Thermoelectric Properties of Ta-doped Zr0.6-xTi0.4TaxNiSn n-type Half-Heusler Materials

Abstract

Half-Heusler (HH) thermoelectric materials are promising for mid- to high-temperature applications, and MNiSn (M = Ti, Zr, Hf) is a representative n-type HH alloy. In general, the M sites are mixed with isoelectronic elements Ti, Zr, and Hf, to lower the lattice thermal conductivity, and the Sn sites are doped with Sb to adjust the electron concentration. However, Hf is a rare element in earth’s crust, and the volatility of Sb makes it difficult to maintain the initial Sb amount during material synthesis. In this study, as an alternative, Ta was added in the M sites along with the host elements Ti and Zr to produce Hf-free Zr0.6-xTi0.4TaxNiSn alloys (0 ≤ x ≤ 0.04), and the effects of Ta doping on the thermoelectric properties were analyzed. The electrical conductivity of Zr0.6-xTi0.4TaxNiSn increased with Ta content, and the electron concentration increased almost linearly, reaching 5.6 × 1020 cm -3 at x = 0.04. By adding Ta, the maximum power factor also increased by approximately 17% to 3.94 × 10-3 W m-1K-2 at x = 0.02. The lowest lattice thermal conductivity (κlat) was observed at x = 0.02, reaching approximately 1.9 W m-1K-1 at 723 K, but overall, a dramatic decrease in κlat was not observed with Ta doping in Zr0.6-xTi0.4TaxNiSn. This is probably due to the existing effect of Zr/Ti mixing at the M sites, which enhances phonon scattering. A maximum figure of merit (zT) of 0.91 was obtained in Zr0.58Ti0.4Ta0.02NiSn at 873 K, which is a high value for ZrNiSn-based Hf-free HH materials. In conclusion, Ta doping is a viable method to replace Sb doping in ZrNiSn-based HH alloys.