Maximizing the scattering of multiwavelength phonons in novel biphasic high-entropy ZrCoSb-based half-Heusler alloys

The thermoelectric(TE) performance of p-type ZrCoSb-based HH alloys has been improved tremendously in recent years; however, it remains challenging to find suitable n-type ZrCoSb-based HH alloys due to their high lattice thermal conductivity (κL). In this work, n-type Zr1−xTaxCo1−xNixSb half-Heusler (HH) alloys were firstly designed by multisite alloying. The evolution of the Raman peak proved that alloy scattering, phonon softening, anharmonicity, entropy-driven disorder, and precipitates had a combined effect on decreasing κL by 46.7% compared to that of pristine ZrCoSb. Subsequently, Hf0.75Zr0.25NiSn0.99Sb0.01 was introduced into Zr0.88Ta0.12Co0.88Ni0.12Sb to further suppress κL. Remarkably, the grain size of the biphasic HH alloys was refined by at least one order of magnitude. A biphasic high-entropy HH alloy with y = 0.2 exhibited the minimum κL of ∼2.44 W/(m·K) at 923 K, reducing by 67.7% compared to that of ZrCoSb. Consequently, (Zr0.88Ta0.12Co0.88Ni0.12Sb)0.9(Hf0.75Zr0.25NiSn0.99Sb0.01)0.1 exhibited the highest TE figure of merit (∼0.38) at 923 K. The cooperation between the entropy and biphasic microstructure resulted in multiscale defects, refined grains, and biphasic interfaces, which maximized the scattering of the multiwavelength phonons in HH alloys. This work provides a new strategy for further reducing the grain size and κL of medium- and high-entropy HH alloys.