Thermal stability and thermoelectric properties of Mg₂Si_0.4Sn_0.6 and Mg₂Si_0.6Sn_0.4

Compounds of Mg₂Si_1−x Sn _x are environmentally friendly, inexpensive and high-efficiency thermoelectric materials for energy conversion in the temperature range 300–550 °C. In this study, the thermal stability is investigated of fine powders and sintered pellets of the compounds Mg₂Si_0.4Sn_0.6 and Mg₂Si_0.6Sn_0.4 by heating the samples from room temperature to ~400 °C in air, while measuring powder X-ray diffraction patterns. The diffractograms of the pellets show no significant changes upon heating for several hours, while the powder samples show increasing emergence of a Mg₂Sn-rich, Mg₂Si_1−x Sn _x phase, and other impurities upon heating for only several minutes. This is attributed to the larger amount of surface area in the powder samples. The appearance of the Mg₂Sn-rich phase is most pronounced for the Sn-rich composition. In addition, the thermal expansion coefficients were extracted from the powder diffraction patterns. All materials have been synthesized by induction-melting followed by ball milling and spark plasma sintering. The thermal conductivity, Seebeck coefficient, electrical resistivity and Hall carrier concentrations have been measured from room temperature to 400 °C on the pellets.