Study On Thermoelectric Propertices Of Bi₂Se₃ Based Alloys

Thermoelectric?TE?material is a functional material which can realize the direct conversion between thermal energy and electrical energy for waste heat recovery or refrigeration.The TE devices are extensible,stable,reliable,and have no moving parts.They can be applied to the areas where conventional heat engines cannot be used.Bi₂Se₃,a V₂VI₃ semiconductor material,is a potential room temperature thermoelectric material,having the same spatial structure as Bi₂Te₃ alloy.The storage of Se on the earth is much larger than that of Te.Therefore,study on the Bi₂Se₃ alloy for the replacement of Bi₂Te₃ is of great significance for the commercial application of thermoelectric materials.In this paper,we prepared the Bi₂Se₃ based TE materials by smelting,ball milling and hot-pressing.The influence of the morphology and carrier concentration on the thermoelectric properties of Bi₂Se₃ alloys is studied.The p-type doping effect of Bi₂Se₃alloys is also explored.The main results are as follows:The effect of hot-forging on the thermoelectric properties of Bi₂Se₃ alloy is investigated.The Bi₂Se₃ alloy powder was prepared by a high-energy ball milling method,and then hot-pressing and hot-forging were tried.It was found that the grain size of the sample was reduced by ball milling,but the grain grew after hot-pressing,decreasing the phonon scattering and the anisotropy of the samples.The lattice thermal conductivity along the hot-pressing direction increased.The TE properties improved when hot-forging for the strengthened of the anisotropy.The carrier concentration of the Bi₂Se₃ alloy is tuned by Cu doping.We doped Cu into the interstitial site?Cu_xBi₂Se₃?and Bi site(Cu_xBi_2-xSe₃),respectively.It was found that the electrical conductivity increased and the Seebeck coefficient decreased with increasing content of Cu when interstitial doping.The electrical conductivity decreased and the Seebeck coefficient increased when x>0.01,suggesting the doping limit of Cu.When Cu substituded with Bi,the Hall carrier concentration decreased and the Seebeck coefficient increased as the doping amount of Cu increased.This may be due to the cancellation of the eletron concentration by holes after Cu¹⁺replaced Bi³⁺.The optimum ZT value is⁰.3 at 375 K for Cu_0.0125Bi_1.9875Se₃,which increased to⁰.35?375 K?after hot-forging.The acceptor doping in Bi₂Se₃ alloy is realized.We chose Mn,Na,and Pb as acceptors to prepare p-type A_xBi_2-xSe₃ alloy?A=Mn,Na,and Pb?.The experiments show that Na doping can reduce the electrical conductivity,and thermal conductivity,and increase the Seebeck coefficient.The ZT value reaches⁰.35 at 525 K when x=0.01.This is mainly due to the decrease in electron concentration after hole doping.But Na_0.01Bi_1.99Se₃ is still an n-type semiconductor.Then we increased doping content of Na to x=0.03 considering the mechanical properties of the materials.But the alloy is still n-type.Both Mn and Pb doping can realize the transition of Bi₂Se₃ from n-type to p-type.When increasing the doping content of Mn to x=0.05,the alloy shows p-type.The room temperature Seebeck coefficient changed from-50?V K^-1for undoped sample to 150?V K^-1for Mn_0.05Bi_1.95Se₃.When increating the doping concentration of Pb to x?0.03,the alloy changed to p-type semiconductor.The room temperature S changed from-100?V K^-1for Pb_0.01Bi_1.99Se₃ to 100?V K^-1for Pb_0.03Bi_1.97Se₃.