| Summary: | 博士 === 淡江大學 === 物理學系博士班 === 100 === The misfit-layered cobaltites Ca3Co4O9+δ have shown large thermoelectric power and low electrical resistivity and thermal conductivity at room temperature. These cobaltites exhibit good chemical stability in air at high temperatures in comparing with intermetallic compounds, e.g., Bi2Te3, Zn4Sb3, PbTe and Si1-xGex alloys. The polycrystalline cobaltites are more reliable for practical applications even with a relatively lower performance compared to single crystalline cobaltites. However, single crystalline samples are too expensive for the practical fabrication of TE devices. Partial substitutions of various metal elements for Ca and/or Co to optimize the carrier concentration and to improve other electronic transport properties can result in better thermoelectric properties of these materials. We have performed systematic x-ray absorption spectroscopy measurements on two series of doped misfit-layered cobaltites, Ca3Co4-xMx(M = Fe, Mn)O9+δ and Ca2.9Ln0.1(Ln = Ca, Dy, Ho, Er and Lu)Co4O9+δ, in order to gain a deeper understanding of the correlations between the electronic structure and the thermoelectric properties in these materials. We found that reducing the ratio of Co4+/Co3+ will increase thermoelectric power. Increasing the hybridization of Co4+ (3d5) – O (2p) will decrease the electric resistivity. Introducing lattice distortion will decrease the thermal conductivity. We also developed the in-situ electrochemistry flow cell which can be performed with liquid samples in high vacuum environment.
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