Three kinds of single crystalline whiskers of Bi2Sr2CoO6 (BC-221), (Bi2Sr2O4)x(CoO2)2 (BC-222), and (Ca2CoO3)xCoO2 (Co-225) phases were grown from 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 Average compositions of the whiskers are Bi2.5(Sr, Ca)3.3CoO6+δ, Bi3.0(Sr, Ca)3.1Co2.0O9+δ, and (Ca, Sr, Bi)1.9Co2.0O5+δ for the BC-221, the BC-222, and the Co-225 whiskers, respectively. Although the Co-225 whiskers are 1.2mm at the longest, the BC-221 and the BC-222 whiskers reach lengths of as much as 10mm. Because the growth points of the whiskers are their bases, the length of the whiskers depends on the microstructure of the precursor plates. Both BC-222 and Co-225 whiskers show excellent thermoelectric properties at high temperature even in air.
