Studying half-Heusler based materials for thermoelectric application

Abstract

Since the fossil fuels are limited, and are fast depleting, there is an urgent need to search for alternative energy sources that are, both, clean and economically viable. Thermoelectric (TE) devices can play a crucial role in this direction as they have the potential to convert "waste heat" into electricity. Here, the term "waste heat" refers to the heat which is generated by the exhaust of an industrial chimney or an automobile, or even the microprocessor chip of a laptop. Tapping waste heat and converting it into useful energy using TE devices is a seemingly good idea; however, its widespread use is limited by the low efficiency of the current TE devices, which makes the energy production cost economically unviable. To come out of this conundrum it is, therefore, necessary to search for new TE materials to improve the conversion efficiency. In this work, we investigate the half-Heusler (HH) compounds TaCoSn and NbFeSb for their potential as p-type thermoelectrics. Using theoretical calculations it was shown that TaCoSn should crystallize in a HH structure with superior TE properties. However, as of today, there are no reports on the single phase synthesis of TaCoSn. We studied the possibility of HH phase formation in TaCoSn using the solid state reaction route under various synthesis conditions. On the other hand, NbFeSb is known to crystallize with the HH structure. It also shows promising TE properties, which we plan to improve further by doping at the Fe site which has not been explored that well. The sign of thermopower of undoped NbFeSb shows a sample dependence. We have, therefore, investigated the effect of Nb off- stoichiometry on the thermopower and TE properties. In a bid to increase the TE performance, we also investigated the effect of Ru doping in Nb0.86Hf0.14FeSb. The highest TE figure of merit (ZT) of 0.88 near 900K.