Preparation And Performance Optimization Of Weak Topological Insulator Bi₂TeI Thermoelectric Materials

Topological insulators have attracted increasing attention in thermoelectric community due to its unique electronic structure and low thermal conductivity.The weak topological insulators that are alternately stacked by quantum spin Hall layers and normal insulating layers,have very low lattice thermal conductivity,which has been considered as a kind of potential thermoelectric materials.In this thesis,the preparation of bulk materials and the electrical and thermal transport properties of weak topological insulator Bi₂TeI are studied.In order to overcome the difficulty in preparation of Bi₂TeI bulk materials,the preparation technique that combines with melting,annealing and spark plasma sintering methods was explored and the thermoelectric properties of Bi₂TeI was investigated.A further optimization of thermoelectric properties of Bi₂TeI was carred out by doping Cu and Zn and by compositing CuI with the Bi₂TeI matrix.The following main conclusions have been drawn:?1?Bi₂TeI materials were prepared by melting,annealing,and spark plasma sintering methods.Single-phase Bi₂TeI materials were obtained at the optimal conditions including a melting temperature of 723K,a melting time of 8h,and an excessive 10 at%of I.Compared with two kinds of layered materials Bi2Te3 and BiTeI,Bi₂TeI shows high conductivity and low Seebeck coefficient due to high carrier concentration and small band gap.Because of the strong lattice anharmonic effect caused by the unique stacking layer structure of Bi₂TeI,it shows the lowest lattice thermal conductivity.?2?The thermoelectric properties of p-type Bi₂TeI materials were optimized by doping Cu and Zn.The microstructure analysis results of CuxBi2TeI and ZnyBi2TeI indicate that Cu and Zn has entered into the layered lattice of Bi₂TeI.When the addition amount of Zn is 0.015,the doping limit is reached which leads to the formation of ZnI₂ impurity phase.The carrier concentration is reduced by Cu doping,which results in the decrease of electrical conductivity and power factor.Zn doping increases the carrier concentration,which is related to the increased hole numbers due to the substitution of Zn for Bi sites.As a result,the electrical conductivity is increased.The power factor of Zn_0.01Bi₂TeI material reaches the maximum value of0.15 mW m^-1 K^-2.With increasing the doping content,the thermal conductivity of doped Cu and Zn samples decreases first and then increases,and the first decrease of thermal conductivity is related to the defect scattering and disorder scattering.The increase of thermal conductivity may be due to the enhancement of interlayer coupling between quantum spin Hall layers and normal insulating layers and the influence of ZnI₂ secondary impurity.The ZT value of Zn_yBi₂TeI increases significantly in the doping range of 0²%.The maximum ZT value of Zn_0.01Bi₂TeI samples reaches 0.078,impoved by 70%as compared with that of undoped sample.?3?A series of p-type xCuI/Bi₂TeI bulk composite materials were prepared by melting,annealing,ball milling and spark plasma sintering methods.The microstructure analysis results indicate that the CuI particles distributed at grian boundary of the matrix.The carrier concentration of xCuI/Bi₂TeI composites decreases with the increase of CuI content,which is related to the doping effect produced by CuI.The thermal conductivity of xCuI/Bi₂TeI composites gradually decreases,which is resulted from the interfacial phonon scattering from the CuI secondary phase distributed at the grian boundary and point defect scattering due to the doping.The maximum ZT value of 3 wt%CuI/Bi₂TeI composite is about 0.11,improved by 67%as compared with that of matrix material.