Enhanced Thermoelectric Performance of Zr_1–xTa_xNiSn Half-Heusler Alloys by Diagonal-Rule Doping

Although Sb doping is regarded as the most effective method to regulate the carrier concentration within the optimum range for ZrNiSn-based half-Heusler (HH) alloys, the resulting thermal conductivity remains high. Hence, the aim of this study was to investigate the effect of "diagonal-rule" doping; that is, the Zr site was displaced by Ta, which can simultaneously enhance the electrical conductivity and reduce the lattice thermal conductivity. The solid-solubility limit of Ta in the ZrNiSn matrix was determined to be x = 0.04. The highest ZT, 0.72, was achieved at 923 K for Zr_0.98Ta_0.02NiSn. In addition, ZT_avg increased by 10.2% for Zr_0.98Ta_0.02NiSn compared with that for ZrNiSn_0.99Sb_0.01 at 873 K, which was mainly attributed to the reduced lattice thermal conductivity of Zr_0.98Ta_0.02NiSn. These results suggest that Ta doping is more effective than Sb doping in ZrNiSn-based HH alloys. In addition, the microhardness of Zr_1-xTa_xNiSn was substantially improved with increasing Ta content and was also much higher than that of other traditional thermoelectric materials.