Symposium FD
Advances in Materials and Technologies for Efficient Direct Thermal-to-Electrical Energy Conversion

Session FD-1 - Theoretical Concepts and Basic Approaches for High Efficiency Thermal-to-electrical Energy Conversion

FD-1:IL01  A Search for Nonequilibrium Thermoelectrics 
I. TERASAKI, Department of Physics, Nagoya University, Nagoya, Japan

In spite of recent development, the energy conversion efficiency is still poor in thermoelectric materials.
Themoelectric materials are difficult to make, because resistivity, Seebeck coefficient, thermal conductivity should be optimized at the same time. Based on conventional semiconductor theories, we do not find any reliable prediction for ZT>4.
Thus we will try to find a new direction from the conventional approach. In this talk, we show two examples in which non-equilibrium states enhance ZT from thermal equilibrium states. One example is the photo-doping in wide-gap semiconductors, and the second example is the enhancement of the Seebeck coefficient in high electrical currents. We find the photo-Seebeck effect in ZnO, and also find that the carrier doping of the order of 10^19 cm^-3 is possible. We further find that the Mott insulator Ca2RuO4 shows largely enhanced Seebeck coefficient in a large current density of 10 A/cm2.
For details, see J. Phys. Soc. Jpn. 81 (2012) 114722, and 82 (2013) 103702.


FD-1:IL02  Improved Thermoelectric Efficiency by Introducing New Doping Schemes
M. ZEBARJADI, K. ESFARJANI, Rutgers University, Piscataway, NJ, USA: BOLIN LIAO, GANG CHEN, Massachusetts Institute of Technology, USA

It is known that the thermoelectric power factor has an optimum versus doping concentration. Increasing the doping level increases the level of conduction carriers. At the same time, background ionized dopants scatter conduction carriers and limit their mobility. In this talk we discuss possible strategies to minimize the deleterious effect of convectional doping. By rearranging the doping centers in cluster forms and away from the transport channel, one can remotely dope the transport channel to increase the carrier mobility. Furthermore it is possible to coat the doping centers by a cloaking layer to hide the scattering regions from conduction carriers to further minimize the scattering center and increase the carrier mobility.


FD-1:IL03  Computational Materials Design of Earth-abundant Thermoelectrics
V. OZOLINS, FEI ZHOU, YI XIA, W. NIELSON, Department of Materials Science and Engineering, University of California, Los Angeles, CA, USA; M.D. NIELSEN, J.P. HEREMANS, Department of Mechanical and Aerospace Engineering, The Ohio State University, Columbus, OH, USA; XU LU, D.T. MORELLI, Department of Physics and Astronomy Michigan State University, East Lansing, MI, USA

Many known good thermoelectric materials are comprised of elements that are in low abundance and require complex doping and synthesis procedures. Thermoelectrics comprised of earth-abundant elements would pave the way to many new, low cost energy generation opportunities. We have used first-principles calculations to identify cubic Cu12Sb4S13 as a promising thermoelectric; subsequent experiments showed dimensionless thermoelectric figure of merit near unity in Cu12-xMxSb4S13, where M is Zn or Fe, for wide ranges of x. These compounds span the range of compositions of the natural mineral family of tetrahedrites, the most widespread sulfosalts on Earth. In related work, we have predicted ultra-low thermal conductivity in new rocksalt-based I-V-VI2 semiconductors, where the group I elements are Cu, Ag, Au, or alkali metals, the group V elements are P, As, or Bi, and the group VI elements are S, Se, or Te. Many of these materials are found to exhibit soft phonon modes and large Grüneisen parameters due to the strong hybridization and repulsion between the lone-pair electrons of the group V cations and the valence p orbitals of the group VI anions. It is shown experimentally that in many of these cases Umklapp scattering reduces lattice thermal conductivity to the amorphous limit.


FD-1:IL04  Utilizing NMR and Advanced Carrier Analysis for Development of Thermoelectric Materials
E.M. LEVIN, U.S. DOE Ames Laboratory & Department of Physics and Astronomy, Iowa State University, Ames, IA, USA

Nuclear magnetic resonance (NMR) is an excellent probe to better understand the chemistry and physics of complex thermoelectric materials by studying the effects of localized and mobile electrons on nuclear spins. Two groups of tellurides, PbTe and GeTe alloyed with various elements (Ag, Sb, Bi, Dy, and other), have been studied by 125Te NMR along with XRD, SEM, EDS, Seebeck effect, electrical resistivity, and thermal conductivity. 125Te NMR can be used to detect various phases in complex tellurides and to obtain mobile charge carrier concentration, one of most important parameters of thermoelectric materials. By using 125Te NMR spin-lattice relaxation time, T1, and Seebeck and Hall effects data for reference materials, the 1/T1 vs. carrier concentration relation has been established for n- and p-type tellurides in the range of concentrations from ~10^17 to 10^21 cm^-3. 125Te NMR spin-lattice relaxation measurements allow us to observe electronic inhomogeneity and obtain fractional carrier concentrations in single- and multi-phase tellurides. Electronic inhomogeneity can be detected by NMR even if tellurides are single-phase according to XRD. Data obtained by NMR along with other measurements are used for development of more efficient thermoelectric materials.


FD-1:IL05  Ab-initio Calculations of the Lattice Thermal Conductivity
L. CHAPUT,  Institut Jean Lamour, UMR CNRS 7198, Université de Lorraine, Vandoeuvre Les Nancy Cedex, France

In this work we present ab-initio calculations of the phonon-phonon interactions and lattice thermal conductivity and for several semiconductors of interest in energy transport and thermoelectricity. Excellent agreements with experiments are found.
To obtain these results a new method is proposed to obtain a numerically exact and fast solution to the Boltzmann-Peierls equation [1]. This is made possible using at best the symmetry of the systems. At first the collision kernel of the equation is constructed using an efficient parallelization of the code over the irreducible triplets of phonon wavevectors [2] involved in the different possible collisions. These irreducible triplets are in fact the equivalent of the irreducible part of the Brillouin zone used for single particle quantities like the density of states. Therefore a formulation of the self energy and collision kernel based on their use drastically reduce the computational time.
In a second step the ther mal conductivity is calculated from a spectral representation that allow obtaining the dynamical thermal conductivity as well [1]. Several examples, including Mg2Si and half heusler will be discussed.
[1] L. Chaput, Phys. Rev. Lett.. 110, 265506
[2] L. Chaput, A. Togo, I. Tanaka and G. Hug, Phys. Rev. B 84, 094302


FD-1:L06  Effects of Vanadium Substitution on the Crystal Structure and Thermoelectric Properties of Higher Manganese Silicide
Y. KIKUCHI, T. NAKAJO, K. HAYASHI, Y. MIYAZAKI, Department of Applied Physics, Tohoku University, Sendai, Japan; K. YUBUTA, Institute of Material Research, Tohoku University, Sendai, Japan

A partial substitution of Cr for Mn in the Higher Manganese Silicide (HMS) phases is known to increase ZT values because of the decrease in Valence Electron Counts (VEC).[1] Based on the 14-electron rule, V should also be effective to further increase ZT of HMS phases. Therefore, we prepared V-substituted HMS phases and measured their thermoelectric properties.
Polycrystalline samples were prepared by the arc-melting of metallic elements in an Ar atmosphere. One half of obtained button samples were annealed at 1273 K for 1 week in a vacuum encapsulated quartz tube and other half of samples were sintered using a spark plasma sintering machine at 1200 K in vacuum.
Solubility limit of V for Mn in the HMS phases was determined to be less than 10% from the results of X-ray diffraction patterns and refined lattice parameters. With increasing x, Seebeck coefficient and electrical resistivity of the samples gradually decreased owing to the decrease in the estimated VEC of the samples. As a result, thermoelectric power factor of the x = 0.06 sample exhibited 1.6 mW/K2m at 800 K (i.e. 1.3 times higher than that of the V-free sample).
[1] Y. Kikuchi, et al., Jpn. J. Appl. Phys., 51 (2012) 085801.


Session FD-2 - Novel Materials for High Efficiency Thermal-to-electrical Energy Conversion

FD-2:IL01  Thermoelectrics Research: New Materials-related Approaches, Theoretical Guidance and Device Development
G.S. NOLAS, Department of Physics, University of South Florida, Tampa, FL, USA

The concept of incorporating nano-scale effects for enhancement of the thermoelectric (TE) properties of bulk materials is a relatively recent focus in the TE community. Polycrystalline TE materials with nanoscale domains, nanostructured TE materials or nanocomposites, have been theoretically predicted and experimentally shown to enhance TE properties of specific, but not all, materials. Ball milling, melt spinning or other "top-down" approaches are typically employed while the "bottom-up" approach appears to be gaining interest due to several advantages over that of other methods. Research on nanocomposite materials, however, has mainly focused on existing TE materials with good bulk properties. New material developments, specifically the incorporation of novel ideas from other fields with materials properties requirements that, to some extent, overlap with the requirements for superior TE properties, have not been widely employed. Theoretical modeling and device design strategies must also be part of this effort. Our recent developments in these areas are presented. These efforts were initiated in the context of research into cost-effective, scalable, and reproducible TE materials research and processing. No less important is the development of a fundamental understanding of nano-scale effects as well as new materials research strategies that result in enhanced TE properties. This approach also has the potential to extend and expand the scope of materials research in general.


FD-2:IL03  Borides as Thermoelectrics
TAKAO MORI, National Institute for Materials Science (NIMS) & University of Tsukuba, Japan

Efforts are intensifying to find viable TE materials [1]. One need exists for high temperature (T), possibly even ultra-high T for topping cycles. Covalent compounds like borides, silicides, are promising and some structures engender low thermal conductivity [2].
B6S1-x may be a lower processing T replacement to "B4C", one of the few commercialized TE materials. Through Tr metal doping into voids, Seebeck coefficients α of YB22C2N, the n-type counterpart, could be increased by 220% while resistivity was reduced by x100. Recently, excellent (|α|>200 μV/K) p-type or n-type was achieved in YAlB14 by varying occupancy of Al sites, i.e., p, n control with matching structure and no foreign doping (i.e. no migration problems). Theory reveals the stable configuration and DOS variation behind TE properties.
Borides may not just be limited to high T application since ZT~3 at RT was reported for "B4C" QW structures. I will present our investigations into this, utilizing a state-of-the-art high T MBE apparatus.
[1] Thermoelectric Nanomaterials, ed. K. Koumoto and T. Mori, (Springer, Heidelberg, 2013).
[2] Modules, Systems, and Applications in Thermoelectrics, ed. D. M. Rowe, (CRC Press, London) 14 (2012), J. Appl. Phys. 102, 073510 (2007).
[3] Appl. Phys. Lett. 101, 152101 (2012).


FD-2:IL05  High Performance Mg2(Si,Sn) Based Thermoelectric Materials: Synthesis, Structure and Transport
TIEJUN ZHU, X.B. ZHAO, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, P.R. China

Mg2(Si,Sn) based solid solutions are cost-effective and eco-friendly candidates for future large-scale commercial application in mid-temperature thermoelectric (TE) power generation. We develop a facile B2O3 flux method to synthesize Sb doped Mg2(Si,Sn) solid solutions with high quality. By using B2O3 as liquid-seal, the sublimation and oxidation of Mg can be significantly inhibited. The Mg interstitials provide an extra tuning parameter in optimizing the thermoelectric properties of Mg2(Si,Sn)-based materials. While adding extra Mg is intended to compensate for the Mg loss during the synthesis, it often leads to Mg interstitials in Mg2(Si,Sn) materials and profoundly affects their thermoelectric properties. Furthermore, usually the high thermoelectric performance of Mg2(Si,Sn) solid solutions with ZT ~ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. By characterizing and modeling of thermoelectric transport properties, we propose that the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.


FD-2:IL06  Development, Synthesis and Characterisation of Advanced Thermoelectric Materials for High Temperature Energy Conversion
A. WEIDENKAFF, WENJIE XIE, XINGXING XAO, FAN FU, Materials Chemistry, Institute for Materials Science, University of Stuttgart, Stuttgart; Germany

Complex Oxides and Heusler based compounds are promising candidates for high temperature conversion of heat [1]. The broad application of thermoelectric converters in future energy technologies requires the development of thermoelectric active, stable, low cost and sustainable materials.
Suitable candidates are being selected from the synthesized products according to their temperature dependent ZT values, and compatibility factors to produce performing thermoelectric converters delivering a high energy density.
Theses converters are tested under ambient air at temperatures of T > 900 °C and applied in an exhaust gas stream of a custom made hybrid vehicle, concentrated solar thermal converters, metal casting furnaces and solid oxide fuel cells.
References
1. Koumoto, K.; Funahashi, R.; Guilmeau, E.; Miyazaki, Y.; Weidenkaff, A.; Wang, Y.; Wan, C. Journal of the American Ceramic Society 96 (2013) 1
2. Sagarna, L., Shkabko, A., Populoh, S., Karvonen, L., Weidenkaff, A., Electronic Structure and Thermoelectric Properties of Nanostructured EuTi1-xNbxO3-δ (x = 0.00; 0.02), Appl. Phys. Lett. 101, (2012) 033908.


FD-2:IL07  Abnormal Thermoelectric Properties in Copper Chalcogenides
XUN SHI, WENQING ZHANG, LIDONG CHEN, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, China

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat. The key to the development of advanced TE technologies is to find highly efficient TE materials. In current commercial materials, the zTs are limited to values around unity. Recently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials. In this presentation, the thermoelectric properties of bulk copper chalcogenides (Cu2-δX) are reported in a wide temperature range, where X could be S, Se, or Te. We will show these materials possess extremely abnormal thermoelectric properties with very low thermal conductivity and good thermoelectric figure of merit. In particularly Cu2-δSe is taken as an example to demonstrate the abnormal properties of the low temperature phase, the high temperature phase, and the transition states during the phase transitions. The mechanisms behind these abnormal thermoelectric properties will also be discussed.


FD-2:IL08  Stability, Structure and Properties of Sb2Te3-Bi2Te3 Intergrowths and Superlattices
M. WINKLER¹, J. KÖNIG¹, A.L. HANSEN², T. DANKWORT³, J.D. KÖNIG¹, H. BÖTTNER¹, K. BARTHOLOMɹ, W. BENSCH², L. KIENLE³, ¹Fraunhofer Institute for Physical Measurement Techniques IPM, Thermoelectric Systems, Freiburg, Germany; ²Institute of Inorganic Chemistry, Christian-Albrechts University Kiel, Kiel, Germany; ³Institute for Materials Science, Synthesis and Real Structure, Christian-Albrechts University Kiel, Kiel, Germany

In 2001, outstanding results (ZT = 2.4) were reported for Bi2Te3/Sb2Te3 MOCVD-grown thin film superlattice (SL) structures. In order to investigate the underlying mechanisms, synthesis of such SLs was carried out by 1.) Nanoalloying, i.e. the deposition of element films and subsequent annealing to induce compound formation and 2.) Epitaxial deposition by MBE.
We present a comprehensive study for different growth conditions comprising transport property and structural characterization by XRD, SIMS, SEM and (S)TEM. Special attention was paid to the role of interdiffusion for the stability of the SL structures. The nanoalloyed films are easy to preparate on different substrates and display a high crystal quality, high power factors of up to 40-50 µW/cmK² and low thermal conductivities. The epitaxially deposited films display a structural stability down to period lengths as small as 6 nm and low lattice thermal conductivities down to ~ 0.3 W/mK, being the first reported reproduction of such low-periodic sharp-interfaced SLs since 2001. Electrical properties suffer from carrier compensation effects but can be significantly improved by annealing.
Additionally, recent theoretical calculations concerning the electrical properties of such SLs will be presented.


FD-2:IL09  High Performance Thermoelectric Materials and their Applications in Energy Conversion
ZHIFENG REN, University of Houston, Houston, TX, USA

Thermoelectric materials for energy conversion are more and more promising due to the recent breakthroughs in enhancing the dimensionless thermoelectric figure-of-merit (ZT) by nanostructuring approach. I will first introduce the nanostructure approach to enhance the ZT, then present a few cases to show the generality of the nanostructure approach, finally focus on the development of half-Heuslers and waste heat recovery using these ZT enhanced materials. At the end if time permits, I will show our very recent success on ZT enhancement on a couple of new materials. The main scheme is to enhance the ZT in these materials systems by studying the compositions and creating nanostructures to reduce the thermal conductivity and simultaneously increase the power factor.


FD-2:IL10  Synthesis of Silicides Using a Na flux and their Thermoelectric Properties
T. YAMADA, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Japan

Powder and bulk of thermoelectric transition metal silicides such as CrSi2, MnSi1.7+d, beta-FeSi2, and CoSi were prepared by heating mixtures of Si and metal powders or Si powders and compact body of metal powders with Na at 823-1173 K. The existence of Na-Si melt above ca. 723 K was revealed by the study of Na-Si phase diagram. It indicates that the metal silicides are prepared by a reaction of Si (liquid) in the melt with the metals (solid). The maximum ZT values of the samples were similar to or modestly smaller than those reported for the samples prepared by conventional methods.
Powders of a high-melting point metal silicide alpha-MoSi2 and a metastable phase beta-MoSi2 were also prepared at 1073 and 858 K, respectively, using Na. Details of the crystal structure of beta-MoSi2 (C40 type structure) was determined by Rietveld analysis of the powder X-ray powder diffraction. The electrical resistivity of the sample with a relative density of 65% of the theoretical one, sintered at 723 K and 600 MPa, was 2.5 mΩ cm at 300 K, and slightly increased with increasing temperature up to 725 K. The Seebeck coefficients changed from +60 to +89 µV/K in the temperature range from 330 to 725 K. The maximum thermoelectric power factor was 2.2 × 10-4 W m-1 K-2 at 725 K.


FD-2:IL13  Unconventional Superlattice-structured Bulk Thermoelectric Materials
K. KOUMOTO, C.L. WAN, F. DANG, K. TSURUTA, Nagoya University, Japan; Y.F. WANG, Nanjing University of Technology, China; R.Z. ZHANG, Northwest University, China; W.S. SEO, KICET, Korea

Novel high-efficiency TE materials composed of naturally-abundant non-toxic elements usable in air atmosphere need to be exploited in order to harvest electricity from solar energy as well as waste heat coming out of industries, vehicles, and public welfare, and thus contributing to reducing carbon dioxide emission and becoming free from nuclear power.
We are currently investigating unconventional TE materials from the above-mentioned point of view; 3D superlattice ceramics of strontium titanate (STO) and TiS2-based inorganic/organic hybrid superlattices for low to mid-temperature materials. Power factor of 3D superlattice ceramics of STO reaches ~6 x 10-3 W/mK2 at 293-353 K which is far larger than those reported for current nanostructured TE materials. Such high performance must be based on quantum confinement of charge carriers at grain boundaries. TiS2-based inorganic/organic 2D superlattices show rather high ZT ranging from 0.21@300 K to 0.28@375 K in air, which is also derived from quantum confinement of carriers in TiS2 layers as well as low thermal conductivity of organic layers.


FD-2:L14  Development of High ZT Skutterudites for Thermoelectric Applications
G. ROGL^{1, 2}, A. GRYTSIV¹, E. BAUER¹, M. ZEHETBAUER², P. ROGL¹, ¹Christian Doppler Laboratory for Thermoelectrics, University and University of Technology, Vienna, Austria; ²Research Group Physics of Nanostructured Materials, University of Vienna, Austria

Skutterudites are among the best candidates for thermoelectric (TE) applications, particularly in automotives, because they cover the temperature range needed for TE devices, the starting material is cheap and abundant and they can be produced easily and fast. In this paper the development of the design and preparation of p-type skutterudites with ZT = 1.2 and n-type skutterudites with ZT = 1.6 will be discussed. Furthermore the enhancement of these values to 1.6 and 1.9 respectively after severe plastic deformation via high-pressure torsion will be shown. In addition the presentation will summarize thermal expansion and mechanical properties of this class of compounds.


FD-2:L15  Lithium as a Dopant for p-type Mg2Si
P. NIERODA¹, A. KOLEZYNSKI², M. SITARZ², K.T. WOJCIECHOWSKI¹, ^1,2AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Krakow, Poland, ¹Department of Inorganic Chemistry, Thermoelectric Research Laboratory, ²Department of Silicate Chemistry and Macromolecular Compounds

The aim of this study was to analyze structural and transport properties of Li doped Mg2Si as a new p-type thermoelectric material.
The theoretical studies of electronic structure (FP-LAPW method), electron density topology and bonding (QTAIM analysis) were used for the analysis of the influence of Li impurity on electronic properties of Mg2Si. It was shown that for two cases i.e.: Li substituting Si (4a) or Li located in interstitial region (4b), the doping leads to n-type conduction. However, if Li is located in Mg position (8c), the materials should exhibit p-type properties.
A series of samples with the nominal compositions of Mg2-xLixSi, (x=0÷0.5) were prepared using the PECS method. The structure and microstructure of materials were examined with the use of X-ray diffraction and SEM microscopy. IR and Raman spectroscopy studies were carried out for the analysis of the influence of Li dopant on vibrational spectra of Mg2Si. Experimental results were compared with corresponding ab initio calculations performed by means of Crystal09 program.
The experimental data of electrical conductivity, Seebeck coefficient and thermal conductivity in the range from 300 to 720 K, were used to determine the temperature dependences of the thermoelectric figure of merit ZT of a new material.


FD-2:L16  Nb-doped SrTiO3 Thermoelectric Nanocomposite
K. VANDAELE, P. VAN DER VOORT, K. DE BUYSSER, Department of Inorganic and Physical Chemistry, Ghent University, Gent, Belgium

Although very high ZT values have been reported for low dimensional thermoelectric structures, the advances achieved in nanostructured thermoelectrics are not easily transferrable towards industrially useful materials.
In this research we synthesized a novel nanocomposite that consists of low dimensional structures of thermoelectric SrTiO3 imbedded in a mesoporous silica template. The nanocomposite powder can be compressed into pellets and subsequently processed and used as bulk material.
The influence of the precursor solution characteristics, pH and solvent nature on the impregnation efficiency and the quality of the final nanocomposite were investigated. An optimized synthesis route was based on the impregnation of a precursor solution containing SrCl2, NbCl5 and TiCl4 into SBA-16 mesoporous silica as template material. Phase pure crystalline Nb-doped SrTiO3 could be obtained after a thermal treatment at 850C in reducing atmosphere while the silica template remained amorphous. The pore filling behaviour of the precursor into the silica template and the crystallinity were studied by means of N2 sorption measurements, XRF, TEM and XRD. In addition, the influence of the pore structure and pore diameter on the thermoelectric properties were investigated.


FD-2:IL17  Tuning Thermoelectric Properties of TiS2 Compounds through Intercalation, Mass Fluctuation, and Non-stochiometry
E. GUILMEAU¹, M. BEAUMALE¹, T. BARBIER¹, O. LEBEDEV¹, Y. BRÉARD¹, S. HÉBERT¹, Y. KINEMUCHI², A. MAIGNAN¹, ¹CRISMAT Lab., UMR 6508 CNRS-ENSICAEN, Caen, France; ²National Institute of Advanced Industrial Science and Technology (AIST), AIST Chubu, Nagoya, Japan

TiS2 is a one of the layered transition metal dichalcogenides (TMDC's) TX2 (T is a transition metal atom from the group IVb, Vb or VIb columns of the periodic table, X = S, Se, or Te) which have been fascinating compounds due to the rich variety of the physical properties. Among the large spectrum of materials in TX2 family, TiS2 is usually described as a semiconductor or semimetal. TiS2 has an anisotropic structure with a trigonal crystal symmetry related to the layered CdI2 structure type, a ubiquitous prototype for TX2 stochiometries. Due to its layered structure, TiS2 offers the enormous capacity for chemical substitution and intercalation to provide a mechanism for optimizing the thermoelectric response through tuning of the transport properties and in the same time decreasing of the lattice thermal conductivity.
For that purpose, we have recently investigated the effect of i) intercalation, ii) substitution and iii) non-stochiometry on the electrical and thermal properties of TiS2 based compounds. We report here on the structural, microstructural and thermoelectric properties in (Cu,Ag)xTiS2, Ti1-x(Nb,Ta)xS2 and Ti1+xS2 series.


FD-2:IL18  Phase Separation as the Key to Thermoelectric Highly Efficient Heusler Compounds
B. BALKE, Johannes Gutenberg University, Mainz, Germany

The Half-Heusler compounds are one of the most promising candidates among the high-temperature thermoelectric materials being investigated for automotive and industrial waste heat recovery applications. For n- as well as p-type materials ZT values with peak values larger than one have been published recently and first modules have been built.
In this talk, I will give an overview about our recent investigations about phase separation in the thermoelectric Heusler compounds. I will present our studies on the phase separations in the quasi-ternary system TiNiSn-ZrNiSn-HfNiSn. Studying patents and publications the last two years carefully one could read a lot about not-single phase samples inside of the TiNiSn-ZrNiSn-HfNiSn system. Furthermore, we will show how we adapted this knowledge to design a p-type Heusler compound which led to a ZT increase of 50% compared to the best published bulk p-type Heusler compound in the literature.
This results strongly underline the importance of phase separations as an important tool in the design of highly thermoelectric efficient materials which fulfill the industrial demands for a thermoelectric converter.


FD-2:L19  Fine Bi Wires Prepared by the Glass Coated Melt Spinning Method
H. KOHRI, T. YAGASAKI, Kogakuin University, Hachioji, Tokyo, Japan

Thermoelectric generator is expected as an energy converter for co-generation with waste heat and so on. Thermoelectric materials are required with high Seebeck coefficient, low electrical resistivity and low thermal conductivity. Authors focus attention on Bi nanowire. It is expected that pseudo one dimensional structure of nanowire to show high Seebeck coefficient due to the quantum confinement of electron in Bi. A glass coated melt spinning process was attempted to fabricate Bi nanowire. The Bi was encapsulated with Ar of 33 kPa in the soda glass tube. The tube was set in a BN die. The tube filled with melted Bi was drawn through a die at a constant speed of 150 mm/h by using a low speed motor so that the melted Bi became a fine wire with thinning of the glass tube. The temperature of the tube was kept constant at 913 K during drawing. N2 gas was supplied to cool the thinned glass tube at the exit of the die. The glass coated fine Bi wire obtained by the above-mentioned process was encapsulated with an empty glass tube in another glass tube. The tube was drawn at same condition. These processes were repeated several times. Bi wires of 180 nm in diameter were obtained. The Seebeck coefficient of Bi wire obtained at four times increased than the one of the single crystal of Bi.


FD-2:L20  Temperature Influence on the Purity, Crystallographic Structure and Thermoelectric Properties of Cu12Sb4S13 Substituted Tetrahedrite Compounds
P. LEMOINE, T. BARBIER, S. GASCOIN, O. LEBEDEV, E. GUILMEAU, CRISMAT Laboratory, ENSICAEN-CNRS UMR 6508, Caen Cedex, France; A. KALTZOGLOU, A. POWELL, University of Reading, Reading, UK

Tetrahedrites are members of the sulfosalts minerals with general formula A6B6-xCxSb4S13 (x ≤ 2), where A is Cu in triangular coordination, B is Cu in tetrahedral coordination, and C is a divalent metal in the same tetrahedral coordination. These compounds were recently reported as promising thermoelectric materials due to their complex cubic structure providing low thermal conductivity. Due to the absence of reported data on the phase stability against temperature, we have studied the influence of the temperature on purity, structure and TE properties of some tetrahedrite phases. High temperature powder X-ray diffraction measurements (up to 830 K) evidence that single phase tetrahedrite samples can be obtained only in a narrow temperature interval. For T ≥ 800 K, compounds decompose irreversibly, leading to a modification of TE properties. The optimization of synthesis and sintering parameters allows to improve the TE properties with a maximum ZT value of 0.75 at 700K obtained for the Cu10.6Ni1.4Sb4S13 composition.


FD-2:L21  Targeted Use of SPS Method for Improvement of Thermoelectrics
L.P. BULAT¹, I.A. NEFEDOVA¹, D.A. PSHENAY-SEVERIN², ¹ITMO University, St. Petersburg, Russia; ²Ioffe Physical Technical Institute, St. Petersburg, Russia

Use of spark plasma sintering (SPS) allows improving thermoelectric figure of merit Z of bulk nanothermoelectrics but required parameters of SPS process for achievement of best Z can be defined only empirically.
In the present study the finite elements method for investigation of electric and thermal processes which occur in volume and on boundaries of sintering particles is applied. As a geometrical model a structural cell of a sintered sample, containing contact "a truncated cone - a plate" has been chosen. Temperature distributions in the volume of a sample depending on amplitude, on-off time ratio and duration of impact of the electric current has been obtained for solid solution based on bismuth telluride using the energy balance equation and the equation of electric current continuity. Under certain conditions nonlinear and nonlocal processes start to arise.
The calculated temperature distributions at different sintering conditions were compared with empirically defined experimental parameters that lead to improved value of Z. The comparison allows formulating recommendations to achieve best conditions of SPS process for increase of Z.
The present method can be used for management of SPS fabrication process for different application, not only for thermoelectrics.


FD-2:IL22  New Routes to High Performance, Low Cost Thermoelectrics Based on Natural Minerals
D.T. MORELLI, Michigan State University, East Lansing, MI, USA

Concerns over element toxicity and the low abundance of some elemental components such as tellurium and cobalt have compelled us to study earth-abundant materials for thermoelectricity. Guided by density functional theory calculations of lattice dynamics and electronic structure calculations, we have identified the tetrahedrite family of minerals as potential high performance thermoelectric materials. Importantly, the tetrahedrites comprise the most widespread sulfosalts on Earth. An unusual band structure gives rise to large Seebeck coefficient over a wide range of composition, spanning those compositions of tetrahedrite that occur in nature. Meanwhile, large anharmonicity drives the lattice thermal conductivity down to near minimum values. As a result, tetrahedrites can possess dimensionless figures of merit exceeding unity at 400C, comparable or even exceeding that of some of the best synthetic thermoelectric materials in this temperature range. We also show that samples synthesized using natural mineral tetrahedrite as a source material possess similar values of figure of merit. The results suggest a new paradigm for thermoelectric material development - the use of natural minerals directly as sources for highly efficient thermoelectric materials.


FD-2:L23  Enhanced Phonon Scattering in "Rattling" Oxides for Thermoelectric Energy Conversion
M. OHTAKI, K. MIZUTA, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Fukuoka, Japan

Increasing energy demands in worldwide definitely require safer energy security options. Thermoelectric (TE) conversion is becoming more and more of vital importance for higher overall energy efficiency.
In terms of durability at high temperature in air, metal oxides are most attractive. Since the early 1990s, several oxide materials such as CaMnO3, ZnO, SrTiO3, NaCoO2, and Ca3Co4O9 have been reported to show a promising thermoelectric performance. However, strongly ionic characters and the light constituent element, oxygen, of metal oxides are apparently against the conventional guiding principles for higher ZT.
In this paper, we report the thermal and electrical properties of defect pyrochlore (β-pyrochlore) oxides AB2O6 with the "rattler-in-a-cage" structure, in which small A cations sit in an oversized cage-like crystal structure. Extremely shortened phonon mean free path with smaller A cations of in the oxides will be reported. Moreover, improvement in the electrical conduction in the B2O6 framework of the oxides will be discussed.


FD-2:L24  Thermoelectric Clathrates - The State of the Art
P. ROGL, Christian Doppler Laboratory for Thermoelectrics; Institute of Physical Chemistry, University of Vienna, Wien, Austria

Automotive applications of thermoelectric generators (TEGs) for the conversion of the waste heat of combustion engines into electricity is a most timely issue. Among the manifold of "intermetallic" clathrates, hitherto three series of clathrate compounds have shown interesting thermoelectric properties: type I compounds such as EA8M16Ge30 and EA8MxGe46-x-yy (EA=earth alkaline metal, M=transition element,  stands for a vacancy) and type VIII clathrates EA8Ga16{Ge,Sn}46. The presentation will summarize the systematic investigations of clathrate formation (phase equilibria in isothermal sections, isopleths, liquidus projections), clathrate structures, bonding and structure-property relation in multicomponent type I clathrate materials. The present analysis will evaluate issues in preparation as well as the materials design of optimized thermoelectric clathrate compounds backed by DFT calculations. For the temperature region of 300 to 850 K the quinary clathrate-I Ba8NixZnyGe46-x-y-zSnz, synthesized in our laboratory, exhibits hitherto the highest average ZT-value achieved for an n-type clathrate I in polycrystalline bulk samples.


FD-2:L25  Application of Strong Gravitational Field for the Preparation of Gradient Thermoelectric Materials
K. JANUSZKO, A. STABRAWA, K.T. WOJCIECHOWSKI, AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Thermoelectric Research Laboratory, Cracow, Poland; Y. OGATA, T. MASHIMO, Kumamoto University, Shock Wave and Condensed Matter Research Center, Kumamoto, Japan

In this study a novel method of preparation of functionally graded thermoelectric materials is presented. Application of strong gravitational field (up to 10^6 G) created in a special ultracentrifuge and elevated temperatures (up to 400-500°C) allowed sedimentation of chemical elements in selected materials. In our work we present the results of preliminary gradation experiments on Se-Te, Bi-Sb and BiSbTe3 alloys. For the analysis of the influence of high gravity on phase and chemical composition changes XRD diffraction and scanning electron microscopy SEM with EPMA analysis were applied. The corresponding alterations of thermoelectric properties were investigated by a scanning thermoelectric microprobe STM. The results of our studies show that the new technique can be an effective method for preparation of the graded thermoelectric materials with a desired profile of carrier concentration.

 
Session FD-3 - Devices Technologies and Applications for Thermoelectrics, Thermionics, and Thermophotovoltaics

FD-3:IL01  High Efficiency Thermoelectric Topping Cycle Systems
A. SHAKOURI, K. YAZAWA, Purdue University, West Lafayette, IN, USA; A. SILAEN, BIN WU, DONG FU, CHENN ZHOU, Purdue University Calumet, USA

A mismatch between the fuel combustion temperature 1800-2000K and the high pressure steam temperature up to 800-850K, results in a large amount of thermodynamic losses in power plants. A solid-state thermoelectric (TE) placed on top of a Rankine cycle on the wall of the boiler will produce additional electrical power. By selecting the right materials for the TE generator for high temperature operation, the energy production can increase by 5-7%. We analytically studied the optimum point-of-operation between the maximum power output for minimizing the payback and the maximum efficiency to obtain the maximum fuel economy for each generator.


FD-3:IL03  High Temperature Thermoelectric Generators for Automotive Applications
J.D. KÖNIG, Fraunhofer Institute for Physical Measurement Techniques IPM, Thermoelectric Energy Converters, Freiburg, Germany

Power efficiency is one of the most important challenges for the society and industry of the 21st century. The direct conversion of lost heat into electricity using thermoelectricity can be one solution for automotive applications. Currently, thermoelectric energy harvesting is enjoying high popularity. It has triggered a worldwide research race for materials, modules and systems with better conversion capabilities in particular for high temperatures.
As a first order assessment of the recent and current material development it is reasonable to expect “high-temperature material” in technical ripeness in a in sufficient quantities within a few years. Therefore additional aspects like availability of raw materials, commodity prices, environmental and recycling aspects will play a decisive role for the choice of the finally “best” high temperature material besides topics like high thermoelectric figure of merit, processability, long-term stability. Here, a survey of most recent achievements in material development will be presented including an updated assessment on their technical ripeness.
Parallel to the progress in thermoelectric material development, increasing efforts have been started to process new materials to modules. In addition the state of art of high temperature module development based on Half-Heusler compounds, silicides and Skutterudites at Fraunhofer IPM will be summarized and some aspects of high temperature system integration for thermoelectric will be discussed.


FD-3:IL04  Nanotechnology to Unlock the Multi-billion Dollar Potential of Thermoelectricity
R.J. MEHTA¹, G. RAMANATH^{1, 2}, T. BORCA-TASCIUC^{1, 2}, R. FREDERICK¹, ¹ThermoAura Inc., Rensselaer, NY, USA; ²Rensselaer Polytechnic Institute, Troy, NY, USA

Thermoelectric materials facilitate the conversion of thermal-to-electrical gradients and vice versa and presently, the conversion efficiencies are low (<10%), because the figure of merit ZT≤1 for the state-of-the-art commercial thermoelectrics. Nanostructuring to suppress phonon transport has emerged as new paradigm for engineering both novel and well-known thermoelectrics yielding substantial ZT gains, but has been hampered by difficulties in translating successes outside the laboratory. Further tangible advances are predicated on the ability to manufacture on industrial scales and tune thermoelectric power factor of nanocrystalline materials. We present a novel synthetic process that yields thermoelectric nanomaterials with ZT>1 through synergistic chemical doping and nanostructuring. We have scaled-up our process to obtain kilogram-to-ton-scale production, identified processing for ZT control, and will discuss optimization of doping and stoichiometry using strategies that we have recently shown can yield ZT>1.5. We anticipate the application of nanotechnology will pave the way for the large-scale manufacture of transformative thermoelectric materials for cooling and heat-harvesting technologies.


FD-3:IL05  Development of Thermoelectric Generation for Waste Recovery
R. FUNAHASHI, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan

Although many kinds of thermoelectric materials have been developed, thermoelectric power application has not been succeeded yet, since the matters of toxicity, abundance, and oxidation have never been solved. Oxides should be one of the strongest candidates for thermoelectric power application because of oxidation resistance, no toxic and rare elements, low processing cost, etc. Especially, some kinds of layered cobaltites have been reported as promising thermoelectric materials. Many efforts; microscopic optimization, formation of junction, fabrication of modules, heat collection, and cooling, generation tests, were carried out to achieve power application using thermoelectric oxides.
The materials with higher conversion efficiency and oxidation resistant around 673-873 K are indispensable. A new n-type silicide possessing a composition of Mn2.7Cr0.3Si4Al2 and hexagonal CrSi2 structure has been discovered. The dimensionless thermoelectric figure of merit ZT reaches 0.3 at 573 K. A thermoelectric module consisting of 64 pairs of legs has been fabricated using MnSi1.7 and Mn3Si4Al2 devices as p- and n-type legs, respectively. Output power reaches 9.4 W corresponding to 2.3 kW/m2 of power density against surface area of the substrate, for a heat source temperature of 873 K in air.


FD-3:IL06  Development of High-efficiency Segmented Thermoelectric Couples for Radioisotope Thermoelectric Generators
T. CAILLAT, S. FIRDOSY, B.C-Y. LI, C.-K. HUANG, V. RAVI, N. KEYAWA, P. GOGNA, J. PAIK, J. CHASE, D. UHL, J. NI, K. SMITH, J.-P. FLEURIAL, Jet Propulsion Laboratory/Caltech, MS 277-207, Pasadena CA, USA

Radioisotope Thermoelectric Generators have been successfully used to power spacecrafts for deep space missions. They have consistently demonstrated their extraordinary reliability and longevity (more than 30 years of life). Over the last few years several advanced high-temperature thermoelectric materials, including n-type La3-xTe4, p-type Yb14MnSb11, and n- and p-type filled skutterudites, have been developed at the Jet Propulsion Laboratory for integration into advanced power generation devices. The stability of their thermoelectric properties has been demonstrated for over 18,000 hours up to 1323K. Stable metallization and sublimation suppression barriers/coatings have been successfully developed. JPL is now focusing on developing segmented and skutterudite only couples based on these high-temperature materials to achieve high conversion efficiencies. Recent performance tests have demonstrated 11 to 15% conversion efficiencies with cold and hot-junction temperatures in the 423-473K and 973-1273K range, respectively. An overview of the progress in the development of the couples is provided and options for integration into advanced RTGs are described.


FD-3:L07  Spacer-inserted Thermoelectric Device with Enhanced Power Generation and Material Efficiency
HOON KIM, WOOCHUL KIM, School of Mechanical Engineering, Yonsei University, Seoul, Republic of Korea

Much of the research in thermoelectrics has focused on the enhancement of the thermoelectric figure of merit, an index that represents performance in power generation and refrigeration. Additionally, there has been significant work in the optimization of thermoelectric devices by modulation of their dimensional parameters. This work suggests a new type of thermoelectric element which consists of the thermoelectric material surrounding a spacer material of low thermal conductivity; we term this design the spacer-inserted thermoelectric device (SITED). This device separates the thermal path and the electrical path within the thermoelectric elements and thereby allows power generation to be optimized by using a certain cross-sectional area ratio of thermoelectric material to spacer material, given a certain practical constraint for the fill factor, that is, the area coverage ratio of each thermoelectric element. Based on a commercial device of fill factor 50%, it is found that the power generation and the power generation per volume of material consumed can both be drastically enhanced, by 2.73 times and 24.54 times, respectively.


FD-3:L08  Preparation of Mg2Si Film by Friction Film Forming Method
M. TAKAHASHI, Graduate School, Kogakuin University, Hachioji, Tokyo, Japan; H. Kohri, T. Yagasaki, Faculty of Engineering, Kogakuin University, Japan

Thermophotovoltaic_(TPV) power generation is being proposed as an efficient energy conversion. In the TPV generating system, GaSb, InGaAs, etc. are used as energy conversion materials for the photovoltaic cell. These compounds, however, have a low Clarke number and/or toxicity to human body. Mg2Si is the most promising for the replacement being an environmentally friendly semiconductor. In this study, the optimal forming conditions of Mg2Si film by the friction film forming method were investigated.
The bench drilling machine was used for the friction film forming. Mg2Si powder was pressed on the SiO2 glass or Al substrates by the indenter, which was fixed by the chuck and rotated, in the atmosphere. The optical properties of the sample on the SiO2 glass were measured by a spectrophotometer.
Mg2Si film was formed by shear force between powder and the substrate. From the results of XRD, it was found that the generation of MgO was reduced by shortening the forming time from 60s to 10s. However, much oxygen was detected on the surface by AES. This oxygen may have been adsorbed to Mg2Si powder before film forming. The bandgap of Mg2Si was 0.67 eV from the results of the transmittance. This value suggests that Mg2Si is suitable to use for TPV system.


FD-3:L09  Advanced Thermionic and Thermoelectric Conversion Module for Concentrating Solar Systems
D.M. TRUCCHI, A. BELLUCCI, P. CALVANI, E. CAPPELLI, V. VALENTINI, IMIP-CNR, Monterotondo Scalo (RM), Italy; S. ORLANDO, IMIP-CNR, Potenza Section (PZ), Italy; L. SILVESTRONI, D. SCITI, ISTEC-CNR, Faenza (RA), Italy; R. YOGEV, A. KRIBUS, Tel Aviv University, Dept. of Mechanical Eng, Israel

A solid-state power generation technology for solar concentrators based on a combined thermionic-thermoelectric converter able to cogenerate electric and thermal energy is herein proposed. A conversion module, working under vacuum, was fabricated and composed by a radiation absorber made of a hafnium carbide-based material with properties of thermal and mechanical stability at the operating temperatures. The absorber surface was properly tailored by a femtosecond laser treatment to induce the formation of periodic ripples which enhance the solar absorbance up to values >90%. CVD diamond films were deposited on the absorber surface not exposed to the radiation. The films, characterized by a hydrogen surface termination, act as thermionic emitter operating at lower temperatures than standard emitters. Electrons emitted by CVD diamond are collected by a metal anode, which is thermally connected to a commercial thermoelectric generator, that provides an additional combined thermal-to-electric conversion.
For the first time, in the framework of the European Project E2PHEST2US, the conversion module has been designed, fabricated and tested under concentrated solar radiation. Technological solutions to overwhelm present performance and limitations are discussed.
 
 
Poster Presentations

FD:P01  Theoretical Study of Layered Oxychalcogenides as Thermoelectric Materials
H. FUNASHIMA, H. KATAYAMA-YOSHIDA, Department of Materials Engineering Science Graduate School of Engineering Science, Osaka University, Osaka, Japan

(LaO)MCh (M=Cu,Ag,Au, Ch=S,Se,Te) are known as transparent narrow gap p-type semiconductors, which give an excitonic absorption/emission near the band edge even at room temperature¥cite{kawazoe}. These compounds have P4/nmm structure and are natural superlattice semiconductor.
In this study, we first calculated the band structure for these compounds, using the Full Potential Augmented Planewave (FLAPW) method based on LDA/DFT. Our results show that these compounds have large anisotropy in the kz direction. We analyzed the electronic structure using group theory.
Secondly, we calculated the conductivity tensors and Seebeck coefficient using semi-classical Bloch-Boltzmann equations. The Bloch-Boltzmann equations show that materials that have small dispersion, so-called ''flat'', band structure, have a large Seebeck-coefficient. As mentioned, we showed that (LaO)MCh also have flat-band structure in the kz direction, these large Seebeck coefficient are large. In addition, these compounds have a small hole-pocket in valence band, thus they have large electric conductivity.
Finally, we show the electronic structure and thermoelectric properties of the multi-block-layered concept Oxychalcogenides.


FD:P02  Solid-State Synthesis and Thermoelectric Properties of Mg2+xSi0.7Sn0.3Sbm
SIN-WOOK YOU, IL-HO KIM, Department of Materials Science and Engineering, Korea National University of Transportation, Chungju, Chungbuk, Korea

Magnesium compounds Mg2X (X = Si, Ge, Sn) and their solid solutions have attracted increasing attention as promising thermoelectric materials at temperatures ranging from 500 to 800 K. Higher ZT (dimensionless thermoelectric figure of merit) can be obtained with Mg2Si1-xSnx, because of the greater difference in atomic mass between Si and Sn. The content of Mg and Sb has a significant impact on the electron concentration and thermoelectric properties of n-type Mg2Si1-xSnx solid solutions. In this study, in order to reduce the changes in composition due to the volatilization and oxidation caused by Mg, Mg2+xSi0.7Sn0.3Sbm (0 ≤ x ≤ 0.2, m = 0 or 0.01) solid solutions have been successfully prepared by mechanical alloying and hot pressing as a solid-state synthesis route. The electrical conductivity of Mg-excess solid solutions was enhanced due to increased electron concentrations. The absolute values of the Seebeck coefficient varied substantially with Sb doping and excess Mg, which was attributed to the change in carrier concentration. The onset temperature of bipolar conduction was shifted higher with Sb doping and excess Mg. The lowest thermal conductivity of 1.3 W/mK was obtained for Mg2Si0.7Sn0.3Sb0.01. A maximum ZT of 0.64 was achieved at 723 K for Mg2.2Si0.7Sn0.3Sb0.01.


FD:P03  First Principles Studies for Thermoelectric Properties of TAGS
H. SHINYA, H. FUNASHIMA, A. MASAGO, T. FUKUSHIMA, H. KATAYAMA-YOSHIDA, Graduate School of Engineering Science, Osaka University, Osaka, Japan

In this session, we report computational materials design of highly efficient thermoelectric conversion materials by choosing GeTe as a host material. As it is well known, PbTe is a good candidate for highly efficient thermoelectric conversion material. While the n-type PbTe exhibits good stability, the p-type PbTe formulations have many practical issues such as instability, poorer mechanical properties and poisonous properties. Therefore, alternative materials of the p-type PbTe have been eagerly anticipated.
Recently, AgSbTe2-GeTe alloys (TAGS) have attracted much attention as the alternative materials. Skrabek and Trimmer have observed that TAGS shows a discontinuous decreasing of the thermal conductivity without changing the electric conductivity around the 80% GeTe concentration. In this sense, TAGS has high thermoelectric efficiency. However, the origin of the high efficiency has yet to be understood theoretically and is still under discussion. The correlation between the crystal structure and the high thermoelectric efficiency based on the results of our first principles calculations is discussed.


FD:P05  Electronic Structure and Thermoelectric Performance of Ternary Yb-Mg-Si Phases
M. KUBOUCHI, K. HAYASHI, Y. MIYAZAKI, Department of Applied Physics, Graduate School of Engineering, Tohoku University, Sendai, Japan

Mg2Si is well known as a promising thermoelectric (TE) material due to lightweight, less-toxic and natural abundance. The n-type Sb-doped Mg2Si exhibited relatively high dimensionless figure of merit, ZT = 0.98.1 On the other hand, high-performance p-type Mg-Si phases are not fully developed. Recently, we succeeded in preparing new p-type materials, A-Mg-Si (A = Sr, Ba) phases. In particular, Sr2Mg4Si3 exhibited ZTmax = 0.25.2 However, the samples were more likely to effloresce with increasing the ratio of alkali earth metal. Therefore, the thermal stability of Sr2Mg4Si3 was not enough for practical use. For further improvement of stability and ZT, we have searched prospective materials using first principle calculation. Yb2Mg4Si3 whose structure is the same with Sr2Mg4Si3 is expected to exhibit high p-type TE performance due to the sharp gradient in the vicinity of the Fermi level on density of states. We have prepared Yb-Mg-Si samples by a spark plasma sintering technique. Their TE performance will be reported in the presentation.
1 Y. Hayatsu et al., J. Solid State Chem., 193 (2012) 161.
2 T. Kajitani et al., J. Electr. Mater., 42 (2013) 1855.