Thermoelectric properties in Bi2-xPbxSr3-yYyCo2O9-delta ceramics

Thermoelectric (TE) properties (resistivity and thermopower) of polycrystal line ceramics in the Bi2-x PbxSr3-yYyCo2O9-delta system prepared by a solid -state reaction in air have been investigated as a function of temperature up to about 300 K, changing the combination of x and y. These oxides are ch emically very stable, and TE properties and the lattice structures do not c hange even after they are heated at high temperatures. Y doping has a profo und effect in reducing resistivity. When y (the amount of Y) is small, liga nd holes created by O 2p-Co 3d hybridization are mainly responsible for ins ulating conduction. With increasing y, resistivity decreases rapidly and in sulating conduction transfers to metallic conduction. This is due to the sh rinkage in the lattice constant by the Y doping, which leads to band crossi ng of the O 2p and Co 3d levels. Measurements of TE properties including th ermal conductivity have been carried out on the material with x = y = 0.5 i n the high-temperature range up to 800 K. The TE figure of merit temperatur e increases from 2 x 10(-5) K-1 to 7 x 10(-5) K-1 as the temperature increa ses from 400 K to 800 K, which indicates that this compound has high potent ial as a good TE material at high temperatures.