Preparation And Thermoelectric Properties Of Mg-Si-Sn Based Materials Doped With Bi

Mg₂Si_1-xSn_x is a widely used thermoelectric material for medium temperature?400K-800K?,Mg₂Si_1-xSn_x has many advantages,such as excellent thermoelectric performance,rich in composition elements,no pollution to the environment,etc.Mg₂Si_1-xSn_x is also widely concerned by researchers because of these advantages.In this paper,Mg,Si,Sn and Bi powders were mixed,according to Mg₂Si_0.4Sn_0.6-xBi_x ratio,milled,cold-pressed and vacuum sintered.The sintered materials were analyzed by XRD,SEM and LSR-3?Seebeck coefficient/electric resistance tester?,and then the influences of different sintering temperature,different Bi doping amount and different annealing time on the structure and properties of sintered materials were investigated.The main conclusions are as follows.As different sintering temperature of the material,the Mg₂Si and Mg₂Sn phases exist in the vacuum sintering materials of 500?-2h,there are Mg₂Si,Mg₂Sn and Mg₂?Si,Sn?solid solution phases in vacuum sintering of 580?-2h,there are Mg₂Si,Mg₂Sn,Mg₂?Si,Sn?solid solution and metal Sn phase in vacuum sintering of 620?-2h.The vacuum sintered materials at 580?-2h and 620?-2h were vacuum densified at 500?for 2h,and Mg₂?Si,Sn?solid solution phases gradually decomposed into Mg₂Si and Mg₂Sn phases,but not completely decomposed,there were still some solid solutions.The material is then treated with solid solution at 580?-2h.The material contains Mg₂Si,Mg₂Sn,Mg₂?Si,Sn?solid solution and a small amount of MgO phase,and no single Mg₂?Si,Sn?solid solution phases has been obtained.500?-2h sintered material annealed at 600?,annealing time extended from 0 to 1.5h,Mg₂Si phase decreased gradually,while the Mg₂?Si,Sn?solid solution is gradually increased,meanwhile,with Mg₂Si and Mg₂Sn decomposition,a small number of MgO and metal Sn phases are also increasing in the material.With the increase of the test temperature,the resistivity of the materials decreases sharply.This is typed characteristics of the semiconductor.With the increase of Bi content,the resistivity of Mg-Si-Sn-Bi sintered material decreases first and then increases,while the absolute value of Seebeck coefficient changes irregularly,and the optimum amount of Bi doping exists,which makes the maximum power factor.The existence of metal Sn in sintering material can reduce the resistivity,but it greatly reduces the absolute value of Seebeck coefficient.A small amount of nano metal Sn can be added to the material to reduce the resistivity of the material,but it can not affect the absolute value of the Seebeck coefficient at the same time,so as to obtain a larger power factor.The content of Mg₂?Si,Sn?solid solution has a good effect on the Seebeck coefficient of the material,the more content,the greater the absolute value of Seebeck coefficient.Few MgO does not have much influence on the thermoelectrical properties of the materials.