Synthesis and thermoelectric properties of Co-based oxide whiskers

Two kinds of single crystalline whiskers of (Bi₂Sr₂O₄)_x(CoO₂) ₂ (BC-222) and (Ca₂CoO₃)_xCoO ₂ (Co-225) phases were grown from the surfaces of precursor plates by annealing in an O₂ gas flow. The phases of the whiskers can be controlled by the Co content of the precursor plates. The average compositions of the whiskers are Bi_3.0(Sr, Ca) _3.1Co_2.0O_9+δ and (Ca, Sr, Bi)_1.9Co_2.0O_5+δ for the BC-222 and the Co-225, respectively. Although the Co-225 whiskers are 1.2 mm at the longest, the BC-222 whiskers reach lengths of as much as 1.0 cm. The difference in the whisker length is due to the microstructure of the precursor plates. Seebeck coefficient and resistivity of both whiskers are higher than 200 μVK^-1 and suppressed to a few mΩcm at high temperature in air, respectively. These phases seem to have pseudogap at the Fermi level. κ of polycrystalline samples of both phases is suppressed to low values at high temperature. These oxides are promising thermoelectric materials for high temperature application