Bismuth doped Mg2Si with improved homogeneity: Synthesis, characterization and optimization of thermoelectric properties

Recent investigations on Bi doped Mg2Si have shown huge differences of the optimum doping level with respect to maximization of thermoelectric performance. A possible discrepancy among the published results can have origin in different homogeneity of the samples examined in different studies, but it is impossible to judge because of lack of the microstructural studies. Therefore, the aim of the study was to develop a method for obtaining a homogeneous Mg2Si doped with Bi samples and determine the influence of dopant on their thermoelectric properties as well as the solubility limit. The results of theoretical studies of the electronic structure employing FP-LAPW (Full Potential Linearized Augmented Plane Wave) method calculations within density functional theory DFT using the WIEN2k package in Bi-doped Mg2Si are presented. A series of samples with nominal composition Mg2Si1−xBix (x=0-0.06) were prepared using the spark plasma sintering (SPS) method and subsequent annealing. Structural, phase and chemical composition analyses were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and scanning thermoelectric microprobe (STM). The solubility limit was found to be higher than in the previous reports. Carrier concentration was measured using the Hall method. The investigations of the influence of Bi dopant on the transport properties i.e.: electrical conductivity, the Seebeck coefficient and the thermal conductivity were carried out in the temperature range from 300 to 720 K. On the basis of the experimental data, the temperature dependencies of the thermoelectric figure of merit ZT were calculated. Detailed analysis of all obtained results was carried out providing additional insight into the role of the homogeneity of studied materials on their thermoelectric properties.