Diffusion Paths for the Formation of Half-Heusler Type Thermoelectric Compound TiNiSn
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1128-U05-08
Diffusion Paths for the Formation of Half-Heusler Type Thermoelectric Compound TiNiSn Chihiro Asami1, Yoshisato Kimura1, Takuji Kita2 and Yoshinao Mishima1 1 Tokyo Institute of Technology, Materials Science and Engineering, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. 2 Toyota Motor Corporation, Higashifuji Technical Center, 1200 Mishuku, Susono, Shizuoka 410-1193, Japan. ABSTRACT Diffusion paths for the formation of the Half-Heusler phase TiNiSn were determined using solid/liquid diffusion couples. The most interesting result is that a single-phase TiNiSn layer forms at the TiNi/Sn(L) interface during annealing at 1073 K for only 1 h. Moreover, faceted grains of TiNiSn single-crystals grow at the interface towards the liquid Sn presumably by the solidification during isothermal holding at 1073 K. The TiNiSn phase layer which forms on the TiNi side of the interface consists of very fine sub-micron grains, and faceted TiNiSn single-crystal grains which form on the other side of the interface all have the same crystallographic orientation. While only TiNiSn forms at the TiNi/Sn(L) interface, the Heusler phase TiNi2Sn also forms with TiNiSn at the TiNi3/Sn(L) interface and Ti6Sn5 is observed at the Ti2Ni/Sn(L) interface. INTRODUCTION The Half-Heusler compound TiNiSn is one of the most promising candidates for thermoelectric materials which can be used to directly convert waste heat to clean electric energy at high temperatures (around 1000 K) [1-3]. Half-Heusler compounds have the cubic C1b type ordered structure denoted as ABX which consists of four interpenetrating fcc sub-lattices of the elements A, B, X and vacancies [4]. Of these compounds TiNiSn is the most attractive one because not only it has excellent thermoelectric properties but also it consists of eco-friendly elements which are neither toxic nor costly. However, for TiNiSn the problem that fabrication of single-phase TiNiSn alloys is quite difficult since the melting temperature of Sn, 505 K, is much lower than that of the other constituent elements. Therefore it is hard to avoid heterogeneous solidification and hence the formation of coexisting impurity phases which tend to degrade the thermoelectric properties of the TiNiSn alloy. The isothermal section at 770 K reported by Stadnyk et al. is the only available phase diagram of the ternary Ti-Ni-Sn system [5]. There exist two ternary compounds, Half-Heusler TiNiSn and Heusler TiNi2Sn, which are in equilibrium with each other. Both are nearly line compounds having quite restricted off-stoichiometric composition ranges. Existing phase diagram information is insufficient to understand the formation of TiNiSn. We fabricated nominally stoichiometric TiNiSn alloys using arc melting and subsequent annealing at 1073 K for 2 weeks [2], though, considerable amounts of metallic Ti6Sn5 and Sn remain as impurity phases. Since TiNiSn single-phase alloy fabrication requires long annealing times and a tremendous amount of energy, we believe it is necessary to develop new fabrication processes.
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