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

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 ${\mathrm{(Bi, Pb)}}_{\mathrm{2.2}}{\mathrm{(Sr, Ca)}}_{\mathrm{2.8}}{\mathrm{Co}}_{\mathrm{2}}{\mathrm{O}}_y$ (abbreviated to BC-232 whisker) and a layered structure in which two different sublattices, ${\mathrm{CoO}}_{\mathrm{2}}$ and rocksalt ${\mathrm{(Bi, Pb)}}_{\mathrm{2}}{\mathrm{(Sr, Ca)}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{4}},$ alternate in the $c$ -axis direction. Edge-sharing ${\mathrm{CoO}}_{\mathrm{2}}$ layers act as conducting and thermoelectric units. Although a plateau is observed at around 500 K, the Seebeck coefficient of the whiskers is about $\text{100\hspace{0.05em}μ}{\mathrm{V K}}^{\mathrm{-1}}$ 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 $\text{0.5\hspace{0.05em}}{\mathrm{mWm}}^{\mathrm{-1}}{\mathrm{ K}}^{\mathrm{-2}}$ at temperatures higher than 650 K and reaches $\text{0.9\hspace{0.05em}}{\mathrm{mWm}}^{\mathrm{-1}}{\mathrm{ K}}^{\mathrm{-2}}$ at 973 K.