High-Performance of Half-Heusler MNiSn (M=Hf,Zr) Single-Phase Thermoelectric Alloys Fabricated using Optical Floating Zo
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0886-F09-02.1
High-Performance of Half-Heusler MNiSn (M=Hf,Zr) Single-Phase Thermoelectric Alloys Fabricated using Optical Floating Zone Melting Yoshisato Kimura1, Tomoya Kuji2,3, Akihisa Zama2, Yasufumi Shibata4 and Yoshinao Mishima1 1 Tokyo Institute of Technology, Materials Science and Engineering, 4259-G3-23, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 2 Graduate student, Tokyo Institute of Technology, Materials Science and Engineering. 3 now with Hitachi Cable, Ltd., 5-1-1 Hidaka, Hitachi, Ibaraki 319-1414, Japan 4 Toyota Motor Corporation, Higashifuji Technical Center, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan. ABSTRACT We have succeeded to grow almost single-phase of Half-Heusler intermetallic compounds MNiSn, where M = (Hfx ,Zr1-x) and x varies from 0 to 1, for the first time by directional solidification using optical floating zone melting (OFZ). Thermoelectric power and electrical resistivity can be dramatically improved since OFZ process effectively reduces solidification defects such as micro-cracks and cavities as well as unfavorable coexisting phases. Dimensionless thermoelectric figure of merit, ZT, of OFZ (Hf,Zr)NiSn alloys can be improved effectively by lowering the lattice thermal conductivity through the solid solution effects due to the substitution of Hf and Zr with each other. The maximum ZT value of 0.9 is achieved in (Hf0.5Zr0.5)NiSn at 963 K. INTRODUCTION Thermoelectric materials allow us to directly convert waste heat to clean electric energy without sacrificing the environment. Half-Heusler type intermetallic compounds are attractive candidate thermoelectric materials that are applicable at high temperatures up to around 1000 K. The ordered crystal structure of Half-Heusler, ABX, consists of four fcc sublattices of each element A, B, X and vacancy, where A and B are transition metals, and X is usually Sn, Sb and so forth. It is well known that Half-Heusler MNiSn (M=Hf,Zr,Ti) compounds exhibit n-type semi-conducting behavior and excellent thermoelectric properties, high thermoelectric power and low electrical resistivity [1-11]. We have conducted a series of investigations of Half-Heusler compounds: designing and developing (Ti,Hf)Ni(Sn,Sb) based alloys and seeking widely for both n- and p-type promising candidates of Half-Heusler compounds. [3-5]. A drawback of relatively high thermal conductivity can effectively be improved, for instance, by the solid solution effects due to substituting Hf, Zr and Ti with each other on the M-site [6,7,9,10,11]. Lots of efforts to improve thermoelectric properties have been performed through the optimization of carrier concentrations, for instance, by the doping of Sb on the Sn site [2,6,10,11]. Recently, ZT values of exceeding 1.5 were reported for multi-component system of (Hf,Zr,Ti)Ni(Sn,Sb) [10].
0886-F09-02.2
In the present work, the fabrication of Half-Heusler MNiSn (M=Hf,Zr) single-phase alloys, or alloys consisting of almost single-phase, was conducted by the traveling solvent floating zone melting method using optical floating zone (
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