We report recent progress in development of efficient interpolation schemes for computing electron relaxation times in bulk crystalline materials from first principles. This opens up the possibility for ab initio calculations of electronic transport coefficients: the electrical conductivity, the electronic part of thermal conductivity, and the Seebeck coefficient. We find that the electronic transport coefficients are very sensitive to carrier concentration, and their accurate prediction is necessary for computational optimization of thermoelectric material composition. For a given thermoelectric material, we are able to determine the optimal carrier concentration which maximizes ZT at a target temperature. With this methodology at hand, computational screening of optimized ZT values is performed in the compositional space of half-Heusler materials selected from materials databases and consisting of cheap earth-abundant elements. Good agreement is found with the available experimental data for previously synthesized half-Heusler compounds, and several new promising candidates for thermoelectric applications are identified. Based on the results of our calculations, we also discuss the validity and applicability limits of the Wiedemann-Franz law for thermoelectric materials.

Creating resonant states by doping would lead to a density of states (DOS) hump near the edge of valence or conduction band, which has been proved as an effect way for power factor improving in thermoelectric materials, such as Tl-doped PbTe, In-doped SnTe for p-type and Al-doped PbSe for n-type. Other than this way of band engineering, recent first principle calculations showed that by increasing of the Ge site vacancies in GeTe, the slope of the DOS near the maxima of valence band (VBM) can be increased, and the peaks in DOS spectra formed by heavy and light hole respectively will move to VBM. These interesting features can help to increase the hole effective mass and improve the Seebeck coefficient, which has been experimentally proved in Pb and Bi co-doped GeTe samples prepared by ball milling and hot pressing method.

Recent works have demonstrated that the thermal phonons responsible for heat conduction possess a broad spectrum, yet this spectrum remains unknown for most materials and is not always accounted for in simulations due to computational cost. In this talk, I will describe our efforts to directly measure and engineer the thermal phonon spectrum using computation and experiment. Experimentally, I will describe our efforts to directly measure the phonon mean free path spectrum using a new experimental technique. Computationally, I will demonstrate the importance of considering the size distribution of nanostructures to achieve the minimum thermal conductivity in solids.

Nano-structured Zinc-Chalcogenides are interesting for thermoelectric applications due to their low lattice thermal conductivity(κ_l).[1] A simple theoretical model study has reported the κ_l cross-over of ZnS, ZnSe and ZnTe while going from bulk to nanoscale.[2] We have performed a systematic first principles study of the Zinc-Chalcogenides to understand their thermal transport behaviour at the nanoscale. We have applied the Boltzmann transport equation in the relaxation time approximation to calculate the κ_l of Zinc-Chalcogenides. We find a κ_l cross-over between ZnS and ZnSe around 2-3mu;m and also show interesting κ_l sim; 1W/m/K for ZnS nanostructures of 100nm size. We explain these findings in terms of the different contributions of phonon modes in these materials. Furthermore, the calculated κ_l is found to be strongly dependent on the volume and we explain the observed differences between LDA and GGA calculations. We compare further calculated thermal properties, such as thermal expansion coefficient, to experiment for validating our approach.
[1] L. Zhen, S. Qiao, D. Y. Xiang, H. Z. Zhong, and Q. L. Gao, J. Mater. Chem. 22, 22821 (2012).
[2] N. Mingo and D. Broido, Phys. Rev. Lett. 93, 246106 (2004).

Thermoelectric materials can conveniently convert heat directly into electricity without moving part due to their enhanced coupling of heat and charge transport. The efficiency is a function of hot and cold side temperatures and the material&’s non-dimensional figure of merit, zT. Significant progress has been made on thermoelectric materials in the last decade showing great promise for high thermoelectric conversion efficiency. However, in addition to good and stable thermoelectric materials, good electrical and thermal contacts are essential to reach the high potential in device efficiency which oftentimes requires an additional metallization layer on the contact surfaces of the thermoelectric material. In this work, we experimentally demonstrate a thermoelectric conversion efficiency of 8.5 % at a temperature difference of 225 °C with a p-type single thermoelectric leg based on a recently reported p-type doped MgAgSb-compound. The sample is fabricated together with silver contact pads using a hot-press technique eliminating an additional sample metallization process. This significantly simplifies the fabrication of thermoelectric elements with low electrical and thermal contact resistances which can readily be used in the device assembling process. This work was partially funded by ‘Solid State Solar-Thermal Energy Conversion Center (S³TEC)&’, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577 (for method development) and by DOE EERE under Award number: DE-EE0005806 (for thermoelectric material characterization).

Materials costs are an important theme in contemporary thermoelectrics research, for example as a critique of sophisticated nanostructuring for high ZT, or as a motivation for polymers of moderate ZT and very low cost. Here we consider the cost-performance optimization of an energy scavenging system (free heat), accounting for the costs of the material, manufacturing, and heat exchangers [1,2]. The analytical treatment [1] couples the economic, thermal, and electrical problems to minimize the system-level cost per watt, which can be related to an equivalent levelized cost of electricity (LCOE). This analysis builds on prior works which considered the materials cost alone [3], used a simplified manufacturing cost [4], had one-way coupling from thermoelectric performance to cost [5], or were purely numerical [6]. We considered realistic cost and property values for 30 bulk and thin film materials, as well as manufacturing and heat exchanger costs [2]. For the large majority of materials we find that the total cost of the $/W optimized system is dominated by the heat exchangers. In this regime we obtain simple expressions for the optimal leg length and fill factor, compatible with realistic contact resistances and other parasitics, and below which there is little additional reduction in the system $/W. The materials goal in this most common regime may be summarized as the pursuit of “high ZT at any cost,” at least up until materials costs of ~$100/cm³.
[1] S. K. Yee et al., Energy & Environmental Science 6, 2561 (2013).
[2] S. LeBlanc et al., Renewable & Sustainable Energy Reviews 32, 313 (2014).
[3] G. G. Yadav et al., Nanoscale 3, 3555 (2011).
[4] D. M. Rowe and G. Min, IEE Proc.: Sci. Meas. Technol. 143, 351 (1996).
[5] K. Yazawa and A. Shakouri, Environmental Science & Technology 45, 7548 (2011).
[6] N. R. Kristiansen et al., Journal of Electronic Materials 41, 1024 (2012).

Recently, great progresses have been achieved in developing high performance thermoelectric materials. However, for the practical applications of thermoelectric conversion technology, especially for thermoelectric power generation, one faces big challenges such as realization of high conversion efficiency, reliable service behavior, scalable assembly with competitive cost, and so on. Beyond the design optimization of structural geometry and using high ZT materials, lowering interfacial thermal and electrical resistances is the most critical technical issue for achieving high conversion efficiency. The service behavior, mainly regarding the structural stability and the performance degradation at high temperatures and under large temperature gradients, is determined by complex factors including the intrinsic stability of TE materials, the resistance of device components to the harsh environment (such as oxidation, element sublimation, large temperature drop, vibration, etc.), and the system compatibility with TE devices. In this talk, recent efforts on skutterudite and bismuth telluride based device fabrication for power generation will be presented. The technical issues on device measurement, especially on evaluating the output performance, service behavior, and life period prediction will be also reviewed.

Thermoelectric materials are being actively developed now, utilizing state-of-the-art nanotechnology and nanomaterials [1]. Boron rich compounds possess atomic networks which are formed through the covalent bonding of boron. Through the insertion of metal atoms and incorporation of elements like C, N and Si in the network, network structure and physical properties can be controlled to some degree [2]. Aluminoboride Y_xAl_yB₁₄ (x sim; 0.56, 0.3 le; y le; 0.6) has been found to show p-n control with large absolute values of the Seebeck coefficient through y variations of the y occupancy of Al site [3]. To further study Al effects on the thermoelectric properties, the Al-free compound Y_xB₁₄ (x sim; 0.56; YB₂₅) was synthesized and its thermoelectric properties were investigated. In addition, to enhance properties of Y_xAl_yB₁₄, C doping into B sites were carried out and their thermoelectric properties were also investigated. We observed that Y_xB₁₄ exhibited large positive Seebeck coefficients, sim; 1000 mu;V/K, around room temperature. Although absolute values of the Seebeck coefficient of Y_xAl_yB₁₄ were proportional to T^1/2, that of Y_xB₁₄ were largely decreasing with increase of the temperature and exhibited 380 mu;V/K at 1000 K. This indicates a strong effect of Al on the electronic structure around the Fermi level. We discovered that C can dope into B sites in Y_xAl_yB₁₄ up to around 4 at.%. Above 4 at.% C doping, the secondary phase of B₄C appears. As the thermal conductivity of 4 at.% C doped Y_xAl_yB₁₄ exhibits 2.9 Wm/K at 1000 K, which is around 1 Wm/K lower than that of non-doped Y_xAl_yB₁₄, C doping into B sites seems to be effective to decrease the thermal conductivity. We will discuss about doping or insertion effects on the thermoelectric properties of Y_xAl_yB₁₄ samples.
[1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, Springer, Heidelberg, (2013).
[2] T. Mori, Handbook on the Physics and Chemistry of Rare-earths, 38, ed. K. A. Gschneidner Jr., J. -C. Bunzli and V. Pecharsky, North-Holland, Amsterdam (2008) 105.
[3] S. Maruyama, Y. Miyazaki, K. Hayashi, T. Kajitani and T. Mori, Appl. Phys. Lett. 101 (2012) 152101.

We study the coupled transport of phonons and magnons in a single-domain ferromagnet under concurrent temperature and external magnetic field gradients, and show it is a close analogue to the conventional coupled electron-phonon transport that leads to the thermoelectric effect. Working within the framework of Boltzmann transport equation, we derive the constitutive equations for the coupled phonon-magnon transport, and the corresponding conservation laws. Especially we show both the similarity and the difference between the roles played by the electrochemical potential for electrons and the external magnetic field for magnons. Our equations reduce to the original Sanders-Walton two-temperature model under a uniform external field, but predict a new magnon cooling effect driven by a non-uniform magnetic field, which resembles the conventional Peltier effect. We estimate the magnitude of the cooling effect in yttrium iron garnet, and show it is within current experimental reach. With properly optimized materials, the predicted cooling effect can potentially supplement the conventional magnetocaloric effect in cryogenic applications in the future. This work is supported partially by S³TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, and partially by the Air Force Office of Scientific Research Multidisciplinary Research Program of the University Research Initiative (AFOSR MURI) via Ohio State University.

This talk will discuss our recent work on using first-principles simulations to investigate both electron and phonon thermoelectric transport in silicon. Electron-phonon and electron-impurity scatterings are computed from first principles calculations to obtain electron relaxation times due to both phonon and impurity scattering. The energy dependent relaxation times are then used in the Boltzmann transport theory to obtain the electrical conductivity, Seebeck coefficient and electronic thermal conductivity. The anharmonic force constants are derived from first-principles and used to compute phonon relaxation times based on Fermi&’s golden rule. The energy dependent mean free paths are computed for both electrons and phonons. The electrical and thermal conductivity accumulation functions in silicon with respect to electron and phonon mean free paths are compared. The electron-filtering concept is examined using our energy dependent transport data. It demonstrates the capability to quantitatively investigate various electron engineering approaches. By combining all electron and phonon transport properties from first-principles, we predict the full thermoelectric properties of the bulk and nanostructure of silicon over a range of temperatures from 100 to 400 K and doping levels from to cm^-3, showing good agreement with experiments. This work is supported by 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.

Low lattice thermal conductivity, k_L, is a requirement for high thermoelectric efficiency in materials [1, 2]. Over the past several years, we have developed first principles theoretical approaches to calculate k_L in semiconductors and insulators, which combine accurate determination of interatomic force constants (IFCs) from density functional theory with a full solution of the phonon Boltzmann transport equation [3-6]. Using these approaches, we have investigated k_L in thermoelectric alloys such as Si_xGe_1minus;x and Mg₂Si_xSn_1minus;x and in nanoparticle-embedded-in-alloy thermoelectric materials. More recently, we have performed first principles calculations for the prototypical thermoelectric material, Bi₂Te₃. We demonstrated that the calculated specific heat, thermal expansion coefficient and thermal conductivity of Bi₂Te₃ are in better agreement with measured values if the temperature dependence of the IFCs is explicitly included via ab initio molecular dynamics [7], a consequence of the large anharmonicity in Bi₂Te₃ and related materials. The possibility of tailoring point defect scattering as a way to achieve lower k_L and better thermoelectric performance will also be discussed. Results for which phonon-point defect scattering is calculated within the Born approximation will be compared with those obtained using an exact Green&’s function method.
[1] H. J. Goldsmid, Thermoelectric Refrigeration (Plenum, New York, 1964) [2] Bed Poudel et al., Science 320, 634 (2008); [3] D. A. Broido et al, Appl. Phys. Lett., 91, 231922 (2007); [4] A. Kundu et al, Phys. Rev. B, 84, 125426 (2011); [5] W. Li et al, Phys. Rev. B 86, 174307 (2012); [6] L. Lindsay et al, Phys. Rev. B 87, 165301 (2013); [7] Olle Hellman and I. A. Abrikosov, Phys. Rev. B 88, 144301 (2013).

Large deployment and penetration of waste heat recovery devices for low to medium temperature application require the identification and development of new non-toxic, low cost and earth abundant thermoelectric (TE) materials. To ensure their competitiveness, these materials have to show TE performances at least comparable to the ones of the current reference system, namely Sb or Se doped Bi₂Te₃. Accelerating the discovery of such new materials appears crucial for fulfilling the demand of the current energy market pull.
In the present study, we employ an integrated computational and experimental approach to search for new thermoelectric materials conforming to the boundary conditions of abundance and non-dangerousness. First principles calculations of thermoelectric transport coefficients and substitutional defect thermochemistry are used to screen materials with a high throughput. SnS doped with monovalent cations is identified as having favorable transport properties at reachable doping levels, i.e. lower than a few 10¹⁹cm^-3. We observed that highly doped SnS synthesized in ampoule and subsequently processed in pellets by Spark Plasma Sintering allows to reach electric resistivity lower than 40 mOmega;.cm at 300K. Combined with Seebeck coefficient measured up to 300 mV.K^-1, these numbers translate into a Power Factor (PF) of 200µW.K^-2.m^-1 at room temperature. This PF is comparable to new state-of-the-art thermoelectric materials like SnSe which recently attracted a lot of attention in spite of its non-innocuousness and much higher cost.

Nanostructures, including quantum dots, nanowires and artificially-grown superlattices have been shown to exhibit enhanced thermoelectric properties. This may be traced to a combination of a more highly structured electronic density of states and increased interface scattering. However, the atomic-scale deposition processes used for the fabrication of such materials are slow, expensive and yield relatively small amounts of material. For this reason, we have sought to investigate materials, in which the advantages of reduced dimensionality may be carried through into a bulk material that can be fabricated in sufficient quantities for large-scale applications. One approach we have adopted is to exploit mixed-metal sulphides containing low-dimensional structural building blocks.
Here, we will describe recent work on pseudo two-dimensional materials, A_xMS₂, containing metal dichalcogenide (MS₂) building blocks, in which the electron count may be continuously tuned and shandite-related materials, A₃Sn₂S₂, containing kagome-like sheets. In the latter, we have explored the effects of both hole-doping through chemical substitution at the main-group metal site in A₃Sn₂S₂ and electron doping though substitution at the A-cation site. Results demonstrate that variations in chemical composition can increase the figure of merit by 30 - 50% at temperatures in the range 300 - 500 °C. We also show that further improvements in the figure of merit (5 - 10 %) are achievable through control of the nanostructure in these bulk materials, either through particle size reduction or through the formation of a composite with nanoparticles of a second phase.

Electron-phonon interaction is an important factor determining transport properties of materials. Weak electron-phonon interaction is beneficial to thermoelectric properties as it can also lead to non-equilibrium phenomena. Strong coupling, on the other hand, can lead to phonon drag, which is also believed to positively affect the thermopower in FeSb2. To investigate these phenomena on a first-principles basis, using accurate treatment of transport properties, we have developed a Boltzmann solver for the coupled problem of electrons and phonons. For given electron and phonon Hamiltonians, we solve for their non-equilibrium distribution functions as a function of the coupling strength, going from the weak to strong coupling limits. Two typical crystalline structures, zincblende and rocksalt are considered and the contribution of the phase space on the relaxation rates is also investigated. This study will guide the materials scientists when trying to design a material with strong or weak electron-phonon interactions for different purposes.

A series of new ideas has been proposed during the last two decades for enhancing the figure of merit of thermoelectric materials, but the interdependences of the Seebeck coefficient, the electrical conductivity, and the thermal conductivity make the problem very difficult to solve. A new idea, proposed by Shuang Tang, which suggests an approach to achieving a more convergent path towards enhancing the thermoelectric figure of merit will be discussed. Recent progress made with pursuing this approach will be illustrated and discussed.

Advanced thermoelectric (TE) materials are useful for energy conversion, waste energy recovery and refriegeration technologies. Thermoelectric alloys and compounds with higher figure of merit are desirable for improved performance in the energy conversion. While it is well known that the nanoscale microstructure of thermoelectric materials plays a major role in the TE behavior, the control of the nanostructure is not always easy from processing point of view, reproducibility aspects, manufacturability aspects, or cost-effectiveness point of view. In this talk, some facile methods for producing the desired nano-grain structures, especially with grain boundary compositional and structural modifications such as in Bi-Sb based or other TE alloys, will be discussed. Materials processing approaches for modulation-doping to introduce different energy bandgap materials or gradient compositions at the particle-particle or grain-grain interfaces will also be described and their effects on TE properties will be discussed.

Recent improvements in the performance of thermoelectric materials have resulted from adding nano-structures in order to scatter heat carrying phonons. While this approach effectively reduces the lattice thermal conductivity, it is often accompanied by large drops in the electrical conductivity caused by mobility reductions. In this work we show that bulk forms of Half-Heusler (HH) alloys can be combined with nano-scale Full-Heusler (FH) inclusions to simultaneously improve the power factor and reduce thermal conductivity. HH structures are of the form MNiSn and MCoSb (M= Ti, Zr, or Hf) and the FH counterparts are created by filling the vacancies on the Ni or Co planes respectively, resulting in MNi₂Sn and MCo₂Sb. Previous experimental results have shown the FH nano-inclusions being coherently integrated into the matrix HH material resulting in enhanced ZT which has been attributed to an energy filtering mechanism that occurs at the HH-FH semi-coherent boundaries as well as moderate reductions in thermal conductivity by nano-inclusion phonon scattering. Ab Initio calculations, in combination with a cluster expansion, are used to test the stability of FH structures in the HH matrix and create a thermodynamic pseudo-binary phase diagram for MNiSn-MNi₂Sn compositions. In addition, electronic structure and lattice dynamics are investigated in order to elucidate possibilities for future approaches to enhance ZT. This research is supported by the Department of Energy, Office of Basic Energy Sciences under Award # DE-SC-0008574.

According to the phonon glass-electron crystal concept [1], a good thermoelectric material has glass-like thermal properties but the electronic behaviour of a crystal. Type-I clathrates and filled skutterudites possess extremely low phonon thermal conductivity. Although this has been much investigated, the physical origin is still debated. Recent inelastic neutron scattering studies [2, 3, 4] have brought important insight to the phonon dynamics of filled cage compounds. Based on these results, we present a model for the interpretation of the phonon thermal conductivity of such materials. Our model will be presented in the context of experimental thermal conductivity data of Ge-based single crystalline type-I clathrates. The thermal conductivity measurements at low temperatures were performed with the help of a standard steady-state heat-flow setup. In order to correct for radiation errors inherent to this technique at elevated temperatures, in addition, precise 3omega;-thermal conductivity experiments were performed in the temperature range between 80 and 330 K using a home-built setup.
In particular, the influence of host-framework vacancies on the phonon thermal conductivity of type-I clathrates is discussed by characterizing a series of single crystals with the composition Ba₈Cu_4.2Ge_41.2-x-y#9632;_yGa_x (x = 0, 0.2, 0.5, 1.0 and y = 1.2 - x). The study presented here includes thermal conductivity, specifc heat, electrical resistivity and Hall effect measurements.
[1] G. A. Slack, New materials and performance limits for thermoelectric cooling, CRC Handbook of Thermoelectrics, (CRC Press; Boca Raton, 1995).
[2] C. H. Lee et al., J. Phys. Soc. Jpn. 75, 123602 (2006).
[3] M. Christensen et al., Nature Mater. 7, 811-815 (2008).
[4] H. Euchner et al., Phys. Rev. B 86, 224303 (2012).
^* This work was supported by the project nanOcla within the DFG project SPP1386.

Filled skutterudites Ep_yT₄X₁₂ (T is a group VIII transition element; X is a pnictide and the electropositive element E_p fills the icosahedral voids in Wyckoff position 2a of space group Im-3) are among the most promising thermoelectric materials fulfilling the PGEC concept as worked out by G. Slack. While the rigid host structure of skutterudites maintains electronic transport, the guest atoms in the icosahedral voids of the skutterudite structure exhibit localised vibrational (“rattling”) modes. This perturbs the propagation of phonons and thereby significantly reduces the phonon contribution to the thermal conductivity. Besides a large variety of interesting ground state properties like superconductivity, or heavy fermion behaviour - some of them will be reviewed here - the possibility of substituting and doping at the various lattice sites of the crystalline unit cell allows fine-tuning the charge carrier density of this material class in order to reach those optimal levels required for superior thermoelectricity. Further improvements of the thermoelectric performance, which is measured by the dimensionless figure of merit, are possible considering nanostructuring of bulk materials as previously proposed by M. Dresselhaus.
In the present work we will show how promising thermoelectric materials can be derived within the family of filled skutterudites and how auspicious routes for further improvements of this material class can be read-off from ab-initio electronic structure calculations carried out for some members of the family of filled skutterudites. Furthermore, we will show how this knowledge, initially derived on a laboratory scale, can be used to set-up an industrial production process of filled skutterudites.
Research supported by the “Christian Doppler Laboratory for Thermoelectricity”and by the Austrian FWF, P24380.

Segmentation of thermoelectric (TE) materials is a widely used solution to improve the efficiency of thermoelectric generators over a wide working temperature range. Recently, we provided an overview of the theoretical efficiency of the best per-forming unicouples designed from segmenting the state-of-the-art TE materials [1]. In this paper, the experimental results of a high-performance segmented oxide-based thermoelectric module using segmentation of half-Heusler and misfit-layered cobaltite as the p-leg and Al-doped ZnO as the n-leg will be presented. The results of this study showed that the power density of the segmented module is three times higher than that of a non-segmented oxide module under the same condition. These finding indicated that choosing the TE materials carefully is rewarded by a significant improvement in the total module efficiency.
[1] Ngan PH, Christensen DV, Snyder GJ, Hung LT, Linderoth S, Nong N Van, et al. Towards high efficiency segmented thermoelectric unicouples. Phys Status Solidi 2014;211:9-17.

The CoSb₃ (skutterudites) based materials are promising candidates as thermoelectric materials for the middle temperature range. Several research reports have been published on skutterudites and have reported high figure of merit (ZT) [1-4], with a major feature of these materials being the filling of voids with Alkaline earth metals or Rare Earths. This helps to decrease the thermal conductivity by phonon scattering due to rattling of the guest atom [5]. But a disadvantage is the air sensivity of these guest atoms, which makes the processing difficult, especially on the bulk scale and their elevated price, making the bulk producers think of alternatives.
We report here CoSb₃-based thermoelectric materials doped with Si and Te simultaneously. This helped, not only in lowering the thermal conductivity but also produced a high power factor (>4 mW/m*K²). And an attractive ZT for Rare earths-free skutterudites is found.
High density materials were prepared by solid state synthesis followed by grinding, compacting in Cold Isostatic Press (CIP), sealing, reannealing and then sintering in Spark Plasma Sintering (SPS) under Argon atomosphere. Crystallographic analysis and thermoelectric properties of all the samples are presented.
References:
[1] G. Rogl, P. Rogl, E. Bauer and M. Zehetbauer, in “Thermoelectric Nanomaterials”, eds. K. Koumoto and T. Mori (Springer, Heidelberg, 2013) p. 193-254.
[2] X. Shi, J. Yang, J. R. Salvador, M. Chi, J. Y. Cho, H. Wang, S. Bai, J. Yang, W. Zhang, and L. Chen, J. Am. Chem. Soc. 133, (2011), p. 7837-7846.
[3] M. W. Gaultois, T. D. Sparks, C. K. H. Borg, R. Seshadri, W. D. Bonificio, and D. R. Clarke, Chem. Mater. 25, (2013), p. 2911minus;2920.
[4] Jing-Feng Li¹, Wei-Shu Liu^1,2, Li-Dong Zhao^1,2 and M. Zhou, NPG Asia Materials2, (2010), p. 152-158.
[5] G. S. Nolas, D. T. Morelli, T. M. Tritt, Annu. Rev. Mater. Sci. 29, (1999), p. 89-116.

Yb-doped n-type skutterudites have been widely studied in the past decade due to their good thermoelectric properties, yet there has been debate about the solubility limit of Yb in skutterudite phase. In this paper, a series of isothermal sections of the ternary phase diagram of Yb-Co-Sb system were studied, and different phase regions near CoSb₃ were identified. The three-phase region on the Co-rich side of Yb-doped skutterudites (CoSb₃, CoSb₂ and YbSb₂) results in a fixed skutterudite composition Yb_xCo₄Sb₁₂ with maximum Yb content at a certain temperature (for example x = 0.44 ± 0.01 at 700), whereas the three-phase region (CoSb₃, YbSb₂ and liquid) on the Sb-rich side leads to a fixed composition Yb_xCo₄Sb₁₂ with relatively lower Yb content at a given temperature (x = 0.26 ± 0.02 at 700). As temperature increases, the Yb content in both fixed compositions increases as well. This gives guideline for synthesizing samples with the optimum carrier concentration and thus optimized thermoelectric properties in a flexible composition range. The phase regions near CoSb₃ and the temperature dependence of Yb solubility in skutterudite phase also explain the debate about Yb solubility.

When the electron-engineering approach in the development of the thermoelectric materials is well established technique, the phonon-engineering part is still offering a play yard for new chemical thoughts [1]. Especially crystallographic features which reflect structural complexity are considered as one of the factors effecting thermoelectric ability of substances. Structural complexity of materials and thermoelectrica in particular may be described taking into account either their basic crystallographic characteristics as point symmetry, number of atoms per unit cell, or considering chemical and positional order/disorder, or even including into considerations thermodynamic phase diagrams and formation conditions of the compounds [2]. Large number of atoms in the unit cell was one of the first of such descriptors, which was introduced to the consideration of the thermal conductivity of thermoelectric materials [3]. To get more insight into the thermoelectric behavior, the chemical bonding descriptors were found to be suitable analytical tool. Spatial separation of the regions with different kind of atomic interactions in the crystal structure is a fingerprint for enhanced thermoelectric ability. For the compounds with the characteristic structural and bonding features, e.g. clathrates with their cage structures or oxides with crystallographic share planes, structural and bonding complexity opens an opportunity to influence more directly the thermal conductivity separating - at least partially - its lattice and electronic parts.
[1] S. Bühler-Paschen, C. Godart, Yu. Grin. In: Complex Metallic Alloys: Fundamentals and Applications, WILEY-VCH, 2011, 365ff.
[2] G. Kreiner, Yu. Grin. Chemie&More (2011) 1.
[3] G. A. Slack. In: Thermoelectric Handbook, CRC, Boca Raton, FL, 1995, 407ff.

Thermoelectric materials are usually doped with external impurity atoms which provide the required level of carrier concentration (electrons/holes) for a good electronic performance. These impurity atoms scatter the conduction carriers and limit their mobility. Such limitation can be improved by introducing new doping schemes. We have recently demonstrated three dimensional modulation doping scheme in nanostructured SiGe materials and observed about 40% enhancement in the carrier mobility compared to uniform doping. The enhancement could be much larger if a complete separation of carriers and ions is achieved e.g. by addition of a spacer layer It is possible to shield the nanoparticles with a coating layer to minimize the conduction carrier scattering and reduce the scattering cross section by 4 orders of magnitudes below the physical cross section to cloak the nanoclusters and to design invisible dopants. Extension of such a design to realistic materials can increase the carrier mobility by orders of magnitude especially at low temperatures, and can potentially increase the thermoelectric performance by two orders of magnitude. We have recently identified several materials combinations at which electronic cloaking is achievable.

Skutterudites are interesting materials for thermoelectric applications. Filling foreign atoms into the cagelike structure of a CoSb₃ skutterudite is beneficial to its thermoelectric properties. Here we demonstrate the generation and detection of coherent acoustic phonons in thin films of CoSb₃ and partially filled Yb_xCo₄Sb₁₂ skutterudites[1] using high-speed asynchronous optical sampling (ASOPS).[2] ASOPS is a femtosecond pump-probe technique that uses two mode-locked Ti:sapphire lasers to investigate the ultrafast dynamic processes in a measurement windows of ~1.25 ns.
We have measured the amorphous and polycrystalline CoSb₃ films of different thicknesses. By using a pulse echo method, the longitudinal sound velocities of amorphous and polycrystalline CoSb₃ films are obtained. In order to study the effect of foreign atoms on the heat transport in CoSb₃ skutterudite, Yb_xCo₄Sb₁₂ thin films with different filling fraction x (annealed at 300 0C, x = 0, 0.08, 0.27, 0.68; annealed at 500 0C, x = 0, 0.05, 0.12, 0.57) were also investigated. The confinement of acoustical vibrations in the Yb_xCo₄Sb₁₂ films is seen; when x = 0, a series of phonon modes up to the 5th harmonic is observed. As x increases, the strong damping of the high frequency phonon modes gives evidence of the scattering of acoustic phonons in the presence of Yb atoms. The relationship between the discrete mode frequency #402;_m and the film thickness d is #402;_m = mv/2d. Provided the thicknesses d, the longitudinal sound velocities v of the Yb_xCo₄Sb₁₂ films are also obtained. At higher filling fraction, an obvious decrease of v is observed. Since the structure of skutterudites is well maintained after filling, the decrease of v is primarily ascribed to the presence of Yb atoms.
The lattice thermal conductivity κ can be estimated using the kinetic theory of gases:[3] κ = (1/3)C_Vv_m²tau;, where C_V is the heat capacity per unit volume, v_m the mean sound velocity of the phonons, and tau; the phonon relaxation time. In the low filling fraction region (03, so the reduction of κ is mainly achieved by the strong scattering of the high frequency acoustic phonons. At high filling fractions of x = 0.57 and 0.68, the significant drop of v, together with the strong scattering of acoustic phonons, could lead to a further reduction of κ. But even at x = 0.68, v is still comparable to that of polycrystalline CoSb₃, so despite the drop of v at high filling fractions, the dominant mechanism in the reduction of κ after filling is the stronger scattering of acoustic phonons, which shortens the phonon relaxation time.
[1] C. He, M. Daniel, M. Grossmann, O. Ristow, D. Brick, M. Schubert, M. Albrecht, and T. Dekorsy, Phys. Rev. B 89, 174303 (2014).
[2] R. Gebs, G. Klatt, C. Janke, T. Dekorsy, A. Bartels, Opt. Express 18, 5974-5983, 2010.
[3] E. S. Toberer, A. Zevalkink, G. J. Snyder, J. Mater. Chem. 21, 15843-15852, 2011.

The possibility to generate electricity from an unused heat source is in particular an attractive feature of thermoelectric materials. Despite possessing a significant advantage by being environmentally friendly, a great weakness of TE materials is their limited efficiency. Therefore present research focuses very strongly on enhancing the figure of merit ZT=S²σκ^-1T. Improved efficiency will open new routes of commercial usage, and allow TE materials to become more competitive with respect to other renewable energy sources.
One of the promising ideas is to fabricate a multilayer stack, composed of alternating layers of two different materials, called a superlattice (SL). An essential concept of the SL is to reduce the thermal conductivity (κ). Reduction of κ takes place at the interfaces, where phonons scatter at the boundary between two materials. Therefore on increasing the number of interfaces one expects a decrease in the thermal conductivity. It is equally important to at the same time keep the electrical parameters (S²σ) more or less unchanged. This requirement is fulfilled by employing Half-Heusler (HH) materials to build the SL. The HH materials used in this research have the elemental formula MNiSn, where M=Ti, Zr, Hf. Since all M elements lie in the same group of the periodic table, the electronic structure of adjacent HH layers should be similar. Therefore the movement of electrons is not expected to be significantly altered.
The main emphasis of this talk will be concerning the reduced κ in SLs. The thermal conductivity of thin films is measured by the differential 3omega; method. After a brief introduction to the method, a systematic study of the TiNiSn/HfNiSn and TiNiSn/Zr_0.5Hf_0.5NiSn systems will be presented. For the SLs with a periodicity between 2 nm and 15 nm we observe a reduced κ as compared to the value measured for a HfNiSn single film, which is the material with lower thermal conductivity (κ_TiNiSn>κ_HfNiSn>κ_{SL(2-15) nm}). We in particular highlight the enormously improved crystallographic quality of our TiNiSn/HfNiSn nanostructures compared to TiNiSn/Zr_0.5Hf_0.5NiSn SLs. This high quality is clearly reflected in XRD data which show several orders of satellite peaks. Moreover, the XRD patterns are in an excellent agreement with the calculations of coherent x-ray scattering. TEM images, showing smooth interfaces are further evidence for the high quality of the studied samples. In addition to this, a presentation of the thermoelectric measurements will complete the whole picture.

Intermetallic clathrates are the representative of nano-cage structured material showing low thermal conductivity [1]. A dimorphic clathrate Ba₈Ga₁₆Sn₃₀ possesses type-I and type-VIII structures. The type-I phase transforms to the type-VIII phase on heating above 739 K. The type-I compound displays the glasslike thermal conductivity with a plateau at 4-12 K, which is ascribed to the strong interaction of acoustic phonons with the low-energy off-center rattling mode. In the type-VIII compound, the Ba guest is vibrating nearly on center of the distorted dodecahedron. Nevertheless, the thermal conductivity is as low as 0.7W/Km for T > 300 K. The carrier type and its density in single crystals can be tuned by controlling the initial amount of flux, Ga or Sn. Further optimization of carrier density by alloying has led to high ZT of 1.0 and 1.45 for p- and n-type, respectively, at around 500 K, where the conventional materials based on Bi-Te and Pb-Te had a valley in ZT. By assembling p- and n-type legs made of Ba₈Ga₁₆Sn₃₀, a module was prepared with the segments of Bi-Te legs. The conversion efficiency reached 7.5% at the temperature difference of 390 K.
[1] T. Takabatake et al., Rev. Mod. Phys. 86, 669-716 (2014).

Increasing the thermoelectric figure of merit can be accomplished via two general and effective approaches, nano- and meso-structuring to reduce the lattice thermal conductivity and altering the band structure to improve the power factor. Multiple methods of band structure engineering have been studied in this field but those with the ability to change the relative energy levels of the band near the Fermi energy and capable of aligning the energy level of the band structure of the a second phase added to the matrix are most effective. By aligning the valence band of the matrix and precipitate, high power factors can be maintained while reducing the lattice thermal conductivity by increasing phonon scattering with nanoprecipitates of the secondary phase. In the valence band of p-type PbQ (Q=S, Se, Te) and SnQ (Q= Se, Te) thermoelectric materials have shown large improvements in ZT by adding second phases which perform the functions of nano- and meso-structuring and at the same time present good band alignment. This strategy has been demonstrated in several systems including p-type PbTe-SrTe, PbSe-(CdS/ZnS), and PbS-CdS systems. Progress using this hierarchical panoscopic approach will be reviewed. Effects on carrier mobility, power factor and reductions in lattice thermal conductivity will be discussed and compared.

Thermolectrics are promising to address energy issues but their exploitation is still hampered by low efficiencies. So far, much improvement has been achieved by reducing the thermal conductivity but less by maximizing the power factor. The latter imposes apparently conflicting requirements on the band structure: a narrow energy distribution and a low effective mass. Quantum confinement in nanostructures or the introduction of resonant states were suggested as possible solutions to this paradox but with limited success. Here, we propose an original approach to fulfill both requirements in bulk semiconductors. We exploit the highly-directional character of some orbitals to produce a type of low-dimensional transport similar to that targeted in nanostructures. The method is demonstrated in Fe₂YZ Heusler compounds yielding isotropic power factors 4-5 times larger than in classical thermoelectrics at room temperature. Our concept is general and rationalizes the search for good alternative thermoelectrics. [see: http://arxiv.org/abs/1405.4685]. Work supported by the ARC project TheMoTherm and FNRS project HiT4FiT. PhG acknowledges a Francqui Research Professorship.

The p-type half-Heusler thermoelectric(TE) material, Hf_0.44Zr_0.44Ti_0.12CoSb_0.8Sn_0.2 exhibits a peak ZT of ~1 at ~800°C. In addition, previous nanoindentation studies predict a more robust mechanical behavior of this compound in comparison to other TE materials. Sample preparation involves arc-melting, high energy ball-milling and uniaxial hot-pressing. Here we report the flexural strength and fracture toughness of the half-Heusler through 800°C. Based upon the findings, the thermal shock resistance of the material is also quantified.

Zintl phases as typically small band gap semiconductors possessing complex crystal structures are considered to be ideal candidates for high efficient thermoelectric materials. These phases allow fine tuning of carrier concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric efficiency. Among bulk materials, Zintl compounds exhibit some of the lowest reported lattice thermal conductivities. Apparently, structural complexity leads to extremely low lattice thermal conductivity due to confinement of heat in low velocity, optical phonon modes, as well as additional opportunities for scattering events. Here, we will present the preparation, chemical characterization and the thermoelectric properties of several Zintl phases in the system Ba-In-Sb.

TiS₂-based superlattices are good candidate TE materials for low to mid-temperature applications. Especially, TiS₂-based inorganic/organic superlattice seems to be well suited for low-temperature TE materials as we have proposed recently. However, the ZT so far obtained is still far lower than that of conventional Bi₂Te₃ materials, even though ultralow thermal conductivity can be easily achieved by superlattice formation. Therefore, its power factor must be enhanced somehow to get higher TE performance. Here we propose a new strategy to enhance its power factor through enhancing carrier mobility and hence electrical conductivity while maintaining Seebeck coefficient.
Polar molecules coexisting with organic cation molecules in the van der Waals gap of layer-structured TiS₂ were found to affect the mobility of carrier electrons present within TiS₂ layers. The higher dielectric constant of polar molecules gives rise to the higher carrier mobility and the higher electrical conductivity, while Seebeck coefficient remained unchanged. This phenomenon may be associated with the charge shielding by polar molecules which would weaken the Coulomb attractive force between organic cations and carrier electrons. Ultralow thermal conductivity is always observed for inorganic/organic superlattices due to possible phonon anharmonicity and low thermal conductivity of the intercalated organic molecules. The highest ZT so far obtained is 0.28 at 373 K in air, but further optimization of carrier concentration and mobility must enhance the overall TE performance.

Good thermoelectric materials must have high Seebeck coefficient, good electrical conductivity and low thermal conductivity, but it is very difficult to control these three parameters independently. Creating nanostructured composites is an effective approach to enhance the thermoelectric performance. There are mainly two kinds of composite structures, which are formed by annealing or mixing, respectively. The mixed composites with exteriorly embedded nanoscale dispersions are expected to possess higher thermal stability than annealing-processed composites, in which nanoscopic precipitates may grow or coalesce when used at high temperature. However, it is of importance to select the dispersed nanoparticles for different thermoelectric matrix. We studied the effect of SiC nanodispersion on Bi₂Te₃-based and PbTe-based thermoelectric materials. It was found that mixing SiC nanoparticles into the BiSbTe matrix is effective for its performance enhancement; a high dimensionless figure of merit (ZT) value up to 1.33 at 373 K is obtained in Bi_0.3Sb_1.7Te₃ dispersed with only 0.4vol% SiC nanoparticles. Mixing SiC nanoparticles in AgPb_mSbTe_m+2 alloys also can lead to a ZT enhancement, but its mechanism is different from the case of BiSbTe alloys. Nevertheless, for both materials, SiC-dispersion could improve their mechanical property. In particular, SiC-whiskers can significantly improve the thermal stability of AgPb_mSbTe_m+2 alloys at high temperatures.

The half-Heusler materials TiNiSn and TiCoSb are very promising for thermoelectric applications. One major goal is to increase their figure of merit by reducing the thermal conductivity, which can be achieved by filling the Ti sublattice with a composition of metals. In the past, excellent results have been achieved for (Ti,Zr,Hf)NiSn alloys [1]. A reduction of the thermal conductivity has also been found for (Ti,Mn)CoSb alloys, but the presence of manganese leads to a strong reduction of the Seebeck coefficient. Experimental studies indicated that the lowering of the heat conductivity might result from the formation of domains, due to a bad miscibility of the components in the Ti sublattice [2].
We combine ab-shy;initio calculations and Monte Carlo simulations to study the thermodynamic conditions for phase separation in the mentioned families of Half-shy;Heusler alloys. In the first family, NiM_1-shy;xN_xSn (M, N=Ti, Hf, Zr), we find that phase separation is achieved at temperatures 300-shy;450 K, while for CoTi_1-xZ_xSb (Z=Sc, Cr, Mn, Fe, Cu) much higher transition temperatures are found. With our studies, we investigate the domain structures that form for various compositions, and analyse the electronic properties. On a larger scale the results can be used to study the domain formation with the help of phase field calculations. The results of our studies provide a deeper understanding of the mechanisms that lead to the reduction of thermal conductivity and help to find optimized materials in a systematic way.
1. L.L. Wang, L.Miao, Z. Y. Wang, W. Wie, R. Xiong, H.J. Liu, H.J. Liu, J. Shi. And X.F. Tang. Journal of Applied Physics, 105, 013709 (2009)
2. Tanja Graf, Peter Klaer, Joachim Barth, Benjamin Balke, Hans-Joaquim Elmers and Claudia Felser. Scripta Materialia 63 (2010) 1216-1219

Tin monoselenide (SnSe) has a layered structure at room temperature and a phase transition about 800K. It is reported to have a high ZT at the high temperature phase.¹ In that study the material was undoped; doping studies² using Ag have not resulted in good material. In this study we tried to improve the performance before phase transition by doping SnSe p-type. Band structure is calculated by KKR CPA method and a group of acceptors impurities (Ag, Na, Tl etc) are experimented with. Doped SnSe is prepared by melting-grinding-SPS process. Thermoelectric properties of low temperature are measured in a cryostat down to 80K. High temperature Seebeck and resistivity are measured in a Linseis LSR3 up to 500⁰C. The best sample has the carrier concentration of 10¹⁹/cm³ with a resistivity 20 times smaller than intrinsic SnSe at room temperature. According to the Pisarenko plot, a hole density-of-states mass of 0.8m_e is confirmed. Thermal conductivity is measured in the Anter 3000. The best ZT is 0.7 at temperatures below the phase transition.
Work supported by the US DOE “U.S.-China Clean Energy Research Center-Clean Vehicles (CERC-CV) - Energy conversion” program and the AFOSR MURI “Cryogenic Peltier Cooling”, FA9550-10-1-0533.
1. Lidong Zhao et al., Nature 508 (2014)
2. Cheng-Lung Chen et al., Journal of Materials Chemistry A (2014) Advance Article

Nanostructuring has been shown to be an effective approach in reducing lattice thermal conductivity and improving efficiency of thermoelectric materials. A challenge for experimental measurements of thermal conductivity is separating the contributions from both carriers and phonons. Building on the work of Lukas et al. ^dagger; we report measurements of thermal and electrical conductivity of single crystal metals: Cu (100), Zn (001) and Al (100) in a transverse magnetic field up to 9 Tesla. Our experiments provide a separation of the lattice/electronic components and make possible a better theoretical model of the lattice portion of the thermal conductivity in materials.
This work is supported by the Solid State Solar-Thermal Energy Conversion Center (S³TEC), an Energy Frontier Research Center sponsored by the DOE, Office of Basic Energy Science, Award No. DE-SC0001299/DE-FG02-09ER46577.
^dagger;K. Lukas et al., Phys. Rev. B 85, 205410 (2012).

Nanoscale engineering of thermoelectric materials is an important approach to enhance the thermoelectric performance, as nanostructuring potentially allows us to suppress material's thermal conductivity without significantly reducing its electrical conductivity. We have employed a combined ALD (atomic layer deposition) and MLD (molecular layer deposition) technique (for a review of the technique, see [1]) to fabricate oxide-organic thin-film superlattices in which periodically introduced single/thin organic layers between the thicker thermoelectric oxide layers are anticipated to hinder the phonon transport and/or bring about charge confinement effects thereby enhancing the material's thermoelectric figure-of-merit. Here I present our recent proof-of-the-concept data for the (Zn,Al)O:HQ [2,3] and (Ti,Nb)O₂:HQ [4] systems (HQ stands for hydroquinone) showing significantly suppressed thermal conductivities, and discuss the future potential of the hybrid inorganic-organic superlattice and related nanolaminate thin-film structures in thermoelectrics [5]. The ALD/MLD technique provides us an elegant and versatile tool to realize such new materials in a highly controlled and industrially feasible way.
[1] P. Sundberg & M. Karppinen, Beilstein J. Nanotechnology, in press (2014).
[2] T. Tynell, I. Terasaki, H. Yamauchi & M. Karppinen, J. Mater. Chem. A1, 13619 (2013).
[3] T. Tynell, A. Giri, J. Gaskins, P.E. Hopkins, P. Mele, K. Miyazaki & M. Karppinen, J. Mater. Chem. A, in press (2014).
[4] J.-P. Niemelä & M. Karppinen, submitted manuscript (2014).
[5] A.J. Karttunen, T. Tynell & M. Karppinen, unpublished works at Aalto University (2014).

Thermoelectric materials can be used to convert the heat of the sun into electricity. Under one sun of illuminaion, these solar thermoelectric generators (STEGs) have reached a conversion efficiency of 4.6%. By incorporating optical concentration, STEGs can achieve higher temperatures with lower losses, thus allowing for a higher conversion efficiency. Our models have predicted that we can reach 10% efficiency with less than 100 suns concentration. The challenge of such a system is achieving high performance, thermally stable selective surfaces and thermoelectric generators that operate at 600 C. We use segmented thermoelectric generators comprising skutterudite and bismuth telluride compounds for our current prototype. Here we report our latest demonstrated performance toward our goal to exceed 10% efficiency. This work is supported by the DOE EERE SunShot Initiative under award number DE-EE0005806.

Zn₄Sb₃ is among the cheapest high performance thermoelectric materials, and it is made of relatively non-toxic elements [1]. The ZnSb phase also has an excellent power factor, but for this material the thermal conductivity must be lowered to reach high zT values. Strong activities are aimed at developing commercial power generation modules based on Zn₄Sb₃, but the extremely complex Zn-Sb phase diagram keeps revealing new surprises [2]. In the course of developing new large scale synthesis methods for Zn₄Sb₃ and ZnSb based on Spark Plasma Sintering new bulk phases have been identified [3, 4, 5]. Likewise new metastable phases have been identified during development of very high performing zinc antimonide thin films [6]. The talk will discuss latest results on zinc antimonide thermoelectric materials.
[1] B. B. Iversen, J. Mater. Chem.2010, 20, 10778-10787
[2] J. Lin et al., J. Am. Chem. Soc.2014, 136, 1497-1504
[3] H. Yin et al., ACS Appl. Mater.2014, in press
[4] H. Yin et al., Appl. Phys. Lett.2012, 101, 043901
[5] T. Dasgupta et al., J. Appl. Phys.2013, 113, 103708
[6] Y. Sun et al., Adv. Mater.2012, 24, 1693-1696

Thanks to the recent advances in materials research, we have several good candidates of thermoelectric (TE) materials with ZT exceeding 1. These TE materials are expected to play essential roles in power generation from the waste heat especially at high temperatures. On the other hand, quite a good portion of the waste heat resides at the relatively low temperature range below 200 °C. In order to make use of such heat, desired properties for TE materials are earth-abundance, environmentally-friendliness, and stability in air, because such low-temperature heat is spread widely including in places close to our residential area. Furthermore, a high power factor is also required to recover energy from such low-temperature heat. Here we focus on mineral-based compounds, especially magnetic semiconductors, to meet these criteria. Mineral-based materials are attracting attention because they are basically composed of earth-abundant and environmentally-friendly elements, and are mostly stable in air and water [1,2]. We have been working on CuFeS₂, known as chalcopyrite [3]. We have successfully prepared heavily n-doped samples. The samples show low electrical resistivities of several mOmega;cm with large negative Seebeck coefficients around -200 to -300 mu;V/K around room temperature. This gives a power factor of about 1 mW/K²m from room temperature to about 500 K. The estimated effective mass of electrons is as large as 3 to 6 times that of a free electron, indicating a significantly enhanced mass. We interpret this mass enhancement to be due to the strong magnetic coupling of carriers with the antiferromagnetic moment of Fe³⁺. We thus propose that using magnetic semiconductors, especially mineral-based ones, can be a new and easy guideline to obtain Green TE materials ; composed of abundant and safe elements, which potentially have high power factors and are applicable to the energy-harvesting from the ubiquitous heat near room temperature.
[1] K. Suekuni et al., Appl. Phys. Exp. 5 (2012) 051201.
[2] X. Lu, D.T. Morelli, et al., Adv. Energ. Mater. 3 (2013) 342.
[3] N. Tsujii and T. Mori, Appl. Phys. Exp. 5 (2013) 051201.

The P-Type half-Heusler material, Hf_0.44Zr_0.44Ti_0.12CbSb_0.8Sn_0.2, offers a promising combination of thermoelectric and mechanical properties. Samples are prepared by arc-melting, high energy ball milling and DC hot-pressing. Compression studies are carried out on the samples through 800°C. The bulk modulus is experimentally measured and compared with elastic constants previously obtained by our group.

Transition and main group metal chalcogenides exhibit useful physical and chemical properties that are of great interest for technological applications. Materials with low thermal conductivity play a key role in many technically significant applications. In the case of thermoelectrics, a low thermal conductivity is important and research on new low thermal conductivity materials is part of our focus. Several material systems, quaternary chalcogenides such as Cu₂ZnSnSe₄ and related compositions for example, are under investigation in our laboratory. Fundamental studies on their structural features, transport properties and electronic structures considered in terms of first principles simulations for representative chalcogenides will be presented. In addition, different approaches to achieving enhanced thermoelectric properties for certain material systems will be discussed. This work aims to further the fundamental investigation of low thermal conductivity materials for potential thermoelectrics applications.

Advanced nano materials have shown great potential for high-efficient thermoelectrics (TE), but it still remains as challenges to produce such materials in scalable, reliable and economical way. Solution-phase reaction, with its advantages of low cost, energy efficiency, scalability, is a promising path to take. In this presentation, we will briefly describe our synthetic method to large-scale synthesize advanced telluride nanowires and nanowire heterostructures through solution-phase reaction and the investigation of thermoelectric properties of their nanocomposites fabricated by hot press. Particularly, we successfully mass-produced Bi₂Te₃ nanowires by more than 17 gram per batch at yield of 94.2 % in an industrial standard 1-liter reactor while still maintaining the thinness and uniformity of nanowires. It was found that as-synthesized Bi₂Te₃ nanowires have much higher carrier concentration (n-type) than optimal value. To reduce the carrier concentration, different amounts of Se was introduced and proved to be capable of optimizing the power factor and ZT of as-synthesized Bi₂Te₃ nanowires. Built on the nanowire synthesis, we also developed a general method of synthesizing telluride nanowire heterostructures using Te nanowire as sacrificial template and sit-selective conversion strategy. Their morphology imitates “barbell” of which the bar and bell parts are two different tellurides . Sharp interfaces between two tellurides can be observed under high-resolution transmission electron microscope. Through tuning the amounts of precursors and critical reactants, different compositions of two tellurides can be synthesized and found to have great impact on thermoelectric properties. Extremely small lattice thermal conductivity compared to bulk has been observed in the nanocomposites fabricated from nanowire heterostructures, leading to enhanced ZT. The most recent results show that Ag₂Te (majority phase) and Bi₂Te₃ (minority phase) nanowire heterostructure (p-type) has been able to achieve ZT of 0.41 at 400 K, which is among the best of p-type Ag₂Te.

The performance of thermoelectric materials is governed by the figure of merit ZT. To maximize ZT it is desirable to reduce the thermal conductivity, while increasing both the Seebeck coefficient and electrical conductivity. However, these physical properties are interrelated in such a manner that it is difficult to change one without negatively impacting the other(s). Here we describe attempts to tune thermoelectric properties in the Nb—Co—Sn ternary system through the use of coherent inclusions, which impede the thermal conductivity without negatively impacting the Seebeck coefficient and the electrical conductivity. The 18 electron half-Heulser compound NbCoSn is a thermoelectric material that has been relatively poorly studied in contrast to the better-known TiNiSn system. The compounds are been prepared using a combination of techniques including arc-melting, microwave methods, and levitation/induction methods, followed by annealing. Second phase precipitates of the magnetic, metallic full-Heusler NbCo₂Sn are introduced through the use of excess Co in the starting reaction mix. We report on structure, microstructure and property evolution, complemented with first-principles electronic structure calculations.

The incomplete growth of nanowires that are synthesized by template-assisted electrodeposition presents a major challenge for nanowire-based devices targeting energy and electronic applications. In template-assisted electrodeposition, the growth of nanowires in pores of the template is complex and unstable, which can result in significant variability. Here we show theoretically and experimentally that the dynamics of this process is diffusion-limited, which results in morphological growth instabilities. Moreover, we use our findings to devise a method to control the growth instability. By introducing a temperature gradient across the porous template, the spatial dependence in ion diffusion triggers a self-controlled growth of the nanowires that can control the growth instability. This strategy significantly increases the fraction of long nanowires by reducing the length variation between them. In addition to shedding light on a key nanotechnology, our results provide fundamental insights into a variety of interfacial growth processes in materials science.

A simple scalable solvothermal route has been employed to synthesize Bi₂Te₃ nanostructures using different solvents. Methanol as a solvent has shown better phase purity as compared to other solvents confirmed by XRD, Raman spectroscopy. TE property measurements of as prepared Bi₂Te₃ samples show the possibility of further improvement in ZT. This was achieved by incorporating nanoprecipitates of BiTe (50-100nm) in different mol% into Bi₂Te₃ matrix. We have shown with the Reitveld refinement for XRD patterns of pure Bi2Te3, and 8 mol% BiTe doped Bi₂Te₃ that the former has rhombohedral BiTe3 phase, and the latter has to lattice parameters corresponding to hexagonal BiTe phase in addition to rhombohedral BiTe3 phase. We have also substiantated the claim with Raman spectroscopic studies. Using thermoelectric property measurement the samples with BiTe in 8mol% shows reduced thermal conductivity significantly thereby enhancing thermoelectric performance with a record ZT=1.1 at 470K for n-type Bi-Te compound.

Gadolinium doped strontium titanate, Gd_.08Sr_.92TiO_3-δ (GST), is a known n-type thermoelectric material with a reasonably high Seebeck coefficient that is stable at high temperatures and reducing environments. However GST has not been utilized in thermoelectric devices due to its high thermal conductivity. Part of the mechanism for the electronic conductivity in GST is through and the formation of oxygen vacancies under reducing atmospheres, which are also scattering centers for phonons. Therefore an increase in the oxygen vacancy concentration should lead to a decrease the thermal conductivity due to phonon scattering, as well as an increase in the electrical conductivity due to the increase of electrons present in the bulk. However the oxygen ion conductivity is very slow in strontium titanate, making the reduction of the bulk of the material kinetically limited. A second phase material, such as Gadolinium doped ceria, Gd_.2Ce_.8O_2-δ, (GDC), which is an excellent oxygen ion conductor, would facilitate a deeper reduction by allowing the diffusion of oxygen out of the bulk and would also contribute to phonon scattering at the grain boundaries. The thermoelectric and transport properties of bulk GDC-GST as a function of the volume fraction of GDC have been measured under reducing conditions (10^-29 to 10^-21 P_O2) and elevated temperatures (600-800#730;C). DC conductivity and conductivity relaxation experiments were done to determine the optimum volume fraction of GDC in GST. Both conventional solid-state synthesis techniques and combustion synthesis techniques were used to produce composites with either micron or nanometer sized grains with the ultimate goal of maximizing ZT

Nanowires are active research field in various materials toward expression of new function. In thermoelectrics, significant enhancement of the dimensionless figure of merit ZT is predicted theoretically in nanowires. An increase in the Seebeck coefficient due to quantum confinement effect of wave function, and a decrease in the thermal conductivity due to phonon scattering at the boundary of the nanowire is expected. The enhancement of ZT was confirmed experimentally in the 50-nm-diameter rough silicon nanowire [1] owing to decreased thermal conductivity. However, the increase of the Seebeck coefficient driven from quantum confinement effect has not been reported yet. Since the ZT value enhances directly with the square of the Seebeck coefficient, the increase of the Seebeck coefficient is hoped for.
In our study, we developed bismuth nanowires encapsulated in quartz templates. Bismuth nanowires were fabricated by injecting molten bismuth into a nano-scale hole of the quartz template and then recrystallized by cooling the temperature gradually. From 50-nm to 1-mu;m-diameter bismuth nanowires were successfully obtained by utilizing this fabrication technique. Crystallinity and purity were evaluated by single-crystal XRD and Shubnikov-de Haas oscillation, and it was confirmed that fabricated nanowires were high quality. We have successfully measured the Seebeck coefficient of bismuth nanowires because the temperature difference could be determined precisely due to advantage of length over 1 mm. Furthermore, the electrical resistivity was also evaluated because the diameter and length of bismuth nanowire could be determined precisely. The measurement of the Seebeck coefficient and the electrical resistivity in the same nanowire sample is the first in the world. Differences between the thermoelectric properties of nanowires and bulk were observed in detail. A decrease of the carrier mobility was predicted by analyzing these thermoelectric properties with theoretical calculation considering the mean free path limitation. Therefore, new electrodes fabrication procedure for measurements of the Hall coefficient and the four-wire resistance of bismuth wires was established utilizing focused-ion-beam processing [2] in order to evaluate the carrier mobility.
[1] A. I. Hochbaum et al., Nature451, 163 (2008)
[2] M. Murata et al., Nanoscale Research Letters8, 400 (2013)

Environmentally friendly Fe-based full Heusler alloys have been attracting much interest as promising thermoelectric materials. Recently, we have succeeded in reducing the thermal conductivity (κ) to 3.3-4.4 W/Km, which is about one-fifth of that in the bulk reported so far, in the Fe₂VAl thin films with the small grain size of less than 50 nm[1]. The observed thermoelectric power factor (P), however, was not enough high (0.5 mW/K²m) in those films. To maximize the P in the films, the composition which determines the valence electron count (VEC) should be controlled. In this study, we fabricated Fe₂VAl-based films with a wide range of the composition by off-axis sputtering method and investigated the VEC dependence on the P.
Heusler alloy thin films were deposited on (100) MgO substrates by sputtering technique from the Fe₂VAl_0.9Si_0.1 target with Si tips at room temperature. The films were post-annealed at 800°C for 1 hour. The crystalline structure was identified by X-ray diffraction (XRD) using Cu-Kα radiation. The Seebeck coefficient (S) and the resistivity (ρ) were measured by differential and 4 probe methods, respectively. The film composition was analyzed by induction coupled plasma method. We prepared films with various compositions by the off-axis sputtering method, in which the substrate offsets the target during sputtering. This method enables the film composition to be varied gradually in a wide range because the sputtering plasma distribution differs depending on each element.
The XRD 2theta;-theta; profiles revealed that the films have polycrystalline structures with the space group of Fm3m. The grain size of all the films was estimated to be approximately 50 nm by applying Scherre&’s equation to the (400) diffraction peak. The film compositions were found to vary depending on the substrate location; the Al composition in the film decreases as increasing the offset distance from the target. As a result, we succeeded in varying the VEC between 23.6 and 24.4 continuously. Next we investigated VEC dependence of the S. The S exhibits +64 mu;V/K with the positive sign at the VEC of 23.6. Once the VEC goes across 24, at which the ρ takes a maximum value, the sign of S inverts from positive to negative. This indicates that the polarity of the carrier changes from p-type (hole) to n-type (electron) and the carrier concentration decreases in the transition region. When the VEC becomes over 24, the S shows a maximum value of -86 mu;V/K at the VEC of 24.4 (Fe_2.04V_0.98Al_0.76Si_0.21). In the film with expected low κ, the highest P is 1.71 W/mK² (ρ = 4.3 mu;Omega;m), which is approximately equal to that of the bulk with the same composition[2]. We will discuss the possible large thermoelectric figure of merit, ZT, in the Fe₂VAl-based films.
[1] A. Nishide, Y. Kurosaki, H. Yamamoto, S. Yabuuchi, M. Okamoto, and J. Hayakawa, J. Japan Inst. Metals, 76, 541, (2012).
[2] Y. Nishino, S. Deguchi and U. Mizutani, Phys. Rev. B, 74, 115115 (2006).

We apply the notion of modulation doping into n-type bulk BiAgSeS by constructing heterogeneous mesoscale composites (BiAgSeS)_0.5(BiAgSeS_0.97Cl_0.03)_0.5 and (BiAgSeS)_0.7(BiAgSeS_0.95Cl_0.05)_0.3 consisting of both undoped BiAgSeS grains and doped BiAgSeS_1-xCl_x (x=0.03, 0.05) grains. Without deteriorating the thermal power, we obtain significant enhancements both on carrier mobility and carrier concentration, compared with the uniformly doped counterpart BiAgSeS_0.985Cl_0.015; the enhanced carrier mobility validates the success of modulation doping, while the promoted carrier concentration is attributed to the possible charge transfer from Cl-rich nanoscale precipitates on the heterogeneous grain boundaries as observed under Transmission Electron Microscope to the undoped BiAgSeS grains. The synergetic effect of enhanced carrier mobility and carrier concentration results in an extraordinary figure of merit ZT ~1.23 at 773K in the modulation doped composite (BiAgSeS)_0.5(BiAgSeS_0.97Cl_0.03)_0.5, indicating modulation doping a promising means for lifting the performance of thermoelectrics.

Nanocrystalline thermoelectric compounds synthesized by mechanical alloying have improved performance, due to enhanced grain boundary scattering. This approach is particularly promising for Bismuth Telluride and related alloys, which are currently front-running materials for low-temperature applications. However, little is known about the mechanistic specifics of mechanical alloying and the conditions under which nanostructure formation is optimized. Our systematic studies in the Bi₂Te₃ system have elucidated some unusual processing trends. Firstly, the reaction of elemental powders to form the compound is shown to occur via an unexpected reaction mechanism, not reported previously in the literature. Secondly, the steady-state grain size is governed unusually strongly by the processing conditions. Crucially, the finest grain sizes are obtained under the lowest energy milling conditions, which runs counter to most literature of mechanical milling. These uncommon trends can be explained by the unique thermal and thermodynamic properties of Bi₂Te₃. From a device efficiency point of view, this work provides guidelines for optimized processing of nanocrystalline thermoelectric materials based on Bi₂Te₃ and its solid solutions.

Vacuum thermionic energy conversion is an efficient process to directly transform heat into electricity. It is based on the release of electrons from a solid by thermal energy which can be described by the law of Richardson - Dushman. Application of this relation to electron emission from a solid allows extraction of the materials work function and Richardson constant. The work function defines the operating temperature of the energy converter where a lower work function is required for a practical device. We have prepared a nitrogen incorporated ultra-nanocrystalline diamond, (N)UNCD, film structure and characterized its emission with respect to the Richardson - Dushman formalism where we extracted a work function of ~ 1.4 eV. In a vacuum thermionic converter cell similar (N)UNCD electrodes with different work function demonstrated energy conversion at temperatures > 500 °C with a significant output voltage of ~ 0.3 V saturating at ~ 600 °C. Introducing gaseous species like atomic hydrogen and ammonia into the interelectrode gap resulted in a significant enhancement in the power output of the device. This was attributed to surface ionization of the gaseous species at the low work function diamond surface. The ionization current, described by the Saha - Langmuir relation, contributes to the thermionic electron current and increases the total current in the device resulting in the observed power output increase. Surface ionization occurs when a particle as it approaches the emitter surface has its vacuum level aligned with the surface vacuum level and an electron tunnels from an electronic state in the solid to the affinity level of the molecule if they are sufficiently aligned.
This research is supported by the Office of Naval Research.

Advances in thermoelectric materials in recent years have led to significant improvements in thermoelectric device performance and thus, give rise to many new potential applications. In order to optimize a thermoelectric device for specific applications and to accurately predict its performance ideally the material&’s figure of merit ZT as well as the individual temperature-dependent material properties should be known with high accuracy. For that matter, we developed an experimental method in which the ZT as well as the individual leg properties can be measured directly in the p/n-type thermoelectric couple device. This has the advantage that all material properties are measured in the same sample direction after the thermoelectric legs have been mounted in the final device. Therefore, possible effects from crystal anisotropy and from the device fabrication process are accounted for. The Seebeck coefficients, electrical resistivities, and thermal conductivities are measured with differential methods to minimize measurement uncertainties to below 3%. The thermoelectric couple ZT is directly measured with a differential Harman method which is in excellent agreement with the calculated ZT from the individual leg properties. The errors in both the directly measured and calculated thermoelectric couple ZT are below 5% which is significantly lower than typical uncertainties using commercial methods. Thus, the developed technique is ideal for characterizing assembled couple devices and individual thermoelectric materials and enables accurate device optimization and performance predictions. We demonstrate the methods by measuring a p/n-type thermoelectric couple device based on commercial bulk thermoelectric Bi₂Te₃ in the temperature range of 30 - 150 #8304;C and discuss the performance of the couple thermoelectric generator in terms of its efficiency and materials&’ self-compatibility. This work was partially funded by ‘Solid State Solar-Thermal Energy Conversion Center (S³TEC)&’, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number: DE-SC0001299/DE-FG02-09ER46577 and by DOE EERE under Award number: DE-EE0005806 (for Bi₂Te₃ characterization).

Energy converters that make use of thermoelectric alloys have attracted attention due to their numerous interesting features such as solid-state operation, zero-emissions, vast scalability, low maintenance, and a long operating lifetime. Electrodeposition of thermoelectric materials, including binary and ternary compounds, have been widely investigated because the techniques has many advantages such as low cost, rapid deposition rate, and the ability to control microstructures and crystallinities of the deposited materials by adjusting the parameters of the electrodeposition process. Although the feasibility of synthesizing thin films and nanowires through electrodeposition was demonstrated, the thermoelectric characteristics and morphologies of electrodeposited structures were poorer than those of their corresponding bulk materials. Recently, electrodeposited thin films using surfactant and pulsed electrodeposition exhibited extremely smooth surfaces with a dense structure and with a lower Seebeck coefficient than that of bulk counterparts. In addition, carrier energy filtering effect has not been observed in the electrodeposited films. In the present study, we performed systematic studies of Bi-Te-Se films deposited by pulsed electrodeposition at room temperature and correlated their material/structural properties to their thermoelectrical/electrical properties. In addition, post-annealing in a reducing environment resulted in an improvement in the crystal structure without the evaporation of the Te element. This result was demonstrated by a reduction in the electrical resistance and decrease in the FWHM of the main diffraction peaks as well as the power factor (σS²) due to carrier energy filtering effect. Details will be presented.

Thermoelectric devices are a promising technology for reclaiming energy that would otherwise be lost as waste heat and converting it to usable electricity. A major hurdle to this goal is the limited performance of thermoelectric materials and the evaluation of new high performance materials. Determination of the constituent properties of the figure of merit ZT is generally done via separate measurements of the thermal conductivity, the electrical conductivity, and the Seebeck coefficient. Separate measurements of these properties not only slows down testing, but also allows more opportunity for error to be introduced when comparing different materials. As an alternative to separate measurement of the individual properties that comprise ZT, the properties can be simultaneously measured on the same sample. As all measurements are conducted simultaneously on the same sample, the test conditions for each property are truly identical ensuring the accuracy of the ZT characterization. Numerical simulations are used to investigate the impact of device design on measurement accuracy. Then, the device and methodology are validated using standard thermoelectric materials characterized using a high throughput testing apparatus. The testing rig is designed to allow rapid measurement of multiple samples at once. This design provides a platform for rapid, high accuracy characterization and comparison of materials.

We have observed melting of current-carrying silicon micro-wires starting at one end and shown that Thomson heat in semiconductors is very significant under extreme thermal gradients (> 1 K/m) [1]. We have identified generation-transport-recombination of minority carriers as the main contributor to Thomson heat in semiconductors at elevated temperatures. The analysis presented in [1] assumes equilibrium carrier concentrations. To determine exact dynamic carrier concentration profile and thermoelectric contributions, we have developed a new model that evaluates electron and hole concentrations, potential profile, and band structure by self consistently solving drift-diffusion, current continuity and Poisson&’s equations including appropriate carrier recombination mechanisms (SRH, Auger etc.). We are able to model nonhomogeneous material systems by using electron affinity as a material parameter in this framework. This model is particularly useful to simulate nanoscale electro-thermal devices, such as phase change memory [2] where significant thermoelectric effect is present due to high operating temperature with extreme thermal gradient [3].
References
[1] G. Bakan, N. Khan, H. Silva and A. Gokirmak, "High-temperature thermoelectric transport at small scales: Thermal generation, transport and recombination of minority carriers," Scientific Reports, vol. 3, 2013.
[2] A. Faraclas, N. Williams, A. Gokirmak and H. Silva, "Modeling of Set and Reset Operations of Phase-Change Memory Cells," Electron Device Letters, IEEE, vol. 32, pp. 1737-1739, 2011.
[3] A. Faraclas, G. Bakan, L. Adnane, F. Dirisaglik, N. Williams, A. Gokirmak and H. Silva, "Modeling of Thermoelectric Effects in Phase Change Memory Cells," Electron Devices, IEEE Transactions On, vol. 61, pp. 372-378, 2014.

Engineering materials with improved thermoelectric performance requires efficient and accurate simulation of thermal transport processes in the solid state. We present and discuss two approaches for improving the computational efficiency of methods for solving the phonon Boltzmann transport equation, in the context of applications to problems involving disparate time and length scales, such as simulation of Transient Thermoreflectance experiments and calculation of the effective thermal conductivity of heterogeneous materials.
We first present a deviational Monte Carlo method for solving the linearized Boltzmann equation that treats deviational phonons independently and as a result, requires no time or space discretization. In addition to being several orders of magnitude more efficient than the recently developed deviational simulation methods, this algorithm is particularly efficient for problems evolving over a wide range of timescales, since each particle is evolved at its own characteristc timescale. The second class of algorithms is based on an adjoint formulation of the linearized Boltzmann equation and is particularly suited to problems requiring enhanced resolution in certain regions of the solution domain. Such methods are useful for resolving individual modal contributions to the effective thermal conductivity that is of particular interest in the context of engineeering materials with optimal thermoelectric performance. Simulation methods and their performance will be discussed in the context of applications to materials and devices of practical interest.

Because thermoelectric efficiency improves with increased Z, the ratio of the thermoelectric material&’s electronic (σ_e) to thermal (κ) conductivity, quadratically amplified by its Seebeck coefficient (S, in units of a few mV K^-1 for metals to ~10² mV K^-1 for semiconductors), researchers seek to modify materials properties to increase σ_e or decrease κ. Historically, creating a composite comprising a mix of a good thermal insulator with a good electronic conductor does not improve Z. We find, however, that manipulation of function using macroscale composite materials in which key components are nanometric offers a means to pry apart the interdependency of all three key thermoelectric properties found in Z [1]. We have created a composite object in which the low thermal conductivity of the primary substrate (silica fiber paper) establishes the thermal transport character of the object while the electronic conductivity necessary for device performance is carried by an ultrathin, nanometric shell of conductive RuO₂ (2-3-nm thick) wrapped around the 10²-10³-nm diameter silica fibers [2]. Because both RuO₂ and RuO₂-wrapped SiO₂ paper [RuO₂(SiO₂)] express the low Seebeck coefficient expected of a metallic conductor, a second shell comprising a nanometric conformal layer of a thermoelectric material (Bi_xTe_y) is electrodeposited atop the RuO₂ nanoshell in order to augment S. The Seebeck coefficient of the triaxial object increases an order of magnitude without affecting the object electronic conductivity of the core: the RuO₂(SiO₂) paper. This promising result indicates that each component of the composite paper dictates one key thermoelectric property. We can now explore the possibility of dialing into a macroscale object#8213;in an independent fashion#8213;the magnitude of the normally interdependent properties required to raise ZT above 1.
This work is supported by the Office of Naval Research and Wayne State University.
[1] D.R. Rolison, U.S. Patent Application S/N 13/433620, 2011.
[2] C.N. Chervin, A.M. Lubers, K.A. Pettigrew, J.W. Long, M. Westgate, J.J. Fontanella, J.C. Lytle, and D.R. Rolison, Nano Lett. 2009, 9, 2316.

Thermal resistance is a critical component of thermoelectric materials and devices. Here we present a fundamental experimental study of the thermal resistance of the interface between two single crystals of silicon. Two bonding configurations are characterized: bonds formed between a pair of hydrogen-terminated surfaces, and bonds formed between a hydrogen-terminated surface and an oxide-terminated surface. We are able to extract the thermal resistance of both types of single interfaces, and we find that the oxide-to-hydrogen interface is 3 times less resistive than the interface formed from two hydrogen-terminated surfaces. The method we use to access such a single interface involves ultra-thin single-crystal sheets of silicon known as nanomembranes. These semiconducting sheets of material have been shown to be readily transferrable, resulting in robust positioning, alignment, and stacking. Because of the thinness of nanomembranes, the interfaces between crystal regions are expected to play an extremely important role in device performance when compared to bulk samples. The thinness also enables us to bring these interfaces very close to the surface of the semiconductor samples for better measurement. In this work we study interfaces created by mechanically transferring and stacking single-crystal silicon nanomembranes onto bulk silicon substrates. We transfer hydrophobic nanomembranes onto both hydrophobic and hydrophilic substrates and anneal at low temperature. We measure the thermal resistivity through the single interfaces formed, as well as that of two interfaces in series with a thin sheet of single crystal silicon intervening. We find that the inclusion of hydroxyl groups at the interface of the hydrophobic-to-hydrophilic samples lowers the thermal resistance through the interface when compared to a hydrophobic-to-hydrophobic interface. The resistance through the hydrophobic-hydrophilic interface is 2.79 ± 0.87 m²K/GW, while the resistance through the hydrophobic-hydrophobic interface is 9.16 ± 2.31 m²K/GW. We discuss these results using a hybrid theoretical model that includes diffuse mismatch at the interface as well as van der Waals and covalent bonding. This work is supported by DOE (DE-FG02-03ER46028).

The effect of Bi doping on the nanostructure and thermoelectric (TE) performance of the polycrystalline calcium cobaltite Ca_3-xBi_xCo₄O₉ (x=0, 0.1, 0.2, 0.3 and 0.4) is reported. The polycrystalline pellets were prepared using conventional uniaxial pressing and sintering of powders which were made by a chemical sol-gel route. From room temperature to 1073 K, the electrical resistivity decreases as the Bi doping level increases up to 0.3, while the Seebeck coefficient has minor change, especially at high temperature. The power factor of the Bi doped calcium cobaltite is over 0.65 mWm^-1k^-2 at 298 K, which is currently the highest power factor reported for polycrystalline calcium cobaltite with various dopants. Thermal conductivity decreases slightly as the Bi doping level increases up to 0.3. Further increasing Bi doping to x=0.4 results in significant increment of both the electrical resistivity and thermal conductivity. Significant Bi segregation at the grain boundaries was observed under analytical Scanning Transmission Electron Microscopy. The grain boundaries themselves and the segregation of Bi at the grain boundaries play favorable roles in reducing the thermal conductivity of the bulk calcium cobaltite.

Thermoelectric (TE) device architectures usually follow a conventional bulk thermo-element design where heat transfer is across the plane of pellet-shaped elements, or a thin film thermo-element design where heat transfer is often in the plane of the film. Here, we fabricate and analyze the performance of a hybrid device architecture that employs thin film thermoelements supported on a plastic substrate that are oriented in a corrugated fashion to resemble a convention bulk thermoelectric architecture. The newly developed structure consists of thermoelectric elements deposited onto a polyimide substrate. Uniaxial heat shrink PET is then adhered to the polyimide in select locations. Heat is applied to the structure, causing the PET sheets to shrink and forcing the polyimide interlayer to form a sinusoidal shape. This process represents a low-cost, scalable fabrication process for large area thermoelectric applications. Thermoelectric elements consisting of Ni and Ag are used as a model system and are deposited onto the polyimide by thermal evaporation. The experimental performance is then accurately modeled using finite element modeling, which is used for geometric optimization of the corrugated TE architecture. Advanced thermoelectric materials that meet the flexibility and low processing temperature requirements are also explored to determine the performance limits of this system. As expected the power density of the device is significantly lower than conventional bulk device architectures, however this also lowers thermal interface requirements. Through design optimization, performance advantages are found is large area applications, particularly when considering the low-cost processing approach and lower heat sink demands.

A simple, surfactant-free, and high-efficient solvothermal approach has been developed to synthesize AgPbmSbTem+2 multicomponent thermoelectric sub-microwires. The AgPbmSbTem+2 nanostructures were prepared using Sb2(NO3)3, Pb(NO3)2, AgNO3 and Na2TeO3 as the reactants, ethylenediamine and ethylene diamine tetraacetic acid as the reducing and complexing agents. A diffusion-limited reaction mechanism was proposed to explain the formation of the nanostructures. The Seebeck coefficient of the bulk pellet pressed by the obtained AgPb18SbTe20 sub-microwires exhibits 22% enhancement over that of the corresponding bulk at room temperature. The synthetic route can be applied to gain other multicomponent telluride nanostructures.

This talk will present results of experiments intended to map out the thermomagnetic transport tensors of single crystals of elemental heavy rare-earth metals. Emphasis has been placed on the evolution of transport behavior between their various magnetic phases, especially their unusual helical antiferromagnetic state. Nernst data on elemental heavy rare-earth metals contain an ordinary and an anomalous component in the paramagnetic phase; the latter provides evidence for field-induced local ordering in this phase. Results of magneto-thermopower, magneto-thermal conductivity, and Nernst coefficients on single crystal samples of these elements will be reported.
This work is supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-0822215 and the ARO Materials with Extraordinary Spin/Heat Coupling MURI under Grant No. W911NF-14-1-0016.

Thermoelectric materials have been widely investigated for one of the green energies in our life. Application devices consisted of Bi-Te and Pb-Te based thermoelectric materials are used owing to their superior power factor, thermo conductivity and figure of merit (ZT). However, those materials include toxic element and are not suitable for environmental-friendly. From a lot of candidate thermoelectric materials have been studied, one of the most important parameter, thermal conductivity, should be needed to control for getting superior ZT. This parameter is basically depended on phonon and career (electron and hole) in the structure. In this study, we focus on conductive hexagonal BaTiO₃ (h-BTO) with various oxygen vacancies because h-BTO has complex stacking structure. We could expect that c-axis oriented h-BTO shows low thermal conductivity. Orientation dependence of thermoelectric properties in h-BTO single crystal was investigated. Single crystal h-BTO was prepared by Floating-Zone (FZ) method. h-BTO single crystal was annealed at 800^oC, 20h under oxygen atmosphere and then, annealed under hydrogen atmosphere with various time and temperature. We obtained h-BTO single crystals with various oxygen concentrations. Crystal structure and oxygen concentration of h-BTO was measured by XRD and gravimetric method, respectively. As comparison with c- and a-axes oriented h-BTO, c-axis oriented h-BTO showed smaller thermal conductivity than a-axis oriented h-BTO because of difference of stacking structure. This result is caused by complex structure consisted of Ti₂O₉ and TiO₆ along c-axis oriented h-BTO. On the other word, phonon dispersion of c-axis oriented h-BTO is larger than a-axis one. Electrical and thermal conductivities increased with increasing oxygen vacancy, however Seebeck coefficient decreased. Totally, ZT was increased with increasing oxygen vacancy.

The conversion efficiency of thermoelectric devices are highly material-dependent through a parameter called the figure of merit (ZT = S²T/ρκ, where ρ is the electrical resistivity, S is the Seebeck coefficient, κ is the total thermal conductivity and T is a temperature). Considering that the ZT value is scaled by the temperature, good thermoelectric materials also need high thermal stability apart from other fundamental properties. Therefore transition metal nitrides are interesting candidates for thermoelectric applications, since they have good mechanical properties and are thermally stable. We have demonstrated that ScN thin films is a promising thermoelectric material because of its high power factor (S²/ρ) of 2.5×10^-3 W/mK² at 800 K [1]. This result can be explained by nitrogen vacancies generating an asymmetric sharp feature in the ScN density of states which allows low electrical resistivity with a retained, relatively large, Seebeck coefficient [2]. However, ScN still has high thermal conductivity, and thus its figure of merit is rather low, about 0.2 at 800 K. With this is consideration, there are suggestions to reduce lattice thermal conductivity contribution through nanostructuring (alloying, nanoinclusions or spinodal decomposition). Therefore CrN is chosen to study as an alloying material due to the report from Quintela et al. which showed that CrN on its own also have a high Seebeck coefficient and low thermal conductivity [3]. Due to the material needs to operate at high temperature, it is important to first know the mixing thermodynamic trends in Sc_xCr_1-xN mixtures. Therefore, we have investigated the trends in mixing thermodynamics of Sc_xCr_1-xN solid solutions in the cubic B1 structure by first-principle calculations. Moreover, experimental studies have been carried out synthezising Sc_xCr_1-xN solid solution thin films by reactive magnetron sputtering, and their structure, thermoelectric properties and thermal stability are investigated. The results are used to discuss suitable candidate materials and general strategies to reduce the high thermal conductivity in ScN while aiming to retain the high power factor.
[1] S. Kerdsongpanya et al, Appl. Phys. Lett. 99, 232113 (2011).
[2] S. Kerdsongpanya et al, Phys. Rev. B 86, 195140 (2012).
[3] C. X. Quintela et al, Appl. Phys. Lett. 94, 152103 (2009)

As discussed in the invited talk submitted to the Symposium BB [1], the most constraining limitation in using composite thermoelectric materials is given by the effective medium theory [2,3]. When one considers a composite made from two thermoelectric materials, A and B, in the absence of interactions between them the thermoelectric figure of merit of the composite cannot exceed the highest of the figures of merit of either A or B [2]. However, it is possible for thermoelectric power factor of the composite to exceed the highest of the power factors of either A or B [3]. In this talk, we focus on one specific case, based on the study on BiSb alloys which are the best n-type thermoelectric materials in the cryogenic temperature range. Polycrystalline n-type BiSb alloy composites made by inserting electrically insulating medium show that phonons are scattered more strongly than electrons under certain circumstances. Detailed experimental procedure and data are presented.
Work supported by AFOSR MURI “Cryogenic Peltier Cooling” Contract #FA9550-10-1-0533
[1] J. P. Heremans and H. Jin, “Overcoming the limitations of the effective medium theory in thermoelectric materials”, Symposium BB: Molecular, Polymer and Hybrid Materials for Thermoelectrics, MRS 2014 Fall Meeting, Boston.
[2] D. J. Bergman and O. Levy, J. Appl. Phys.70, 6821 (1991).
[3] D. J. Bergman and L. G. Fel, J. Appl. Phys.85, 8205 (1999).

Rocksalt based compounds composed of groups I-V-VI₂ have shown¹ minimal lattice thermal conductivity caused by phonon-phonon scattering linked to the very high Grüneisen parameter induced by lone pair electrons. Although lattice instabilities cause many of the compounds to crystallize in structures other than rocksalt, the marginally stable compounds are promising in the design of thermoelectric materials because not only of their low lattice thermal conductivity but also of their favorable valence band structure. In this application, the electronic properties of these compounds must be optimized by adjusting the p-type doping level. Here we examine the electronic properties of an alloy of two alkali based compounds, LiSbSe₂ and LiSbS₂. We report the effects of off-stoichiometry on the hole concentration. We further use Sn substitution for Sb as an acceptor impurity. All studies are driven by theoretical calculations. This work is supported as part of the “Revolutionary Materials for Solid State Energy Conversion (RMSSEC),” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science 61-3212B
1. M. D. Nielsen, V. Ozolins and J. P. Heremans, Energy Environ. Sci., 6, 570 (2013)

Higher manganese silicides (HMS), MnSi_δ (δ ~ 1.7), have attracted great interest as promising thermoelectric materials owing to the natural abundance of constituents. However, the difficulty in obtaining a single crystal of HMS due to the peritectic nature prevents an improvement of the thermoelectric performance [1]. One solution is utilizing nanostructures realized in thin films especially for reducing a thermal conductance (κ). In thin films, small grains would work as scattering sources of phonon. Furthermore the material composition is easily controlled. We thus studied both thermoelectric properties and crystal structures of HMS thin films by varying the Si/Mn composition.
HMS thin films with the thickness of 100 nm were deposited on thermally oxidized Si wafers by RF magnetron sputtering technique with a base pressure of 10^-7 Pa. MnSi_1.75 and Si targets were simultaneously sputtered and the Si/Mn ratio was controlled by changing the Si sputtering power up to 450W while that of MnSi_1.75 was kept at 300W. The films were annealed at 800°C in a vacuum of 10^-6 Pa for 1 hour. The film composition was determined by inductively coupled plasma method. The in-plane resistivity (ρ) and Seebeck coefficient (S) were measured by 4 probe and differential methods, respectively. The out-of-plane k was evaluated by pico-second laser flash method. And the crystal structure was characterized by X-ray diffraction (XRD) and cross-sectional transmission electron microsopy (TEM).
We evaluated the change of textures in the obtained films by increasing the Si amount. XRD measurements clarified an existence of MnSi with small amount of HMS when the Si/Mn ratio is 1.2. The fraction of HMS increases with Si amount and MnSi disappears when the ratio is over 1.65. These results indicate that we successfully changed silicide phases towards the single phase of HMS. Next, we investigated nanostructures of the sample with the Si/Mn ratio of 1.2 from cross-sectional TEM images. The thin film consists of MnSi grains with the size ranging from 50 to 100 nm while even finer HMS particles of about 5 nm distribute in the part of MnSi matrix.
In addition to the film texture, thermoelectric properties were also measured. In the film with Si/Mn ratio of 1.2, both the ρ and S are small: ρ = 4.1 mu;Omega;m and S = 36 mu;V/K at room temperature. This is because metallic MnSi exists as a mother phase. On the other hand, the κ is 5.6 W/mK, which is rather smaller than that of bulk MnSi possibly due to phonon scattering in the multi-scale nano-composite. Both ρ and S show an enhancement with increasing the Si amount and the S reaches 210 mu;V/K with the high ρ over 300 mu;Omega;m when the ratio is over 1.65. We will also discuss the thermoelectric feature of the obtained films at high temperatures.
This work was supported by TherMat, Future Pioneering Projects of METI, Japan.
[1] T. Itoh and M. Yamada, J. Elec. Mater. 38 (2009) 925-929.

SrTiO₃ (STO) is a promising n-type oxide for high-temperature thermoelectric applications [1]. We report the effect of STO-based superlattice structures in enhancing the thermoelectric figure of merit (ZT) of STO. Superlattices with alternating layers of Pr-doped SrTiO_3-δ (SPTO) and Nb-doped SrTiO_3-δ (STNO) have been successfully fabricated on LaAlO₃ substrate using pulsed laser deposition (PLD). The stability of these superlattices, as a function of time, temperature, and individual layer thickness, is reported. In addition, we extracted the in-plane thermal conductivity of superlattices by using the continuum analysis along with the measured thermal conductivities of the individual SPTO and STNO layers using the 3omega; method. The optimum superlattice exhibits a high ZT of 0.46 at 1000 K. Our work demonstrates the importance of nano-structuring approach in improving ZT of metal oxides which are useful for high temperature energy harvesting applications.
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[1] Hiromichi Ohta, Materials Today 10 (10), 44 (2007).

Microstructure refinement of Mg₂Si-Si eutectic alloy through solidification was investigated in the context of improving its figure of merit for thermoelectric applications. Influences of rapid cooling (injection mold cast and melt-spinning), directional solidification, and addition of alloying elements on the microstructure have been investigated. Only rapid cooling is effective in microstructure refinement among the three approaches. Rapid cooling can produce a microstructure of distorted Si rods with a diameter of ~100 nm near ingot surfaces where the most rapid cooling occurs. Directional solidification produces microstructure on the order of micrometers or larger and the variation in cooling rate does not significantly change the length scale of microstructure. Cu, Ce, and Na additions show no effect on microstructural refinement for directional solidification. However, Na addition modifies the microstructure resulting in a colony morphology. The general microstructural feature is isolated Si in a continuous Mg₂Si matrix, regardless of whether Si is elongated or not. The Si in rapidly cooled alloys becomes less elongated and significantly coarsens after annealing at 600 °C for 1 week.

A comparison between reported analytical results with experimental data of the magnetic flux density on cylindrical tin inclusions of elliptical cross-section embedded in a copper matrix under external thermal excitation is presented. By changing the aspect ratios b/a designated by e of the elliptical inclusions, a wide range of real situation such as slender inclusions can be simulated. The experimental magnetic field distribution illustrated the potential for the non contacting thermoelectric method to detect and characterize metallic inclusions of different geometries based on their magnetic signature. Preliminary results on a cylindrical hard alpha (TiN) inclusion embedded in Ti-6Al-4V matrix is also presented to demonstrate that the proposed non-destructive method might be applicable to a wide range of alloys including high-strength, high-temperature engine materials.

In the transverse thermoelectric (TransTE) effects, directions of electrical current and heat flow are decoupled, providing an opportunity to individually tune the electrical and thermal properties. However, efficiencies of TransTE materials are inferior to their counterparts operating in the conventional mode, which seems to be true as demonstrated by a few recent experimental studies. In this presentation, the concept of engineering layered composite materials for TransTE is revisited. Using analytical and numerical tools, we will show the performance of transverse TE materials can be significantly improved. When appropriate materials and design parameters are selected, the ZT (dimensionless figure of merit) values of TransTE materials may approach or even surpass those of the constituting phases. For example, the theoretic ZT of a material consisting of CNT-doped Bi_0.5Sb_1.5Te₃ and Pb can be as high as 0.89 at 300 K. Therefore, we believe the potential of TransTE is overlooked, and deserves more attention.

Bismuth telluride (Bi₂Te₃) thermoelectric (TE) materials is considered to be best materials under 3000C. The efficiency of TE materials is denoted as “figure of merit Z=S²σ/κ”, where, S is Seebeck coefficient, σ is electric conductivity, and κ is thermal conductivity. This equation indicated that, to improve efficiency, simultaneous achievement of high electrical conductivity and low thermal conductivity should need, nevertheless it was not compatible in common materials. On the other hand, L.D.Hicks et.al reported that reducing the grain size of TE materials into nanometer level will satisfy the requirements. Therefore, many efforts have been made to increase the ZT value of Bi₂Te₃ materials.
Here, we reported that well crystallized and uniform metal alloy nanoparticles could be synthesized by controlling metal complex in the aqueous solution based on metal complex calculations using the critical stability constants [1]. Needless to say, redox potential of metal ion and/or complex in solution varies according to their condition. This means that the reduction route could be controlled by controlling the metal complexes. Therefore, redox potentials of Bi and Te species are controlled by adding complex agent into aqueous solution to optimize the homogeneous redox potentials. As a results, nano sized and well crystallized Bi₂Te₃ nanoparticles are synthesized at near room temperature, in the aqueous solution. To increasing the Z, at the next step, size control within nm level should need.
Therefore, in this study, size controlled synthesis method of Bi₂Te₃ nanoparticles in aqueous solution for improvement of thermoelectric properties was developed.
10ml of 0.5mmol Bi(NO₃)₃#12539;H₂O with 0.03M 2Na#12539;EDTA aqueous solution, 10ml of 0.75mmol Na₂TeO₃ aqueous solution, and 1-50 ml of 0.2M ascorbic acid aqueous solution as reducing reagent, was independently adjusted. The pH of all solutions with no precipitation was controlled to 10 by using 1.0M NaOH solution. Then adding condition and treatment temperature was changed. After that samples were filtered and washed with purified water and dried for 12 hours at 60#730;C. Samples were analyzed by SEM-EDX and TEM EDX.
Results of XRD analysis showed that many peaks corresponding to Bi₂Te₃ (PDF#15-086) was apparently observed, and also peaks of metal Te (PDF#36-1452) was appeared with increasing the treatment time. From these results, it expected that reduction rate of Te complex will be classify into two, one is that for the synthesis of Bi₂Te₃, and other is that for the synthesis of Te. Average particle size was decreased with increasing the treatment temperature from 50nm (500C, 1.8M) to 42nm (70 0C, 1.8M), or with increasing the concentration from 50nm (70 0C, 1.1M) to 42nm (70 0C, 1.8M). Other results will reported in our presentation. This work was supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (Start-up) 25886001. [1] Applied Catalysis A: General, 392, 80-85 (2011)

Thermoelectric properties of a compound are defined by its crystal structure. The optimization of the charge carriers concentration is a challenging task since it is often accompanied with structural transformations and/or phase decomposition. Clathrates Ba₈M₁₆P₃₀ (M = Cu, Au) exhibit low lattice thermal conductivity. Type and concentration of the charge carriers in these materials can be adjusted by the aliovalent substitutions in the clathrate framework. The substitution of Zn for M in Ba₈M₁₆P₃₀ results in a series of structural transformation from orthorhombic to cubic to rhombohedral symmetry. Detailed analysis of these transformations and associated re-distribution of Zn and M atoms over framework positions by X-ray and neutron powder and single crystal diffraction will be presented. The correlation between the crystal structure, distribution of the metal and phosphorus atoms over the clathrate framework and thermoelectric properties will be discussed.

Theoretical calculations [1] have revealed the presence marginally stable acoustic phonons which have extremely large Grüneisen parameters resulting in strong anharmonicity in heat-carrying acoustic phonon branches of select rocksalt I-V-VI₂. Here, we expand on our findings of a new simple method of predicting Grüneisen parameters in this class of materials, using readily available information on the ionization and the hybridization of the chemical bonds involved which avoids extensive numerical simulations. We also present current advances in understanding and doping of Noble Metal and Alkali based I-V-VI₂ materials based on theoretical calculations and experimental work.
This work is supported as part of the “Revolutionary Materials for Solid State Energy Conversion (RMSSEC),” an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
[1] Michele D. Nielsen, Vidvuds Ozolins and Joseph P. Heremans, Lone pair electrons minimize lattice thermal conductivity, Energy Environ. Sci., 6, 570 - 578 (2013)

We present a new concept of electron emission from semiconductors we call "beta-enhanced thermoelectron emission," or BETE. BETE is the combination of two phenomena: thermoelectron emission in which electrons are emitted from a heated body, and the generation of electron-hole pairs within a semiconductor under beta irradiation. Incident radiation generates electron-hole pairs in a semiconductor and at elevated temperatures this generation enhances the thermoelectron emission above simple Richardson-Dushman emission. BETE is a minority carrier effect for which electrons are the minority carrier. BETE could be incorporated in a vacuum thermoelectron engine for a novel, high efficiency direct energy conversion device for a number of different applications.
An electron diffusion model is presented to quantify BETE. We show results of the model applied to a III-nitride material under beta irradiation from tritium and 63-Ni. The output power and efficiency of a BETE-based thermoelectron engine is then calculated using these results.

Thermal rectification in nanostructured materials is an active topic of research and development. Here we suggest that porous semiconductor materials can offer unmatched tailoring of its structural properties, resulting in both the ability to study the effects of nanoscale morphology on thermal rectification as well as the potential to achieve large thermal rectification over wide temperature ranges in combination with other beneficial properties, such as wide tunability of thermal conductivity or optical transparency of the thermally rectifying structure. In this contribution we will present what is to our knowledge the first experimental demonstration of thermal rectification in mesoporous silicon. The influence of pore morphology controlled via Si substrate crystallographic orientation and etching conditions on thermal rectification are studied. The effects of porosity modulation as well as oxidation of the porous material will be presented as well. Experimental results will be further compared with several recently published theoretical predictions of thermal rectification in porous semiconductor materials. Based on experimental results and theoretical considerations the levels of potentially achievable rectification in such materials will be analyzed and a pathway to further improvements will be discussed. The potentials of such materials for multiple applications ranging from energy efficient windows to higher efficiency thermoelectric materials will be discussed.

Transport processes contributing to thermoelectric energy conversion involve both electrons and phonons, and their intricate interactions. For electron-dependent thermoelectric properties (such as thermopower or electronic thermal conductivity), the challenging part in computing the transport coefficients is the determination of the energy-dependent relaxation time functions that govern the fundamental characteristics of energy transmission of the electrons. Nonetheless, due to the complexity of dealing with energy dependence of the relaxation time functions, often a constant relaxation time approximation is used in theoretical or computation studies. Presence of defects or deformation arising from distribution of defects, make the exploration of energy dependence of relaxation time functions even more ambitious. In this work, to advance our understanding of the role of the coupling between stress and relaxation time functions on electron-dependent properties, we use a combination of first-principles calculations and the Boltzmann transport formalism and examine electron-dependent thermoelectric properties of a range of material systems including crystalline, polycrystalline, alloys and hetero-structures. This talk will discuss how energy dependence affects Lorenz number and Seebeck coefficient of these material systems from a computational perspective, and show how it influences transport properties of electrons with energies higher or lower than the Fermi energy of the corresponding system.

Since the 1950s, thermoelectric materials have offered the unique property of converting temperature gradients into an electrical current. Bismuth telluride is one of the most popular thermoelectric materials and has been widely studied due to its large figure of merit compared to other bulk materials at room temperature. Recently, it has been shown that low dimensional thermoelectric materials can perform much better than bulk samples due to quantum confinement effects and increased phonon scattering at the grain boundaries in nanostructured materials. Examining the growth mechanisms of nanostructured bismuth telluride can improve understanding of how grains at the nanoscale affect bulk material properties, as well as improve the functionality of thermoelectric devices. As such, we have grown nanostructured bismuth telluride on silicon and copper substrates via pulsed laser deposition (PLD). This growth mechanism is highly dependent on several experimental parameters, such as substrate temperature and laser power. By adjusting the substrate temperature and deposition pressure, we have grown a range of morphologies from thin films to nanorods to hexagonal nanoplatelets. We also explore how the substrate material affects the as-grown film characteristics, as well as how the wavelength and flux of the incident laser beam affects the growth mechanism of the ablated material.

Tin monoselenide (SnSe) belongs to the group IV- VI layered binary semi-conducting compounds with an orthorhombic structure. This compound has potential in thermoelectric applications because it is both a stable material and made of from earth abundant elements. Recent results by Zhao, et al. [1] show single crystal SnSe to have an ultra-low thermal conductivity and highest figure- of- merit (ZT=2.6, at T=923 K) reported along its b-axis. In our work, we report the thermoelectric properties of Ge doped SnSe samples to understand how changes in both carrier concentration and scattering regime modify thermoelectric efficiency. Temperature-dependent thermoelectric transport properties as a function of Ge doping will be discussed.
[1] Zhao, et al, Nature 508, 373-377, 17 April 2014.

Nano- and microcrystalline substrate-free diamond foils manufactured by hot-filament chemical vapour deposition (HFCVD) have some unique advantages over other thermoelectric materials. Diamond has the highest atom density of all solid materials and thus there is virtually no diffusion of the dopant even at high temperatures. Unless Te or Ge based materials diamond foils are abundantly available due to synthetic production. Diamond foils are thermally stable up to at least 1000°C in inert atmosphere and up to 500 -600 °C when exposed to air.
It has been show in previous works that an electrical conductivity of 40000 S m^-1 and Seebeck-coefficients of 280 µV/K has been reached by boron doping.
However the resulting figures of merit are currently lower than zT=0.01 due to the low mobility of charge carriers (about 1 cm² V^-1 s^-1) and to the high thermal conductivity
(between 20 and 60 W m^-1 K^-1).
The presented work will focus on different ways of improving the thermoelectric properties. By changing the setup of the diamond coating machine and by using a time modulated HFCVD process, diamond foils with thinner grain boundaries and thus higher sp3 content at a certain grain size can be be obtained. This leads to a much higher electrical conductivity, while the loss in Seebeck coefficient is moderate.
A way to lower the thermal conductivity is to constrict the movement of phonons by doping the diamond foil with impurity atoms. The impurities hinder the phonon movement but have little effect on the mobility of the charge carriers. This is especially true for the synthetic enrichment with other isotopes.
With these actions it will be investigated if there is a perspective to reach the theoretically calculated zT values of more than 3, found in literature, in a real material.

In the context of environmental issues that become more and more prevalent in our society, there has been recently an increase of interest for thermoelectric (TE) materials, which have the property to convert heat into electricity, and vice-versa. Although they do not display exceptional thermopower (in comparison to best thermoelectric like bismuth telluride), oxide materials have attracted some attention for high-temperature TE applications, due to their high stability. Amongst them, CoO₂-layered compounds were proposed as good p-type TE candidates. Still, these compounds have been only poorly characterized both theoretically and experimentally. In this work we report a first-principles study of misfit calcium cobaltite (Ca₂CoO₃)(CoO2)_1,618 based on density functional theory and an hybrid functional. The computed structural, electronic and magnetic properties match well the avalaible experimental data. Then the thermoelectric properties can be deduced using the Boltzmann transport formalism within the constant relaxation time approximation and will be discussed.

Silicon-germanium (SiGe) superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocompoosites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we employ a momentum-dependent specularity parameter. Our results show highly anisotropic thermal transport in SiGe nanocomposites below their bulk alloy counterparts. SiGe nanowires (NWs) have also attracted attention due to their unique properties such as unusually long phonon mean-free-paths and the possiblity of quasi-ballistic transport over micron lenghts. This counterintuitive feature is a consequence of alloy scattering having a strong frequency dependence so that, while most phonons are strongly scattered by mass disorder, long wavelength phonons having low vibrational frequencies are left contribute the majority of the thermal conductivity. In order to capture the interplay between alloy, anharmonic, and boundary scattering in SiGe nanowires, we develop and compare results based on the improved Callaway model and the Monte Carlo method, both utilizing the full phonon dispersion and a momentum-dependent roughness scattering model. We find good agreement between the two models and show that they capture the increased specularity of low-angle phonon scattering at the boundaries, which leads to long mean-free-paths for phonons directed along the wire. We conclude that our Monte Carlo simulations accurately capture the quasi-ballistic transport in SiGeNWs and that heat is carried predominantly by quasi-ballistic long-wavelenth phonons travelling along the direction of the wire, thus producing a pronounced length dependence, especially in NWs having smooth surfaces.

Bulk chalcogenides Bi₂Te₃ and Sb₂Te₃ as well as related heterostructures are well known to be efficient thermoelectric materials [1,2]. Recent research revealed Bi₂Te₃ and Sb₂Te₃ to be strong topological insulators, i.e. the bulk is insulating, while the surfaces are metallic due to the presence of robust gapless surface states [3]. While the spin structure and the low-temperature electrical transport gained much attention, the physics of the thermoelectric transport is still under debate. To contribute on this, we studied the electronic structure of Sb₂Te₃ thin films with a fully relativistic screened Korringa-Kohn-Rostoker Green&’s function method. The thermoelectric transport properties were calculated within the relaxation time approximation of the Boltzmann theory. The influence of temperature, doping and film thickness on the thermoelectric transport were analyzed in detail and clear signatures of the surface state in the total thermoelectric transport can be revealed up to room temperature. Comparisons to Bi₂Te₃ thin films will be made [4]. References: [1] T. M. Tritt et al., MRS bulletin 31, 188 (2006) [2] N. F. Hinsche et al., Phys. Rev. B 86, 085323 (2012) [3] H. Zhang et al., Nature Phys. 5, 438 (2009) [4] F. Rittweger et al., Phys. Rev. B 89, 035439 (2014)

Nanostructuring of thermoelectric materials is typically intended to reduce contribution to thermal conduction from low frequency phonons that are weakly scattered by anharmonicity and point defects. An experimental method for determining the distribution of phonon mean-free-paths and therefore quantifying the effects of various nanostructures on the lattice thermal conductivity would be a significant advance. Over the past several years, several approaches have been described for so-called “mean-free-path spectroscopy” based on the fact that the heat diffusion equation fails to describe the relationship between temperature fields and heat fluxes when heat carriers have mean#8208;free#8208;paths smaller than the important length#8208;scales of the problem. However, there are no clear criteria for predicting whether Fourier theory will apply in a nanoscale#8208;thermal#8208;transport problem. To address this issue, we characterize the relationship between the failure of Fourier theory, phonon MFPs, important length#8208;scales of the temperature#8208;profile, and interfacial#8208; phonon scattering by time#8208;domain thermoreflectance (TDTR) experiments on Si, Si0.99Ge0.01, boron doped Si, and MgO crystals between 40 and 300 K. The failure of Fourier theory results in anisotropic thermal transport in Si and MgO. In TDTR measurements of materials whose high wavevector phonons have a low thermal diffusivity, Fourier theory will fail as a function of modulation frequency. In TDTR measurements materials whose high#8208;wavevector phonons have high thermal diffusivity, Fourier theory will fail as a function of the spot radius. In situations where Fourier theory fails, the effect of interfacial#8208;phonon scattering cannot be accurately described with a simple thermal boundary resistance.

A variety of complex crystals contain quasi-one dimensional (1D) substructures, which give rise to distinctive electronic, spintronic, optoelectronic, and thermoelectric properties. Among them, higher manganese silicides (HMS) with a Nowotny Ladder Structure have been studied for several decades as a p-type thermoelectric material that is made of non-toxic and earth-abundant elements. However, there is a lack of understanding of the lattice dynamics that influences the functional properties of this broad class of complex crystals including HMS. Over the past several years, we have been engaged in a collaborative effort to elucidate the mechanisms that result in the low and anisotropic thermal conductivities of HMS, and explore different approaches to enhancing the thermoelectric figure of merit. Inelastic neutron scattering measurements and density functional theory calculations have been employed to discover numerous low-lying optical phonon modes, including unusually low-frequency twisting motions of the silicon helical ladders inside the manganese chimneys of HMS. These low-lying optical phonons provide large phase space for scattering acoustic phonons, and result in the low and anisotropic thermal conductivities of HMS crystals. The obtained phonon dispersion has further enabled us to calculate a glass-like thermal conductivity in nanostructured HMS, whereas both HMS nanowires and bulk HMS with nanoscale grains and precipitations have been studied in experiments to examine the size effects on phonon and thermoelectric transport. In addition, heavy element substation is found to be an effective method of suppressing the thermal conductivity to approach the amorphous limit of Rhenium-substituted HMS. While the theoretical limit in the thermoelectric power factor of these nanostructured HMS and Rhenium-substituted HMS remains an outstanding question, the findings on the lattice dynamics of HMS can be applicable to different complex crystals with internal quasi-1D structures and promising functional properties.

The fundamental role played by the number, size and orientation of twins in nanoscale heat conduction was studied by molecular dynamics simulation. This talk will present an anomalous thermal transport behavior in model silicon nanowires and bulk silicon containing nanoscale twin boundaries oriented either perpendicular or parallel to the transport direction. We find that both cross-plane and in-plane thermal conductivities decrease to a minimum limit as the twin size decreases, but either remain constant or increase below a critical twin size. Atomic-scale phonon scattering calculations reveal that the twin-size dependence of thermal conductivity in nanotwinned Si materials stems from a change from interface phonon scattering to homogeneous heat conduction induced by the intrinsic contribution of the hexagonal twin boundary structure, and independent from surface effects. New avenues for structurally designing common crystalline materials with coherent interfaces for high-performance thermoelectric applications in microscale devices will be discussed.

Embedded nanoparticles have been shown to reduce thermal conductivity of semiconductors below the alloy limit while preserving electron mobility and the thermoelectric power factor. A wide distribution of phonon mean free paths contribute to thermal conductivity of III-V alloys such as InGaAlAs. Changing the modulation frequency in time domain thermoreflectance (TDTR) has been used to extract contribution of ballistic phonons as the phonons with mean free path larger than the thermal penetration depth don&’t contribute to the “apparent” thermal conductivity of the material. The apparent thermal conductivity of InGaAlAs alloy (20% Al, lattice matched to InP substrate) reduces by more than 40% when the modulation frequency is increased from 1MHz (~ 1 microns thermal penetration length) to 10MHz (~0.3 microns thermal penetration length). This explanation is based on quasi-ballistic description that considers “ballistic” heat transport near the metal transducer and a transition to pure “diffusive” transport at larger distances.
An alternative explanation is based on truncated Levy flight where ballistic “jumps” and Brownian “diffusion” are mixed. This gives rise to a fractional diffusion at early times (short distances). The transition to regular diffusion happens after a ballistic-diffusive transition length. While modified Fourier and truncated Levy can both explain the top transducer temperature profile accurately, their prediction for internal temperature distribution inside the semiconductor alloy can be significantly different. In this study, TDTR is used to investigate the effect of embedded nanoparticles on the Levy random walk as well as the thermal conductivity. Six samples of randomly distributed ErAs nanoparticles in InGaAlAs matrix with nanoparticles concentration ranging from 0.01 to 10% were characterized.
Consistent with previous observations [1], a drop in the thermal conductivity by a factor of 2.5 is observed with an increase in the nanoparticles concentration. More nanoparticles leads to more phonon scattering which reduces the effective phonon mean free path as well as the thermal conductivity. By varying the nanoparticles concentration from 0.01% to 10%, the extracted ballistic to diffusive transition length is reduced by almost an order of magnitude. This confirms that transition length is correlated to the effective phonon free path. On the other hand, the Levy exponent is almost constant (~1.55-1.67) in all of the samples. This suggests that superdiffusive exponent is inherent to the properties of the host material.
[1]W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A. Shakouri and A. Majumdar, "Thermal Conductivity Reduction and Thermoelectric Figure of Merit Increase by Embedding Nanoparticles in Crystalline Semiconductors," Physical Review Letters, vol. 96, 2006.

There has been extensive research on the lattice dynamics of thermoelectric materials, aiming at unraveling the mechanisms that lower the thermal transport. Open framework structures with guest atoms and the influence of these guests on the thermal transport have been investigated, as well as materials with a large unit cell, in which a low relative amount of vibrational modes participate to heat transport. In addition to selecting and tuning the material's crystal structure, further tuning is possible by controlling the dimensions of the material in nanostructures. An efficiency enhanshy;cement through improved electronic properties and through reduction of the phononic heat transport can be achieved by such tuning of the size. Characterizing the lattice dynamics in these nanoobjects is a challenging task that inelastic scattering techniques such as inelastic neutron scattering and nuclear inelastic scattering can tackle elegantly.
Phonon properties obtained in bulk thermoelectric antimonides and tellurides[1,2] as well as in matched compounds with confined geometry such as nanowires[3], thin films[4] and nanopowders[5] will be reviewed. First experimental insights in the specificity of lattice dynamics in nanoscaled thermoelectrics obtained both by inelastic neutron and nuclear inelastic scattering will be presented, notably in Si [6], in (Sb,Bi)₂Te₃ nanowires[3], and in FeSb₂, ZnSb, and NiSb nanopowders[7].
The Helmholtz Gemeinschaft Deutscher Forschungsshy;zentren is acknowledged for funding VH NG-407 “Lattice dynamics in emerging functional materials”. The DFG is acknowledged for funding SPP1386 'Nanostructured thermoelectrics'. The MLZ, ILL, SINQ-PSI, and the SNS are acknowledged for provision of neutron beam time; the ESRF and the APS for synchrotron radiation beam time. I am greatly indebted to all coauthors for the lasting fruitful collaborations.
[1] Diakhate M. et al., Phys. Rev. B 84, 125210 (2011); Moechel A. et al._,PRB84,184303 (2011).
[2] Bessas D. et al., Phys. Rev. B 86, 224301 (2012).
[3] Bessas D. et al., Nanoscale 5,10629 (2013).
[4] Aabdin Z. et al., J. Electr. Mater. 41, 1493 (2012).
[5] Birkel C. et al., Phys. Stat. Sol. A208, 1913 (2011); Inorg. Chem.50, 11807 (2011)
[6] Claudio T. et al., J. Mater. Sci. 48, 2836 (2013); PCCP, DOI 10.1039/C3CP53749H, (2014).
[7] Claudio T. et al., Phys. Stat. Sol. B 251, 919 (2014).

Near-field radiative thermal transport has attracted increasing attention recently, with orders-of magnitude heat transfer enhancement already demonstrated between bulk materials. In this talk I will describe our recent experiments, performed using a novel custom-built experimental platform, to systematically investigate the effect of film thickness on the near-field heat transport properties. By studying thermal radiation between a hot silica microsphere and thin silica films of varying thicknesses (50 nm to 3 microns), at different gap sizes (30 nm to 10 microns), we found substantial enhancements in heat transport properties due to near-field effects, even for the thinnest films when the gaps size was comparable to the film thickness. Further, we found that at larger separations (~1 micron), the thicker films show substantially larger near-field enhancement than thinner films. These results provide the first direct evidence of a distance dependent penetration depth in thin films. We will also describe recent measurements of near-field radiative heat transfer between planar surfaces with nanoscale gaps that show dramatic enhancements in radiative heat transfer. These results have potential implications for future thermo-photovoltaics and thermionic technologies.

PbTe-PbSe alloys are of special interest to thermoelectric applications. The low thermal conductivity is desired to enhance the efficiency of thermoelectric materials. Although first-principle calculations have made tremendous progress in accurately capturing phonon transport properties in bulk materials, the proper treatment of phonons in alloys is still difficult due to the broken of long-range translational symmetry. We thus study the phonon dispersion and density of states of PbTe-PbSe alloys of different compositions using inelastic x-ray scattering. With insights into the spectra change in alloy systems and the validity of the virtual crystal approach, it can pave the way for deeper understanding of phonon transport in alloy systems and guide the thermoelectric material design.
This material was supported by 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. Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357

Thermoelectric power generation is one of the most promising solutions against the global energy challenge. Thermoelectric cooling is currently the most preferred refrigeration and super-cooling approach. Thus, thermoelectrics has been intensively focused on by contemporary researchers. Though much progress has been made during the recent two decades, the thermoelectric figure of merit has not yet come to a level that is industrially competitive with traditional energy source. The future development of thermoelectrics has come to a challenge point.
Our present work first points out why the problem of enhancing ZT is barely soluble and how can we reformulate the problem? We then find out what are the variables that really decides ZT of a system, which are actually not the electrical conductivity, Seebeck coefficient and thermal conductivity as are used in the current literature. We then continue to analyze why the old proposals only work in specific systems, and fail in others, including the low-dimension method, the sharp density of states method, the pipe-shaped Fermi surface method, etc. We here introduce the concept of pseudo-ZT, which can be used to analyze how the various variables influence the thermoelectric performance, including the dimension, the form of dispersion relation, the carrier scattering mechanism, density of states, etc. Furthermore, we point out that the variables that can really enhance ZT convergently are the band asymmetry factor and the band gap. Finally, we have summarized a frame work of principles for thermoelectric ZT enhancement, which can be used for future guidance for thermoelectric performance improvement.

Thermally Regenerative Electrochemical Cycle (TREC) can directly convert heat to electricity. This strategy is based on the temperature dependence of electrode potential in an electrochemical cell. For a half reaction, A + n e- --> B, the temperature coefficient is defined as alpha = dV/dT = ΔS(A,B)/nF, where V is the electrochemical potential, T is temperature, n is the number of electrons transferred in the reaction, F is Faraday&’s constant, and ΔS(A,B) is the partial molar entropy change for the half cell reaction in isothermal condition. This effect indicates that the voltage of a battery depends on temperature; thus a thermodynamic cycle can be constructed by discharging the battery at T1 and charging back at T2 with a lower voltage. The electricity produced originates from heat absorbed at the higher temperature of T₁ and T₂, similar to a thermomechanical engine whose theoretical efficiency is limited by Carnot efficiency. Previously we demonstrate a system based on copper hexacyanoferrate cathode and Cu/Cu²⁺ anode separated by an ion-selective membrane.¹ The system shows a high heat-to-electricity efficiency of 5.7% when cycled between 10 and 60 ^oC with an assumed heat recuperation efficiency of 50%. In this talk, new progress on lowering charging voltage and removing ion-selective membrane will be discussed. Reference: 1. Seok Woo Lee, Yuan. Yang, Hyun-Wook Lee, Hadi Ghasemi, Daniel Kraemer, Gang Chen, Yi Cui. Nature Communications 2014, 5, 3942.

Constantan is a good metal-alloy thermoelectric material. It has been used in thermocouples because of its good mechanical properties, wide applicable temperature range, large thermopower and low cost. Formation of resonant levels due to Ni 3d levels is known as the cause of large Seebeck coefficient. By adding doping atoms with large Pauling electronegativity, such as bromine in this case, it is found that one can improve the efficiency of the resonance, much as described by the theory of Mahan and Sofo¹and our own work on semiconductors². As a result, an increase in Seebeck coefficient and resistivity is seen, leading to an enhancement in thermopower from room temperature to 900K. The new alloy can have potential applications in various thermoelectric devices where the mechanical strength of metals confer them an advantage over semiconductors. Work supported by the AFOSR MURI “Cryogenic Peltier Cooling”, FA9550-10-1-0533.
1. G. D. Mahan and J. O. Sofo, Proc. Natl. Acad. Sci. U. S. A.,93, 7436 (1996).
2. Joseph P. Heremans, Bartlomiej Wiendlocha and Audrey M. Chamoire, Energy Environ. Sci., 5, 5510-5530 (2012).

Radioisotope Thermoelectric Generators (RTGs) generate electrical power by converting the heat released from the nuclear decay of radioactive isotopes (typically plutonium-238) into electricity using a thermoelectric (TE) converter. RTGs have been successfully used to power a number of space missions including the Apollo lunar missions, the Viking Mars landers, Pioneer 10 and 11, and the Voyager, Ulysses, Galileo, Cassini, and New Horizons outer planet spacecrafts. MSL&’s Curiosity rover is powered by the Multi-Mission Radioisotope Generator (MMRTG). Teledyne Energy Systems Inc. (TESI) and prime contractor, Pratt & Whitney Rocketdyne, working in partnership with the Department of Energy, produced this generator for Curiosity. RTGs have demonstrated their reliability over extended periods of time (tens of years) and are compact, rugged, radiation resistant, scalable, and produce no noise, vibration or torque during operation. NASA&’s Radioisotope Power Systems Technology Advancement Program is pursuing the development of more efficient TE technologies that can increase performance over state-of-practice RTGs, which are limited to device-level thermal-to-electrical energy conversion efficiencies of 7.5% or less, and system-level specific power of 2.4 to 5.1 W/kg. The Jet Propulsion Laboratory (JPL), under funding from the NASA Radioisotope Power Systems Project, under the Advanced Thermoelectric Couple (ATEC) task, has developed couples based on advanced skutterudite (SKD) thermoelectric materials. Conversion efficiency values on the order of 9% have been demonstrated for SKD-based un-segmented couples when operating at a hot-junction of 873K and a cold-junction of 473K. This represents ~ a 25% improvement over the conversion efficiency of PbTe/TAGS MMRTG couples. JPL, in collaboration with TESI, has initiated a project to transfer the technology to TESI, to further mature this technology, to develop the manufacturing capabilities for SKD TE materials, couples, and modules at TESI, and to demonstrate their performance and lifetime potential for insertion into an enhanced-MMRTG (eMMRTG). This paper provides a status of the technology development at JPL, the initial development work at TESI, as well as a brief description of the technology maturation plan.

Phase change materials are identified for their ability to rapidly alternate between amorphous and crystalline phases and have large contrast in the optical/electrical properties of the respective phases. The materials therefore are primarily used in memory storage applications, but recently phase change materials have been under the eye of thermoelectrics&’ research. Many of the phase change materials studied today can be found on the pseudo-binary tie-line of (GeTe)_1-x(Sb₂Te₃)_x. Though many compounds found on this tie-line have exhibited adequate thermoelectric properties, the majority of them contain vacancies, which is non-desirable for structural stability. Ge₄SbTe_5, which is a single phase compound just off of the (GeTe)_1-x(Sb₂Te₃)_x tie-line, forms a stable and filled fcc crystal structure at room temperature. The current work has studied the thermal and electrical transport properties of water-quenched Ge₄SbTe₅. Ge₄SbTe₅ exhibits a thermal conductivity of ~1.2 W/m-K at high temperature and a moderate Seebeck coefficient of ~300 mu;V/K at high temperature. The resistivity decreases dramatically at 350#8304;C due to a structural phase transition. This lends to a large enhancement in power factor and a ZT of approximately 1.6 at 550#8304;C.

Predicting heat transport in nanostructured materials is challenging because heat travels non-diffusively and non-local effects have to be taken into account. By using the recently developed phonon mean free path Boltzmann transport equation [1], we compute heat transport in nanoporous materials with pores of various shapes, including circles, squares and triangles, for both aligned and staggered configurations. We study the correlations between the thermal conductivity, the pore-pore distance and the view-factor, i.e. the possibility of having straight phonon trajectories connecting the hot side with the cold side. We deduce that the pore-pore distance plays a major role in tuning thermal transport whereas the view factor plays a less important role for these particular systems [2]. Based on this finding, we show that a pore configuration composed of two misaligned rows of triangular pores corresponds to a reduction in the thermal conductivity of ~60 % with respect the case with circular pores in the aligned configuration with the same porosity. As we consider length scales where electrons can still travel diffusively, the electrical conductivity depends on the porosity and only weakly on the pore arrangement. Consequently, this type of phonon-boundary engineering can lead to a substantial increase in the thermoelectric figure-of-merit for such materials.
[1] G. Romano and J. Grossman, arXiv (http://arxiv.org/abs/1312.7849)
[2] G. Romano and J. Grossman, arXiv (http://arxiv.org/abs/1406.4379)

Thermoelectricity has been regarded as one of the most promising strategies for clean, low-cost and environmental friendly sustainable energy solution for decades. Many efforts have been devoted to the improvement of the performances of thermoelectric materials, i.e., to maximize their figure of merit (ZT). Recently, a new class of MgAgSb-based (MAS) room-temperature thermoelectric materials has been discovered with ZT up to ~1.4 at 475K.^[1] In this work, we present a theoretical study on the structural, electronic and thermoelectric properties of MAS by combining first-principles calculations and semi-classical Boltzmann transport theory. Our predicted Seebeck coefficients are in reasonably good agreement with experimental data. The effects of spin-orbit coupling and atomic structure on the electronic and thermoelectric properties of MAS are discussed. Our results intend to provide a theoretical support for the future improvement of the thermoelectric performance of MAS-related materials.
[1] H Zhao, J Sui, Z Tang, et.al, Nano Energy (2014) 7, 97-103

The recent focus in thermal management in nanostructures and energy harvesting using thermolectric devices has motivated the interest towards understanding the role of phononic thermal transport in nanostructured materials. Engineered nanostructured semiconductors have been shown to improve the efficiency of thermoelectric systems, by reducing the thermal conductivity of the crystalline materials while preserving their electronic properties. Recent experiments have reported a reduction in the group velocities of phonons, thereby leading to a strong reduction in the thermal conductivity in sub-10 nm free-standing Si membranes [1]. However, a microscopic understanding of thermal transport in ultra-thin membranes especially the correlation between phonon surface scattering and thermal transport, is still lacking. The theoretical studies reported, mostly rely on bulk phonon properties and use of specularity parameters to model surface scattering of phonons[2]. A detailed understanding of the role and behavior of phonons in confined structures is necessary in the design of nanostructured materials with tailored thermal transport properties.
We use lattice dynamics (LD) and classical molecular dynamics (MD) to investigate the nature of phononic thermal transport in nanostructured silicon membranes with thicknesses of the order of 20 nm and below. We find that dimensionality reduction has a significant effect on
the phonon dispersion and has the direct consequence of suppression of group velocities of phonons in the silicon membranes. The dimensional reduction leads to a 3-fold reduction in the thermal conductivity of the membranes with respect to bulk silicon. The presence of surface nanostructures, by means of pattern formation and surface oxidation, has even a stronger influence on the phonon dispersion, leading to a 25-fold reduction in the in-plane thermal conductivity of the rough oxidized membranes, implying a 25-fold enhancement of the thermoelectric figure of merit at room temperature. Such figures make nanostructured silicon membranes viable materials for thermoelectric units.
[1] J. Cuffe et al, “Phonons in slow motion: dispersion relations in ultra-thin Si membranes.,” Nano Lett.,12, 3569-3573, (2012).
[2] J. E. Turney et al, “In-plane phonon transport in thin films,” J. Appl. Phys.,107, 024317 (2010).
Acknowledgment: This project is funded by the program FP7-ENERGY-2012-1-2STAGE under contract number 309150.

Material design for direct heat-to-electricity conversion with substantial efficiency essentially requires cooperative control of electrical and thermal transport. Bismuth telluride (Bi₂Te₃) and antimony telluride (Sb₂Te₃), displaying the highest thermoelectric power at room temperature, are also known as topological insulators (TIs) whose electronic structures are modified by electronic confinements and strong spin-orbit interaction in a-few-monolayers thickness regime, thus possibly providing another degree of freedom for electron and phonon transport at surfaces. Here, we explore novel thermoelectric conversion in the atomic monolayer steps of a-few-layer topological insulating Bi₂Te₃ (n-type) and Sb₂Te₃ (p-type). Specifically, by scanning photoinduced thermoelectric current imaging at the monolayer steps, we show that efficient thermoelectric conversion is accomplished by optothermal motion of hot electrons (Bi₂Te₃) and holes (Sb₂Te₃) through 2D subbands and topologically protected surface states in a geometrically deterministic manner. Our discovery suggests that the thermoelectric conversion can be interiorly achieved at the atomic steps of a homogeneous medium by direct exploiting of quantum nature of TIs, thus providing a new design rule for the compact thermoelectric circuitry at the ultimate size limit.

We demonstrated an enhancement in thermopower of nano-crystalline SbTe thin films with a mixture of two phases. Study on systematically engineered crystallinity of as-electrodeposited SbTe thin films from amorphous to polycrystalline using a post heat treatment led to the optimized two-phase system with the highest power factor of ~590 µW/mK². A complete measurement of electrical conductivity, Hall mobility, carrier concentration, and Seebeck coefficient as a function of the cyrstallintiy gives a clear understanding of the enhancement originating from carrier energy filtering effect induced by the interface of the two phases. This work highlights that the mixture of two phases in the system can play an important role for superior thermoelectric properties.

Thermoelectric devices with greater efficiencies could fulfill the growing need for alternative energy resources. One way of improving the thermoelectric efficiency of a material is by significantly decreasing its lattice thermal conductivity. Recent molecular dynamic simulations using reverse non-equilibrium techniques have shown that Silicon based pseudomorphic heterostructures in which randomly selected atomic planes from the host silicon lattice are replaced with planes of more massive atoms, a structure known as a Random Multi-Layer (RML), can have lattice thermal conductivities below 0.10 [W/m-K]. This reduction to the lattice thermal conductivity results from the localization of vibrational modes and requires almost atomically abrupt interfaces between that of the host lattice planes and the mass-altered planes. A Si/Sn RML heterostructure system, with a mass mismatch of approximately 4, was fabricated by sputtering techniques to experimentally investigate this claim. The thermal conductivity of Si/Sn RML thin films deposited at room temperature was measured to be up to a factor of ten below that of a-Si. Rapid thermal annealing of the RML thin films at temperatures up to 450°C produced improved silicon crystallinity with little apparent Sn agglomeration. However, this thermal treatment was found to increase the lattice thermal conductivity to values similar to that of a-Si. These results suggest that thermal annealing results in a loss of atomic abruptness at the Si-Sn interface. Optical absorption studies of these samples revealed that the Si/Sn RML band gap is approximately equal to that of crystalline Silicon (1.1 [eV]) and preliminary cross-plane Seebeck coefficient measurements indicated values near 1 [mV/K]. Room temperature Hall measurements indicated a charge carrier mobility of 2 [cm²/V-s] at carrier concentrations of 10¹⁷ [cm^-3]. If these values can be maintained at higher carrier concentrations, the thermoelectric figure of merit should approach two.

Grain boundaries play a key role in determining the charge and thermal transport properties of polycrystalline thin films. The effects of grain sizes and boundary height have been explored for LPCVD-grown Si_(1-x)Ge_x thin films to optimize the thermoelectric figure of merit ZT. It is shown that a small grain size can suppress the thermal conductivity, but also reduce the power factor by lowering the carrier mobility, without benefiting ZT. Boundary height is almost independent on grain sizes and peaks at the moderate doping level, leading to an unusual variation of in-plane charge mobility with respect to the doping level. The existence of boundary barriers also makes the Seebeck coefficient less sensitive to the doping level. The thermal conductivity shows different variation trends in the cross-plane and in-plane directions with the increasing doping levels, verifying the dual roles of grain boundaries in both scattering phonons and facilitating electronic thermal conductivity.

Currently 60% of the conventional energy sources such as coal, oil, and gas is wasted in the form of heat. Thermoelectric (TE) materials show great promise for converting this wasted heat into useful electricity. TE systems have many unique advantages such as silent operation, time realiability, and dimensional scalability. In the last 60 years, MnO₂ powders have been used as positive electrodes in dry-cell alkaline batteries and not too much attention has been paid to their thermoelectric properties. This has been changed recently due to the discovery of giant Seebeck coefficient of S = 20 mV/K [Ref. Song et al.] which is 100 times higher than bismuth telluride, one of the best TE materials. In this work, we present preliminary results of the Figure of Merit (ZT) as a function of particle electrical resistance (R ~ 10 - 80Omega;) and packing density (d_P = 2.4 - 3.5 g/cm³). These thermoelectric properties were also measured as a function of MnO₂ particle size (d ~ 5 nm - 150 mu;m). The results show that the Seebeck coefficient for R ~ 10 - 80Omega; are consistent with bulk MnO₂ values reported in the literature. The observed exponential behavior for electrical conductivity and thermoelectric power factor is induced by the correlation between electrical resistance and tube length. The packing density reaches an upper plateau for sample S6 (d ~ 79 mu;m) and the highest power factor (and ZT) were measured for sample S4 (i.e., P_F = 5.7 * 10^-7 W/(m K²), and 6.8 x 10^-4) at a packing density value of d_P ~ 2.7 g/cm³.
F. F. Song, L. Wu and S. Liang, Nanotech, 23, 085401 (2012).

Strontium titanate (SrTiO₃) nanoparticle is a perovskite-type material with cubic crystal structure, has attracted an increasing interest in the synthesis of nano-sized-particles because of their scientific importance and widespread applications. In recent years, heavily doped SrTiO₃ is considered a promising n-type thermoelectric oxide candidate because it exhibits high Seebeck coefficient (S). However, an enhancement of the ZT value of SrTiO₃ bulk material is limited because of a large thermal conductivity. Few researchers have demonstrated that the nanoparticles below 10 nm significantly lower the thermal conductivity by enhanced phonon scattering at grain boundaries. The crystallinity in nanostructured materials is also important, which effects the Seebeck coefficient and electrical conductivity. In present work, La- and Nb-doped SrTiO₃ nanocubes with high crystallinity have been synthesized via hydrothermal method with different doping levels of La and Nb and thin films were prepared via slow evaporation process. Thin films were characterized by XRD, FE-SEM, TEM and AFM. The shape and size of La- and Nb-doped SrTiO₃ nanoparticles were cubic and about 100 nm, respectively. Furthermore, thermoelectric properties were investigated. For the thermoelectric measurement, the thin films of La- and Nb-doped SrTiO₃ were heated at the temperature range of 900-1100 ^oC under 5%H₂/N₂ atmosphere to reduce La and Nb.

The integration of thin film thermoelectric generators on the basis of misfit cobaltate p-type [Ca₂CoO₃]_0.62[CoO₂] (Ca₃Co₄O₉) and n-type perovskite Nb-doped SrTiO₃ into silicon technology is a prerequisite for their application in miniaturized electric circuits and energy harvesting devices. In thin film thermoelectric generators using waste heat reduction of the electrical resistivity is mandatory. The power factor of Ca₃Co₄C₉ reaches 1 mW/K²m and is as high as those of classical thermoelectrics based on harmful and less abundant elements. In this work we report on epitaxial Ca₃Co₄O₉ and Nb-doped SrTiO₃ thin films grown by pulsed laser deposition (PLD) on (001)-silicon buffered with a thin epitaxial yttria-stabilized zirconia (YSZ) layer. X-ray diffraction (XRD) and cross-sectional high resolution transmission electron microscopy (HRTEM) analysis show that high quality c-axis oriented Ca₃Co₄O₉ films with a 12-fold in-plane rotational symmetry can be grown. This result is explained by the nearly equivalent lattice misfits in symmetry directions of the pseudo hexagonal [CoO2] sublayer in monoclinic Ca₃Co₄O₉ on the cubic (001)-YSZ surface. The charge transporting [CoO₂] sublayer domains form only two types of sublayer domain boundaries in these layers with the hexagons aligned or rotated by 30° and provide nearly ideal electronic coupling. The in-plane resistivity in these Ca₃Co₄O₉ thin films reaches that of single crystalline material. In a next step epitaxial Ir thin films on YSZ buffered silicon were investigated. It is found that Ca₃Co₄O₉ again grows with a 12-fold in-plane rotational symmetry, but in this case 15° rotated against the YSZ buffer layer orientation. The electrical I-V characteristic of the Ir-[Ca₂CoO₃]_0.62[CoO₂] contacts is purely ohmic and ideally suited for high temperature stable low-resistive ohmic contacts.

A thermoelectric module is a fully solid-state generator that can be used either for power generation or for refrigeration. The compactness of thermoelectric modules makes these devices an alternative option to conventional heat engines, especially in applications with severe space limitations. For this reason, generators using thermoelectric modules are considered as potential candidates for vehicular waste heat recovery systems.
The effeiciency and power output of conventional thermoelectric generators is relatively low compared to traditional devices for energy conversion, which makes it important to optimize and control the operating condition of a thermoelectric generator to the best, optimal point under given external thermal conditions. Since the external condition in which a generator operates is subject to changes, a close-loop control becomes essential for this purpose. In order to implement an adequate strategy of control, one must first identify an appropriate model of the transient behavior of the thermoelectric module. Unfortunately, most transient models for thermoelectric modules are complex high-dimensional dynamical systems sometimes including partial differential equations, which are improper for being used during the design of a control strategy.
The main objective of the study is to develop an appropriate model that reasonably describes the transient behavior of thermoelectric modules. Reduced-order modeling of a thermoelectric element is first attempted by using a simple projection scheme on the governing partial differential equation. The resulting simplified model is cross-validated by comparing the performance to the original partial differential equation. We further incorporate the simplified model into the module-heat sink assembly and study the transient bahavior of a full-scale generator.

A thermoelectric power generator is a solid-state device that relies on a temperature differential to produce electricity.¹ For a given temperature differential, the thermal to electrical conversion efficiency of a thermoelectric material depends on the dimensionless figure of merit ZT = S² σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the temperature in Kelvin, and κ is the total thermal conductivity, which includes both lattice, κ_ph, and electronic, κ_el, contributions. An ideal thermoelectric material would possess a large S, high σ, and low κ. Recently, magnesium silicide (Mg₂Si) has attracted much interest as an n-type thermoelectric material because it is eco-friendly, non-toxic, light, and relatively abundant compared with other thermoelectric materials. ^1-3
In this study, we tried to improve the thermoelectric performance by doping Sb in the Mg₂Si, to cause phonon scattering and optimize x in the carrier concentration, and Ge, to cause phonon scattering. A high purity Mg₂Si was synthesized from metal Mg and Sb doped Si-Ge alloy by using spark plasma sintering (SPS) equipment. A bulk of Sb doped Si-Ge alloy Sb was fabricated by using an arc-melting method in a vacuum condition. The bulk alloy was crushed and sieved to a powder-particle size of 75 mu;m or less in a dry box filled with Ar gas. The alloy powder and metal Mg were mixed in the inert gas condition. The mixture was placed in a graphite die and then heated at 923 K at 20 MPa in an Ar atmosphere for 10 min by using the SPS equipment at a heating rate of ~100 K/min, and in the next step, the temperature was increased up to 1113 K at 30 MPa. The sintered samples were cut and polished. They were evaluated by using X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses. The carrier concentration of the samples was measured by using Hall measurement equipment. The electrical conductivity and Seebeck coefficient were measured by using a standard four-probe method in a He atmosphere. The thermal conductivity was measured by using a laser-flash system. We succeeded in obtaining a Sb doped Mg₂Si_0.95Ge_0.05 sintered body easily without any impurities with the SPS equipment. The electrical conductivity of the sample was increased, and thermal conductivity was decreased by increasing the amount of doped Sb. The dimensionless figure of merit ZT became 0.74 at 733 K in the sample with x = 0.005.
References
1 Sabah K. Bux, et al. J. Mater. Chem., 21, 12259 (2011)
2 M. W. Heller and G.C. Denielson, J. Phys. Chem. Solids, 23, 601 (1962)
3 M. Akasaka, et al. Journal of Crystal Growth, 304, 196-201 (2007)

Thermoelectric power generation is an attractive method for converting waste heat into electric power. Recently, magnesium silicide (Mg₂Si) has been applied such as n-type thermoelectric material that operates in a temperature range from 500 to 800 K [1-5]. In comparison with other thermoelectric materials, Mg₂Si has many benefits such as being non-poisonous, light, and abundant. Generally, Mg₂Si is synthesized by using an all-molten synthesis [1-5]. However, this synthesis method has some problems, for example, the melting point of Mg₂Si is very close to the boiling point of Mg, synthesizing Mg₂Si causes incorporation of impurities, and composition deviation occurs. Kajikawa et al. [6] reported preparing Mg₂Si without generating MgO and any impurities by using spark plasma sintering (SPS) method in a lower temperature range compared with that of the all-molten method.
In this study, we fabricated Mg₂Si with metal Mg and Si with different particle sizes (200 mu;m or less). Additionally, Mg₂Si formation was investigated. Si powders with different particle sizes were readied by using a planetary ball mill with a tungsten carbide grinding jar and ball and then sieved to a powder-particle size in a dry box filled with Ar gas. Then, the sieved powder was mixed with a metal Mg powder in the inert gas atmosphere. The mixture was placed in a graphite die in an Ar atmosphere and subjected to SPS at 923 K in an Ar atmosphere. The obtained sintering bodies prepared were Si particles 100 nm or less in size. Then, the sintered samples were evaluated by X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermoelectric performance.
References
1) M. W. Heller and G. C. Danielson, J. Phys. Chem. Solids 23, 601 (1962)
2) R. J. LaBotz, D. R. Mason, and D. F. O&’Kane, J. Electrochem. Soc. 110, 121 (1963)
3) Y. Noda, H. Kon, Y. Furukawa, N. Otsuka, I. Nishida, and K. Matsumoto, Mater. Trans. JIM 33, 845 (1992)
4) J. Tani and H. Kido, Physica B 223, 364 (2005)
5) V. K. Zaitsev, M. I. Fedorv, E. A. Gurieva, I. S. Eremin, P. P. Konstantinov, A. Yu. Samunin, and M. V. Vedernikov, Phys. Rev. B74, 045207 (2006)
6) T. Kajlkawa, K. Shida, S. Sugihara. International Conference on Thermoelectrics Proc., 275-278 (1997)

Several compositions of the n-type half-Heusler (HH), Ti_xZr_1-xNiSn_0.975Ge_0.025 were synthesized by reacting elemental powders at high temperature using induction melting. The resulting products were annealed at 900 °C for 2 weeks to improve the crystallinity. The resulting polycrystalline powders were finally mechanical alloyed to achieve fine grain size. X-ray powder diffraction revealed the formation of single phase products with HH structure. The effect of band gap engineering through isoelectronic substitution of Sn by Ge and mass fluctuation arising from the intermixing of Ti and Zr in the HH structure on the electronic and thermal transports was investigated in the temperature range from 300 to 775 K. A large reduction in the lattice thermal conductivity (4.9 W/Km to 2.8 W/Km at 300 K) was observed with increasing Zr to Ti substitution. For the composition with x = 0.1, 2.5%Ge-substitution at Sn sites led to an increase in the thermopower from -150 mV/K to -180 mV/K at 775 K simultaneously with an increase in the electrical conductivity from 350 S/cm to 700 S/cm at the same temperature. The combined large reduction in lattice thermal conductivity via mass fluctuation at Ti/Zr site and optimization of the thermopower and electrical conductivity through Ge doping at Sn site drastically increase the figure of merit from 0.05 to 0.45 at 775 K.

The materials class of filled skutterudites, which is constructed by accommodating guest atoms into the cage-like crystal structure of unfilled skutterudites, is one of the most promising thermoelectric materials. It is believed that the guest atoms filling the voids within the host structure have beneficial effects of reducing thermal conductivity by providing an additional scattering channel for heat-conducting acoustic phonons, while maintaining decent electrical properties.
In this work, we investigate the vibrational properties of alkaline-earth-filled pnictogen-substitued skutterudites (M_xCo₄X₆Te₆ where M=Ca, Sr, or Ba, X=Ge or Sn, and x=0.5 or 1). First-principles calculations have been performed in order to understand the role of both fillers and the chemical substitutions in terms of lattice vibrational features. Phonon dispersions and vibrational density of states of M_xCo₄X₆Te₆ are analyzed. The signature of the filling element in the lowest optical phonons confirms the typical picture of the filling element&’s role as the center of largely decoupled random vibration, i.e., rattling, that scatters heat-conducting acoustic phonons off, which may be beneficial for thermoelectric applications.
Additionally, the Born effective charges of the atoms inside the unit cell and the mode-resolved effective charges for optical phonons are presented. The result is further analyzed to probe the feasibility of reducing the polarity of low-lying optical phonons, which was suspected as one of the most important causes for the low electrical conductivity of pnictogen-substituted skutterudites. We show that such reduction of polarity is not apparent in alkaline-earth-filled pnictogen-subtituted skutterudites, because the charge donated by alkaline-earth fillers has little interference to the nearby group IV (X) and group VI (Te) atoms.

In recent years, it has been reported that Si nanocrysltals prepared by ball-milling followed by sintering exhibit significantly improved thermoelectric performance. This improvement is attributed to the reduction of the thermal conductivity, which is caused by phonon scattering at grain boundaries. While nanocrystallization is an effective method to reduce the thermal conductivity, it has been recognized that a reduction of the electrical conductivity is a difficult problem in this method. This is because electrical carriers are also scattered at grain boundaries, resulting in the reduced carrier mobility. One of the cause of this reduction is considered to be due to impurity atoms, which tend to segregate at grain boundaries and cause the grain boundaries to have higher resistance than those without impurities. While ball-milling is a simple and efficient method for the synthesis of nanopowders, impurity contamination from milling media is inevitable in this method. The present study is an attempt to remove the impurity atoms incorporated during ball-milling by means of acid washing. We used ZrO₂ vial and balls for ball-milling since the ZrO₂ contamination together with the surface oxidation of Si nanopowder can be removed by using hydrofluoric acid (HF).
Heavily doped Si (Si_0.98B_0.02) ingot was prepared by Arc melting. This ingot was ball-milled for 5 to 30 hours with ZrO₂ vial and balls to synthesize nanopowders. The averaged grain size of the resulting powder was estimated from the broadening of X-ray diffraction peaks. The estimated grain size was 20 nm for 5 hours of milling, and lowered down to 6.5 nm for 30 hours of milling. The powder was washed by 5% HF for unit hour to remove the ZrO₂ and SiO₂ impurities. The washed powder was sintered by spark plasma sintering technique, resulting in dense pellets with 90% of the theoretical density. We measured X-ray diffraction of the obtained pellets and found that no impurity peaks were observed. The results indicate that ball milling with ZrO2 vial and balls conjunction with HF washing is effective to prepare Si nanopowder without impurities. The grain size of the pellet prepared from the powder milled for 30 hours was estimated and found to be 19.7 nm. The thermal conductivity of the corresponding pellet was 8.7 W/mK at room temperature, which is significantly lower than that of single crystal Si. The thermoelectric properties of the heavily doped nanocrystalline silicon prepared by this method will be discussed.

In recent years, several groups have reported that nanostructured Si-based thermoelectric materials exhibited significantly improved thermoelectric figure of merit ZT. Si-based thermoelectric materials have a high affinity for semiconductor devices since these devices have been based on silicon. When Si-based thermoelectric materials with higher ZT is developed, they can be used in the semiconductor devices in order to recover waste heat by Seebeck effect and to control the local temperature by Peltier effect.
Recently, Ge has attracted growing interest among the semiconductor industry since it has higher mobility (1900 cm²/Vs for hole and 3900 cm²/Vs for electron) than Si, which makes it promising channel material for the high performance devices. In terms of thermoelectric materials, Ge is also attractive material since it has lower thermal conductivity and higher mobility than Si. Hence, nanostructured Ge could exhibit superior thermoelectric properties than nanostructured Si. For the development of the Ge-based thermoelectric materials, it is essential to understand basic thermoelectric properties of Ge. Little is known, however, about the thermoelectric properties of Ge, and it is of great importance to study its thermoelectric properties.
We prepared four kinds of Ge single crystal specimens, which were cut from Ge wafers: Ga doped p-type wafers with career concentrations of 5.7×10¹⁶, 3.4×10¹⁸ and 1.0×10¹⁹ cm^-3, and Sb doped n-type with a carrier concentration of 9.1×10¹⁶ cm^-3. The carrier concentration was determined by Hall measurements. The thermal conductivity was calculated from heat capacity, density and thermal diffusivity. The thermal diffusivity was measured by a laser flash technique from 300 to 850 K. The electrical conductivity and Seebeck coefficient were measured from 300 to 850 K. The thermal conductivity was 25 to 30 W m^-1K^-1 at 470 K. The power factor increased with carrier concentration, and reached 2.5 mWm^-1K^-2 for the most heavily doped Ge at room temperature, which is significantly higher than that of Si with the same carrier concentration. The thermoelectric properties of Ge will be discussed in comparison with those of Si.

While today&’s thermoelectric materials, e.g. PbTe and Bi₂Te₃, show high thermoelectric performance, they are not yet widely used since these materials contain toxic and rare elements. To allow the thermoelectric materials to be widely used in a variety of applications, we should realize thermoelectric materials composed only of environmentally-conscious and abundant elements such as Si and Fe. Although single-crystal Si is a poor thermoelectric material because of its high thermal conductivity, nanostructured Si is known to possess significantly improved thermoelectric performance. This is because the thermal conductivity is markedly reduced by enhanced phonon scattering at the nanocrystal grain boundaries. This induced authors to synthesize nanostructured high performance thermoelectric materials composed of Si and Fe. In recent years we synthesized dense Si and metal-silicide nanocrystals by phase separation from amorphous silicon-metal alloy through thermal annealing. Si and Ni-silicide nanocrystal composite films prepared by this method exhibited much lower thermal conductivity of 4.4 Wm^-1K^-1 and higher ZT of 0.13 than those of single-crystal Si. Here, we adopted this method to Si and Fe system. We have synthesized the semiconducting composite films of Si and Fe-silicide nanocrystals and evaluated their thermoelectric properties.
The amorphous Si-Fe films were deposited on silica (SiO₂) substrates by DC magnetron sputtering of the target with a composition of Si:Fe = 20:1 in the pure Ar atmosphere. As a p-type dopant, 2 mol. % of boron was incorporated to the sputtering target. The amorphous Si-Fe films were thermally annealed at 1473 K for 10 seconds to form nanocrystals. The X-ray diffraction patterns of the films indicated that Si and α-FeSi₂ crystals were generated. Hall effect measurements showed that the films were p-type semiconductor with carrier concentration of 2.1×10²⁰ cm^-3. We measured temperature dependence of the electrical conductivity and the Seebeck coefficient from room temperature to above 650 K and the thermal conductivity at room temperature. The power factors of the films were 1.1×10^-3 Wm^-1K^-2 at room temperature and 1.2×10^-3 Wm^-1K^-2 at 669 K. This power factor, combined with the low thermal conductivity value of 4.8 Wm^-1K^-1 measured by scaning thermal probe micro-image system, results in a ZT of 0.068 at room temperature.
In summary, we have succeeded in synthesizing nanocrystal composite films only from environmental friendly elements (Si, Fe); the ZT estimated in the Fe-Si nanocomposite films is about half of the ZT for the Ni-Si nanocomposite film, but it is still significantly higher than that of single-crystal Si.

Recently, BiCuOQ (Q = chalcogen) oxychalcogenides have attracted much attention as promising thermoelectric materials for power generation using waste heat. It has a ZrCuSiAs type structure (space group = P4/nmm) and it consists of alternately stacked (Bi₂O₂)²⁺ insulating layers and (Cu₂Q₂)^2- conducting layers along the c-axis. It exhibits extremely low lattice thermal conductivity, while the electrical conductivity in BiCuOQ, whose valence band minima is comprised of Cu 3d and Q p, is relatively poor due to its low hole mobility. However, the electrical conductivity can be significantly improved by doping of various elements, and it led to the remarkable enhancement of its thermoelectric performances.
In this work, we report anomalous doping effects of Pb on thermoelectric properties in Bi_1-xPb_xCuOTe composites. Although the substitution of trivalent Bi with divalent Pb could generate only one hole per dopant atom to the maximum, we observed the generation of 1.75 holes per a Pb atom in Bi_0.94Pb_0.06CuOTe composite. Density functional theory calculations revealed that this anomalous doping effect was closely related with the changes in the formation energies of native point defects (V_Bi and V_Cu) due to the large difference in ionic radii between Bi³⁺(0.117 nm) and Pb²⁺(0.133 nm). Details on the anomalouns doping mechanism and its effects on thermoelectric transport properties in Bi_1-xPb_xCuOTe composites will be discussed.

The thermoelectric figure of merit ZT measures the thermoelectric efficiency of a thermoelectric material. To obtain a high ZT, a material must have a high thermo power and a low lattice contribution to thermal conductivity. However, the non-mutually exclusive nature of these material properties makes it extremely difficult to optimize one without affecting the other. Recent research trend have been focused on reducing lattice contribution to thermal conduction without affecting the Seebeck coefficient and electrical conductivity. Development of laminated structure and multilayers has been reported to significantly enhance phonon scattering leading to a low thermal conductivity. In this report, we investigate the effect of eutectic volume fraction on the overall thermoelectric performance of BiSbTe alloy. Pure elements of Bi, Sb and Te were loaded in a quartz tube and melted under argon with a handheld flame three times. X-ray diffraction technique was used to determine the phases formed and the morphology and phase composition were investigated using Scanning electron microscope and electron probe microanalyzer. The transport properties were measured using commercially available setups for Seebeck and electrical conductivity ULVAC-RIKKO ZEM-3 while the thermal properties were measured using the laser flash technique by TA instruments. This presentation will highlight the salient feature of the evolution of the microstructure and will attempt to correlate the microstructure with the prevailing overall thermoelectric properties observed.

Lead-telluride based compounds continue to show some of the best performances in thermoelectrics for mid to high temperatures. However, the complications associated with lead's toxicity, and the relatively high price of tellurium leads us to consider alternative materials. Recent work on chalcopyrite structure compounds, namely CuInTe₂ and CuGaTe₂, has shown much promise, with ZT in excess of unity for both compounds. However, they still suffer from relying on large amounts of expensive tellurium. In this work we look at sulfur based chalcopyrite materials, specifically the naturally occurring mineral for which the structure is named, CuFeS₂. This compound is highly abundant in the earth&’s crust and contains nontoxic elements, making it a good alternative to lead-based materials.
Initial studies on CuFeS₂ have shown much promise, with a large powerfactor that is flat over a wide temperature range, and thermal conductivity that is moderate, but almost entirely lattice. This leaves room for improvement with grain size reduction and increased phonon scattering. The work shown here focuses on the latter, by substituting selenium on the sulfur site to increase scattering from point defects.

Thermoelectric materials are being actively developed now, utilizing new concepts, state-of-the-art nanotechnology, and nanomaterials [1]. One need exists to develop materials which can function at high temperature, for applications utilizing high temperature heat sources, such as topping cycles, focused solar TEG, waste heat of incinerators, plants, steelworks, etc. Boron-rich compounds are attractive materials for their stability, typically exhibiting melting points above 2200 K, and they have been found to possess intrinsic low thermal conductivity, κ, even for single crystals [2]. In the course of further developing one of the series of n-type counterparts [3] to p-type boron carbide (one of the few previously commercialized TE materials), we discovered an interesting effect. Through a combination of doping certain transition metals (Co or V) into YB₂₂C₂N, and heat treatment, the absolute values of Seebeck coefficients could be enhanced to 220% while simultaneously, electrical resistivity was reduced by ~10,000% [4]. This is a striking result since it largely overcomes the traditional trade-off between Seebeck coefficient and electrical conductivity which has been a huge obstacle for enhancing thermoelectric performance. It is surmised that this is due to a hybrid/composite effect where highly conductive paths were established in the material of which the host material has the electronic structure, i.e. Seebeck coefficient, modified by intrinsic doping. This hybrid effect may be applicable to a broad spectrum of thermoelectric materials.
[1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, (Springer, Heidelberg, 2013).
[2] G. Slack et al., Phys. Rev. B 4 (1971) 1714, T. Mori, “Higher Borides” in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 38, (North-Holland, Amsterdam, 2008) pp. 105-173, J. Appl. Phys. 102 (2007) 073510.
[3] T. Mori et al., J. Solid State Chem. 179 (2006) 2908, J. Appl. Phys. 101 (2007) 093714, Dalton Trans. to be published (2014).
[4] A. Prytuliak et al., J. Electron. Mat. 40 (2011) 920, Mat. Res. Bull. 48 (2013) 1972

Solid state devices based on thermoelectric materials, not only offer a solution to meet the increasing global demand of energy but also provide a pollution free environment. However, the present challenge is that most of the good thermoelectric materials discovered so far, either have toxic elements or have expensive and rare earth elements making it less reliable for commercial acceptance. We report here synthesis of new ternary clathrate compound K₈Al_xSi_46-x using elements which are non-toxic and abundant in earth crust, and its thermoelectric properties. X-Ray diffraction (XRD) measurements indicate that the synthesized sample consists of type-I clathrate with a lattice parameter a = 10.45 (3) Å. Induction-coupled plasma optical emission spectroscopy (ICP-OES) shows the chemical compositions of the synthesized sample are 19.4 wt. % K, 12.0 wt. % Al, and 68.3 wt. % Si, which corresponds to the chemical formula K₈Al₇Si₃₉. Its electrical resistivity shows metallic conduction below 390K: the resistivity increases with temperature. The Seebeck coefficient is found to be 87 µV/K at 390 K.

The performance of the thermoelectric device is largely determined by the contact resistance properties. In this study, we examined one of the thermoelectric device aspects, contact resistance of thermoelectric films and their electrodes. We investigated the contact properties of thermoelectric bismuth-tellurium (Bi-Te) and antimony-tellurium (Sb-Te) films with various electrodes (Au, Ti, TiN and Sb) and post-annealing effects on them. To measure contact resistance, we employed the Transfer length method (TLM). The Bi-Te and Sb-Te films were co-evaporated on the patterned substrate at room temperature. The thickness of the films was found around 1000 nm. Subsequently, the electrode was also patterned using lift-off method and various electrodes materials were sputtered on the thermoelectric films at room temperature. The thickness of the electrode was fixed at around 300 nm. Then, the samples were annealed at 250 °C for 1 h under vacuum atmosphere. The TLM samples consist of various metal electrodes of 260 × 80 mu;m sizes were parted by 30, 40, 50, 60, 70, 80, and 90 mu;m on the thermoelectric films. The current input and output probes were located on each end of metal contact and the voltage probes were placed on the metal pad. The resistance was measured at various current values from -0.5 to 0.5 mA for the adjacent metal contact.
The Au contact shows the best performance for both Bi-Te and Sb-Te thermoelectric films. The specific contact resistance between 1 h-annealed Bi-Te and Sb-Te films with Au electrode were 4.07x10^-11 #8486;m² and 1.91 x 10^-11 #8486;m², respectively.

Over the past decades, bismuth (Bi) is well-known material as the most interesting thermoelectric (TE) applications at room temperature because of its anisotropic transport characteristics [1]. It is now widely recognized that bulk Bi is a semimetal and electrical conductivity of Bi thin films decreases with increasing temperature, in contrast to that of bulk Bi. It was suggested by quantum size effects that in the state of Bi from a semimetal into a semiconductor [2]. Thus, both the thermal conductivities of electronic (κ_e) and lattice (κ_l) of Bi thin films are affected by film thickness as well as surface morphology. Consequently, TE properties of Bi have been studied in micro/nanostructures that it can be associated with a significant reduction of thermal conductivity.
In this work, we present on the in-plane thermal conductivities of nanoporous Bi thin films with a thickness of 50 nm, which were prepared using e-beam evaporation of Bi metal. For this structure, we utilized polystyrene beads ranging from 200 to 750 nm in diameter as an etch mask. At room temperature, total thermal conductivity (κ =κ_e + κ_l) of Bi of 1.40, 0.92, and 0.46 W/m-K for nanoporous Bi thin films, with hole/neck size of 490/260 nm, 200/90 nm, and 135/65 nm as reported in our previous publication, respectively [3]. Subsequently, we successfully investigated the temperature-dependent thermal conductivities of nanoporous Bi thin films with different pore sizes via four-point probe 3-omega; method in the temperature range of 20 to 300K. In consequence, the measured κ of the nanoporous Bi thin films were gradually decreased by controlling pore size, and it is lower than the planar Bi thin film because of the greatly enhanced boundary scattering through charge carriers and phonons at the pore surface.
[1] J. W. Lim, K. Hippalgaonkar, S. C. Andrews, A. Majumdar, and P. D. Yang, Nano Lett. 12, 2475, (2012)
[2] M. S. Dresselhaus, T. Koga, X. Sun, S. B. Cronin, K. L. Wang, and G. Chen, 16th IEEE International Conference on Thermoelectrics, 112, (1997)
[3] G. S. Kim, M. R. Lee, S. Y. Lee, J. H. Hyung, N. W. Park, E. S. Lee, and S. K. Lee, Nanoscale Res Lett. 8, 371, (2013)

It was well noticed that Tthermoelectric device has the a low-degree efficiency and high potential to covert heat to electricity now. Its efficiency highly depends on thermoelectric figure-of-merit, ZT, which consists of Seebeck coefficient (S), electrical conductivity (σ), thermal conductivity (k), and absolute temperature (T). During last few decades, most effects have been made on increasing ZT at room temperature. From the definition of ZT, higher ZT requires higher S and σ, and lower k. Howerver, it has been changing to improve ZT in given bulk thermalelectric materials, because these three physical properties are coupled with each other. Thus, a low dimension nanostructures could be one of the best candidate materials for thermoelectric device applications.
In this study, we investigated a thermal transport and measured temperature-dependent thermal conductivities of 2-dimensional (2D) antimony telluride (Sb₂Te₃) at a temperature 20 K - 300 K using 3-omega; technique. A number of research groups, including our groups, have studied the thermoelectric properties of the Sb₂Te₃ thin films using various techniques such as platinum-resistance-thermometer (PRT), laser heating, and 3-omega; technique.
In this study, we intensively studied on the out-of-plane thermal conductivities of epitaxial Sb₂Te₃ thin films with thicknesses of 100 and 500 nm, which were prepared using pulsed laser deposition (PLD) on SiO₂ (300 nm)/Si substrates. We found that their thermal conductivities significantly reduced with decreasing grain size and thickness of the films. The thermal conductivities of the Sb₂Te₃ films were found to be in the range of 0.18 to 1.6 W/m middot; K at 300 K. Furthermore, we theoretically calculated the thermal conductivities of these Sb₂Te₃ films at a temperature ranging from 20 K to 300 K using a simple theoretical Callaway model and compared with our temperature-dependent thermal conducities. We then found that the Callaway model agrees reasonably with the experimental data.

In the preparation of Mg₂Si thermoelectric (TE) legs, the donor impurity Sb (Group 15 element) is known to be one of the most thermally stable dopants in Mg₂Si, and provides sufficient thermal and electrical conductivities at elevated temperatures. However, the largest sintered sample size so far that exhibits good TE properties has been up to 40 mm in diameter because Sb-doped of Mg₂Si causes cracks to form during sintering. We have examined the use of metallic Ni, Cu, and Zn powders as binders for the sintering process of Sb-doped Mg₂Si with the aim of developing a scalable and reproducible fabrication of TE legs, which is a technology that will be needed in order for Mg₂Si TE generators to move into production. The current maximum possible doping concentration of Sb in Mg₂Si has been 0.5 at%, while even if we utilize the metallic binder process, doping of Sb at concentrations beyond 0.5 at% has still not enabled scalable sintered pellet fabrication in a reproducible manner. However, we have witnessed that the incorporation of Sb at >1.0 at% in Mg₂Si has effected a reduction in thermal conductivity at elevated temperatures in past experiments. We have been investigating the co-doping of impurities into Mg₂Si using Sb and Al as donors, and also the isoelectric impurity Zn, which is expected to predominantly substitute in place of Mg, in order to obtain better TE properties, thermal stabilities, and process scalability. Instead of the addition of a metallic binder prior to the sintering process, the supplementary dopant Zn was introduced into a congruent melt of Sb-/Al-incorporated Mg₂Si using the “All Molten Synthesis” method. The concentrations of co-doped Sb+Zn or Al+Zn were varied from 0.5 to 1.0 at%, respectively. The resultant polycrystalline Mg₂Si was pulverized and then sintered using a “Plasma Activated Sintering” (PAS) technique. Co-doping improved the sinterability and the reproducibility with a yield of >90 %, which was much better than that of solely Sb-doped samples, while co-doping enabled a power factor of >2.5x10^-3 W/mK² (600~900 K) and a ZT value of ~0.85 at 873 K. A thermally stable TE chip is basically needed for the industrialization of Mg₂Si TE generators, while Sb-doping exhibited satisfactory thermal durability, showing low and stable electrical resistivity characteristics at 873 K in atmospheric conditions for up to 5000 h. In this report, we also present the results of atmospheric aging tests for co-doped specimens in terms of thermal stability, such as changes in resistivity, variations in surface morphology, and deterioration behavior.

In this study, a novel methodology of incorporating nanoscale secondary phase at crystallite boundaries has been used to modify electron and phonon transport for improving the properties of a thermoelectric materials. The effect of crystallite size and grain boundary phase on the structural, electrical and thermoelectric properties of Bi₂Te₃ nanocomposite thin films has been investigated. Bi₂Te₃:Silicon and Bi₂Te₃:Carbon nanocomposite films have been grown by co-sputtering of silicon and carbon with Bi₂Te₃. Growth temperature and presence of Si & C phase at inter-crystallite boundaries are observed to have a strong effect on the topography, orientation of crystallites and microstructure. X-ray diffraction study demonstrates Bi₂Te₃ and Bi₂Te₃:C samples have preferred 0 0 15 orientation in comparison to Bi₂Te₃:Si samples which have randomly oriented crystallites. Atomic Force Microscopy, Conducting Atomic Force Microscopy and Scanning Thermal Microscopy were carried out on composite samples. Seebeck coefficient and electrical conductivity values have been determined to study the power factor. High value of power factor (3.71mWK^-2m^-1) for Bi₂Te₃:Si_300 sample is attained as appreciable Seebeck coefficient is achieved simultaneously with high electrical conductivity. The effect of tellurium (Te) concentration on thermoelectric performance of Bi₂Te₃ is studied to further improve the Seebeck coefficient by pre-depositing a thin Te layer before Bi₂Te₃ deposition. For comparison and to study the effect of secondary phase on thermal conductivity, bulk samples of Bi₂Te₃ with different concentration of carbon have been used. This study shows that by incorporating a secondary phase along crystallite boundaries, microstructural, electrical and thermoelectric properties can be controlled.

Organic thermoelectric has been a popular research direction due to their flexibility, low manufacturing cost and suitability for large-area fabrication. One of the key parameters in adjusting the figure of merit (ZT) of organic materials is the thermal conductivity. In metal/organic hybrid materials, the thermal boundary conductance across the metal/organic interface plays a significant role in the total thermal conductivity of the thin film. Here by using an “intermixing layer” to simulate the Ag/ pentacene interface, we apply lattice Boltzmann method (LBM) to evaluate the thermal boundary conductance (TBC) of Ag/pentacene thin film. In the LBM simulation, an interface model involving the interfacial bonding, thickness and composition are considered and we show that interfacial bonding has a significant influence on the TBC of Ag/pentacene thin film. For a 50% Ag and 50% pentacene intermixing layer, the TBC of Ag/pentacene thin film decreases from 35.3MW/m²-K to 26.9MW/m²-K, while the intermixing layer thickness increases from 2nm to 20nm. The cross-sectional TEM images indicate that the interface has a larger Ag/pentacene mixing when deposition temperature is higher. Simultaneously, 3-omega; method is used to experimentally measure the thermal boundary conductance of the interface and the total thermal conductivity of the Ag/pentacene thin film with different deposition conditions. The experimental results will be compared with the simulation findings. The detailed effects of intermixing layer will be studied and discussed in the future research. The simulation and experiment findings will provide important information about modulating the thermal conductivity of metal/organic hybrid materials and use it as the potential application for organic thermoelectric devices.

In order to fabricate Mg₂Si thermoelectric (TE) power generation modules, metal electrodes are indispensable in order to extract the generated electricity efficiently. However, electrode materials that exhibit a thermal expansion coefficient that is matched to that of Mg₂Si, which have high durability and no serious reactions with Mg₂Si at elevated temperature, and which exhibit low ohmic-contact resistance to Mg₂Si of less than 10^-10 Omega;-m², need to be identified to accelerate the advent of the Mg₂Si TE power generators. We have been trying to form Ni electrodes to obtain lower contact resistance on n-type Mg₂Si by means of a monobloc plasma activated sintering (PAS) technique that enables the simultaneous formation of metal electrodes during sintering of Mg₂Si. So far, Ni is currently the electrode material of choice for n-type Mg₂Si, with low contact resistance values of 3x10^-10 Omega;-m² for the monobloc sintering and 10x10^-10 Omega;-m² for electro-less plating methods. One more important aspect of Ni is that it exhibited a stable boundary between the Mg₂Si and Ni layers, and its inter-layer adhesion properties were adequate at elevated temperatures of up to 900 K. However, our recent experimental data clearly indicate that the thermal stability of Ni at the interface between the Mg₂Si and the Ni electrode layer can be affected by the presence of impurities in the Mg₂Si matrix, and thus an alternative electrode material could help us to design a Mg₂Si TE power generator for various heat source conditions in a flexible manner. To carry out a thorough examination of other electrode materials for the Mg₂Si matrix, multiple elemental materials, compounds and alloys could be candidates for the electrodes. However, the present monobloc sintering method is not a suitable process for forming electrodes, because it is not easy to control thickness of the electrodes precisely, and the process temperature required for the preparation of the electrodes has to match a sintering condition of Mg₂Si at about 1123 to 1173 K. In this report, we attempt to identify electrode materials which possess better interfacial junctions to n-type Mg₂Si, which have sufficient persistence at high-temperature, and which exhibit lower contact resistance. We used a plasma activated sintering method to form the Mg₂Si matrix, and we used vacuum evaporation and sputtering methods to form the electrodes (materials are Al, Mo, Au, Ti, W and Ni). In order to identify an appropriate electrode material, we examined (i) the I-V and C-V measurement characteristics and calculated the Schottky barrier height and carrier density (ii) aging tests for thermal durability at elevated temperatures, and (iii) post-annealing for sufficiently low contact resistance at interfacial junction. In terms of identifying possible alternative electrode materials to Ni, our results for the electrical properties, the durability and the thermal behavior at the interface will be discussed in the report.

The fabrication of Mg₂Si thermoelectric power generators (TEGs) using a conventional Π-structurerequires a counterpart p-type material such as MnSi_x or β-FeSi₂, while the use of n-type Mg₂Si alone is another possibility for fabricating a unileg TEG structure. One concern about the p-type conductivity of Mg₂Si is that, although they are practical, the thermoelectric (TE) properties of p-type material are not equivalent to those of n-type Mg₂Si. Nevertheless, Mg₂Si has several promising features, such as the abundance of its constituent elements, its non-toxicity, and the facts that it is light-weight and has the capability of becoming a candidate material for TEG applications. We have attempted to realize a unileg TEG by using solely n-type Mg₂Si. We are currently tuning the TE chip power generation ability by modifying the type of dopant and the contents of the matrix, the sintering sequence, the metal electrode adaptability, and the TE chip dimensions. For a practical TEG module, we need to give more consideration to mechanical toughness, thermal impedance matching, thermal interface material, and thermal heat flow management.
Here, we report on the power generation characteristics of a TE chip as a function of chip dimensions by monitoring the open circuit voltage (V_OC) and the power output (P) under over a temperature difference between 873 K and 373 K (ΔT= 500 K). The elemental n-type Mg₂Si TE chips that were examined had dimensions of between 3x3x5.0 and 5x5x10 mm³. Under the present Mg₂Si matrix conditions, the 5x5x7.5 mm³ chip showed a maximum output power of 511 mW (2.0 W/cm²) when a temperature difference of ΔT= 500 K was maintained, while the TE chip with 5 mm height and a 5x5 mm² cross-section exhibited a lowered V_OC value resulting in insufficient ΔT retention due to higher heat flow. The expected power generation characteristics, with reference to V_OC, the short circuit current (I_SC), and the power curve of the TE chip, were measured and then evaluated using finite-element modeling with the ANSYS code. For the ANSYS calculation, the fundamental TE data of the Seebeck coefficient, the electrical conductivity, and the thermal conductivity as a function of temperature were initiated from the measured values of a fabricated TE chip consisting of Sb-doped and Sb+Zn co-doped n-type Mg₂Si prepared by a plasma-activated sintering process. A prospective unileg structure TE module based onan n-type Mg₂Si chip was also fabricated and measured. We also report the results of power curve measurements of basic unileg modules consisting of between 2 and 6 TE chips. Regarding the ANSYS calculation, unileg structures consisting of multi-TE-chips as a basic TEG model were evaluated in terms of possible maximum output power and thermal loss peculiar to the unileg configuration by comparing the measured data with the calculated results.

In the present study, we use the first-principles calculation based on a full-potential linearized augmented plane-wave (FLAPW) method to investigate the effects that doping of magnesium silicide (Mg₂Si) with Al, Sb, and Zn has on its structural and thermoelectric properties, and we compare the theoretical results with experimental data.
Mg₂Si is a promising candidate for mid-temperature (600 K-900 K) thermoelectric applications and is therefore expected to be suitable for a waste-heat-recovery system in the automotive industry. This material also possesses attributes that are advantageous for practical applications in terms of cost and environmental protection: it is nontoxic, cheap, lightweight, and composed of elements that are abundant in the Earth&’s crust. In order to optimize the thermoelectric properties of Mg₂Si for practical applications, impurity doping is necessary. This can bring about changes in carrier concentration as well as in the structural parameters, e.g., the lattice constant and the atomic positions in a crystal. Site preference of impurity atoms, which is associated with carrier concentration, is also related to the electronic structure and transport properties. However, it is time-consuming and sometimes impractical to obtain a direct measure of the contributions of these impurities to the thermoelectric performance. A theoretical method, therefore, can be used to understand the mechanism of impurity doping and can thereby contribute to material design.
In the present study, we clarify the doping effect on the thermoelectric properties of Mg₂Si containing several impurities, such as Al, Sb, and Zn, from both the theoretical and experimental points of view. We first examine the impurity concentration dependence of the lattice constant using the All electron Band structure CAlculation Package (ABCAP), which employs the FLAPW method based on density functional theory within the local density approximation. The calculation results are compared with experimental results obtained from Rietveld analysis of X-Ray diffraction data. Moreover, we focus on a quantitative feature, i.e., the solubility limit of the dopants. We calculate the total energy of a super cell containing impurities and evaluate the formation energies of the impurity-doped systems. We then discuss the dependence of the Seebeck coefficient on the impurity concentration within the solubility limit using the Boltzmann transport equation, and elucidate the contributions of the structural and electronic properties. We compare the calculation results with experimental data obtained using the ULVAC-RIKO ZEM-3 over a temperature range of room temperature to 873 K and discuss the validity of our theoretical analysis.

From the standpoint of reducing our dependence on fossil fuels and reducing greenhouse gas emissions, thermoelectric (TE) generation is a very attractive technology because it can convert waste heat directly into electricity. Magnesium silicide (Mg₂Si) has been recognized as a promising environmentally benign thermoelectric material for applications in the temperature range from 600 K to 900 K due to several attractive features, such as its lightweight, the abundance of its constituent elements, and its non-toxicity. Focusing on systems that are appropriate for automotive applications, TE power generation modules should have the capability to operate well even under conditions of thermal stress and vibration. In order to realize a practical TE power generation module, better understanding of the mechanical properties of TE materials is essential to facilitate the structural design of TE power generators. One of the crucial problems that affect many of TE materials is their low mechanical strength and brittleness. As is the case for many TE materials, Mg₂Si also suffers from these problems. The bending strength of Mg₂Si is about 50 MPa, and its fracture toughness is about 1.0 MPa#12539;m^1/2.Thus, the mechanical properties of Mg₂Si should be improved if we wish to design more reliable module structures.
The mechanical properties of sintered bodies are closely related to grain size. It has been reported that the strength of polycrystalline metals and ceramics has been increased by refining their grain size. From this point of view, we investigated the influence of grain size on the bending strength and fracture toughness of Mg₂Si. Specimens were sintered from Mg₂Si powder with four different particle sizes(75~25 mu;m, 10 mu;m, 500 nm and 100 nm). Each of the specimens was sintered by PAS (plasma activated sintering) under a pressure of 60 MPa. The strength and the fracture toughness of the sintered specimens were measured by employing a 3-point bending test and an indentation fracture toughness test, respectively.

There is a large need for development of sustainable thermoelectric materials with high conversion efficiency. Bi-Te based materials have emerged as ideal thermoelectric materials with relatively high zT value. But due to the rarity of such elements and toxic nature has limited their use. As an alternative, CuFeS₂ nanoparticles are synthesized as building blocks for nanostructured thermoelectric materials which meet the requirement for sustainability as these elements are in abundance in earth&’s crust and are non-toxic in nature. Bottom up wet chemical approach has been employed to synthesize copper iron sulfide nanoparticles which allow the opportunity to tune size, shape and composition of nanoparticles. In this presentation, we focus on the structural characterization of the resulting nanoparticles and compositional effect on the Seebeck value of thermoelectric material. The results will be discussed in terms of the nanoparticle characterization techniques such as TEM, XPS, STEM-HAADF, XRD, which reveals detailed information about the particles size, shape, composition structure and other characteristics which dictate the resulting thermoelectric properties such as seebeck coefficient.

Zintl phase clathrates are characterized by the cage framework mainly composed of Si, Ge, or Sn with alkali metal or alkaline-earth metal elements residing inside as guest atoms. These materials show low thermal conductivity because of the large scattering of the acoustic phonons by the rattling phonons arising from the anomalous vibrations of the guest atoms [1,2]. Therefore, clathrates are considered to be promising thermoelectric materials, from the view point of thermoelectric figure of merit , where S is the Seebeck coefficient, is the electrical conductivity and is the thermal conductivity.
The fact has been known that, in type I clathrate#12288;Sr₈Ga₁₆Ge₃₀, showing off-centered displacement of encapsulated elements, is suppressed even stronger via the scattering of acoustic phonons by anharmonic phonons of the rattler. Consequently, further detailed understanding on the anharmonic potentials realized in clathrates is important, and their related physical properties as well as the material design for achieving high ZT efficiency have been very interesting research topics.
In this meeting, we will present a systematic study on the low temperature heat capacity C_p for single-crystal type-I clathrates: n-type Ba₈Ga₁₆Ge₃₀, p-type Ba₈Ga₁₆Ge₃₀, n-type Sr₈Ga₁₆Ge₃₀, n-type K₈Ga₁₆Sn₃₀, and n-type Ba₈Ga₁₆Sn₃₀ [3]. The low-T linear term ^obsγT of C_p, including the tunneling term of the atoms accommodated in the host cages (γ_phT = αT ) and the Sommerfeld itinerant-electron term (γ_eT ), are successfully estimated through careful measurements. The electron phonon interaction strength and the tunneling density of anharmonic potentials have successfully been evaluated on a basis of the analyses.
[1] J. Tang, et al., Phys. Rev. Lett., 105, 176402 (2010).
[2] J.-T. Xu, et al., Phys. Rev. B, 82, 085206 (2010).
[3] J. Wu, et al., Phys. Rev. B, 89, 214301 (2014).

The harnessing of the catalytic flameless combustion of hydrocarbons as heat sources constitutes an important pathway for exploring chemical-to-thermal and thermal-to-electric energy conversion. This type of thermoelectric energy conversion depends on the catalytic activity and heat generation. Pt and Pt-based nanoalloy catalysts have demonstrated high activity for catalytic oxidation of hydrocarbons. This work aims at a fundamental understanding of catalytic mechanism of methanol and ethanol oxidations over supported nanoalloy catalysts on a solid state device with thermoelectric materials. Both the catalytic activity and the heat production over different nanoalloy catalysts on different supports are determined. Results from the characterization of the nanoalloy structures, and the support-nanoalloy interactions in correlation with the catalytic properties will be discussed. The implications for the design of effective nanocatalytic-thermoelectric energy conversion will also be discussed.

The conversion of a temperature difference to an electrical potential/voltage difference via the Seebeck effect is fundamental to various schemes of thermal to electrical energy conversion. Conventionally, the Seebeck coefficient (S), at a given temperature, has been regarded as a scalar quantity with a definite positive/negative value, and material specific.
However, on deeper inquiry, it is observed that the S (as the ratio between the electric field and the temperature gradient vectors) is actually a second rank tensor. Consequently, it would be necessary to examine both the diagonal and off-diagonal components of the S, with interesting consequences, e.g., the off-diagonal components would have the implication that a voltage could be obtained in a direction perpendicular to the temperature gradient, In this talk, we will show through analytical and experimental methodologies, and based on the principles of the invariance of material properties to coordinate transformations, that it should be possible to create, control, and transform the S tensor and associated characteristics. For proof of principle, we have fabricated a metamaterial consisting of alternating 2 mm thick layers of copper (with a S ~ 6 mV/K) and stainless steel (S ~ 5 mV/K), forming an effective thermal media (ETM). The S tensor of such a metamaterial was probed and the measured components conformed to computational estimates.
Our work introduces the notion that the Seebeck coefficient (S) is no longer a quantity particular to a given material, and proposes a new paradigm in that both the magnitude and tensorial character of S can now be manipulated per will through suitable materials arrangements. Other intriguing consequences extend to the possibility of constructing a material with a zero S at room temperature, which will be discussed in the talk.

Thermoelectric materials are materials that present the capacity of generate electric energy when submitted to a temperature gradient. These materials have been studied because they have the possibility to recover the lost heat in workable electric energy. We have been studying the synthesis of thermoelectric nanostructures of Ca₃Co₄O₉ by microwave-assisted hydrothermal method. For these, samples were prepared using Co(NO₃)₂.6H₂O and CaCO₃ as started reagents, and KOH as a mineralization agent with the concentration of 4M. The synthesis were performed using different times (from 60 min to 90 min) and temperature of 200 °C. As it could be verified by means of X-ray diffraction results, the first experiments using time of 60 min indicate that samples crystallize in the Ca₃Co₄O₉ phase, but peaks of additional phases are also identified. Although we have observed that the amount of addition phase change depend on the time used during the hydrothermal synthesis. Than, we are studying the effect of these synthesis parameters in order to obtain single-phase samples. The samples prepared have been also characterized by thermal conductivity, electric resistivity, and Seebeck coefficient measurements as a function of temperature.

Thin film solar cells offer lower materials and manufacturing costs essential to the large scale deployment of solar arrays. Materials such as Cu(In,Ga)Se₂ (CIGS), CdTe and amorphous Si (aSi) have been the dominating light absorbing layers in thin film PV. However, elements present in these alloys present certain issues. A candidate absorber layer is the quaternary compound Cu₂ZnSnS₄ (CZTS) which is composed of earth abundant materials while its electronic and optical properties are almost ideal for solar devices. In this work, the precursor layer for the CZTS absorber was deposited by co-sputtering of Cu, SnS and ZnS followed by annealing in H₂S/N₂. This resulted in grain enhancement and near single phase CZTS crystals. The films were deposited on molybdenum (Mo) coated glass and silicon substrates and had their physical and optical properties measured. Composition is characterized with x-ray photoelectron spectroscopy (XPS). The optical properties, like band gap and dielectric constants, are characterized through ultraviolet-visible spectroscopy (UV-VIS) and spectroscopic ellipsometry. Initial measurements of film composition show that the films consist of 23 at. % Cu, 15 at. % Zn, 12 at. % Sn, and 50 at. % S.

In this work, we investigate the influence of core-shell architecture on nanowire (1D) thermal conductivity using molecular dynamics simulations with reliable Si-Ge interaction potential of Tersoff type. This provides an opportunity to design structures with desired thermal conductivity for thermoelectric device or electronic cooling applications. To explore the parameter space, we have calculated thermal conductivity values of Si-Ge core shell nanowires at different temperatures for different cross-sectional sizes and different Ge contents. Our results indicate that (1) increasing the cross-sectional area of pure Si nanowire causes an increase in thermal conductivity (2) increasing the Ge core size in the Si/Ge structure results in a decrease in the thermal conductivity values at 300 K (3) there is no significant variation in the thermal conductivity of Si nanowire for temperature values larger than 300 K (4) the predicted thermal conductivity around 10 W/m.K is still larger than the value convenient for thermoelectric applications.

We present results of modeling and experimental characterization of thermoelectric materials built on new fabrication principles, involving the coating of three-dimensionally structured quantum well super-lattice substrates with PbTe/PbSe. A new system for wafer-scale electrochemical deposition of such structures was specifically developed and will be described in this paper. Scanning electron microscopy (SEM) was used to measure film thickness and electron diffraction spectroscopy (EDS) was used to determine film material concentration. By adjusting deposition parameters, we were able to build stoichiometric PbSe, PbTe and stacked PbSe/PbTe super-lattice films on planar and pre-structured surfaces. The films were thermoelectrically modelled using Comsol and then characterized using an infrared Seebeck effect measurement system which measured surface heating of the film while measuring the voltage associated with the temperature gradient. Thermal conductivity was measured with ultrafast laser reflectometry. We report advances in the design and fabrication of thermoelectric materials which improve cost-effectiveness and thermoelectric efficiency.

Thermophotovoltaic (TPV) system is an energy conversion scheme which converts heat energy from high-temperature thermal emitter into electrical energy via blackbody radiation propagating towards the low-band gap photovoltaic (PV) cell. Recent researches could improve the system&’s conversion efficiency critically by adopting photonic crystal (PhC) to the thermal emitter. PhC is a periodic optical nanostructure, which can prohibit the propagation of electromagnetic wave for some frequency range in certain direction. The thermal emitter composed of the PhC can manipulate its radiation spectrum to match the desired bandgap of the PV cell. The possibilities of emission tailoring by the nanophotonics have been certified theoretically and the selective thermal emitters with metallic materials (such as tungsten and tantalum) have been realized. However, we focused on the emitter&’s highest possible temperature ensuring its long-term reliability. We deducted system efficiency as a function of the emitter&’s temperature theoretically, and set a target temperature of 1500K. We considered the boron nitride (BN) as a refractory ceramic material to achieve the target temperature because it has not only high melting point and low vapor pressure but also the highest thermal shock resistance. It is expected that there is rarely atomic transfer in the BN nanostructure even at the high temperature even above the 1200K. The finite difference time domain (FDTD) simulator has been used to calculate the expected emissivity of the BN PhC thermal emitter. We optimized the nanostructure (rectangular array of cylindrical holes) to achieve cut-off wavelength of 2mu;m. The excellent thermal and chemical stability of boton nitride are very desirable for the long-term reliability of the application, but the properties cause difficulty in tailoring its nanostructure. In this study, various nano-patterning methods have been conducted in order to manipulate surface nanostructure of commercial BN sheet (5mm x 5mm x 0.2mm). The fabrication methods including focused ion beam, reactive ion etching, and wet etching with alkaline molten salts were compared in terms of the long-term thermal stability from scanning electron microscope (SEM) analysis and emissivity measurement.

Nanostructuring has been considered as an effect strategy to largely create a variety of interfaces or surfaces for not only significantly decreasing the thermal conductivity, but improving the power factor through the induced quantum confinement. In this work, we applied this concept to antimony telluride (Sb₂Te₃), one of the antimony chalcogenide materials which typically have a narrow bandgap and possess excellent thermoelectric properties near room temperature, for obtaining further improvement in the thermoelectric figure of merit. To realize the purpose, a series of nanostructured Sb₂Te₃ were deposited on SiO₂/Si substrates by pulsed laser deposition techniques. It was found that, along with the very high electrical conductivity of ~3'10⁵ S/m, an excellent Seebeck coefficient of ~140 mVK^-1 was obtained to have a comparable power factor of ~33 mu;W/cmK² at room temperature.

Thermoelectric materials allow for direct conversion of thermal energy into electrical energy and may play an important role in the search for alternative energy technologies. The efficiency of a thermoelectric device is governed by the thermoelectric figure of merit, ZT. According to the phonon-glass electron crystal (PGEC) concept, a good thermoelectric material requires an optimized charge carrier concentration and a low lattice thermal conductivity.(1) In recent publications the PGEC concept has been applied to superionic compounds like Cu₂Se, Cu₂S, Ag₂Se and related compounds.(2,3,4) In this context the concept of phonon-liquid electron-crystal (PLEC) thermoelectrics has been introduced.(3,4)
Inspired by the promising thermoelectric properties of these binary compounds, we present here the thermoelectric properties of Cu₇PSe_6, as the first representative of the class of argyrodite-type superionic thermoelectrics, which is a class of more complex superionic conductors.
Argyrodites constitute a large family of compounds and have been studied because of their structural and physical properties, in particular their ionic conductivity. Many argyrodites are known to date due to a manifold of substitution possibilities on the different cation and anion sites. Most ternary argyrodites are semiconductors. In the structure of Cu₇PSe₆ the Se atoms form a 3D network of close packed tetrahedra providing a crystalline pathway for semiconducting electrons (or more precisely holes) partially occupied by P atoms in an ordered fashion. The copper ions are highly disordered with liquid-like mobility in the Se sublattice and superionic even at low temperatures. This extraordinary "liquid-like" behavior of copper ions around a crystalline sublattice of Se in Cu₇PSe₆ leads to an intrinsically low lattice thermal conductivity and a very promising maximum zT of 0.35 at 575 K. The multitude of options for site substitution make argyrodite-type compounds an excellent model system to study phonon-liquid electron-crystal thermoelectrics.
We describe the synthesis and chemical characterization as well as the characterization of the thermoelectric transport properties of Cu₇PSe₆. The obtained thermoelectric data are compared with those of the closely related superionic thermoelectric material Cu₂Se in order to obtain a deeper understanding of the thermoelectric transport in superionic conductors within the phonon-liquid electron-crystal concept.
[1] G. A. Slack, in CRC Handbook Of Thermoelectrics, edited by D. M. Rowe, 1995.
[2] Drymiotis, F.; Day, T. W.; Brown, D. R.; Heinz, N. a.; Jeffrey Snyder, G. Appl. Phys. Lett. 2013, 103, 143906.
[3] Liu, H.; Shi, X.; Xu, F.; Zhang, L.; Zhang, W.; Chen, L.; Li, Q.; Uher, C.; Day,T.; Snyder, G. J. Nat. Mater. 2012, 11, 422-425.
[4] He, Y. et al. Adv. Mater. 2014, n/a-n/a. doi:10.1002/adma.201400515

Thermoelectric devices that make a direct conversion between thermal and electrical energies have attracted great attention in the last years. In particular, nanowire superlattices have a band structure by design, which with a properly placed Fermi level could significantly enhance the thermoelectric power. Moreover, the phonon scattering at nanowire surface and compositional interfaces leads to a lower thermal conductivity and hence an increase of the thermoelectric figure-of-merit (ZT). In this work, based on the Kubo-Greenwood formula, the transport of electrons and phonons in segmented nanowires is studied by means of a real-space renormalization plus convolution method [1]. This method has the advantage of being computationally efficient, without introducing additional approximations, and capable to analyze aperiodic nanowires with truly macroscopic length. The tight-binding and Born models are respectively used for the calculations of electronic and lattice thermal conductivities [2]. For the periodic case, an analytical solution of ZT is found, which is compared with those obtained from periodic and quasiperiodically segmented nanowires. Our calculations show a clear enhancement of ZT in quasiperiodic nanowires, mainly due to its multifractal band structure and phonon-scattering coherency from quasiperiodically located interfaces. Finally, the numerical results of ZT are compared with experimental data.
This work has been partially supported by CONACyT 131596, UNAM IN113813 and IN113714. Computations were performed at Miztli of DGTIC-UNAM.
[1] V. Sanchez and C. Wang, Phys. Rev. B70, 144207 (2004).
[2] C. Wang, F. Salazar and V. Sanchez, Nano Lett.8, 4205 (2008).

The compounds (LaS)_1+xTS₂ (T: Cr, Nb) belong to the misfit layered family having a general formula (MX)_1+xTX₂ (M: Sn, Pb, Sb, Bi, rare earths; X: S, Se; T: Ti, V, Cr, Nb, Ta; and x: 0.07 - 0.28) and are built of an intercalated LaS layer responsible for disorder and phonon scattering sandwiched between the TS₂ layer with high carrier mobility responsible for electron transport. Being perfect examples of a phonon-glass-electron-crystal (PGEC) behaviour, these misfit sulfides hold a good potential as high temperature thermoelectric materials. In this study, we investigated the high-temperature thermoelectric properties of misfit layered n-type (LaS)_1.20CrS₂ and p-type (LaS)_1.14NbS₂. The samples were prepared by CS₂ sulfurization of 6 h or 12 h duration and then consolidated using pressure-assisted sintering to produce highly and randomly oriented samples with tunable microstructures. The randomly and highly oriented natural superlattices provided ultralow lattice thermal conductivities (as low as ~0.9 W K^minus;1 m^minus;1 and ~0.5 W K^minus;1 m^minus;1, respectively, at 950 K) perpendicular to the pressing direction. The highly oriented texture produced the highest ZT of 0.14 at 950 K among the (LaS)_1.20CrS₂ system, whereas the weakly/randomly oriented texture produced the highest ZT of 0.15 at 950 K among the (LaS)_1.14NbS₂ system. The off-stoichiometry of La was studied in the (LaS)_1.14NbS₂ system. A 25% improvement in ZT was observed for the composition (La_1.05S_1.05)_1.14NbS₂ mainly due to the increase in power factor arising from higher carrier concentration and mobility.

Thermoelectric (TE) materials are all solid-state materials which interconvert thermal energy into electrical energy and vice versa. Having no moving parts and/or fluids they have great potential in the cooling and power generation applications. These advantages have gained the interest of the scientific community to investigate TE materials from research to the products.
Depending on their application temperature TE materials are classified in three main categories; as low (up to 250°C), intermediate (up to 550°C) and high (more than 600°C) temperature. Currently, Skutterudites (CoSb₃) based materials have shown promising results in the intermediate temperature range (300-500°C). This family of material is highly suitable for automotive, marine transportation and industrial power generation applications to recover the waste heat from the exhaust and generate electricity. Conventional TE modules need p- and n-type semiconductor materials and for the skutterudite family, iron (Fe) has proven to be among the best candidates for the substitution of cobalt sites. Additionally, rare earths are doped as rattlers in the crystal cages of the skutterudite to decrease the thermal conductivity, thus improving the figure of merit ZT of the TE material.
For industrial purposes, stability of these materials is of great importance. Compositional stability is being addressed as the material decomposes above certain temperature (>600°C). Temperature dependent x-ray diffraction study was performed on Fe substituted, Yb-filled skutterudites, using Beam Line I711 at MAX LAB [1], to observe the crystal structure as a function of temperature. 2D diffraction patterns were collected from room temperature to 500°C by utilizing Huber furnace. Results of these studies as well as thermoelectric transport evaluations will be presented in detail.
[1] Y Cerenius, K Staring;hl, LA Svensson, T Ursby, Å Oskarsson, J Albertsson, A Liljas, The Crystallography beamline I711 at MAX II, J Synchrotron Rad (2000) 7, 203-208

Thermoelectric (TE) materials exhibit numerous interesting features such as solid-state operation, zero-emissions, vast scalability, low maintenance, and a long operating lifetime. However, its low efficiency, expressed as a dimensionless figure of merit, zT, limits many applications. Recent research into low-dimensional nanoengineered TE has identified classical and quantum mechanical size effects on electrons and phonons, which provide additional mechanisms to tune the ZT. Recent reports on the high peak zT of PbTe from 1.5 to 2.2 have raised interest in the material as thermoelectric applications. PbSe, compared with PbTe, has not been as frequently investigated for thermoelectrics due to its higher thermal conductivity and narrower bandgap. However, recent calculation and experiments from Parker, Singh, and Snyder showed the possibile application of of heavily doped PbSe in highly efficient thermoelectrics. In this report, we fabricated electrodeposited PbTe and PbSe nanowires patterned with photolithography, a method widely known as lithographically patterned nanowire electrodeposition. Nanowires fabricated in this manner do not need to be dispersed and aligned for measurements. Yang and Penner fabricated PbTe nanowires in this way, but they synthesized PbTe by cyclic electrodeposition-stripping. They electrodeposited PbTe and elemental lead simultaneously, and the excess lead was etched selectively. We synthesized few atomic layers of Pb and Te (Se) layer by layer using surface limited reactions, generally called as underpotential deposition (UPD). UPD is a phenomenon where few atomic layers of element are electrodeposited at a potential before the required potential for bulk deposits. Because underpotentially deposited Pb and Te(Se) have few atomic layers, a compound is formed spontaneously without annealing process. Thermoelectric properties of fabricated nanowires were measured and analyzed. Details will be presented.

Power generation characteristics of a sandwich-type thermoelectric generator in which heat source is embedded into thermoelectric elements are investigated. Our previous work on a similar concept only considered a uniform heat source distribution inside thermoelectric elements. In this work, the effect of the spatial distribution of heat source is examined. In particular, the effect of the concentration of heat source near the hot end is intensively studied as a potential way of improving the efficiency of the device.
While the effects of heat source concentration in impractical cases without any heat transfer limitation on the cold side remain ambiguous, it becomes clear that heat source concentration indeed has positive effects in more realistic cases with finite heat transfer coefficients imposed on the cold side. Due to the relatively low efficiency of typical thermoelectric generation, significant amount of heat must be dissipated from the cold end of the thermoelectric element. A higher level of heat source concentration near the hot end leads to more effective utilization of available heat source, reduces the amount of heat ejected at the cold end, and lowers the hot-end temperature of the thermoelectric element. Overall, it is suggested that heat source concentration can be used as a method to achieve more efficient operation and better structural integrity of the system.

We present our results on phonon and electron transport across nanostructured interfaces and the resulting impact on thermoelectric properties. For the electron transport we focus our investigations on the energy filtering at grain boundaries [1]. 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 introduced by Seto [2]. In this model a doping dependent 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. For low doping concentration we obtain such an energy filtering effect, but for high doping concentration that are necessary for effective thermoelectric materials 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. For the phonon transport we use an atomistic Greens function method [3] to investigate the phonon scattering at ²⁸Si/²⁹Si and ²⁸Si/³⁰ Si isotope-multilayer. Our calculations show that a periodic arrangement of the layer-system cannot decrease the phonon thermal conductivity substantially, whereas a random arrangement of the layer-system can lead to a strong decrease in the phonon conductivity. We also show that small deviations from the periodic arrangement are enough to end up in the random regime.
[1] M.Bachmann, M. Czener, and C.Heiliger, Phys. Rev. B86, 115320 (2012)
[2] J. Seto, J.Appl. Phys.46, 5247 (1975)
[3] W. Zhang, T. Fisher, and N. Mingo, Numerical Heat Transfer, Part B51, 333 (2007)

Lead telluride (PbTe), one promising thermoelectric material candidate, has been extensively studied because of the atoms that deviate from the frozen rigid lattice approximation at finite temperatures. It has been observed that the mean square displacements of Pb atoms are significantly larger compared to those of the Te atoms at elevated temperatures. It is also believed that at high temperatures, lead chalcogenides form local structural dipoles from the undistorted ground state. Using first-principles calculation, we studied this important problem by determining the stable crystal configurations of PbTe in the temperature range of 0 K to 400 K. The displacements of the Pb nuclei are found predominantly along <100> direction at the temperature above 200 K. Based on such configurations, the electronic properties, such as the direct energy bandgap and the local density of states (LDOS), were studied to understand the electronic hybridization. The dipole moments in the PbTe crystal at 0 K and 400 K were also evaluated based on the charge density distribution. Local structural dipoles were proved to arise as the temperature increases.

Zintl phase based materials are considered as a potential candidate for high performance thermoelectric materials due to their characteristic feature of “Phonon Glass Electron Crystal (PGEG)” which is essential criteria for a material to possess a high thermoelectric performance. In this work, thermoelectric properties of Zn substituted Mg₃Sb₂ - based Zintl compound with stoichiometric compositions Mg_3-xZn_xSb₂ (0 le; x le; 0.10) has been investigated. These materials are synthesized directly by spark plasma sintering (SPS) technique as one step synthesis process. X-ray diffraction analysis and Scanning electron microscopy observations reveal a single-phase of β-Mg₃Sb₂ in all compositions of Mg_3-xZn_xSb₂ (0 le; x le; 0.10). Thermoelectric properties are characterized by measuring the Seebeck coefficient, electrical resistivity, and thermal conductivity in a temperature range of 300 K to 773 K. It is found that the Zn substitution on Mg site leads to decrease in thermal conductivity and significant enhancement in the Seebeck coefficient. As a result, a maximum ZT of 0.36 at 773 K was optimized for Mg_2.9Zn_0.1Sb₂ composition. This enhancement in ZT is about ~ 40% higher than the value of Mg₃Sb₂. Relatively high ZT, non-toxicity and abundance of their constituents make these materials a promising cost effective thermoelectric materials for waste heat recovery.
Corresponding author. E-mail address: [email protected], [email protected] (DKM)

Si has attracted big attention as an environment-conscious thermoelectric material. While Si has the high power factor, there is a problem of the quite high thermal conductivity. In the recent years, it was reported that the nanostructured Si has the extremely low thermal conductivity. Previously, Si-SiNix nanocomposite thin film is reported to have high power factor and uncommonly low thermal conductivity1). Nanosized grains are formed because metal-silicide crystals inhibit the grain growth. However the grain size is larger than 100 nm, therefore the low thermal conductivity may attribute to the planar defects in Si matrix.
To clarify this phenomenon, we fabricate Si-C nanocomposite by the mechanical alloying (MA) method by high energy ball milling process. Nanometric-sized C or SiC particles has been embedded into agglomerated amorphous Si matrix. The mixtures result in amorphous silicon and nanocrystalline silicon carbide as confirm by XRD. The SPS process is used for densification of the powder. Then samples are thermally annealed at 973K#65374;1373K to form and disperse the second phase SiC into the Si matix. The SiC second phase formation generates a volume change in the matrix, which provides the planar defects. The second phase formation effectively decreases the thermal conductivity as well as the Si-Ni system.
1) N. Uchida,T. Tada, Y. Ohishi, Y. Miyazaki, K. Kurosaki, S. Yamanaka, J. Appl. Phys., 114, 134311/1-134311/6, 2013.

We study the thermal and electronic transport through epitaxial GaAs nanopillars that are about 10 nm long and 100 nm in diameter. The pillars are epitaxially embedded between three-dimensional GaAs contact reservoirs on their front and back ends and in an AlGaAs matrix along their circumference. They can be considered as very short nanowires that are lattice matched to the contacts. The pillars represent quantum point contacts between two three-dimensional GaAs charge and heat reservoirs. The AlGaAs barrier can be removed in a selective wet etching process. In this way thin GaAs membranes arise that are supported by the pillars [1]. In such gap structures temperature gradients in the range of 10⁷ K/m are realizable along the pillars [2,3]. Here we will report about thermal transport through pillars in such gap structures, as well as about first electronic transport experiments on pillars that are still embedded in an AlGaAs matrix.
The thermal transport along the pillars is ballistic in a wide temperature range as has been shown in previous studies [2-4]. This is given, because of short pillar length, the high crystalline quality and the epitaxial connection of the pillars to the phonon reservoirs. Here we will focus on first electronic transport studies with pillars that are still embedded in an AlGaAs matrix. Reference measurements on samples that contain just the AlGaAs matrix layer without pillars show that tunneling transport through the AlGaAs barrier is negligible at a layer thickness of 16 nm. Current-voltage characteristics of samples with pillars show distinctive asymmetries [5] that we associate with the conical shape of the pillars. Although contact reservoirs and pillars are made from the same material, the transport through the pillars is dominated by tunneling across shallow barriers. This is explained by the quantum size effect on the electronic states within the pillars. Thus the transport in our pillars is similar to the transport of planar quantum point contacts in two-dimensional electron systems at gate voltages close to the threshold [6]. Molenkamp et al. reported relatively large thermo-voltages in this transport regime, which are caused by a hot electron filtering effect [6]. Due to the good thermal conductivity of their structures Molenkamp et al. were not able to apply large temperature gradients. In our structures large temperature gradients can be established if the AlGaAs matrix is removed [2,3]. Such experiments to the electron and thermo-electric transport through pillars in structures with removed AlGaAs matrix are plant for the near future.
[1] Ch. Heyn et al., Appl. Phys. Lett. 98, 033105 (2011)
[2] Th. Bartsch et al., Phys. Rev. Lett. 108, 075901 (2012)
[3] Th. Bartsch et al., Phys. Status Solidi A 210, 161 (2013)
[4] C. Jeong and M. Lundstrom, Appl. Phys. Lett. 100, 233109 (2012)
[5] Th. Bartsch et al., J. of Elec. Mat. 43, 1972 (2014)
[6] L.W.Molenkamp et al., Physica Scripta. T49, 441 (1993)

Bismuth telluride is well-established in room temperature thermoelectrics. In order to increase the thermoelectric efficiency rough nanowires have been of particular interest in recent years. In such, an increase of surface phonon scattering rate is discussed as the origin for the observed suppression of thermal conductivity. To resolve the role of morphology in bismuth telluride nanowires, a comprehensive full-thermoelectrical, morphological, structural and chemical characterization of individual nanowires was applied. We present the electrical and thermal conductivity as well as the Seebeck coefficient of individual bismuth telluride nanowires which were grown by electro-deposition. The measured electrical conductivity of the nanowire was comparable to the bulk value at room temperature. The Seebeck coefficient showed a reduced absolute value and n-type behavior, which is in agreement to the literature of as-grown nanowires. Furthermore, a reduced thermal conductivity by a factor of three was measured. Transmission electron microscopy was applied to investigate the morphology, the crystallographic orientation and the chemical composition of the full-thermoelectrically characterized nanowires. The nanowires exhibited a distinct diameter variation between 180 nm and 320 nm which distinguishes them from previous investigations on nanowires with smooth surfaces. Moreover, the nanowires showed an oriented growth along the [110] direction and a rough amorphous oxide shell. The determination of the nanowires crystallographic orientation, composition and detailed morphology allowed to consider the nanowires as model system and to distinguish between their intrinsic and extrinsic properties. Hence, we attribute the reduction of the thermal conductivity to a reduction of the lattice thermal conductivity due to the strong diameter variation of the nanowire. In particular, the incorporation of interfaces to the vacuum which are nearly perpendicular to the direction of transport can enhance diffusive phonon backscattering.[1]
[1] A. L. Moore, S. K. Saha, R. S. Prasher, and L. Shi, ‘Phonon backscattering and thermal conductivity suppression in sawtooth nanowires&’, APL 93, 083112 (2008).

Nanostructuring of thermoelectric materials has been widely employed in the last two decades with the aim to reduce the thermal conductivity k and/or to increase the power factor S²σ, thus enhancing the energy conversion efficiency. Among different nanostructuring strategies, interfaces in superlattices or nanodots in a host matrix have emerged as promising pathways to reduce the thermal conductivity through phonon scattering at multiple length-scales. Single-crystalline SiGe superlattices with and without nanodots are an ideal platform to study the effects of interfaces and nanodots on thermal transport not only because they are simple enough to permit a comparison of theoretical calculations and experiments, but also because the structural parameters, such as the interface spacing length (i.e. period length), nanodot density and matrix concentration can be precisely controlled during epitaxial growth. Despite previous investigations that showed that k can be lowered by shortening the period length and/or by increasing the nanodot areal density of SiGe superlattices, the understanding of the effects produced by interfaces and nanodots on thermal transport is still not satisfactory. Here we present systematic experimental studies on thermal transport through SiGe superlattices, which were grown by molecular beam epitaxy. First, the experiments clearly show that k of Ge/Si nanodot superlattices linearly decreases with decreasing the period length [1]. The results indicate that phonon scattering at the Ge/Si interfaces is diffusive while phonon transport through Si spacers is quasi-ballistic. Second, we show that Ge-segregation-driven intermixing around the interfaces play a crucial role in reducing k below the alloy limit [2]. Third, we explore the limits of lowering k by decreasing the period length to values down to ~1.5 nm and the role of SiGe nanodots on the reduction of k [3]. Our results reveal a weakening of the interface effect on phonon scattering and imply a lower limit for k when the period length is shorter than 2 nm. The experiments also suggest that SiGe nanodots with ‘‘pyramid&’&’-shape have an effect comparable to nominally planar wetting layers on the cross-plane thermal transport. Fourth, we elucidate how k changes during structural evolution from superlattices to alloy thin films through post-growth annealing [4]. The experiments reveal that the effect of Ge/Si interfaces on phonon transport can be weakened and eliminated by enhancing Si-Ge intermixing. The presented results are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures. Reference: [1] G. Pernot, et al., Nature Materials, 9, 491 (2010). [2] Peixuan Chen, et al., Phys. Rev. Lett., 111, 115901 (2013). [3] Peixuan Chen, et al., J. Appl. Phys., 115, 044312 (2014). [4] Peixuan Chen, et al., in preparation.

Thermoelectric materials can serve for the conversion of thermal to electrical energy by the Seebeck effect or for the solid-state cooling e.g. of electrical devices by the Peltier effect. The performance of these materials is defined by the figure of merit ZT = T * S²*σ/lambda; with T= absolute temperature, S = Seebeck coefficient, σ = electrical conductivity and lambda; = thermal conductivity.
In 200, outstanding properties were reported for Bi₂Te₃/Sb₂Te₃ thin film superlattice (SL) structures grown by metal-organic chemical vapor deposition (MOCVD) [[1]]. In this work, results on the fabrication of such structures with high thermoelectric performance, lower costs and less effort compared to epitaxial growth are shown. A comprehensive study on growth, structural and electrical properties of the SLs that significantly exceeds the current state of the art is presented.
Two synthesis methods were used: 1.) Nanoalloying, i.e. the deposition of element films and subsequent annealing to induce compound formation by sputtering / Molecular Beam Epitaxy (MBE) setup and 2.) Epitaxial deposition by MBE. We present a comprehensive study on transport properties, structural properties and thermal stability on the binaries Bi₂Te₃ and Sb₂Te₃ and of the SLs grown by these two methods.
The nanoalloyed SLs are easy to fabricate on many substrate materials, display a high structural quality and very pronounced c-axis texture, high PFs of up to 40-50 µW/cmK², thermal conductivities down to 0.40 W/mK and estimated ZT values of 1-1.9. However, the thermal stability of the nanostructuring is inferior to epitaxial material.
The epitaxially deposited Bi₂Te₃/Sb₂Te₃ SL films display very sharply defined interfaces down to period lengths as small as 6 nm. This is the first reported reproduction of Bi₂Te₃/Sb₂Te₃ SLs with such small period lengths. Low lattice thermal conductivities down to ~ 0.3 W/mK were determined. Remarkably, due to charge carrier compensation effects only low Seebeck coefficients and carrier mobilities were observed, preventing high ZT values.
Special attention was paid to the interconnection of texture, thermal stability of the nanostructure and its influence on thermal conductivity. A survey on these results will be presented.
[1] R. Venkatasubramanian, E. Siivola, T. Colpitts & B. O'Quinn., Nature 413, 597 (2001)

Silicon (Si) is widely used for electronic applications but for thermoelectric applications its thermal conductivity is unfortunately too high [1,2]. Nanostructured semiconductors open the opportunity to independently tailor electric and thermal conductivity by manipulation of the phonon transport [3]. This is a highly promising strategy for engineering thermoelectric devices with improved efficiency [4].
The concept of reducing thermal conductivity without degrading the electrical conductivity is most ideally realized by controlled isotope doping. The thermal conductivity of Si is effectively reduced by means of isotopic nanostructuring [5,6]. In this work we report measurements on the thermal conductivity of periodic and aperiodic alternating Si layers enriched with different stable isotopes. The mono crystalline structures were grown by means of molecular beam epitaxy on (100)-oriented natural Si substrates. Thermal conductivity in the cross plane direction was measured with time-resolved X-ray scattering (TRXS) at the beamline ID09B of the European Synchrotron Radiation Facility (ESRF). For these measurements the isotope structures were covered with a 2 nm thick chromium layer followed by a 30 nm thick gold (Au) film. The cooling of the metal layer after pulse-laser heating with a femtosecond laser was followed by measuring the lattice constant of Au with X-ray pulses synchronized with the laser beam. The Au lattice expansion is a direct measure for the temperature of the Au layer [7]. The effective thermal conductivity of the isotopically modulated multilayer is determined from numerical simulations of the heat transport problem. Measurements were also conducted by means of the three omega technique. Different isotopically enriched silicon nanostructures were investigated in order to determine the impact of layer ordering, number of layers, broadening at the layer interfaces, and of lateral confinement on thermal conductivity.
[1] AI. Boukai et al. Nature 451, 168-171 (2008)
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[3] L. D. Hicks, M. S. Dresselhaus Phys. Rev. B 47, 12727 (1993)
[4] R. Venkatasubramanian et al. Nature 413, 597-602 (2001)
[5] H. Bracht et al. Appl. Phys. Lett. 101, 064103 (2012)
[6] H. Bracht et al. New J. Phys. 16, 015021 (2014)
[7] D. Issenmann et al. Thin Solid Films 541, 28-31 (2013)

The efficiency of thermoelectric energy conversion can be improved by reducing the phonon contribution to thermal conductivity κ. Epitaxial multilayers and superlattices represent a promising approach because they allow for tuning the phonon density of states as well as controlling the mean free path of phonons.
In this study we focus on thin film multilayer samples of the oxides SrTiO₃ (STO) and Pr_1-xCa_xMnO₃ (PCMO, xasymp; 0.34). STO is a promising thermoelectric material. For Nb-doped samples ZT values of 0.03 at room temperature and 0.27 at 1073 K are reported [1, 2]. In PCMO a strong electron-phonon coupling results to the charge transport controlled by the hopping of small polarons. Since phonons can be efficiently scattered at the lattice distortions introduced by polaron carriers, this may offer new possibilities for tuning thermoelectric properties. In addition, the combination of these two materials reveals a large acoustic impedance mismatch (AIM) of 1.8 and thus potentially large band gaps in the phonon density of states.
In order to study the influence of the layered structure on thermal conductivity, the coherent effect of phonon interference must be separated from incoherent scattering at the interfaces and other types of defects. Consequently, a systematic study of κ(T) as a function of layer thicknesses, preparation-induced defects [3] and interface roughness is presented. These properties can be tuned by varying deposition conditions and choosing three different thin film deposition techniques: pulsed-laser-deposition (PLD), ion-beam sputtering (IBS) and metalorganic aerosol deposition (MAD). The microstructure of these samples is analyzed by means of X-ray diffraction and transmission electron microscopy. Temperature-dependent cross-plane thermal conductivity κ(T) is measured by means of the 3omega; method. In order to analyze possible scattering mechanisms, the experimental results will be compared with the trends in thermal conductivity due to phonon band gap formation as revealed by a continuum elastic wave model.
[1] K. Koumoto, Y. Wang, R. Zhang, A. Kosuga, and R. Funahashi, Annu. Rev. Mater. Res. 40, 363 (2010).
[2] S. Ohta, T. Nomura, H. Ohta, and K. Koumotoa, J. Appl. Phys. 97, 034106 (2005).
[3] S.Wiedigen, T.Kramer, M.Feuchter, I.Knorr, N.Nee, J.Hoffmann, M.Kamlah, C.A. Volkert, and Ch. Jooss, Appl. Phys. Lett. 100, 061904 (2012)

We will present our latest research on mass production of ultrathin telluride nanowires, such as Bi2Te3, PbTe, Ag2Te, Cu1.75Te, as well as the nanowire heterostructures between these compositions. By tuning the growth condition, nanowires with diameter below 10nm can be produced at tens of grams level within 2 hour window. These nanowires can be sintered into nanocomposite disks and have demonstrated improved properties in certain cases. Most importantly, by adjusting the composition modulation within the nanowire heterostructures, we can identify new platforms to study the energy filtering effect and phonon scattering at material interfaces. This work is supported by United States Air Force of Scientific Research.

The theoretically predicted enhancement of the Seebeck coefficient S or the thermoelectric power factor σS² upon quantum confinement in one-dimensional nanowires (NWs) is experimentally hard to verify. InAs NWs are an excellent experimental model system because of a relatively large Bohr radius of ~ 34 nm. Hence, ballistic electron transport has already been demonstrated at moderate diameters of around 20 nm.
In this study, we investigate crystalline InAs NWs grown by metal-organic vapor phase epitaxy using Au nanoparticles as seed. The n-type InAs NWs with a diameter between 15nm and 25nm were coated with a thin layer of Al₂O₃ for surface passivation. Subsequently, the NWs were deposited on a highly doped Si wafer covered with a 30nm-thick HfO₂ layer and contacted with multiple Ni electrodes defined by electron-beam lithography and lift-off technique. The Si wafer acted as a back gate to control the carrier concentration in the NW. A resistive heater was positioned close to the NW to establish a temperature gradient. Seebeck coefficient and conductance G of the NW were measured as a function of temperature and gate voltage.
Room temperature measurements show a maximum of the power factor of nearly 2x10^-3W/K²m. This value is even as high as in bulk n-InAs. Taking into account that the charge carriers in NWs are much more affected by extrinsic scattering effects than carriers in bulk semiconductors, this high power factor is a clear indication for an enhanced Seebeck coefficient due to one-dimensional quantum confinement. Assuming the same thermal conductivity in these 20nm-thin InAs NWs as we measured in thicker InAs NWs [1,2] i.e. ~2 W/Km, a room-temperature value for ZT of ~0.3 is obtained.
The one-dimensional confinement was verified using conductance measurement at low temperatures (<150K) and short electrode distance (<300nm). There, the NW conductance showed clear plateaus when plotted as a function of the gate voltage V_G. Typically, a G value of 25-50% of the conductance quantum G₀shy; was obtained for the first sub-band. Together with the conductance, the Seebeck coefficient of the InAs NWs was measured as a function of V_G. The shape of S vs. V_G exhibited a peak correlated with the end of the conductance plateau. Using model calculations based on the Boltzmann transport formalism we confirmed the features in the shape of G and S and derived quantitative insights, e.g. mean-free path, relaxation time, or scattering parameter.
[1] S. Karg, et al., J. Electron Mat. 2013, 42, 2409-2414
[2] S. Karg, et al., Nanotechnology 2014, in press

Nanostructured materials have been demonstrated with great success in improving the thermoelectric figure of merit (ZT=S²σ/κ) owing to their remarkably reduced thermal conductivity κ. Recently, with κ being engineered close to their low limits, further boosting the dimensionless ZT over unity in nanostructures relies more and more on enhancing the power factor (S²σ). However, due to the overwhelming surface effects, traditional strategies that aim to decouple S and σ for optimizing the power factor, i.e. by resonant scattering or energy filtering, can hardly be applied in nanostructured materials, such as quasi-1D nanowires.
Here, we demonstrate a modulation of thermoelectric power factor in Si nanowires by increasing the hole transport mobility via rationally designed radial dopant inhomogeneity. The nanowires were B-doped and were configured to exhibited a heavily doped surface but lightly doped core. Such modulated dopant distribution facilitated surface-to-core diffusion of mobile carriers, and hence decoupled the axial/longitudinal carrier transport from the dominant surface and dopant impurity scatterings. Tailoring the radial dopant profile under this strategy to be δ-doping-like allowed us to achieve apparently increased power factors over that of homogeneously doped systems. As evidenced by field-effect measurements, the enhancement was clearly related to the increased hole mobility in such nanowires. In the rationally designed δ-doped Si nanowires, the extracted hole mobility at the relatively heavily doped concentration of ~10¹⁹ cm^-3 was even 4 times higher that of bulk Si, showing the great potential in optimizing the power factor under this strategy.
In distinct from the earlier heterostructured nanocomposites or nanowires, the present strategy induces dopant modulation while without creating heterointerfaces. Thus, the possible mobility degradation by interface related mid-band trapping states can be avoided. Regarding to such advantage, this strategy may find universal prospects in many thermoelectric materials.
This work was supported by NEXT Project. T.K. was supported by FIRST program. T.Y and K.U thank the financial support of CREST.

Recent theoretical and experimental investigations have demonstrated that individual nanowires exhibit enhanced thermoelectric performance, relative to their bulk counterparts. Extension of these properties to large assemblies of nanowires ideally requires the formation of nanowires networks, where the nanowires and bridges connecting the nanowires in the network are of the same chemical composition. Such welded nanowire networks prevent the formation of oxide interfaces between the nanowires when assembled, which is typical of the current strategies for assembling nanowires. In this context, a simple and versatile strategy for synthesizing large scale, welded metal silicide nanowire networks was developed. This strategy involved the solid state phase transformation of mixtures of silicon nanowires decorated with silica nanoparticles. Experiments performed using Mg₂Si materials system as an illustrative example indicated that the formation of welded Mg₂Si nanowire networks is possible by the solid-state phase transformation of silica nanoparticle decorated silicon nanowires. These networks offer multiple levers for reducing the lattice thermal conductivities of Mg₂Si nanowire assemblies, while having no detrimental effect on their electrical conductivities. Experimentation performed also indicated that the use of welded nanowire networks enhances the thermoelectric performance of Mg₂Si at least 2-3 times, relative to that is currently possible. In this talk, experimental strategies relating to the synthesis of the welded Mg₂Si nanowire networks will be presented. The thermoelectric performance of both doped and undoped Mg₂Si nanowire network assemblies will also be discussed.

The confluence of energy demands and environmental preservation has spurred research into renewable technologies that will allow for an eventual transition away from fossil fuels as the primary source of energy. One such avenue lies in the area of thermoelectrics(TE). Recently, CuInTe_2, a member of ternary I-III-VI₂ compounds with diamond-like structure (I = Cu, Ag; III = Al, Ga, In; VI = S,Se,Te) has been reported to show promising TE properties due to the extremely low thermal conductivity. In this study, we report the rapid fabrication of CuxInTe₂ compounds with a home-made melt-spinning system, which is assembled into the glove box with high energy ball milling. By employing this integrated synthesis system, comparing with the tradition solid states method, the processing time is reduced at a large degree. On the other hand, the effect of doping Cu with Ag, In with Yb and Zn, and Cu vacancy on the thermoelectric performance of CuxInTe₂ compounds were also investigated. A figure of merit up to 1.0 was achieved in this study. The enhanced thermoelectric performance coupled with the dramatically reduced processing time will be of considerable significance to the commercial-scale production of CuInTe₂-based thermoelectric materials.

In this study, we investigated the effect of the structure of microporous p-type (Bi_0.4Te₃Sb_1.6) and n-type (Bi_2.0Te_2.7Se_0.3) bismuth telluride based thin films on their thermoelectric performance. A prototype thermoelectric module consisting of 20 pairs of p- and n-type strips over an area of 30cm² was fabricated on the porous PI substrate. Porous PI substrate with pore diameter ranging from 300 nm to 500 nm was prepared by oxygen plasma etching of polyimide (PI) layers capped with a heat-resistant block copolymer. We deposited bismuth telluride thin films on the porous PI substrate by arc plasma deposition method through the shadow masks. This module produced an output power of 0.1 mW and an output voltage of 0.6 V for a temperature difference of 130 degree C. The output power of the submicrostructured module was 1.5 times greater than that of a module based on smooth bismuth telluride thin films due to the low thermal conductivity of the submicrostructured bismuth telluride thin films.

Silicon nanowires (NWs) are a well-assessed example of the beneficial role that dimensional constraints may impart to the thermoelectric properties of otherwise inefficient materials. Incoherent phonon scattering at NW walls leads to a decrease of the thermal conductivity while marginally affecting both the electrical conductivity and the Seebeck coefficient. Further improvements of ZT may be obtained if the power factor is increased by keeping the charge carrier density at its optimal value (asymp; 10¹⁹minus;10²⁰ cm^-3 in Si) while increasing mobility. To this aim, polarization doping by molecular grafting at NW walls following an approach pioneered by Tour [1] in ultrathin silicon layers proves to be an interesting strategy. A top-down fabrication process for the massive production of silicon NWs, organized in large area arrays, was implemented. Using silicon-on-insulator substrates the simultaneous fabrication of more than 10⁵ nanowires/mm² could be achieved by lithography, anisotropic etching, and oxidation [2]. Conventional optical lithography could be used for pattern definition, followed by NW width reduction by stress-limited oxidation. The NWs, smaller than 50 nm and several micrometers long, were interconnected in an array and then positioned between contacts for electrical and thermal measurements. For this experiment, virtually undoped NW arrays were fabricated and characterized. A resistivity of 0.012 Omega; m was measured, reporting an equivalent doping of asymp; 10¹⁵ cm^-3, higher than the nominal doping value possibly because of the band bending at the NW surface. Either 1-ethynyl-nitrobenzene or 4-ethynyl-aniline were grafted onto the NW walls by thermal hydrosilation in mesitylene (130 °C, 16 h) to impart p and n-type polarization doping, resp. [3, 4]. After removal of the unreacted (physisorbed) organics, thermoelectric characteristics were reassessed, showing in all cases a relevant increase of the electrical conductivity along with an expected decrease of the absolute value of the Seebeck coefficient. As in the pristine NW array, surface polarization leads to an increase of the carrier density (either holes or electrons) in the absence of dopants. As a result, no ionized scattering centers limit the carrier mobility, ultimately leading to enhanced electric conductivities and power factors.
References
[1] T. He, D.A. Corley, M. Lu, N.H. Di Spigna, J. He, D.P. Nackashi, P.D. Franzon, and J.M. Tour, J. Amer. Chem. Soc.. 2009, 131, 10023.
[2] G. Pennelli, M. Totaro, M. Piotto, and P. Bruschi, Nano Lett., 2013, 13, 2592.
[3] A. Taffurelli and D. Narducci, Surf. Sci., 2007, 601 2840.
[4] G.F. Cerofolini, D. Narducci, and E. Romano, ‘Silicon Functionalization for Molecular Electronics&’, in Dekker Encyclopedia of Nanoscience and Nanotechnology, 3^rd edition, CRC Press: New York, 2014, p. 2663.

The electronic and phonon properties are investigated for thermal electric material BiCuSeO. The soft mode in the phonon spectrum is studied, which contributes to the intrinsic low thermal conductivity of this layered material. By searching the zero-soft-mode structure and analyzing the electronic behavior of it, connection between the lattice vibration and the dielectric properties is build, which gives the explanation of the abnormal behavior of the history sensitive electronic conductivity.

The cobaltite Ca₃Co₄O₉ (CCO) with misfit layered structure is a promising p-type thermoelectric material with high Seebeck coefficient, low electrical resistivity and high thermal stability in air. To further improver the energy conversion efficiency of CCO ceramic, the effect of Cu stoichiometric substitution of Co and non-stoichiometric addition Cu addition on the electrical transport properties of CCO ceramics were studied. It has found non-stoichiometric Cu addition and stoichiometric Cu substitution are having different effects on the electrical transport properties. Cu substitution of Ca₃Co_4-xCu_xO₉ at optimum Cu doping level has resulted in a 50% decrease of the electrical resistivity, and no change in the Seebeck coefficient. Over-doping resulted in the increase of the electrical resistivity, and no change in the Seebeck coeficient. Cu addition of Ca₃Co₄Cu_xO₉ at optimum level has resulted in the 50% decrease of the electrical resistivity, and simultaneous increase in the Seebeck coefficient. Over-doping at higher level resulted in the increase of the electrical resistivity. Cu addition of Ca₃Co₄Cu_xO₉ at optimum level is with the enhanced Seebeck coefficient, decreased electrical resistivity, and increased power factor. Furthermore, Cu addition of Ca₃Co₄Cu_xO₉ at optimum levels have resulted in the formation of the nano-lamella secondary phase. Such secondary phase is evidenced by the electron diffraction patterns and the atomic resolution high resolution TEM images. The simultaneous formation of the secondary phase is expected to lower the thermal conductivity dramatically.

Thermoelectric devices convert waste heat into electrical energy and hence can provide an efficient supplemental source of energy. Perovskite materials are promising oxides for thermoelectric application as they can be doped with a wide range of elements resulting in n-type and p-type materials. They are also known to be generally stable at high temperatures and can be synthesized with clean and sustainable metals. The figure of merit (ZT=TσS²/(κ_e+κ_l)) is an elegant way to evaluate the thermoelectric value of materials. ZT is indeed the measure of the thermoelectric efficiency and its optimal value is achieved with high Seebeck coefficient, high electrical conductivity and low thermal conductivity. Optimisation of these parameters can be obtained via a number of methods, most importantly nanostructuring (i.e. introduction of grain boundaries and/or nanopores) and band engineering (i.e. doping).
Here we focus on band engineering using a combination of plane wave pseudopotential and all-electron calculations to study the two promising perovskite oxides CaMnO₃ (CMO) and SrTiO₃ (STO). The transport properties are evaluated using the Boltzmann transport theory. The effect of doping and co-doping and the role of defects on thermoelectric properties of (Nb,Sr) and (Mo,Sr) co-doped CMO and La doped STO will be discussed.

Oxide materials have attracted increased attention in the past decade for thermoelectric applications due to their high chemical and thermal stability. Research of new oxide systems can inform the structural and electronic properties necessary to achieve higher performance. Complex palladium oxides are rare examples of oxides that can be easily hole-doped to undergo metal-insulator transitions. We report favorable thermoelectric performance in complex palladium oxides upon hole doping. The materials were prepared by a conventional solid state method and densified through Spark Plasma Sintering (SPS), which was found to improve the electrical properties compared to samples sintered in a furnace. The oxides display p-type conductivity with a Seebeck coefficient of 100 mu;VK^-1 at room temperature and resistivity of 0.8 Omega;cm. Upon hole-doping with Li, the resistivity decreases by several orders of magnitude, with only a minimal decrease in the Seebeck coefficient. This improvement in electrical conductivity and thermal conductivities around 2 Wm^-1K^-1 at elevated temperatures lead to ZT values of 0.1 at 600 K. Above this temperature, the material begins to decompose in the He under-pressure required for measurements. In air, the compound is stable above 1200 K leading to an expected ZT greater than 0.25.

Boosted by the global energy issues and carbon dioxide emission problems, thermoelectric energy conversion is becoming more and more of vital importance for recuperating decentralized waste heat energy aiming at better energy efficiency. Nevertheless, conventional TE materials such as Bi₂Te₃ and PbTe based on Bi, Te, and Pb are unlikely to satisfy wide commercialization because of the toxicity, poor heat durability, and low abundance of the comprising elements, particularly for those containing Te. In terms of durability at high temperature in air, metal oxides are most attractive, which are highly durable particularly at the temperature range of > 400 °C, where all the non-oxide candidate materials will eventually be oxidized under aerobic conditions. However, the light constituting element, oxygen, in oxides results in a high sound velocity of acoustic phonons carrying heat, leading to the high lattice thermal conductivity of oxide materials.
Recently, however, carefully chosen multinary doping and/or nanostructure control have been revealed to be successful in improving the thermoelectric performance and hence the ZT values of oxides particularly in polycrystalline ceramic samples. Microstructure control is one of the prospective ways to effectively reduce the thermal conductivity with maintaining a sufficiently high electrical conductivity.
In this paper, thermoelectric properties of ZnO-based oxides with multinary doping will be discussed. Upon the binary doping of ZnO with Al and Ga [1], the amount of a Ga-related impurity phase is revealed to show an excellent quantitative agreement with the decrease in the thermal diffusivity. Furthermore, multinary doping with Al, Cu, and Ga results in a substantially low thermal conductivity values just 2-3 times higher than the theoretical minimum of the thermal conductivity [2], showcasing an effective thermal conductivity reduction by a nanocomposite structure in bulk oxides. The multinary doping is revealed to be effective for compensating the carrier electrons depleted by Cu-doping. The phonon scattering will be discussed in terms of the mean free path and microstructure in the oxides.
References
[1] M. Ohtaki, K. Araki, and K. Yamamoto, J. Electron. Mater. 38, 1234 (2009).
[2] D. G. Cahill, S. K. Watson, and R. O. Pohl, Phys. Rev. B 46, 6131 (1992).

High thermoelectric figure of merit (ZT) requires an unusual type of material: a good electrical conductor with high thermopower, but low thermal conductivity. Good thermoelectrics are therefore usually crystalline materials with structural features that scatter phonons (to minimize lattice contribution to thermal conductivity) without impeding electronic transport; this is referred to as a ‘phonon-glass electron-crystal&’. Ca₃Co₄O₉, which consists of alternate stacking of Ca₂CoO₃ layer, acting as blocking layer for phonon, and CoO₂ layer, acting as conducting channel for electrons, along c-axis, can serve the same purpose as phonon-glass electron-crystal. Due to such inherent layered structure the transport properties of Ca₃Co₄O₉ are anisotropic and electrical conductivity in ab-plane is higher than along c-direction. So, for full realization of thermoelectric performance from this class of material system recent effort is on the fabrication of perfectly textured Ca₃Co₄O₉ thin-film. We report a novel two step sputtering/annealing method for the growth of Ca₃Co₄O₉ thin film. First, CaO-CoO thin films were deposited on 0001-sapphire (Al₂O₃) substrates by reactive co-sputtering from Ca and Co targets. Second, the CaO-CoO thin films were annealed under O₂-gas flow in order to form the final phase of Ca₃Co₄O₉. The advantage of the present method is that it allows control over the composition of the film. The thermal-induced phase transformation from CaO-CoO film into Ca₃Co₄O₉ film have been investigated by ex-situ X-ray diffraction and in-situ time resolved synchrotron radiation annealing experiments. The effects of irregular distribution of CaO and CoO nano phases in the as-deposited film, substrate surface roughness, and the off-stoichiometry on film quality have been investigated. The elemental ratio Ca:Co is found to play a role in the phase transformation processes during annealing. Although, heat treatment at temperature 730 °C converts CaO-CoO film with Ca:Co=0.75 into Ca₃Co₄O₉ film, CaO-CoO film with CaO:Co =0.5 is found to result in Ca_xCoO₂ (x ~ 0.5). This indicates that the present method may be applicable for the growth of all other phases of this material system.

We have developed a high-performance thermoelectric generator using nanostructured PbTe bulk thermoelectric materials. In the generator, the sintered samples of PbTe-MgTe doped with Na₂Te and those of PbTe doped with PbI₂ were used for p- and n-type legs, respectively. The samples were prepared by melting stoichiometric amounts of the constituent elements in the evacuated and sealed quartz tubes at 1323 K, followed by pulsed electric current sintering (SPS) at 773 K to obtain the high density compacts. The thermoelectric figure of merit ZT of PbTe-MgTe doped with Na₂Te was sim;1.8 at 810 K and that of PbTe doped with PbI₂ was sim;1.2 at 760 K. The sintered compacts were cut into p- and n-type legs with height of 2.1 mm and cross-sectional area of 1.9 mm by 1.9 mm. The thermoelectric generator including eight-thermocouples (sixteen-legs) were fabricated. The conversion efficiency and electrical power output were evaluated at the cold-side temperature of 303 K and hot-side temperature up to 873 K in vacuum. The highest conversion efficiency and electrical power output were observed at the temperature difference of 570 K.

Recent concerns associated to energy use and storage are calling for more efforts in research for alternative energy sources. In this context, the interest for thermoelectric materials, in which thermal and electrical currents are prone to transfer processes, is constantly raising. Indeed, the discovery of efficient bulk thermoelectrics may lead to promising industrial applications, especially for designing heat-to-electricity devices (e.g. design of better solar panels) and in the automotive industry (e.g. for recycling waste heat into electricity).
In molybdenum cluster chemistry, the Chevrel phases have been extensively studied mainly because of their superconducting properties. Theses phases consist of a stacking of Mo₆Q₈ (Q = S, Se, and Te) units and contain channels where additional metal atoms can be inserted, forming M_xMo₆Q₈ compounds where M can be a variety of atoms. Investigations of their transport properties have shown that they are interesting candidates at high temperature (1000K).¹
Molybdenum clusters with a nuclearity higher than six can also be obtained and derive from the one-dimensional trans-face sharing of Mo₆ octahedra² . Among them the ternary Ag_xMo₉Se₁₁ (with x ~ 3.6 - 3.8) show outstanding low lattice thermal conductivities that give rise to a rather high value of the dimensionless thermoelectric #64257;gure of merit ZT of 0.65 at 800 K for x = 3.8 - 3.9.² Since this family of materials seems to be promising for thermoelectric power generation applications, several additional molybdenum cluster selenides such as Ag₃In₂Mo₁₅Se₁₉ ³ and Ag₂Tl₂Mo₉Se₁₁⁴ have been synthesized and their crystal structures have been studied by single crystal X-ray diffractions techniques. Transport properties were measured to assess the thermoelectric potential of these compounds. For a better understanding of their structural and electronic properties, band structure calculations were carried out within the framework of density functional theory. Electronic transport properties have been computed as well using the semiclassical Boltzmann equations.
I clear that the reduced molybdenum chalcogenides are promising candidates as high-ZT thermoelectric materilas in the temparatre range of 600-1200 K, because of their complexe structure, reasonably mu; and low κ_L. These results demonstrated by the Mo-cluster chalcogenides make theim a successful thermoelectric materials fo industrial applications.
References
¹T. Caillat, J.-P. Fleurial and G. J. Snyder, Solid State Sci. 1, 535 (1999).
²T. Zhou, B. Lenoir, M. Colin, A. Dauscher, R. Al Rahal Al Orabi, P. Gougeon, M. Potel, E. Guilmeau, App. Phys. Lett. 98, 162106 (2011).
³P. Gougeon, P. Gall, R. Al Rahal Al Orabi, B. Fontaine, R. Gautier, M. Potel, T. Zhou, B. Lenoir, M. Colin, C. Candolfi, A. Dauscher, Chem Mater. 24, 2899 (2012).
⁴ R. Al Rahal Al Orabi, P. Gougeon, P. Gall, B. Fontaine, R. Gautier, M. Potel, B. Lenoir, M. Colin, C. Candolfi, A. Dauscher, submitted.

Thermoelectric materials enable a two-way conversion between thermal energy and electrical energy, and hence provide routes for both power generation and thermoelectric cooling. Current successful thermoelectric modules are mainly based on Bi2Te3 and PbTe, and but there is a high interest in finding candidate materials that avoid or reduce the usage of the toxic Pb as well as the expensive Te. Very recently, a thermoelectric figure of merit ZT as high as 2.6 has been reported for SnSe single crystals, which was attributed to its ultralow thermal conductivity.[1] SnS and SnSe assume an orthorhombic structure which can be described as a distorted rock-salt structure, whereas PbTe has the rocksalt structure as the ground-state. Compared with the orthorhombic structure, the high-symmetry rocksalt structure potentially allows for a larger number of energy pockets near the band edge, which could enhance the thermoelectric transport properties. In order to investigate the dependence of the thermoelectric properties on the lattice distortion, we are considering the alloying between the orthorhombic SnS and SnSe with rocksalt chalcogenides such as SnTe, SrS, and SrSe. Using alloy theory, bulk and thin-film synthesis, we have investigated the phase diagram of the mixed-cation alloys (Sn,Sr)S and (Sn,Sr)Se, as wells as that of the mixed-anion-alloys Sn(S,Te) and Sn(Se,Te). The fact that the end compounds have different crystal structures is found to have a profound impact on the composition range where the alloys are metastable against phase separation. Further investigations of the thermoelectric properties for these alloys are ongoing.
[1] L.-D. Zhao, S.-H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid, and M. G. Kanatzidis, Nature 508, 373 (2014).

PbTe nanostructuring thermoelectric materials are attractive options to enhance the thermoelectric figure-of-merit ZT. In the PbTe system, SrTe nanostructure precipitates is added in order to decrease the thermal conductivity by enhancing phonon scattering at the interface between the SrTe precipitates, and the host matrix PbTe. However, it seems that the electron mobility is not affected by this high number density of interfaces. Using first-principles calculations, we studied the effect of the different interfaces between the SrTe precipitates and the PbTe matrix on the band alignments, showing that the low valence band offset (VBO) of the system has a minor impact on the mobility of the carriers. In addition, we found that the VBO and the interfaces energies are correlated. The highest interface energy and the highest the VBO occur in the same direction.

SnSe is reported to have a high zT under high temperature.¹ The performance of its low-temperature crystallographic phase can be improved because undoped SnSe in that phase has a high electrical resistivity. Alloying with Te reduces the band gap and increases the carrier concentration, presumably via the holes introduced by SnTe. This improves electrical conductivity. We investigate the effects of Te alloying of SnSe in the Se-rich phase. Samples are prepared using traditional melting-grinding-SPS methods. We observe decreased resistivity by a factor of three, decrease of the Seebeck coefficient, and doubling of the power factor at room temperature. We also observe a turnover of the Seebeck coefficient and resistivity near 600K. We obtain low temperature data in a cryostat and conduct thermal conductivity measurements to report zT. Results are analyzed in comparison with theoretical band structure calculations.
Work supported by the US DOE “U.S.-China Clean Energy Research Center-Clean Vehicles (CERC-CV) - Energy conversion” program and the AFOSR MURI “Cryogenic Peltier Cooling”, FA9550-10-1-0533.
1. Lidong Zhao et al., Nature 408 (2014)

In thermoelectric study, nanostructuring has recently drawn much attention because of the enhancement of the dimensionless figure of merit in thermoelectric conversion. We proposed a nanostructure for thermoelectric application: stacking structure of epitaxial nanodots (NDs) with ultrasmall size and high density. Our idea is that the epitaxial growth can keep electric conductivity and ultrasmall NDs can work as phonon scatters. In our previous report [1], we formed epitaxial Si NDs which were stacked on Si substrates and demonstrated the reduction of thermal conductivity. In this paper, to develop Si-based thermoelectric nanomaterials with high power factor, we focus on the NDs of the iron silicide that has relatively high Seebeck coefficient and the Si layer that has relatively high carrier mobility. We develop the epitaxial growth technique of the stacking structures of iron silicide NDs/Si on Si substrates by applying the ultrathin SiO₂ film technique [2]. We also measure their thermoelectric properties
Clean Si surfaces were prepared by Si buffer layers on Si (111) substrate at 550 °C after degassing for 550 °C for several hours. These clean Si surfaces were oxidized in the chamber at 600°C for 10 min at 2×10^-4 Pa to form ultrathin SiO₂ films with ~0.3 nm thickness. First, 1 monolayer (1ML) Si was deposited onto the ultrathin SiO₂ films at 450°C leading to the formation of the ultrahigh density nanowindows in the ultrathin SiO₂ films. Second, we codeposited Fe and Si onto the ultrathin SiO₂ films at 450°C to form epitaxial β-FeSi₂ NDs on the nanowindows. Finally, we deposited Si onto β-FeSi₂ NDs at 400°C and then oxidized at 450°C for 10 min at 2×10^-4 Pa. These processes were repeated to fabricate stacking structures of epitaxial β-FeSi₂ NDs/Si. Thermoelectric properties were measured after boron doping by implantation (<1×10¹⁵ cm^-2).
Reflection high energy electron diffraction patterns exhibited epitaxial growth of β-FeSi₂ NDs and Si layers through all growth stages. Transmission electron microscope observation revealed that the stacking structure of β-FeSi₂ NDs with Si layers. For the samples with 180-ML Si layer, electric conductivity increased with carrier density which was similar tendency to that of a bulk Si. This could be attributed to the existence of Si layer. In the case of the stacking structure of 18-ML β-FeSi₂ NDs/180-ML Si layer, Seebeck coefficient was measured to be ~400-500 mVK^-1 near the hole density of ~ 10¹⁹ cm^-3. This value was similar to that of the bulk β-FeSi₂ near the hole density of 10¹⁹ cm^-3, which can be attributed to the existence of β-FeSi₂ NDs. These results demonstrated that further thermoelectric property enhancement could be expected by controlling NDs and Si layer.
This work was supported by PRESTO-JST program.
[1] Y. Nakamura, M. Isogawa, T. Ueda, J. Kikkawa, A. Sakai, 2012 MRS Fall meeting
[2] Y. Nakamura, Y. Nagadomi, S.-Pyo Cho, N. Tanaka, and M. Ichikawa, J. Appl. Phys. 100, 044313 (2006).