Thermoelectric Properties of Half-Heusler Compounds N-type MNiSn and P-type MPtSn (M = Hf, Zr)
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0980-II04-03
Thermoelectric Properties of Half-Heusler Compounds N-type MNiSn and P-type MPtSn (M = Hf, Zr) Yoshisato Kimura1, Tomoya Kuji2, Akihisa Zama2, Taiki Lee2, and Yoshinao Mishima1 1
Materials Science and Engineering, Tokyo Institute of Technology, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan 2
Graduate student, Materials Science and Engineering, Tokyo Institute of Technology, 4259-G3-23 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan ABSTRACT To design and to develop Half-Heusler based high-temperature thermoelectric materials, thermoelectric properties of n-type MNiSn and p-type MPtSn (M = Hf, Zr) were investigated based on two respective strategies. For the n-type (Hf, Zr)NiSn, a combined process of optical floating zone melting and hot-pressing was applied aiming to reduce thermal conduction through the lattice contribution. For the p-type HfPtSn, power factor and hence figure of merit ZT were dramatically improved by the p-type doping of Ir and Co targeting for Pt-site, which effectively lower electrical resistivity. The additions of Ir and Co are expected not only to increase carrier concentration but also to suppress the lattice thermal conduction by substituting for Pt. INTRODUCTION Thermoelectric materials can directly convert waste heat to clean electric energy. Half-Heusler type intermetallic compound is one of attractive thermoelectric materials that are applicable at elevated temperatures up to around 1000 K. It is well known that Half-Heusler MNiSn (M= Hf, Zr, Ti) with the valence electron count (VEC) of 18 shows excellent n-type thermoelectric properties, especially high Seebeck coefficient and low electrical resistivity [1-6,8-10]. Recently, we have found that Half-Heusler MPtSn (M = Hf, Zr) with the same VEC of 18 exhibits p-type thermoelectric properties [7]. To fabricate thermoelectric modules used at high temperatures, n-type and p-type materials should be designed on the same compounds with similar physical and chemical properties. We are focusing on the design to develop Half-Heusler thermoelectric materials of both n-type MNiSn and p-type MPtSn. We fabricated nearly single-phase Half-Heusler alloys by unidirectional solidification using the optical floating zone melting (OFZ) method, which is effective to improve thermoelectric properties not only because of single-phase microstructure but also of suppression of solidification defects such as micro cracks and cavities. The power factor of OFZ alloys is improved about four times higher than those counterparts prepared by arc-melting, hot-pressing and spark plasma sintering. The highest
value of dimensionless figure of merit ZT = 0.9 was achieved in (Hf0.5Zr0.5)NiSn at 963 K. It is possible to improve electrical properties through the optimization of carrier concentration by doping elements. A typical drawback of Half-Heusler compounds is relatively high thermal conductivity which can be improved by the solid solution effects through substituting constituent elements and introducing interfaces such as grain boundaries
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