Theoretical Calculations On The Thermoelectric Properties Of Bulk And Low-dimensional Materials

Due to increasing energy crisis and environment pollution, there is currently growing interests in searching for advanced energy materials. Among them, the thermoelectric materials which can directly convert heat into electricity or vice versa have attracted much attention. The ZT value of traditional theremoelectric materials is about 1.0, which limit the widely use of these materials. In the past two decades, people have been looking for high-efficiency thermoelectric materials. In recent years, the good thermoelectric performance of CuInTe2 has been reported experimentally. Besides, Hicks et al. first predicted that one-and two-dimensional structures could have significantly larger ZT values than the corresponding bulk materials because of decreased thermal conductivity caused by enhanced phonon boundary scattering as well as improved power factor caused by quantum confinement. In this dissertation, we used a combination of density functional theory (DFT), semi-classical Boltzmann theory and molecular dynamics (MD) simulations to investigate the thermoelectric properties of CuInTe2 compounds and other two-and one-dimensional structures, such as the combination of graphene nanoribbon with BN sheet systems and SiGe nanotubes.We first study the thermoelectric properties of CuInTe2 chalcopyrite. The electronic and transport properties of CuInTe2 chalcopyrite are investigated using density functional calculations combined with Boltzmann theory. The band gap predicted from hybrid functional is 0.92 eV, which agrees well with experimental data and leads to relatively larger Seebeck coefficient compared with those of narrow-gap thermoelectric materials. By fine tuning the carrier concentration, the electrical conductivity and power factor of the system can be significantly optimized. Together with the inherent low thermal conductivity, the ZT values of CuInTe2 compound can be enhanced to as high as 1.72 at 850 K, which is obviously larger than those measured experimentally and suggests there is still room to improve the thermoelectric performance of this chalcopyrite compound.We then calculate the lattice thermal conductivity of thermoelectric material CuInTe2. It is predicted using classical molecular dynamics simulations, where a simple but effective Morse-type interatomic potential is constructed by fitting first-principles total energy calculations. In a broad temperature range from 300 to 900 K, our simulated results agree well with those measured experimentally, as well as those obtained from phonon Boltzmann transport equation. By introducing the Cu vacancy and Cd impurity, the thermal conductivity of CuInTe2 can be effectively reduced to optimize the thermoelectric performance of this chalcopyrite compound.We also investigate the thermoelectric properties of systems that two kinds of graphene nanoribbons embedded in boron nitride sheets. It is found that the relaxation time of carriers in two kinds structure have larger values which can be comparable to that of graphene nanoribbons. Combining with the relatively high Seebeck coefficient and low lattice thermal conductivity, two systems are found to exhibit good thermoelectric properties. Moreover, the peak of ZT values for these systems correspond to different temperature. If we combine with two structures, the ZT values can be keep a high values at broad temperature region.Finally, we investigate the thermoelectric properties of two typical SiGe nanotubes using a combination of density functional theory, Boltzmann transport theory, and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe nanotubes tend to have gear-like geometry, and both the (6,6) and (10,0) tubes are semiconducting with direct band gaps. The calculated Seebeck coefficients as well as the relaxation time of these SiGe nanotubes are significantly larger than those of bulk thermoelectric materials. Together with smaller lattice thermal conductivity caused by phonon boundary and alloy scattering, these SiGe nanotubes can exhibit very good thermoelectric performance. Moreover, there are strong chirality, temperature and diameter dependence of the ZT values, which can be optimized to 4.9 at room temperature and further enhanced to 5.4 at 400 K for the armchair (6,6) tube.