Study on Thermoelectric Properties of N-type Half-Heusler Compounds



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Thermoelectric materials have been attracting intensive attentions for the ability of directly converting heat into electricity and vice versa, which can be potentially applied to waste-heat recovery and solid-state cooling. Among the various thermoelectric materials, half-Heusler compounds stand out for their high thermal stability, good mechanical strength, and low toxicity. In this dissertation, focus is placed on the n-type half-Heusler compounds. A series of experimental studies on the thermoelectric properties of n-type half-Heuslers are discussed, the goals of which consist of two aspects: (1) improving the thermoelectric properties of the conventional MNiSn (M refers to Hf, Zr, Ti or their combination) based n-type half-Heulsers; (2) exploring the thermoelectric properties of other unconventional half-Heulser compositions to search for new potential candidates for thermoelectric applications. Three strategies are employed to improve the thermoelectric properties of the nanostructured half-Heusler Hf0.25Zr0.75NiSn. First, doping with adjacent elements to optimize the electrical properties is carried out. Different from the conventional Sb doping at the Sn site which may be not stable at high temperatures due to the high vapor pressure of Sb, Nb, V, and Ta doping at the Hf/Zr site is studied. Enhanced thermoelectric performance is achieved. Second, alloy effects have long been used as an effective approach to reduce the thermal conductivity. Thus the effects of further alloying Ti at the Zr site in Hf0.25Zr0.75NiSn are studied. Finally, incorporating nanoinclusions is another way to improve the thermoelectric properties of a material. Here, InSb nanoinclusions are introduced into the half-Heulser matrix, and the results are discussed. Besides, the thermal stability of the Hf0.25Zr0.75NiSn0.99Sb0.01 sample is studied as well. To search for unconventional half-Heusler compositions, which may possess good thermoelectric properties, the half-Heusler compound VCoSb was studied at first. With VEC = 19, VCoSb is thought to be a metal, so it’s never been studied as a thermoelectric material before. Here, the thermoelectric properties of VCoSb are reported for the first time. Another two half-Heusler compounds VFeSb and YNiSb are synthesized and their thermoelectric properties are studied too. Doping in VFeSb and YNiSb is also tried to optimize their thermoelectric performance.



Thermoelectrics, Half-Heusler


Portions of this document appear in: Zhang, Hao, Yumei Wang, Keshab Dahal, Jun Mao, Lihong Huang, Qinyong Zhang, and Zhifeng Ren. "Thermoelectric properties of n-type half-Heusler compounds (Hf0. 25Zr0. 75) 1–xNbxNiSn." Acta Materialia 113 (2016): 41-47. And in: Zhang, Hao, Yumei Wang, Lihong Huang, Shuo Chen, Heshab Dahal, Dezhi Wang, and Zhifeng Ren. "Synthesis and thermoelectric properties of n-type half-Heusler compound VCoSb with valence electron count of 19." Journal of Alloys and Compounds 654 (2016): 321-326.