Transmission Electron Microscopy Study of Mg₂Si_0.5Sn_0.5 Solid Solution for High-Performance Thermoelectrics

Abstract

A Mg₂Si_0.5Sn_0.5 solid solution was prepared by mixing Mg₂Si and Mg₂Sn powders and hot-pressing the mixture. The Mg₂Si_0.5Sn_0.5 samples exhibited a much lower thermal conductivity (1.92 W m⁻¹ K⁻¹ at 300 K) than the parent Mg₂Si (8.75 W m⁻¹ K⁻¹) and Mg₂Sn compounds (6.28 W m⁻¹ K⁻¹). X-ray diffraction measurements confirmed the successful synthesis of the Mg₂Si_0.5Sn_0.5 solid solution. Electron microscopy observations revealed that the grains were mainly 10–20 μm in size and had clean grain boundaries without obvious inclusions and precipitates. The major phase was cubic Mg₂Si_0.5Sn_0.5. MgO nanoparticles 10–20 nm in diameter were evenly dispersed in the Mg₂Si_0.5Sn_0.5 matrix, which probably reduced its thermal conductivity; moreover, uneven structures containing pure Si and Sn particles were found in the Mg₂Si_0.5Sn_0.5 grains. The origin and the formation mechanisms of the MgO and other impurity particles, and their effect on thermoelectric properties of Mg₂Si_0.5Sn_0.5, are discussed. The low thermal conductivity of Mg₂Si_0.5Sn_0.5 resulted in a relatively high dimensionless figure of merit ZT = 0.0132 at 300 K, which may be further increased by optimizing the synthesis procedure, alloy composition, and doping level. This work provides information on the structure and chemistry and their relationship with the thermoelectric properties of the Mg₂Si_0.5Sn_0.5 solid solution; it may help in developing other Mg₂Si_1−x Sn _x compounds with superior thermoelectric properties.