Synthesis and Thermoelectric Properties of ZrxTi1−xNiSn0.98Sb0.02 n-Type Half-Heusler Materials

Abstract

Hf-free ZrxTi1−xNiSn0.98Sb0.02 (x = 0.25, 0.5, 0.75) n-type half-Heusler (HH) thermoelectric materials were synthesized by a serial processing method including induction melting (IM), annealing, ball milling, and spark plasma sintering (SPS). For comparison, a Hf-containing half-Heusler Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 ingot was also alloyed by arc melting, and the effects of Hf on the thermoelectric properties were estimated. The ZrxTi1−xNiSn0.98Sb0.02 HH materials were nearly pure according to the x-ray diffraction analysis, but microscopic investigation revealed impurity phase inclusions of unalloyed Sn, Zr, and Ti. The power factor (PF) of the Hf-free HH materials reached the maximum value of 4.31 mWm−1 K−2 at 823 K in Zr0.75Ti0.25NiSn0.98Sb0.02, which was higher than Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (4.01 mWm−1 K−2 at 773 K) in this study. However, the thermal conductivity of the Hf-free samples was significantly higher, by which the maximum dimensionless figure of merit was slightly lower (ZTmax = 0.92 in Zr0.75Ti0.25NiSn0.98Sb0.02 at 873 K) than that of Hf0.25Zr0.25Ti0.5NiSn0.98Sb0.02 (ZTmax = 1.03 at 873 K). The thermal conductivity was decomposed into lattice and electronic contributions, and the possible correlation with Ni off-stoichiometry is discussed.