Synthesis Of SiGe Alloy By MS-PAS And Its Thermoelectric Properties

Si Ge alloy was and still is the main thermoelectric materials for high temperature. However, because of its big difference between liquidus and solidus and the low interdiffusion coefficients, it has been a big challenge to synthesis homogeneous Si Ge alloy. After more than half century, the ZT of Si Ge alloy has been improved dramatically, but the current Si Ge applied in RTGs still bear low thermoelectric performance, for the maximum of ZT are 0.93@1200 K and 0.5@1100 K for n-typed Si Ge and p-typed Si Ge respectively. Rapid and economical method to synthesize homogeneous is of important meaning for practical application. My thesis aim to synthesis homogeneous Si Ge alloy with high thermoelectric performance by adopting Melt Mpinning technique together with Plasma Activated Sintering, during with the influence of MS process on phase composition, microstructure and final thermoelectric performance have been studied. Specific research content and conclusions are listed as followed:The first research is synthesis of intrinsic Si80Ge20 alloy by MS-PAS to weigh the feasibility of this method. Systematic study of different crucibles and ejecting pressure on MS product has been carried out and it turns out that MS with S5 crucible and ejecting pressure of 0.02 MPa produce relative homogenegous ribbons while bear higher yield that MS does with other crubible types. The intrinsic Si80Ge20 bears poor thermoelectric performance due to its low carrier concentration, which requires optimization by doping.The second research is synthesis n-typed and p-typed Si80Ge20 alloy by MS-PAS, during which impact of MS process on phase composition, microstructure and final thermoelectric performance has been carefully studied and conclusion drawn in this part is that high MS speed is able to suppress the phase splitting or composition segregation during solidification and thus improve the homogeneity of the alloy, which result in the evenly diffusion of doping elements in the matrix. Finally, n-typed Si80Ge20P2 alloy processed by MS speed of 30 m s-1 bears high ZT of 0.9@950 K while the p-typed Si80Ge20B1.7 processed by MS speed of 50 m s-1 bears high ZT of0.6@950 K, an improvement of almost 40 percent for both relative to Si Ge alloy used by RTGs.The third part is to synthesize homogeneous Si Ge alloy by using alloy with high Si/Ge ratio and it has been successfully achieved. Free side of MS ribbon processed by high MS speed has clear surface with Ge-riched phase coated. As a consequence, the doping elements diffuse more evenly in the alloy matrix. Finally n-typed Si90Ge10 processed by MS with speed of 30 m s-1 bears ZT of 0.7@950 K while the p-typed Si90Ge10 processed by MS with speed of 50 m s-1 bears ZT of 0.5@950 K, lower than that of optimized Si80Ge20 but still higher than that of Si Ge used by RTGs. Most importantly, costing Ge has been reduced in the alloy matrix which is of practical meaning for Si Ge alloy.