Enhanced Thermoelectric Properties of Zr0.85–xHfxNb0.15–yTayCoSb Medium-Entropy Alloys: Tradeoff between “What to Alloy” and “How Much to Alloy”

Abstract

Although configurational entropy is regarded to be a gene-like performance indicator for thermoelectric (TE) materials, increasing the configurational entropy alone does not guarantee a high TE figure of merit (ZT). Therefore, determining protocols for designing medium- and high-entropy TE materials with high ZT values is imperative. Herein, we provide a strategy for designing high ZT n-type ZrCoSb-based medium-entropy (ME) half-Heusler (HH) alloys and highlight the tradeoff between “what to alloy” and “how much to alloy”. As expected, an intrinsically low lattice thermal conductivity of ∼2.24 W m–1 K–1 was obtained at 923 K for the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy owing to the synergy between Zr-site disorder, anharmonicity, refined grain size, and entropy-driven multiscale defects. Entropy-driven quantum traps lead to energy-filter effects that concurrently provide optimized Seebeck coefficients and power factors. These favorable modifications enable the Zr0.45Hf0.4Nb0.06Ta0.09CoSb ME HH alloy to exhibit a high peak ZT of ∼0.57. The developed design concept provides an effective strategy for enhancing the ZT values of medium- and high-entropy TE materials.