Theoretical Investigation On The Electronic Structure And Thermoelectric Properties Of Bi₂Te₃ And AgGaX₂（X=S、Se、 Te） Compounds

Thermoelectric materials can realize the direct conversion between heat and electricity, and there is on noise and pollution during the process of transformation. Thermoelectric materials can make use of industrial waste heat and solar energy to generate electricity, thus relieve the energy crisis and environmental problems to a certain extent. For the figure of merit of the existing thermoelectric materials are not high enough, the thermoelectric conversion efficiency is not ideal. In this paper, we studied the conformation and electronic structure of a total 11 kinds of compounds including Bi2Te3 and AgGaX2 (X=S, Se, Te) system to achieve a deep understanding on the microscopic mechanism of thermoelectric materials and their influence factors. Meanwhile, we predicted the thermoelectric properties of these systems in expectation to provide theoretical guidance for the development of thermoelectric materials.For the existence of a heavy elements Bi in layered Bi2Te3 compound system, spin-orbit coupling effect has a significant influence on the electronic structure. Its not only affects the energy band structure near Fermi energy, but also affect the minimum band gap value and the band gap type. The calculation results of transport properties showed that Bi2Te3 has a good thermal performance, and the ideal figure of merit can reach 2.2. Further research results showed that the figure of merit of Bi2Te3 increased with the decrease of the distance between the two layers without considering thermal conductivity changes. On this basis, we also studied the electronic structure of two types of doped Bi2Te3 system and the results showed that Sb doping affected both the top of the valence band and the bottom of the conduction band. But when Te atom was replaced by Se, only the bottom of the conduction band would be influenced. Therefore, it can be expected that the effect of the thermoelectric properties of the Bi2Te3 by Sb doping is more apparent.For AgGaX2 system, we not only calculated the electronic structure and thermoelectric properties, but also calculated the elastic properties and thermodynamic properties. Calculation results showed that the elastic properties of three kinds of materials all meet the requirement of mechanical stability, and the three kinds of crystals are anisotropic materials with an easy extension feature, and the ductility reduce gradually from S to Te. Thermodynamic properties calculation results showed that the thermodynamic property of AgGaTe2 is obviously different from AgGaS2 and AgGaSe2 system. AgGaTe2 has a relatively high thermal expansion coefficient, lower Debye temperature and an especially lower lattice thermal conductivity. The calculation results of the electronic transport properties indicates that there was little difference in Seebeck coefficient, σ/τ and PF/τ between the three systems. AgGaTe2 showed a relatively high ZT value for its low lattice thermal conductivity, and the biggest ZT value can reach about 1.4 when the temperature is 900K.