First-principles Study On The Electronic And Transport Properties Of Thermoelectric Materials

The aim of this thesis is to investigate the electronic, transport and other physical properties of some bulk and low-dimensional thermoelectric materials from first-principles calculations. In combination with the Boltzmann theory, we are able to predict a large number of potential candidates which may exhibit higher thermoelectric performance. Our theoretical work serves as a guide to the experimental study searching for high-performance thermoelectric materials.We first focus on the half-Heusler compounds TiNiSn and TiCoSb. At room and elevated temperature, it is found that the p-type doping may be more favorable than the n-type doping for both compounds. The optimized doping concentration corresponds to a chemical potential of -0.34 eV and -0.61 eV for the TiNiSn and TiCoSb, respectively. By using the density functional perturbation theory (DFPT), the phonon dispersion relations of TiNiSn and TiCoSb are obtained and the corresponding thermal conductivity are examined with respect to the size of these two compounds.The electronic structure and transport properties of bulk Bi2Te3 are then investigated. Within the rigid-band picture, the optimal doping atom and the corresponding doping concentration are obtained. We find that the power factor of p-type doped Bi2Te3 is higher than that of n-type at high temperature. On the other hand, we construct a series of Bi and Sb nanotube structures. It is found that most of the Bi-tubes are semiconducting and the energy gap decreases when the diameters of these tubes are increased. The power factor of the Bi(10,0) and Sb(6,0) tubes will be enhanced for the p-and n-type doping, respectively. If the power factor and thermal conductivity between them can be adjusted experimentally, one can realize thermoelectric devices with nanosize.We also discuss the possibility to using carbon nanotubes as thermoelectric materials. When doped with As/Sb/Bi elements, there are obvious changes of the band structures of three kinds of 4 A carbon nanotubes. Moreover, the band gap of (8,0) and (10,0) carbon nanotubes decrease when doped with As atom. The relatively higher power factor of these two nanotubes suggests that carbon nanotubes may be a new kind of high performance thermoelectric materials if one can effectively reduce their thermal conductivity.