Growth of Co-Based Oxide Whiskers

Three kinds of single-crystalline whiskers of Bi 2 Sr 2 CoO 6 (BC-221), (Bi 2 Sr 2 O 4 ) x (CoO 2 ) 2 (BC-222), and (Ca 2 CoO 3 ) x CoO 2 (Co-225) phases were grown from the surfaces of precursor plates by annealing in an O 2 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 2.5 (Sr, Ca) 3.3 CoO 6+ δ, Bi 3.0 (Sr, Ca) 3.1 Co 2.0 O 9+ δ, and (Ca, Sr, Bi) 1.9 Co 2.0 O 5+ δ . Although the Co-225 whiskers are 1.2 mm at the longest, the BC-221 and the BC-222 whiskers reach lengths of as much as 8.0 mm. Microstructural observations indicate that the growth points of the whiskers are their bases. The length of the whiskers depends on the crystallization status of the precursor plates.