Design, Preparation And Properties Of Bi₂Te₃/CoSb₃ Graded Thermoelectric Materials

Thermoelectric materials are new functional materials that can be used to convert thermal energy into electrical energy directly. The introduction of Functionally Graded Materials in the research course of thermoelectrics can greatly improve the optimal temperature range of thermoelectric materials and conversion efficiency of a thermoelectric device.In this paper, firstly, monolithic materials CoSb3 and Bi2Te3 were prepared by Sparkle Plasma Sintering (SPS) respectively, and at the same time the microstructure of CoSb3 and Bi2Te3 were studied by SEM; the Seebeck coefficients and electrical conductivities of monolithic materials were measured by standard-four-probe method (ULVAC ZEM-700) in a He atmosphere simultaneously, and their thermal conductivities were investigated by laser flash method (TC-7000) in vacuum. Secondly, the junction temperature of graded Bi2Te3/CoSb3 thermoelectric materials was optimized based on the thermoelectric transport properties of monolithic materials, also when graded materials were used in the temperature difference ranging from 300K to 800K, the length ratio of monolithic materials CoSb3 and Bi2Te3 were optimized in theory. Thirdly, graded Bi2Te3/CoSb3 thermoelectric materials were prepared by two-step SPS sintering, and the relationship between its average Seebeck coefficients and temperature were calculated by theory model. Finally, in to order to verify the validity of optimal design the open voltages and power outputs of graded Bi2Te3/CoSb3 thermoelectric materials in the temperature difference ranging from 300K to 800K were investigated.The research results show that: as for monolithic materials, Bi2Te3 alloys process higher figure-of-merit in the temperature ranging from room temperature to 450K, then it begins to worsen with temperature increasing. When the temperature exceeds 500K, CoSb3 compounds have better figure-of-merits so graded Bi2Te3/CoSb3 thermoelectric materials will not only benefit from the better performance of Bi2Te3 alloys at the lower temperature,but also make full use of the properties of CoSb3 compounds at the higher temperature side. The junction temperature optimized by the relationships between figure-of-merits of monolithic materials and temperature is about 500K, and the optimal length ratio of CoSbs and Bi2Te3 is about 15:2 when it was used in the temperature difference from 300K to 800K. Graded BijTea/CoSbs thermoelectric materials were prepared by two-step SPS sintering, the SEM results illustrated that these two kinds of monolithic materials bonded well at the junction. The average Seebeck coefficients of graded materials possess higher values over a wide temperature difference. The open voltages and power outputs of graded materials are much higher than monolithic materials, and when the length was kept at 15:2, the maximum values were achieved. The measured maximum power outputs of graded materials is about 320 Wm"2 when the loaded is fixed at 0.1 , which is about 1.8 times than that of CoSbj compounds (178Wm"2). Under the same measuring conditions, the power outputs of graded materials are higher than monolithic materials...