Entropy-driven multiscale defects enhance the thermoelectric properties of ZrCoSb-based half-Heusler alloys

As a new method of alloying, entropy engineering has proven to be an effective strategy to decrease the lattice thermal conductivity (κL). However, the majority of entropy engineering applications of half-Heusler alloys (HHs) are suboptimal. The poor Seebeck coefficients (S) and obscure underlying electron and phonon transport mechanisms of medium- and high-entropy HHs hinder the further optimization of their thermoelectric properties. Herein, a systematic synthesis of n-type ZrCoSb-based medium-entropy HHs is reported, along with a corresponding structural, theoretical, and thermoelectric study. It is demonstrated that the effectively decreased κL in medium-entropy HHs was mainly due to the scattering of atomic disorder, in addition to the vacancies, stacking faults, dislocations, and nano-domains/precipitates. Using density functional theory calculations, we attribute the lower S to the lower density-of-states (DOS) effective mass and the slowly changing DOS at Fermi level. By optimizing the spark plasma sintering temperature, an ultralow κL of 1.27 W m−1 K−1 was achieved in the Zr0.6(NbTa)0.4CoSb medium-entropy HH alloy at 923 K. In conjunction with the improved power factor, the highest peak figure-of-merit value of ∼0.42 was achieved for the Zr0.6(NbTa)0.4CoSb medium-entropy HH alloy. This study provides a guidance for the design and further optimization of the thermoelectric properties for medium-entropy HH alloys.