Study On The Preparation And Peoperty Regulation Of Bi₂Te₃ Based Thermoelectric Materials

Due to the shortage of resources and environmental pollution caused by the burning of fossil fuels,the demand for clean energy is particularly urgent.As a green energy material,thermoelectric materials are clean,safe,stable and pollution-free.Thermoelectric materials can realize the direct conversion of heat energy and electric energy,so they have received extensive attention.Bi₂Te₃-based material is a thermoelectric material close to room temperature and it is also the only thermoelectric materials in commercial application at present.This thesis mainly studies the Sb alloyed p-type Bi_0.46Sb_1.54Te₃ material.Due to its ZT value of～1.0 around room temperature,it has broad application prospects in thermoelectric refrigeration and thermoelectric power generation.Therefore,it is favored by researchers.However,Bi_0.46Sb_1.54Te₃ material is a narrow band gap semiconductor with a band gap of only 0.17 e V.This leads to a significant bipolar excitation at higher temperature,resulting in a significant deterioration of thermoelectric properties,seriously hindering the material’s large-scale commercial applications.Therefore,how to widen the application temperature range of materials and further improve the thermoelectric figure of merit ZT has become an urgent problem to be solved.To address this issue,we optimize the thermoelectric properties of Bi_0.46Sb_1.54Te₃ material by carrier regulation,defect engineering and nanostructure construction.The specific research work is as follows:1.It is well known that element doping can regulate the carrier concentration and introduce defects into the matrix.Through literature search,we found that rare earth elements as effective dopants can significantly improve the thermoelectric properties of materials,but there are few relevant studies.Therefore,in this study,we chose rare earth element Er as dopant,and explore its impact on the thermoelectric properties of Bi_0.46Sb_1.54Te₃ material.Due to the low solubility of rare earth element Er in matrix and agglomeration during high energy ball milling,the secondary phase Er nanoparticles are formed.Interestingly,the mismatch between the microstructure formed by the agglomeration of Er nanoparticles and the layered structure of the matrix leads to the formation of pores and then the Bi_0.46Sb_1.54Te₃ material forms a porous structure,which can be observed by SEM.It is worth noting that the micro dispersion of Er nanoparticles does not affect the structure of the material.Er doping inhibits the formation of Te vacancy and Bi _Te antisite deficiency,thereby reducing the hole carrier concentration.At the same time,the resulting nanoprecipitates and pores enhance phonon scattering and significantly reduce the lattice thermal conductivity,thus optimizing the thermal transport performance of Bi_0.46Sb_1.54Te₃ material.Finally,under the combined effects of carrier concentration regulation,nanoprecipitates and pores on the thermoelectric performance,a higher ZT value of 1.21 at 350 K is achieved for the sample with the optimal doping concentration of x=0.02.2.Constructing special nanostructures is considered to be an effective method to improve the thermoelectric properties of materials,and doping alloys into the matrix is also an effective way to improve the properties of materials.According to literature reports,the metastable ternary phase promotes the formation of special nanostructures in the matrix due to its unstable and easily decomposed characteristics.It is well known that Ag,Cu and Te elements have favorable effects on the thermoelectric properties of Bi_0.46Sb_1.54Te₃ materials.By combining these two factors,we prepared metastable ternary phase Ag Cu Te which was then doped into Bi_0.46Sb_1.54Te₃.Since the metastable ternary phase Ag Cu Te is unstable at the room temperature,there are always secondary phases Ag₂Te and Cu₂Te.At the same time,Ag and Cu with relatively smaller atomic radius are more likely to enter lattice sites and then replace Bi/Sb sites,resulting in a significant increase in carrier concentration,realizing carrier regulation and optimizing the electrical transport performance of the material.The excess Te in the matrix is caused by Ag entering the lattice position,which self-assemble along the direction of the minimum crystallization energy to generate the 3D flower-like dendrites and micro-spherically flocculated crystals nanostructure.In addition,Ag Cu Te,Cu₂Te,and Ag₂Te all have relatively larger band gaps,which effectively improve the effective band gap of Bi_0.46Sb_1.54Te₃materials.The doping of the metastable ternary phase AgCuTe also introduces abundant lattice defects into the matrix,which significantly improves the thermal transport performance of the material.Finally,a high ZT value of 1.46 at 425 K is obtained for the Bi_0.46Sb_1.54Te₃-0.1 wt%Ag Cu Te sample.3.Through literature survey,it is found that the nanoporous structure is beneficial to improve the thermoelectric properties of materials.It is well known that S element has a low sublimation point and can be used as a pore-making agent.Therefore,during material preparation,a part of S will sublimate and leave nanopore in the matrix,and an other part of S can be doped into the lattice to achieve isoelectric atomic substitution.It can be seen that S element has the potential to introduce nanopores and point defects into the matrix.At the same time,the atomic radius of Ag atom is relatively small,and it is easier to enter the lattice position,threrby playing a role in the regulation of carrier concentration.Based on the favorable effects of Ag and S elements,Ag₂S compound was prepared and doped into Bi_0.46Sb_1.54Te₃ material,and the experimental results were in line with expectations.Nanopores are formed in the composite materials due to the partial sublimation of element S.At the same time,the presence of Ag also effectively adjusts the carrier concentration.In addition,Ag₂S doping also introduce multi-scale lattice defects into the matrix,thus significantly enhances the full-frequency phonon scattering and significantly reduces the lattice thermal conductivity.Finally,excellent thermoelectric performance with a ZT value of 1.30 at 325 K is achieved for the Bi_0.46Sb_1.54Te₃-0.3 wt%Ag₂S sample.