Conventional Half-Heusler alloys advance state-of-the-art thermoelectric properties

Abstract

Half-Heusler phases have garnered much attention as thermally stable and non-toxic thermoelectric materials for power conversion in the mid-to-high temperature domain. The most studied half-Heusler alloys to date utilize the refractory metals Hf, Zr, and Ti as principal components. These alloys can quite often achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies have advanced the thermoelectric performance of half-Heusler alloys by employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5. Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT. Specifically, we present the well-studied p-type Hf0.3Zr0·7CoSn0·3Sb0.7 alloys, previously reported to have a ZT near 0.8, resonantly doped with less than 1 atomic percent of metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This is achieved through a significant increase in power factor, by ∼65%, and a notable but appreciably smaller decrease in thermal conductivity, by ∼13%, at high temperatures. These favorable thermoelectric properties are discussed in terms of a local anomaly in the density of states near the Fermi energy designed to enhance the Seebeck coefficient, as revealed by first-principles calculations, as well as the emergence of a highly heterogeneous grain structure that can scatter phonons across different length scales, effectively suppressing the lattice thermal conductivity. Consequently, the effective mass is significantly enhanced from ∼7 to 10me within a single parabolic band model, consistent with the result from first-principles calculations. The discovery of high ZT in a commonly studied half-Heusler alloy obtained through a conventional and non-complex approach opens a new path for further discoveries in similar types of alloys. Furthermore, it is reasonable to believe that the present study will reinvigorate effort in the exploration of high thermoelectric performance in conventional alloy systems.