Electronic Structure, Preparation And Thermoelectric Properties Of Mg₂Si_1-xGe_x

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 since the last decade. It was known that Mg2Si_1-xGe_x solid solutions showed much better thermoelectric properties than the pure compound of Mg₂Si, which have long been expected as an ideal candidate for thermoelectric energy conversion in a middle temperature.The electronic structures of Mg2Si_1-xGe_x(x=0, 0.2) are studied by quantum chemical calculation method using a first-principle Pseudopotential, which could theoretically explain why the thermoelectric properties varies with the structure microscopically. Calculation of band structure reveals that Mg2Si_1-xGe_x(x=0,0.2) has lighter effective conducting mass of electron than that of hole , consequently ,it is n-type semiconductor .It is also found that ,because of the solution of Ge ,the forbidden gap of Mg₂Si_0.8Ge_0.2 becomes narrower than that of Mg2Si,which results in higher electrical conductivity, while the phonon scattering becomes strengthened ,which reduces the thermoelectric conductivity .DTA, XRD and SEM analysis are applied to explore the processing conditions of Solid State Reaction for Mg2Si_1-xGe_x(x=0,0.2,0.4,l) .The results show that the mixed compounds of Mg₂Si (Ge)and Mg2Ge(Si) could be synthesized in the temperature range of 750-823 K with the holding time of 8h .To obtain the homogenous Mg2Si_1-xGe_x(x=0.2,0.4) solid solutions ,the mixeds powders should be heat treated at 1073K for 16h . Ag and Sb, Te were chose as p -type and n-type doping agents, two-step solid state reaction was used to synthesis p-type and n-type Mg₂Si₀.8Ge₀.2 powders. SPS was selected to fabricate the undoped and doped dense bulk thermoelectric Mg₂Si₀.8Ge₀.2 at 1103K.In the temperature range of 400-823K, the dimensionless figure of merit (ZT) of Mg₂Si₀.8Ge₀.2 is always higher than that of Mg₂Si., and the maximum value of ZT for Mg₂Si and Mg₂Si₀.8Ge₀.2 are 0.207(577K) and 0.14(640K) respectively .The resultproved that the solution of Ge has an important effect on the electronic structure and thermoelectric properties of Mg₂Si_1-xGe_x, which is consistent with the theoretical deducement. All the Ag-doped specimens show p-type semiconducting behaviors, which means the dominant carrier is hole. For the Ag-doped (20000ppm) sample, the power factor reaches the maximum value of 6.1×10^-4 W/mK2 (811K).For both the Sb-doped and Te-doped samples, the thermoelectric properties were improved with the increasing concentration of doping agent , which also indicate that the sb is more befitting n-type dopant compared with Te .The maximum power factor for Sb-doped(5000ppm) samples reaches 1.65×10^-3 W/mK²(620K),whereas for 4000ppm Te-doped samples the maximum power factor is 1.4×10^-3 W/mK²(520K).