| Summary: | 碩士 === 國立彰化師範大學 === 物理學系 === 98 === The misfit layered structure Ca3Co4O9 is composed with two subsystems of [CoO2] and [Ca2CoO3]. This compound has different b parameters of lattice constants between the [Ca2CoO3] block and [CoO2] sheet, so it is made up [Ca2CoO3]q[CoO2], q=bCoO2/bCa2CoO3～0.62. The triple layered rock salt type [Ca2CoO3] blocks can be written [CaO][CoO][CaO] that to serve as electronic transport layers. The CdI2 type [CoO2] sheets store carrier charges and provide electrons to [Ca2CoO3]. The single crystal Ca3Co4O9 exhibits a high thermoelectric power, low electric resistivity, and low thermal conductivity. We attempt to improve the thermoelectric properties by substituting transition metals. Using solid state reaction to prepare Ca3-xRexCo3.95Ga0.05O9+δ, aCa3-xRexCo4O9+δ (Re = Yb and Eu),Ca3-xAgxCo3.95Fe0.05O9+δ and Ca3Co4-xSixO9+δ samples. The electrical resistivity and thermoelectric power as a function of temperature were measured between 10K and 300K. The measuring temperature of thermal conductivity is at room temperature (～290K). The average valence of cobalt and oxygen content are determined by using the iodometric titration and the thermopower can be calculated by Heikes formula. When substituting Yb and Eu for Ca, the thermopower increase and thermal conductivity decrease with x in Ca3-xRexCo4O9+δ. The Ga-doped compared with Ca3-xRexCo4O9+δ samples affect the electrical resistivity decreased. For the Si-doped samples, the electrical resistivity and thermopower are improved simultaneously.
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