Novel Synthesis and Processing Effects on the Figure of Merit for NbCoSn, NbFeSb, and ZrNiSn Based Half-Heusler Thermoelectrics

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The n-type NbCoSn, ZrNiSn and p-type NbFeSb half-Heuslers are promising thermoelectric materials with high figures of merit (ZT) for applications between 575 and 1000 K. By eliminating hafnium and utilizing low cost scalable production methods, the widespread commercialization of high temperature thermoelectrics could be achieved. In this work, half-Heuslers containing refractory metals (Nb and Zr) were produced by high energy planetary ball milling and their thermoelectric performance compared to half-Heuslers prepared by the conventional arc-melting synthesis route. Starting materials, as-milled powders, and consolidated samples were characterized by XRD, SEM, and EDS. The thermoelectric performance parameters were characterized using simultaneous Seebeck and electrical resistivity measurements and laser flash analysis. The mechanically alloyed NbCoSn and NbFeSb achieved ZT of approximately 0.53 (873 K) and 0.72 (873 K), respectively, comparing well to literature and prepared arc-melted monoliths. The results demonstrate mechanical alloying as a viable synthesis technique for refractory metal containing half-Heusler thermoelectrics.