Thermoelectric Properties of Nearly Single-Phase Half-Heusler NbCoSn Alloys and Importance of Microstructures for Improv
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1044-U07-06
Thermoelectric Properties of Nearly Single-Phase Half-Heusler NbCoSn Alloys and Importance of Microstructures for Improving Performance Yoshisato Kimura1, Yukio Tamura1, and Takuji Kita2 1
Materials Science and Engineering, Tokyo Institute of Technology, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan 2
Toyota Motor Corporation, 1200 Mishuku, Susono, Shizuoka, 410-1193, Japan
ABSTRACT In order to evaluate the thermoelectric properties of n-type half-Heusler compound NbCoSn, single-phase NbCoSn alloy was prepared using directional solidification based on the phase diagram information of the Nb-Co-Sn ternary system which was investigated in this work. The isotherm at 1273 K and the reaction scheme together with the projection of liquidus surface were determined. Thermoelectric properties of NbCoSn based alloys were evaluated by Seebeck coefficient, electrical resistivity and power factor, which were measured in a temperature range from 300 K to 1073 K. NbCoSn has excellent Seebeck coefficient exceeding -250 µV/K at around 900 K and shows the metal-like temperature dependence of electrical resistivity. INTRODUCTION Thermoelectric power generation can help to improve the efficiency of power generation systems which helps to protect and to conserve the global environment. Our interest is especially placed on the direct conversion of the waste heat to clean electric energy, for instance, those which are usable in automobiles, airplane and so forth. Half-Heusler compounds with valence electron count around 18 show semi- conducting behavior, and thus they are promising candidates of thermoelectric materials [1-10]. The authors of this paper have been conducting a systematic investigation aimed to design and develop high-temperature thermoelectric materials based on half-Heusler compounds such as n-type MNiSn (M = Zr, Hf), n-type TiPtSn, p-type MPtSn (M = Zr, Hf) and so forth [4-6]. Single-phase alloys, including nearly single-crystals, were successfully grown for all of above mentioned half-Heusler compounds by directional solidification using the optical floating zone melting method (OFZ-DS). Therefore we were able to evaluated their actual thermoelectric properties and to prove that half-Heusler compounds have high potential as thermoelectric materials [4-6]. Information from a phase diagram, measured in this study, has been used to determine optimal alloy compositions to fabricate single-crystals and single-phase alloys of the intermetallic compounds of interest. The traveling solvent float zone method enables us to grow
single-crystals of a phase which forms through a peritectic reaction if we know a compositional range where the target phase is the primary phase to solidify. One good example is a half-Heusler compound NbCoSn which is known to have n-type thermoelectric properties and is usable at high temperatures around 1000 K [1,2]. In the present work, the authors have focused on n-type half-Heusler NbCoSn for which the fabrication of single-phase alloys using OFZ-DS is very difficult si
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