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

• Published in: Journal of electronic materials 2022-07, Vol.51 (7), p.3485-3494
• Main Authors: Park, Gitae, Lee, Ho Seong, Yi, Seonghoon
• Format: Article
• Language: English
• Subjects: Annealing; Carrier density; Characterization and Evaluation of Materials; Chemistry and Materials Science; Defect annealing; Electric arc melting; Electrical resistivity; Electronics and Microelectronics; Instrumentation; Materials Science; Microstructure; Optical and Electronic Materials; Original Research Article; Reduction; Seebeck effect; Solid State Physics; Thermoelectricity; Zirconium
• ISSN: 0361-5235; 1543-186X
• DOI: 10.1007/s11664-022-09627-2

Half-Heusler M NiSn ( 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 0.26 Zr 0.37 Hf 0.37 NiSn. The Ti 0.26 Zr 0.37 Hf 0.37 NiSn 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 Ti 6 Sn 5 . 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 M NiSn 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