NbFeSb based p-type half-Heusler for power generation applications*

We report a peak dimensionless figure-of-merit (ZT) of \textasciitilde 1 at 700 $^{o}$C in nanostructured p-type Nb$_{0.6}$Ti$_{0.4}$FeSb$_{0.95}$Sn$_{0.05\, }$composition. Even though the power factor of the Nb$_{0.6}$Ti$_{0.4}$FeSb$_{0.95}$Sn$_{0.05}$ composition is improved by 25{\%} in comparison to the previously reported p-type Hf$_{0.44}$Zr$_{0.44}$Ti$_{0.12}$CoSb$_{0.8}$Sn$_{0.2}$, the ZT value is not increased due to a higher thermal conductivity. However, the higher power factor of the Nb$_{0.6}$Ti$_{0.4}$FeSb$_{0.95}$Sn$_{0.05}$ composition led to a 15{\%} increase in power output of a thermoelectric device in comparison to a device made from the previous best material Hf$_{0.44}$Zr$_{0.44}$Ti$_{0.12}$CoSb$_{0.8}$Sn$_{0.2}$. The n-type material used to make the unicouple device is the best reported nanostructured Hf$_{0.25}$Zr$_{0.75}$NiSn$_{0.99}$Sb$_{0.01}$ composition with the lowest hafnium (Hf) content. Both the p- and n-type nanostructured samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. Moreover, the raw material cost of the Nb$_{0.6}$Ti$_{0.4}$FeSb$_{0.95}$Sn$_{0.05}$ composition is more than six times lower compared to the cost of the previous best p-type Hf$_{0.44}$Zr$_{0.44}$Ti$_{0.12}$CoSb$_{0.8}$Sn$_{0.2}$. This cost reduction is crucial for these materials to be used in large-scale quantities for vehicle and industrial waste heat recovery applications.