DEVELOPMENT OF THERMOELECTRIC Co-XIDES

From the environmental viewpoint，we need the research and development of new thermoelectric oxides that can transform the high temperature heat in the gases ejected from automobiles，thermal plants and so on to electricity。In a solid state material that has the temperature difference between two edges，there is the electromotive force proportional to the temperature difference。The high effective thermoelectric material contains low electric resistivity，low thermal conductivity and colossal thermopower.At high temperature(500℃-800℃)，usually materials are oxidized.If oxides are employed，however，there should be no problems。Furthermore，the discovery of high Tc oxides leads to the successful syntheses of high conductive Co-perovskite oxides.From this point of view，the present project has treated Bi_(2-x)Pb_XSr_(3-y)Y_yCo_2O(_9-δ)polycrystalline ceramics，explored the best thermopower material by changing the x/y-ratio and then elucidated the kinetics of the thermoelectric effect。As a result，Y-doping has the significant effect in the thermoelectric properties，reducing the resistivity by three orders and transforming this material from the insulator to the conductor。Since this compound contains impurity phases that interfere with the elucidation of the kinetics of the thermopower effect，Bi_(1.5)Pb_(0.5)Ca_(2-x)M_xCo_2O_(8-δ)has been prepared because this oxide does not contain impurity phases.By Sc^(3+)，Y^(3+)或La^(3+)·substitution for Mand changing x from0to0.3，the Co-O spacing has been changed。Then thermoelectric properties(Resistivity And Thermopower)and magnetic properties have been measured。The effective magnetic moments estimated from the magnetic measurements indicate that the charge-transfer-energy between Co e_g and O2p levels can mainly account for the thermoelectric properties.Besides this，oxygen solubility and attice deformation in the specimens contain also important effects on the thermoelectric features.

From the environmental viewpoint, we need the research and development of new thermoelectric oxides that can transform the high temperature heat in the gases ejected from automobiles, thermal plants and so on to electricity. In a solid state material that has the temperature difference between two edges, there is the electromotive force proportional to the temperature difference. The high effective thermoelectric material contains low electric resistivity, low thermal conductivity and colossal thermopower. At high temperature (500℃-800℃), usually materials are oxidized. If oxides are employed, however, there should be no problems. Furthermore, the discovery of high Tc oxides leads to the successful syntheses of high conductive Co-perovskite oxides. From this point of view, the present project has treated Bi_(2-x)Pb_XSr_(3-y)Y_yCo_2O(_9-δ) polycrystalline ceramics, explored the best thermopower material by changing the x/y-ratio and then elucidated the kinetics of the thermoelectric effect. As a result, Y-doping has the significant effect in the thermoelectric properties, reducing the resistivity by three orders and transforming this material from the insulator to the conductor. Since this compound contains impurity phases that interfere with the elucidation of the kinetics of the thermopower effect, Bi_(1.5)Pb_(0.5)Ca_(2-x)M_xCo_2O_(8-δ)has been prepared because this oxide does not contain impurity phases. By Sc^(3+)、Y^(3+) or La^(3+) ・ substitution for Mand changing x from 0 to 0.3, the Co-O spacing has been changed. Then thermoelectric properties (resistivity and thermopower) and magnetic properties have been measured. The effective magnetic moments estimated from the magnetic measurements indicate that the charge-transfer-energy between Co e_g and O 2p levels can mainly account for the thermoelectric properties. Besides this, oxygen solubility and attice deformation in the specimens contain also important effects on the thermoelectric features.