Alloys and Compounds for Thermoelectric and Solar Cell Applications V: Session IV
Sponsored by: TMS Functional Materials Division, TMS: Alloy Phases Committee
Program Organizers: Sinn-wen Chen, National Tsing Hua University; Franck Gascoin, Ensicaen University of Caen; Soon-jik Hong, Kongju National University; Philippe Jund, Université de Montpellier; Lan Li, Boise State University; Takao Mori, National Institute for Materials Science; Ce-Wen Nan, Tsinghua University; Hsin-jay Wu, National Sun Yat-Sen University

Tuesday 2:00 PM
February 28, 2017
Room: 21
Location: San Diego Convention Ctr

Session Chair: Hsin-jay Wu, ational Sun Yat-sen University; Teruyuki Ikeda, Ibaraki University

2:00 PM  Invited
Engineering (ZT)eng and Efficiency, High Power Factor in Half-Heusler, and New Zintl Materials: Zhifeng Ren1; 1University of Houston
    A new parameter, engineering (ZT)eng and a new efficiency formula based on (ZT)eng for accurate prediction of efficiency of any materials are developed. Is high peak ZT good enough? Does a high peak ZT necessary translate into a high efficiency? In this talk, I will demonstrate how a high peak ZT cannot warrantee a high efficiency, and how a newly defined engineering (ZT)eng should be considered as the most relevant parameter. I will also demonstrate how a high conversion efficiency does not warrantee high output power, but the high engineering power factor (PF)eng does, and this is what matters most for thermoelectric power generators. A power factor higher than 100 μW cm-1 K-2 is found in half-Heuslers, which produced the highest output energy density. A peak ZT of 1.3 is recently achieved in a new Zintl phase, which will stimulate more research on Zintl phases for even higher ZT.

2:20 PM  Invited
Microstructural Size and Morphology Control of Si Base Thermoelectric Composites: Teruyuki Ikeda1; 1Ibaraki University
    Reduction of lattice thermal conductivity from nanostructuring has recently been regarded as a promising way to improved thermoelectric figures of merit zT. Structural control of thermoelectric materials has been recognized as a important tool to enhance performance of thermoelectric materials. Self-assembled nanostructures have been introduced into thermoelectric materials utilizing phase transformations. In this paper, we discuss the strategy to tune microstructural size and morophology of silicon base thermoelectric nanocomposites to reduce thermal conductivity and get additional benefits to thermoelectric performance.

2:40 PM  Cancelled
Microstructure and Performance of Mg2(Si,Sn,Ge) Materials Prepared by Different Processing Methods: Theodora Kyratsi1; 1University of Cyprus
    In the field of thermoelectrics, it is important to have highly efficient materials, but also take sustainability into account since the overall ecological backpack for certain materials can be surprisingly high. Silicide compounds seem to be an advantageous choice not only because of their ample availability in nature, but also because of their non-toxicity that is consistent with the priority for environment- and human- friendly technology. Among silicide compounds, Mg2Si system seems to be the best with the ternary series Mg2(Si,Sn) presenting ZT higher than 1.4. In this presentation, our recent findings on our best Mg2(Si,Sn,Ge) material are presented with emphasis on the fabrication process. Different processing techniques and/or conditions, such as hot pressing or SPS sintering, and their effect on the thermoelectric performance are studied. Moreover, the potential use of Si waste from other applications into thermoelectrics is also discussed.

3:00 PM  
Synthesis, Processing and Transport Properties of Metastable Phases in the Mg-Si-Sn System: Pathikumar Sellappan1; Anthony Fong1; Masayuki Murata2; Yasuhiro Kodera1; Javier Garay1; 1University of California San Diego; 2National Institute of Advanced Industrial Science and Technology (AIST)
    Compounds in the Mg-S-Sn recently attracted considerable attention as thermoelectric materials due to their low-cost, earth abundant, environmentally friendly constituents. We demonstrate the synthesis of trigonal form of Mg-Sn and Mg-Si-Sn, phases which are not found on the Mg-Sn or Mg-Si-Sn phase diagram. We investigated the phase transformation (cubic to trigonal) while increasing the Si content on the system and optimized the high-energy ball milling parameters. We employed current activated pressure assisted densification (CAPAD) on the phase transformed powder to consolidate them into a near theoretical dense form while maintaining the phase as well as fine nano-structure. We present the effect of CAPAD parameter on the density, microstructure and phase composition of the material. We also show the influence of processing parameters on the transport properties and thermoelectric properties.

3:20 PM  
Phase Equilibria of Ternary Sn-Sb-Co Systems and Ge-Co-Sb and Thermoelectric Properties of Sn/Ge Doped Skutterudite CoSb3: Ping-Yuan Deng1; Hsin-Jay Wu1; 1Department of Materials and Optoelectronic science, National Sun Yat-sen University
    Thermoelectric materials and devices can generate electricity from thermal energy directly, and can be useful in waste heat recovery. The skutterudite CoSb3 has been a promising candidate for mid-temperature thermoelectric generator, and proper dopants, such as Ge and Sn, might have to be introduced into the CoSb3 for optimizing the thermal/electrical transport properties. Phase equilibria, which illustrates relationship between the phase stability, microstructures and thermal-to-electricity conversion, could be crucial and the phase diagrams of ternary Sn-Sb-Co and Ge-Co-Sb systems have been determined by experiments. Herein the liquidus projection of ternary Ge-Co-Sb and Sn-Co-Sb system are constructed; various ternary Ge-Co-Sb and Sn-Co-Sb alloys are either thermally-equilibrated at 400℃ for 45 days or solidified by water-quenching to investigate the microstructural evolution. On the basis of phase diagram determined in this study, selective ternary Ge-doped/Sn-doped CoSb3 alloys are synthesized and their thermoelectric properties are measured within 300K-700K.

3:40 PM Break

4:00 PM  Invited
Phase Stability and Vacancy-site Occupation of Half-Heusler Compounds in Multi-component System M(Ni, X)Sn (M: Ti, Zr, Hf and X: Co, Ir): Yoshisato Kimura1; Yaw Wang Chai1; 1Tokyo Institute of Technology
    Half-Heusler compounds are well-known high potential thermoelectric materials which can be used at relatively high temperature. We focus on the multi-component M(Ni,X)Sn (M: Ti, Zr, Hf and X: Co, Ir) system to systematically control not only excellent thermoelectric properties but also the conduction type between n and p. Objective of the present work is to understand distinctive characteristic lattice defects and interfaces formed in Half-Heusler M(Ni,X)Sn alloys, and to evaluate their effects on thermoelectric properties. These lattice defects and interfaces are formed due to the close relationship between Half-Heusler and Heusler (M(Ni,X)2Sn) in terms of crystallographic and thermodynamic similarity. Slightly Ni-rich composition of Half-Heusler leads to the nano-scaled Heusler precipitation which affects thermoelectric properties. On the other hand, Co and Ir atoms are soluble in the vacancy-site to some extent while the conduction behavior changes from n- to p-type, where M is Zr and Ti.

4:20 PM  Invited
Thermoelectric Enhancement of Cu2Se by CuInSe2 Incorporation: Pierre Ferdinand P. Poudeu1; 1University of Michigan
    The intrinsic coupling of a material’s electronic and thermal properties has long been a limitation of thermoelectric energy conversion. Recent developments have challenged this obstacle by introducing concepts such as nanostructuring, resonant level impurities, and electron energy filtering. It has been previously reported that Cu2-xSe can reach a maximum zT of 1.5 at 1000K, due to low lattice thermal conductivity arising from liquid-like behavior of copper ions and superior electronic transport provided by the rigid selenium framework. Here we show that introducing CuInSe2 nanoparticles into a bulk Cu2Se matrix significantly increase the zT to 2.2 at 850K. At low concentrations of indium, a simultaneous increase in electrical conductivity and reduction of thermal conductivity is observed, suggesting that indium acts as a dopant further disrupting phonon transport, while increasing carrier mobility. This work demonstrates the importance of multicomponent systems as a means to strategically increase thermoelectric efficiency.

4:40 PM  Invited
Phase Diagram of Ternary Cu-Ga-Te System and Thermoelectric Properties of Chalcopyrite CuGaTe2 Materials: Hsin-jay Wu1; Zong-jin Dong2; 1 National Sun Yat-sen University; 2National Sun Yat-sen University
    Chalcopyrite CuGaTe2 has been viewed as a promising candidate for use of thermoelectric generator. The 923K isothermal section of ternary Cu-Ga-Te system is determined, and ternary CuGaTe2 phase is stabilized within the compositional region of 48.0-53.0at%Te and 25.0at%-30.0at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe2, provides options for precisely locating the compositions of CuGaTe2-based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu25Ga26Te49 alloy at 750K, which is nearly eight times higher than the neighboring Cu28Ga25Te47 alloy, presumably due to the fact that the Cu25Ga26Te49 alloy, which exhibits high phase purity of CuGaTe2, has lower lattice thermal conductivity (κL~1.3 (W/mK) and higher power factor (PF~11.2 (μW/mK2)), comparing with than that of Cu28Ga25Te47 alloy (κL~1.8 (W/mK) and PF~1.9 (μW/mK2)), which locates in a three-phased CuGaTe2+Cu2Te+Cu9Ga4 region, with only a slight deviation in the starting composition.

5:00 PM  
Understanding the Role of Secondary Phases in Enhancing the Figure-of-Merit in Ge-Sb-Te Alloys: Jared Williams1; Donald Morelli1; 1Michigan State University
    Thermoelectric materials are the leading candidate today for applications in solid-state waste-heat recovery applications. Novel research and engineering has pushed the figure-of-merit (ZT), and overall conversion efficiency, of these materials to values which can be deemed practical for commercialization. However, many of the state-of-the-art thermoelectric materials utilize elements which are expensive and/or toxic, such as Ag, Pb, Tl, and Cd. Alloys of GeTe and Sb2Te3 were first explored for their applications in phase-change memory, and recently have been identified as materials with ZT much greater than unity. This presentation describes the effects of various materials as a secondary phase on the transport properties of Ge17Sb2Te20, a known phase-change material. We find that these secondary phases act as electron filters thereby enhancing power factor values. The thermal conductivity remains relatively unchanged, which results in ZT values in excess of 2 for a number of secondary phases.

5:20 PM  
Transient Liquid Phase Bonding of Cu/Ga/Ni and Cu/Ga/Co and Phase Equilibria of Cu-Ga-Ni and Co-Cu-Ga Ternary Systems: Ji-min Lin1; Sinn-wen Chen1; Tsu-ching Yang1; 1National Tsing Hua University
    Thermoelectric modules have attracted a lot research interests because they can convert heat directly into electricity, and have great potential in waste heat recovery. Usually thermoelectric modules are composed of arrays of P-N thermoelectric devices, and there are thus many joints in thermoelectric modules. Transient liquid phase bonding using Ga is a potential joining technique for thermoelectric models which are joined at lower temperatures and can be used at higher temperatures. The interfacial reactions at the Cu/Ga/Ni and Cu/Ga/Co contacts are examined at 200°C, 350°C and 500°C. For one hour reaction, a 23-µm-thick Cu9Ga4 phase layer is formed at the Cu/Ga contact when reacted at 350°C while a 13-µm-thick CuGa2 phase layer is formed at 200°C. A 38-µm-thick CoGa3 phase layer is formed at the Ga/Co contact when reacted at 350°C for 1 hour. The reaction paths are determined, and are illustrated with the determined Cu-Ga-Ni and Co-Cu-Ga isothermal sections.