Transmission Electron Microscopy Study of Mg2Si0.5Sn0.5 Solid Solution for High-Performance Thermoelectrics

Abstract

A Mg2Si0.5Sn0.5 solid solution was prepared by mixing Mg2Si and Mg2Sn powders and hot-pressing the mixture. The Mg2Si0.5Sn0.5 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 Mg2Si0.5Sn0.5 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 Mg2Si0.5Sn0.5. MgO nanoparticles 10–20 nm in diameter were evenly dispersed in the Mg2Si0.5Sn0.5 matrix, which probably reduced its thermal conductivity; moreover, uneven structures containing pure Si and Sn particles were found in the Mg2Si0.5Sn0.5 grains. The origin and the formation mechanisms of the MgO and other impurity particles, and their effect on thermoelectric properties of Mg2Si0.5Sn0.5, are discussed. The low thermal conductivity of Mg2Si0.5Sn0.5 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 Mg2Si0.5Sn0.5 solid solution; it may help in developing other Mg2Si1−xSnx compounds with superior thermoelectric properties.