Thermoelectric properties of Pb- and Ca-doped (Bi2Sr2O4)xCoO2 whiskers

Abstract

An oxide single crystalline whisker has been prepared which displays superior thermoelectric properties at high temperature in air. The whisker has an average composition of (Bi, Pb)2.2(Sr, Ca)2.8Co2Oy (abbreviated to BC-232 whisker) and a layered structure in which two different sublattices, CoO2 and rocksalt (Bi, Pb)2(Sr, Ca)2O4, alternate in the c-axis direction. Edge-sharing CoO2 layers act as conducting and thermoelectric units. Although a plateau is observed at around 500 K, the Seebeck coefficient of the whiskers is about 100 μV K−1 at 100 K, basically increases with temperature up to 773 K, and is saturated at temperatures higher than 773 K. Temperature dependence of electric resistivity shows semiconducting-like behavior but its value is very low compared with general semiconductors. These electrical 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 estimated 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.