B23-O-16Correlation of thermoelectric properties to microstructure of an annealed Sb-doped Mg2Si0.5Sn0.5solid solution with TEM

Abstract

The effect of annealing (1068 K, 250 h) on microstructure and thermoelectric properties of a compound, a 0.33 mol% Sb-doped Mg2Si0.5Sn0.5 solid solution, was studied with TEM, thermoelectric measurements, XRD, SEM, and EPMA (electron probe microanalysis). The solid solution was synthesized by combining a liquid-solid reaction and hot-pressing process. The as-prepared sample contained Si, Sn and MgO inclusions tens of nm in size. After annealing, Si and Sn inclusions disappeared, while the MgO nanoparticles remained almost unchanged; the charge carrier concentration and electrical conductivity decreased and the lattice thermal conductivity increased. As a result, the thermoelectric figure of merit ZT ~ 0.34 at 394 K for the as-prepared sample deteriorated to ~ 0.24 at 388 K after the annealing. The results suggest the presence of a high density of point defects, such as Mg interstitials in the as-prepared sample. The density of these Mg interstitials was reduced by the annealing, thereby affecting the charge carrier concentration and electrical conductivity. The increase in the lattice thermal conductivity upon annealing is attributed to the disappearance of point defects, grain boundaries (grain growth) and Si and Sn inclusions, which all act as phonon scattering centers. Thus point defects and nanoinclusions might be important for optimizing the thermoelectric properties of a material. This work provides new insights into the effect of annealing on the microstructure and its relationship with the thermoelectric properties of Sb-doped Mg2Si0.5Sn0.5 solid solutions. It also provides hints for developing Mg2Si0.5Sn0.5-based materials with superior thermoelectric properties. B23-O-15 doi:10.1093/jmicro/dfv174