Investigation On Microstructure Control And Thermoelectric Properties Of SnTe-based Compound Semiconductors

Thermoelectric materials can realize direct conversion between heat and electricity,have been considered to be apromising solution to relieve the energy crisis and environmental pollution.However,they are still prevented from extensive application by some obstacles,such as the low conversion efficiency as well as some toxic or expensive metal components.Therefore,it is of great significance to develop environmental-friendly and cost-effective new TE materials.Recently,SnTe-based compound semiconductors have been paid more and more attention to as a promising envioronmentally-benign thermoelectric material.However,its thermoelectric properties are not so satisfying due to the following factors:firstly,a large number of Sn intrinsic vacancies(10²⁰-10²¹cm^-3)lead to the excessive hole concentration,which deteriorates electrical properties;secondly,the band gap is too narrow,and the energy of the heavy band and light band splits greatly?0.3-0.4eV?,which is not favorable to enhance the Seebeck coefficient;thirdly,the lattice thermal conductivity is large.To improve the thermoelectric performance of SnTe,the above-mentioned problems must be solved.Therefore,point defect engineering,energy band engineering as well as microstructure engineering have been employed to control the carrier concentration,band structure as well as lattice thermal conductivity so that achieving the large improvement of TE properties of the compound.The main contents and results of the dissertation are as follows:1.The influence of Sn vacancy on SnTe compound's carrier concentration as well as the thermal and electrical transport has been studied.The carrier concentration can be lowered to a certain extent by proper adjustment of the ratio Sn/Te.However,less improvement has been obtained on the Seebeck coefficient and lattice thermal conductivity.Thus,the effect of Sn vacancy on the whole thermal properties is not remarkable.2.By mean of hydrothermal synthesis,SnTe nanopowders with different morphologies have been fabricated and they were composited with those SnTe powders prepared by melting method to adjust the thermal and electrical transport of SnTe compound.The effect of SnTe nanoadditives on the electrical properties of SnTe parent phase is not remarkable.However,those SnTe nanocrystals can act as the phonon scattering centers and decreases the phonon thermal conductivity effectively.Finally,the ZT_maxvalue of 0.71 was achieved at 873 K,compared to the matrix increased by 65%.3.To enhance the thermoelectric performance of SnTe,ZnO nanoparticles have also been added into the SnTe matrix.On one hand,a large number of of ZnO/SnTe heterojunction interfaces can filter some lower energy carriers and thus effectively increases the density of states near the Fermi level and the Seebeck coefficient;on the other hand,the multiscale phonon scattering centers formed by uniformly dispersed spherical Zn O nanoparticles and ZnO aggregates can block the transport of phonons and thus reduce the thermal conductivity of SnTe effectively.Finally,the ZT_maxof SnTe+0.8wt%ZnO sample is about 0.9 at 873 K,which is 112%higher than the matrix.4.The electrical and thermal transport of WSe₂ compositing SnTe has been studied.The electron-blocking layer can be formed with the introduction of WSe₂,which filtered some minority carriers,so that the net Seebeck coefficient of the material can be improved effectively.At the same time,the introduction of WSe₂ with extremely low intrinsic thermal conductivity greatly reduce the lattice thermal conductivity,and the resulted high density stacking faults in the process of hot pressing further block the phonon transport.Finally,the ZT_maxof SnTe+6wt%WSe₂ sample is about 0.96 at 873 K,which is 123%higher than the SnTe matrix.5.The influence of Cl substitution of Te on thermoelectric properties of SnTe compounds has been studied,the substitution of Cl can improve the electrical properties and decrease the thermal conductivity effectively,by which a maximum ZT of 0.75 was achieved at 873 K,which is 75%higher than the matrix;then,on the basis of Cl substitution,ZnO nanoinclusions were further added into the matrix to regulate the TE performance.As a result,a maximum ZT of 1.0 has been achieved at 873 K for the sample of SnTe_0.88?SnCl₂?_0.06+0.5%ZnO,which is 33%higher than the result from the first stage and 133%higher than the matrix.6.The influence of Bi doping on the thermoelectric properties of SnTe compounds has been studied.Bi occupys the position of Sn can significantly reduce the hole concentration and increase the Seebeck coefficient when the Bi doping content is over the limit of solid solubility,excess Bi precipitates in the form of the second phaseand can significantly reduce the lattice thermal conductivity.Therefore,it is precisely because of the multiple influence of Bi in SnTe compounds that greatly improves the thermoelectric performance of SnTe and a maximum ZT of 1.1 has been achieved at 873 K for the sample of Sn_0.94Bi_0.06Te.7.The influence of MCl₃?M=Bi,Ce?double doping on the thermoelectric properties of SnTe compounds has been studied.The substitution of MCl₃ contribute electrons both in the anion sites and the cation sites synchronously,thus greatly reduce the hole concentration of the material.Meanwhile,the significant enhancement of the density of states near the Fermi level further increases the Seebeck coefficient,which greatly improves the electrical transport properties of the material;while the thermal conductivity is effectively reduced by the supersaturated solution precipitation of Bi and multiscale defects such as point defects and dislocations,finally the thermoelectric performance has greatly improved effectively,a maximum ZT of 1.27 was achieved at 873 K for the sample of Sn_0.96Bi_0.04Te_0.88Cl_0.12,which is 182%higher than the matrix.8.MnO₂ nanoparticles have been added into the SnTe matrix and they reacted in situ with the matrix to form the Mn doped SnTe matrix and a large amount of dispersed SnO₂nanoparticles.On the one hand,the solid solution of Mn effectively reduces the energy split between the heavy band and the light band significantly thus increases the band degeneracy and Seebeck coefficient,on the other hand,the point defects,nanocrystalline SnO₂ and excessive Mn precipitates greatly scatter phonons and thus reduce the thermal conductivity significantly.As a result,a maximum ZT of 1.46 has been achieved at 873 K,which is 224%higher than the matrix.9.The influence of hot working process on the formability and thermoelectric properties of SnTe compounds has also been studied preliminarily.By controlling the deformation times,deformation temperature,deformation parameters and so on,the dislocation density,grain size and grain orientation can be regulated.It can improve the thermoelectric properties andand offer new insight for to the improvement of TE performance.