The structural, microstructural and thermoelectric properties of Mg2Si synthesized by SPS method under excess Mg content conditions

Abstract

The aim of the study was to determine the influence of Mg excess on the physicochemical properties with particular emphasis on thermoelectric properties of synthesized Mg2Si. Available literature devoted to this issue suggests very often a need to use an excess of Mg during the synthesis process in order to obtain homogeneous material [1–13]. Samples with 0–10 wt% of excessive Mg were prepared using the Spark Plasma Sintering (SPS) method. The structural and phase compositions were examined using X-ray diffraction XRD and scanning electron microscopy SEM coupled with EDS detector for the chemical composition analysis. The influence of Mg content on the impedance spectrum and transport properties, i.e.: electrical conductivity, the Seebeck coefficient and the thermal conductivity was studied in temperature range from 300 to 720 K. Carrier concentration was measured using Hall method. Scanning thermoelectric microprobe (STM) was used to determine the uniformity of Seebeck coefficient. Furthermore, the temperature dependences of the thermoelectric figure of merit ZT were calculated. The experiments performed, allow concluding that Mg excess which is in liquid phase during the sintering process gathers mostly at the grain boundaries, enhancing sintering process and decreasing total porosity of the samples. Moreover, higher Mg content increases electrical and thermal conductivity of Mg2Si, what causes simultaneous decrease of Seebeck coefficient. Step change in the electrical conductivity between the samples without and with the excess of magnesium (e.g. σ = 30 S m−1 for 0 wt% and σ = 1.26 · 105 S m−1 for 10 wt%) indicates the presence of a percolating mechanism.