First Principle Calculations And Mechanical Tuning Of Thermoelectric Properties Of Novel Oxychalcogenides

Thermoelectric devices can be used to convert heat energy directly into electric power, and they are promising in easing world energy shortage and environmental pollution. Recently, it has been found in experiments that novel oxychalcogenides, such as BiCuOCh (Ch: S, Se, Te), CuGaTe2and SrTiO3, exhibit good thermoelectric properties, though theoretical understanding on these materials is rather poor. In this disserttion, in order to understand the microscopic mechanisms of thermoelectric transport of these compounds, the electronic structures of these oxychalcogenides are investigated using first-principles calculations, and their thermoelectric properties are semi-classic Boltzmann transport theory. The optimal doping concentrations have been estimated, and the effects of external stress or misfit strain have been demonstrated. The major work can be summarized as the follows:(1) The microscopic mechanisms of thermoelectric transport of oxychalcogenides BiCuOCh, CuGaTe2and SrTiO3have been investigated by electronic structure calculations using first-principles. By analyzing the characteristics of the electronic structures, it was found that the antibonding states of Cu3d-Ch np determine the transport properties of BiCuOCh. It was also found that the electronic structure of CuGaTe2has a mixture of heavy and light bands near the valence band maximum, which is highly desirable for good thermoelectric performance. The relatively high Seebeck coefficient of n-type SrTiO3was also analyzed.(2) The optimal carrier concentrations of BiCuOCh, CuGaTe2and SrTiO3have been calculated. The chemical potential, temperature and doping levels dependence of thermoelectric transport properties of these compounds were calculated and compared with experimental data, and good agreements are observed. Based on the calculated maximum power factor with respect to relaxation time of these oxychalcogenides, the corresponding optimal doping concentrations were estimated. The thermoelectric properties of these p-doped oxychalcogenides were also compared with these n-doped compounds.(3) The effects of external stress on the thermoelectric properties of BiCuOSe and CuGaTe2have been analyzed by first-principles calculations in conjunction with the semi-classical Boltzmann theory. The phenomenon that electrical conductivities of these compounds can be improved by external stress was observed, and the microscopic mechanisms of thermoelectric transport of these compounds tuned by external stress were investigated.(4) The thermoelectric properties of CuGaTe2and SrTiO3tuned by strain were studied. The electronic structures and thermoelectric properties of these oxychalcogenides under biaxial strain were calculated, and the variation of electronic structures and thermoelectric properties of these compounds under strain were analyzed. It was found that strain-induced effects in Seebeck coefficient and electrical conductivity tend to counter each other, and microscopic mechanisms of thermoelectric transport of these oxychalcogenides under strain were estimated.In summary, the relationship between microscopic mechanisms and thermoelectric properties of these novel oxychalcogenides was investigated in this dissertation. It was found that thermoelectric properties of these compounds can be tuned by band engineering, such as doping, external stress and strain. This research offers useful guidelines for improving the thermoelectric performance of these compounds, and lays the foundation for further study of more complicated thermoelectric material systems.