Study On Performance Optimization Of Bi₂Te_2.7Se_0.3 Thermoelectric Materials

Thermoelectric?TE?materials have a wide application prospect in the field of power generation and solid refrigeration,because it can convert thermal energy to electric energy directly and reversely based on Seebeck effect and Peltier effect.Bi₂Te₃-based alloys are the most typical thermoelectric material used near room temperature and have been put into commercial application.However,compaered to high-performance p-type Bi₂Te₃-based alloys,n-type Bi₂Te₃ has lower TE performance.How to effectively improve TE performance of n-type Bi₂Te₃ is the key to achieve wide application for Bi₂Te₃-based TE materials.The n-type Bi₂Te_2.7Se_0.3 is selected as research object in this paper.The main research contents and obtained conclusions are as following:?1?Due to the small band gap?about 0.15 eV?of Bi₂Te₃,the intrinsic excitation occurring near room temperature causes that its TE performance decreases sharply with increasing temperature.In this paper,Bi₂Te_2.7Se_0.3 with excess Te are prepared by an energy-saving and time-saving method?high-energy ball milling method and DC rapid hot pressing?,and the intrinsic point defects are adjusted by controlling Te content.Excess Te atoms occupy Bi sites to form positively charged antisite defect Te^·_Bi,which significantly increases the carrier concentration,thereby enhancing the electrical conductivity and delaying the intrinsic excitation.Because the intrinsic excitation is effectively delayed,the bipolar thermal conductivity decreases significantly.In addition,the antisite defect Te^·_Bi enhances the phonon scattering,thereby reducing the lattice thermal conductivity.Finally,the ZT peak of Bi₂Te_2.71Se_0.3 reaches 0.9 at 373 K,and the average ZT reaches 0.82 between 300 K and 498 K,which is 20%and 61%higher than that of Bi₂Te_2.7Se_0.3,respectively,indicating its application potential in TE power generation?below 500 K?.?2?The commercial Bi₂Te_2.7Se_0.3 is selected as the matrix material,and the Bi₂Te_2.7Se_0.3/Bi₂S₃ heterostructure is constructed in situ using an ion exchange reaction.The sulfur-containing group?-CS-?is first adsorbed on the surface of Bi₂Te_2.7Se_0.3 powder particles,and then hydrolyzed in situ to form S^2-ions.Under the solvothermal condition,anion exchange reaction occurs with Bi₂Te_2.7Se_0.3 to form Bi₂S₃.Bi₂Te_2.7Se_0.3/Bi₂S₃ hetero-interface with appropriate concentration has no significant effect on the electrical conductivity.And the Seebeck coefficient is enhanced due to the potential barrier filering effect of the hetero-interface,which improves the power factor to a certain extent.In addition,the Bi₂Te_2.7Se_0.3/Bi₂S₃ hetero interface significantly enhances phonon scattering,thereby effectively reducing the lattice thermal conductivity.Finally,the maximum ZT reaches 0.7 at 473 K,and the average ZT value reaches 0.61 between 300 K and 498 K,which is 23%and 25%higher than the matrix,respectively.?3?After commercial Bi₂Te_2.7Se_0.3 and S powder are uniformly mixed at a certain ratio,the mixed powder is subjected to DC rapid hot-pressing sintering.During the high-temperature solidification process,part of the S is sublimated,leaving nano-scale pores in the material,and the other part of the S is doped into the Bi₂Te_2.7Se_0.3 lattice to form point defects in atomic-scale.The electrical conductivity is slightly decreased because of the introducing of defects.However,the Seebeck coefficient is significantly enhanced due to the improvement of the effective mass,thereby improving the power factor.In addition,multi-scale defects significantly enhance phonon scattering,thereby effectively reducing lattice thermal conductivity.Finally,the ZT value reaches 0.85 at 473 K,and the average ZT value reaches 0.74 between 300 K and 548 K,which is 49%and 51%higher than the matrix,respectively.