Effect of Processing Route and Antimony Content on the Thermoelectric Properties of Nb₂FeCoSnSb Double Half-Heusler Alloy

The concept of high entropy has been explored in half-Heusler alloys by either introducing multiple elements to a half-Heusler (HH) alloy or combining 2 half-Heusler alloys to form a double half-Heusler (DHH) alloy. The thermoelectric properties of HH alloys have been well studied, but very little work has been performed on DHH alloys. Here DHH Nb₂FeCoSnSb was synthesized using arc melting + milling + sintering (VAM-DM-SPS) as well as arc melting + melt spinning + sintering (VAM-RSP-SPS) routes to study the effect of processing conditions. The effect of Sb content was also investigated. The VAM-RSP-SPS processing led to coarser grains and higher HH phase content. The lower the Sb content, the greater the Fe₂Nb phase content, resulting in higher electrical conductivity at the expense of thermopower. The VAM-DM-SPS samples, despite the finer grain size, exhibited higher thermal conductivity due to their higher metallic phase content. The Nb₂FeCoSnSb sample had a figure of merit (zT) of 0.138 at 873 K which was close to that of the zT of Ti₂NiCoSnSb. The Nb₂FeCoSn_0.5Sb_1.5 sample synthesized by the VAM-RSP-SPS route had the highest zT of 0.15 at 823 K, owing to its low thermal conductivity.