Crystalline Structures and Superconducting Properties of Rapidly Quenched BiSrCaCu₂O_x Ceramics

Phase changes and superconducting properties of rapidly quenched BiSrCaCu₂O_x ceramics have been studied. The rapidly quenched sample consists of an amorphous state which begins to crystallize around 510°C. The crystal system was found to be tetragonal with a=5.39 Å and c=24.5 Å. By annealing above 700°C, a tetragonal system with a=5.41 Å and c=30.8 Å was produced. The former crystal was a semiconductor and the latter was a superconductor with T_c=75 K. A superconducting phase with T_c=105 K was observed in a sample annealed at 840°C. The superconducting volume ratio in the samples had a maximum at 803–840°C. The decrease of the volume ratio at the higher temperatures is considered to result from a decomposition of the superconducting phase (75 K) into a nonsuperconducting phase.