Enhanced thermoelectric properties of Hf-free half-Heusler compounds prepared via highly fast process

Abstract

Hf-free n-type half-Heusler with a nominal composition of Ti0.5Zr0.5NiSn0.98Sb0.02 has been reported to have a high ZT value of almost 1.2. However, the synthesis process requires a long annealing time to achieve single-phase structure, which contributes to high product costs due to energy and time consumption. Here we introduce a new route to prepare (Ti0.5Zr0.5)1−xNbxNiSn (x = 0, 0.0050, 0.0075, 0.0100, 0.0125, 0.0150, 0.0175 and 0.0200) compounds for high thermoelectric (TE) performance along with shortening time for sample preparation. The samples were prepared by a combination of arc-melting (AM) and melt-spinning (MS) followed by spark plasma sintering process (SPS). The combination of these synthetic methods produced (Ti0.5Zr0.5)1−xNbxNiSn samples with high chemical homogeneity, single-phase structure, and fine grain about 300 nm in size, which are preferred for both charge and phonon transport properties. As a result, a maximum power factor of 44.5 µW cm−1 K−2 at 817 K and a maximum ZT of 1.19 at 874 K were achieved for the sample with x = 0.015, which are comparable to the highest ZT value reported so far for the Hf-free n-type MNiSn (M = Ti, Zr) compounds. The calculated output power density Pd and efficiency η based on a single-leg device showed an excellent performance, which yields the maximum Pd of 16.2 W cm−2 and η of 12.08% at the cold side temperature TC ≈ 305 K and the hot side temperature TH ≈ 875 K for the optimized composition with x = 0.0125. Furthermore, it is noted that the synthetic process here does not require a long-annealing time and it can be easily applied to mass production.