Effects of Annealing on the Microstructure and Thermoelectric Properties of Half-Heusler MNiSn (M = Ti, Zr, Hf)

Abstract: Half-Heusler MNiSn (M = Ti, Zr, Hf) shows a good thermoelectric performance in a medium temperature range of 500–1100 K. In this study, we investigate the effect of annealing on the microstructure and thermoelectric properties of half-Heusler Ti_0.26Zr_0.37Hf_0.37NiSn. The Ti_0.26Zr_0.37Hf_0.37NiSn composition which was designed to improve phonon scattering via mass difference and strain fluctuation. The samples were fabricated through arc melting and then annealed at 1173 K for 24–216 h. The as-cast sample showed a microstructure in which the Ti-rich and Ti-poor half-Heuslers were mixed with some secondary phases like Hf, Sn, and Ti₆Sn₅. The secondary phases disappeared, and the difference in the Ti fraction between the Ti-rich and Ti-poor half-Heuslers was reduced with increasing annealing time. In other words, the samples become homogeneous with annealing, leading to the reduction of the anti-site defects commonly observed in MNiSn alloys. This was confirmed by the increase of the band gap calculated from the Seebeck coefficient. The electrical conductivity decreased, and the Seebeck coefficient increased as the annealing time increased. This is attributed to the decrease of the carrier concentration caused by the reduction of the anti-site defects. The figure-of-merit value for the sample annealed for 216 h reached 0.74 at 773 K, showing an increase of approximately 20% compared with that of the as-cast sample. Graphical Abstract: [Figure not available: see fulltext.]