Enhanced Thermoelectric Properties of Zr<sub>0.85–<i>x</i></sub>Hf<sub><i>x</i></sub>Nb<sub>0.15–<i>y</i></sub>Ta<sub><i>y</i></sub>CoSb Medium-Entropy Alloys: Tradeoff between “What to Alloy” and “How Much to Alloy”
Rongchun Chen, Yu Yan, Wen Zhang, Fuyuan Liu, Huijun Kang, Enyu Guo, Zongning Chen, Tongmin Wang
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 Zr 0.45 Hf 0.4 Nb 0.06 Ta 0.09 CoSb 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 Zr 0.45 Hf 0.4 Nb 0.06 Ta 0.09 CoSb 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.