2 Prepared by Sulfurization of TiO2 With CS2 Gas.
Shuhei Satoh 1 , Toshihiro Kuzuya 1 , Michihiro Ohta 2 , Shinji Hirai 1
1 Material Science and Engineering, Muroran Institute of Technology, Muroran, Hokkaido, Japan, 2 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, Japan
Show AbstractIt has been reported that (SnS)1.2(TiS2)2, where SnS layers are intercalated in the layered structure of TiS2, is synthesized by the direct reaction of Sn, S, and Ti followed by sintering. The ZT of this sintered material is enhanced to 0.37 because of the reduction of thermal conductivity by the layered structure (natural superlattice). It is also known that TiS2 tends to be deficient in the S content at high temperatures. In the present work, sintered compacts with a composition close to stoichiometry was fabricated by pulse electric current sintering followed by CS2 sulfurization using TiO2 powder (mean grain size: 100 nm) as a starting material and its ZT was measured. Pulse electric current sintering was carried out at 973 K for 3.6 ks with 50 MPa pressure under vacuum. In case the sample was deficient in sulfur content, sulfur was replenished either by the vaporization of sulfur by heating sulfur powder in the CS2 gas flow during sulfurization or by the addition of sulfur powder during sintering. Thermoelectric power was determined from a slope obtained by the plot of the Seebeck voltage versus the temperature gradient. Thermal conductivity and electrical resistivity were measured by the laser-flash method and the four- probe DC technique, respectively. X-ray diffractometry (XRD) showed that the synthesis powder varied on the basis of sulfurization temperature. For a constant sulfurization time of 14.4 ks, the synthesis powder was a mixture of Ti1.08S2 and Ti3S4 at 1273 K, and a mixture contained Ti2S3 in addition to Ti1.08S2 and Ti3S4 at 1173 K. The synthesis powder was single phase Ti1.08S2 at a range of 1073~1123 K, and TiO2 and Ti1.08S2 were formed at a lower temperature of 973 K. In the case of sulfurization at 1073 K, the deficiency of sulfur was avoided by sulfurization with sulfur vapors. Consequently, the lattice parameter of the synthesis powder was close to that of TiS2. In the instance when the Ti1.08S2 single phase powder or the powder mixture of the material, which has a lattice constant close to that of TiS2 and sulfur (4 mass%), was sintered at 973 K, the lattice parameter of these sintered compact with additional sulfur was not much different from those of synthesis powders, and the relative density was greatly improved compared to the one without additional sulfur reaching almost the theoretical value. The thermoelectric properties of the sintered compacts that had the lattice parameter closest to that of TiS2 were measured. The sintered compact without additional sulfur showed ZT of 0.1 at 400 K and 0.23 at 673 K and the sintered compacts that were deficient in sulfur showed ZT lower than these values.
9:00 PM - DD10.11
Aluminium-doped Zinc Oxide Thin Films: Thermoelectric Properties and Characterization.
Nina Schaeuble 1 , Yaroslav Romanyuk 2 , Myriam Aguirre 1 , Anke Weidenkaff 1 , Andrey Shkabko 1 , Petr Tomes 1 , Sascha Populoh 1
1 Solid State Chemistry and Catalysis, Empa, Duebendorf Switzerland, 2 Thin Films and Photovoltaics, Empa, Duebendorf Switzerland
Show AbstractApart from a high Seebeck coefficient and a high electrical conductivity, a low thermal conductivity (κ) is inevitable to reach good ZT values for thermoelectric materials. In bulk samples of many materials, the high thermal conductivity is the limiting factor. One possibility to lower κ is to structure the material in nano-scale either by synthesizing nano-particles or by growing thin films. This leads to an increase in phonon scattering at the grain boundaries and the film layers [1].Contrarily to the conventional TE materials which need to be protected against oxidation at high temperature and which are toxic and expensive, oxide materials have a high stability in air beyond 500 °C and they are harmless.Aluminium-doped zinc oxide is a promising thermoelectric material due to its high electrical conductivity (ca. 300 S/cm) and a Seebeck coefficient between -150 and -200 μV/K (both from RT to 1000°C for bulk) [2]. In this work, thin films of aluminium-doped zinc oxide were grown by RF magnetron sputtering in an Ar/O2 plasma. Bulk polycrystalline material was synthesised by a soft chemistry method using citric acid and ethylene glycol as a network builder. The thermoelectric properties of the films are measured and compared to the results from bulk sample measurements. Microstructure and crystal structure of bulk and thin film samples are evaluated by scanning and transmission electron microscopy and by X-ray diffraction. The interrelation between microstructure and transport properties will be discussed.[1] Dresselhaus, S. et al., New directions for low-dimensional thermoelectric materials, Adv. Mater. 19, 1043 (2007)[2] Tsubota, T. et al., Thermoelectric properties of Al-doped ZnO as a promising oxide material for high-temperature thermoelectric conversion, J. Mater. Chem.7 (1), 85-90 (1997)
9:00 PM - DD10.12
Characterization and Thermoelectric Properties of the Type-I Clathrate Sr1Ba7AlxSi46-x (14< x <15) Prepared by Aluminum Flux.
John Roudebush 1 , Eric Toberer 2 , Jeff Snyder 2 , Susan Kauzlarich 1
1 Chemistry, The University of California, Davis, Davis, California, United States, 2 Materials Science, California Institute of Technology, Pasadena, California, United States
Show AbstractThermoelectric materials have the potential to increase energy efficiency through waste heat recovery or co-generation across a diverse range of applications where heat is lost. Aluminum and silicon are relatively inexpensive, light, abundant and their alloys exhibit high melting points (> 1000 °C), rendering semiconductors based on these materials attractive for high temperature and transportation applications.In this study we conduct a detailed compositional analysis of the aluminum – silicon based type-I clathrate Sr1Ba7AlxSi46-x (14< x <15), prepared by aluminum flux. Microprobe analysis of the sample before and after hot pressing reveals that a solid solution with varying Sr content is formed. Using the microprobe data, nominal compositions were calculated for each data point using both a cation and framework vacancy model. The resulting compositions suggest fully occupied cation sites and vacancies in the Al-Si framework. Single-crystal x-ray diffraction has also been employed as well and results are discussed in light of the microprobe findings. Thermoelectric properties have been measured on a pressed pellet of the crystals grown by flux. As also found in other Al-Si based clathrates, the Seebeck coefficient is negative, indicating n-type conductivity and the resistivity of the materials increases with temperature, characteristic of a poor metal.
9:00 PM - DD10.13
Grain Size Reduction in Thermoelectric Rare Earth Sesquisulfides via Phase Transformations.
Toshihiro Kuzuya 1 , Hideto Sasaki 1 , Michihiro Ohta 2 , Shinji Hirai 1 , Toshiyuki Nishimura 3
1 Material Science and Engineering, Muroran Institute of Technology, Muroran, Hokkaido, Japan, 2 Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba , Ibaraki, Japan, 3 Advanced Materials Labiratory, National Insitute for Materials Science, Tsukuba , Ibaraki, Japan
Show AbstractCubic Th3P4-type γ-NdGdS3 exhibits its highest ZT of 0.51 at 950 K in the NdGd1.02S3 composition, in which the compound has an optimal carrier concentration. A further increase in ZT is expected from the grain size reduction of sintered γ-NdGd1.02S3 due to the scattering of phonons at grain boundaries. This work proposes a method for reducing the grain size of sintered γ-LnLn'S3 compounds via phase transformation. Ln2S3 (Ln: La, Ce, Pr, Nd, Sm) transforms from the α phase (orthorhombic) to the γphase (cubic; Th3P4 type) via theβphase (tetrahedral). In contrast, Ln'2S3 (Ln': Gd, Tb) directly transforms from the α phase to the γphase. It has been found that γ-LnLn'S3 with high oxygen content transforms from the α phase to the γ phase with fine grains via the βphase, which is supposed to be Ln5Ln'5S14O, during the sintering process. In previous studies, γ-NdGd1.02S3 has been synthesized from commercial Nd2O3 and Gd2O3 by sulfurization using CS2 gas. In contrast, in the present study, LnLn'S3 powder was prepared by the sulfurization of an oxycarbonate containing Ln and Ln', which was previously synthesized by a complex polymerization method, at 1273 K for 28.8 ks under CS2 gas. After sulfurization, the powder was annealed under vacuum with an oxygen partial pressure less than 0.25 × 10-3 Pa at 1723-1773 K for 21.6-43.2 ks. The annealed powder was consolidated at 1723 K for 3.6 ks under vacuum by pulse electric current sintering at a pressure of 50 MPa. X-ray diffractometry (XRD) revealed that after sulfurization, the powder was in the α phase. On the other hand, after annealing, the powder was in a single γ phase. In addition, the carbon and oxygen contents were observed to decrease and increase, respectively, with increasing annealing time. When the oxygen content was large, the linear shrinkage curve indicated the stagnation of the shrinkage during the sintering process. XRD measurements of the sintered samples before and after the stagnation of the shrinkage revealed the existence of the α phase before stagnation and both the α and β phases after the stagnation. In the samples with a single γphase that underwent such stagnation of shrinkage during the sintering process, a reduction in the grain size was observed. In contrast, the stagnation of the shrinkage was not observed during the sintering of the α phase powder (which contains less oxygen) without annealing after sulfurization, and hence the grain size was not reduced even if the powder consisted of the γ phase after sintering. In particular, in the cases of γ-NdGdS3 and γ-PrGdS3, the grain size was reduced significantly. While the respective grain sizes of γ -NdGdS3 and γ -PrGdS3 samples sintered without annealing were approximately 40 and 50μm, they were approximately 5 and 6μm in the samples sintered after annealing.
9:00 PM - DD10.14
Study of Nanostructure Inclusions on the Thermoelectric Behavior of Ca3Co4O9 Thin Films Grown by Pulsed Laser Deposition.
Evan Thomas 1 , Yonggao (Jack) Yan 2 , Xueyan Song 3 , Joshua Martin 2 , Margaret Ratcliff 4 , Winnie Wong-Ng 2 , Paul Barnes 4
1 Metals and Ceramics Division, University of Dayton Research Institute-Air Force Research Laboratory, Dayton, Ohio, United States, 2 Materials Science and Engineering Laboratory, Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Department of Mechanical and Aerospace Engineering, West Virginia University, Morgantown, West Virginia, United States, 4 Propulsion Directorate-RZPG, Air Force Research Laboratory, Wright Patterson Air Force Base, Ohio, United States
Show AbstractThe misfit cobaltite Ca3Co4O9 (CCO) is considered a good thermoelectric (TE) material with its enhanced Seebeck coefficient, low electrical resistivity and high thermal stability in air. Nanostructure approaches to forming nanocomposites within bulk materials have effectively shown enhanced TE behavior, as the small dimensions of the nanostructures results in boundary scattering of phonons, thus effectively reducing thermal conductivity. In this study, high quality c-axis oriented thin films have been grown on Si (100), TiO2 (100), LaAlO3 (100), and Al2O3 (0001) substrates by means of pulsed laser deposition, using sector targets consisting of compatible oxides to induce the self-assembly of nanoinclusions in a CCO matrix. Detailed structural analyses and observations of the nanoinclusions formed as a result of lattice mismatching, have been performed using scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, transmission electron microscopy, and X-ray diffraction. Thermoelectric property behavior was determined using a custom-built screening device at room temperature, a Quantum Design Physical Property Measurement System (T = 10 – 390 K), and a thermal effusivity measurement system. The effects of controlled second-phase nanostructural additions on the thermoelectric behavior of Ca3Co4O9 thin films are compared and presented.
9:00 PM - DD10.15
Thermal Conductivities of SixGe1-x Nanowires for Thermoelectric Application.
Hyoungjoon Kim 1 , Ilsoo Kim 2 , Heon-jin Choi 2 , Woochul Kim 1
1 School of Mechanical Engineering, Yonsei University, Seoul Korea (the Republic of), 2 School of Advanced Material Engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractSilicon germanium (SixGe1-x) has been known as a good thermoelectric material for high temperature application. The thermoelectric effect is the direct conversion of temperature difference into electrical energy or vice versa. There exist thermoelectric figure of merit as indicator for conversion efficiency. The thermoelectric figure of merit (ZT) is defined as S^2σT/κ, where S, σ, T and κ are Seebeck coefficient, electrical conductivity, absolute temperature and thermal conductivity. If material possesses high ZT, a device made out of it should have high conversion efficiency. Many research efforts are being focused on improving the ZT of SixGe1-x. One of them is increasing ZT by reducing thermal conductivity of SixGe1-x. Li et al. (Appl. Phys. Lett. 84, 2934 (2003)) shows that Si nanowires have lower thermal conductivity than that of bulk Si because of the phonon boundary scattering. We applied this concept to the SixGe1-x material. We measured thermal conductivities of Si0.9Ge0.1 nanowires with different diameters in the temperature range of 40-400K. Result suggests that thermal conductivity reduced as diameter did. Also, thermal conductivity of Si0.9Ge0.1 nanowire is lower thermal conductivity than that of its thin film and bulk counterpart. This result shows the possibility of increasing ZT out of SixGe1-x nanowires.
9:00 PM - DD10.16
Microthermocouples of Glass-coated Bifilar Microwires Based on Bi2Te3.
Dragosh Meglei 1 , Marya Dyntu 1
1 Academy Science, Institute of Electronic Engineering and Industrial Technologies, Chisinau Moldova (the Republic of)
Show AbstractThe paper presents the results of researches related to the technology of obtaining of glass-insulated bifilar microwires based on Bi2Te3 of the n- and p-type conductivity, the study of their mechanical properties by the strain method, the microscopic analysis of the morphology of structural defects and thermoelectric characteristics with a view to prepare microthermocouples on the basis of these microwires.Mechanical properties were determined by the methods of bending and rupture strain. The bending strength as a measure of elasticity was determined by the critical bending radius of samples. It is found that the bifilar microwires are more flexible in comparison with single ones with the same external diameters. For example, a bifilar microwire with an external diameter of 86 µm has a critical radius of 0.565 mm; for an n-type microwire with the same diameter is 0.654 mm.Microscopic studies of rupture faces of the bifilar microwires showed that the rupture is brittle; however, in samples with small diameters, the rupture occurs along the cleavage plane and the cleavages have smooth surfaces; the cleavage of thick samples is accompanied by the formation of steps and twins. The microscopic study of transverse and longitudinal sections of the samples obtained by mechanical and chemical polishing showed that bifilar microwires of Bi2Te3 are disoriented single-crystal blocks; their typical structural defects are dislocations, microcracks, and twin. The dislocation density ranges within 105-107 cm-2. It is also observed that the tendency to twinning weakens with decreasing diameter of bifilar microwires both of n- and p-type conductivity; at the same time, their homogeneity increases. Thus, as a result of study of mechanical properties of the bifilar microwires, it is found that, despite the relatively large external diameters, they are stronger and more flexible than single microwires of the same thermoelectric material Bi2Te3, which makes it possible to use the bifilar microwires for the production of thermoelectric microthermocouples.On the basis of the bifilar microwires, microthermocouples with the integral signal value of 9.8 mV at 36.6°C and 26.0 mV at 88°C are prepared.
9:00 PM - DD10.17
Thermoelectric Properties of Bulk In2Te3 and Nano Bi2Te3 Composite.
Heejin Kim 1 2 , Chanyoung Kang 3 , Mi-Kyung Han 1 , Woochul Kim 3 , Sung-Jin Kim 1
1 Department of Chemistry Nano Science, Ewha womans University, Seoul Korea (the Republic of), 2 Department of Metallurgical Engineering, Yonsei University, Seoul Korea (the Republic of), 3 School of mechanical engineering, Yonsei University, Seoul Korea (the Republic of)
Show AbstractThe ability of thermoelectric device is measured by figure of merit (ZT). ZT is given by ZT=S2σT /κ, where σ is the electrical conductivity, S the thermo power, and κ is the thermal conductivity. For several decades, the ZT of the thermoelectric materials have been less than 1. Current research results often are reported to be ZT>2 due to quantum confinement effect. We studied thermoelectric properties of In2Te3 prepared by insertion of Bi2Te3 nanoparticles, which were prepared by colloidal method.The thermal conductivity and the electrical conductivity decrease with increasing Bi2Te3 nanoparticles contents. The phonons and electrons would be scattered by Bi2Te3 nanoparticles depending on their contents and size. We observed that the contents and sizes of Bi2Te3 nanoparticles are critical factor for achieving an increase in ZT comparing to bulk In2Te3. An increase in ZT could be achieved for an optimal contents and sizes of Bi2Te3 nanoparticles.Seebeck coefficient and electrical conductivity have been measured by ZEM-3(Ulvac) and the thermoelectric figure of merit calculated.The microstructure and morphologies of In2Te3 composite imbedded by Bi2Te3 nanoparticles were investigated by Powder X-ray diffraction and HR-TEM.
9:00 PM - DD10.18
Correlation of Microstructural Properties With Thermoelectric Performance of Bi0.5Sb1.5Te3 Films Fabricated by Electroplating.
Elena Koukharenko 1 , Xiaohong Li 3 , Jekaterina Kuleshova 2 , Marcel Fowler 4 , Nicole Frety 5 , John Tudor 1 , Steve Beeby 1 , Iris Nandhakumar 2 , Neil White 1
1 ECS, Southampton University, Southampton, Hampshire, United Kingdom, 3 School of Engineering Science, Southampton University, Southampton United Kingdom, 2 School of Chemistry, Southampton University, Southampton United Kingdom, 4 School of Physics & Astronomy, Southampton University, Southampton United Kingdom, 5 LPMC, Universite Montpellier II, Montpellier France
Show AbstractBismuth telluride-based materials are well known for their thermoelectric applications at near room temperature (thermopiles, thermal sensors, laser diodes, power generation and refrigeration devices). They are commonly obtained by using inorganic materials fabrication synthesis, which have the major drawback of chemical homogeneity. This is an important point as thermoelectric properties are greatly correlated with chemical homogeneity of the materials. Electroplating process provides an attractive low cost, room temperature route to the fabrication of high homogeneity films of Bi2-xSbxTe3. Although Bi2-xSbxTe3 alloys properties have been the subject of many research papers, there was very limited information regarding to a correlation of microstructure with physical properties. This paper presents chemical composition and its homogeneity and their influence on thermoelectric properties performance for p-type Bi0.5Sb1.5Te3 electroplated films (10-15 μm thick).Two sets of electrodeposited samples have been fabricated and characterised: the first set of samples, made using a conventional electroplating recipe, was deposited under potentiostatic control at room temperature in unstirred electrolyte solutions containing 0.001 M Bi3+, 0.01 M HTeO2+, 1 M HNO3, 0.02M Sb3+, 0.1 M H3Cit and 0.05 M Na3Cit; the second set was fabricated using an optimised electroplating recipe by adding lignin sulfonate as surfactant agent to improve microstructural electroplated film properties.The chemical composition and its homogeneity across the film surface of Bi0.5Sb1.5Te3 were determined using microprobe analysis (Cameca, Castaing) on raw electroplated film surface. Chemical composition for desired Bi0.5Sb1.5Te3 without surfactant resulted in: Bi content of 5.49 at %, Sb content of 27.99at% and Te content of 66.52 at% giving an important deviation from stoichiometry (Bi0.25Sb1.26Te3) and significant variations in compositions across the sampleS areas. Chemical composition for desired Bi0.5Sb1.5Te3 with surfactant resulted in Bi content of 6.8 at %, Sb content of 29at% and Te content of 65 at%, closer to stoichiometry (Bi0.32Sb1.33Te3) and with very homogeneous compositions across the samples areas.While thermoelectric properties of electrodeposited Bi0.5Sb1.5Te3 films obtained using the first recipe shown modest thermoelectric properties (Seebeck coefficient of 50-120 μm/K; Hall mobility of ~ 11 V-1s-1, carrier concentration (p) of 1018cm-3), the electrodeposited Bi0.5Sb1.5Te3 by the second recipe showed very promising physical properties results (the Seebeck coefficient was 220-250 μm/K ; Hall mobility ~30 V-1s-1, (p) of 1021cm-3). As a result, chemical homogeneity of electroplated Bi0.5Sb1.5Te3 is a crucial parameter for its thermoelectric performance.
9:00 PM - DD10.19
Thermal Conductivity in ZnSe Bulk Crystals Grown With Nano-ordered Rotational Twins.
Haruhiko Udono 1 , Takuro Yamashita 1 , Dongyan Xu 2 , Joseph Feser 2 , Arun Majumdar 2
1 Electrical and Electronic Engineering, Ibaraki University, Hitachi Japan, 2 Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractRecent development of nanostructured thermoelectric materials has realized to enhance the thermoelectric figure of merit ZT=(S2σ/κ) by mainly reducing their thermal conductivity, which is attributed to phonon scattering by their high density of interfaces. There are several experimental approaches to make nanostructured thermoelectric materials, such as superlattice films and quantum dot superlattice films, and more recently developed nanowires and nanocomposites. However, these approaches are still difficult to make large-sized bulk thermoelectric materials for the industrial use. Introducing of nano-ordered crystal defects, such as twins is alternative approach to make high density of interface for phonon scattering and to decrease the thermal conductivity of the bulk crystals. In this work, we have investigated the thermal conductivity of ZnSe bulk crystals which had nano-ordered twin-structure. Totally twinned ZnSe bulk crystals (φ10mm × ~50mm) were grown by the vertical Bridgman method using Zn-partial pressure. The type of twins was mainly 120° rotational twins along <111> direction, and the periods of twin boundaries determined by TEM observations were between 10nm and 60nm. The thermal conductivity was measured by 3ω method using Pt-heater/thermometer or steady state method between 40K and 320K. The measured thermal conductivity of twinned ZnSe bulk crystal (average period ~40nm) was lower than that of non-twinned ZnSe bulk crystal especially below 200K. The values were 19W/m-K at 300K and 215W/m-K at 40K for the twinned crystal and 19W/m-K at 300K and 309W/m-K at 40K for non-twinned crystal. From the analysis based on the Debye-Callaway model, we concluded that the reduction of the thermal conductivity would be mainly due to the boundary effect of twins.
9:00 PM - DD10.2
Seebeck Coefficient Measurement Using a Kelvin-probe Force Microscopy Technique.
Hiroya Ikeda 1 , Faiz Salleh 1
1 Research Institute of Electronics, Shizuoka University, Hamamatsu Japan
Show AbstractThe introduction of nanometer-scale structures into thermoelectric materials has been expected to lead to breakthroughs for enhancing the thermoelectric figure-of-merit. However, it is very difficult to measure the thermoelectric characteristics of these materials because of the very small dimensions. External disturbances such as lead-wire contact essential to the conventional thermoelectric-motive force (TEMF) measurement affect accurate evaluation. In order to measure the Seebeck coefficient of nanometer-scale thermoelectric materials, we propose a new technique in which the TEMF is evaluated by Kelvin-probe force microscopy (KFM). Using this technique, it is possible to obtain the work-function difference between the cantilever and the sample, that is, the surface potential of the sample. In other words, the Fermi energy of the sample relative to that of the cantilever metal can be evaluated on a nanometer scale. Therefore, the TEMF can be obtained from the Fermi energies at the high- and low-temperature regions on a sample. This allows for evaluation of the Seebeck coefficient of the sample. One of the crucial advantages to be emphasized of this technique is that the cantilever never touches the sample surface during the measurement. Therefore, the TEMF measurement is not perturbed by external factors such as the metallic probe and the lead wire. Another advantage is that we can use commercial KFM equipment by adjustments of the sample holder. In our present experiment, we evaluated the Seebeck coefficient of an n-type Si wafer with a P concentration of 1x10^{18} cm^{-3} by the KFM technique. Time evolution of the surface potential was measured by KFM equipment and monitored by a digital multimeter, simultaneous with temperature measurement at the high- and low-temperature regions on the Si surface. It was found that the surface-potential difference between the high- and low-temperature regions increased with increasing temperature difference. This indicates that the TEMF can be measured by KFM. The Seebeck coefficient evaluated from the surface-potential difference was 0.71±0.08 mV/K, which was close to that obtained by the conventional method. Consequently, the Seebeck coefficient can indeed be measured by KFM with no contact between the probe and the sample, leading to realization of accurate Seebeck-coefficient measurement for nanometer-scale materials.
9:00 PM - DD10.20
A Study on Self-assembled Nanostructure and Thermoelectric Properties in Bi2Te3-PbTe System.
Kyooho Jung 1 , JuHyuk Im 1 , KwangChon Kim 1 3 , HyunWoo You 1 2 , JinSang Kim 1
1 , Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 , Yonsei University, Seoul Korea (the Republic of), 2 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe microstructure and properties of alloys in the pseudo-binary Bi2Te3-PbTe system were investigated as a first step towards the design of nanostructured materials with enhanced thermoelectric properties. The liquid alloys were cooled by water quenching method. Dendritic and lamellar structures were observed clearly by using environmental scanning electron microscope(eSEM) and electron probe micro analyzer(EPMA) take into account composition ratio between Bi2Te3 and PbTe. The compound Pb2Bi6Te11 precipitated as a metastable phase under all conditions. The structure of those samples changed from dendritic to lamellar by increasing Bi2Te3 ratio of composition.
9:00 PM - DD10.21
Analysis of SiGe Thermoelectric Modules in Wood Stove Application.
Zhixi Bian 1 , Ali Shakouri 1
1 Electrical Engineering Department, University of California Santa Cruz, Santa Cruz, California, United States
Show AbstractWood stoves are popular for cooking in the rural areas of developing countries. These stoves are much more inefficient than the gas fired ones and produce a lot of Carbon monoxide and particle emissions which results in millions of deaths each year. Philips research has invented an efficient wood stove with forced air circulation which is powered by BiTe thermoelectric energy conversion modules. We present analysis of an alternative design using nontoxic Silicon Germanium thermoelectric modules. Taking into account the heat flux boundary conditions, the element geometry is optimized. We show that it is possible to improve the thermoelectric energy conversion efficiency significantly and generate several times larger electrical power for air circulation. This design also has the potential to reduce the cost and improve the reliability.
9:00 PM - DD10.22
Thermoelectric Properties of Sintered Mg2Si Fabricated Using Commercial Al-doped n-type Polycrystalline Sources.
Takahiro Miyata 1 , Tatuya Sakamoto 1 , Tsutomu Iida 1 , Shiro Sakuragi 2 , Yutaka Taguchi 2 , Yohiko Mito 3 , Hirohisa Taguchi 1 , Yoshifumi Takanashi 1
1 , Tokyo University of Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan Japan, 2 , Union Material Inc., 1640 Oshido-jyoudai, Tone-machi, Kitasouma, Ibarak Japan, 3 , Showa KDE Co.,Ltd., 5-17-14 Nishi-ikebukuro Toshima-ku, Tokyo 171-0021 Japan
Show AbstractMagnesium silicide (Mg2Si) has been identified as a promising thermoelectric material with a moderate thermal-to-electric energy-conversion performance operating in the temperature range from 500 to 900 K. Compared with other thermoelectric (TE) materials that operate in the same conversion temperature range, such as PbTe, TAGS (Ge-Te-Ag-Sb) and CoSb3, Mg2Si shows benign aspects, such as the abundance of its constituent elements in the earth’s crust and the non-toxicity of its processing by-products, resulting in freedom from care regarding prospective extended restriction on hazardous substances. One of the reasons why TE devices are not more widely used at present is that the cost-per-watt of TE power generation has been too high to allow it to displace existing technologies. Mg2Si has significant advantages for TE applications in terms of both lower raw materials costs due to the abundance of its constituent elements, and its lighter weight compared with conventional TE materials such as Bi2Te3 and CoSb3. It is necessary to dope the raw material in order to optimize the TE properties of Mg2Si for practical applications. Bismuth (Bi), antimony (Sb) and aluminum (Al) are well known as n-type dopants for Mg2Si. Sb is considered to be a dopant that substitutes more stably into Si-sites in Mg2Si compared with the other elements by a first principle calculation. We have been examined for Bi and Sb doping in Mg2Si to obtain both higher TE performance and sufficient stability at elevated temperature. In the past experiments, we succeeded to obtain the ZT value of 1.08 with Bi doping, while the diffusion of Bi atom degraded durability at expecting operation temperature. As for the Sb-doped specimens, practical thermal stability and comparable TE characteristics to Bi-doped ones were obtained, however, the Sb is concerned about the hazardous substance. We have now tried to examine Al-doped Mg2Si in order to obtain practical TE performance at elevated temperatures and to realize a completely toxicity-free Mg2Si TE device.The starting Mg2Si sources are pre-synthesized commercial poly-crystalline provided by Showa KDE CO., LTD. and the doping concentrations of Al were varied between 0.5, 1.0, 2.0, 3.0 and 5.6 at.%. The polycrystalline Mg2Si sources were pulverized and then sintered using a plasma activated sintering (PAS) technique using ELENIX CO., LTD Ed-PAS III. For the thermoelectric properties, the Seebeck coefficient and the electrical conductivity were evaluated using ULVAC-RIKO ZEM-2 equipment, while the thermal conductivity was evaluated by the laser flash method. In conjunction, we are also working on the formation of nickel (Ni) and tungsten (W) electrodes on Mg2Si by monobloc sintering in order to extract the generated output power. In this report, we will discuss the TE properties of these samples and will evaluate the output power and durability of the samples fabricated with electrodes at as high as 900 K operation.
9:00 PM - DD10.23
Effect of Vacancies on the Thermoelectric Properties of Mg2Si Containing Sb and Bi Substitution.
Titas Dasgupta 1 , C. Stiewe 1 , R. Hassdorf 1 , L. Boettcher 1 , E. Mueller 1
1 Institute of Materials Research, German Aerospace Center (DLR), Cologne Germany
Show AbstractMg2Si and its solid solutions are promising materials for thermoelectric power generation [1]. Recent reports on these materials show a thermoelectric figure of merit (ZT) greater than unity [2] which is comparable to the state of the art materials [3]. Efforts to improve ZT in this material system rely on successful reduction of the lattice thermal conductivity (κL). Recently, a report [4] on Sb substituted Mg2Si showed significant reduction of κL) (by a factor of six at room temperature) at high Sb concentrations. This was due to the formation of Mg vacancies at high Sb concentrations. Since vacancies can effectively scatter phonons, the effect of Sb and Bi substitutions on the vacancy formation and its subsequent effect on the high temperature thermoelectric properties were investigated. Compositions with initial stoichiometries of Mg2Si1-xSbx and Mg2Si1-xBix (0≤x≤0.1) were synthesised by induction melting followed by hot uniaxial pressing. The phase purity and compositional analysis have been carried out using X-ray diffraction and EDAX, respectively. The formation and effect of the Mg site vacancies on the transport and thermal properties have been measured from room temperature to 750 K and the suitability for thermogenerator application is discussed. [1] V.K. Zaitsev, M.I. Fedorov, I.S. Eremin and E.A. Gurieva, Chapter 29, Thermoelectric Handbook – Macro to Nano, 2006, Taylor and Francis Group.[2] V.K. Zaitsev, M.I. Fedorov, E.A. Gurieva, I.S. Eremin, P.P. Konstantinov, A.Yu. Samunin, and M.V. Vedernikov, Phys. Rev. B 74, p-045207, 2006.[3] G. Jeffrey Snyder and Eric S. Toberer, Nature Materials 7, p-105, 2008.[4] G.S. Nolas, D. Wang and M. Beekman, Phys. Rev. B 76, 235204, 2007.
9:00 PM - DD10.24
Thermal Conductances of Self-assembled Monolayers with Different Functional End Group.
Buyoung Jung 1 , Chanyoung Kang 1 , Yongjun Choi 1 , Joon-Sung Kim 2 , Sung-Yeon Jang 2 , Woochul Kim 1
1 School of Mechanical Engineering, Yonsei University , Seoul Korea (the Republic of), 2 Center for Energy Materials Research, Korea Institute of Science and Technology, Seoul Korea (the Republic of)
Show Abstract The molecular electronics, especially self-assembled monolayers(SAMs), has gained attention since the last decade because it was proposed as one of the future electronics. Also, it is predicted that molecules could be used as future thermoelectrics (J. Chem. Phys. 129, 044708 (2008)). Most of studies have been focused on its electronic properties beside how to synthesize it. Only a couple of papers dealt with its thermal properties. Robert Y. Wang et al.(Appl. Phys. Lett. 89, 173113 (2006)) found the independence of thermal conductance from its chain length and Zhaohui Wang et al.(Science 317, 787 (2007)) researched the ultrafast thermal conductance of molecular chains. We measured the thermal conductance of ODT(Octanedithiol, HS(CH2)8SH) and MPTMS((3-mercaptopropyl)trimethoxysilane, HS(CH2)3Si(OCH3)3) SAMs which have different functional groups. The ODT has thiol(-SH) functional groups at both ends and the MPTMS has silane(-Si(OCH3)3) group at one end and thiol group at the other end. The thermal conductances of those SAMs were measured by the 3ω method from 120K to 330K. The ODT SAMs were grown onto a GaAs substrate by the solution. Au patterns for thermal conductance measurement were evaporated on a Si substrate. The patterns were transferred to the ODT/GaAs by the nanotransfer printing. In this way, we made the Au/ODT/GaAs junction for thermal conductance measurement. Also the Au/MPTMS/Si junctions were made by similar method but MPTMS SAMs were formed on the Si substrate. The thermal conductances of Au/ODT/GaAs and Au/MPTMS/Si were 12~14 and 12~17 MW/m2-K at room temperature while those of Au/GaAs and Au/Si are 150 and 120 MW/m2-K. By including SAMs between solids, thermal conductance has been reduced by an order of magnitude. However, we did not observe differences in thermal conductance due to the differences in functional end groups.
9:00 PM - DD10.25
High Temperature (300K – 1000K) Thermoelectricity in TiNiSn and ZrNiSn Half Heusler Compounds.
Sascha Populoh 1 , Andrey Shkabko 1 , Myriam Aguirre 1 , Anke Weidenkaff 1 , Siham Ouardi 2 , Benjamin Balke 2 , Claudia Felser 2
1 Solid state chemistry and catalysis, EMPA, Duebendorf Switzerland, 2 Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität, Mainz Germany
Show AbstractThe half Heusler compounds as e.g. TiNiSn, ZrNiSn with zT ≈ 1.5 at 800K [1] are very promising candidates for high thermoelectric figures of merits. Half Heusler compounds are materials of the composition XYZ and crystallise in C1b structure. Many of them are semiconductors with a small band gap [2, 3] depending on the number of valence electrons, which is favourable for thermoelectric applications. As a model system they are especially suited since there are already more than 250 known compounds of this structure type and the electronic structure can be easily tuned to p- or n-type conductivity by appropriate substitution. Systematic investigations have been performed only on a few of these compounds. We will present a study of the electrical and thermal conductivity and the Seebeck coefficient in the high temperature region for several TiNiSn and ZrNiSn derived compounds doped with different amounts of Nb, Zr, Hf, Sb, and V. Transmission electron microscopy study will be used to check the interrelation between structure and microstructure and the transport properties.[1] S. Sakurada and N. Shutoh, Appl. Phys. Lett. 86 (2005) 2105[2] S. Ogut, K.M. Rabe, Phys. Rev. B 51 (1995) 10443[3] H. C. Kandpal, C. Felser and R. Seshadri, J. Phys. D: Appl. Phys. 39 (2006) 776
9:00 PM - DD10.26
Fabrication of Nanotubules of Thermoelectric γ-Na0.7CoO2 Using Porous Aluminum Oxide Membrane as Supporting Template.
Chia-Jyi Liu 1 , Shu-Yo Chen 1 , Long-Jiann Shih 1 , Hsueh-Jung Huang 1
1 Physics, National Changhua University of Education, Changhua Taiwan
Show AbstractWe report the successful synthesis of nanotubules of thermoelectric materials γ-NaxCoO2 using two different sol-gel routes aided by porous anodized aluminium oxide (AAO) membrane as supporting templates. The γ-NaxCoO2 nanotubule using urea-based route can be achieved at 650 degree C at a heating rate of 1 degree C/min and held for 4h. The γ-NaxCoO2 nanotubule using citric acid-based route can be achieved at 500 degree C using a rapid-heat-up procedure and held for 30 min. The products were investigated using various techniques including XRD, SEM and TEM. Electron diffraction pattern taken along [001] zone axis direction on the nanotubule shows that all the diffraction spots can be indexed using a hexagonal unit cell with a = b = 0.56 nm, which can be considered as a superstructure with cell doubling within the ab plane.
9:00 PM - DD10.33
Direct Conversion of Simulated Solar Radiation into Electrical Energy by Thermoelectric Oxide Modules (TOM) With Respect to an Efficient Heat Transfer.
Petr Tomes 1 , Matthias Trottmann 1 , Clemens Suter 2 , Anke Weidenkaff 1
1 Solid State Chemistry and Catalysis, Empa - Swiss Federal Laboratories for Materials Testing and Research, Duebendorf Switzerland, 2 Institute of Energy Technology, ETH, Zurich Switzerland
Show AbstractMaximum output power Pmax and conversion efficiency η of solar heat into electrical energy was measured on a series of a four – leg thermoelectric oxide modules (TOM) with leg length of 4, 5 and 10 mm. TOM were made by combining two p- (La1.98Sr0.02CuO4) and two n-type (CaMn0.98Nb0.02O3) [1] thermoelements connected electrically in series and thermally in parallel. The electrical contacts were made by metallisation technique with DuPontTM conductor paste and by brazing with 0.1 mm thick Ag sheet. The thermal contacts were provided by two Al2O3 substrates. Temperature gradient ΔT was generated by applying solar radiation (heat flux) from the High – Flux Solar Simulator source (HFSS) which generates radiation spectral distribution close to the sun radiation [2]. The influence of the coated graphite layer on the hot side of the Al2O3 substrate compared to the uncoated surface on ΔT, Pmax and η was studied. Temperature distributions in the TOM were measured by 6 attached 0.5 mm K - type thermocouples. The measurements show almost linear temperature profile along the thermoelectric legs. The highest maximum output power of 88.8 mW was reached for TOM with leg length of 5 mm at ΔT = 622 K. The highest η was found for the optimum heat flux between 4 – 8 W cm-2 and the dependence of η on the leg length was investigated. The experimental data were compared with the theoretical heat transfer model to examine the effect of the radiation and the geometrical parameters of the TOM on η.[1] L. Bocher et al., Inorg. Chem. 47 (18), 8077 (2008).[2] P. Tomeš et al., Materials Science and Technology Conference and Exhibition, MS&T08, Pittsburgh, USA, Vol. 1: 429 (2008).
9:00 PM - DD10.4
High-temperature Thermoelectric Properties of Ni-added Ca3Co4-xNixO9 Ceramics.
S. Nam 1 , H. Hwang 1 , Kyeongsoon Park 1
1 , Sejong University, Seoul Korea (the Republic of)
Show AbstractNano-sized Ca3Co4-xNixO9 (0≦x≦0.3) thermoelectric powders were synthesized by solution combustion method, using aspartic acid as fuel. The synthesized Ca3Co4-xNixO9 nano-powders exhibited the homogeneous distribution of the size and the shape. The microstructure and high-temperature (773-1073K) thermoelectric properties of the Ca3Co4-xNixO9 were investigated, depending on Ni content. Higher Ni content resulted in a smaller grain size and a higher porosity. The Ca3Co4-xNixO9 was a misfit-layered oxide consisting of two monoclinic subsystems. The added Ni gave rise to a decrease in the conductivity because it led to a reduction in the grain size and density. The sign of the Seebeck coefficient was positive over the measured temperature range, indicating that the major conductivity carriers were holes. The Seebeck coefficient of the Ni-added Ca3Co4-xNixO9 was much higher than that of the Ni-free Ca3Co4O9. The power factor increased gradually with temperature. The highest value of the power factor was attained for Ca3Co0.38Ni0.2O9, i.e., 3.3×10-4Wm-1K-2 at 1073K. In addition, the Ca3Co4-xNixO9 oxides possessed excellent thermal and chemical stability under air or oxidizing atmospheres at high temperatures because they are sintered at 1123K in air.
9:00 PM - DD10.5
Opacified, Reinforced Silica Aerogel for Thermal Insulation of Thermoelectric Generators.
Ryan Maloney 1 , Jeffrey Sakamoto 1
1 Chemical Engineering & Materials Science, Michigan State University, East Lansing, Michigan, United States
Show AbstractAn efficient thermoelectric material relies on the non-dimensional figure of merit ZT, and maximizing this value is the subject of much current research. The efficiency of a thermoelectric generator, however, also relies on maintaining a large temperature gradient across the relatively short height of the thermoelectric legs. Thus, the application of a thermoelectric device for waste heat recovery requires high-performance thermal insulation. Unfortunately, the very nature of a thermoelectric generator makes finding an appropriate insulation material a challenge. The insulation must be able to be cast into place around the thermoelectric legs, such that intimate contact with the device prevents parasitic heat loss through gaps in insulation. While there are many materials that satisfy this first requirement, thermoelectrics must also operate at high temperatures (above 600°C), which is well above the maximum operating temperature of most cast-in-place materials. Finally, the material must be mechanically stable over the lifetime of the device, enduring both the vibratory forces and large heat fluxes associated with power generation.Silica aerogels, with their tortuous microporosity, low solids content and sol-gel chemistry, are uniquely poised for this effort, having the lowest thermal conductivity of any non-evacuated solid (10-30 mW/m*K) and a relatively high sintering resistance (600°C before modification). In this work, we present the results of our optimization of opacified, reinforced silica aerogel and its use in a 50W skutterudite-based thermoelectric generator for vehicle waste heat recovery. The aerogel can withstand the high temperature of a vehicle exhaust stream (650°C), and can maintain a large thermal gradient across a short distance (550°C over 10mm). Moreover, aerogel can be cast into place, ensuring intimate contact with the thermoelectric legs and preventing the formation of line-of-sight thermal shorts. The use of aerogel in a 50W generator is demonstrated in providing a 25% reduction in heat flow as compared to high-temperature wool insulation.
9:00 PM - DD10.6
Thermoelectric Properties of Bi2Te3 Sintered Body Fabricated by a Combination of Mechanical Milling and Mixing Processes.
GilGeun Lee 1 , GookHyun Ha 2 , KyungTae Kim 2
1 Division of Materials Science and Engineering, Pukyong National University, Busan Korea (the Republic of), 2 , Korea Institute of Materials Science, Changwon Korea (the Republic of)
Show AbstractThe present study aimed at investigating application possibility of the percolation concept to the microstructure control of thermoelectric materials for improving thermoelectric properties. Two kinds of Bi2Te3 powders, pure Bi2Te3 and Bi2Te3/2vol.%ZrO2, have been prepared by a mechanical milling process. Effect of mixing of the these powders on thermoelectric properties of the sintered body was studied by measuring Seebeck coefficient, specific electric resistivity and thermal conductivity. The Bi2Te3 sintered body has a lower specific electric resistivity and higher thermal conductivity than the values of the Bi2Te3/2vol.%ZrO2 sintered body. The sintered body of the mixed powder showed percolation transition behavior on the specific electric resistivity at a specified powder mixing ratio. This transition behavior can be predicted by a two-dimensional particle configuration model based on the percolation concept. The sintered bodies of mixed powder based on the percolation concept have a higher figure of merit than the value of the one of unmixed powder.
9:00 PM - DD10.7
Methods for Studying Telluride-based Thermoelectrics Using Atom Probe Tomography.
Michelle Hekmaty 1 , Jessica Lensch-Falk 1 , Mark Homer 1 , Nick Teslich 2 , Doug Medlin 1
1 , Sandia National Laboratories, Livermore, California, United States, 2 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractIn the development of nanostructured thermoelectric materials, it is important to understand the nanoscale distribution of compositional inhomogeneities. Atom probe tomography (APT) provides a powerful means to study the distribution of dopants, composition of buried precipitates, and segregation at interfaces. APT combines the field evaporation of ions at the specimen surface with time-of-flight mass spectroscopy and a position sensitive detector to determine the identity and position of ions in a material achieving sub-nanometer resolution in 3D. However, obtaining and interpreting APT data on thermoelectric materials poses a number of challenges due to their poor thermal conductivity, brittle mechanical properties, and complex mass spectra. In this presentation, we discuss our work developing methods for sample preparation, data collection, and spectral analysis of telluride-based thermoelectric materials. We have explored three methods to prepare telluride specimens for APT: electropolishing, Focused-Ion-Beam (FIB) milling and low energy argon ion-milling. We discuss the merits and drawbacks of each of these techniques. Additionally, we have resolved several issues related to APT data collection and analysis of tellurides. For instance, we find that telluride-based alloys typically exhibit much poorer mass resolution than do typical metals. By investigating different run conditions, we learned the best conditions for these materials to optimize the mass resolution. Additionally, we find that analyzing the mass spectra of these materials is challenging because of the field evaporation of singly and multiply charged Te-based complex ions and the overlap of peaks from different ion species. By running a variety of materials and employing multi-peak fitting, we are able to reliably identify the peaks and quantify the contributing amounts of overlapped peaks from different ion species. By correctly identifying the peaks, we can confidently interpret 3-D reconstructions of the atom probe data and the chemical composition. We illustrate these methods and the valuable insight they provide, in conjunction with TEM data, concerning composition of the matrix, precipitates, and interfaces in several relevant TE systems including PbTe, PbTe/Ag2Te, (AgSbTe2)(PbTe) 3, AgSbTe2 and (Bi0.2Sb0.8) 2Te3.
9:00 PM - DD10.8
Synthesis and Characterization of Bulk La/Nb-doped Strontium Titanates for Thermoelectric Energy Conversion.
Hongjoo Yang 1 , Choongho Yu 1
1 , Texas A&M University, College station, Texas, United States
Show AbstractComplex oxides have been reported as one of promising thermoelectric materials due to their wide tunability in electrical and thermal transport properties. However, previous works report for atomic layers or thin films that are not easy to use for real applications. This work presents bulk strontium titanates that are doped with La and Nb, which are substituted for Sr and Ti sites, respectively. Samples were prepared by pressing oxide mixtures of SrO, TiO2, La2O3, and Nb2O5 into pellets. Then, they were sintered at 1450 oC for 18 hours. Structural and compositional analyses were performed on samples via scanning and transmission electron microscopies and x-ray diffraction. Thermoelectric transport properties including electrical and thermal conductivity, Seebeck coefficient were measured as a function of temperature and concentration of the doping elements.
9:00 PM - DD10.9
Single-crystal Wires Based on Doped Bi for Anisotropic Thermoelectric Microgenerators.
Albina Nikolaeva 1 2 , Leonid Konopko 1 2 , Tito Huber 3 , Ana Tsurkan 1
1 , Institute of Electronic Engineering and Industrial Technology, Chisinau Moldova (the Republic of), 2 , International Laboratory of High Magnetic Fields and Low Temperatures, Wroclaw Poland, 3 Department of Chemistry, Howard University, Washington, Washington, United States
Show AbstractThe appearance of new more efficient materials for anisotropic thermoelements (ATs) is reviving interest in the transverse thermoelectric effect. The efficiency of ATs is determined, to a considerable extent, by thermopower anisotropy value. Single crystals of Bi have the thermopower anisotropy of ≈ 50 μV/K in the temperature range of 100 - 400 K, which makes it possible to design ATs with a sensitivity of ≈ 10 – 15 mV/W and rapid response time τ = 10 – 2 s, which are applied, in particular, as heat flow meters, in microcalorimetry [1]. We have studied the possibility to use a microwire of BiSn to design an anisotropic thermoelectric generator. The glass-coated microwire of pure and Sn-doped bismuth was obtained by the Ulitovsky method; it was a cylindrical single-crystal with orientation [1011] along the wire axis; the C3 axis was deflected at an angle of 70° to the microwire axis. It is known that the size effect significantly changes the thermoelectric properties of microwires and leads to an increase in thermoelectric efficiency [2,3]. The developed technology allows obtaining a glass-insulated single-crystal microwire of Bi and its alloys with Sn with a length up to a few tens of meters and with a given diameter from 100 nm to 50 μm. To study the thermopower anisotropy of the wires, we used samples with the orientation of C3 along the wire axis, which were obtained by zone and laser recrystallization [4]. It is found that doping of bismuth wires with tin increases the thermopower anisotropy in comparison with Bi by a factor of 2-3 in the temperature range of 200-300 K. According to the preliminary results, for a Bi microwire with a diameter of 10 μm with a glass coating of 35 μm, the transverse thermopower is ~150 μV/(K*cm); for BiSn, 300 μV/(K*cm). The design of an anisotropic microthermogenerator based on BiSn microwire is proposed. The miniature thermogenerator will be efficient for power supply of devices with low useful current.In addition to the considerable thermopower anisotropy of BiSn wires in a glass coating, they exhibit stable thermoelectric properties, high mechanical strength and flexibility, which allows designing thermoelectric devices of various configurations on their basis.References[1]. Anatychuk L.I. Termoelementy i termoelektricheskie ustroistva. Reference book. Kiev: Naukova dumka 1979, 766 p.[2]. Lin Y.M., Sun X.Z., Dresselhaus M.S. Phys. Rev. B.: Condens. Matter Mater.Phys., 62, 4610 (2000).[3]. Gitsu D., Konopko L., Nikolaeva A. and Huber T. Appl. Phys. Lett. 86, 10210 (2005).[4]. Patent MD 3693 2008.08.31
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Symposium Organizers
John D. Baniecki Fujitsu Laboratories Ltd.
Jonathan A. Malen Carnegie Mellon University
G. Jeffrey Snyder California Institute of Technology
Harry L. Tuller Massachusetts Institute of Technology
DD11: Thermoelectric Oxides
Session Chairs
Kunihito Koumoto
Anke Weidenkaff
Friday AM, April 09, 2010
Room 2002 (Moscone West)
9:15 AM - DD11.0
Direct Measurement of the Absolute Seebeck Coefficient for Pb and Cu at 300 K to 450 K.
Patrick Garrity 1 , Kevin Stokes 1
1 Department of Physics, Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractThe utilization of thermal fluctuations or Johnson/Nyquist noise as a spectroscopic technique to experimentally measure transport properties is applied to Pb and Cu metal films. Through cross-correlation and autocorrelation functions obtained from power spectral density measurements, multiple transport coefficients are obtained through the Green-Kubo formalism. Supported rigorously by the underlying fluctuation-dissipation theory, this new experimental technique provides a direct measurement of absolute thermoelectric coefficients in addition to the electrical resistivity, electronic contribution to thermal conductivity, Lorentz number and various diffusion coefficients. This work reports the validation results of the experiment accomplished through the use of materials with thermoelectric properties widely accepted by the thermoelectric community, Pb and Cu. Further validation of the data was accomplished by comparing resistivity results to standard collinear four-probe resistivity measurements. Thermal fluctuation data for Pb resulted in 2.6% and 4.1% agreement with the published Seebeck and four-probe resistivity data respectively. The Cu thermal fluctuation measurements showed agreement within 3.4% and 5.2% for the published Seebeck and four-probe data respectively thus lending further credibility to the experimental method and underlying theory. The Pb and Cu electronic thermal conductivity measured 29.6 W/m-K and 321.3 W/m-K respectively. Interestingly, the Lorentz number was approximately 81% of the theoretical value for both Pb and Cu.
9:30 AM - **DD11.1
Nanostructure Control to Enhance Thermoelectric Performance of SrTiO3 and TiS2.
Kunihito Koumoto 1 2
1 , Nagoya University, Nagoya Japan, 2 , Japan Science and Technology Agency, CREST, Tokyo Japan
Show Abstract Most of the recent studies on nanostructuring have attempted to introduce nanointerfaces into bulk polycrystals or composites to reduce thermal conductivity by enhancing boundary phonon scattering and improve the thermoelectric performance. However, in order to further increase ZT, not only nanointerfaces to reduce thermal conductivity but also quantum nanostructures to enhance power factor should be incorporated simultaneously into one bulk material. This “synergistic nanostructuring” concept has been applied to our SrTiO3 material, and 3D superlattice (nano Rubik’s cube structure) was designed and was verified by numerical simulation to be capable of showing ZT>0.8 @300K. Nanosizing of grains was experimentally verified to reduce lattice thermal conductivity through diminishing the phonon mean free path. Our challenge to realize the designed nanostructure will be presented. We have recently proposed TiS2-based misfit-layered compounds as novel TE materials. Insertion of misfit-layers into the van der Waals gaps in layer-structured TiS2 gives rise to internal nanointerfaces and dramatically reduce its lattice thermal conductivity. ZT value reaches 0.37 at 673 K. Our challenge to further increase ZT by controlling the nanostructure will be presented.
10:00 AM - DD11.2
The Effect of Rh and Sr Substitution on the Thermoelectric Performance of LaCoO3.
Andrew Smith 2 , Mas Subramanian 2 , Kyei-Sing Kwong 1
2 Department of Chemistry, Oregon State University, Corvallis, Oregon, United States, 1 , National Energy Technology Laboratory, Albany, Oregon, United States
Show AbstractA series of LaCo1-xRhxO3 (x = 0-1) samples and La1-ySryCo1-xRhxO3 (y = 0.05, 0.15 and x = 0.1-0.3) samples were prepared to study the effect of Rh substituion for in the four component system and Sr substitution for La in the five component system on the crystal structure and thermoelectric performance of the LaCoO3. As Rh for Co substitution increases to x = 0.7 the crystal structure shifts from rhombohederal (LaCoO3) to orthorhombic (LaRhO3). Thermoelectric evaluation revealed that Rh doped samples (0.3〈 x〈 1) show large positive seebeck coefficients indicating a P-type conduction in the temperature range of the tests (300 to 775 K). Rh substitution for Co decreases thermal conductivity, increases electrical conductivity and consequently increases the theroelectric figure of merit ZT. Sr substitution for La increases thermal and electrical conductivity and consquenently negligiblely decreases the seebeck coefficient. A promising thermoelectric figure-of-merit (ZT) around 0.075 has been achieved for LaCo0.5Rh0.5O3 at 775 K, and is expected to reach 0.15 at 1000 K. Sr substitution improved the TE properties throughout the lower temperature range with a ZT = 0.045 observed for La0.95Sr0.05Co0.9Rh0.1O3 at 425 K and ZT = 0.05 for La0.85Sr0.15Co0.5Rh0.5O3 at 775 K. These findings provide new insight into thermoelectric perovskite oxides containing rhodium and strontium.
10:15 AM - DD11.3
Enhancement of Seebeckcoefficients in Functional Oxides by Heterostructure Formation.
Hanns-Ulrich Habermeier 1 , Stefan Heinze 1
1 , MPI-FKF, Stuttgart Germany
Show AbstractIn addition to low thermal and high electrical conductivity large Seebeckcoefficients are mandatory to construct materials with a large thermoelectric figure of merit. We explore the approach of superlattice fabrication of complex oxide constituents to artificially manipulate the sign and size of Sebbeckcoefficients. In our experiments we explored the feasibility of this concept by using YBCO and LaCaMnO thin film superlattices and obtained Sebbeckcoefficients several times larger than the indicidual layers. We attribute this to modifications of the electronic structure at the interface of the multilayers.
10:30 AM - DD11.4
Synthesis and Thermoelectric Properties of Y-doped Ca3Co4O9 Ceramics.
Julio E. Rodriguez 1 , Luis C. Moreno 2
1 Department of Physics, Universidad Nacional de Colombia, Bogota Colombia, 2 Department of Chemistry, Universidad Nacional de Colombia, Bogota Colombia
Show AbstractPolycrystalline ceramics with nominal composition of Ca3-xYxCo4O9+δ (0≤x≤0.10) were grown by using the citrate-complex method. The thermoelectric properties were studied from Seebeck coefficient S(T) and electrical resistivity ρ(T) measurements. These transport properties were studied in the temperature range between 100 and 290K. For low doping levels Y substituted samples (x≤0.06) the magnitude of S(T) and ρ(T) decreases with the yttrium content. The temperature behavior of S(T) and ρ(T) was interpreted in terms of small-polaron hopping mechanism. From S(T) and ρ(T) data it was possible to calculate the thermoelectric power factor PF, which reaches maximum values close to 20 μW/K2-cm, these values become these compounds promissory thermoelectric compounds to be used in thermoelectric devices for room temperature thermoelectric applications.
10:45 AM - DD11: Oxides
BREAK
11:15 AM - **DD11.5
Development of Perovskite-type Oxides and Oxynitrides for Thermoelectric Solar Converters.
Anke Weidenkaff 1 , Myriam Aguirre 1 , Sascha Populoh 1 , Laura Bocher 1 , Matthias Trottmann 1 , Petr Tomes 1
1 Solid State Chemistry and Catalysis, Empa, Duebendorf Switzerland
Show AbstractHigh temperature heat from concentrated solar insolation can be used to run thermoelectric converters and to provide electricity. For the realisation of innovative, lightweight and environmentally harmless ceramic thermoelectric converters stable p- and n- type thermoelectric oxides have to be developed and tested in Thermoelectric Oxide Modules (TOM). The thermopower of systems with strongly correlated electronic systems can be substantially enhanced by a spin-orbital-entropy factor. A promising approach to reduce the thermal conductivity is to increase the amount of grain boundaries by producing nano-scaled thermoelectric materials. Suitable candidates for the p- and n-legs are perovskite-type materials. Tailor-made compounds with various compositions are chosen following theoretical considerations and synthesised by chimie douce methods. Here the thermoelectric figure of merit is tuned by appropriate variations of the composition, morphology and crystallographic structure. The improved thermoelectric materials are finally tested in demonstration devices.
11:45 AM - DD11.6
High Temperature Thermoelectric Properties of Bi2Sr2Co2Oy Thin Films Grown by Pulsed Laser Deposition.
Jayakanth Ravichandran 1 , Wolter Siemons 2 , Joseph Feser 4 , Herman Heijmerikx 3 , Arun Majumdar 5 , R. Ramesh 2
1 Applied Science and Technology, University of California, Berkeley, Berkeley, California, United States, 2 Department of Physics, University of California, Berkeley, Berkeley, California, United States, 4 Department of Mechanical Engineering, University of California, Berkeley, Berkeley, California, United States, 3 Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands, 5 ARPA-E, Department of Energy, Washington DC, District of Columbia, United States
Show AbstractMisfit cobaltates are strongly correlated materials showing exceptional thermoelectric properties. Sodium cobaltate is the model system for this class of materials, which show high thermopower in the limit of large carrier concentration. Even though the exact origin of enhanced thermoelectricity is debatable, there is a general consensus that strong correlation plays a huge role in this effect. In order to better understand the nature of these materials, we chose to investigate Bi2Sr2Co2Oy, which is one of the least studied materials in thin film form. We grew c-axis oriented thin films of Bi2Sr2Co2Oy on c-plane sapphire substrates by Pulsed Laser Deposition method. The thermoelectric properties of the films namely thermopower, electrical and thermal conductivities were measured over a temperature range of 300-800 K. The effect of the growth parameters particularly cooling pressure on the thermoelectric properties is discussed. Magnetic measurements, Nuclear Magnetic Resonance (NMR) and Electron Spin Resonance (ESR) were performed to determine the valence and spin states of Cobalt and the validity of Heikes formula is checked for this compound.
12:00 PM - DD11.7
Preparation of High Seebeck Coefficient Calcium Cobaltite Themoelectric Powders.
Sidney Lin 1 , Jiri Selig 1
1 Chemical Engineering, Lamar University, Beaumont, Texas, United States
Show AbstractWhen a difference in temperature exists on either side of a thermoelectric device, a voltage is created, and when a voltage is applied to a thermoelectric device, a difference in temperature is created. In order to have a high figure of merit (ZT=σTS2/κ, where σ is the electrical conductivity, T is the absolute temperature, S is the Seebeck coefficient, and κ is the thermal conductivity), a good thermoelectric material should have a high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity.Alloy semiconductors have proven to have a high figure of merit because of high electrical conductivity, but they will oxidize in air/oxygen at high temperatures, which lowers their figure of merit and limits their application in high-temperature oxidizing environments. Oxides are good candidates for high-temperature thermoelectric material because they have low thermal conductivity and will not oxidize at high temperatures. Recently, Co-based oxides with a misfit-layered structure have shown high Seebeck coefficients. Ca3Co4O9 is one of these compounds that has been reported to have a high figure of merit of about 0.87 at 973 K. A low-cost way of producing this material would allow it to be produced efficiently. A way of doing this is by using Self-propagating High-temperature Synthesis (SHS).Self-propagating High-temperature Synthesis (SHS) is a special kind of solid state reaction using a highly exothermic reaction to produce a solid product. After ignition by an external heat source, the heat released from the reaction is sufficient to sustain the movement of the reaction front until the reactant is completely converted into the product. In this work, the SHS process is used to produce high-quality thermoelectric materials at a low cost by the following reactions:1.24CaO2 + 1.62Co + 0.69O2 → Ca1.24Co1.62O3.86 and1.24CaO2 + 1.62Co + 0.35NaClO4 → Ca1.24Co1.62O3.86.In the first reaction, atmospheric pressure oxygen is used, and a solid oxidizer (NaClO4) is used as the oxygen source in the second reaction. To study how different product purities are formed, many variables are investigated, such as pellet diameter, density, and oxygen flow rate. K type thermocouples are used to measure the temperature history during the combustion and the velocity of combustion front movement. Synthesized samples were analyzed by XRD for their phase purity and TG/DSC was used to determine the reaction pathway for SHS of Ca1.24Co1.62O3.86 and thermal stability of Ca1.24Co1.62O3.86 under various oxygen partial pressures. Thermographic analysis was performed to study reaction propagation in pellets during SHS reactions and to determine the surface temperature of the sample during reaction. Measured temperature history and TG/DSC results were used to develop a finite element model of SHS of calcium cobalt oxide. This model can be used to predict reaction temperatures for various initial conditions.
12:15 PM - DD11.8
Microwave Processing of Ca3Co4O9.
Murat Gunes 1 , Ahmet Ozenbas 2 , Mehmet Parlak 3
1 Micro and Nano Technology, Middle East Technical University, Ankara Turkey, 2 Metallurgical and Materials Engineering, Middle East Technical University, Ankara Turkey, 3 Physics, Middle East Technical University, Ankara Turkey
Show AbstractA microwave processing method has been applied to synthesize Ca3Co4O9 by using calcium and cobalt nitrates as raw materials and citric acid as complexing agent. The formation process of Ca3Co4O9 and the characterization of powders were investigated by TG-DTA, XRD and FE-SEM. The results showed that Ca3Co4O9 powders can be prepared after drying the solution in the modified microwave oven at 540 watt for 2x20 minutes. The dried gel was ground and calcined at 750 oC for 2 h. Ethylene glycol as a dispersant was added to the nitrate solution during the formation of sol precursor. A sol-gel method has also been applied to synthesize Ca3Co4O9 by using the same raw materials as used in microwave processing. The sol-gel precursor was heated at 80oC for 6-7 h in order to obtain the gel followed by drying at 120oC for 12 h. The dried gel was also ground and calcined at 750 oC for 2 h. The calcined powders were compressed into pellets with a diameter of 10 mm under a pressure of 10 MPa, and then these pellets were sintered at 750 oC for 2 h. The surface area of powders obtained by microwave heating and sol-gel method were determined as 13.97 and 10.15 m2g-1, respectively, using BET analysis. The bulk densities of these samples measured by helium pycnometer were 4.273 and 6.257 gcc-1 corresponding to microwave and sol-gel methods. Compared with the sol-gel method, microwave processed samples exhibited higher Seebeck coefficient as 225.7 µVK-1 at 170 oC against 201.8 µVK-1. The resistivity of the microwave processed and sol-gel samples were 0.07 and 0.08 ohm-cm at room temperature, respectively. These results showed that microwave processing provides an efficient method for the preparation of Ca3Co4O9 thermoelectric composition.
12:30 PM - DD11.9
Thermoelectric Properties of a Single Electrospun NaCo2O4 Nanofiber.
Feiyue Ma 1 , Jiangyu Li 1
1 , University of Washington, Seattle, Washington, United States
Show AbstractNaCo2O4 nanofibers are synthesized by sol-gel based electrospinning method, and their morphology and crystalline structure are examined by SEM, TEM, and XRD. It is demonstrated that after appropriate treatment, the diameter of the nanofibers is around 200nm, and the grain size is around 10nm, much smaller than NaCo2O4 ceramics or thin films synthesized by conventional techniques. This makes it very promising for thermoelectric energy conversion, taking advantages of phonon scattering at grain boundaries for substantially reduced thermal conductivity. To characterize the thermoelectric performance of NaCo2O4 nanofibers, we measure electrical conductivity of a single nanofiber using conductive atomic force microscopy, and confirm its thermoelectric effect using electrostatic force microscopy under a highly localized heating. Thermal conductivity of a bulk NaCo2O4 ceramics sintered from nanofibers is also measured, confirming the expected reduction in thermal conductivity.
12:45 PM - DD11.10
Structure-Strain-Thermoelectric Property Relationships of Conducting Epitaxial Perovskite Thin Films for Thermoelectric Device Applications.
John Baniecki 1 , Masatoshi Ishii 1 , Kazunori Yamanaka 1 , Kazuaki Kurihara 1 , Tetsuya Kaneko 2 , Paul McIntyre 2 , Arturas Vailionis 2
1 Energy Technology Laboratory, Fujitsu Laboratories, Atsugi, Kanagawa, Japan, 2 Department of Materials Science and Engineering, Stanford University, Stanford , California, United States
Show AbstractThermoelectric materials have gained extensive attention as energy-conversion materials in order to recycle waste heat from power plants, automobiles, and computers into usable electrical energy. Recently there has been significant progress in realizing thermoelectric materials with improved thermal to electric energy conversion efficiencies owing, in part, to the ability to nanostructure thin film materials. Donor doped strontium titanate (STO) has emerged as an important thermoelectric thin film material candidate owing to the ability to manipulate film structure and defect concentrations in these materials, yet the thermoelectric properties of other perovskite structure materials, in thin film form, remain largely unexplored. While film structure and defects (such as vacancies and dislocations) have been suggested to influence thermoelectric properties of conducting perovskite structure thin films, detailed interrelationships among them remain poorly understood. In the presentation, we will present the results of studies characterizing the nanostructure and strain state of conducting perovskite thin films including donor doped strontium titanate (STO) and rare-earth nickelates, grown epitaxially by pulsed laser deposition and RF sputtering over a wide range of film thicknesses (10-200 nm) and growth conditions. Correlations among the film structure, strain state, and thermoelectric properties, such as electrical conductivity and Seebeck coefficient, will be presented and the role of structure, defects, and strain on the thermoelectric performance discussed.
DD12: Thermoelectric Measurement Methods
Session Chairs
John Baniecki
Harry Tuller
Friday PM, April 09, 2010
Room 2002 (Moscone West)
2:30 PM - DD12.1
In-plane Thermal Conductivity Determination in Silicon on Insulator (SOI) Structures Through Thermoreflectance Measurements.
Max Aubain 1 , Prabhakar Bandaru 1
1 Materials Science Program, Mechanical and Aerospace Engineering Department, University of California, San Diego, La Jolla, California, United States
Show AbstractSignificant effort has been expended in the measurement of the thermal conductivity of thin film structures to evaluate low-dimensional effects such as phonon confinement, surface scattering, and thermal boundary interface resistance. In this context, various techniques for determination of cross-plane thin film thermal conductivity, e.g., the 3-omega method, have been developed. However, in these techniques the in-plane component of the thermal conductivity is not measured, but accounted for through an anisotropy factor. The validity of such adaptation is not clear. Alternatively, non-contact optical methods, such as thermoreflectance, could be used to directly measure the in-plane thermal transport of optically active thin films. In this paper we report on our experiments where, by using a scanning thermoreflectance technique, we have measured thermal profiles, induced by on-chip alternating electrical currents in the range of 1kHz to 45kHz, in a multilayered silicon-on-insulator structure. Additionally, several heaters, with widths ranging from 5μm – 90μm, were used as additional variables to vary heat flow conditions. Information on the thermal state of the structure, both with respect to lateral distance away from the heater and depth with respect to the layer interfaces, was then obtained. At heating frequencies below 10kHz, the profiles were fitted using a transient one-dimensional heat flow equation and the in-plane thermal conductivity of the buried oxide was determined. At higher frequencies, the thermal profiles suggest that heat trapping occurs in the buried oxide due to slow lateral heat dissipation. The experimental requirements and practicality of this method to determine the in-plane thermal conductivity of thin films will be discussed.
2:45 PM - DD12.2
New Method for a Complete 4-point Thermoelectric Characterization.
Volker Schmidt 1 , Johannes de Boor 1 , Ulrich Goesele 1
1 , MPI of Microstructure Physics, Halle Germany
Show AbstractContact resistances - in particular thermal contact resistances - are a frequently encountered problem when it comes to the characterization of thermoelectric materials. A characterization method that does not depend on the quality of the contacts is therefore highly desirable. We present a novel and simple method for a complete 4-point characterization of macroscopic samples. By complete it is meant that the method allows for the independent determination of the electrical conductivity, the thermal conductivity, and the Seebeck coefficient, with all three parameters measured in a 4-point manner. Using these data the figure of merit ZT can be determined. The method can also easily be extended for a direct measurement of ZT via the Harman method, which is a valuable extension, because one can thus cross-check the consistency of the data.The method is easy to adopt and does neither require extensive preparation nor expensive instrumentation and may therefore be a valuable tool in the strive for improved thermoelectrica. The method has been successfully tested on Ni and InSb bulk samples. Temperature dependent measurements of the electrical conductivity, the thermal conductivity, and the Seebeck coefficient were conducted. The results show good agreement with respective reference data.
3:00 PM - DD12.3
Thermomechanical Imaging and Contact Potential Analysis on Thermoelectric Materials With Heated Atomic Force Microscope Tips.
Jessica Remmert 1 2 , Michael Check 1 2 , Harry Seibel II 1 2 , John Jones 1 , Douglas Dudis 1 , Andrey Voevodin 1 , William King 3
1 Materials and Manufacturing Directorate, AFRL/RX, Air Force Research Laboratory, WPAFB, Ohio, United States, 2 , Universal Technology Corporation, Dayton, Ohio, United States, 3 Department of Mechanical Science and Engineering, University of Illinois, Urbana-Champaign, Illinois, United States
Show AbstractAtomic force microscopy (AFM) can offer unique mapping capabilities for superlattice structures when a temperature gradient is applied at the tip-sample contact and the emerging thermoelectric potential is measured. Broadly speaking, scanning probe techniques have used this approach to characterize Seebeck coefficients at molecular junctions1 and across doped interfaces2 with nanometer-scale resolution. A heated AFM tip can also operate as a Kelvin probe3 to enable surface potential mapping with a contributing thermal voltage. This experiment builds upon prior work which demonstrated temperature-dependent contact potential measurement using doped silicon cantilevers, as a function of distance-to-contact and applied bias offsets.4 Dual ac resonance tracking5 (DART™) mode on an Asylum AFM was adapted to modulate the driving voltage across the cantilever legs and amplify the mechanical response on resonance. Driving frequencies within the range 0.1-1 MHz were tested so that heating rates exceeded the 280 µs thermal time constant and restricted temperature oscillations near the cantilever tip.6 Thermal DART imaging was performed on thermoelectric materials with an independently controlled substrate bias. The amplitude and phase of the cantilever response were captured while the tip was held in contact at set point force by the static (dc) deflection. The ac mechanical response was proportional to the electrostatic field strength, suggesting that DART is a feasible tool for mapping superlattice interfaces and other surfaces with nonuniform thermoelectric properties.1P. Reddy, S.Y. Jang, R.A. Segalman, and A. Majumdar, "Thermoelectricity in molecular junctions," Science 315, 1568 (2007).2H.K. Lyeo, A.A. Khajetoorians, L. Shi, K.P. Pipe, R. J. Ram, A. Shakouri, and C.K. Shih, "Profiling the thermoelectric power of semiconductor junctions with nanometer resolution," Science 303, 816 (2004). 3M. Nonnenmacher, M.P. O'Boyle, and H.K. Wickramasinghe, "Kelvin probe force microscopy," Applied Physics Letters 58, 2921 (1991). 4J. L. Remmert, Y. Wu, J. Lee, M.A. Shannon, and W.P. King, Appl. Phys. Lett. 91, 143111 (2007). 5B. J. Rodriguez, C. Callahan, S. V. Kalinin, and R. Proksch, “Dual-frequency resonance-tracking atomic force microscopy”, Nanotechnology 18, 475504 (2007). 6K. Park, J. Lee, Z. M. Zhang, W. P. King, “Frequency-dependent electrical and thermal response of heated atomic force microscope cantilevers”, J. Microelectromech. Syst. 16, 213 (2007).
3:15 PM - DD12.4
Valence Band Edge Offset Measurements between Bi2Te3, Sb2Te3 Thin Films and Their Superlattices for Thermoelectric Applications.
Fang Fang 1 , Robert Opila 1 , Rama Venkatasubramanian 2
1 Materials Science & Engineering, University of Delaware, Newark, Delaware, United States, 2 Center for Solid State Energetics (CSSE), RTI International, Research Triangle Park, North Carolina, United States
Show AbstractPrevious results indicate that in pursuit of an enhancing ZT>1 at room temperature, we need to tune the phonon and hole (charge carriers) transport to achieve a material structure that is phonon-blocking as well as carrier-transmitting. It has been reported that high-quality superlattices of Bi2Te3/Sb2Te3 can offer a value of ZT>1, owing to their significantly higher carrier mobility from absence of alloy scattering as well as significantly reduced lattice thermal conductivity. The carrier transport perpendicular to superlattice interfaces were as good as in-plane in some specific superlattices. However, there have been no direct measurements of band alignment at the Bi2Te3/Sb2Te3 hetero-interface to estimate the potential barrier to efficient carrier transmission, i.e., that valence-band offsets are expected to be less than the average thermal energy of carriers. Photoemission spectroscopy using UV-light (UPS) is a powerful technique to directly measure the valence band maximum (VBM) at film surface; thereby we can calculate the band energy offset between these alternative films in the superlattices at their hetero-interfaces. UPS measurements were carried out in Brookhaven National Laboratory and Stanford Synchrotron Radiation Laboratory, using synchrotron radiation UV source. The results demonstrate that VBMs of p-type Sb2Te3 and n-type Bi2Te3 films almost overlap with each other (±0.1eV), and they have a 0.17eV±0.1eV offset with p-type Bi2Te3. Also, p-type Bi2Te3/Sb2Te3 superlattice has almost the same VBM as p-type Sb2Te3 film; while n-type Bi2Te3/Bi2Te3-xSex superlattice has a VBM same as p-type Bi2Te3. Thereby, the measured VBM offsets are equivalent to the thermal energy of kT/2 per degree of freedom, 0.026 eV at 300 K, which is expected to be the average thermal energy of carriers. Therefore, we can conclude that stacking Bi2Te3/Sb2Te3 thin films and a stacking structure of p-type Bi2Te3/Sb2Te3 superlattice and p-type Sb2Te3, n-type Bi2Te3/Bi2Te3-xSex superlattice and p-type Bi2Te3 are likely to be carrier-transmitting.
3:30 PM - DD12.5
In-plane Thermal Characterization of Thin-film Using Thermoreflectance Thermal Imaging.
Xi Wang 1 , Ali Shakouri 1 , Huijun Kong 2 , Li Shi 2
1 Electrical Engineering, Univ. of California Santa Cruz, Santa Cruz, California, United States, 2 Mechanical Engineering, University of Texas Austin, Austin, Texas, United States
Show AbstractThin film materials have attracted extensive investigation in the past decade. The main driving force is to alter the thermal transport properties and/or to improve the energy efficiency in Peltier refrigeration and thermoelectric energy generation. Thin films are known often have very anisotropic thermal properties in the cross-plane and in-plane direction. Time domain laser pump-probe technique and 3omega method have been successfully used in measurements of thin film cross-plane thermal conductivity. However, the thin film in-plane thermal conductivity measurement remains as a challenge, because it is very difficult to separate the parasitic heat conduction in the substrate. A novel method based on suspended microheater devices has been developed to characterize the in-plane thermal conductivity of films as thin as tens of nanometers. Similar suspended microdevices were previously used in the measurement of nanowire thermal conductivity. One of the issues is that the thermal contact resistance could be substantial. Also the stress and non-uniform temperature distribution can affect the measurement results. High resolution thermal imaging using thermoreflectance technique is used to accurately quantify the contribution of interface thermal resistance in the overall thermal network and in addition it can show the non-uniformity in the film or nanowire. Temperature distribution along the 400nm wide, 3-5 micron long suspended thin film can be clearly measured. We use this to extract the thin film in-plane thermal conductivity as well as the contact thermal resistances with the silicon nitride membranes. In the full manuscript, we will provide detailed analysis of the thermal transport properties.
3:45 PM - DD12.6
Electron Emission Enhancement from Phosphorus Doped Diamond Films by Surface Ionization for Thermionic Energy Conversion Application.
Franz Koeck 1 , Yasodhaadevi Balasubramaniam 3 , Jeff Sharp 2 , Ken Haenen 3 4 , Robert Nemanich 1
1 Department of Physics, Arizona State University, Tempe, Arizona, United States, 3 Institute for Materials Research, Hasselt University, Diepenbeek Belgium, 2 , Marlow Industries, Inc., Subsidiary of II-VI Incorporated, Dallas, Texas, United States, 4 Division IMOMEC, IMEC vzw, Diepenbeek Belgium
Show AbstractThis study presents a new approach for thermionic energy conversion which employs molecules in the vacuum gap between the emitter and collector electrodes. Novel thermionic electron emitters based on diamond rely on effects related to the emission barrier at the surface, band bending and negative electron affinity (NEA) properties. With H-terminated diamond surfaces the surface barrier for emission can efficiently be removed as the vacuum level becomes located below the conduction band minimum (CBM). For phosphorus doped diamond films we have measured an effective work function for thermionic emission of ~ 1 eV where an onset of emission is observed at ~ 300 °C. This is significantly lower than corresponding emission barriers for conventional metallic electron sources. In a novel approach we propose amplified emission by utilizing surface ionization processes where charge is transferred from the conduction band of the emitter surface to the affinity level of the scattered molecule. It has been shown that the negative electron affinity of diamond surfaces can provide efficient means for charge transfer as electrons can readily be released into vacuum. We present results on electron emission from phosphorus doped diamond films in a variable environment of gaseous species where a significant enhancement of the emission characteristics is observed. This strong increase in the emission current coincides with an increase in emission stability at elevated temperatures. These emission enhancing phenomena can be utilized in a thermionic energy converter configuration and simple analysis indicates a high efficiency at operating temperatures of ~ 650 °C.
4:00 PM - DD12: Methods
BREAK
4:30 PM - DD12.7
Electron Interface Scattering in Thin Metal Films During Electron-phonon Nonequilibrium.
Patrick Hopkins 1
1 , Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractElectron-interface scattering during electron-phonon nonequilibrium in thin films creates another pathway for electron system energy loss as characteristic lengths of thin films continue to decrease. As power densities in nanodevices increase, excitations of electrons from sub-conduction-band energy levels will become more probable. These sub-conduction-band electronic excitations significantly affect the material’s thermophysical properties. In this work, the effects of d-band electronic excitations are considered in electron energy transfer processes in thin metal films. In thin films with thicknesses less than the electron mean free path, ballistic electron transport leads to electron-interface scattering. The ballistic component of electron transport, leading to electron-interface scattering, is studied by a ballistic-diffusive approximation of the Boltzmann Transport Equation. The effect of d-band excitations on electron-interface energy transfer is analyzed during electron-phonon nonequilibrium after short pulsed laser heating in thin films.Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract DE-AC04-94AL85000.
4:45 PM - DD12.8
Raman Scattering as a Probe of the Anharmonicity of Phonon Modes and of the Phase Stability of Thermoelectric Materials.
Romain Viennois 1 2 , Jean-Claude Tedenac 1 , Didier Ravot 1 , Rose-Marie Ayral 1 , Denis Machon 2 , Alfonso San Miguel 2 , Pierre Toulemonde 2 3 , Michael Koza 4 , Mutka Hannu 4 , Tetsuji Kume 5 , Hiroyasu Shimizu 5 , Minako Komura 5
1 Institut Gerhardt, Universite Montpellier II, Montpellier France, 2 Laboratoire PMCN, Universite Lyon 1, Villeurbanne France, 3 Département MCMF, Institut Néel, CNRS and Université Joseph Fourier, Grenoble France, 4 , Institut Laue-Langevin, Grenoble France, 5 Department of Materials Science, Gifu University, Gifu Japan
Show AbstractThe improvement and determination of the stability of thermoelectric materials for high temperatures applications necessitate a thorough study of lattice dynamics which can give insight into the nature of the lattice vibrations whose knowledge is indispensable for the understanding of the scattering mechanism of heat-carrying phonons and, hence, of the reduction of lattice thermal conductivity. In order to reach this goal, spectroscopic probes associated with lattice dynamics calculations are very powerful tools, as we have recently shown for the case of powder inelastic neutron scattering experiments on filled skutterudites (1). Raman scattering is as well a powerful spectroscopic technique, as it gives information about the anharmonicity of lattice Gamma point modes (accessing Grüneisen parameters and life time of modes), about presence and nature of defects in the material (with the breaking of the selection rules) and phase stability of the studied materials (with experiments under various conditions such as under pressure and variable temperatures) (2). Indeed, Raman scattering is for practical reasons a convenient spectroscopic probe of lattice dynamics at high pressures. It also permits to probe indirectly electronic properties from the phenomenon of Resonance as in Carbon nanotubes (3). In the present communication, we will present a large number of experimental results from Raman scattering experiments on different promising thermoelectric materials for high temperature applications (skutterudites, clathrates, titanates, silicides, antimonides such as Zn4Sb3, tellurides such as AgSbTe2). We will illustrate the wide band of applications of this experimental technique by studies performed under various conditions (pressure, temperature, wavelength of the incident energy). A comparison of our results with other spectroscopic probes such as inelastic neutron scattering as well as with lattice dynamics calculations will be given.(1) M. M. Koza et al, Nature Materials 7, 805 (2008)(2) See e. g. : G. Lucazeau, J. Raman Spectrosc. 34 478 (, 2003)(3) See e. g. : E. Anglaret et al, Phys. Rev. B 65, 165426 (2002)
5:00 PM - DD12.9
Thermal Boundary Resistance of Organic-metal Interfaces.
Yansha Jin 1 , Abhishek Yadav 2 , Huarui Sun 2 , Max Shtein 1 , Kevin Pipe 2
1 Material Science and Engineering, University of Michigan,Ann Arbor, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan,Ann Arbor, Ann Arbor, Michigan, United States
Show AbstractOrganic-metal interfaces are indispensable in a number of important technologies, including high-performance thermal epoxies, organic LEDs, solar cells, and transistors. Because the majority heat carriers on either side of an organic-metal interface are of different type (phonons and electrons) and because there is a significant acoustic mismatch at the interface, considerable thermal resistance arises. The resulting barrier to heat transfer can undermine the efficiency and reliability in the aforementioned applications. However, a fundamental understanding of interfacial heat transport in these systems is still in its infancy. Developing such an understanding could also aid in the development of novel hybrid thermoelectric devices that take advantage of the low thermal conductivity and discrete energy levels for electrical carriers in molecular organic solids. Using the 3-omega technique, we have measured the thermal resistance at interfaces between thin films of silver and CuPc. Our samples consist of multilayer films deposited by vacuum thermal evaporation on a temperature-controlled stage, where each layer can be precisely controlled in thickness over the range of 5-50 nm, with considerable control of interface roughness. We explore the effects of roughness on the interface thermal resistance, comparing measured data to thermal resistance models, and verifying data gathered in 3-omega measurements using time-domain thermoreflectance.