Filling vacancies in the skutterudite structure with rare earth atoms has been used extensively in cobalt and iron based skutterudites. State of the art filled n-type CoSb3 skutterudite has a ZT approximately 30% greater than unfilled, substitutionally doped CoSb3 at equivalent carrier concentration. IrSb3 is a potentially attractive thermoelectric material because it is more refractory than cobalt-based skutterudites and can therefore take advantage of higher operating temperatures, as well as fill some of the gap in performance in segmented device configurations between state of the art filled iron- and cobalt-based skutterudite antimonides and higher temperature thermoelectric materials such as Yb14MnSb11 and La3-xTe4. In the past, it has been found quite challenging to prepare highly doped n-type IrSb3 compositions by doping with impurities such as Pd and Pt. Peak ZT values obtained at elevated temperatures only ranged from 0.1 to 0.15 at best. In contrast, alkaline and rare earth filling of IrSb3 skutterudite has produced greater carrier concentrations and carrier mobilities than ever achieved through substitutional doping alone. As a result, a large increase in ZT values is reported here, with a peak of nearly 0.9 at 1000 K.

Filled skutterudites are one of the most promising materials for thermoelectric (TE) power generation applications in the intermediate temperatures, due to their superior TE and thermomechanical performance as compared to other materials [1-2]. In the past, we have demonstrated that n-type skutterudites can be optimized so that their maximum TE figure of merit reaches 1.7 at 850 K [1]. TE performance of the p-type, however, is lagging behind, which hinders the optimization of skutterudites-based TE module development. The underlying reasons for this are related to the skutterudites electronic band structures, which results in higher thermal conductivity for the p-type at elevated temperatures due to bipolar lattice thermal conduction; and lower power factor because of the heavy valence bands unlike the conduction bands with beneficial 3-fold degeneracy. In this talk, I will review our recent theoretical and experimental effort on modifying the valence bands of p-type skutterudites and highlight means of improving their TE properties. 1. X. Shi, Jiong Yang, J. R. Salvador, M. Chi, J. Cho, H. Wang, S. Bai, J. Yang, W. Zhang, and L. Chen, “Multiple-Filled Skutterudites: High Thermoelectric Figure of Merit through Separately Optimizing Electrical and Thermal Transports”, J. Am. Chem. Soc. 133, 7837 (2011). 2. J. R. Salvador, J. Yang, A. A. Wereszczak, H. Wang, and J. Y. Chi, “Temperature Dependent Tensile Fracture Stress of n- and p-Type Filled-Skutterudite Materials”, Sci. Adv. Mater. 3, 1 (2011).

Filled skutterudite materials are very promising for mid-temperature thermoelectric power generation and waste heat recovery, because of their good thermoelectric and mechanical properties. Traditional preparation method needs a very long time annealing (usually 7 to 14 days) to form the right skutterudite phase. The annealing is especially critical for p-type filled skutterudites, since Fe₄Sb₁₂ need filler atoms to enter the cage to form a stable phase. In this work, we prepared Ce/Nd double filled p-type skutterudite material by directly ball-milling alloyed and quenched ingot. The results show that, by breaking the ingot into nano-sized particles, ball-milling greatly reduced the distance which filler atoms need to travel and hence accelerated the phase formation. With appropriate ball-milling and handling, pure p-type filled skutterudite phase can be obtained by hot-pressing the ball-milled powder for just 5 minutes, although the powder is still a mixture of FeSb₂ and Sb phases. The samples prepared by this way have the same high quality as the samples prepared by the traditional way, and show a peak ZT value above 1 at 750 K. This method greatly saves the processing time and is suitable for large scale industrial production. This research was supported by Bosch and the Solid State Solar-Thermal Energy Conversion Center (S3TEC), and Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0001299/DE-FG02-09ER46577 (GC and ZFR).

A model, test setup, and preliminary test results are presented for concentrating solar thermoelectric generators. In these devices, sunlight is concentrated onto an absorbing surface connected to a thermoelectric generator attached to a heat sink. Our previous experiment demonstrated 4.6% solar-to-electricity conversion efficiency without any optical concentration. Optically concentrated solar radiation can create high absorber temperatures with higher conversion efficiency. An efficiency exceeding 10% is possible with current materials. Progress toward achieving this efficiency will be presented.

The demand for primary energy in the world was 12,013 million tons of oil per year in 2007. The average total thermal efficiency of the systems utilizing this fuel is as low as 30%, with about 70% of the heat exhausted to the air as waste heat. It is clear that improving the efficiencies of these systems could have a significant impact on energy consumption. Electricity is a convenient form of energy that is easily transported and stored; thus there are a number of advantages to converting the waste heat emitted from our living and industrial activities to electricity. Thermoelectric conversion is attracting attention because it is the strongest candidate to generate electricity from dilute waste heat sources. Oxide and silicide thermoelectric materials are considered to be promising ones because of their durability against high temperature, cost, no content of toxic elements, and so on. Many types of modules using p-type Ca₃Co₄O₉ and n-type CaMnO₃ have been produced. They show good generation power density higher than 4.0 kW/m². An n-type Mn₃Si₄Al₂ discovered recently shows good oxidation resistance at 973 K in air. Thermoelectric modules have been produced with p-type MnSi_1.7. The silicide module can generate 9.4 W, which corresponds to 2.3 kW/m² at 873 K of the hot side temperature.

As thermoelectric (TE) element length decreases, the impact of contact resistance on TE device performance grows more significant. In fact for a TE device containing 100 µm tall TE elements, the figure of merit ratio (ZT_Device/ZT_Material) drops from 0.9 to 0.5 as the contact resistivity increases from 5 x 10^-07 to 5 x 10^-06 Omega;-cm². To improve and understand the effects of contact resistance on bulk TE device performance, a reliable experimental measurement method is needed. There are many popular methods to extract contact resistance such as Transmission Line Measurements (TLM) and Kelvin Cross Bridge Resistor method (KCBR), but they are only well-suited for measuring metal contacts on thin films and do not necessarily translate to measuring contact resistance on bulk thermoelectric material. The authors present a new measurement technique that precisely measures contact resistance (on the order of 5 x 10^-07 Omega;-cm²) on bulk thermoelectric materials by processing stacks of bulk, metal-coated TE wafers using TE industry standard processes. One advantage of this technique is that it exploits realistic TE device manufacturing techniques and results in an almost device-like structure, therefore representing a realistic value for electrical contact resistance in a bulk TE device. Contact resistance values for different manufacturing processes and an estimate of the accuracy of the measurements will be presented.

Thermoelectric power sources have consistently demonstrated their extraordinary reliability and longevity for deep space missions (67 missions to date, more than 30 years of life) as well as terrestrial applications where unattended operation in remote locations is required. The successful application of high temperature thermoelectric technology has relied on only a few materials, PbTe, GeTe-AgSbTe2 (TAGS) alloys and Si-Ge alloys, identified in the late 1950s and 1960s. We provide a brief overview of key technologies and system approaches that have enabled such an excellent track record, and reflect on some key lessons learned as we consider infusing new higher performance materials into next generation systems. NASA&’s Radioisotope Power Systems Technology Advancement Program is pursuing the development of more efficient thermoelectric technologies that can increase performance by a factor of 2 to 4X over state-of-practice systems. After several years of maturing new high temperature thermoelectric materials, JPL has started developing segmented couple and module technology using high temperature n-type La3-xTe4 and p-type Yb14MnSb11 Zintl materials combined with lower temperature filled skutterudite compounds. Recent performance tests have demonstrated 11 to 15% conversion efficiencies in the 1273 - 473 K temperature range. The stability of the thermoelectric properties of these new materials have been tested successfully for over 13,000 hours under normal and accelerated operating conditions. Single couple performance tests have been conducted for nearly 9000 hours and technical challenges pertaining to the development and system integration of highly reliable high temperature segmented devices are summarized. The potential for application of this promising new technology to next generation space systems is discussed.

Optimally hybridizing photovoltaic (PV) and thermoelectric (TE) has long been quested as an ideal candidate for more efficiently harnessing solar energy. This focuses mostly on partitioning of the solar spectrum into ultraviolet/visible region for PV and infrared region for TE; especially for PV/TE stack approaches, the TE part aims mainly for converting the transmission and thermalization losses derived from a sun-sided PV device. Cost-ineffectiveness to their conversion performances, however, has been a main obstacle which is still far short of competing with not only fossil fuels but also the sole devices of highly efficient PVs. Here we focus on the Seebeck effect, which is the conversion of temperature differences directly into electricity, depending upon the resistance matching in a serially-connected hybrid circuit of PV and TE devices. We demonstrate that a linear power output of hybrid current-voltage operations can be equal to a nonlinear sum of maximum powers produced separately from each PV and TE circuits when the temperature difference (ΔT) between hot and cold sides of a TE device reaches a threshold value, typically 10~20 °C. This feature is based upon the lossless coupling between open circuit voltages of a serial PV-TE circuit in which the internal TE resistance is low enough to convey the amount of photogenerated current without serious degradation of fill factors in the resistance-matched hybrid operation. At ΔT=16 °C, we could obtain the conversion efficiency of 16.3% in serial operation of the PV/TE stack showing the PV conversion efficiency of 12.5%. This result is, in particular, effective for a position-separated, remote serial operation in which PV and TE parts can share the one set of a control unit, dc-dc converter, charging battery pack, and dc-ac inverter for further utilizing exhaust gas in PV-integrated electrical vehicles. The hybrid generation system consists of crystalline Si PV and bismuth telluride (Bi₂Te₃) TE, which were placed in tandem and electrically connected in series. To confirm the lossless power coupling in the hybrid circuit, three kinds of TE modules were used; 1) TE87 represents the internal resistance of TE (R_i) of 8.7 Omega;cm² with the number of thermocouple pairs (N) of 127; 2) TE38 represents the R_i of 3.8 Omega;cm² with N of 31; TE300 is for R_i=30 Omega;cm² with N=127. The I-V characteristics were independently recorded for PV, TE, and hybrid circuits to compare the integration results while understanding the operating status of PV and TE parts in the hybrid circuit. To gain further insight into the behavior of the hybrid circuit, output powers were calculated for individual circuit and hybrid circuit. Our theoretical and experimental results suggest that the resistance matching is of significant importance for optimal operation in a hybrid circuit in which the fill factor of a hybrid circuit and a voltage gain are controllable using ΔT and TE variables such as R_i, N, and Seebeck coefficient, S.

The US Department of Energy initiated the application of thermoelectric generators (TEG's) to vehicles in 1994. This TEG was built by Hi-Z Technologies evaluated on a dynamometer test stand. It was then installed on a fully loaded Heavy Duty Diesel truck on the PACCAR test track and run for the equivalence of 550,000 miles. Today every major automobile manufacturer is investigating thermoelectric applications. The US Department of Energy is supporting the development of production prototype TEG's with teams headed by BSST and GM to integrate TEG's to directly convert engine waste heat directly to electricity in the BMW X6, the Ford's Lincoln MKT and the Chevy Suburban. These TEG's will provide a nominal 5 percent improvement in on-highway fuel economy by allowing the alternator to be downsized by at least 1/3. DOE/NETL conducted a completive procurement for automotive thermoelectric air conditioners/heaters (TE HVAC) development and selected teams headed by Ford and GM to develop this technology. Current air conditioners use the R134a refrigerant gas, which has 1300 times the "Greenhouse Gas Effect" as carbon dioxide (CO2)., the primary "Greenhouse Gas". Approximately 41 Million Metric tons of CO2 equivalent (CO2e) are released to the atmosphere in the US annually from air conditioner compressor seal leakage and frontal collisions wherein the the R134a refrigerant gas containment was ruptured. The TE HVAC's are candidates to eliminate refrigerant gases from vehicles. A problem with maintaining occupant comfort in an electrically assisted vehicle was illustrated by Bob Lutz, Vice Chairman, General Motors , who drove a Chevy Volt in January in Detroit and to obtain occupant comfort had to turn on the 5 kW resistive heater which reduced the battery only propulsion mileage from 40 to 28. Preliminary analysis indicates that with TE HVAC a single occupant can be made comfortable using about 630 Watts. Whereas current compressed refrigerant gas air conditioners typically use 3500 to 4,500 Watts. The TE HVAC uses design advantages afforded by Thermoelectrics as a dispersed or zonal system wherein only the occupants are cooled/heated, not the whole cabin. AsTE HVAC is a DC electrical system it only requires a switch to go from the cooling mode to heating. The Zonal System will consist of a thermoelectric seat, thermoelectric units in the overhead and dashboard, A&B pillars focused on each occupant . There will be a cooling loop with either a dedicated radiator or the engine's radiator. TE HVAC is vehicle specific. In this program they are the Cadillac SRX, the Chevy Volt and the Ford Fusion. The latter 2 will also have TEG's. The Department of Energy has initiated a jointly funded program with the National Science Foundation (NSF) to fund university and industrial teams to develop advanced commercially viable Thermoelectrics for 2nd generation automotive thermoelectric applications. Awards were made to 9 of the 48 universities who, with their industrial partners, responded to the DOE/NSF announcement. In 2011 the Department of Energy with it's partner, the Army's TARDEC conducted a competitive procurement to accelerate scale up and manufacture of advanced automotive TEG&’s. The Teams selected and their approaches will be presented.in Boston.

There is increasing interest in thermoelectric materials motivated in part by recent progress and in part by the potential of these materials in various energy technologies. Thermoelectric performance is a multiply contra-indicated property of matter. For example, it requires (1) high thermopower and high electrical conductivity, (2) high electrical conductivity and low thermal conductivity and (3) low thermal conductivity and high melting point. The keys to progress are finding an optimal balance and finding ways of using complex electronic and phononic structures to avoid the counter-indications mentioned above. In this talk, I discuss some of the issues involved in the context of recent results. These include the surprising doping dependence of the thermopower in PbTe and PbSe, and the interplay between acoustic and optical phonons in PbTe. The potential of some new materials is discussed. This work was supported by the Department of Energy through the Office of Science S3TEC Energy Frontier Research Center and EERE Vehicle Technologies, Propulsion Materials Program.

The thermoelectric transport in bismuth is examined. Although this material has been studied for many years, bismuth is still a great source of new discoveries. The anisotropic transport, Dirac nature of its electronic structure, and the presence of two types of carriers enable us to find some surprising results related to bismuth thermoelectricity. The intricate relationship between the characteristics of the charge and heat transport together with the role of imbedded nanostructures and/or applied an external magnetic field allow us to provide theoretical guidelines for practical advantages and limitations of thermoelectric enhancement in this material. We also demonstrate that an imbalance in the population of the bismuth energy bandstructure can be achieved via a rotating magnetic field in the binary-bisectrix plane. This results in selective carrier filtering and excitations of particular Dirac valleys with specific signatures not only in the charge, but also in the heat transport. These findings suggest novel opportunities for thermoelectricity tuning in a rather wide temperature regime.

Computational models for the transport properties of nanostructured thermoelectric materials predicted vast improvements in the thermoelectric power factor (PF) values over bulk due to discretization of the electron density-of-states function as the result of confinement. We have developed a model that bridges bulk and nanostructure PF data. The model is analyzed in the framework of the relaxation time approximation, considering different scattering mechanisms. The model shows that the PF of nanowires and thin films in fact falls below the bulk value for most of the experimentally-accessible size range. Under the constant relaxation time approximation, universal scaling relations are obtained for all single-carrier semiconductors. An improvement over bulk is only seen for film thicknesses and nanowire widths associated with quantization energies exceeding ~10 times the thermal energy. The power factor increases with size for most of the size-range investigated. With the consideration of specific scattering mechanisms with energy-dependent scattering times, the size-dependence of the PF of thin films and nanowires becomes material-specific. However, we find that the non-monotonic relationship between the thermoelectric PF and the system size is recurring in all systems studied. The effects of size, dimensionality, temperature, doping, impurity scattering, and phonon scattering in single-carrier semiconductors will be discussed.

In this presentation we review recent advances in thermoelectric modules and power generation systems [1, 2]. During the last decade, nanostructured thermoelectric materials have shown improvements in thermoelectric figure-of-merit, ZT. Values from 1.5 to 1.8 have been demonstrated in several systems [1, 2]. Nevertheless, thermoelectric modules with significant improvement in maximum cooling, coefficient of performance, or efficiency, have not been commercialized. In this presentation we describe the efficiency/cost tradeoff in thermoelectric power generation systems [3]. Co-optimization of the thermoelectric module with the heat sink leads to a reduction in the amount of material used in the system and reduces the overall payback time. On the other hand, thermal stresses play a big role in terrestrial thermoelectric systems that are subjected to large temperature gradients and repeated thermal cycles. A new analytic theory is described where the shearing stress can be estimated as a function of the module parameters [4]. We show that there is a significant opportunity to reduce the stress and increase the thermoelectric module lifetime with fractional area coverage of thermoelectric elements in the module. [1] C.J. Vineis, A. Shakouri, A. Majumdar, M.G. Kanatzidis, "Nanostructured Thermoelectrics: Big Efficiency Gains from Small Features", Advanced Materials, Vol. 22, pp. 3970-3980, 2010. [2] A. Shakouri, “Recent developments in semiconductor thermoelectric physics and materials”, Annual Review of Materials Research, July 2011. [3] K. Yazawa and A. Shakouri, "Optimizing Cost-efficiency Trade-offs in the Design of Thermoelectric Power Generators", Environmental Science and Technology, July 2011. [4] E. Suhir and A. Shakouri, “Assembly bonded at the ends: Could thinner and longer legs result in a lower thermal stress in a thermoelectric module design?”, Journal of Applied Mechanics, to appear in 2012

There are numerous potential commercial applications for bismuth antimony telluride BiSbTe thermoelectric bulk alloys for cooling, heating, or power generating application near room temperature, preferably if the dimensionless thermoelectric figure of merit ZT reaches to around 2. In this talk, we summarize our effort in Samsung to reach this goal by commercially-viable and scalable techniques. The ZT value of BiSbTe alloys was enhanced either by enhancing power factor through slight doping/substitution or by reducing thermal conductivity through nanostructuring. The power factor was enhanced by slight doping/substitution by In/Ga. The notable ZT improvement has been achieved by significant reduction of lattice thermal conductivity by forming nanostructured grain boundaries with high-density periodic dislocations. By these approaches combined together, a relatively large amount of BiSbTe alloys was produced with high ZT value higher than 1.50 at around room temperature in lab scale. The main advantage is that these approaches only involves with commercially scalable processes. The results on maximum heat pumping capacity Qc,max measurement of cooling modules made from these materials will be discussed. High-ZT BiSbTe bulk alloy is now commercially-viable technologically, and wider commercialization of thermoelectric BiSbTe materials is closer.

The binary narrow band gap semiconductor PbSe combines several attractive features for potential thermoelectric applications such as a favorable electronic valence band structure that is comprised of two sub-bands with very different effective masses similarly to that of PbTe. The addition of a few percent of alkaline earth selenide into p-type PbSe generates the second phases that produce a reduction of the lattice thermal conductivity compared to pristine PbSe, but without appreciably affecting the power factors. Here we present a systematic study with respect to characterization and thermoelectric effects of p-type PbSe-MSe systems with PECS samples. The electrical conductivity, thermoelectric power, and thermal conductivity as a function of temperature and carrier density were measured. Our data indicate that the addition of alkaline earth selenides leads to an increase in ZT values (~1.2) compared to those of pristine PbSe (~0.95) around 900 K.

Energy is an indispensable part of our daily lives, and about 60% of the energy we produce is lost as waste heat. Thus, thermoelectric (TE) materials are of particular interest as they can tap on these waste heat and convert them directly into electricity. Recent developments in TE materials have been focused on multiphase TE materials, due to the ability to concurrently decrease thermal conductivity and tune electrical properties, resulting in improved TE properties. In this work, melt spinning was employed to produce various compositions of (SnTe)1-x-(SnSe)x (10 < x < 50), which to the best of the authors&’ knowledge have not been reported by other groups before. Due to the metastable nature of the samples formed via this processing technique, a second phase was found to precipitate simply by mechanical grinding and then compaction into a pellet, forming a multiphase composite. Experiments suggested that the mass fraction of the second phase can be controlled by the grinding process, and preliminary data showed that a power factor of about 1 mW/m.K^2 can be obtained at 546 K for one of the compositions, which is a 1.2 times improvement in the power factor compared to pure SnTe phase, and about 65 times improvement compared to pure SnSe phase. The large difference in improvement is due to the high Seebeck coefficient but low electrical conductivity of SnSe and high electrical conductivity but low Seebeck coefficient of SnTe, resulting in low and average power factor for SnSe and SnTe respectively. By combining these two phases, a synergistic effect can be obtained, indicating the importance of the contribution of the second phase in enhancing the power factor. Additional characterization, e.g., X-ray diffraction and electron microscopy, were performed to correlate microstructure and phases present with the TE properties obtained. The ease of fabrication of such multiphase materials is attractive for large scale production.

Single-crystal nanoplates of bismuth telluride synthesized by the vapor solid method have been assembled directly onto a suspended device consisting of two adjacent low-stress silicon nitride membranes, each with a platinum resistance thermometer and two platinum electrodes and suspended by six silicon nitride beams. The device allows for accurate measurement of themoelectrical (Seebeck), electrical, and thermal transport in the nanostructures and quantification of the electrical and thermal contact resistances in the temperature range of 4-500 K and 60-500 K, respectively. Measurements of the thermoelectric properties, particularly the dimensionless figure of merit (ZT), of four Bi₂Te₃ nanoplates and one (Bi_1-xSb_x)₂Te₃ nanoplate, x~0.1, have been conducted, with thicknesses between 7.6-19.6 nm. All five samples show n-type Seebeck coefficient and ZT < 0.25 due to low Seebeck coefficients in these “as-grown” nanoplates. Moreover, the Seebeck coefficient appears to decrease with decreasing thickness, which can be attributed to two degenerately n-type surfaces that have not yet hybridized to form the band gap suggested in theoretical reports.

Extended crystallographic defects, such as dislocations, grain boundaries, and stacking faults, can strongly affect the thermal and electronic transport properties of thermoelectric materials. At present, however, our understanding of the structural details of such defects in thermoelectrics is in its infancy, even for such widely used materials as bismuth telluride. In this presentation, we will discuss our experimental analyses of grain boundary defect arrangements in bismuth telluride. We start with a discussion of low-angle grain boundaries in bismuth telluride. In general, such interfaces can be thought of as ordered arrays of individual crystal lattice dislocations. Our HAADF-STEM observations of a <1,0,-1,0> tilt boundary help to clarify the topological details of the relevant defects in bismuth telluride, including their Burgers vectors and relationship to intergranular misorientation. In contrast to low-angle grain boundaries, in many materials, the structure of high angle boundaries (e.g., with misorientations greater than ~15°) is better considered with reference to specific interfaces arising at crystal orientations and interface inclinations with low energy, singular structures. Thus, as a starting point, we consider the (0001) basal twin in bismuth telluride and show how defects in interfaces vicinal to this relatively simple and low energy interface can be interpreted in terms of interfacial disconnections (i.e. step defects possessing dislocation content). For instance, our ab initio, density functional theory calculations indicate a strong energetic preference for terminating (0001) twins at the Te(1)-Te(1) sites of the crystal structure, a result that is consistent with our HAADF-STEM observations. This energetic preference also imposes strong constraints on the possible disconnection arrangements. Our observations of more complex high angle boundaries also identify faceted step formation on tellurium-terminated planes, suggesting such analyses can be extended more generally in this and related layered tetradymite-type materials.

The creation of nanoscale precipitates via phase separation provides a mechanism for decreasing the lattice thermal conductivity and increasing the figure of merit of some bulk thermoelectric materials, such as PbS-PbTe. It has recently been found that the addition of Na to PbS-PbTe drastically alters the morphology of precipitates in the system. To better understand the phase tranformations giving rise to precipitates in this system, and in particular, the effects of Na doping on these precipitates, we use first-principles density functional theory (DFT) calculations to study the thermodynamics of mixing in PbS-PbTe-Na. We model the PbS-PbTe solid solution with special quasirandom structures (SQS), and treat the Na solubility in PbS and PbTe as dilute. To calculate the dilute-limit solubility of Na in PbS and PbTe, we calculate a large variety of Na, Pb, S, and Te defects in PbS and PbTe, from which we find that Na prefers to substitute for Pb in both PbS and PbTe. Due to the large lattice mismatch between PbS and PbTe, and the anharmonic nature of phonons in PbTe, vibrational entropy is expected to play a large role in the free energy of mixing in PbS-PbTe. As such, we directly obtain the vibrational entropy of mixing for PbS-PbTe through frozen phonon calculations of the SQS&’s. With the free energies of mixing in PbS-PbTe-Na, we calculate the miscibility gap between PbS and PbTe, as well as the solubility limits of Na in PbS and PbTe. We find that vibrational effects play a very important role in the thermodynamics of PbS-PbTe, bringing the calculated miscibility gap into good agreement with experiment. We also find that the solubility of Na in PbTe and PbS depend on the off-stoichiometry of the parent compound and the manner in which Na is added to the system (e.g. doping with Na metal vs. Na2Te).

PbTe is one of the leading thermoelectric materials for power generation in the temperature range 600-900 K. A concurrent optimization of the power factor (PF) and the thermal conductivity (κ) is highly desirable to further improve the ZT of PbTe-based materials. It has been shown that Na doping in PbTe-PbS systems modifies the electronic structure to achieve a high power factor over a wide temperature range while concurrently reducing the lattice thermal conductivity due to shape controlled phase separation on the nanoscale. The complex valence structure of PbTe and other Pb chalcogenides have also been shown to enhance the PF resulting in high ZT. To maximize the synergism between band structure engineering and nanostructuring, here-in we studies the effects of Na doping in a PbTe-Se-S system (PbTe1-x-ySxSey). Transmission electron microscopy (TEM) and local electron atom probe (LEAP) tomography characterization of the nanoscale precipitates show the formation of two nanoscale secondary phases, namely PbSe- rich and PbS- rich, within the Na- doped PbTe matrix. Additionally tuning the location of the valance structure with Se and S substitution for Te results in enhanced power factors over a wide temperature range from 500-800K. These simultaneous enhancements in the PF and reduction of κlatt result in a high ZT for 2% Na-doped PbTe-PbSe 5%-PbS 2% at 800K.

In this work, a comprehensive study of thermoelectric chalcogenide materials is presented and the systematic optimization of n-type Bi₂Te₃, p-type Sbi₂Te₃ and their ternary compounds is performed. Thermoelectric materials are synthesized by potentiostatic electrodeposition on Au/Pt and stainless steel substrates. The influence of the preparative parameters such as the composition of the electrolyte bath and the deposition potential is investigated in a nitric acid solution. A novel deposition method is developed using millisecond potentiostatic pulses. As a post-deposition step, the influence of annealing of the films is investigated. The optimized p-doped (Bi_xSb_1minus;x)₂Te₃ and the n-doped Bi₂(Te_xSe_1minus;x)₃ films are annealed for a period of about 1 h under helium atmosphere and also under tellurium atmosphere at 250°C for 60 h. The use of a potential-controlled millisecond-pulsed deposition method improves both the morphology and the composition of the films. The samples are characterized in terms of composition, crystallinity, Seebeck coefficient, thermal- and electrical resistivity. p-doped pulsed deposited films exhibit Seebeck coefficients up to approximately +160 µV/K (Sb₂Te₃) and +208 µV//K ((Bi_xSb_1minus;x)₂Te₃). For n-doped films, approximately -100 µV/K (Bi₂Te₃) and -130 µV/K (Bi₂(Te_xSe_1minus;x)₃) are achieved. Power factors and ZT values for p-type and n-type ternary alloys of up to 1325 µW/mK² (ZT=0.4) and 825 µW/mK² (ZT=0.3) are realized, respectively. In conclusions, the thermoelectric performance has been improved by more than one order of magnitude in comparison to previously electrochemically synthesized thermoelectric layers. The authors gratefully thank the funding of the German Ministry of Science and Educations (BMBF).

The nanostructuring approach to highly efficient thermoelectrics has produced a paradigm shift and ushered in a new era of investigation for bulk thermoelectrics. The new approach shows considerable promise to enhance the “contra-indicated” parameters of high electrical conductivity and low thermal conductivity. Currently lead chalcogenides hold the record in figure of merit for high temperature power generation applications. This is achieved by introducing endotaxial nanostructures in bulk host materials to significantly reduce lattice thermal conductivity via effective scattering of heat carrying phonon through hierarchical architecture of nanostructured thermoelectrics. Band alignment strategies are then applied to maximize the charge transport and the power factor. In band alignment hole transport is controlled by minimizing band offsets of the valence bands between the host material and the embedded second phases. The smaller valence band offset allows better carrier transmission between two endotaxial components, thus minimizing hole mobility deterioration and allowing a larger power factor to be achieved. The presentation will highlight recent advances in our group and the move toward tellurium free systems.

An ideal thermoelectric (TE) material would be a semiconductor with high electrical conductivity and low thermal conductivity, which is described as an “electron crystal, phonon glass (ECPG)”. ECPG behavior can be achieved successfully using nano-grains to decrease the thermal conductivity. For example, the ball milled nanoparticles are used as building blocks to approach bulk materials with nano-structures. Besides nanoparticles, another nanostructure approach is to use a mesostructured matrix made up of nanocrystalline grains that are assembled as the framework structure to give one more desirable characteristic: meso-pores. Nanocrystalline grains generate more boundaries and are expected to reduce thermal conductivity, and meso-pores are likely to further reduce the thermal conductivity via crystalline wall-pore interfaces. In addition, a continuous mesostructure nanocrystalline framework can maintain the high electrical conductivity. However, studies on such porous TE materials have been limited to theoretical calculations so far and synthetic examples are rarely reported. In this work, we report a novel and simple synthetic strategy to prepare mesoporous self-doped n-Bi2Te3 monoliths. Different from previous reports, most of the porosity in the monolith results from the synthetic mesopores that are created by the hard template instead of inter-grain voids formed during consolidation process. The continuous mesoporous nanocrystalline framework is highly effective for phonon scattering. More specifically, the generated interface and boundaries scatter phonons with a high efficiency, which leads to a substantially reduced thermal conductivity, as low as 1.2 W m-1K-1 at room temperature. This huge reduction compensates for the loss of electrical conductivity caused by the mesopores. A maximum zT of 0.7 in the direction perpendicular to the press is obtained at 480 K, which is comparable with the zT of self-doped, bulk n-type Bi2Te3 at a similar temperature. As the first reported mesoporous monolith, the mesopores in the n-type Bi2Te3 monolith suggest a new viable avenue for the heterostructured synthesis of efficient TE materials.

Chalcogenide and its alloys are the most promising materials for thermoelectric (TE) applications. It is important to understand electrical and thermal stability of TE for reliability because high current density and temperature is the inevitable to TE application. These conditions can lead to current-induced atomic migration because of high mobility of atoms and driving force to migrate. Electromigration (EM), the atomic displacement due to momentum transfer from charged carriers to atoms, can occur at high current density and temperature. Localized compositional variation induced by EM can lead to the failure of the devices by the de-mixing of atoms. In this study, we investigated failure mechanism induced by electric current in the crystalline of Bi₂Te₃ which was intermetallic compound of Bi-Te system. DC current densities of range from 0.17 to 1.17 MA/cm² at 30 to 200 °C were applied to a line-shaped Bi₂Te₃ samples during 30 hours. The electrical resistance was measured during the test. Shape change and compositional variation were observed using SEM and TEM analysis. Three different failure modes could be determined for the current stressing test of BT line specimen; abrupt fail, gradual fail, and no fail by time to failure in resistance and morphological change according to its current density. The sample of high current density (over 1 MA/cm²) failed in short time with largely agglomerated voids formed in its molten state, while the sample of low current density (below 0.5 MA/cm²) exhibits no failure and no voids. Unlike the two failure mode, gradual increase of resistance during current stressing was observed in intermediate current density. Nano-scaled voids (about 10 nm) responsible for the gradual change of resistance were observed in the sample of intermediate current density. In TEM analysis, ratios of local composition of Bi:Te are 53:47 in near voids, 89:11 far from voids, respectively. However, compositional variation along the overall line was only 5 at.%. These results show that composition change dominant localized, rather than generalized. The observed data indicate that the current-induced atomic migration leads to the formation of localized Te-rich phase. In sequence, local melting occurred in Te-rich phase due to its low melting point and results in gradual changes of resistance. Consequently, TE device can be degraded even in their operation conditions of solid state due to the local melting on the compositional variation.

While the type-I clathrates Ba8Ga16Ge30 and Eu8Ga16Ge30 have excellent thermoelectric properties at several hundred degrees above room temperature, a number of obstacles have to be overcome before clathrates can be commercialized. (1) Ge is an expensive element. - Full or at least partial substitutions of Ge by Si or Sn would solve this problem. (2) In the above ternary clathrates it is difficult to tune the charge carrier concentration to optimum performance of both the n- and the p-type leg. - This problem can be overcome by replacing Ga by a transition metal element. (3) Many clathrates can be fabricated in phase pure form only after extensive annealing, which increases the price of the production process. - As recently demonstrated [1], melt spinning can be used as extremely fast and thus energy- and cost-effective alternative method. (4) The solution of problem 1 by Si substitution leads to the undesired effect of enhanced lattice thermal conductivity. - A possible way out is to nanostructure the material. I shall review our activities along all four lines. Financial support from the Austrian Science Fund (FWF project TRP 176-N22) is gratefully acknowledged. [1] ``Method for producing clathrate compounds'', Utility patent AT: 10749 U1 2009-09-05, DE: 20 2008 006 946.7, patent applications US: 12/231,183, JP: 135994/2008.

Clathrates are inclusion compounds in which covalently bonded frameworks encage guest atoms that undergo large thermal displacement amplitudes and effectively scatter phonons. Their low thermal conductivity together with the potential for tuning their electrical properties with composition makes them attractive for a range of applications, including thermoelectrics. Assessment of these materials for potential technological applications requires investigations of the intrinsic properties of a range of compositions, preferably in a single-crystal form, for which the development of novel synthetic routes is necessary. Single-crystal NaxSi136 clathrates were synthesized and characterized for the first time by a two-step process that takes advantage of our crystal growth technique together with the traditional thermal decomposition approach. This approach also allowed for low temperature measurements on polycrystalline NaxSi136 specimens. A metal-to-semiconductor transition was experimentally observed for Na ~ 8, thus opening another potential route towards thermoelectric clathrates research.

The thermoelectric performance of p-type skutterudites continues to lag that of the corresponding n-type materials. Moreover, many of the best performing p-type skutterudite formulations have high weight percentages of rare earth metals. This high rare earth content has the potential risk of high cost and of uncertainly in the supply chain of raw materials, both of which would be destabilizing to a burgeoning thermoelectrics industry. It is possible, however, to synthesize n-type skutterudites without rare earths while maintaining reasonable thermoelectric properties. Recent reports have also indicated high thermoelectric performance in p-type skutterudite materials without any rare earth content. We present the synthesis and high temperature transport properties of a series of rare earth free p-type skutterudites including their dependence on carrier concentration, and we examine the cause of the somewhat low thermal conductivity observed for these materials. We found during this investigation that the onset of bipolar conduction occurs at a rather low temperature and that the onset temperature could be pushed to higher temperature through an increase in carrier concentration. We conclude by examining the cost and performance tradeoffs of these rare earth-free formulations.

Nanostructuring has been the foremost approach for the manufacture of high-performance thermoelectric materials for nearly a decade.1 Nanostructuring is often achieved by harnessing the thermodynamic instability of metastable compositions to produce nano-sized precipitates, long heat-treatments, however, are often required. This study explores a novel technique - attrition-enhanced nanocomposite synthesis - to rapidly produce thermoelectric nanocomposites from metastably filled InxCo4Sb12 skutterudites. Attrition-enhanced nano-composite synthesis has been employed to seed indium-based nano-sized inclusions from the matrix of metastably indium-filled skutterudites:2 The technique exploits both the open cage-like skutterudite crystal structure and the intrinsic metastability of the indium-fillers to rapidly precipitate the interstitial from the crystal lattice; moreover, it is known that the kinetics of precipitation is strongly dependent on mechanical attrition. Since mechanical attrition of skutterudites results in a fine-grained microstructure with increased dislocation and other defect concentrations, it has been theorized that these features are crucial to the enhanced precipitation of the metastable interstitial.2 In this study, optimized techniques have been used to maximize the dislocation concentration in metastably filled InxCo4Sb12 in order to enhance the in situ synthesis of nano-sized indium-based inclusions. A complete analysis of the structural and transport properties led to an enhanced understanding of the efficacy of this technique - generating the genuine prospect of producing high-performance nanostructured thermoelectric materials. (1) Kanatzidis, M. G. Chemistry of Materials 2010, 22, 648. (2) Eilertsen, J.; Rouvimov, S.; Subramanian, M. A. Acta Materialia 2012, 60, 2178.

Lattice phonons have been the important issue for understanding temperature evolution of both thermal and electric transports in terms of electron and phonon scatterings for long years. Recently, the concept of phonon is broadly generalized from a collective mode to a localized one. The former is the typical Debye-mode phonons with a linear k-omega; phonon dispersion relation, while the latter is the Einstein-mode phonons with the same frequency of all oscillators. Both are generally categorized into the harmonic phonons. Recently the phonons, created in cage-structured materials such as clathrates, have begun to be considered to play an important role [1-4] and provide a way of tailoring phonons as the phonon engineering. Thanks to the large spaces inside the polyhedral network crystals that can accommodate atoms, a large degree of freedom in motions of atomic oscillations can be generated and this is recently drawing much attention as a new mode of phonons to be categorized as anharmonic oscillations. Clathrate crystals have nano cage structure consisting of IVth group elements with face shared, and these crystals have sufficiently large inner spaces for atomic elements to be confined. Such endohedral atoms move under the anharmonic potentials made by cages and give rise to unique phonons recently known as rattling phonons. These phonons greatly differ from the conventional lattice phonons and produce exotic electron-phonon interactions [2, 3]. In this talk, we would like to describe how electron-phonon interactions are modified and how heat transport can be influenced by anharmonic phonons. These factors are very important for designing excellent thermoelectric materials, a class of materials that convert heat difference to electric voltage, and this has recently become a subject of intensive scientific researches. We will discuss how these phonons can be tailored by modifying elements constructing the cage. [1] K. Tanigaki, et al, Nature Materials, 2, 653 (2003). [2] Y. Kohama, T. Rachi, K. Tanigaki et al., , Phys. Rev. Lett., 102, 013001-013004 (2009). [3] J.Tang, J-T Xu, S. Heguri, K. Tanigaki et al., Phys. Rev. Lett., 105, 176402_1-4 (2010). [4] J-T Xu, J. Tang, K. Sato, Y. Tanabe, K. Tanigaki et al., Phys. Rev. B, 82, 085206_1-6 (2010).

Thermoelectric materials attract a surge of interest, as they are considered to be used as materials for producing electric energy efficiently from waste heat as future technology and this has become one of the most important topics in this meeting. Thermoelectric efficiency is commonly evaluated quantitatively by the figure of merit ZT=TS^2 σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity. Zintl phase type I clathrates, which are featured by the cage framework mainly composed of Si, Ge, or Sn and alkali metal or alkaline-earth metal elements accommodated inside as gust atoms, show low thermal conductivity caused by the rattling phonon modes of these guests. The fact has been known so far that, in type I clathrates showing off-centered displacement of encapsulated elements such as Sr8Ga16Ge30, κ is suppressed even stronger via scattering of acoustic phonons transferring heats by anharmonic phonons of the rattler. Consequently, further detailed understanding on the anharmonic potential realized in clathrates and their related physical properties as well as the material design for achieving high efficiency have been very interesting research themes. In the present meeting, we will present our recent studies on low temperature specific heat of type I clathrate compounds Ba8Ga16Sn30 and K8Ga8Sn38, being compared to those reported data of Ba8Ga16Ge30 and Sr8Ga16Ge30 elsewhere [1]. The discussion will mainly be focused on the separation of the apparent linear temperature dependent terms between via rattling phonons and via conduction electrons. The electron phonon interaction strength and the tunneling density of anharmonic potentials will be described on a basis of the analyses. [1] J. T. Xu, K. Tanigaki et al, Phys. Rev. B 82, 085206 (2010)

Magnesium Silicide (Mg2Si) is an environmentally friendly thermoelectric semiconductor material. Because the primary constituent elements are non-toxic and readily abundant, Mg2Si has recently attracted much interest in the area of thermal energy-harvesting research. As-deposited Mg2Si (non-intentionally doped) fabricated using various techniques usually shows n-type semiconductor behavior. In this work we have deposited thin-film Mg2Si using sputtering. To generate stable p-type Mg2Si, dopants such as Cr are introduced into the intrinsic film by co-sputtering, diffusion or ion-implantation, depending on the required dopant concentration. The impact of the dopant on the electrical conductivity and Seebeck coefficient of thin-film Mg2Si will be discussed, and a possible theoretical explanation based on first- principle calculations will be presented.

The layered crystal structure of sodium-cobalt oxides (Na_xCo_yO₂, or NCO) allows the combination of good electrical conductivity, through the CoO₂ layers, with low thermal conductivity, due to the Na layers, which makes NCO a good candidate for thermoelectric applications. NCO can exist in several polymorphs, depending on the concentration of Na and Co vacancies; the most promising polymorph for thermoelectric applications is hexagonal γ-NCO. The purpose of this research was to investigate the high-temperature behavior of γ-NCO up to 1100 K, specifically by studying its thermal expansion and phase transitions, by non-ambient X-ray diffraction. The thermal expansion was found to be highly anisotropic, with the c-axis exhibiting negative thermal expansion. The role of Co vacancies on the stability of the γ polymorph will be discussed. This research was supported as part of the Revolutionary Materials for Solid State Energy Conversion EFRC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Measurements were performed at the High Temperature Materials Laboratory, Oak Ridge National Laboratory, which is sponsored by the Vehicle Technologies Program, Energy Efficiency and Renewable Energy Program Office, U.S. Department of Energy

In the present study, a comparison of thermo power of PbTe thin film under thermal and athermal annealing has been investigated. It has been shown that athermal annealing induced by swift heavy ion (SHI) irradiation enhances the thermoelectric properties while it deteriorates with thermal annealing. X-ray diffraction (XRD), Atomic force microscopy (AFM) and Rutherford backscattering spectrometry (RBS) have been used to characterize the sample for phase formation, surface morphology and elemental composition respectively. Electrical conductivity (σ) and thermo power (S) measurement of all samples have been measured using four probe method. The thermo power (S) is enhanced ~40% under ion irradiation at high temperature (~ 525K) whereas it decreases after annealing treatment. These findings may be explained on the basis of density of states enhancement or carrier scattering due to the point defects after SHI irradiation.

Nb doped SrTiO₃ (STNO) is a promising thermoelectric perovskite with a wide band gap and tunable electrical properties. Epitaxial thin films of STNO with 12% and 20% Nb doping have been grown on LaAlO₃ (LAO) substrates using pulsed laser deposition. Oxygen vacancy concentration of the films is varied by growing films in oxygen and argon partial pressures. With Nb doping, the films become degenerate and an enhancement in electrical conductivity together with a decrease in Seebeck coefficient is observed. Undoped films grown in oxygen background gas yielded high Seebeck coefficients (~350 mu;V/K at 300 K), but significantly lower electrical conductivity compared to doped films as well as films grown in argon. For Nb doped films, a linear temperature dependence of Seebeck coefficient with no Heikes limit was observed. Our results bring out the role of Nb dopants and oxygen vacancies in tuning the electron effective mass of SrTiO₃. This work was funded by the faculty initiated collaboration (FIC) grant from KAUST.

SrTiO₃ (STO) is a promising n-type oxide suitable for high-temperature thermoelectric applications [1]. Here, we present the effect of the laser fluence on high temperature thermoelectric properties of La doped STO (SLTO) thin films grown epitaxially on LaAlO₃ <100> substrates by pulsed laser deposition (PLD). In PLD process, the coating of laser window by the ablated species with sequential depositions is a source of uncertainty of actual laser energy at the target. For wide band-gap materials such as SLTO, this coating effect results in a significant drop in energy of photons arriving at the target, but often goes undetected to the bare eye. We show that oxygen vacancies that influence the effective mass (1.88-3.86 m_omicron;) of carriers in SLTO films can be tuned by varying the laser fluence (5-7 J cm^-2 pulse^-1). As the laser fluence increases, the oxygen concentration decreases, resulting in an enhancement in carrier concentration and electron effective mass which determines the thermoelectric power factor of the films. The highest power factor of 0.433 W K^-1 m^-1 has been achieved at 636 K for a film deposited using a higher laser fluence of 7 J cm^-2 pulse^-1[2]. This work was funded by the faculty initiated collaboration (FIC) grant from KAUST. References [1] Hiromichi Ohta, Materials Today 10 (10), 44 (2007). [2] A.I. Abutaha, S.R. Sarath Kumar, and H.N. Alshareef, Appl. Phys. Lett. 100, 162106 (2012).

Electrochemical deposition is a cost-efficient method to grow thermoelectric thin films on large substrates. Therefore, the development and optimization of electrochemical deposition procedures of n-type Bi2Te3 thin films were investigated. The deposition process was developed in collaboration with the Institute of Applied Physics of the University of Hamburg on small scale substrates. For upscaling a temperature controlled electroplating setup for 4-inch substrates with a peristaltic pump for electrolyte circulation during the electrochemical deposition was used. The first results of potentiostatically deposited Bi2Te3 films on 4-inch substrates are shown. Besides their chemical composition, surface properties and layer thicknesses were determined. In addition, simultaneous temperature dependent measurements of Seebeck coefficient and electrical conductivity were made on a measurement platform developed at the IPM. With this measurement setup, in-situ-annealing of Bi2Te3 thin films under online control of the power-factor is possible. Furthermore, the thermal conductivity was determined.

Although it is one of the most important and abundant materials, silicon is unsuitable for applications such as thermoelectrics because of its high thermal conductivity κ. Porous silicon, with its significantly reduced thermal conductivity, could potentially make silicon a strong candidate for thermoelectric applications. Along this line, low κ values were found in silicon nanomesh structures with ~10 nm feature sizes, whereas electrical conductivities were still preserved from bulk silicon in the high-doping range [1]. The observed κ reduction was attributed to the altered phonon bands and the associated phonon confinement in a periodic structure. Although the same conclusion was also reached for much larger feature sizes (>300 nm) [2], periodicity required for phonon confinement was shown to be unnecessary for κ reduction in silicon membranes with randomly distributed pores [3]. This may suggest that diffuse phonon scattering on pore edges, instead of phonon confinement, is still the dominant mechanism for κ reduction in nanoporous silicon. In theory, however, simply considering pore edge scattering of phonons may not be sufficient to explain the reported low κ values [2]. In recent molecular and lattice dynamics calculations, it was pointed out that the amorphization of pore surfaces could also play an important role in κ reduction [4]. Still assuming diffuse pore-edge scattering of phonons, in this work κ values of nano-porous silicon thin films are re-investigated via phonon Monte Carlo simulations with more accurate frequency-dependent phonon lifetimes from recent first-principles calculations on bulk silicon [5]. The influence of amorphous pore surfaces is simply treated as an expansion of “effective” pore sizes. Unrestricted to very small feature sizes (le;10 nm) in molecular dynamics calculations [4], computations are carried out for a much wider range of feature sizes to be compared with existing experimental results. More generally, the influence of pore misalignment, impurity (dopant, isotope) scattering and film-boundary scattering of phonons will also be addressed. References: 1 J.-K. Yu, S. Mitrovic, D. Tham, J. Varghese, and J. R. Heath, Nat. Nanotech. 5, 718 (2010). 2 P. E. Hopkins, C. M. Reinke, M. F. Su, R. H. Olsson, E. A. Shaner, Z. C. Leseman, J. R. Serrano, L. M. Phinney, and I. El-Kady, Nano Lett. 11, 107 (2010). 3 J. Tang, H.-T. Wang, D. H. Lee, M. Fardy, Z. Huo, T. P. Russell, and P. Yang, Nano Lett. 10, 4279 (2010). 4 Y. He, D. Donadio, J.-H. Lee, J. C. Grossman, and G. Galli, ACS Nano 5, 1839 (2011). 5 A. Ward and D. A. Broido, Phys. Rev. B 81, 085205 (2010).

Phase change of the Ge2Sb2Te5 chalcogenide alloy (GST) presents a high electrical resistivity contrast between the amorphous and crystalline phases. This property is used for data storage in phase change memory (PCM) devices, where switching between amorphous and crystalline phases is induced by an electrical pulse that heats the GST above crystallization (~ 150-200 C) or melting (~ 600 C) temperature. Temperature gradients during PCM device operation are very large (~10 K/nm and higher), which results in strong thermoelectric effects. However, there is limited data on high temperature characterization of thermoelectric properties of phase change materials, which is important to understand for PCM device operation. We have performed Seebeck coefficient (S) and resistivity (ρ) measurements of thin film GST with different thicknesses, deposited on silicon dioxide, from room temperature to ~ 600 C, under small temperature gradients. The two parameters are simultaneously extracted from (I-V) measurements obtained using a semiconductor parameter analyzer. The slope and the intercept of the I-V characteristic represent the sample resistance (R) and the Seebeck voltage, respectively. The measured R(T) is then scaled to ρ(T) by the geometry factor obtained from the room temperature resistivity measurement on the film. Details of the measurement techniques and the measured S(T) and ρ(T) data for the GST thin film samples will be presented and discussed. 1. H. Wong et al., Proc IEEE 98, 2201, 2010.

Reverse nonequilibrium molecular dynamics simulations have recently shown that silicon based pseudomorphic heterostructures in the form of Random Multi-Layers (RML) can have lattice thermal conductivities below 0.050 W/m-K. An experimental investigation of this claim as well as an experimental determination of the Seebeck coefficient available from such structures is being undertaken. In particular, RML heterostructures in the Si/Sn system have been produced by room temperature sputtering followed by rapid thermal annealing at temperatures up to 875 K. TEM images indicates no Sn agglomeration in these structures, while optical absorption studies reveal the band gap remains similar to silicon. The thermal conductivity of unannealed samples has been measured to be approximately an order of magnitude smaller than that of a-Si, while Hall effect measurements indicate that the electron mobility is reduced to values of 10 cm²/V-s at carrier concentrations below 10¹⁹ cm^-3. The impact of rapid thermal annealing on these properties as well as heterostructure Seebeck coefficient will be discussed. If the heterostructure Seebeck coefficient can be maintained at values similar to silicon, thin films with ZT greater than three should be possible.

Bismuth telluride based alloys with perfect thermoelectric properties have been widely used in either cooling or power generating applications around room temperature. In this study, the structural and thermoelectric properties were investigated on Bi2Te3-xSex alloy thin films grown at different deposition conditions. Continuous composition-spread Bi2Te3-xSex material libraries were deposited by DC magnetron sputtering from Bi2Te3 and Bi2Se3 targets. The crystalline phase and composition of Bi2Te3-xSex thin films can be validly controlled which confirmed by X-ray diffraction (XRD) and energy dispersion spectrum (EDS). We investigate the thin film structures and the thermoelectric properties at room temperature. The value of x in Bi2Te3-xSex film ranged from 0 to 1.05. It has been observed that the Seebeck coefficient,electrical resistivity and power factor of Bi2Te3-xSex thin films were -63~-70 mu;V/K,2.22~4.54 mOmega;-cm and 0.985~2.186 mu;W/K2cm, respectively. The power factor of Bi2Te3-xSex (2.18 mu;w/K2cm) is higher than Bi2Te3 (1.4 mu;w/K2cm), even though the Seebeck coefficient of Bi2Te3-xSex (-70.8 mu;V/K) is slightly less than Bi2Te3 (-96.6 mu;V/K). Consequently, the maximum power factor of the films was located at x = 0.097.

The focus of this talk will be on our work using ion beam to change the thermal and electrical properties of thin films, during the past seven years. The presented results will be(1,2) mostly on measurements of the thermoelectric properties of ion beam modified thin films with specific focus on properties of the interacting nano-materials. Some thin films, such as SiGe, after bombardment, showed reduction in their thermal conductivity and increased Seebeck Coefficient and enhanced figure of Merit at 300K to 350K. Similarly, the thin films containing pseudo-crystals of nano-materials1, produced from energy deposited by ionization to produce quantum dots or nano-structures, showed enhanced figure of merit above 3.5(2) at 300K to 350K. The interacting nanocrystals enhance the electrical conductivity, measured using cross-plane conductivity, reduce thermal conductivity, measured using 3Omega; technique, and increase the Seebeck coefficient, resulting in room temperature highly efficient thermoelectric thin films(1,2): Periodic nanolayers of SiO2Ag /SiO2Au containing Au and Ag nanocrystals, periodic nanolayers of SiO2/SiO2Au containing Au nanocrystals and bulk SiO2Au containing Au nanocrystals with high volume fraction. Theoretically, the regimented quantum dot superlattice/ pseudo-crystals consisting of nanostructures of any materials produces new physical properties such as new electrical band structure, phonon mini-bands, as well as improved mechanical properties. A proper choice of nanocrystals and host results in production of a highly efficient thermoelectric generator (TEG) with efficiencies as high as 30%, corresponding to figure of merit above 4.0. In addition to above, such systems are in a unique position to be used both for electrical generation from heat, and/or other forms of radiation, as well as cooling the structures, thus enhance the applicability for hybrid power systems. The interaction of nanostructures results in phonon mini-band formation, reducing the thermal conductivity, while increasing the electrical conductivity, resulting in the synthesis of a TEG with much higher efficiency than reported to this date. We will review a series of materials selected for investigation, some operating at temperatures around 300K and some at about 1000K. 1- Patents Awarded. 2- Patent Pending

Mg-Si thin films were fabricated on glass, Si(100), Si(111), and polycrystalline Al2O3 substrates by RF magnetron-sputtering deposition using an elemental composite target composed of a Mg disk and Si chips. The effect of deposition condition such as Mg/Si target area, substrate temperature, sputtering power, Ar gas pressure, and substrate were investigated. The relationship between the deposition conditions and the structural and electrical properties has been investigated in detail. By controlling deposition condition, pure-phase Mg2Si polycrystalline films were successfully fabricated at room temperature. The crystalline orientation of the films was strongly influenced by the Mg/Si elemental ratio in the films. The electron concentration of Mg2Si films drastically increased by impurity doping with Al and Bi.

In times of dwindling resources, the conversion of waste heat into electricity on a large scale is an inevitable step to overcome the world's energy problem. Thermoelectric devices can be used to convert waste heat into electric energy. Copper oxide is a promising material for thermoelectric applications since it is a binary p-type semiconductor and stable up to high temperatures, environmentally friendly and available in abundance. We investigated and compared the thermoelectric properties of thin films of the three different phases of copper oxide CuO, Cu₂O and Cu₄O₃. The samples were fabricated by rf-sputtering on glass substrates with varying oxygen flow. The electric conductivity σ and the charge carrier concentration were determined using Hall-effect measurements. In conjunction with measurements of the Seebeck-coefficient the Power factor of the samples was estimated. Using the 3-Omega method the thermal conductivity of the samples was measured.

Si has attracted big attention as an environment-conscious thermoelectric material. While Si shows the high power factor, there is a problem of the quite high thermal conductivity. In recent years, it was reported that the nanostructured Si has the extremely low thermal conductivity. However, the power factor is slightly deteriorated because they include low-density regions and oxide films at the nanocrystal interface. In this study, we focused on Si and metal-silicide nanocomposite thin film, which has nanostructures without low-density or oxide regions. Si and metal-silicide nanocomposite thin film is fabricated by depositing amorphous film and heat-treatment for the crystallization. The nanosized grains indicate the strong phonon scattering at grain boundaries. Additionally it has the continuous interface without impurities. Previously in Si-MoSi2 nanocomposite thin film, nanosized grains has been successfully formed and the thermal conductivity decreased to κ=2.2 Wm-1K-1 (@300 K). This Si and metal-silicide nanocomposite thin film can be fabricated by using various metal-silicides. We selected Ti as the metal and prepared Si-TiSi2 nanocomposite thin film. Si and Ti were deposited on a SiO2 substrate with the sputtering method using TiSi12 target prepared by SPS. Ar gas pressure and plasma power were controlled appropriately at the sputtering. Next, amorphous Si and Ti were crystallized by annealing at 1073 K,1173 K and 1273 K. The film thickness, crystallinity and grain size were evaluated by the FE-SEM. The grains of several dozen nanometers were observed.

Bi2Te3 is narrow band-gap semiconductor and has been widely used as thermoelectric materials due to its high Seebeck coefficient and low thermal conductivity. In this study, Bi2Te3 was deposited on SiO2/Si substrates by pulsed-plasma-enhanced metal organic chemical vapor deposition at low-temperature. The chemical composition of the films was controlled by varying the precursor injection times of Bi and Te. Annealing was performed in a N2 atmosphere at different temperatures for a given time of 120 min. The evolution of film properties over annealing time is also studied at single temperature. The grown films were characterized by using X-ray diffractometry (XRD), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The thermoelectric properties, i.e. electrical conductivity, carrier concentration, carrier mobility, Seebeck coefficient and thermal conductivity, were determined at room temperature before and after samples experienced annealing.

In the previous study, we fabricated and examined a series of multiphase type composites constructed of Nb-doped SrTiO3 / TiO2 fine particles1). The composites were prepared via two processing steps: (1) precursor oxide preparation by wet processes; (2) sintering by using spark plasma sintering (SPS) apparatus. The composition of the composites and the sintering temperatures were selected in a region where a perovskite SrTiO3 and a rutile TiO2 phases coexist in stable. Such multi-phase composite was found to be effective to reduce the thermal conductivity, contributing good thermoelectric performance, e.g. ZT=0.24 at 600°C. However, this value is not satisfactory for practical uses at lower temperatures. Higher electrical conductivity at lower temperature region is required as well as larger Seebeck coefficient. Here, we have conducted a study to find an effective way to enhance the TE performance of the composites. We examined the effect of addition of titanium nitride in the SPS sintering process. The precursor oxide powder containing Ti, Sr and Nb in proper proportion prepared by the same wet processing was mixed with 5 wt% of TiN powder, followed by the SPS sintering under a pressure of 50MPa at 1300°C in vacuo for 5min. Composite oxide partly containing nitrogen was obtained, showing a relatively high electrical conductivity especially at low temperatures below 300°C. On the other hands, Seebeck coefficient and thermal conductivity were not affected by TiN addition, resulting higher ZT values at 300°C. The highest value was found to be 0.38 for the sample with a metallic molar ratio of Sr:Ti:Nb=50:80:20. Reference: 1)K. Fuda, et.al., Mater. Res. Soc. Symp. Proc. Vol. 1166, N03-05,(2009,Sanfrancisco)

Abstract: Double doping in PbSe with Al and Cl has been achieved by ball milling and hot pressing. By tuning the composition of Al and Cl in Pb and Se sites, respectively, high electrical conductivity and Seebeck coefficient are simultaneously achieved, which are the results of resonant states in the conduction band for the enhancement of local density of states around Fermi level by Al doping and increase of carrier concentration by Cl doping. Combined with the increased phonon scattering induced low lattice thermal conductivity by ball milling and hot pressing, a figure of merit > 1.4 at about 823 K has been obtained in Al0.01Pb0.99Se0.998Cl0.002.

The rocksalt-structured PbSe is of considerable scientific interest as a thermoelectric material for high-temperature power generation. The thermoelectric properties are strongly related to the shape and the widths of bands near the Fermi level. In this work we focus on the valence band structure of PbSe by a synergetic study of the optical and transport properties of the p-type Pb_1-xNa_xSe system, with 0 le; x le; 0.04, yielding carrier densities in a wide concentration range 10¹⁸ - 10²⁰ cm^-3. The room temperature susceptibility effective mass of holes, obtained by the analysis of the infrared reflectivity spectra, was found to increase with increasing hole density, demonstrating that the valence band is non-parabolic. The optically extracted values are compared with the density of states effective mass, obtained by the room temperature Seebeck coefficient measurements and the band parameters as the band degeneracy and the band anisotropy factor are determined. The temperature dependence of the Hall coefficient R_H revealed gradually increased values reaching a maximum at about 650 K, situation that may be attributed to carriers transferring from a light-hole to a heavy-hole band. The results are discussed within the framework of the Kane and the two-valence-band models.

TiS_2-x crystal with a layered structure is a prominent candidate for a n-type thermoelectric material because of its low thermal conductivity and high concentration of charge carrier. The synthesis of TiS_2-x single crystal via chemical-vapor-transport with iodine as a transport agent, and characterization of its thermoelectric properties were reported in the former literature [1, 2]. In this study, we synthesized TiS_2-x crystal by annealing TiS_1.80 powder in sulfur vapor. Typical procedure was summarized as the follows. Commercial TiS_1.80 and sulfur powder were put in a silica ampoule (inner diameter: 8mm; length: 370 mm). Then the silica ampoule was sealed under vacuum (<1.0 × 10^-4 Pa). It was heated for 2 ~ 7 days in an electric furnace under a temperature gradient of 6.1 K / cm. The mixture of TiS_1.80 and sulfur powder was put on the hot zone of the ampoule (1173 K). The chemical composition of TiS_2-x crystal was estimated from the mass change caused by the oxidation of the sample into TiO₂ at 1073 K [Saeki&’s Method]. Our results indicated that the growth rate of TiS_2-x crystal depended upon the partial pressure of sulfur dimmer S₂ (P_S2) [3]. In the case of P_S2 = 14 atm, plate-like crystals with dimensions of several millimeters were observed on the cool zone of the ampoule, 2 days later, while TiS_1.80 powder on the hot zone was sublimated. Therefore, mass transfer occurred from the hot to the cool zone of the ampoule under high partial pressure of sulfur dimmer. However, under P_S2 = 6.6 atm, the TiS_2-x crystals were not observed 7 days later. The function (T vs P_S2) in equilibrium with TiS_2-x is represented as logP_S2 = A - 2ΔH⁰/RT, where A, ΔH⁰ and R were fitting constant, enthalpy of the reaction; “S _{in lattice} = sulfur vacancy + 1/2S_2(g)” and gas constant. In order to obtain A and ΔH⁰, the partial pressure of S₂ in equilibrium with TiS_2-x has been measured by using Bourdon gauge technique. This relationship indicated that under P_S2 = 14 atm TiS_1.97 and TiS_1.93 were formed at 946 (cold zone) and 1173 K (hot zone), respectively. The Seebeck coefficient of TiS_1.97 (5 × 4 mm) was measured by using the scanning probe type apparatus. The Seebeck coefficient(S) in c-axis direction of the single crystals was estimated to be -111 mu;V/K. [1] E.Logothetis et. al.: Physica B,99 (1980) , 193. [2] H. Imai et al., Phys. Rev. B, 64(2001), 241104. [3] The partial pressure of S2 was calculated by Wakihara&’s method. M.Wakihara et. al.: J. Less-Common Metals, 105(1985), 311.

Magnesium silicide (Mg2Si) has been identified as a relevant thermoelectric material that covers the temperature range from 500 to 800 K, and one that fits well with the operating temperatures of automobile exhausts, incinerators and boilers. Although thermoelectric devices have obvious merits in terms of power generation, one of the reasons why thermoelectric devices are not more widely used at present is that the cost-per-watt of thermoelectric power generation has been too high to allow it to displace existing technologies. Mg2Si has significant advantages for automobile applications in terms of both lower raw materials costs due to the abundance of its constituent elements associated with no national risk to material supply, and its lighter weight than other dominant TE materials. It is necessary to dope the raw material in order to optimize the thermoelectric properties of Mg2Si for practical applications, and typical device operating temperatures require high stability from any substitutional elements used as dopants. Aluminum (Al), Bismuth (Bi) and Antimony (Sb) are well known as n-type dopants for Mg2Si. First principle calculations show Sb to be a more stable dopant in substitutional Si-sites in Mg2Si compared with other elements. It is well known that doping Mg2Si with appropriate impurity elements can enhance the TE performance, increasing the power factor or decreasing the thermal conductivity. In the case of Bi doping, ZT ~1.1 has been obtained, while the doped matrix was found to deteriorate rapidly at elevated temperatures. Al-doped Mg2Si is easy to process but gives only a moderate ZT of ~0.8. The current prominent dopant for Mg2Si in terms of good power factor and thermal conductivity is Sb, even though it is toxic and Sb-doped Mg2Si is difficult to sinter. To further elevate the TE capability of Mg2Si, it needs to be doped, and typical device operating temperatures require that any element used as an impurity be highly stable and electrically active in substitutional sites. Stable and substitutional impurity elements in Mg2Si are needed to ensure advancement of the TE characteristics and long lifetime operation at elevated temperatures. In our past experiments, it was revealed that doping Mg2Si with Sb contributed to a decrease in thermal conductivity and an increase in electrical conductivity, while doping it with Al exhibited an increase in electrical conductivity, and doping it with the isoelectric impurity Zn indicated a decrease in thermal conductivity. In order to examine the effects of co-doping with these impurities, samples of Mg2Si doped with Sb/Zn, Sb/Al and Sb/Al/Zn were fabricated using an all-molten polycrystalline synthesis process at 1423 K. The ensuing specimens were examined with regard to their power factor, thermal conductivity as a function of temperature, and their durability over several hundred hours at the operating temperature.

Zintl compounds have been recognized as promising candidates for thermoelectric applications due to their favorable combination of electronic and thermal transport properties. Recent research has revealed several such examples, including Yb14MnSb11, EuCd2Sb2, and Ca5Al2Sb6. In this study, a new Zintl compound Rb2Cd5As4 has been synthesized from high-temperature reaction and its structure has been established by single-crystal X-ray diffraction. This compound crystallizes in an orthorhombic structure with space group Cmcm (No. 63) and cell parameters: a=12.432(4)Å, b=7.587(2)Å, c=12.507(4)Å, and V=1179.7(7)Å3. In the structure, Cd and As form CdAs4 tetrahedra, which are further interconnected into a complex 3-dimensional network by sharing common edges and corners, and Rb cations reside in the channels formed by this polyanionic network. Electronic structure calculations confirm the Zintl formalism of this compound. Thermoelectric property measurements have been conducted on pressed pellets of the polycrystalline sample. The solid solution Rb2Cd5(As,Sb)4 has been synthesized and studied, too, which demonstrates that such an approach could be an effective way to fine-tune the transport properties.

Oxide thermoelectric materials have been studied actively by many researchers. However, there are few reports for a simple transition metal oxides such as CuO and ZnO. Ohtaki et al. reported that ZnO showed the high thermoelectric performance [1]. The improvement of the electrical conductivity of CuO by substituing Al or Li for Cu-site was already reported by Suda et al. [2]. We have studied the potential of CuO for the application as thermoelectric materials, because it is inexpensive, rich and non-toxic. Although CuO has a high thermoelectric power S, an electric resistivity ρ and a thermal conductivity κ are also high. Therefore, we attempted to introduce the carriers into CuO by doping a univalent or a trivalent ion in order to solve the problem mentioned above. The introduction of the alkali metals (Li, Na, K), which acts as the univalent ion, can result in the decrease of ρ and κ, and then the highest dimensionless figure of merit ZT = 3.2×10^-2 at 1247 K was achieved for Cu_0.97Li_0.03O [3]. In this study, to obtain the further reduction in κ, we examined the substitution effect of Ni for the Cu-site on the thermoelectric properties. A peak of κ drastically decreased from 1315 mW/cmK for CuO at 68 K to 918 mW/cmK for Cu_0.95Ni_0.05O at 56 K. This resulted from the lattice distortion caused by a difference in ionic radii between Cu (0.73 nm) and Ni (0.69 nm). We also revealed that κ additionally decreased by substituting both Ni and Li ion for the Cu-site. As a result, Cu_0.92Ni_0.05Li_0.03 showed ZT = 4.2×10^-2 at 1247 K, which was comparatively high among p-type oxide thermoelectric materials. In the presentation, we report the improving effect of the thermoelectric performance on the doped metal ion other than Ni for Cu-site, and the carrier number and mobility in the oxide materials. References [1] M. Ohtaki et al., High-temperature thermoelectric properties of (Zn_1-xAl_x)O, J. Appl. Phys. 79 (1996), 1816-1819 [2] S. Suda et al., The Effect of Atmosphere and Doping on Electrical Conductivity, Jpn. J. Appl. Phys. 31 (1992), 2488-2491 [3] N. Yoshida et al., in preparation

The discovery of colossal enhancement of thermoelectric figure of merit in bismuth-based chalcogenide nanostructures has identified them as potential candidates for thermoelectric applications. Especially one shot facile synthesis of p-type bismuth antimony telluride which has been reported to show a large thermoelectric power around the room temperature still remains as a challenge. We report a surfactant-assisted wet chemical synthesis of nanostructures of single crystalline ternary Bi_2-xSb_xTe₃ (0.5 le; x le; 1.5) alloy by a microwave-assisted method. The powder X-ray diffraction confirmed formation of single phase ternary alloys. The atomic percentage of ternary compositions is also confirmed using energy-dispersive spectroscopy (XEDS). Transmission electron microscopy demonstrated single crystalline nature of the hexagonal flakes. The thicknesses of the flakes were determined using atomic force microscopy. Different morphologies have been observed in scanning electron microscope while varying the tellurium-source precursor from telluric acid to tellurium metal. The hexagonal flakes with cross section of around 1 mu;m, with thickness around 50 nm have formed using telluric acid while octahedral crystals of nearly 20 nm size have been found using the highly pure Te-metal with tri-octyl-phospine as the tellurium sources. The effect of temperature and microwave doses on morphologies is also investigated in this work. The difference in morphology observed in this two microwave assisted synthesis procedure is advantageous for tuning the thermal conductivity of nanostructure and thereby enhancing the thermoelectric figure of merit.

ZrNiSn samples were prepared by a time-efficient levitation melting and spark plasma sintering procedure. High-resolution synchrotron radiation powder X-ray diffraction shows that a single phase half-Heusler compound has been obtained. Rietveld refinements were carried out for both unannealed and annealed ZrNiSn samples to study the possible structural disorders. It is found that the generally accepted Zr/Sn antisite defects are not likely to exist. Instead, the refinements revealed interstitial fractional occupancy of Ni on the (½, ½, ½) site, which is normally empty in the half-Heusler phases, but filled in full Heusler materials. The electrical conductivity and Seebeck coefficient from 300 to 900 K of the unannealed and annealed ZrNiSn displayed no obvious distinction, and the room temperature electrical resistivity and absolute Seebeck coefficient of the annealed ZrNiSn even decreased slightly compared to those of the unannealed one, which implies no obvious Zr/Sn disorder transition during the annealing procedure. A slight increase in the thermal conductivity was observed after a long time annealing, possibly due to reduced Ni atomic disorder.

Ruddlesden-Popper type (RE, Sr)_n+1Co_nO_3n+1 (RE=rare earth elements) has a layered structure consisting of a conducting (RE, Sr)CoO₃ and insulating (RE, Sr)O layers; the number of the (RE, Sr)CoO₃ layer between the (RE, Sr)O ones increases with increasing n. Although it is difficult to fabricate (RE, Sr)_n+1Co_nO_3n+1 with the same RE for the different n, we succeeded in preparing the RE=Eu and Gd samples for n=1, 2, and infin; and various Sr concentration. Since a Co ion in this system is known to take a different valence depending on n and the Sr concentration in terms of the charge neutrality, the thermoelectric properties should be evaluated using the samples with the same average valence of Co ion. We measured the Seebeck coefficient S and electrical resistivity ρ, and then evaluated the power factor P (=S²/ρ) for the samples with the average valence of Co³⁺, such as RESrCoO₄ (n=1), RE₂SrCo₂O₇ (n=2) and RECoO₃ (n=infin;). Both Seebeck coefficient and resistivity of Gd₂SrCo₂O₇ were smaller than those of GdSrCoO₄ for the whole temperature. On the other hand, The Seebeck coefficient of GdCoO₃ was rather larger than that of other two samples below 600 K. Although the resistivity of GdCoO₃ was also large below 400 K, it rapidly decreased with increasing temperature above 400 K which originated from the spin state transition of Co³⁺ from the intermediate spin state to the high spin state, and finally was smaller than that of other two samples. The maximum of the power factor of GdSrCoO₄, Gd₂SrCo₂O₇ and GdCoO₃, respectively, were 1.2×10^-5 W/K²m at 1250 K, 1.5×10^-5 W/K²m at 740 K at and 1.4×10^-4 W/K²m at 740 K. For the RE=Eu samples, the similar results were obtained. We discuss the thermoelectric properties of (RE, Sr)_n+1Co_nO_3n+1 (RE=Eu, Gd) system in view of the structure, spin state of Co ion and carrier doping.

When subject to a change of electric field, the dielectric materials will undergo a polarization change, leading to a change in entropy and thus temperature, which is defined as electrocaloric (EC) effect. Compared with the traditional vapor-compression method, cooling with EC effect is more efficient and environmentally friendly. The poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) based ferroelectric and relaxor materials have been proved to be good EC materials. To further enhance the EC effect in ferroelectric relaxor terpolymer poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)), composites such as polymer-polymer blends and nanocomposites filled with inorganic nanoparticles are fabricated and investigated. It is found that the addition of small amount of P(VDF-TrFE) can increase terpolymer&’s crystallinity and enhance its relaxor behavior through interface couplings. Similar results are also observed in the nanocomposites. The increased crystallinity and enhanced relaxor behavior together result in enhanced electrocaloric effect. For example, a ~30% increase in the adiabatic temperature change over the entire experimental temperature range (293K~333K) is observed in the blends with 10 wt.% P(VDF-TrFE). Moreover, the ferroelectric-paraelectric transition for P(VDF-TrFE) copolymer in the blends is not observed, indicating that the ferroelectric response of the crystalline phase of the copolymer is strongly influenced by the defects, i.e. the bulky CFE, in the terpolymer through the interfaces in the blends. The interface couplings with the terpolymer convert the nano-crystallites of the copolymer from a normal ferroelectric to a relaxor ferroelectric. The blends system provide a model to study how the random defects in the terpolymer influence the polarization response in the copolymer and consequently ECE in the blends.

Beta-gallia rutile intergrowths, a homologous series expressed as Ga4Tin-4O2n-2, are being investigated for high-temperature thermoelectric applications. Their versatile chemistry and natural superlattice structure provides an excellent opportunity optimize and decouple the electrical and thermal transport properties. In this study, solid state and combustion synthesis methods were used to make powders with a range of Ga/Ti ratios. Powders were consolidated using spark plasma sintering and conventional sintering under oxidizing and reducing atmospheres. Electrical conductivity and Seebeck coefficient were measured simultaneously from room temperature to 1000oC. Thermal conductivity was measured as a function of temperature using the laser flash method. The role of Ti reduction on the observed thermoelectric properties will be discussed.

Our team has recently shown that layered oxychalcogenide materials, with general formula RCuChO (with R a trivalent cation and Ch a chalcogen element) are very promising p-type thermoelectric materials [1-5] with a thermoelectric figure of merit ZTsim;1 around 600°C [5]. These materials, with parent compound BiCuSeO, share the same layered crystal structure as the well-known 1111 iron-based superconductors, with an oxide Bi₂O₂ charge reservoir layer and an intermetallic covalent Cu₂Se₂ layer. They are moderate (Bi) to large (rare-earth) band gap semiconductors, and they can be easily hole doped by substituting A²⁺ on the Bi site [1, 3-5] or by Cu vacancies [2] to optimize the charge carriers concentration. Although their thermoelectric power factor S²σ keeps moderate due to the low holes mobility, intrinsically very low lattice thermal conductivity values, of the order of 0.3 W.m^-1.K^-1 at high temperature, lead to high ZT values around 600°C that could compete with the best p-type materials in this medium temperature range. The most interesting characteristic of the layered oxychalcogenides is that they can constitute a bridge between oxides and intermetallics by combining the main advantages of distinct classes of materials, with the moderate electrical resistivity and good thermopower of chalcogenide semiconductors, the low thermal conductivity of layered compounds, and a thermal stability under air which is strongly enhanced as compared to intermetallic materials due to the oxide layer. I will present the thermoelectric properties of these materials and how they can be explained by the crystal structure and the electronic band structure. I will also discuss their potential in medium temperature (T_hot sim; 500-600°C) power generation modules as compared to both oxides and intermetallics, and last, I will show that there is still a large place to improve their thermoelectric properties, especially the power factor. Ref: 1. Zhao L.D. et al, Appl. Phys. Lett. 97, 092118 (2010) 2. Liu Y. et al, JACS 133, 20112 (2011) 3. Zhao L.D. et al, Energ. & Environ. Sci. Submitted (2012) 4. Barreteau C. et al, Chem. Mat. Submitted (2012) 5. Li J. et al, to be submitted (2012)

In order to look for new performing thermoelectric materials, discovering new-materials and /or re-visiting known compounds lead to an intensive research around the world. According to their chemical stability at high T (up to 1200K for some of them) and especially to their robustness against oxidation, metal transition oxides with layered structure such as Ca3Co4O9, a “misfit” cobaltite, possess attractive properties. Nevertheless, their chemical bonds being more ionic than, for instance, those of chalcogenides, their too high electrical resistivity values are responsible for rather poor power factor values (PF ~few 10-4Wm-1K-2, with PF=S2/ρ where S and ρ are for the Seebeck coefficient and the electrical resistivity, respectively). This difference between chalcogenides and oxides will be illustrated by comparing Cr-based ACrX2 layered compounds (X=O, S, Se) which structure can be described as a 1:1 regular staking of CdI2-type CrX2 slab with a layer of A+ cation. In the case of X=Se, one advantage of this 2D structure deals with the impact on the thermal conductivity induced by disordering of the A cations over the tetrahedral crystallographic sites (zT=1 at 800K for AgCrSe2 [1]). A similar liquid type of thermal conductivity ( =0.9 Wm-1K-1 at 800K) has been recently reported in Cu2Se [2]. The Cr chromium sulfides such as Cr2S3 provide also a class of materials where the structure can be described as the 1:1 intergrowth of CrS2 CdI2-type layers bridged together by partially occupied layers of Cr in sixfold coordination. Interestingly, the sequence of these Cr deficient layers along the stacking direction can be changed by rapid cooling during the SPS synthesis leading to a κ value of only κ =2Wm-1K-1 at 700K in Cr2S3 [3]. A comparison will also be made between the latter and Cr5S6. Finally, the hollandites and pseudo-hollandites (sulfides) compounds, which structure exhibits a marked 1D feature, will also be compared to illustrate the major differences in their thermoelectric properties. This opportunity of comparing a sulfide with an isostructural selenide will be given in the case of the AxCr5X8 compounds. [1] F. Gascoin and A. Maignan, Chem Mater 23, 2510 (2011) [2] H. Liu et al, Nat. Mater, DOI: 10.1038/NMAT 3273 [3] A. Maignan, Y. Bréard, E. Guilmeau and F. Gascoin, J. Appl. Phys. (in press)

Perovskite oxides display a rich variety of electronic properties as metals, ferroelectrics, ferromagnetics, multiferroics, and thermoelectrics. Due to their diverse range of properties, temperature stability, and robust chemistry, perovskite oxides have garnered interest from the scientific community for potential application as thermoelectric materials. The cross-plane electron filtering transport in metal/semiconductor superlattices provides a potential technique to increase the thermoelectric figure of merit (ZT)[1]. P-type La_0.67Sr_0.33MnO₃(LSMO)/LaMnO₃ (LMO) metal/semiconductor superlattices deposited on 100-strontium titanate (STO) substrates by pulsed laser deposition (PLD) have been investigated. X-ray diffraction and TEM indicate that the superlattices are epitaxial and pseudomorphic without any obvious sign of interlayer diffusion. Preliminary work focused on cross-plane transport of high resistivity p-type LSMO/LMO superlattices. The high resistivity helped suppress electrical and thermal parasitics in cross-plane transport measurements, thereby allowing interpretation of measurements of thermionic transport, barrier height and lattice thermal conductivity. LSMO/LMO superlattices exhibited substantially lower thermal conductivity (0.89 W/m-K) than those of the constituent materials, which indicates that cross-plane phonon scattering reduces the lattice contribution to the thermal conductivity. The extracted cross-plane conductivity of the superlattice structure from I_V_T measurements of etched pillars suggests a contribution from thermionic behavior, and the extracted effective barrier height of 300 ± 15 meV is consistent with the theoretically expected LSMO/LMO Schottky barrier height (Phi;_B) of ~300 meV at 300K. In spite of the suppressed thermal conductivity, the ZT achieved is low due to a low power factor (S2σ) that is a consequence of the high resistivies of the constituent materials combined with a high barrier height relative to kT at room temperature. Current work is focused on depositing low resistivity heterostructures to evaluate the potential of these oxide superlattices for thermoelectric applications. LSMO and LMO films were deposited under conditions that yielded a 100x increase in electrical conductivity, approaching the conductivity of a good thermoelectric material (~1000/Ohm-cm). Preliminary measurements of cross-plane thermal conductivity indicate that the suppression of thermal conductivities due to the interfaces is preserved. The cross-plane electronic transport properties of low resistivity LSMO/LMO superlattices will be discussed in light of the prior measurements of the high-resistivity materials, and the potential for tuning this system or similar perovskite oxide superlattices for applications as thermoelectric materials at moderate temperatures will be evaluated. [1] A. Shakouri et.al., APL 1997

The heavy band feature in 3d p-type skutterudites causes strong electron-phonon interactions, and reduces the power factor. We present band structures of Ru- and Os-containing skutterudites, which show that valence band near the Fermi levels composed of 4d or 5d states is much lighter. In these skutterudites, high Seebeck coefficient can be achieved at low hole concentrations, while the corresponding density of states at the Fermi level as well as electron-phonon interactions, are greatly reduced. We demonstrate that LaIr₁Os₃Sb₁₂ could possess sufficiently high Seebeck coefficient, and its power factor at high temperatures is estimated to be over 50 mu;W cm^-1K^-2. Fe-substitution for these light VB skutterudites should provide additional degrees of freedom in adjusting d state characteristics and the density of states at the Fermi level, and balance the cost of materials as well.

Doping the SrTiO3 cubic perovskite lattice has been demonstrated as an effective mechanism to improve its thermoelectric performance. However, a more pronounced increase in the power factor and/or decrease in lattice thermal conductivity is still necessary in order for this oxide thermoelectric material to compete with other high-temperature thermoelectrics. In this study we present the first report on the electronic transport properties of Pr-doped SrTiO3 ceramics and show that Pr-doping can bring about additional enhancement in the thermoelectric performance of doped SrTiO3. Samples were synthesized using a combined solid-state reaction and spark plasma sintering (SPS) technique to achieve high density polycrystalline bulk materials. Through doping with Pr and optimization of the oxygen deficiency using various process parameters, the charge-compensating mechanisms were modified to benefit the thermoelectric transport properties. Using this doping strategy, bulk polycystalline Sr1- xPrxTiO3 samples with x = 0.075 show, to the best of our knowledge, the highest ever reported power factor (PF = α2σT) of ~ 1.1 W/m-K at 500°C among n-type SrTiO3 ceramics. Electrical transport measurement results for these samples will be presented and discussed along with the specific synthesis techniques we used to make these materials. This collaborative research was funded by a Faculty Initiated Collaboration (FIC) grant from KAUST.

Development of the novel materials where charge and heat transport are partially de-coupled is a key factor for the next generation of thermoelectric materials. One of the effective approaches is to create thermoelectric nanocomposites that incorporate nanoparticle precipitates within a bulk matrix. The improved performance notwithstanding, the nanocomposite materials exhibit only fractional improvement of the parent bulk materials properties. Thus, if significant breakthroughs in the area of TE materials (ZT > 2) are to be achieved, novel bulk materials need to be discovered. New compounds with high density of states near Fermi level and low thermal conductivity are required for efficient thermoelectrics. We propose to use ternary phosphides of late transition metal (Ni, Cu, Zn) and electropositive metals (Ca-Ba; La-Pr) as a base for new thermoelectric materials. Electron balance in these compounds is achieved in a way similar to classical Zintl phases assuming closed shell d¹⁰ configuration of transition metal. Synthesis of new materials, their crystal and electronic structure as well as thermoelectric properties will be discussed.

The Zintl compound, Ca₅Al₂Sb₆, has extremely low lattice thermal conductivity (< 0.6 W/mK at 1000 K) and tunable electronic properties, making it a promising thermoelectric material for high temperature waste-heat recovery. The current study investigates trends in the chemical and transport properties of the Ca₅M₂Sb₆ compounds (M = Al, Ga, or In), revealing routes toward improved thermoelectric properties in this system. Here, we show that isoelectronic M-site substitutions can be used to “fine-tune" the electronic properties of the Ca₅M₂Sb₆ system, without inducing electronic doping effects. Electronic structure calculations reveal that the electronegativity of the M element is a good indicator for the energy level of M electronic states. The effects of M-site substitutions on the effective mass and band gap are reflected in measurements of the high temperature electronic properties of Ca₅M₂Sb₆ samples (M= Al, Ga, and In) which reveal increased hole mobility in the Ga and In analogues, and a smaller thermal band gap in the Ga analogue, relative to Ca₅Al₂Sb₆. Optical absorption measurements reveal a trend in the direct band gaps consistent with both calculations and transport measurements. Additionally, a direct benefit of substituting heavier elements on the Al site arises from the increased density and softer lattice, which leads to reduced sound velocity and lattice thermal conductivity. Ca₅In₂Sb₆, which exhibits both enhanced mobility and reduced thermal conductivity while still retaining a sufficiently large band gap, exhibits the most favorable properties of the three compounds. We show that doping with Zn²⁺ on the In³⁺ site can be used to increase the p-type carrier concentration in Ca₅In_2-xZn_xSb₆. Upon optimization of the electronic properties in accordance with a single band model, a peak zT of greater than 0.7 was obtained at 1000 K.

As highly promising thermoelectric (TE) materials, electron-doped binary Mg2B (B=Si, Ge, Sn) compounds, especially Mg2Si-Mg2Sn and Mg2Si-Mg2Ge binary solid solutions, have received increasing interest. Popular methods used for synthesizing these materials include the solid-state reaction in a sealed quartz tube at high temperatures and the mechanical alloying at room temperature. Here we report a simple procedure for the material synthesis. As verified by x-ray powder diffraction, single-phase samples can be obtained by one-step solid-state reaction in an atmosphere of Ar/H2 mixture within a short period of time. We have employed this method to synthesize the ternary Mg2Si-Mg2Sn-Mg2Ge system. The carrier concentration of the system is tuned by varying Sb doping concentration. The density and electrical conductivity of Sb doped samples are enhanced by Spark Plasma Sintering of the as-synthesized powder. By mapping out thermoelectric properties over the entire ternary system, we aim to identify the compositions with the optimum band structure and Fermi energy that is further tuned by Sb doping.

Mg2X (X = Si, Ge, Sn) phases and their solid solutions are known to be very promising thermoelectric materials with exhibit high zT values. For Mg2Si a zT = 0.7 was published recently [1, 2], for the solid solution Mg2Si0.4Sn0.6 a value of zT = 1.1 has been reported [3]. A promising way to enhance the thermoelectric properties of these materials is to modify their fairly high thermal conductivity. Lowering thermal conductivity can be achieved essentially by two different ways, doping the material with nano-inclusions or heavy elements (point defect scattering). Divalent rare-earth cations therefore should be a promising isovalent replacement for Mg2+ in the structure and should effectively scatter heat carrying phonons due to their high atomic mass. Since the replacement is isovalent the dopant should have no influence on the carrier concentration of the material. Using metal hydrides has been shown recently to be a viable approach for synthesizing the Mg2X matrix and doping the sample [4]. The use of the hydrides forms a clean, oxide free sample and gives easy access is to the doped solid solutions of Mg2X. During the formation of these materials using spark-plasma sintering (SPS) or hot-pressing techniques rare-earth intermetallic inclusions are formed. These islands should be according to Mingo et al. one of the reasons for the decreased thermal conductivity [5] since inclusions of 5 nm and above have a strong effect on the properties. Recent calculations have shown that inclusions lighter than the matrix could be effective too, in reducing thermal conductivity [6]. Synthesis and characterization of the phases will be presented and the optimization of their thermoelectric properties will be discussed. [1] S. K. Bux, M. T. Yeung, E. S. Toberer, G. J. Snyder, R. B. Kaner, J.-P. Fleurial, J. Mater. Chem. 2011, 21, 12259-12266. [2] J. Tani, H. Kido, Physica B 2005, 364, 218-224. [3] V. K. Zaitsev, M. I. Fedorov, E. A. Gurieva, I. S. Eremin, P. P. Konstantinov, A. Y. Samunin, M. V. Vedernikov, Phys. Rev. B 2006, 74, 045207. [4] T. Yi, S. Chen, S. Li, H. Yang, S. K. Bux, Z. Bian, N. A. Katcho, A. Shakouri, N. Mingo, J. P. Fleurial, N. D. Browning, S. M. Kauzlarich, Energy Env. Sci. 2012, submitted. [5] N. Mingo, D. Hauser, N. P. Kobayashi, M. Plissonnier, A. Shakouri, Nano Lett. 2009, 9, 711-715. [6] A. Kundu, N. Mingo, D. A. Broido, D. A. Stewart, Phys. Rev. B 2011, 84, 125426.

One of the promising thermoelectric materials is Mg₂Sn, which is cheap, light, and non-toxic. Pure Mg₂Sn compound has a high energy gap and high mobility, but its thermal conductivity is too high for it to be used as a thermoelectric material.¹ Alloys of Mg₂Sn_1-ySi_y have higher zTmax = 1.1² reported: here the thermal conductivity is reduced by alloy scattering. Because the silicide is difficult to prepare, we concentrate on the binary stannide. The valence band of Mg₂Sn appears theoretically favorable for generating a high zT. Here, we report on p-type doping of polycrystalline binary Mg₂Sn, and the quest to find resonant impurities, which are known to increase the zT by increasing the thermopower at a given carrier concentration.³ We synthesize and characterize p-type-doped Mg₂Sn with various acceptors. Thermomagnetic and galvanomagnetic properties (electrical resistivity, Seebeck, Hall, and Nernst coefficients) are measured, along with thermal conductivity, and zT up to 700K is reported. The work is supported by the joint NSF/DOE program on thermoelectrics, NSF-CBET-1048622 1 V. K. Zaitsev. et al., Thermoelectrics Handbook (CRC press, New York, 2005), Chap. 29. 2 V. K. Zaitsev. et al., Phys. Rev. B. 74, 045207 (2006). 3 C. M. Jaworski. et al., Energy & Environ. Sci. 4, 4155 (2011).

La3-xTe4 is an n-type state of the art thermoelectric material with a ZTmax~1.4 at 1273 K with a demonstrated reliability and performance at the device level. However, the material behaves as a brittle ceramic and suffers from chipping and cracking during processing and service. Efforts to develop La3-xTe4 as a metal composite is discussed as a means to improve material robustness and device yield. Samples are produced through mechanical alloying and spark plasma sintering. They are characterized with XRD and electron microprobe analysis. The high temperature thermoelectric properties of the composite materials are measured through Van der Pauw geometry Hall Effect, small temperature differential light pipe Seebeck, laser flash diffusivity, and compared with single leg in-thermal-gradient testing. Results on the Vickers hardness testing and equibiaxial flexural strength are discussed for the various composites. The effects of particle loading, particle morphology, and particle size are examined. Optimization between thermoelectric and mechanical performance is discussed.

Direct conversion of heat into electricity relies on efficient electron sources which can be realized by low work function doped diamond emitters. Performance of a thermionic energy converter (TEC) is related to charge released from the emitter and surface ionization at its surface presents means to enhance power output. With nitrogen donors and negative electron affinity (NEA) surface properties, which positions the vacuum level below the conduction band minimum, diamond emitters exhibit a low work function of ~ 1.5 eV. In this research we present an energy converter based on nanostructured diamond electrodes separated by a gap of 25 mu;m which utilizes charge transfer through surface ionization for enhanced power output. As the temperature of the emitter is increased a thermovoltage between the electrodes is established which saturates at a temperature of 650 °C with a value of ~0.325 V. Introduction of atomic hydrogen at a pressure of 1 x 10^-4 Torr enhances the charge transport which is attributed to surface ionization effects as the affinity level (0.75eV) of the atom becomes populated through a tunneling process from states of the diamond emitter. Efficient charge transfer from the emitter surface occurs through ionized hydrogen atoms moving with the velocity of the gas. The output power of the converter is then prominently enhanced by a factor of 5. The hydrogen in the gap enables stable operation at elevated temperatures achieving a 10 fold increase in the output power. While the output power is apparently limited by space charge effects, the critical result of this observation substantiates the stability of diamond based TEC at elevated temperatures under atomic hydrogen. This research is supported by the Office of Naval Research.

In modern stationary gas turbines used for power generation, the combustion temperature of the fuel source often exceeds the metallurgical limit of the turbine machinery and must be lowered before the heat is utilized in a Brayton cycle. This lowering of the hot side temperature leads to high exergy destruction and degrades the thermal efficiency potential of the system. One method to better utilize the high-grade heat of the combustion process is the addition of a topping cycle that takes advantage of the high temperature before delivering heat to the base Brayton cycle at a more suitable temperature. This paper presents a theoretical model for a thermoelectric (TEG) topping cycle integrated with a Brayton engine. The thermoelectric cycle is presented in two configurations: a topping cycle and a preheating topping cycle. For the topping cycle configuration, the thermoelectric generator receives heat from a high-temperature heat source and produces electrical work before rejecting heat to a Brayton cycle. For the preheating topping cycle, the rejected heat from the thermoelectric generator partially heats the compressed working fluid before a secondary heater, which directly heats the working fluid. The model identifies the optimum range of operating conditions of the thermoelectric and Brayton cycles to obtain the maximum thermal efficiencies of the combined cycles. When the thermoelectric generator is utilized as a topping cycle, efficiency gains are realized for low-temperature Brayton cycles (<1200 K). For high-temperature Brayton cycles, however, a thermoelectric topping cycle is unable to improve the thermal efficiency of the system. As a preheating topping cycle, the thermoelectric is able to improve the efficiency of both low- and high-temperature Brayton cycles. In both configurations, the thermoelectric topping cycle is a more effective enhancement for low-temperature turbines. A design to achieve the preheating thermoelectric topping cycle is also presented.

We are building MEMS-based prototypes of heat-to-electricity and solar-to-electricity energy converters that are based on evaporation of electrons from solid surfaces (thermionic effect). Such microfabricated thermionic energy converters (mu;-TECs) can convert very high-temperature heat (>1000 C) directly to electricity. Microfabrication is the optimal manufacturing approach for thermionic energy converters because, as we show, the optimal cathode-anode gap for TECs is in the range of 1-10 mu;m, which is highly suitable for MEMS-based fabrication processes. In addition, the aggressive introduction of through-silicon vias (TSV), wafer-stacking, and wafer-scale vacuum encapsulation technology by the semiconductor and MEMS industries provides a route to large-scale fabrication of thermionic converters with high efficiency, low cost, and low maintenance requirements. We have recently demonstrated a simple prototype of a mechanically and thermally robust encapsulated mu;-TEC. The devices are fabricated out of silicon carbide and include barium-oxide coatings to reduce the work function. Small arrays of converters were encapsulated under a glass lid using wafer-scale anodic bonding. We will discuss the experimental demonstrations and the possible applications in concentrated solar power (CSP) and residential combined heat and power (CHP) systems.

Intermediate valence (IV) compounds such as CePd₃ offer a possible avenue for the development of high ZT materials for cooling applications due to the unusual combination of a relatively large Seebeck coefficient with metallic electrical resistivity. To search for improved thermoelectric performance in these compounds, we have successfully synthesized multiple series of nanogranular IV materials through vibratory ball milling and Pulsed Electric Current Sintering in an effort to reduce their lattice thermal conductivity. In this talk we will present some results on powder-processed CePd₃- and CeNi₂Al₃-based IV compounds. Measurements of the thermoelectric and magnetic properties of these alloys show a strong correlation among these properties and the grain size, indicating that nanostructuring appears to have a significant effect on both the lattice thermal conductivity as well as the intermediate valence state.

Solid-state thermoelectric devices can convert waste heat directly into electricity without emitting any additional greenhouse gases. They are portable, reliable, longer lasting, inexpensive to maintain and have no moving parts. The fabrication of highly efficient thermoelectrics requires materials that exhibit high electrical conductivities, but low thermal conductivities. Owing to the constraints imposed by Wiedemann-Franz law, this can only be accomplished through a reduction in the lattice thermal conductivity of materials. Theoretical predictions indicate that materials in one-dimensional morphology offer lower lattice thermal conductivities because of enhanced phonon scattering at their boundaries. Further, it is well established that single-crystalline one-dimensional nanostructures exhibit enhanced electrical conductivities compared to their nanoparticle counterparts. If one-dimensional nanostructures were to be employed for the fabrication of highly efficient thermoelectric cells, then the following are required: i) strategies for the large-scale synthesis of nanowire powders in the required compositions and in a contaminant-free manner, and ii) strategies for the large-scale assembly of nanowires that also offer precise control over electrical and thermal transport across nanowire interfaces. In this context, the aim of this talk is to demonstrate that self-catalytic schemes can be scaled-up for the mass production of nanowire powders of many compound semiconductors in a contaminant-free manner. Specifically, the use of self-catalytic schemes for the large-scale synthesis of Zn3P2 nanowire powder will be discussed. Zn3P2 nanowire powder was simply synthesized by exposing large area zinc foils to phosphorus vapors. Secondly, large-scale assembly of the obtained nanowire powder was accomplished by simply pressing it at elevated temperatures and pressures. These nanowire synthesis and assembly strategies were also employed for obtained inorganic-organic hybrids. Obtaining inorganic-organic hybrids involved a two-step procedure: i) in-situ functionalization of Zn3P2 nanowires with bi-functional organic molecules immediately after their synthesis, and ii) pelletization of the organic-molecule functionalized nanowire powders. When these in-situ functionalized nanowires were pressed into pellets, the conjugated functional molecules bridged the interfaces between the nanowires, and essentially allowed for tuning the electrical and thermal transport across interfaces. Thermoelectric properties of Zn3P2 nanowire pellets and hybrid pellets comprised of conjugated molecule functionalized Zn3P2 nanowires will be discussed.

Sodium cobalt oxide (Na_xCoO₂ or NCO) has attracted interest as a p-type thermoelectric material since it has a layered structure that exhibits good electrical conduction in the CoO₂ octahedral layer and low thermal conductivity in the disordered sodium layer. This leads to anisotropic properties, especially the electrical conductivity. In order to have comparable thermoelectric properties in bulk polycrystalline materials to that of single crystals, a highly textured and well-connected microstructure is required. Additionally, reports show that the thermoelectric properties are highly dependent on the sodium non-stoichiometry. Since sodium is volatile, it is difficult to control the sodium content during calcination using traditional techniques such as solid-state reactions. The purpose of this research was to develop new solution based synthetic techniques to achieve highly non-soichiometric, phase pure Na_xCoO₂ powder with sodium content optimized for thermoelectric applications. These powders were compacted with a new field-assisted sintering technique known as Rapid Induction Hot-pressing (RIHP). This sintering method provides very fast heating rates enabling the preservation of nanostructures. This research was supported by the Revolutionary Materials for Solid State Energy Conversion Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy Office of Science.

Electric power from solar thermoelectric converters offers an attractive high energy density alternative to established photovoltaics technologies. For the realisation of such devices highly efficient, stable, cheap, and environmentally harmless thermoelectric materials have to be developed for better converters covering large hot surfaces. Dense and highly nanostructured thin films were produced by chimie douce [1]and plasma methods. Compositions and morphologies were changed in order to tune the band structure, and defects in oxides and heusler phases. The thin films provide an alternative way to receive suitable samples to determine the phonon and charge carrier transport and thermoelectric properties. The presentation will provide a materials design strategy for future solar thermoelectric technologies.

Nanostructuring in thermoelectric materials is a promising route towards high dimensionless figure of merit (ZT), but yet this approach is not robust in oxides. Through the nanostructuring in oxides, decrease in thermal conductivity is reliable, while reduction in electrical conductivity is unpredictable and decisive for ZT; thus the design of interfaces being crucial for enhancing ZT. The key is to ensure the itinerancy of carriers, which would be attained in coherent interfaces or band structures with large contribution of s-orbital. Here, we will show typical examples of these two cases, in which nanostructuring accounts for gaining ZT. One example for coherent interface can be found in titanium oxide system. Ti_nO_2n-1 (4le;nle;9), known as magneli phase, is a homologous compound including rutile-like chains of edge-sharing TiO₆ octahedra interrupted every nth octahedron at a shear plane, where octahedra share faces as well as edges and corners. Due to the structure of the plane across which the distorted h.c.p net of oxygens is continuous but the metal nets are in antiphase, the thermal conductivity effectively decreased without a reduction in the electrical conductivity. Another case we will discuss is In₂O₃ whose conduction band minimum is primarily composed of s-orbitals of indium cation. Here, the large spatial spread of the s-orbital enables to overlap with those of neighboring cations, resulting in robust electrical network even for amorphous state. Owing to the situation, the electrical conductivity was not severely hampered by the nanostructuring although interface had random structure, leading to ZT enhancement likewise. Above examples show that moderate ZT enhancement by the tuning of scattering are practical if appropriate interfaces are designed in the nanostructure of oxides.

Homologous compounds RMO3(AO)n (R=In, Sc, Fe, Ga, Yhellip;; M = In, Ga, Al, Fe, A = Mg, Mn, Co, Zn, Fe) with alternated RO2- layers and MO(ZnO)n+ blocks are fascinating thermoelectric materials allowing for control over transport properties. In the present work we report on the synthesis, structure and thermoelectric properties of RMnO3(ZnO)n (R=Y) (n=0-9) oxides. The specimens were synthesized from Y2O3, Mn2O3, ZnO at 1200 and 1300 oC for 12 h in the air. Depending on the composition and the synthesis temperature, the formation of Mn-doped ZnO, spinel-type ZnMn2O4, hexagonal manganate YMnO3 and two-phase compositions with an inversion domain boundary was confirmed by X-ray powder diffraction, TEM and Raman spectroscopy. As found, the material composition and their microstructure determine their thermoelectric properties. Hexagonal manganate YMnO3 possesses large positive thermopower (600 microV/K at 300 K) that decreases with the increase in the temperature. In contrast, YMnO3(ZnO)n oxides (n>1) show negative thermopower that remains nearly constant (200 microV/K) at 300-1200 K that is attributed to the predominant effect of the Mn-doped ZnO. YMnO3(ZnO)n oxides (n=0.5) oxides show transient behavior that is attributed to the spinel (ZnMn2O4) inclusions in the ZnO phase. Thermal conductivity of the specimens increases with the increase in ZnO content, being of about 1 W/(mK) for YMnO3 and of about 15 W/ (mK) for YMnO3(ZnO)n oxides (n=9) at 500 K.

We report the significant enhancement of the power factor of Ca3Co4O9+δ through Yb doping. The pellets were prepared by pressing under 0.5 GPa and 2 GPa. The highest power factor of 553 mu;Wm-1K-2 due to the significant decrease of electrical conductivity was obtained for Ca2.9Yb0.1Co4O9+δ pressed at 0.5 GPa. This is 2.3 times higher than that of Ca3Co4O9+δ (246 mu;Wm-1K-2). Nanostructure examinations show that the pellets pressed at 0.5 and 2 GPa have different nano-lamella structures. This work suggests that Yb is an effective doping element for enhancing the electrical transport properties of Ca3Co4O9+δ, and the optimum doping level is related to the nanostructure of the bulk pellets.

Materials with the half-Heusler structure have long been considered as promising thermoelectric materials for high-temperature waste heat recovery. This was in particular stimulated by the so far irreproducible observation of zT = 1.5 in a series of antimony doped (Ti0.50Zr0.25Hf0.25)NiSn compositions [1]. We have used a combination of synchrotron and laboratory X-ray powder diffraction, high-resolution electron microscopy, and thermoelectric property measurements to investigate the Ti1-xZrxNiSn series. This reveals a Ti/Zr size difference driven phase separation into multiple half-Heusler phases. Interestingly, this phase separation is not detrimental to the electronic properties with large power factors observed (3.5 mW m-1 K-2 for x = 0.5), and may be related to the low thermal conductivities observed in [1]. The presentation will focus on the accurate correlation between synthesis method, composition, structures and thermoelectric properties in this series. [1] S. Sakurada and N. Shutoh, Applied Physics Letters 86 082105 (2005).

Ti-Ni-Sn type half-Heusler alloys which have the versatility to be either p- or n-type depending on the type of substitution, have been synthesized and investigated in the present work. The added advantage of doping them with multiple elements is that they will be amenable to bulk amorphous phase formation. Hence, in the present work, both isoelectronic as well as p- and n-type substituted alloys were designed and experimentally investigated. Substitution of Co for Ni and simultaneous substitution of Sc for Ti and In for Sn should result in the formation of p-type alloys while simultaneous substitution of Nb, Cu and Sb for Ti, Ni and Sn respectively should result in the formation of n-type alloys. A charge compensated alloy, with near zero Seebeck coefficient, can be formed by simultaneous substitution of Mn for Ti/Ni and Bi for Sn. Accordingly, 5 different alloys have been synthesized by vacuum processing followed by annealing. The p-doped alloys were predominantly single phase with a cubic C1b structure, while the n-type and compensated alloys were found to have minor additional phases. All the alloys exhibit an extremely weak metallic-like or degenerate semiconductor transport behavior in the temperature range 20 K to 1000 K. The resistivity of p-type alloys 10-5 Omega; m, exhibits semimetallic-to-semiconducting transition at ~ 500 K while the n-type alloys exhibit a metallic-like behavior in the complete temperature range albeit with a lower resistivity of 5x10-6 Omega; m. These values are an order of magnitude lower compared to base TiNiSn which is an indirect semiconductor with a resistivity of 10-4 Omega; m. The Seebeck coefficient has strong temperature dependence with a maximum of 45 µV K-1 in the temperature range 600-700 K in the p-type alloy which crosses over to n-type behavior at high temperatures. The n-type alloys however exhibit a linear variation of the Seebeck coefficient with temperature. The total thermal conductivity of the alloys increases with increasing temperature without any peak at low temperatures indicating significant disorder induced scattering. The p-type alloys have the lowest thermal conductivity compared to the n-type alloys, commensurate with the electron transport behavior. The thermal conductivity varies from 1.5 to 8 Wm-1K-1 in the p-type alloys and 3 to 15 Wm-1K-1 in the n-type alloys in the temperature from 20 K to 1000 K. All these alloys have been amorphized by pulsed laser deposition and their thermoelectric properties investigated. All these results including their magnetic behavior will be presented and discussed in detail.

One of the major roadblocks to large improvements in the thermoelectric figures of merit (ZT) of leading candidate thermoelectric materials such as the Bi2Te3, PbTe, CoSb3 and half-Heusler (HH) based systems remains the difficulty in making meaningful simultaneous improvements in both the electrical conductivity (σ) and thermopower (S) of these materials through doping and/or substitutional chemistry. In conventional semiconductors, both materials parameters (S and σ) are fundamentally coupled adversely through the concentration, n, of charge carriers. Therefore, the maximization of one parameter by tuning the carrier concentration (n) via doping and/or substitutional chemistry inevitably results in the minimization of the other. Here, we show that by coherently embedding sub-ten nanometer scale inclusions within a semiconducting half-Heusler matrix, large enhancements of the thermopower (S) and the mobility (mu;) can be achieved simultaneously in both n-type and p-type nanocomposites1-3. The enhancement in thermopower originates from large reductions in the effective carrier density (n) coupled presumably with an increase in the carrier effective mass (m*). The surprising enhancement in the mobility is attributed to an increase in the mean-free time (t) between scattering events (phonon-electron scattering, ionized-impurity scattering and electron - electron scattering). Using X-ray powder diffraction, electron microscopy, and electronic transports data, we will discussed the mechanism of phase formation and transformation, at the sub-ten nanometer scale, in bulk half-Heusler (HH) matrix and the mechanism by which the embedded nanostructures regulate electronic charge transport within the semiconducting HH matrix to achieve unprecedented combinations of physical properties such as, large enhancements in the carrier mobility (mu;), thermopower (S) and electrical conductivity (σ) simultaneously with drastic decrease in thermal conductivity (κ) at high temperatures. Emphasis will be placed on the n-type Zr0.25Hf0.75Ni1+xSn1-yBiy and Ti0.1Zr0.9Ni1+xSn, and the p-type Ti0.5Zr0.5Co1+xSb nanocomposites. (1) Makongo, J. P. A.; Misra, D. K.; Zhou, X.; Pant, A.; Shabetai, M. R.; Su, X.; Uher, C.; Stokes, K. L.; Poudeu, P. F. P. J. Am. Chem. Soc. 2011, 133, 18843. (2) Liu, Y.; Makongo, J. P. A.; Zhou, X.; Uher, C.; Poudeu, P. P. F. in preparation 2012. (3) Sahoo, P.; Makongo, J. P. A.; Zhou, X.; Uher, C.; Poudeu, P. P. F. in preparation 2012.

Due to the decent ZT value, high mechanical strength, high thermal stability, and non-toxicity, MNiSn (M=Hf, Zr) based n-type half heuslers are promising thermoelectric materials for mid temperature (500-700 °C) heat to electricity conversion. However, one drawback that potentially hinders the large scale application of MNiSn is the cost of Hf. At $450/kg, it is more than 8 times more expensive than other elements in MNiSn. According to the current best composition of MNiSn (Hf0.75Zr0.25NiSn0.99Sb0.01, with peak ZT ~1 at 700 °C), the cost of Hf is above 90% of the total cost. Here we report our efforts on optimizing the ratio between Hf and Zr to reduce its usage while keeping the thermoelectric property. We found that by reducing Hf usage to merely 1/3 of the best composition, we not only cut the cost by nearly 50%, but also retain the peak ZT of 1. We also found that the peak ZT appears at 600 °C instead of the previous 700 °C, which is closer to the temperature of practically available heat sources such as automobile waste heat. This downshift is likely due to reduced band gap as more Zr replaces Hf. This work performed at Boston College is supported by DOE Award Number: DE-EE0004840, and that at MIT is supported by the “Solid State Solar-Thermal Energy Conversion Center (S3TEC)”, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577 (GC).

For thermoelectric technology to be successful, research is needed on improving the figure-of-merit (ZT) as well as strong contact metal layer to fabricate into a module. In this work, our recent success on both ZT improvement and metallization of half-Heusler (HH) materials will be discussed. Thermoelectric figure-of merit of HH materials has been improved by a simple ball-milling and hot-pressing method. Peak ZTs of about 1 are obtained for both P and N type HH, which are mainly due to thermal conductivity reduction through nanostructures in the sample. Metallization layer on HH having low contact resistance in the range of a few mu;Omega;-cm2 has been developed. The adhesion strength and metal-HH interface of such legs have been investigated through mechanical test (strain test) and SEM, which show very promising results. Combination of high ZT and good metallization make these materials very attractive for waste heat recovery applications.

We report an enhanced thermoelectric figure of merit (ZT) in the undoped Zr0.25Hf0.75NiSn half-Heusler alloy employing nanostructuring techniques. This enhancement is due to both an increase in the thermoelectric power factor with simultaneous decrease in the thermal conductivity yielding a peak ZT = 1.09, which is the highest value reported so far in any chemically undoped half-Heusler alloys. These results suggest that the electronic and lattice properties can be partly decoupled on nanostructuring the half Heusler materials. Cell volume expansion due to nanostructuring and its relation to the enhanced thermoelectric properties in half-Heusler alloys is the novelty of the present work. In conjunction with theoretical calculations, the increase in thermo power has been attributed to an increased localization of the transition metal d states, while decrease in thermal conductivity is primarily due to interface scattering of phonons resulting from surfaces, grains and phase boundaries. It would be, therefore, interesting to explore whether such favorable effects to increase the ZT via nanostructuring could be applied to a variety of thermoelectric materials. *Corresponding author: E-mail address: [email protected] OR [email protected] (D.K. M.)

Heusler compounds with C1b structure are among the most promising material classes for themoelectric applications. The demands for thermoelectric materials are environmental friendliness, low-cost and future availability of raw materials, high efficiency, temperature and mechanical stability, possibility of industrial processing, and a thermoelectric material pair (n- and p-type) with very similar coefficients of thermal expansion and good thermoelectric compatibility. The Heusler materials class does meet nearly all of those requirements including a high power factor. In this talk, we will give an overview about our recent investigations about the thermoelectric Heusler compounds. We will present our studies on the phase separations in the quasi-ternary system TiNiSn-ZrNiSn-HfNiSn. Studying patents and publications the last two years carefully one could read a lot about not-single phase samples inside of the TiNiSn-ZrNiSn-HfNiSn system. We think we solved the mystery of the phase separations and observed several stable phases inside this huge material system. We will present how the knowledge about this phase separations and the stable phases is an important tool in the design of highly thermoelectric efficient materials which fulfill the industrial demands for a thermoelectric converter.

Recently, the thermoelectric performance of conventional thermoelectric materials such as Bi-Te, Pb-Te and Si-Ge based alloys has been remarkably enhanced through the nontraditional processing technology of nanostructuring. The nanostructuring approach has been shown to be one of the most effective ways to improve the thermoelectric figure of merit ZT, as this approach can either reduce the lattice thermal conductivity or enhance power factor. Advances in theories and experiments related to nanostructured thermoelectric materials confirm that it is possible to reduce the lattice thermal conductivity by intensified phonon scattering at interfaces without a severe reduction of the electrical conductivity. In this presentation, the emphasis is on nanostructured Bi-Te based bulk thermoelectric materials with an enhanced ZT. Strategies for high-performance thermoelectric nanocomposites are summarized, after which a few important nanostructuring technologies are presented.

Certain thermoelectric (TE) materials containing nano-scale grains have been shown to possess enhanced TE properties, as compared to that of bulk materials, due to interfacial phonon scattering and charge carrier filtering. These nanoscale effects establish a new means of improving the efficiency of TE devices as well as the potential for broader TE cooling applications. To study the TE properties of nanostructured materials, the bottom-up approach allows for the manipulation of matter at the molecular level, as well as good chemical homogeneity at the nano-scale, resulting in a useful synthetic technique for such investigations.[1] By adjusting different parameters, such as reaction conditions and chemical sources, the composition, crystal size, and size distribution of the nanocrystals can be controlled. A variety of synthetic processes can be employed to achieve phase purity, high yield and high crystalline quality for bottom-up processing of TE nanocomposites. Spark plasma sintering (SPS) is then used to form a dense polycrystalline material with uniform dispersion of nonconglomerated nano-grains. Transport properties of the resulting nanocomposites allow for an investigation into their physical properties, and their potential for TE cooling applications. [1] A. Datta, A. Popescu, L.M. Woods, and G.S. Nolas, 'The bottom-up approach to bulk thermoelectric materials with nano-scale domains', in CRC Handbook on Thermoelectrics and Its Energy Harvesting on Materials, Preparation and Characterization, Taylor & Francis, Ed. David Michael Rowe, 2012.

The photoresponse of optoelectronic devices is governed by relaxation pathways of photoexcited electron-hole pairs energy and heat as well as their optical properties. Nanoscale systems can offer various ways to control energy relaxation via electronic band modification, phonon confinement effects, and of engineering of front surface optical properties, potentially resulting in a more efficient photoconversion process. We report on the photoelectric response of a thermoelectric device based on junctions of 200-nm bismuth nanowire arrays with transparent indium tin oxide electrode. The thermal regime may offer a path to nanostructured thermoelectric materials for direct energy conversion of infrared light and efficient across-the-spectrum solar energy harvesting.

There has been an increased interest in bulk nano-composite thermoelectric (TE) materials due to their ability to offer better control over their electronic and thermal transport properties. Bismuth-based compounds have been widely investigated due to their interesting properties and applications such as the high thermoelectric figure of merit (ZT), presence of topological and surface states, etc. Ball-milling is one of the recent methods used to prepare nanostructures. Such nanostructures, with increased surface-to-volume ratio, lead to pronounced surface effects on various transport properties. In order to prepare bulk samples, Spark Plasma Sintering (SPS) is commonly employed as a densification technique. Further, the electronic nature of such surface states in Bi-based compounds may be severely affected by external parameters such as the applied pressure and structural defects, etc. Such external effects caused by SPS process on the transport properties are not well understood yet. Here, we present a detailed experimental study of the effects of surface modification on transport properties of Bismuth (Bi) via ball-milling and SPS processing. Several samples were prepared by varying the ball-milling time as well as employing different densification techniques; viz. SPS and Hot-press and the ensuing transport properties were measured on these samples. As a result of controlled surface modification in Bi, decoupling in the thermopower and electrical conductivity was observed in polycrystalline Bi, which leads to a greater than six-fold improvement in the power factor. This is a very significant improvement and details of the properties will be presented.

Bi-Sb alloy is one of the most common thermoelectric materials for refrigeration applications. To control coupled transport of electrons and phonons and to enhance zT further, it is essential to understand their transport in the material. However, in bismuth and Bi-Sb alloy, phonon contribution to the total thermal conductivity is not well known since electrons and phonons have comparable contributions to total thermal conductivity. There have been several attempts to experimentally separate electron and phonon thermal conductivity, but some discrepancies exist among the results. To quantify phonon thermal conductivity and understand phonon transport in bismuth, we calculated lattice dynamics in bismuth using first principles and the Boltzmann equation which was very accurate in the case of other thermoelectric materials such as Si-Ge alloy. From the calculation, we found two interesting features of phonon transport in bismuth. First, the 9th nearest neighbors in the structure cannot be ignored even though the interatomic distance is long. This peculiar feature originates from the rhombohedral crystal structure of bismuth, and significantly affects both harmonic and anharmonic interactions between atoms. Second, normal scattering process is stronger than Umklapp process. For this reason, the commonly used single mode relaxation time approximation is not valid for bismuth, and the Boltzmann equation should be solved exactly. Comparison of the results from the single mode relaxation time approximation and full iterative solution of the Boltzmann equation will be presented. Acknowledgment: This work is support by AFOSR MURI through OSU (S.L. and G.C.), S3TEC, a DOE EFRC (K.E. and J.G.).

Introducing nanostructures into thermoelectric materials to reduce lattice thermal conductivity, and therefore increase ZT, has been an effective strategy in recent years in many materials systems. However, despite clear evidence that nanostructures are critical to the performance of many thermoelectric materials, a detailed investigation of the natural defects and nanostructures of conventional thermoelectric materials has not been done in a systematic manner. We have carried out a transmission electron microscopy study on several traditional tellurium based thermoelectric materials, namely AgSbTe₂, Bi₂Te₃, and PbTe. Similar periodic nanostructures were found in all three materials. In addition, the period of the natural nanostructures was found to be directly proportional to lattice thermal conductivity of the materials studied, suggesting that the natural nanostructures may be extremely important to the thermal conductivity of tellurium based thermoelectric materials. Furthermore, optimization of these natural nanostructures in thermoelectrics could lead to higher ZT materials and natural nanostructuring could become an economical replacement for some artificial nanostructuring schemes.

Nanomaterials have drawn much attention as thermoelectric materials with high performance since the introduction of nanostructures was reported to enhance the dimensionless figure of merit, ZT in thermoelectric conversion. In various nanomaterials, we focused on the stacked epitaxial Si nanodot (ND) structures on Si substrates. Here, each epitaxial Si ND has an identical crystal orientation and an ultrathin SiO₂ surrounding barrier, which can keep high electric conductivity, in addition to the reduction of thermal conductivity and the enhancement of power factor due to the quantum effect. In this paper, we develop the formation technique of the above stacked Si ND structures using ultrathin SiO₂ film technique [1] and measure their thermal conductivity. Si(001) wafers were introduced into molecular beam epitaxy chamber (~10^-8 Pa). Clean Si surfaces were oxidized at 500°C for 10 min at the O₂ pressure of 2×10^-4 Pa to form ultrathin (~0.3 nm) SiO₂ films. Si was then deposited to form ultrahigh density (>10¹² cm^-2) Si NDs epitaxially grown on Si substrates as the following. At first stage of Si deposition, nanowindows were created in the ultrathin SiO₂ films through the reaction of Si+SiO₂ →2SiOuarr;, and Si NDs were subsequently formed on the ultrahigh density nanowindows. The above Si ND formation process and oxidization process were repeated to fabricate the stacked Si ND structures. In the stacked structures (13 times) of 10-ML Si NDs, we confirmed the epitaxial growth of Si NDs on Si substrates by reflection high energy electron diffraction. Cross-sectional high resolution transmission electron microscope observation showed that the Si NDs of ~3 nm in diameter were stacked. Unlike the nanocomposite, every NDs had the identical crystal orientation and homogeneously distributed with ultrahigh density. The diameter of NDs was tunable by changing the Si deposition amount. In the case of 40-ML Si NDs, Si NDs of 40 nm in diameter were also stacked and were epitaxially grown on Si substrates. We measured the thermal conductivity, κ of stacked Si ND structures. In the case of 3 nm Si NDs, κ is measured to be ~0.64±0.11 W/mK which is below that of Si amorphous (~2 W/mK). This demonstrates the possibility of our stacked epitaxial Si ND structure as a thermoelectric material with high ZT. This work was supported by PRESTO-JST program. [1] Y. Nakamura, Y. Nagadomi, S.-P. Cho, N. Tanaka, and M. Ichikawa, J. Appl. Phys. 100, 044313 (2006).

CoSi alloys are being investigated because of the possibility of achieving enhanced thermoelectric properties by tuning the Fermi level near the sharp edge of the density of states. In this study the mechanical properties of CoSi specimens prepared by arc melting with different amounts of boron (0-10 at%), were investigated. Prior x-ray diffraction and scanning electron microscopy studies had shown that boron segregates to the grain boundaries in the form of CoB, with no evidence of boron in solution in the CoSi matrix. The Young&’s modulus, hardness and indentation fracture resistance were measured at room temperature by nanoindentation using a cube corner tip. It was found that while the hardness of the CoSi phase was independent of boron concentration, its Young&’s modulus decreased monotonically from ~220 GPa for 0at% boron to ~180 GPa for 10at% boron. The mechanisms responsible for this behavior will be discussed. This research was supported as part of the Revolutionary Materials for Solid State Energy Conversion EFRC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Measurements were performed at the High Temperature Materials Laboratory at Oak Ridge National Laboratory, which is sponsored by the Vehicle Technologies Program, Energy Efficiency and Renewable Energy Program Office, U.S. Department of Energy.

Higher manganese silicide (HMS) is a promising thermoelectric material for use in intermediate-high temperature applications such as vehicle waste heat recovery, due to its relative abundance, low cost, and reasonable ZT values as high at 0.4 at 800 K. The HMS compound constitutes a family of compounds MnSi_x, with x in the range between 1.72 and 1.75, which are characterized by a very long unit cell of 17.5 - 118 Å along the c axis, compared to 5.5 Å along the a axis. The room-temperature thermal conductivity of bulk single crystal HMS is found to be anisotropic, and varies between 2.2 along the c axis and 3.5 W/m-K in the a-b plane at room temperature. The temperature dependence of the thermal conductivity of HMS crystals show a phonon-crystal behavior at low temperature, as well as contributions from the optical modes and bi-polar contribution at intermediate and high temperatures. Here, we report glass like, size-dependent thermal conductivity observed in HMS nanowires and nanoribbons. The recently obtained phonon dispersion of HMS is used to analyze whether the observed thermal conductivity has violated the Casimir limit in phonon scattering by the nanowire surface. The electrical conductivity and Seebeck coefficient have also been measured on the same nanostructures, allowing for the determination of the figure of merit.

We expand our expertise in the chemical synthesis of nanowires (NWs) and nanoribbons (NRs) of higher manganese silicides (HMS) to synthesize nanostructures of HMS in bulk. HMS are interesting semiconducting thermoelectric (TE) materials with reported figures of merit (ZT) up to 0.7-0.8 in bulk crystals. The abundance, low cost, and robustness of silicides make them particularly promising for large scale thermoelectric applications, such as in automobile waste heat recovery. We have previously shown that the free-standing 1D-nanostructures can be successfully synthesized using chemical vapor deposition (CVD) of a single source precursor and that the thermal conductivity of these nanostructures are suppressed from the already low bulk HMS value of 2-4 W/m-K to a value approaching 1 W/m-K and the amorphous limit. However, the demand for large amount of TE material limits the use of NWs and NRs in devices. In our current work, we develop a cheap, vapor phase conversion approach to improve the synthesis of nanostructured HMS, which could potentially be used for practical applications. The morphology and microstructures of the converted nanostructures are characterized using scanning electron microscopy and transmission electron microscopy, while the composition and phase are determined using energy dispersive x-ray spectroscopy, electron diffraction, and powder x-ray diffraction. We also investigate the phase behavior of these nanostructures during heating and cooling using in-situ high resolution powder x-ray diffraction which is crucial for TE applications.

Higher manganese silicides (HMS) are promising p-type thermoelectric materials for power generation from intermediate-temperature heat sources. HMS is stabilized in a narrow range of compositional ratio (MnSix=1.71-1.75) and consists of several complex Nowotny chimney ladder (NCL) phases, including Mn4Si7, Mn11Si19 , Mn15Si26 , and Mn27Si47. Single-crystal HMS shows a large anisotropy in the transport properties at room temperature. Among them, the thermal conductivity varies from about 2.3 W/m K along the c-axis to 4 W/m K in the a-b plane. The phonon dispersion of single crystalline HMS is crucial for understanding the anisotropic and low thermal conductivities in single crystal HMS, but has not been measured or calculated. Here, we report inelastic neutron scattering measurement results of the phonon dispersion of single crystal HMS that is synthesized by the floating zone method. The longitudinal acoustic (LA) branch and transverse acoustic (TA) branch along the a-axis, and the TA branch along c-axis have been determined from the experiment. It is found that the TA mode along the c-axis is very broad and crosses low-energy transverse optic modes at energy around 16-17 meV, which could be related to the lower thermal conductivity along c-axis than that along a-axis. The group velocity is found to be 2978 m/s and 6202 m/s, respectively, for the TA and LA branches along the a-axis, and 3948 m/s for TA branch along c-axis.

Thermoelectric materials can be utilized to convert waste heat into usable electricity. However, many of the common materials for this purpose cannot function at higher temperatures and therefore materials with higher temperature tolerances have found recent popularity. One such material family is the β-gallia-rutile (BGR) structured intergrowths, in which two structural components, the β-gallia and rutile phases, form a homologous system in the ratio Ga₄Ti_n-4O_2n-2, where n>9. This study is the first to synthesize these intergrowths by liquid combustion synthesis and subsequent heat treatment. Three gallium-to-titanium ratios were observed in this study and full structure solutions were completed through X-ray diffraction, so as to determine phases present. The three samples exhibited majority n = 8 structure with other closely matched intergrowth phases, leading towards the stabilization of lower n values without added dopants. Spark plasma sintering aided in the consolidation of the BGR specimens, which exhibited enhanced electrical conductivity above conventionally formed BGR materials. Thermoelectric analysis, including thermal and electrical conductivity coupled with Seebeck coefficient, was conducted on the sintered samples to ascertain the power generation character of the final specimens. Figure of merit (ZT) values of 0.13 at 1273 K were proven for these systems. The materials also show exciting potential for further improvement in efficiency.

Sodium cobalt oxide NaxCoO2 exhibits a large range of interesting phenomena, including surprisingly high thermoelectric performance around x = 0.7. It has previously been suggested that the thermoelectric power may increase during formation of oxygen vacancies. However, the published experimental data on the oxygen non-stoichiometry of this system varies considerably, and the reason for this variation has been unknown. We demonstrate in this study that the intrinsic oxygen vacancy concentration is very low in pure NaxCoO2. To this end, we have combined atomistic density functional calculations with well-controlled thermogravimetric measurements of the oxygen loss as a function of temperature and oxygen partial pressure. To investigate the effect of impurities and to predict the thermoelectric potential of doped NaxCoO2, we have also calculated the thermodynamic stability of NaxCo1-yTMyO2 for a large range of transition metals TM. All these compositions are thermodynamically unstable, but a significant equilibrium concentration can be anticipated in some of the systems. Also, a few compositions were stabilized by oxygen vacancies. Some of the models with combined transition metal doping and oxygen vacancies displayed promising electronic structures for thermoelectricity.

The transport properties across atomic junctions are calculated using the Landauer formalism and the non-equilibrium Green function technique. Thermoelectric performance is enhanced when the transmission function depends strongly on energy near the Fermi level. This suggests that transmission anti-resonances due to quantum interference effects may lead to an enhancement of the thermoelectric figure of merit. The effects of phase breaking processes and inelastic scattering of the carriers on the overall thermoelectric performance are also explored.

Modern computational methods have the potential to guide the design and optimization of thermoelectric materials. We show how density-functional theory (DFT) coupled with the Boltzmann transport equation can deliver predictive accuracy in describing the electrical transport in bulk thermoelectrics. In particular, we stress that a quantitative agreement between measured and calculated electrical transport coefficients is achieved through including corrections to the DFT band structures and explicit energy dependence of the electronic relaxation time to the computational framework. In this work, we present the results of our calculations of electrical transport coefficients for two classes of promising high-temperature thermoelectric materials, skutterudites and half-Heusler alloys. We also perform computational screening of the chemical composition of contacts by evaluating Schottky barriers at the interfaces between thermoelectric materials and metal contacts. Such analysis requires us to consider once again corrections to the DFT band structures of thermoelectric materials.

We introduce the concept of anti-resonance scattering to create a dip in the electron scattering cross section versus energy. Similar to resonance scattering, anti-resonance scattering can improve the Seebeck coefficient when the slope of the scattering rate is large at or near the Fermi-level. Minimization of the scattering rate close to the Fermi level increases the electron mobility. Therefore, using anti-resonance scattering, we can improve the Seebeck coefficient and the electrical conductivity at the same time to enhance the thermoelectric power factor significantly. We show that such anti-resonances are feasible in core-shell nanoparticle-doped thermoelectrics. At the same time, core shell nanoparticles with large acoustic mismatch to the lattice can reduce the thermal conductivity significantly. Thus, anti-resonance scattering points to a strategy to improve all the three parameters in the thermoelectric figure of merit simultaneously. This work is an extension to solids of the Ramsauer-Townsend observation, that the scattering cross section of electrons off noble gas atoms exhibits a minimum versus electron energy.

There is current interest in high temperature thermoelectric materials for space applications, solar-thermal electrical -energy production and electrical power generation from waste heat. These applications require high stability thermoelectric materials with high figure of merit. ZnO is of particular interest in this regard. This compound has a high electrical conductivity, high Seebeck coefficient therefore it is a promising high figure of merit thermoelectric material. In addition, this compound is cheap, non-toxic n-type semiconductor that is stable in air up to very high temperatures in excess of 1300 C, especially when alloyed with electropositive trivalent elements such as Al. Here we report an investigation of the temperature and doping dependent thermoelectric behavior of n-type ZnO. We use a combination of first principles calculations and analysis of existing experimental data to investigate the potential of n-type ZnO as a very high temperature thermoelectric. We find that improvement of the lattice thermal conductivity is essential for obtaining high ZT in n-type ZnO.

Modeling phonon transmission is vital for multiscale modeling of heat transport in nanostructured materials. In this study, we calculate the phonon transmission in three-dimensions via Green's function method. The methodology is applied to silicon/germanium interface for which the force constants are calculated using either a semi-empirical potential developed by Stillinger and Weber (SW), or first-principles density functional potentials. Once the force constants are known, Green's function method is used in conjunction with Landauer formula to calculate the transmission and thermal conductance of the junction. Moreover, the mode dependent transmission is detailed, which allows us to track the trajectory of each mode after encountering the interface. Both the perfect interface and the rough interface are investigated. This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577

For the past two decades void-filling has been extensively studied in the Co4Sb12-based skutterudite lattice structure. Several transition metals, alkali earth metals, and rare earth metals have been used as single, double, and multiple fillers in the Co4Sb12-based skutterudite structure in attempts to lower the lattice thermal conductivity in this high performing electronic system. To this date and to our knowledge; however, Praseodymium (Pr), a rare earth ion, has not been successfully tested for its thermoelectric properties as a “rattler” in the Co4Sb12-based structure. According to literature, the suggested theoretical FFL of Pr is approximately 0.07. However, we have observed that filling the skutterudite cage with Pr < 0.07 at% will not form a stable skutterudite lattice. Recent developments suggest that exceeding the FFL in these skutterudite materials can possibly enhance the thermoelectric properties due to nanosized secondary phases developing during the synthesis process. Single-filled PrxCo4Sb12 (0.1 ge; x ge; 0.4) was synthesized and experimentally tested for high and low temperature thermal and electrical properties. We observe stable structures with an increase in figure of merit and an increase in the temperature range for maximum ZT as the filling fraction of Pr is increased. A maximum of ZT asymp; 0.8 is observed in Pr0.4Co4Sb12 around T asymp; 750K. A more complete set of results on Pr filled Co4Sb12 samples will be presented and discussed.

Materials of general formula Co₃M₂S₂ (M = Sn, In) adopt the shandite structure. This consists of Kagome-like sheets of metal atoms, stacked in an ABC sequence and capped by sulphur anions. Adjacent Kagome layers are linked by metal atoms located in inter-layer sites of trigonal-prismatic coordination geometry. The ternary phases are metallic, with relatively large (for a metal) Seebeck coefficients. We have sought to exploit the combination of a metal-like resistivity and high Seebeck coefficient for the design of new thermoelectric materials containing the abundant element, sulphur. Here we present the results of an investigation of materials Co₃Sn_2-xIn_xS₂ (0 le; x le; 2) in which the progressive replacement of tin by indium is used to effect chemical control over the transport properties. As the indium content is increased, a change from metallic to semiconducting behaviour is observed as the 50% level of substitution is approached. At higher levels of substitution (x > 1.05) metallic behaviour is restored. The changes in transport properties are accompanied by a change in the nature of the dominant charge carriers from electrons to holes with increasing levels of indium incorporation. This behaviour may be understood in terms of the depopulation, with increasing indium content, of an upper partially-filled band of mainly cobalt d-character, leading to semiconduction in Co₃SnInS₂, followed by removal of electrons from a lower filled band, also of cobalt d-states, with further incorporation of indium. The sharpness of the cobalt d-levels, together with the capacity to control their degree of filling by electrons through chemical substitution, leads to promising thermoelectric behaviour. A maximum figure-of-merit (ZT = 0.2) at room temperature is observed at a composition just prior to the onset of semiconducting behaviour.

Poly(vinyl acetate) copolymer latex-based composites were prepared with multi-walled carbon nanotubes (MWNT), stabilized with sodium deoxycholate (DOC) or meso-tetra(4-carboxyphenyl) porphine (TCPP). SEM images show that a segregated MWNT network developed during drying, which resulted in relatively low percolation thresholds (1.62 and 2.17 wt% MWNT for DOC and TCPP, respectively). The electrical conductivity of TCPP-stabilized composites is very similar to that of DOC-stabilized, while the thermopower (or Seebeck coefficient) is five times as large (5 to 25 µV/K, respectively). This enhanced thermopower suggests the MWNT:TCPP composite will have an order of magnitude greater thermoelectric efficiency. The thermal conductivity also remains comparable to typical polymeric materials (< 0.30 W/(m*K)) due to numerous tube - tube connections that act as phonon scattering centers. The universality of this approach was confirmed using double-walled carbon nanotube-filled composites that showed similar improvement with TCPP stabilization. The tailorability of the Seebeck coefficient with different stabilizing agents is an important tool for increasing the thermoelectric efficiencies of polymer composites.

We present results that show the ineffectiveness of energy filtering at grain boundaries. Our results are based on a model that we developed to describe electron transport in nanograined materials. For the band structure we use a one band effective mass model. The transport is calculated using the Landauer formalism. The grain boundaries are described using the model by Seto [1]. In this model additional trapping states in the grain boundary are assumed that causes a space charge accumulation in the grain boundary. This space charge distribution leads to a formation of a double Schottky barrier. It is believed that such barriers can increase the efficiency of thermoelectric materials by energy filtering effects. Since in our model the space charge distribution depends on the doping concentration and therefore also the barrier, we are able to calculate the electron transport in dependence of the doping concentration. For low doping concentration we obtain a energy filtering effect, but for high doping concentration that are necessary for effective thermoelectric material the barrier height and width is too small to have an impact on the transport. Therefore, we conclude that electrostatic barriers play no role for thermoelectric devices. [1] J. Seto, J.Appl. Phys. 46 5247 (1975)

Intermediate valence compounds such as YbAl₃ and CePd₃ have been studied as low temperature thermoelectric materials due to their high Seebeck coefficient and low electrical resistivity values. While the origin of the large Seebeck coefficient in these materials can be understood on the basis of the Anderson model, a clear relationship between intermediate valence and Seebeck coefficient has not yet been demonstrated experimentally. YbAl₃ and the related compound, YbAl₂, can in principle be used for such a demonstration because the Yb valence can be altered by changing either internal (chemical) or external (applied) pressure. For instance, partial substitution of Sc for Yb in these compounds results in chemical pressure that alters the Yb valence. In order to study the influence of this internal pressure on the Seebeck coefficient, we have synthesized a series of alloys of composition Yb_1-xSc_xAl₂ and Yb_1-xSc_xAl₃ using a novel combined arc-melting and pulsed electric sintering technique. We will report upon the thermoelectric properties of these alloys from 80-300 K.

Of particular interest of the TiS2 (and most of TX2 metal dichalcogenide compounds, ) is the possibility to intercalate foreign atoms or molecules into the van-der-Waals gap between the host layers. This method of modifying the physical properties was widely studied and discussed in view of practical applications (mostly for batteries). It is for instance possible to achieve semiconductor-to-metal transitions (or vice versa). The occurring changes are ascribed to a charge transfer from the introduced species to the host lattice. The CdI2 layers of the TiS2 structure can then form a high-mobility semiconductor, whereas the intercalated layer can also create disorder and phonon scaterring. TiS2 can then possess high electron conductivity while keeping relatively low thermal conductivity. Based on this concept, we have recently revisited the synthesis of TiS2 based compounds for studying the effect of intercalation and substitution on the thermoelectric properties. We report here on the structure and transport properties of CuxTiS2 and TiS2-xSex compounds.

Recently, relative large ZT values have been reported in the series of narrow-bandgap intermetallic compounds composed of transition metals (TM: Fe, Ru) and group-13 elements, such as RuAl2- (ZTmax = 0.20) [1,2], RuGa2- (ZTmax = 0.50) [2,3], and RuIn3-based (ZTmax = 0.80) [4-6] samples. These compounds form a narrow-band gap of 0.2 - 0.4 eV near the Fermi level (EF), and the transport properties are, therefore, semiconducting like behavior with a large Seebeck coefficient over 200 mu;V/K. On the other hand, the thermal conductivity of these materials is relative high values of 3 - 5 W/m-K with respect to their simple crystal structures [6]. Therefore, these materials have a strong potential for new high-efficient thermoelectric materials by adjusting the carrier concentration and lowering lattice thermal conductivity. To understand the origin of formation of narrow-bandgap near EF, we have performed the maximum entropy method (MEM/Rietveld) analysis for pure materials [7]. The strong covalent bonding between TM and group 13 elements was observed, which is qualitatively consistent with the band structure calculation employed by WIEN2k package program. Next, to evaluate the potential as thermoelectric materials of TM(Al,Ga)2 and TM(Ga,In)3, the maximum dimensionless figure of merit ZTcalc was calculated based on the semi-classic Boltzmann transport theory under the constant relaxation time. We have found that ZTcalc of these materials would be achieved above unity by carrier-doping [6,8]. According to the calculation, we investigated experimentally the effect of hole- and electron-doping on the thermoelectric properties for some TM(Al,Ga)2 and TM(Ga,In)3 materials. The experimental Seebeck coefficient is consistent with the calculation, indicating a simple assumption of rigid-band-approximation may be applicable in doping samples. The reported high ZTmax are all for p-type material. By electron doping, relative large ZT of 0.31 for n-type was observed in RuGa2 system. In this presentation, details of calculational and experimental thermoelectric properties of these compounds will be presented. Discussions include the validity of the rigid-band-approach and the guiding principle for further enhancement of ZT value. [1] S. Takahashi et al., J. Alloy Compd. 493, 17 (2010). [2] Y. Takagiwa et al., Mater. Trans. 51, 988 (2010). [3] Y. Takagiwa et al., J. Alloy Compd. 507, 364 (2010). [4] M. Wagner et al., J. Mater. Res. 26, 1886 (2011). [5] D. Kasinathan et al., Phys. Rev. B 85, 035207 (2012). [6] Y. Takagiwa et al., J. Appl. Phys. 101, in press (2012). [7] Y. Takagiwa et al., to be submitted. [8] Y. Takagiwa et al., to be submitted.

La3-xTe4 is a high temperature n-type thermoelectric material with maximum zT~1.4 at 1273 K. For this system, the number of vacancies can vary between 1/3ge;xge;0, and previously the carrier concentration has been optimized through controlling the Te:La ratio. Efforts in the past have shown that controllable doping can be also be achieve through non-isoelectronic substitutions of the La atoms with rare earth X2+ metals. Computational modeling suggests the La atoms play a crucial role in defining the density of states for La3-xTe4 and substitutions could play a crucial role in increasing the power factor of this material. The effects of doping with various alkaline earth metals, vacancy versus alkaline earth metal doping, and varying carrier concentration are explored. High purity, oxygen-free samples are produced by ball milling and spark plasma sintering. Powder XRD and electron microprobe analysis are used to characterize the material. High temperature thermoelectric properties are reported and compared with baseline La3-xTe4 compositions.

Ideal thermoelectric materials are semiconductors exhibiting phonon-glass electron-crystal behavior, possessing simultaneously high electronic mobility and low thermal conductivity. Most bulk materials require nano structuring to lower the phonon contribution to thermal conductivity. Our work explores the properties of structurally complex chain based Zintl compounds with intrinsically low lattice thermal conductivity. Our recent investigations of Sr3GaSb3 and Ca3AlSb3, which possess different crystal structures in large unit cells based on chains on M&’Sb4 tetrahedra (with M&’ = Al, Ga), reveal lattice thermal conductivities approaching the minimum allowed value at high temperature and promising figures of merits. Here we report on the chain-based Zintl phase Sr3GaSb3, which exhibits a lattice thermal conductivity of 0.4 W/K-1m-1 at 900 K and relatively high mobility (30 cm2V-1s-1 at room temperature), leading to a figure of merit approaching unity at optimum doping levels. Previous investigations of this material have been limited to crystallographic studies. We employ a combination of transport measurements, Density Functional Theory calculations, and classical transport models to characterize this compound for thermoelectric applications. Furthermore we show the influence of the grain size in Ca3AlSb3 on the thermoelectric transport properties in an intermediate temperature range, which goes counter to conventional wisdom, showing that nano structuring does not always lead to an enhanced figure of merit. An attempt will be made to compare the transport properties of the materials with each other in respect to the difference in their crystal structure. 1. Zevalkink A., Toberer E.S., Zeier W.G., Flage-Larsen E., Snyder G.J., Energy & Environ. Sci. 2011, 4, 510-518 2. Zeier W.G.*, Zevalkink A.*, Schechtel E., Snyder G.J., Tremel W., J. Mater. Chem., 2012, 22, 9826-9830 3. Zevalkink A.*, Zeier W.G.*, Pomrehn G.*, Schechtel E., Tremel W., Snyder G.J., submitted

A Seebeck coefficient Standard Reference Material (SRM) is critical to establish reliable interlaboratory data comparison, to validate the accuracy of numerous measurement apparatus, and to confirm reports of high efficiency thermoelectric materials for commercial application. Following the success of the recently released low temperature (10 K - 390 K) Seebeck coefficient standard (SRM 3451), we discuss our current development of a new SRM for the Seebeck coefficient at high temperature (300 K - 900 K). The development process consists of several complimentary components: first, the construction of an improved experimental Seebeck coefficient measurement apparatus to evaluate and compare different measurement methodologies; second, the selection and validation of optimal Seebeck coefficient metrology protocols, both through experiments and through computational measurement simulations using finite element analysis; and third, the evaluation of candidate materials, comprising cyclical transport property measurements and other correlated physical property characterizations. These candidates include p-type SiGe-based polycrystalline materials and other single crystal systems. This talk will provide a general overview of our apparatus design and instrumentation, with examples on managing sources of error. In addition, we will present representative data from thermal cycling studies of the candidate materials and discuss the challenges in developing a high temperature Seebeck coefficient SRM that is both consistent and stable.

One outstanding challenge for the understanding of the effect of nanoscale inhomogeneities on thermoelectric performance is a probe of thermoelectric energy conversion with a spatial resolution of sub-50 nm down to a few nanometers. These are the length-scales of the nanoscale roughness, extended defects, and unconventional dopants that have recently been demonstrated to significantly enhance thermoelectric efficiency. We will present our recent efforts in developing thermoelectric characterization techniques based on scanning probe microscopy, contrasting our contact and non-contact approaches to nanoscale thermopower measurements [1]. Though conceptually similar, the transport mechanisms behind these two techniques are completely different, and we anticipate that the corresponding thermoelectronic information will likewise be different but complementary. Combining these two techniques will add a degree of certainly and completeness to local thermoelectric characterization. We have used Bi2Te3 cleaved in ultrahigh vacuum as a benchmark material to demonstrate and verify the capabilities of these techniques. The contact approach yielded surprisingly good agreement with the +235 mu;V/K thermopower derived from bulk-measurement, while offering ~10 nm spatial resolution, even without focused improvement of the measurement probes. In contrast, the net thermovoltage signal in tunneling measurements was consistently five- to ten-fold smaller. Based on the comparative analysis of the Mott formula for thermopower and the Tersoff-Hamann theory of tunneling transport, we will argue that the tunneling thermovoltage is actually consistent with the properties of the material studied and corresponds approximately to the energy derivative of the surface density of states (DOS) - i.e. the DOS component of the thermopower. This is unlike several earlier publications where the tunneling signal was used to estimate the net value of thermopower. As a corollary, a direct measurement of the DOS thermopower component will be of value to many of the large number of nanostructuring proposals that aim to enhance its magnitude. Finally, we will discuss the thermoelectric contrast due to localized and extended defects on the Bi2Te3 surface and the range of thermal gradients where SPM-based techniques may be used for systematic measurements. SJ, PM: Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. MM: Supported by the U. S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. [1] S. J. Kelly, M. A. McGuire, and P. Maksymovych, submitted (2012).

The dimensionless figure of merit ZT ranks good thermoelectric materials. Larger electrical conductivity and Seebeck coefficient in conjunction with a low thermal conductivity results in useful thermoelectric materials. Since some of the best thermoelectric materials do have an anisotropic crystal structure and since plasma sintering (SPS) is uniaxial most or all samples produced by SPS should show to some extend anisotropy. In addition, inhomogeneity is also expected in SPS samples because the current density, the temperature field and pressure may not be homogeneous during the sintering process. For the above mentioned reasons, it would be useful to map the transport properties of SPS-samples measured along and perpendicular to the sintering direction. Here we report on the development and capabilities of two new measurement systems developed at Fraunhofer-IPM. The first measurement system is based on an extension of the Van der Pauw method suitable for cube-shaped samples. A mapping of the electrical conductivity tensor of a Skutterudite-SPS samples produced at the Instituto de Microelectroacute;nica de Madrid is presented. The second measurement system is a ZTmeter also developed at the Fraunhofer-IPM. Its enable the simultaneous measurement of the electrical conductivity, Seebeck coefficient and thermal conductivity up to 900 K of cubes at least 5x5x5 mm3 in size. The capacity of this measurement system for measuring the anisotropy of the transport properties of a Bi2(Te,Se)3 SPS samples produced by KTH is demonstrated by simply rotating the samples.

Phonon scattering exhibits a strong influence on the thermal properties of nanostructures. By promoting phonon scattering at surfaces and interfaces, a nanostructured thermoelectric material may achieve reduced thermal conductivity and enhanced thermoelectric efficiency. While phonons over a wide frequency range contribute to energy transport, thermal conductivity measurements capture only their combined effect. However, a window into phonon transport in nanostructures at specific frequencies could provide unique information and also serve as a crucial test platform for phonon transport theories. To this end, we have constructed a microfabricated phonon spectrometer. At a temperature of 0.3K, a superconducting tunnel junction locally generates non-thermal distributions of phonons and transmits them through adjacent silicon micro- and nanostructures.[1] We employ modulation techniques to select narrow frequency bands of phonons at frequencies up to hundreds of GHz.[2] This prototype phonon spectrometer achieves phonon frequency resolution as low as ~10 GHz, more than an order of magnitude lower than comparable thermal methods. We describe the other key parameters of this technique: spatial resolution, frequency range, dynamic range, signal-to-noise ratio and calibration methods. We will discuss the use of this technique to assess phonon mean free paths in nanostructured thermoelectrics. This work was supported in part by the National Science Foundation under Agreement No. DMR-1149036 and the DOE Office of Basic Energy Science under Award Number DE-SC0001086, and performed in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Infrastructure Network, which is supported by the National Science Foundation (Grant ECS-0335765). [1] H. Kinder. Phys. Rev. Lett. 28, 1564 (1972) [2] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011).

Thermoelectrics have known a dramatic growth of interest in the last decade. Many complex structures are being studied for their specific properties to maximize the thermoelectric efficiency. The performance of thermoelectric materials is quantified by the figure of merit ZT=S²T/ρκ (S=Seebeck value, ρ = electric resistivity, κ = thermal conductivity and T the absolute temperature). The ideal material should have, for a given temperature, a lattice thermal conductivity close to the one of an amorphous material, but the electronic properties associated to a crystalline material [1]. This is why a large attention has been focused on phonon glass layered cobalt oxides due to their complex electronic configurations. Among these materials (complex intergrowths between hexagonal perovskite or Rock Salt (RS) type layers and [CoO2] —CdI2 type— layers)[2], the Ca3Co4O9 and the [BaCoO3]_n[BaCo8O11] family are promising both for magnetic and thermoelectric purposes [3-5]. Here we emphasize presence of high order layered cobalt oxide in nanostructured materials synthesized by SPS (Spark Plasma Sintering). We analyze specimen by HAADF-STEM and Atom Probe Tomography and compare the two techniques. For many years APT has been restricted to metallic materials analysis but now, with the apparition of laser assisted APT, insulators and semi-conductors might also be observed [6,7]. Specimens were analyzed by laser-assisted wide-angle tomographic atom probe (LAWATAP) from CAMECA at 80K. We reveal the notable difference of measurement between APT and EDX in STEM which might come either from the experimental conditions and the underlying evaporation process. Nevertheless, HAADF STEM observations corroborate XRD analysis concerning the layered structure. 1. Snyder, G.J. & Toberer, E.S. Complex thermoelectric materials. Nature materials 7, 105-14 (2008). 2. Kobayashi, W., Hébert, S., Pelloquin, D., Pérez, O. & Maignan, a. Enhanced thermoelectric properties in a layered rhodium oxide with a trigonal symmetry. Physical Review B 76, 3-7 (2007). 3. Masset, A., Michel, C., Maignan, A. & Hervieu, M. Misfit-layered cobaltite with an anisotropic giant magnetoresistance: Ca3Co4O9. Physical Review B 62, 166-175 (2000). 4. Pelloquin, D., Perez, O. & Martinet, G. The Oxide Ba6Ga2Co11O26: A New Close-Packed Stacking Derived from the Hexagonal Perovskite. Chemistry of 19, 2658-2662 (2007). 5. David, R. et al. [BaCoO3]n[BaCo8O11] Modular Intergrowths: Singularity of the n = 2 Term. Chemistry of Materials (2011). 6. Larson, D.J. et al. Toward atom probe tomography of microelectronic devices. Journal of Physics: Conference Series 326, 012030 (2011). 7. Lardé, R. et al. Evidence of Superparamagnetic Co Clusters in Pulsed Laser Deposition-Grown Zn0.9Co0.1O Thin Films Using Atom Probe Tomography. Journal of the American Chemical Society 133, 1451-1458 (2011).

Thermoelectric (TE) materials with high ZT is necessary for practical applications of thermoelectric modulus not only power generation, but also cooling applications. Their thermoelectric properties such as Seebeck coefficient, electrical conductivity, resistivity and thermal conductivity measurements are unavoidable for evaluation of ZT. For this reason reliable, accurate, and consistent thermoelectric measurements are significant characterizations. We have designed and established a device for bulk and film materials to perform effective electrical measurements of TE materials. There are similar systems with varying capabilities describing the measurements of those characteristics; however, almost all of them are specifically designed as sample probe and there is necessary to have furnace to be inserted. We have designed and implemented modular thermo-electrical measurement system for bulk and film samples in order to measure thermopower, and electrical conductivity / resistivity simultaneously at temperature range from 300 K to 1000 K. The first advantage of the device is that user can insert and measure two samples simultaneously as bulk and film. One of the samples can be used as reference sample. It is easy to make precise direct contact with thermocouples by mechanical pressure to maintain the pressure with springs which does not lose their properties at high temperature. The second advantage of the device is that any desirable kind of thermocouple types can be used and be changed for maintenance without ordering-off the whole system. Bead improved double thermocouple design is third and the most important advantage of the system by eliminating as called “cold-finger effect” and gives opportunity to user to apply 4-point probe Van der Pauw method for resistivity measurements with non-destructive capability. Moreover, differential temperature measurement can be taken with the help of double thermocouple design. The forth advantage of the device is that micro heaters design. It is built and placed on the sample probe to eliminate magnetic field effect that is created by wires of the heater. The vacuum chamber, was actually built to have good temperature control. It is designed to work in any desirable atmosphere up to 10-5 torr pressure. Device is computer controlled by Labview2010 software written in our laboratory. Differential methods such as steady-state and quasi steady-state conditions can be applied for thermopower measurement.. As the specific equilibrium points that occur in the range that of sim;0.06 K, the system does the resistivity measurements while heating and cooling. Current reversal and four-point technique is applied for resistivity. Standard carbon rods for bulk samples and platinum coated silicon wafer for film samples are used to ensure the accuracy and reliability of the system. As a result, a complete system having unique properties was built and used for thermoelectric measurements.

Zinc oxide (ZnO) is one of the most significant n-type materials that have been investigated. Its abundancy, transparency, non-toxicity, and good electrical conductivity have made it attractive for optoelectronics. The high temperature thermal stability of ZnO has made it a strong candidate for thermoelectric applications in harsh conditions. Chemical synthesis of ZnO thin films is quite appealing, since it offers a cheap and a controlled manufacturing pathway. In this study, the thermoelectric properties of a sol-gel prepared ZnO and 3% Ga doped ZnO (GZO) thin films have been explored as a function of annealing time in Ar/H₂ (95/5) gas mixture. Obtaining stable films, against the creation of the oxygen vacancies and other possible defects, is important to understand the high temperature properties of the films. By increasing the annealing time, not only do the temperature dependent conductivity and thermopower show a stable behavior upon heating and cooling, but also the power factor of GZO, as compared to ZnO, is improved by 66% at 800 K. Annealing for 12 hours in Ar/H₂ at ambient pressure resulted in a metallic behavior for GZO films, a behavior which has not been observed in short annealing times. On the other hand, ZnO films always showed a semiconductor behavior below 400 K, irrespective of the annealing time. Oxygen vacancies are found to be created above 550 K during thermoelectric measurements in Ar/H₂. The role of Ga dopants and oxygen vacancies, formed upon annealing, in driving the complex temperature dependence of the thermoelectric properties have been established using X-ray diffraction, electron microscopy, Hall effect and photoluminescence results.

Reduction of CO2 emission and energy efficiency are important challenges for industry and society in the 21st century. Thermoelectricity is seen therefore as one of the key technologies for waste heat recovery. Decisive factors for the conversion efficiency of the thermoelectric materials are high Seebeck coefficients, high electrical and at the same time low thermal conductivities. Here the classical physics has imposed laws which do not allow an independent optimization of these properties. Nanostructured materials opened a new way to increase the thermoelectric figure of merit (ZT), mainly by reducing their thermal conductivity due to phonon scattering. Modern high ZT materials manage in this way to trick the nature to a certain degree: different nanoscale configurations restrict the mobility of the phonons while the electrons are not hindered to transmit them. Such materials are manufactured by different techniques which can be divided into two groups: The bottom-up concept includes for example superlattices, quantum well structures and nanowires. The top-down concept is more useful for bulk materials using coherent precipitates, spinodal decomposition or embedding inert nanoparticles in a macroscopic matrix. As an example for the bottom-up group nanoscale multilayers based on the most important Bi2Te3-related compounds were presented in order to get more insight into the chemical and physical properties. Also the influence of nanoscale precipitates on the thermal conductivity will be demonstrated for a bulk nanostructured material representing the top-down approach. An overview on the most recent results of the buttom-up and top-down concepts regarding nanostructured thermoelectric materials and an outlook will be given.

Silicon-germanium-based alloy superlattices (SLs) [2] show promise as efficient thermoelectrics because of their low thermal conductivity. We demonstrate the sensitivity of the lattice thermal conductivity in Si/Si_1-xGe_x alloy superlattices (SLs) to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. In order to accurately treat phonon scattering from a series of rough interfaces with a given rms roughness height (Δ), we employ a momentum-dependent SL specularity parameter p(q) [02] over the distribution of many uncorrelated interfaces present between the layers of the superlattice. In the calculation of the total phonon lifetime, surface roughness, Umklapp phonon-phonon, and isotope scattering have been considered. Results for 4 nm SiGe SLs show a strong anisotropy of thermal conductivity due to the directional dependence of the phonon velocity and boundary scattering [5]. The computed values of κ show excellent agreement with the measurements on SiGe SLs [2] at both high and low temperatures. References: [1]. S.-M. Lee et al., Appl. Phys. Lett. 70, 2957 (1997). [2]. S. T. Huxtable et al., Appl. Phys. Lett. 80, 1737 (2002). [3]. G. Chen, Phys. Rev. B 57, 14958 (1998). [4]. Z. Aksamija and I. Knezevic, Phys. Rev. B 82, 045319 (2010). [5]. W. L. Liu et al., J. Nanosci. Nanotech. 1, 39 (2001).

Thermoelectric transport can have a large impact on the performance of semiconductor devices and related nanostructures. The impact is possibly most pronounced in phase-change memory cells, which experience large current densities and temperature excursions exceeding 600 °C. Recent measurements provided evidence of thermoelectric transport in phase-change cells by capturing the bias polarity dependent programming conditions. However, there are few data for the thermoelectric properties of phase-change materials at the film thicknesses relevant for contemporary devices. This work uses a novel silicon-on-insulator experimental structure to measure the phase and temperature-dependent Seebeck and Thomson coefficients of Ge2Sb2Te5 films including the first data for films of thickness down to 25 nm. The Ge2Sb2Te5 films annealed at different temperatures contain varying fractions of the amorphous and crystalline phases which strongly influence the thermoelectric properties. The Seebeck coefficient of amorphous and face-centered cubic Ge2Sb2Te5 films decreases with increasing temperature due to increased carrier concentration, as expected for non-degenerate semiconductors. The Seebeck coefficient of hexagonal close-packed Ge2Sb2Te5 films increases with increasing temperature due to increased scattering with phonons, as expected for degenerate semiconductors. The Ge2Sb2Te5 films with mixed phases are modeled with effective medium theory, and the Seebeck coefficient reduces from 371 µV/K to 206 µV/K as the crystalline fraction increases by a factor of four. The data are consistent with X-ray diffraction analysis and suggest that careful consideration of phase purity is needed to account for thermoelectric transport in phase change memory. The thermoelectric properties identified in this study improve the quality of simulations and provide a detailed view of temperature distributions. Precise knowledge about thermoelectric transports in Ge2Sb2Te5 films can thus allow the development of viable design strategies for novel phase-change memories.

Thermoelectric materials are used in solid state conversion of heat energy into electrical energy and cooling [1]. Strontium titanate (STO) based perovskite oxides exhibit excellent values of electrical conductivity and Seebeck coefficient at high temperatures [2]. Moreover, the excellent compositional and chemical stability of oxides together with the tunability of carrier density make electron doped STO a suitable material for high temperature applications. Though a high Seebeck coefficient (α) can be obtained together with a reasonably high electrical conductivity (σ), the maximum figure of merit (ZT=α² σT/lambda;) obtainable in STO is around 0.3-0.4 only, mainly due to the undesirably high lattice thermal conductivity (lambda;) [3]. With a view to minimize the thermal conductivity, superlattices (denoted by [(NO)_a/(STNO)_b]_c) are fabricated with alternating layers of 20% Nb doped STO (STNO) and a lattice matched nano-oxide (NO), using pulsed laser deposition from corresponding targets. Keeping the number of superlattice pairs c=20, the number of unit cells a and b are varied and the thermoelectric properties in the temperature range 300-1000 K are analyzed. XRD studies revealed the epitaxial growth of the layers on LaAlO₃ (LAO) substrates. Lattice resolved HRTEM together with EDX line profiles and elemental mapping is used to characterize the superlattices. An increase in electrical conductivity and power factor is observed with a decrease in a/b ratio. A maximum power factor of 0.65 Wm^-1K^-1 is obtained at 950 K, beyond which diffusion of oxygen from STNO to the NO layers causes a deterioration of the thermoelectric properties. References [1] D.M. Rowe, Thermoelectrics Handbook: Macro to Nano, CRC Press/Taylor and Francis, Boca Raton, FL, 2006. [2] B. Jalan, S. Stemmer, Large Seebeck coefficients and thermoelectric power factor of La-doped SrTiO3 thin films, Applied Physics Letters, 97 (2010) 042106. [3] C. Yu, M.L. Scullin, M. Huijben, R. Ramesh, A. Majumdar, Thermal conductivity reduction in oxygen-deficient strontium titanates, Applied Physics Letters, 92 (2008) 191191.

Manipulating structure of materials at the atomic level has been the key to controlling thermal transport, either by changing phonon dispersion or scattering mechanisms. Nanostructured systems, such as superlattices, are excellent candidates for studying structurally controlled thermal transport. They exhibit low thermal conductivity sufficient to beat the alloy limit and are good candidates for thermoelectric systems. In order to further reduce the thermal conductivity in these systems, we need to find tunable parameters, which are fundamentally related to the nature of phonon transport. Controllable disorder can be a viable candidate for such a parameter. We chose perovskite oxides, which are excellent thermoelectric materials, as a model material system for this investigation. Randomness was introduced in these structures by varying the thickness of SrTiO3 and CaTiO3 layers, but keeping the average interface density and volume fraction constant. These aperiodic superlattices were fabricated using pulsed laser deposition method, monitored by in-situ reflection high-energy electron diffraction (RHEED). The cross plane thermal conductivity of these films, measured using time domain thermoreflectance (TDTR) technique, showed thermal conductivity lower than corresponding periodic superlattices. Using theoretical models and structural analysis, we found that the correlation parameter is the distribution of strontium and calcium atoms in the random sequence, which determines the thermal conductivity of the structure. Moreover, our result confirms the theoretical prediction [1] that short-range correlations are responsible for bringing drastic changes to the phonon transmission spectrum, while the long-range correlations only changes the fine features of the spectrum. 1. N. Nishiguchi, S. Tamura, and F. Nori, Phys. Rev. B 48, 14426 (1993)

Phonon scattering in nano-structures can lead to a reduction in the thermal conductivity of materials without affecting the electrical conductivity. Therefore, understanding the frequency dependence of phonon scattering and phonon transport in nanostructures is important for designing nanostructured thermoelectric materials. Non-equilibrium phonons provide an effective means to measure the frequency dependence of phonon transport in materials. Superconducting tunnel junctions (STJ) have been previously used to generate and detect narrow frequency bands of non-equilibrium phonons in bulk materials. Recently, our group implemented STJ based phonon spectrometry in micro-scale for the ultimate purpose of measuring frequency dependent phonon transport through nano-structures[1]. In the current work, Monte Carlo simulations are reported for phonon spectrometry through Si nanosheets at low temperatures. The nanosheets are thick (>100 nm) so acoustic confinement does not play a major role, but the rough surfaces and sheet length will influence the phonon transmissivity. The simulations are performed considering phonon behavior at low temperatures and only phonon-boundary scattering is taken into account as the source of deviation in the phonon path. The simulations are performed at the full system scale considering the geometry of the actual experimental arrangement. The phonon frequency range is set to be between 80 GHz to 800 GHz (~75 nm to ~7.5 nm in phonon wavelengths). The important parameters for the simulations are found to be nanosheet surface roughness, nanosheet thickness and nanosheet length. The simulation results are compared with analytical Casimir-Ziman models for simple structures and also actual phonon transport measurements through the nano-fabricated Si nanosheets. This work is supported by the NSF DMR-1149036 and the DOE Office of Basic Energy Science under Award Number DE-SC0001086. [1] J. B. Hertzberg, O. O. Otelaja, N. J. Yoshida, and R. D. Robinson. "Non-equilibrium phonon generation and detection in microstructure devices," Rev. Sci. Inst.82, 104905 (2011).

Alloys of Bi1-xSbx, are considered as one of the best thermoelectric materials for low temperature applications below 200 K. The band structure of Bi1-xSbx varies as a function of stoichiometry. At a temperature below 77 K, it does not change with temperature. At a certain Sb composition (x=0.04), the conduction band and the valence band touch each other at the L point, and the band-crossing occurs. The electronic dispersion relation becomes linear at the L point, which implies that a Dirac cone is formed at each of the three L points. By making the alloys of Bi1-xSbx into thin films, we have two more parameters to vary the band structure, namely film thickness and growth orientation. In our present work, the rich variety of band structure configurations, as well as various phases, of Bi1-xSbx thin films has been revealed. The electronic band structures of Bi1-xSbx thin films for different growth orientations are studied. The results show that by growing the Bi1-xSbx thin film normal to a low symmetry crystalline direction other than the trigonal axis, the three-fold symmetry of the three L points in the bulk Bi1-xSbx can be broken. Specifically, by growing the Bi1-xSbx thin film along the bisectrix axis, anisotropic single-Dirac-cone can be constructed at the L point associated with this bisectrix axis. In similar ways, by choosing proper antimony compositions, growth orientations and film thicknesses, a large variety of Dirac-cone materials can be constructed based on the Bi1-xSbx thin films system, including single-Dirac-cone materials with different aisotropies, bi-Dirac-cone materials, tri-Dirac-cone materials, quasi-Dirac-cone materials and semi-Dirac-cone materials.

We use density-functional perturbation theory to compute the thermal conductivity of GaAs/AlAs superlattices using relaxation times that include both anharmonic and interface scattering effects. We find that interface disorder plays a dominant role in reducing thermal conductivity in GaAs/AlAs superlattices. For example while in perfect superlattices, the thermal conductivity drops by a factor of only about 1.5 compared to bulk GaAs, in superlattices with disordered interfaces the corresponding value is about 5 at the shortest period studied. As the period is increased, the anisotropy ratio between the in-plane and cross-plane thermal conductivity increases and asymptotically approaches that in the perfect superlattices. At 300 K almost 40% of the heat is conducted by phonons with mean freepath longer than 1 micron indicating that coherent phonons play a dominant role in heat conduction in superlattices. This provides avenues to control phonons as waves thereby opening up new pathways to engineer phonon transport and hence thermal conductivity in nanostructured materials. This material is based upon work supported as part of the “Solid State Solar-Thermal Energy Conversion Center (S3TEC), an Energy Frontier Research Center funded by the U.S. Department of Energy.

We use molecular dynamics with internal degrees of freedom (DID) with embedded atom method (EAM) potential to characterize thermal transport in metals and metal-insulator interfaces. The electronic contribution to thermal transport is described with a DID-based two temperature model that captures the local energy exchange between electrons and phonons. The thermal conductivity of pure metallic systems with varying electron-phonon coupling constants and heat fluxes is studied. The experimental thermal conductivity is recovered under conditions where electrons and phonons fully equilibrate. We characterize thermal transport at metal/insulator interfaces and metal/insulator laminates focusing on the role of non-equilibrium transport.

The misfit-layered oxide Ca3Co4O9 exhibits outstanding physical properties including high thermoelectric power, low thermal conductivity, low resistivity and high thermal stability. It was previously suggested that the Seebeck coefficient, S, can be further improved by stabilizing an increased Co-ion spin state in the CoO2 layers. Ca3Co4O9, an incommensurately layered oxide, can be best described as a monoclinic structure with two misfit-layered subsystems, a distorted rocksalt-type Ca2CoO3 layer sandwiched between two CdI2-typed CoO2 layers along the c-axis, which makes it an ideal system for studying effects such as charge transfer, strain and spin-state transitions on the material&’s thermoelectric behavior. Both subsystems share the same lattice parameters with a=4.8339 Å, c=10.8436 Å and β=98.14°, but along b-axis the incommensurate structure results in b1= 2.8238 Å for the CoO2 subsystem and b2=4.5582 Å for the Ca2CoO3 subsystem. In this presentation, a combination of thin film synthesis, atomic-resolution characterization and first-principles modeling will be used to demonstrate a significant increase in the room-temperature in-plane Seebeck coefficient of Ca3Co4O9 films grown by pulsed laser deposition on SrTiO3 substrates. By combing atomic-resolution Z-contrast imaging, annular bright field imaging and EELS spectrum imaging in an aberration-corrected cold-field emission JEOL ARM200CF with density-functional calculations, it will be shown that the increase in the in-plane Seebeck coefficient is caused by CoO2 stacking faults with Co4+-ions in a higher spin state compared to that of bulk Ca3Co4O9. In addition, the effects of oxygen disorder in the rocksalt-type Ca2CoO3 layer will be discussed using first-principles modeling, as well as the role of dopants, such as Bi and Ti. By using different substrates for the PLD synthesis, the effects of interfacial strain on the thermoelectric properties of Ca3Co4O9 thin films will be explored.