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Alloys and Compounds for Thermoelectric and Solar Cell Applications IX: Session IV
Sponsored by: TMS Functional Materials Division, TMS Structural Materials Division, TMS: Alloy Phases Committee
Program Organizers: Hsin-Jay Wu, National Chiao Tung University; Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, CNRS Crismat Unicaen; Philippe Jund, Montpellier University; Yoshisato Kimura, Tokyo Institute of Technology; Lan Li, Boise State University; Takao Mori, National Institute For Materials Science; Tiejun Zhu, Zhejiang University; Alexandra Zevalkink, Michigan State University; Wan-Ting Chiu, Tokyo Institute of Technology

Tuesday 2:00 PM
March 16, 2021
Room: RM 21
Location: TMS2021 Virtual

Session Chair: Wan-Ting Chiu, Tokyo Institute of Technology; Ping-Yuan Deng, National Chiao Tung University

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2:00 PM  Invited
Structure and Physical Properties of Complex Chalcogenides: Fundamental Research with an “Eye” Towards Lower Temperature Applications: George Nolas¹; ¹University of South Florida
    Despite their attractive features, thermoelectrics remains a niche field due to the low conversion efficiency of devices due, in part, to the challenge in overcoming the disadvantages from the interrelation between electric and thermal properties. Nevertheless, advances in computational modeling capabilities have led to new developments by combining experimental and theoretical efforts. In all this progress, materials research for lower temperatures, e.g. in cooling or refrigeration applications, does not seem to be as much of an interest as materials for higher temperature, or power generation, applications. Furthermore, developing a fundamental understanding of the underlying physical properties of materials is often secondary to thermoelectric enhancement efforts. Here I present some of our recent progress on the structure-property relationships of specific materials, provide insight into the search for new materials, and present results that demonstrate strong electron-phonon coupling and atypical transport of new layered chalcogenide materials.

2:20 PM  
Phase Boundary Mapping to Improve Na solubility, Band Convergence, and Thermoelectric Properties in p-type PbTe: James Male¹; Priyanka Jood²; Shashwat Anand¹; G. Snyder¹; ¹Northwestern University; ²National Institute of Advanced Industrial Science and Technology
    Many monumental breakthroughs in p-type PbTe thermoelectrics are driven by optimizing a Pb0.98Na0.02Te matrix. We find high temperature zT near 2 for Pb0.96Na0.04Te, despite doping beyond the expected solubility limit of Na. High temperature X-ray diffraction and carrier concentration measurements show enhanced Na solubility at high temperatures when x > 0.02, but no improvement in carrier concentration, indicating that Na enters the lattice but is electrically compensated by intrinsic defects. Experiments and calculations show that higher Na concentration converges the light L and heavy Σ valence bands in PbTe, suppressing bipolar conduction and increasing the Seebeck coefficient. Further, we apply a phase diagram approach to explain the origins of increased solubility from excess Na doping and offer strategies for repeatable synthesis of high zT Na-doped materials. A starting matrix of simple, high performing Pb0.96Na0.04Te synthesized following our guidelines may be superior to Pb0.98Na0.02Te for continued PbTe optimization.

2:40 PM  
Defect Evolution Enabling Low Thermal Conductivity and High Thermoelectric Performance for n-type PbTe : Ping-Yuan Deng¹; Kuang-Kuo Wang²; Jia-Yu Du³; Hsin-Jay Wu¹; ¹National Chiao Tung University; ²National Sun Yat-sen University; ³National Tsing Hua University
    The PbTe-based alloys have been extensively utilized as mid-temperature thermoelectric materials since the 1960s. During the adventure on chasing high-performance PbTe via doping, it comes to realize that the breakthroughs for n-type PbTe are less impressive, which limits the overall conversion efficiency of a PbTe-based TE device. In light of this obstacle, the Gallium is incorporated into the PbTe, which tunes the carrier concentration by introducing the additional impurity traps within the bandgap. Also, the κL reveals a significantly decreasing tendency, accompanying with the defect evolution that changes from a dislocation loop to nano-precipitation with increasing Ga content. The footmark for the κL reduction can be probed by an equilibrium phase diagram, which opens up a new avenue for locating the high-zT TE materials. The synergy approach of carrier optimization and defect engineering rejuvenate the well-established TE materials and boost their performance even beyond existing records.

3:00 PM  Invited
Thermal and Electrical Transport in Zintl Thermoelectrics: From Ab Initio Understanding to Materials Discovery: Geoffroy Hautier¹; ¹Université catholique de Louvain
    Zintl phases have led to many high performance thermoelectric materials. Ab initio computations are a powerful tool to understand electrical and thermal transport in these complex materials. In this talk, I will focus on two antimonides systems: the M3X2 (M=Mg, Ca, X=Sb, Bi…) and A14MX11 (A=Yb, Ca M=Mn, Mg, X=Sb, Bi…) showing promising thermoelectric properties. Using ab initio computations, I will show how surprising experimental findings (e.g., dependence of transport data on atomic substitutions or on temperature) can be explained combining ab initio and experimental results. I will finish my talk by showing how ab initio computations can not only be used to understand but also predict new thermoelectric materials by performing high-throughput screening computations.

3:20 PM  Invited
The “Grand Challenge” of Thermoelectric Materials: David Parker¹; ¹ORNL
     Despite intensive research, it remains difficult for thermoelectric technology to attain its substantial technological promise.. One main reason is the difficulty of finding sufficiently high performance, based on the “figure-of-merit” ZT. Large ZT requires high electrical conductivity and Seebeck coefficient S and low lattice thermal conductivity. It is easy to meet these requirements separately – there are numerous materials with thermal conductivity as low as 0.4 W/m-K (five times as low as Bii₂Te₃), and several materials with substantial power factors of a few to several mW/m-K². It is finding the simultaneous combination of these properties that is a materials science “grand challenge”. I will discuss three relevant scientific questions: - Does strong anharmonicity necessarily lead to low carrier mobility? - Can electrical conductivity masses and density-of-states masses be decoupled? - Can one use inverse design techniques to design a material with extreme band edge degeneracy?

3:40 PM  Invited
The Origin of Low Thermal Conductivity in Tetrahedrites: A Jahn-Teller Electronic Instability: Paz Vaqueiro¹; ¹University of Reading
    Tetrahedrites are excellent p-type thermoelectric materials with extremely low lattice thermal conductivities. The understanding of the origin of low thermal conductivity in tetrahedrites, which has been attributed to rattling vibrations of the trigonal-planar copper ions, remains limited. Neutron and synchrotron diffraction data collected on Cu₁₂Sb₄S₁₃ as a function of temperature, demonstrate that copper rattling in tetrahedrite is a direct consequence of a tetragonal-to-cubic phase transition at 90 K, which results in a sharp increase, by approximately 200%, of the atomic displacement parameters of the trigonal-planar copper cations. A Jahn-Teller electronic instability leads to the formation of “molecular-like” Cu₅⁷⁺ clusters below 90 K. This suppresses copper rattling vibrations due to the strengthening of direct copper-copper interactions. First-principles calculations demonstrate that the structural phase transition opens a small band gap in the electronic density of states and eliminates the unstable phonon modes found for cubic tetrahedrite.

4:00 PM  Invited
Effect of Phonon Drag on Seebeck Coefficient Based on Linear Response Theory: Masao Ogata¹; Junya Endo¹; Hiroyasu Matsuura¹; Hideaki Maebashi¹; Hidetoshi Fukuyama²; ¹University of Tokyo; ²Tokyo University of Science
    We studied the general framework of the linear response theory to thermal perturbation and clarified the range of validity of Sommerfeld-Bethe relation that leads to the Mott formula at low temperatures. The contributions which do not satisfy the Sommerfeld-Bethe relation are identified, including the well-known phonon drag. Based on this theoretical work, we study the phonon drag effect on Seebeck coefficient in FeSb2 in the presence of strong impurity potentials. It is found that the Seebeck coefficient due to the phonon drag mechanism is enhanced significantly in proportional to the square of the effective mass. This significant enhancement in the Seebeck coefficient is shown to occur in the temperature range where the electrical resistivity has a shoulder structure in FeSb2. A similar effect using magnons, i.e., magnon drag effect is also studied, having the large Seebeck coefficient discovered recently in Fe-Heusler thin film in mind.

4:20 PM  Invited
Strategies for the Balance of Oxide/Metal Composites Towards the Applications of Flexible Solar Energy Harvesters: Wan-Ting Chiu¹; Chang Tso-Fu Mark¹; Masato Sone¹; Agnes Tixier-Mita²; Hiroshi Toshiyoshi²; Hideki Hosoda¹; ¹Tokyo Institute of Technology; ²The University of Tokyo
     Renewable energies are important topics in the critical sustainability issues [1]. Among the green energies, solar-driven energies are considered to be promising due to their low-cost, high efficiency, and portability [2]. This study focuses on the series of decoration strategies between oxides (i.e. ZnO and TiO¬2) and metals (i.e. Au and NiP) for the flexible solar energy applications. Composites were fabricated by various electrochemical techniques such as cathodic deposition, co-deposition, and thermal reduction. Mechanisms for the synthesis of composites were clarified and the rules of the trade-off between the oxides and metals were also constructed for the optimization of energy output efficiency. This study, therefore, provides the guidelines for the fabrication of oxide/metal composites by the electrochemical techniques. Keywords: Solar energy; Photoelectrochemical; Composites. Reference: [1] H. Lund, Renewable energy strategies for sustainable development, Energy 32(6) (2007) 912-919[2] E. W. Dickinson, Solar Energy Technology Handbook, CRC Press, USA, (2018)

4:40 PM  
Effect of Zn and Cr Co-doping on the Thermoelectric Properties of Colusite Cu26V2M6S32 (M=Ge,Sn): Paulina Kaminska¹; Cédric Bourgès²; Piotr Śpiewak¹; Takao Mori²; ¹Warsaw University of Technology; ²National Institute for Materials Science
    Colusite-based materials have attracted significant attention in the field of thermoelectric materials because of their earth-abundant elements and high thermoelectric performance. In this work, we present the enhancement of thermoelectric properties in colusite Cu26V2M6S32 (M=Ge,Sn) by Zn and Cr co-doping effect. The process of synthesis a pure, single phase colusites has been optimized and confirmed by X-ray diffraction. Measurements of the electrical, thermal and magnetic properties of examined compounds have been conducted in order to investigate its thermoelectric properties. Zn doping is responsible for stabilizing the structure and providing electron doping, while Cr affects the magnetic properties of material. We obtained a great improvement of the power factor value for doped colusites, compared to undoped one. For better understanding of the electronic properties, the density functional theory (DFT) calculations were employed.

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