Symposium FF
Progress in Materials and Devices for Direct Thermal-to-Electric Energy Conversion

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

FF-1:IL01  Thermoelectrics in Strongly Correlated Electron Systems
ICHIRO TERASAKI, Department of Physics, Nagoya University, Nagoya, Japan

In strongly correlated electron systems, conduction electrons move in a correlated manner to keep away from each other as far as possible in order to minimize the Coulomb repulsion. As a result, various degrees of freedom can dramatically change the electronic state from a simple band picture. In the case of thermoelectrics, such degrees of freedom can be a source of additional transport entropy to enhance the thermopower. A typical example is a large thermopower in the layered cobalt oxide reported in 1997, which is a milestone of oxide thermoelectrics. Another example is a large thermopower in heavy fermion intermetallics. In this talk, we show our recent three works related to this issue.
(1) Enhanced power factor in Ca2RuO4 far from equilibrium (J. Phys. Soc. Jpn. 86, 093707 (2017)), (2) Kelvin formula in ferromagnetic oxides (J. Phys. Soc. Jpn. 86, 104707 (2017)), (3) Anomalous phonon drag in FeSb2 (Nature Commun. 7, 12372 (2016)).


FF-1:IL02  Electronic Structure Calculations of Energy Converting Alloys by KKR-CPA Method
J. TOBOLA, S. KAPRZYK, M. RYBSKI, B. WIENDLOCHA, AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Krakow, Poland

The Korringa–Kohn–Rostoker (KKR) method based on Green function multiple scattering theory was implemented to calculate electronic band structure and relevant kinetic parameters of electrons on the Fermi surface. The coherent potential approximation (CPA) was employed to account for chemical disorder effects (alloying, vacancy defects) on electronic structure. Recent results of electronic structure calculations as well as modeling of electron transport properties in selected thermoelectric bulk materials are discussed. We focus mostly on effect of unusual electronic structure features (e.g. band convergence or interplay between nonparabolic dispersion relations and remarkable anisotropy of electron transport properties), intentional doping as well as the spin–orbit interactions on TE performance, which can be optimized by proper adjustment of carrier concentration for a given temperature range. Furthermore, experimental and theoretical studies on impact of electronic structure features on discharge curves in AxMO2 (A= Li, Na; M= V, Co) battery systems are presented. Novel approach enlightening apparently different characters of the discharge/charge curve in LixCoO2 (monotonous curve) and NaxCoO2 systems (step-like curve) has been proposed based on the KKR-CPA calculations.


FF-1:IL03  Optimization of Nanostructured Thermoelectics through Computer Simulations
K. TERMENTZIDIS, CETHIL UMR 5008, CNRS, INSA of Lyon, Villeurbanne, France

Elaborating and optimizing innovative thermoelectric materials is the object of intense research the last decade. Nanostructures and nanostructured materials as nanowires-nanotubes, superlattices, materials with nanopores and nanoinclusions, or 2D materials are now possible to be elaborating with tailored transport properties. Current thermoelectric materials often have a low figure of merit (ZT) and yet cannot be used for mass energy production. A way to increase efficiency of these materials is to reduce their thermal conductivity while preserving their electrical conductivity, achieving the holy-grail of thermoelectric materials, the so-called “electron crystal, phonon glass”. In general, nanostructured materials have a much lower thermal conductivity compared to bulk materials, due to phonon confinement and boundary scattering, leading to a higher figure of merit. Such possibilities are studied in the present work through the use of computer simulations, combining modeling and mainly thermal conductivity prediction through ab-initio calculations, molecular dynamics or Monte Carlo simulations. In this review the latest simulations and modeling of nanostructured materials will be given.


FF-1:IL04  High Throughput DFT Calculations - Screening for New TE Compounds
G.K.H. MADSEN, Institut für Materialchemie, TU Wien, Vienna, Austria

With the advances of computational methodologies, theoretical methods promise to play a important role in exploring and understanding the rules underlying high thermoelectric performance, thereby paving the way for the design new materials. In the present talk, we will present the new BoltzTrap2 and almaBTE[1] codes as well as design strategies based on band structure[2,3] and defect engineering[4,5] to maximize the thermoelectric powerfactor and minimize the lattice thermal conductivity. A promising TE material class is the half-Heusler compounds (HHCs). High conversion efficiencies have been demonstrated in the 500-900 K temperature range. Furthermore, they are generally based on noncritical elements. The chemical complexity of the intermetallic phases both offers a great opportunity for systematically optimizing the thermoelectric performance but also a highly complex and many-dimensional phase space for the optimization. We will discuss how simultaneous band structure and defect engineering helped identify a new p-type HHC based TE.[4]
[1] Carrete et al Comp. Phys. Comm. 220, 351 (2017). [2] Bhattacharya et al, PRB 92 , 085205 (2015). [3] Zhang et al Nature Comm. 7 10892 (2016). [4] Bhattacharya et al J. Mater. Chem. C 4, 11261 (2016). [5] Katre et al PRL 119, 075902 (2017).


FF-1:IL05  Electric Power Generation from Waste Heat without Temperature Gradient
SHINJI MUNETOH, YUKI OSAKABE, OSAMU FURUKIMI, Kyushu University, Fukuoka, Japan

The electric power can be generated by the temperature difference between both ends of the thermoelectric materials in the Seebeck effect. However, low conversion efficiency is caused by heat flux from hot side to cold side of sample. In this paper, we have proposed a new thermal power generation mechanism with no temperature difference. Ba8AuxSi46-x clathrate single crystal with n-p junction was synthesized by Czochralski method. The single crystal has a gradient of the gold contents along the growth direction. According to the results of Seebeck coefficient, the electrical properties of the Ba8AuxSi46-x clathrate dramatically changed depending on the gold contents. The band gap of the n-type and ptype Ba8AuxSi46-x clathrate were wider than the intrinsic semiconductor. The single crystal was heated under the uniform temperature and was able to obtain generated electric voltage of around 0.6 mV at 400°C. These results suggested that the obtained electric voltage can be generated from the separation of hole-electron pair excited by heating at the intrinsic part with a narrow band gap along to the energy band curve formed by p-n junction.


FF-1:IL06  Ab Initio Calculations as a Guiding Tool for the Study of Phase Stability of Thermoelectric Materials
D. FUKS, Y. GELBSTEIN, Materials Engineering Department, Ben Gurion University of the Negev, Beer Sheva, Israel

PbTe or TiNiSn half-Heusler alloys are promising materials for application in thermoelectric devices. Their improving may be achieved by increasing the Seebeck coefficient and/or by reduction of thermal conductivity. Doping of these materials may change the type of conductivity and/or may lead to decomposition of alloys. Morphology of material influences the thermal conductivity, and this is an additional way to manage the thermoelectric efficiency. The aim of this presentation is to examine the improving thermoelectric figure of merit by combining Density Functional theory calculations and statistical thermodynamics. The influence of alloying of PbTe with Na, Ti, and Cl and TiNiSn with Cu, Fe or Sc on the type of conductivity is investigated. Decomposition of TiNiSn with growing Ni contents or upon alloying with Sc as well as of PbTe alloyed by Ti is studied for T≠0. The approach bridges the gap between the quantum mechanical calculations of the phase stability in the ground state and the behavior of the alloys at elevated temperatures. It is demonstrated that existence of the miscibility gap in alloyed compounds leads to phase separation in the nano-scale and to reduction of thermal conductivity recently found in experiments.


FF-1:IL07  Structuring Intuition with Theory: The High-throughput Way
M. FORNARI, Department  of Physics and Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, MI, USA

Striking the balance between thermal and charge transport is a main goal when searching for optimized materials for thermoelectric energy conversion. This difficult task can be achieved by controlling detailed features in the band structure as well as by enhancing phonon scattering. We will discuss methodologies developed in the AFLOW consortium (www.aflow.org) to improve the quality and the speed of the theoretical predictions. The talk is organized around the concept of properties descriptors which link computable quantities with functionalities of interestin order to provide a structure to scientists’ intuition. Specific examples including phonon scattering mechanisms in oxychalcogenides, electronic bands convergence due to Ca- and Ga-doping in SnTe, and phenomena associated to warped band structure will be discussed to illustrate the effectiveness of the approach.


FF-1:IL08  Ab Initio Calculations of the Thermal Conductivity, Discovery of New Materials, and Multi-scale Modeling
L. CHAPUT, LEMTA, CNRS UMR-7563, Univ. Lorraine, Vandoeuvre les Nancy, France  

Within the last few years it has been possible to compute the lattice thermal conductivity of bulk materials using ab initio methods. The interactions between the phonons are obtained from density functional theory and this information is incorporated into the Boltzmann to obtain the thermal conductivity. The good accuracy obtained from those calculations allows trying to use them to find new materials. We present several strategies that we used performing such a search. The first method we used is datamining. We screened the entire Material Project library to find materials with ultra low thermal conductivity. The second method is based on polymorphism and was used to study Zn-Sb compounds. Finally, we conclude showing how ab initio calculations can be combined with Monte Carlo simulations to describe thermal conduction at the micron scale.


FF-1:IL09  Thermal Transport and Chemical Bonding in Clathrates
Y. GRIN, Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, Germany

The intermetallic clathrates attracted attention of materials scientists by their particular crystal structures with large cavities (cages) in the three-dimensional framework which may be occupied by filler species [1]. The clathrates with non-occupied cages are called 'empty' [2]. Mainly three types of atomic interactions are present in this family of inorganic materials: polar and non-polar covalent interactions in the framework, ionic forces and strongly polar covalent dative bonds between the filler species and the framework. The coexistence of the different bond kinds (inhomogeneity of the bonding) is one of the reasons of the suppressed thermal transport. One possible mechanism of this behavior is associated with the vibrations (‘rattling’) of the filler atoms within the cage-like crystal structure. Recently was shown, that there are no indications for the formation of isolated oscillators in clathrates system because the low thermal conductivity is characteristic also for the empty clathrates. A new phonon-filter mechanism was proven by the inelastic neutron scattering experiments [3].
1. M. Pouchard, C. Cros. The Physics and Chemistry of Inorganic Clathrates, Springer, 2014, 1. 2. A. M. Guloy et al. Nature 2006, 443, 320. 3. P.-F. Lory et al. Nature Comm. 2017, 8, 491.


FF-1:L10  Advanced Protective Layers for Improved Chemical Stability and Corrosion Resistance in CoSb3 and Mg2Si Based Materials - Experimental and Theoretical Aspects
A. KOLEZYNSKI, J. LESZCZYNSKI, P. NIERODA, AGH University of Science and Technology, Faculty of Materials Science and Ceramics, Krakow, Poland

Despite the considerable success in recent years in thermoelectric (TE) materials development, the wide application of these newly developed materials in devices for energy conversion is still strongly limited by the problems related to their chemical stability (resulting from i.a. corrosion processes undergoing during long-term heating at work temperatures of TE devices) and thus stability of their transport properties. The problems related to chemical stability of modern TE materials remain still - to a large extent - unsolved, making it the crucial obstacle on the way to their practical use and so an improvement of chemical stability of elements used in thermoelectric devices is currently one of the key issues which need to be solved in order to allow further development of technology for thermoelectric conversion of energy. In this context, the aim of our work was to develop new, effective amorphous protective layers for TE materials. Our most recent results of the influence of the application of such layers on chemical stability of selected n and p-type doped Mg2Si and CoSb3 based TE materials are presented and analyzed in detail and future advancements proposed.
Acknowledgments This research was supported by Polish National Science Center [Grant no. 2016/21/B/ST8/00409]


Session FF-2 - Novel Materials for high Efficiency Thermal-to-electrical Energy Conversion

FF-2:IL02  Towards a Magnesium-silicide Based Thermoelectric Generator: Material and Contact Development for n- and p-type Magnesium Silicide Based Solid Solutions
J. DE BOOR¹, H. KAMILA¹, P. PONNUSAMY¹, M. YASSERI^{1, 2}, A. SANKHLA¹, N.H. PHAM¹, N. FARAHI¹, E. MÜLLER^{1, 2}, ¹Institute of Materials Research, German Aerospace Center, Koeln, Germany, ²Institute of Inorganic and Analytical Chemistry, Justus Liebig University Giessen, Giessen, Germany

Mg2(Si,Sn) is among the most promising thermoelectric materials for the temperature range of 500 K to 800 K where a large fraction of the reusable heat is available. Very good thermoelectric properties have been demonstrated, especially for the n-type material. This, combined with a high material availability, low cost of raw materials and environmental compatibility makes this material class suitable for large scale applications. We aim for the development of a Mg2(Si,Sn)-based thermoelectric generator as usage of the same material class for p- and n-type legs improves thermomechanical compatibility and simplifies contact development. We will present our results on the main challenges for the development of such thermoelectric generator. These are: • Synthesis of efficient p- and n-type material by high energy ball milling and subsequent compaction. Using SEM/EDX, XRD and particle size analysis we can show how the phase formation proceeds during the ball milling. We will also discuss the influence of synthesis parameters on the thermoelectric properties. • Optimization of the p-type properties by variation of the Si:Sn ratio • Correlation between stoichiometry and electronic band structure, with emphasize on the valence band. • Contact development for p- and n-type Mg2Si1-xSnx


FF-2:IL03  High Electron Mobility and Stability of n-type Mg3(Sb,Bi)2
TSUTOMU KANNO, H. TAMAKI, H.K. SATO, Panasonic Corporation, Seika, Kyoto, Japan; Y. Miyazaki, Tohoku University, Sendai, Miyagi, Japan

Layered Zintl compound Mg3(Sb,Bi)2 has a CaAl2Si2-type crystal structure, which is made of covalently-bound anionic [Mg2Sb2]2− blocks and cationic Mg2+ sheets stacked along the c direction. Among thermoelectric researchers, the family of Mg3(Sb,Bi)2 has been known as persistent p-type thermoelectric materials with moderate performance; n-type properties with low carrier concentration (~ 10^18 cm−3) and low ZT have only been reported in Mn-substituted single crystals. We discovered high thermoelectric performance (ZT = 1.51 at 716 K) in n-type Mg3(Sb,Bi)2 with exotic functional heterogeneity. The cation layers are responsible for excellent electronic conduction and anionic layers work as effective phonon scatterers by introducing Sb/Bi disorder. I further improved average ZT values of Mg3(Sb,Bi)2 in the whole temperature range from 300 K to 700 K, by texture control of sintered samples. The efficiency of thermoelectric power generation calculated from the measured ZT values reaches 14.3% (ΔT = 450 K), which is unprecedentedly high as a single-material value. We also examined the high temperature stability in air by thermal analysis. The sample was barely oxidized up until 700 deg. C at a heating rate of 20 K min-1. Another measurement showed that the sample was stable at 400 deg.


FF-2:IL04  Thermoelectric Properties in Dirac/Weyl Semimetals
QIANG LI, Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York, USA

Recent discoveries of new phenomena due to interacting Dirac fermions across vastly different energy and length scales have led to a fascinating convergence between condensed matter physics and high energy nuclear physics. Dirac/Weyl semimetals have a linear dispersion that leads to the electrons near the Fermi energy behaving like Dirac fermions. Topological materials, such as ZrTe5 Dirac semimetal, hold promise of transmitting and processing information and energy in new ways. Many of the topological materials originate from the thermoelectric compounds. In this presentation, I will present our studies on the transport properties of Dirac/Weyl semimetals, with a view on thermoelectric applications. Dirac dispersion can give rise to large thermopower in a magnetic field and the Nernst effect. Combined with an ultrahigh carrier mobility, Dirac/Weyl semimetals may be exploited for thermomagnetic refrigeration.


FF-2:IL05  Highly Efficient Silicides Based Thermoelectric Materials
T. KYRATSI, Department of Mechanical and Manufacturing Engineering, University of Cyprus, Nicosia, Cyprus 

Thermoelectric materials for power generation from waste heat have achieved a major breakthrough during the last decades. Amongst the various promising materials used in the field of thermoelectrics, Mg2Si-based binary materials and Mg2Si-Mg2Sn pseudo-binary systems have attracted much attention during the last years due to their high ZT in combination to the fact that they are cheap and environmentally friendly. They contain widely abundant constituent elements, with low cost and low density, and hence meet the requirements for large scale production, especially in bulk forms. This presentation will review recent developments on the Mg2Si-based materials. Reports on binary system show very interesting properties, however, its applicability is limited by the relatively high thermal conductivity. Thermal conductivity may be significantly reduced by alloying Mg2Si with Sn or Ge, resulting in ternary and quaternary Mg2(Si,Sn,Ge). Moreover, compositional variations which extend from nanoscale to macroscale result in materials with high figure-of-merit that reaches 1.4 at 800K.


FF-2:IL06  Transport Properties of Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 1, 2 and 3)
S. SASSI, C. CANDOLFI, A. DAUSCHER, B. LENOIR, Institut Jean Lamour, UMR 7198 CNRS, Université de Lorraine, Campus Artem, Nancy Cedex, France

Homologous compounds are interesting platform for designing novel thermoelectric materials. Such compounds form a series of crystal structure built from similar blocks whose number increases by regular increments. In particular, they allow an easy manipulation of the complexity of the crystal structure by simple variations in the chemical composition. Among them, the series (PbSe)5(Bi2Se3)3m (m = 1, 2, 3) show very interesting physical properties. For instance, a topological insulating state was discovered for m = 2. Another unique feature characterizing these semiconducting n-type materials is their low ability to conduct heat. For this reason, these compounds received recently a special attention for thermoelectric applications. In this communication, we report on the synthesis, structural and chemical characterizations and thermoelectric properties measurements in a broad range of temperatures (2 – 700 K) of the m = 1, 2 and 3 compounds. The influence of alloying element will be also discussed in the m = 1 and 2 compounds.


FF-2:IL07  Renewed Interest for Heusler and Half-Heusler Alloys for Thermoelectric Applications
E. BAUER, B. HINTERLEITNER, I. KNAPP, A. GRYTSIV, Technische Universität Wien, Institute of Solid State Physics, Vienna, Austria; P. ROGL, G. ROGL, University of Vienna, Institute of Material Chemistry; A. TAVASSOLI, University of Vienna, Institute of Material Chemistry and C. Doppler Laboratory for Thermoelectricity, Vienna, Austria

Among the various families of thermoelectric materials, half-Heusler and full-Heusler systems are appreciated for their excellent mechanical properties and an outstanding thermal stability. While half-Heusler materials are also known for their superior thermoelectric performance as characterized by the so-called figure of merit, ZT, reaching ZT values above 1, the thermoelectric efficiency of full-Heusler systems is still moderate and does not exceed ZT ~ 0.1 - 0.2. The latter finding is based on the unfavorable fact that the total thermal conductivity of such Heusler phases is pretty large, exceeding that of well behaving thermoelectric materials by more than one order of magnitude. Nevertheless, the power factor of Heusler systems like those based on Fe2VAl, is comparable, or even exceeds that of well behaving and excellently performing materials based on Bi-Te. In this study, the influence of substitution on different lattice sites (e.g., V/W or Fe/Cr) on the thermoelectric performance is studied, both from experiments as well as from first principles DFT calculations. I addition, thin film preparation of Heusler systems extraordinarily enhances the thermoelectric figure of merit due to a substantial drop of the lattice thermal conductivity.


FF-2:IL08  Development of Thermoelectric Borides toward Topping Cycles
TAKAO MORI, National Institute for Materials Science (NIMS), Tsukuba, Japan

Novel materials and principles are being utilized in the search for viable thermoelectrics (TE) [1]. There are several attractive TE power generation applications which require high temperature materials [2]. Foremost is the topping cycle in power plants which can deliver sizable increase in power output with moderate ZT (e.g. ZT=0.7 can lead to 6% power output enhancement) [2,3]. Higher borides are attractive candidates because of melting points typically more than 2000oC, large Seebeck coefficients α, and intrinsically low lattice thermal conductivity [4]. Several recent advancements have been made. Excellent (|α|>400 μV/K) p-type or n-type was achieved in elemental boron through strategic Zr doping of the voids [5]. A study was made on RB50 to elucidate effects of disorder on TE properties and possible control. 40 times enhancement of ZT was discovered in the Sm and Yb phases of RB66, the first PGEC [6]. Mixed valency was indicated to play a role. Further study to create composites has yielded significant enhancement in the ZT and will be presented.
[1] Small in press doi: 10.1002/smll.201702013 (2017). [2] JOM 68, 2673 (2016). [3] Scripta Mat. 111 58 (2016). [4] J. Appl. Phys. 102 073510 (2007). [5] Acta Mater. 122 378 (2017). [6] J. Materiomics 1 196 (2015).


FF-2:IL09  Thermal Conductivity over Engineered Inorganic-organic Interfaces
M. KARPPINEN, Aalto University, Department of Chemistry and Materials Science, Espoo, Finland 

Nanoscale superlattice structures of mutually different layers may exhibit – once properly designed and fabricated – strongly suppressed thermal boundary conductance over the layer interfaces. Inorganic-organic interfaces are of particular interest owing to the fundamental dissimilarity of the two material constituents. We have demonstrated that the atomic/molecular layer deposition (ALD/MLD) thin-film technique is uniquely suited for the fabrication of such inorganic-organic structures as it readily allows the atomic/molecular layer level control of the thicknesses and arrangement of individual layers. In this talk I will discuss our recent research on various layer-engineered inorganic-organic systems, showing remarkably decreased thermal conductivities for the inorganic metal oxide and sulfide constituents depending on the number and the thickness of the organic barrier layers, as well as the pattern (regular or irregular) of the introduction of these layers within the inorganic matrix. The results underline the attractive possibilities provided by the ALD/MLD technique to enhance the thermoelectric performance for inorganic-organic hybrid thin films.


FF-2:IL10  Solar Thermoelectric Materials Development
A. WEIDENKAFF, WENJIE XIE, XINGXING XIAO, University of Stuttgart, Stuttgart, Germany

Solar energy can be converted by low temperature photovoltaic and photocatalytic processes or by high temperature entropic conversions using concentrated solar radiation. Conventional concentrated solar power plant are based on mechanical converters with challenging stability problems. Thermoelectric conversion at high temperatures with large applied temperature gradient is a prospective alternative to generate electricity from concentrated solar light. This very high temperature application of a thermoelectric converter requires the development of performing, active, stable, low cost and sustainable materials. Perovskite-type ceramics and stable Heusler compounds as well as their nanocomposites are prospective candidates for high temperature thermoelectric energy conversion processes. Their TE performance is based on e.g. their suitable band structures, adjusted charge carrier density, effective mass and - mobility, hindered phonon transport, electron filtering potentials, and strongly correlated electronic systems. These properties are tuneable by changing the composition, structure, crystallites size, interfaces and materials combinations with tailor-made scalable synthesis procedures.


FF-2:IL11  Structural Features and Transport Properties in Ternary and Quaternary Thermoelectric Sulfides
E. GUILMEAU¹, C. BOURGES¹, V. PAVAN KUMAR¹, L. PARADIS-FORTIN^1,2, P. LEMOINE², O.I. LEBEDEV¹, T. BARBIER¹, B. RAVEAU¹, B. MALAMAN³, G. LE CAER⁴, M. OHTA⁵, K. SUEKUNI⁶, A.R. SUPKA⁷, R. AL RAHAL AL ORABI⁷, M. FORNARI⁷, ¹Lab. CRISMAT, Caen, France; ²Institut des Sciences Chimiques de Rennes (ISCR), Rennes, France; ³Institut Jean Lamour, Vandœuvre-lès-Nancy, France; ⁴Institut de Physique de Rennes (IPR), Rennes, France; ⁵Research Institute for Energy Conservation, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan; ⁶Dept. of Applied Science for Electronics and Materials, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan; ⁷Dept. of Physics and Science of Advanced Materials Program, Central Michigan University, USA

Regardless their relatively light masses, most of copper-based ternary and quaternary sulfides with complex structures exhibit low thermal conductivity possibly determined by local structural distortions, rattling phenomena, or strong bond anharmonicity. However, the improvement of the TE performances of these materials remains a challenge, due to the interdependent and contrary effects of their parameters S, ρ and κ. The presence of structural defects, the deviation to stoichiometry, the nature of the chemical bonds, and of the distribution of charges in these complex structures are still a matter of debate, which is of capital importance for the optimization of their TE properties. In this respect, recent progress on some thermoelectric complex sulphides derived from mineral compounds (stannoidite, germanite, colusite) will be described. Synthesis, processing, shaping, as well as structural and microstructural features will be reported, together with electrical, thermal properties. Band structure and vibrational dispersions from first principles calculations will be discussed.


FF-2:L12  Thermoelectric Properties of Mn2V(Al1-xSix) Full-Heusler Alloys
HEZHANG LI, KEI HAYASHI, YUZURU MIYAZAKI, Tohoku University, Sendai, Japan

In order to obtain a high zT value, a high power factor, PF (= S2σ), is preferred. Recently, we reported that a half-metallic Co2MnSi full-Heusler alloy showed a relatively high PF of 2.9 × 10-3 Wm-1K-2 at 550 K. Since the Co2MnSi alloy is n-type, it is necessary to develop a p-type full-Heusler alloy to be paired with it. In this study, we focused on half-metallic Mn-based full-Heusler alloys. We found that a Mn2VAl alloy was p-type and exhibited a little high PF of 2.7 × 10-4 Wm-1K-2 at 717 K. To improve PF of the Mn2VAl sample, partial substitution of Si for the Al site was performed. Mn2V(Al1-xSix) samples in the range of x < 0.1 were in a single phase of a fcc full-Heusler crystal structure. A Mn2V(Al0.95Si0.05) sample showed the highest Seebeck coefficient among the samples. Consequently, we obtained the highest PF of 3.4 × 10-4 Wm-1K-2 at 717 K for the Mn2V(Al0.95Si0.05) sample, which was about 1.3 times higher than that of the Mn2VAl sample.


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

FF-3:IL01  Next Generation Thermionic Energy Conversion for Space Application
V.I. KUZNETSOV, Ioffe Institute, St. Petersburg, Russia

New tasks in near-earth space demand electric power sources of hundred kilowatts and more. To date, only nuclear power plants can claim the role of such powerful sources for the space. As a converter of a heat-to-electricity, the thermionic energy converter (TIC) is best suited. In the last century, the thermionic TOPAZ plant of arc regime of about 5 kW and about 5% efficiency was created in the USSR. Its collector temperature was of 900 K. One of these units operated successfully on board of Cosmos-1867 spacecraft for an year and had specific dimension of a cooler-radiator 1.5 m2/kW. Currently, the Ioffe Institute develops a new generation TIC of collisionless regime with enhancing specific electric power up to 50 W/cm2 and efficiency up to 30% due to increase in the emitter temperature. The collector temperature increases greatly, too. Thus, the cooler-radiator specific dimension reduces essentially to 0.02 m2/kW. Another essential feature of this advanced converter is the opportunity to generate the alternate current in the shape of higher voltage pulses (10…12 V). As a result, alternate voltage is generated within a TIC and there is no need in extra current converters, thus, lowering its cost and improving its weight parameters as well as its reliability as a whole.


FF-3:IL02  Ultra High Temperature Thermophotovoltaic Technology Combined with Thermionic Energy Conversion
A. DATAS, E. ANTOLIN, P.G. LINARES, J. VILLA, A. MARTI, Instituto de Energía Solar, Universidad Politécnica de Madrid, Madrid, Spain; D.M. TRUCCHI, A. BELLUCCI, M. GIROLAMI, Istituto di Struttura della Materia - Consiglio Nazionale delle Ricerche, Monterotondo Scalo, Rome, Italy; A. VITULANO, G. SABBATELLA, Ionvac Process SRL. Colli di Enea, Rome, Italy

Thermophotovoltaics (TPV) and thermionics (TI) are the two main technologies enabling the conversion of heat directly into electricity at very high temperatures. In this work we present a new concept that hybridizes both technologies in a single device. This device is named hybrid thermionic-photovoltaic converter (TIPV) and takes advantage of the simultaneous emission of electrons and photons from an incandescent surface. The use of these two energy carriers simultaneously results in an increased heat flux and electrical power density, which will ultimately result in a lower cost of produced electricity. These converters are currently under development within the EU-funded project AMADEUS (www.amadeus-project.eu), which aims at the development of a new kind of ultra high temperature (>1000ºC) thermal energy storage systems based on molten silicon. At these very high temperatures, radiation is the main heat transfer mechanism; thus, thermionic- and thermophotovoltaic-based devices are the ideal choice. Molten silicon storage plus TIPV conversion has the potential to lead to a new generation of very compact storage devices with one of the highest energy and power densities among the current energy storage options.


FF-3:L03  STEALS a Modular Direct Conversion Thermal System with Integrated Storage
D. GINLEY¹, P. PARILLA¹, J. ALLEMAN¹, J. VIDAL¹, G. GLATZMAIER¹, A. ZAKUTAYEV¹, J. REA², C. OSHMAN², A. SINGH², N. SIEGEL³, J. SHARP⁴, M. WHITE⁵, P. BREHM⁵, S. DRANEY⁵, G. BUCHHOLZ⁵, E. TOBERER², ¹NREL, Golden, CO, USA; ²CSM, Golden, CO, USA; ³Bucknell University; ⁴Marlow, ⁵Infinia Tech Corp.

Solar ThermoElectricity via Advanced Latent heat Storage (STEALS). This systems basic concept integrates a solar receiver, thermal storage, and power block all on top of a small-scale solar power tower. Key to the concept is the use of three novel approaches as part of the overall integration. These include the use of latent heat in metal alloys for thermal storage, the development of a new form of thermal valve for heat control and direct conversion via either a thermoelectric array (demonstrated) or the use of a Stirling engine (in process). By combining subsystems that are separated by pumped loops in traditional concentrating solar power designs, STEALS reduces the number of moving parts required and has a low material cost. With 5 hours of thermal energy storage, it can be used as a complement to photovoltaics by generating electricity during morning and evening hours, during times of low solar insolation, at a capacity factor of near 30%. Analysis of the levelized cost of electricity (LCOE) of STEALS for different locations and deployment strategies, show that system configurations can be unique with competitive LCOE.


FF-3:L04  Radioisotope Thermoelectric Generators for the European Space Nuclear Power Programme
R. AMBROSI¹, H. WILLIAMS¹, E.J. WATKINSON¹, A. BARCO¹, R. MESALAM¹, M. REECE², KAN CHEN², K. SIMPSON³, M. ROBBINS³, R. TULEY³,
C. BURGESS⁴, M.-C. PERKINSON⁴, A. WALTON⁴, C. STROUD⁵, A. GODFREY⁵, S. GIBSON⁵, K. STEPHENSON⁶, T. CRAWFORD¹, C. BICKNELL¹, J. SYKES¹, M. SARSFIELD⁷, T. TINSLEY⁷, C. FONGARLAND⁸, D. KRAMER⁹, ¹University of Leicester, Leicester, UK; ²Queen Mary University of London, London, UK; ³European Thermodynamics, Kibworth, Leicester, UK; ⁴Airbus Defence and Space, Stevenage, UK; ⁵Lockheed Martin, Ampthill, UK; ⁶European Space Agency, ESTEC, Netherlands; ⁷National Nuclear Lab., Sellafield, UK; ⁸Ariane Group, Paris, France; ⁹University of Dayton Research Institute, Dayton, OH, USA

Radioisotope thermoelectric generators (RTG) are under development in Europe as part of a European Space Agency (ESA) funded programme. Aimed at enabling or significantly enhancing space science missions, the development programme relies on the cost effective production of americium-241 as the radiogenic heat source and an iterative engineering approach to developing the systems which include isotope containment architectures and in the case of RTG systems bismuth telluride based thermoelectric generators. The RTG containment systems rely on the use of inner platinum-rhodium alloy cladding, insulation layers and quasi-isotropic carbon-carbon composite outer aeroshells. The RTG heat source configuration is designed to deliver 200 W. A 5% total system conversion efficiency and a modular scalable design imply that electrical power output can range between 10 W and 50 W, with each RTG system housing up to 5 heat sources. This paper describes the most recent updates in system designs and provides further insight into recent laboratory prototype test campaigns of RTG systems.


FF-3:IL05  Variation in Device Design for Low $/W and Flexible System
WOOCHUL KIM, Yonsei University, Seoul, South Korea

In a power generation system, the price per watt ($/W) is an important parameter to be considered for checking the feasibility for practical implementation. In this talk, we experimentally demonstrate that $/W of a thermoelectric device can be reduced to around 60%. We propose that $/W can be reduced by lowering the material consumption ($) with a slight sacrifice in power output by changing the device architecture. A simple calculation suggests that zT ~ 6 is needed for such a reduction in $/W based on the conventional approach. This method can be accompanied by a search for high-zT material so that further reduction in $/W can be achieved with efficient thermoelectric materials. Also, we demonstrate that the bulky and rigid nature of conventional inorganic materials can be applied to flexible systems. In particular, we propose a bracelet-like and mat-like flexible thermoelectric module with a heat sink based on rigid inorganic bulk materials, which is referred to as the flexible thermoelectric system (FTES). We perform experiments and theoretical analysis to verify that the FTES performs like a conventional system, although it is flexible. In addition, we carry out experiments while the FTES is worn on a person’s wrist for body-heat harvesting. This study shows the possibility of using high-performance bulk materials in the design of flexible devices.


FF-3:L06  Importance of Electrical Impedance Matching on Efficiency and Power in Integrated Thermoelectric Generator Circuits
MARK LEE, Department of Physics, The University of Texas at Dallas, Richardson, TX, USA

Thermoelectric (TE) generators are of compelling interest to non-carbon energy generation, thermal management, infrared sensing, and powering microcircuits. Most research in the TE field has focused on developing materials having a high TE figure-of-merit ZT. However, in many cases, but particularly for integrated circuit TE generators, how well a TE generator converts temperature difference into electrical power may be limited not by the ZT of the TE material but by device impedances. In this presentation we present a thorough circuit-level analysis showing that in real TE generator circuits an electrical impedance mismatch between generator source impedance and load resistance can be more important in determining device efficiency and power output than the ZT of the TE material used. We find that unmatched impedances can degrade power conversion efficiency by over an order-of-magnitude, overwhelming any benefit of using a high ZT material if that material cannot be properly impedance matched in the TE circuit. We model the degree of impedance mismatch tolerable to maintain usefully high efficiency and power generation for a given ZT. Consequences and examples for silicon based integrated circuit TE generator circuits will be discussed.
Work supported by NSF award ECCS-1707581.


FF-3:IL09  Thermoelectric Power Generation from Nanostructured PbTe and Colusite: Materials and Modules
MICHIHIRO OHTA, P. JOOD, ATSUSHI YAMAMOTO, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan; KOICHIRO SUEKUNI, Kyushu University, Kasuga, Fukuoka, Japan; M.G. KANATZIDIS, Northwestern University, Evanston, Illinois, USA, and Argonne National Laboratory, Argonne, Illinois, USA

We will present recent results of our comprehensive effort covering all aspects of thermoelectrics, i.e., from materials to module. A high conversion efficiency was realized in newly-developed thermoelectric materials based on nanostructured PbTe [1,2] and colusites Cu26A2E6S32 (A: V, Nb, Ta; E: Ge, Sn) [3]. For PbTe, nanostructuring and controlled doping in PbTe–0.7% Ge–4% Na led to an exceptionally high p-type thermoelectric figure of merit ZT of 1.9 at 825 K. We fabricated thermoelectric generator modules with the Ge added-PbTe and n-type PbTe–0.18% PbI2 (ZT ~ 1.3 at 780 K) legs. The maximum conversion efficiency of ~8.5% and ~12% for a temperature difference of 590 K were achieved in nanostructured PbTe-based and cascaded Bi2Te3/nanostructured PbTe modules, respectively. In colusites, a ZT of 1.0 at 670 K was achieved for Cu26Ta2Sn5.5S32 through Ge-site non-stoichiometry. The Au-based diffusion barrier between colusites and electrodes provides reduced electrical and thermal contact resistances, leading to enhanced conversion efficiency.
This work is supported by NEDO and METI, Japan.
[1] M. Ohta et al., Adv. Energy Mater., 2016, 2, 1117. [2] X.K. Hu et al., Energy Environ. Sci., 2016, 9, 517. [3] Y. Kikuchi et al., J. Mater. Chem. A, 2016, 4, 15207.


FF-3:IL10  Enhancing Solar Energy Conversion by Hybrid Photovoltaic Thermoelectric Cells
D. NARDUCCI, Department of Materials Science, University of Milano Bicocca, Milan, Italy

Photovoltaic (PV) efficiency is intrinsically limited, since a relevant fraction of the available solar power is either transmitted (unconverted) or is partially degraded into heat by carrier relaxation in the PV absorber. However, the significant amount of heat made available thereof might be reused and partially converted into electricity by thermoelectric generators (TEGs), an opportunity made even more attractive by the increasing efficiency of thermoelectric (TE) materials. This talk will focus on the research aimed at developing hybrid photovoltaic−thermoelectric generators (HPVTEGs), namely tandem solar converters wherein a TE stage partially converts into electricity both the heat released by the PV stage and the sub-gap part of the solar spectrum. Strategies being developed to pair PV cells to TEGs will be discussed, addressing the role played by their thermal coupling and by the impact of heat dissipation on the efficiency of the HPVTEG. TEG layout in hybrid cells will be shown to require an ad hoc design critically depending on the PV material. The conclusion will be reached that HPVTEGs could enable the use of low-cost, non-critical PV materials, currently not considered for solar conversion technologies because of their marginal efficiency.


FF-3:IL11  Power Generation and Durability of Oxide Based Thermoelectric Module
RYOJI FUNAHASHI¹, T. URATA¹, Y. MATSUMURA¹, M. SUZUKI¹, H. MURAKAMI¹, H. IKENISHI¹, T. TAKEUCHI¹, R.O. SUZUKI², S. SASAKI³, S. SUGIYAMA³, ¹National Institute of Advanced Industrial Science & Technology, Ikeda, Osaka, Japan; ²Graduate School of Engineering, Hokkaido University, Japan; ³Akita Industrial Technology Center, Japan

Thermoelectric oxides are considered as promising materials because of their durability　at high temperature, low production cost, non-toxicity etc. The layered Ca3Co4O9 (Co-349), shows high thermoelectric efficiency at even 1073 K in air. Thermoelectric modules using p-type Co-349 and n-type CaMnO3 (Mn-113) possessing the perovskite structure have been produced using Ag-based contacts. The Co-349 bulks were prepared by multiple hot-forging to obtain good thermoelectric property. Thrice hot-forging is effective to suppress the electrical resistivity. As the result, the power factor of this sample reaches 0.49 mW/mK2 at 973 K, which corresponds to a factor of 1.4 of the sample hot forged once. Life time test has been carried out for the oxide thermoelectric modules up to 1073 K for the hot-side temperature (TH) in the air atmosphere. No degradation in the output power is observed up to 1073 K of TH for one month. The durability against heat cycling between 873 and 373 K of TH was investigated in the air atmosphere. The output power is almost constant during 1000 times of the heat cycles. Thermoelectric power generators working by air cooling have been developed using the oxide thermoelectric modules.


FF-3:IL12  Integration of Skutterudites in Thermoelectric Devices
D. KENFAUI, I. KOGUT, B. LENOIR, C. CANDOLFI, A. DAUSCHER, Institut Jean Lamour, UMR 7198 CNRS, Université de Lorraine, Campus Artem, Nancy Cedex, France; A. JACQUOT, J. KÖNIG, Fraunhofer IPM, Freiburg, Germany

Thermoelectric power generators could be well suited to convert low-graded heat for charging batteries in low-power autonomous systems (e.g. wireless sensors) and to increase energy efficiency on a global scale. Nevertheless, power density and pressure on natural resources (thermoelectric materials) are critical factors when it comes to application on an industrial scale. In this presentation, we will present our strategy to reach higher power density by using thin thermoelectric converters that are able to produce more power in a compact design with less thermoelectric materials. Research and development effort focused on the production of wafers made of efficient thermoelectric materials (n and p- type skutterudites) able to work in the mid temperature range (up to 600 °C) that are directly processed to further build the thermoelectric converters. Wafers that encompassed diffusion barriers and electrodes on both sides have been produced. The thickness of the thermoelectric materials could be down-sized to 200 µm. Very compact thermoelectric converters (15x15mm2, 12 legs, leg footprint 2x2 mm2 in size) producing 3.6 W have been fabricated. This performance is achieved with a lower amount of thermoelectric material than in prior works.