Enhancing Thermoelectric Performances of Hf-Free ZrNiSn-Based Half-Heusler Alloys by Multisite Element Modulation

Hf alloying at the Zr site is an effective method to suppress the lattice thermal conductivity (κ_l) toward high thermoelectric (TE) performance for ZrNiSn-based half-Heusler TE materials. However, the high cost of the Hf element limits its large-scale application. Herein, we propose the multisite element modulation strategy for enhancing TE performances. At the initial stage, Nb-Ni codoping shifts the Fermi level into the conduction band, and the power factor increases by 25.3% over the pristine ZrNiSn. Subsequently, Ge-Si codoping induces multiscale defects such as point defects, dislocations, and nanoprecipitates to reduce the κ_l. Benefit from the comprehensive optimization of the electric-thermal properties, Zr_0.96Nb_0.04Ni_1.04Sn_0.95Ge_0.025Si_0.025 obtains a figure of merit (zT) of ∼0.78 at 923 K, representing a 29.6% improvement, compared to that of pristine ZrNiSn. Furthermore, due to the solid-solution strengthening and the grain refinement, the Vickers hardness of the ZrNiSn-based half-Heusler alloys (HHs) is further enhanced as well. This work achieves the collaborative optimization of the TE and mechanical performances of TE materials, proposing a new strategy for the development of Hf-free ZrNiSn-based HHs.