Preparation And Thermoelectric Properties Of Mg₂Si_1-xSn_x (0≤x≤1.0) Compound

Semiconducting metal silicide of Mg₂Si, which is composed of less toxic and naturally abundant elements, has attracted more attention for its potential application to thermoelectric device. It is known that Mg₂Si_1-xSn_x solid solutions show much better thermoelectric properties than the pure compound of Mg₂Si and Mg₂Sn.Recently,it is reported that the figure of merit (ZT) of Sb-doped Mg₂Si_1-xSn_x have exceeded l,so Mg₂Si_1-xSn_x solid solutions have long been expected as an ideal candidate for thermoelectric energy conversion in a middle temperature.In order to reduce the oxidizing, volatilizing and carbonizing caused by Mg element in the traditional methods for synthesizing Mg₂Si_1-xSn_x (0≤x≤1.0) compounds, solid state reaction at low temperature (SSR) combined with spark plasma sintering (SPS) were firstly introduced in that system.The influences of parameters during the synthesis processing were discussed. The results suggest that the oxidation of Mg can be restrained by changing heating programs, the separation of Sn can be controlled by adjusting pre-pressure, and the volatility of Mg can be made up by controlling excessive content of Mg.When the excessive content of Mg is about 0.0025mol,the pre-pressure is about 20MPa, and the samples are kept under 893-943K for 23-30 hours, the single phases of Mg₂Si_1-xSn_x (0≤x≤1.0) powders can be obtained under this experimental conditions. Then the powders were sintered by SPS, and the thermoelectric characters of bulks were measured.The result of thermoelectric properties for Mg₂Si_1-xSn_x (0≤x≤1.0) solid solutions show that the lattice constants and thermoelectric properties of Mg₂Si_1-xSn_x are all sensitive to Sn content. The forbidden band of Mg₂Si_1-xSn_x (0≤x≤1.0) solid solutions reduces with the adding of Sn, which is benefits for the electron grading from valence band to conductor band. It resulted the electrical conductivity (σ) increases and the absolute Seebeck coefficient (α) reduces with the increase of Sn.Because of the great different average atomic masses between Si and Sn, it enhances point defect, and scatters the low frequency phonons more effectively to decrease the thermal conductivity (κ).Whenχ=0.2, the highest figure of merit (Z7) of 0.1 was obtained at about 490K, which shows twice as high as that of pure Mg₂Si. Bi is chosen as n-type dapant, the single phase of Bi-doped Mg₂Si_0.8Sn_0.2 and Mg₂Si_0.5Sn_0.5 samples have been successfully fabricated by solid state reaction -spark plasma sintering (SPS). The thermoelectric properties of Bi doped Mg₂Si_1-xSn_x (x=0.2, 0.5) are sensitive to Bi content. When the doping concentration of Bi is up to 3.0at% (nominal molar percent), the sample of Mg₂Si_0.8Sn_0.2 shows the highest power factor and a maximum value of the figure of merit, ZT, is 2.5×10^-3 W/mK² and 1.17 at about 850K respectively. It is a good result for Mg-Si-Sn system, which is similar with the best result reported by Y.Isoda (7500ppm Sb doped Mg₂Si_0.5Sn_0.5, ZT=1.2,620K). When the doping concentration of Bi is up to 2.5at% (nominal molar percent), the sample of Mg₂Si_0.5Sn_0.5 shows the highest power factor and a maximum value of the figure of merit, ZT, is 1.39×10^-3 W/mK² and 0.78 at about 800K respectively.