Enhanced thermoelectric performance of n-type TiCoSb half-Heusler by Ta doping and Hf alloying

The p-type TiCoSb-based half-Heuslers are widely studied due to the good electrical transport properties after hole doping, while the pristine TiCoSb is intrinsically n-type. It is thus desired to obtain a comparable n-type counterpart through optimization of electron concentration. In this work, n-type Ti_0.9−xHf_xTa_0.1CoSb half-Heuslers were fabricated by arc melting, ball milling, and spark plasma sintering. An optimized carrier concentration, together with a decreased lattice thermal conductivity, was obtained by Ta doping at the Ti site, leading to a peak figure of merit (ZT) of 0.7 at 973 K in Ti_0.9Ta_0.1CoSb. By further alloying Hf at the Ti site, the lattice thermal conductivity was significantly reduced without deteriorating the power factor. As a result, a peak ZT of 0.9 at 973 K and an average ZT of 0.54 in the temperature range of 300–973 K were achieved in Ti_0.6Hf_0.3Ti_0.1CoSb. This work demonstrates that n-type TiCoSb-based half-Heuslers are promising thermoelectric materials.