Spinodal decomposition promoting high thermoelectric performance in half-Heusler

Spinodal decomposition typically manifests in partially miscible solid solutions in relevant phase diagrams as primarily dictated by the underlying thermodynamics, which is viewed as a powerful means for enhancing thermoelectric performance. Yet, the incomplete ternary phase diagrams of thermoelectric materials pose a challenge for microstructure design via spinodal decomposition. In addition, experimental investigation of microstructure evolution upon spinodal decomposition in thermoelectric alloys is rare, and its influence on electron and phonon transport remains largely unexplored. Herein, we constructed the (Ti, Zr, Hf)NiSn phase diagram experimentally, revealing a miscibility gap within 973–1,273 K. Spinodal decomposition with anisotropic composition modulation was observed in Ti_0.5Zr_0.25Hf_0.25NiSn_0.99Sb_0.01 by in situ transmission electron microscopy. The phase-field simulation further elucidates the microstructure evolution upon spinodal decomposition and provides insights into the generation of dislocations during further heat treatment. The annealing process not only induces dense dislocation arrays formed by spinodal evolution but also homogenizes the multiphase to facilitate electron transport. Consequently, a record-high average zT of ∼1.1 between 300 and 973 K has been realized in n-type Ti_0.5Zr_0.25Hf_0.25NiSn_0.99Sb_0.01. Importantly, the half-Heusler module achieves a maximum conversion efficiency of ∼12% and an output power density of ∼3.7 W cm⁻² at a temperature difference of 653 K. This “double-high” result outperforms all of the current devices. Our results highlight spinodal decomposition as an effective avenue to advance materials for highly efficient thermoelectric power generation.