Preparation And Characteristics Of Bi₂Te₃ Based Composite Thermoelectric Materials

The research on new thermoelectric materials has always been a hot topic in the field area of new energy materials.Bi₂Te₃-based thermoelectric materials have been commercialized.Recently,it was found that the density of energy states near the Fermi surface of Bi₂Te₃can be increased via introducing a second phase compound or/and a small number of nanoparticles(nanowires)with conductive and thermal properties into the lattice of Bi₂Te₃.In addition,through adding a little bit of other substances,more interfaces and defects can be introduced into the Bi₂Te₃lattice to increase the scattering of phonons and low energy electrons in order to reducing the thermal conductivity.It is noteworthy that the thermal conductivity of Bi₂Te₃based composite thermoelectric material can be further improved by the presence of one-dimensional carbon nanotubes(CNTs)owing to the networks of CNTs,which can exhibit unique electrical properties and mechanical strength.Furthermore,carbon nanotubes dispersed uniformly in the lattice of Bi₂Te₃can introduce abundant interface and growth defects in the lattice structure so that enhanced ZT can be achieved.In order to further improve the thermoelectric properties of Bi₂Te₃thermoelectric materials,in this thesis,the influences of Bi₂Te₃and Bi₂Se₃alloys fabrication by hot die-pressing and sintering methord on their thermoelectric properties were researched systematically.And then the influences of optimum fabrication parameters and different doping ratio of Cu,Si C,MWCNTs on thermoelectric properties of Bi₂(Te_0.95Se_0.05)₃were researched respectively.By comparing and analyzing the Seebeck coefficient,as well as ZT value of the above four kinds of Bi₂Te₃based composite thermoelectric materials,the fabrication parameters of the Bi₂Te₃based composite thermoelectric materials with the good performance were found.The research method has its innovation.The results are of great value to further improve the thermoelectric characteristics,and even expand the application of Bi₂Te₃and Bi₂Se₃based composite thermoelectric materials.The main researches of this thesis are showed in the following:(1)Bi₂(Te_1-xSe_x)₃composite thermoelectric materials were fabricated by hot die-pressing and sintering method by using Bi₂Te₃and Bi₂Se₃powders as source materials.The influences of the experimental parameters(including time,pressure,temperature,etc.)on material sintering quality were investigated.Through orthogonal analysis experiment,the influence of Bi₂Te₃incorporation on the performance of Bi₂(Te_1-xSe_x)₃composite thermoelectric materials was researched,such as differential thermal electromotive force,conductivity,thermal conductivity,Seeback coefficient and ZT values.The optimum mass ratio between Bi₂Te₃and Bi₂Se₃was found.The results indicated that the fabricated Bi₂(Te_1-xSe_x)₃composite thermoelectric materials were confirmed to be n-type at the heat source temperature of 316 K.The thermoelectric properties of the material changed with varying composition“x”of Bi₂Se₃.The Seebeck coefficient and thermoelectric value of the materials with the best values are-208.33?V/K and 0.89@x=0.05,respectively.(2)Si C nano-materials based on Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials were fabricated by hot die-pressing and sintering method.The fabrication of Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials doped with different proportions of Si C nano-materials was carried out.The influence of Si C incorporation on the performance of Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials was researched.The surface morphology,lattice structure and phase composition of Si C doped Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials were analyzed.The results displayed that the optimum value of Seebeck coefficients and ZT can be obtained at the heat source temperature of 316 K.The values are-292.07?V/K and0.973@Si C with the chemical composition of 0.0004,respectively.(3)Based on the above experimental results,Cu/Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials were fabricated by hot die-pressing and sintering method.The fabrication of Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials doped with different mass percentages of purity Cu powder was researched.The influences of the Cu with different incorporation ratios on the performance of Bi₂(Te_0.95Se_0.05)₃thermoelectric composites were investigated,such as differential thermal electromotive force,conductivity,thermal conductivity,Seeback coefficient and ZT values.The Cu/Bi₂(Te_0.95Se_0.05)₃thermoelectric composites with Cu chemical composition of 0.005 exhibited excellent Seebeck coefficients and ZT value at the heat source temperature of 316 K.The values are-294.86?V/K and 1.77,respectively.The value of ZT is improved by about 2 times,compared with Bi₂(Te_1-xSe_x)₃composite thermoelectric materials.(4)Finally,based on the above experimental results,MWCNTs/Bi₂(Te_0.95Se_0.05)₃composite thermoelectric materials were fabricated by hot die-pressing and sintering method.The influences of MWCNTs with different incorporation ratios on the related thermoelectric parameter of Bi₂(Te_0.95Se_0.05)₃were analyzed comparatively.The incorporation ratio of MWCNTs was optimized through orthogonal experimental analysis.The mechanical strength was improved,as well as the thermoelectric properties of Bi₂(Te_0.95Se_0.05)₃were increased.The results showed that the MWCNTs/Bi₂(Te_0.95Se_0.05)₃with MWCNTs chemical composition of 0.003 exhibited the maximum ZT value of 1.39 at the heat source temperature of 316 K.The results are helpful to improve the thermoelectric properties and mechanical properties of Bi₂Te₃based composite thermoelectric materials,which is of great value to further expand their applications.