Kirkendall effect induced ultralow thermal conductivity yields enhanced thermoelectric properties in Mg–Zn–Sb alloys

Abstract

Thermally induced phase change can modify the microstructure of thermoelectric alloys due to the interdiffusion of atomic species, and also to alter the properties of these materials. Herein, the impact of Mg additive on thermoelectric performance and microstructure of Mg-Zn-Sb thin films is studied. The results show complex microstructural changes upon heating of amorphous Mg-Zn-Sb thin films. The crystallization process of the films leads to the formation of Zn-Sb and Mg-Zn-Sb phases at elevated temperatures. Out-diffusion of Zn during crystallization process results in the formation of Kirkendall voids in Mg-Zn-Sb materials, while no such voids are found in pure Zn-Sb films. The maximum figure of merit (ZT) is found to be ∼1.8 at 584 K for optimized Mg24.3(Zn-Sb)75.7 thin films, which is larger than of other similar thermoelectric materials. Transmission electron microscopy reveal distinct microstructures between the pure and Mg-alloyed Zn-Sb materials. Defects and secondary phase in the alloyed films mainly contribute to effective phonon scattering and lead to very low thermal conductivity (∼0.26 W/m·K at 584 K). Overall, this study shows a way for synthesis of high-performance low-cost Mg-Zn-Sb thermoelectric alloy by microstructure optimization and demonstrates that Kirkendall voids is an effective approach for the enhancement of thermoelectric performance of the materials.