Synthesis and Thermoelectric Properties of Zr_xTi_1−xNiSn_0.98Sb_0.02 n-Type Half-Heusler Materials

Hf-free Zr_xTi_1−xNiSn_0.98Sb_0.02 (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 Hf_0.25Zr_0.25Ti_0.5NiSn_0.98Sb_0.02 ingot was also alloyed by arc melting, and the effects of Hf on the thermoelectric properties were estimated. The Zr_xTi_1−xNiSn_0.98Sb_0.02 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 Zr_0.75Ti_0.25NiSn_0.98Sb_0.02, which was higher than Hf_0.25Zr_0.25Ti_0.5NiSn_0.98Sb_0.02 (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 (ZT_max = 0.92 in Zr_0.75Ti_0.25NiSn_0.98Sb_0.02 at 873 K) than that of Hf_0.25Zr_0.25Ti_0.5NiSn_0.98Sb_0.02 (ZT_max = 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.