Ultralow lattice thermal conductivity and improved thermoelectric performance in a Hf-free half-Heusler compound modulated by entropy engineering

Half-Heusler compounds can potentially be applied to medium- to high-temperature power generation. However, most of them have relatively high thermal conductivity, which is considered to be a serious obstacle to improving their thermoelectric performance. Herein, a great improvement in the thermoelectric and mechanical properties of a TiNiSn-based compound has been achieved by entropy engineering. By increasing the configurational entropy, the carrier concentration has been optimized markedly. Meanwhile, the low levels of deformation potential coefficient and alloy scattering potential facilitate a high carrier mobility. The synergistic improvement in carrier concentration and carrier mobility leads to a significant increase in electrical conductivity, thereby enhancing the power factor. Additionally, the introduction of a highly disordered microstructure, in which phase separation, dense dislocations, nanoprecipitates, lattice distortions and point defects are observed, can provide multi-scale phonon scattering centers, and hence a minimum lattice thermal conductivity of 0.48 W m⁻¹ K⁻¹ at 870 K is obtained in Ti_0.57Zr_0.4Al_0.02Ta_0.01NiSn_0.98Sb_0.02. Finally, these favorable factors contribute to a high peak zT of ∼1.4 at 870 K for the half-Heusler alloy without the addition of Hf. Moreover, compared with the pristine TiNiSn, the Vickers microhardness and compressive strength have increased by 20.3% and 82.8%, respectively. This work indicates that the advantages of entropy engineering in improving the overall performance of half-Heuslers are considerable.