Preparation, Properties And Quantum Chemistry Calculation Of Cobaltite Thermoelectric Materials

The thermoelectric effect, theory with the thermoelectric parameters and research progress are discussed in the dissertation. Ca₃Co₂O₆ with 1D Co-O chains and misfit layer Ca₃Co₄O₉ with 2D Co-O chains are selected for research and experiment and theory calculation applied to study these systems.Sol-gel method and no pressure sinter are used in the experiment, some influences are considered in the powder synthesizing progress, Ca₃Co₂O₆ is decomposed by Ca₃Co₄O₉. They have the same tendency of resistivity and Seebeck coefficient with the temperature and are both p-type semiconductors. To their power factors, they increase with the temperature, however, Ca₃Co₄O₉'s increase sharply after 600℃. Cu and Sr atoms are applied to dope Co and Ca site atoms. Doping can decrease the resistivity in some degree, Ca₃(Co_1-xCu_x)₂O₆'s is improved obviously in 100-500℃, but it has no great effect on (Ca_1-xSr_x)₃Co₂O₆ Seebeck coefficient.. The resistivity and Seebeck coefficient of based-Ca₃Co₄O₉ oxides all increase with the temperature, doping can lower the resistivity, and Seebeck coefficient change little.The electronic structure and chemical bonds of Ca₃Co₂O₆ system are studied by the quantum chemical software CAmebride Serial Total Energy Package (CASTEP) which is based on Density Function Theory (DFT) and pseudopotential. It can be found from the band structure that materials belong to p-type semiconductor, which is consistent with conclusions of the experiment, and the highest valence band(HVB) and the lowest conduction band(LCB) are mainly contributed from Co and O atomic orbits. The results indicated that Co-O chains along the c axis mainly cause electric conductivity of Ca₃Co₂O₆. Furthermore, the viewpoint is also confirmed by the calculation of the effective quality of HVB and LCB along the main axis. Ca and Sr atoms have no attribution to the density near the Fermi energy in the Ca_1.5Sr_1.5Co₂O₆ calculation model, and the mutual effect between Col and Co2 is lower than no-doped oxide; the other model is Ca₃Co_1.5Cu_0.5O₆ which Cu replaces the Col site of octahedron. An acceptor impurity band appears in Ca₃Co_1.5Cu_0.5O₆ band structure, and the gap becomes narrower. It can be found from the total density that Cu atomic orbits contribute more energy to the conduction band near Fermi energy than Co atom's, and the effect of Cu-Co2 becomes strong.