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.