(V,Nb)-doped half Heusler alloys based on {Ti,Zr,Hf}NiSn with high ZT

Half Heusler alloys are among the most promising materials for thermoelectric generators as they can be used in a wide temperature range and their starting materials are abundant and cheap, the latter as long as no hafnium is involved. For Sb-doped Ti _0.5Zr _0.25Hf _0.25NiSn Sakurada and Shutoh in 2008 have published ZT _max = 1.5 at 690 K, a value that hitherto was never reproduced independently. In this paper we successfully prepared Ti _0.5Zr _0.25Hf _0.25NiSn with ZT _max = 1.5, however, at higher temperature (825 K). As the main goal is to produce hafnium – free half Heusler alloys, we investigated the influence of niobium or vanadium dopants on Ti _xZr _1−xNiSn _0.98Sb _0.02, reaching ZTs > 1.2 and thermal-electric conversion efficiencies up to 13.1%. For Hf-free n-type TiNiSn-based half Heusler alloys these values are unsurpassed. In order to further improve our thermoelectric materials our study is completed by electrical resistivity and thermal conductivity data in the low temperature range but also by mechanical properties (elastic moduli, hardness) at room temperature. The electrical properties have been discussed in comparison with DFT calculations.