| Summary: | 碩士 === 國立彰化師範大學 === 物理學系 === 98 === This reports includes the crystal structure and high-temperature transport properties of element-doped perovskite calcium manganese oxide. Under high temperature, Calcium manganese oxide series are the most respected N-type thermoelectric materials. CaMnO3+δ exhibits a high thermoelectric power, but the electric resistivity and thermo conductivity exhibits high value as well. Hence, the thermoelectric performance (ZT) of the polycrystalline samples is small and unsuitable for practical utilization. The ZT of calcium manganese oxide would require improvement to become acceptable for practical application. It will become a promising thermoelectric material for high temperature power generation. This experiment attempts to enhance the thermoelectric properties by element doping. The samples used in this experiment includes CaMn1-ySiyO3+δ、Ca1-xBixMnO3+δ、Ca1-xBixMn1-ySiyO3+δ、Ca0.98R0.02MnO3+δ (R = Sm, Eu, Gd, Dy) and Ca1-xGdxMnO3+δ. All the samples are crystallized into orthorombic distorted perovskite structures (space group Pnma(No.62)). All the samples were prepared by conventional solid-state reaction method. The homogeneity was confirmed by X-ray diffraction (XRD), and the XRD data is used to refine the cell parameters. The electric resistivity and thermoelectric power is measure in respect of temperature from 300K to 700K. The thermal conductivity was measured by Kapton sensor at room temperature (~300K) and Mica sensor at high temperature (from 300K to 620K). The average valence of manganite and oxygen content is determined by iodometric titration. SQUID is use to measure some samples from 5K to 400K. From the SQUID data, we yield effective magnetic moment by applying Curie-Weiss law. The electric resistivity is reduces significantly and displays metallic property when Ca is substituted with Bi, Sm, Eu, Gd, or Dy. In the substitution of Si with Mn, electric resistivity data still exhibits semiconductor property. When we combine the substitution of Bi and Si with Ca and Mn, electric resistivity data differs by substitution ratio. At Ca0.98R0.02MnO3+δ (R = Sm, Eu, Gd, Dy) part, the maximum values is recorded with Dy substitution, in which the power factor (PF) was 2.217 μW/cmK2 and dimensionless figure of merit (ZT) was approximately 0.0723 at 618K. In this experiment, Ca0.95Bi0.05MnO3+δ showed the highest value recorded. The power factor (PF) was 3.122 μW/cmK2 at 620K.
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