Thermoelectric Properties of Bi-Doped Mg2Si0.6Sn0.4 Solid Solutions Synthesized by Two-Step Low Temperature Reaction Combined with Hot Pressing

Abstract

The goal of this study is to investigate the influence of the amount of Bi doping on the microstructure and thermoelectric properties of the Mg2Si0.60Sn0.4 material studied by X-ray diffraction, scanning electron microscopy, and thermoelectric measurements. The results of the present study provide strong evidence for the presence of a number of secondary phases with different stoichiometries, including Si-rich and Sn-rich phases, besides the main matrix and the MgO. Phase separation could be attributed to preferential occupancy of the dopant (bismuth in the present case) on tin sites in the lattice, leading to segregated darker regions that are observed in the SEM micrographs, which arose as a result of non-diffusion of Si in the matrix. The electron concentration increases with increasing Bi concentration, similar with Sb. The optimum amount of Bi seems to correspond to Mg2Si0.57Sn0.4Bi0.03 with maximum dimensionless figure of merit of ≈1.2 at 850 K, arising from a high power factor 36 µW cm–1 K−2 and a low lattice thermal conductivity of ≈1.5 W m−1 K−1. These materials exhibit a high thermoelectric figure of merit, which is one of the best among all reported Mg2Si1−xSnx compounds on the Si-rich side.