Enhancing thermoelectric properties of MCoSb-based alloys by entropy-driven energy-filtering effects and band engineering

Abstract

MCoSb-based medium-entropy half-Heusler (HH) alloy with low lattice thermal conductivity (κL) is a promising medium- and high-temperature thermoelectric material. However, the unexpected band structure of the alloy leads to a sudden decrease in the Seebeck coefficient (S), which severely restricts the optimization of the figure-of-merit (ZT). In this study, we demonstrate a new concept for decoupling S and electrical conductivity (σ) based on entropy engineering. This method involves band engineering and the energy-filtering effect, where the entropy-driven quantum confinement effect is manipulated to filter low-energy electrons, and the increasing entropy promotes a rapidly changing density of state near Fermi level. Consequently, an excellent S of ∼ −211.6 μV K−1 at 923 K was achieved for the M0.82Nb0.18CoSb HH alloy, which is 62.3% higher than that of the pristine M0.85N0.15CoSb (M = Ti, Zr, Hf; N= V, Ta; equimolar) HH alloy (∼−130.4 μV K−1). Moreover, benefiting from the optimization of S and κL, a peak ZT was enhanced from ∼0.17 for the pristine M0.85N0.15CoSb medium-entropy HH alloy to 0.58 for the M0.85Nb0.15CoSb high-entropy HH alloy. These results broaden the applications of entropy-engineering to decrease κL and decoupling between S and σ.