Thermoelectric properties of Pb- and Ca-doped (Bi2Sr2O4)(x)CoO2 whiskers

An oxide single crystalline whisker has been prepared which displays superi or thermoelectric properties at high temperature in air. The whisker has an average composition of (Bi, Pb)(2.2)(Sr, Ca)(2.8)Co2Oy (abbreviated to BC- 232 whisker) and a layered structure in which two different sublattices, Co O2 and rocksalt (Bi, Pb)(2)(Sr, Ca)(2)O-4, alternate in the c-axis directio n. Edge-sharing CoO2 layers act as conducting and thermoelectric units. Alt hough a plateau is observed at around 500 K, the Seebeck coefficient of the whiskers is about 100 muV K-1 at 100 K, basically increases with temperatu re up to 773 K, and is saturated at temperatures higher than 773 K. Tempera ture dependence of electric resistivity shows semiconducting-like behavior but its value is very low compared with general semiconductors. These elect rical phenomena would indicate pseudogap formation with its width of a few meV at the Fermi level. The power factor of the BC-232 whiskers is estimate d at over 0.5 mWm(-1) K-2 at temperatures higher than 650 K and reaches 0.9 mWm(-1) K-2 at 973 K. (C) 2001 American Institute of Physics.