Optimization Of Thermoelectric Transport Properties Of Bismuth Telluride-based Polycrystalline Alloy

Thermoelectric conversion technology,as a green and environment-friendly new energy technology,can directly convert heat and electricity or vice versa,and its energy conversion efficiency depends on the dimensionless figure-of-merit?ZT value?.Bismuth telluride-based thermoelectric?TE?materials are the only thermoelectric material for commercial application at room temperature,but the relatively low ZT value seriously limits its industry application.In this paper,Bi_0.5Sb_1.5Te₃ and Bi₂Te_2.7Se_0.3 were used as research objects of p-type and n-type bismuth telluride materials,respectively.The polycrystalline bismuth telluride materials were synthesized by vacuum melting,ball milling and spark plasma sintering technology.We investigated the influence of stoichiometry of Te,doping,and the second metal phase on its electrical and thermal transport properties and explored the mechanism about the improvement of TE performance.We investigated polycrystalline alloy with nominal stoichiometric Bi_0.5Sb_1.5Te_3+x?x=0,0.1,0.2,0.4,0.6?by liquid phase sinter technique.The reduction of point defects and matrix grains rearrangement with the help of molten liquid Te in sintering process,both mechanisms can reduce the carrier scattering which can increase the carrier mobility and improve the thermoelectric performance.As a result,ZT value of 1.2 is obtained for Bi_0.5Sb_1.5Te_3.4 at 323 K,50% increment compared with Bi_0.5Sb_1.5Te₃.Further,group ?A elements Sn and Pb were doped into the bismuth telluride cation site,and the analysis showed that such acceptor dopant can effectively improve carrier concentration and thus the electrical transport properties.And the bipolar effect is also suppressed,so the application temperature ranges of bismuth telluride material has been expanded.Specifically,Sn doping can slightly increase the carrier concentration,enhance the figure of merit of the material near room temperature,and obtain the optimal ratio(Bi_0.25Sb_0.75)_1.97Sn_0.03Te₃.Based on this result,Pb doping can significantly enhance carrier concentration,and suppress the intrinsic excitation,thus the work temperature of the material has been shifted to higher temperature.As a result,(Bi_0.25Sb_0.75)_<sub>1.97Sn_0.02Pb_0.01Te₃ can obtain a ZT value of 1.2 at 423 K.In practice,thermoelectric devices simultaneously require the p-type material and n-type material with similar properties.Therefore,we still investigated the n-type bismuth telluride,different contents of Sb were dispersed into Bi₂Te_2.7Se_0.3 during ball milling process to form second metal phase.The Schottky barrier between metal and semiconductors filtered low energy carriers,at the same time,it enhances the scattering of low-frequency phonons.When the Sb content is 5 wt.%,the ZT value at 400 K is 1.3,40% increment compared with Bi₂Te_2.7Se_0.3 and the thermoelectric performance is enhanced over the entire test temperature range.