Enhanced thermoelectric performance by Hf substitution in p-type half-Heusler TiNi0.8Co0.2Sn

The half-Heusler TiNiSn alloy is a promising candidate for thermoelectric power generation, capable of directly converting waste heat into electric power, due to its high thermoelectric performance over a wide temperature range from 500 to 1000 K. However, the thermoelectric performance of p-type TiNiSn is much lower than that of its n-type counterpart. Here, we demonstrate that Hf substitution in the p-type half-Heusler TiNi0.8Co0.2Sn alloy significantly enhances thermoelectric performance within the miscibility gap region of the binary TiNiSn-HfNiSn phase diagram. The Seebeck coefficient, which is below 80 μV/K for TiNi0.8Co0.2Sn, is substantially improved to 170 μV/K in Ti0.5Hf0.5Ni0.8Co0.2Sn at 650 K, owing to modifications in the electronic band structure induced by Hf substitution. Moreover, the lattice distortion and point defects introduced by Hf substitution effectively reduce thermal conductivity, from 5.0 W/mK for TiNi0.8Co0.2Sn to 3.3 W/mK for Ti0.5Hf0.5Ni0.8Co0.2Sn at 900 K. Consequently, the thermoelectric figure of merit (ZT) significantly increases from 0.03 for TiNi0.8Co0.2Sn to 0.26 for Ti0.5Hf0.5Ni0.8Co0.2Sn at 800 K. This enhancement of thermoelectric performance in p-type TiNiSn enables the construction of thermoelectric modules composed solely of half-Heusler TiNiSn based alloys, which are expected to exhibit high thermal stability useful for long-term thermoelectric power generation.