Electronic Structure Calculations On Bi₂Te₃-and Ca₃Co₂O₆-based Thermoelectric Materials, Preparation Of Thermoelectric Ceramics

Primarily using the Linearized Augmented Planewave (LAPW) method and Quantum Chemistry Calculation method of density function and discrete variation (DFT-DVM), we calculated two important thermoelectric materials: Bi2Te3 based thermoelectric alloys and Ca3Co2O6 based thermoelectric oxide.Bismuth telluride based alloys are the best materials for cooling devices around room temperature. In this thesis, we have calculated bismuth telluride based alloys by LAPW method and DFT-DVM method. The results show, Bi2Te3 and SnBi2Te4 are both narrow-band semiconductor, the spin-orbit interaction plays a important role in determining bands near Feimi level, and their bands have many degeneracy. The conduction band of Bi2Te3 is predominantly Bi 6p in nature whereas the valence band is predominantly Te(I) 5p. For SnBi2Te4 due to doped Sn, the Sn atom predominantly affects the conduction band. Compared Bi2Te3 with SnBi2Te4, the gap and the effect mass of SnBi2Te4 become smaller due to doped Sn atom; the Sn atom only link to Te((II) and the covalent bonds of Sn-Te(II) is stronger than that of Bi-Te(II) while is weaker than that of Bi-Te(I). The calculated results preferably explain the experimental phenomena.Ca3Co2O6 based oxide compounds are a layered compound and are the value materials to study due to according "phonon glass and an electron crystal" (PGEC) theory. We have calculated calcium cobalt oxide and the Ni-doped one by LAPW method and DFT-DVM method. The results indicate, Ca3Co2O6 and Ca3CoNiO6 are both narrow-band semiconductor, the spin-orbit interaction plays a important role in determining gaps, and their bands have many degeneracy; and the transport property of them are both anisotropic. The doped Ni atom changes the band structure of Ca3Co2O6 and the covalent and ionic bonds of Ni-doped ones are both weaker than thoseBased on the results of theoretical calculation we have synthesized Ca3Co2O6 compound and the Ni-doped one by Sol-Gel method and then sintered the powders by spark plasma sintering. The results indicate, the thermoelectric properties of Ca3Co2O6 compound agree well with the theoretical calculations; Because the Ni atom doped toCa3Co2O6, the experimental results do not agree well with theoretical calculations, and this phenomena will be useful to study.