Thermoelectric performance enhancement of Ti0.75NiSb half-Heusler alloy via Co substitution and interfacial phase regulation

The 18-electron Ti 0.75 NiSb compound exhibits intrinsically low lattice thermal conductivity due to the presence of numerous Ti vacancies. However, the high carrier concentration limits further improvement of the thermoelectric performance. In this work, Co atoms are employed to substitute Ni atoms in Ti 0.75 Ni 1- x Co x Sb. The results show that Co substitution effectively tunes the valence electron count of Ti 0.75 NiSb and leads to the formation of a NiSb-based metallic impurity phase, which significantly reduces the carrier concentration from 8.04 × 10 21  cm −3 to 2.31 × 10 21  cm −3 . Consequently, the Seebeck coefficient increases significantly from −94 μV/K for the x  = 0 sample to −189 μV/K for x  = 0.5, while the electronic thermal conductivity drops substantially from 3.52 to 0.87 W/mK at 873 K. Meanwhile, the lattice thermal conductivity increases due to the reduced Ti vacancy concentration (to satisfy the 18-valence-electron rule) and the formation of the metallic impurity phases. Ultimately, a peak zT of ∼0.50 is achieved at 873 K, representing a ∼47% enhancement compared with the Ti 0.75 NiSb alloy. This work demonstrates that the combination of valence electron count modulation and interface engineering is an effective strategy to optimize the thermoelectric performance of half-Heusler systems. Download: Download high-res image (378KB) Download: Download full-size image