Teruo Kumagai

Crystalline structures and superconducting properties of rapidly quenched BiSrCaCu2Ox ceramics

Phase changes and superconducting properties of rapidly quenched BiSrCaCu2Ox 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 A and c=24.5 A. By annealing above 700°C, a tetragonal system with a=5.41 A and c=30.8 A was produced. The former crystal was a semiconductor and the latter was a superconductor with Tc=75 K. A superconducting phase with Tc=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.

Superconducting Bi-Sr-Ca-Cu-O thin films by sputtering

BiSrCaCu2Ox thin films were prepared by rf magnetron sputtering. Superconducting transitions were observed for the films on MgO after postdeposition annealing at 870°C and above. The appearance of a superconducting transition at a temperature of about 120 K for the film necessitated annealing at close to, but not above, its melting point near 900°C: the appropriate annealing temperature range was from 875 to 900°C. It was noteworthy that the 910°C annealed film showed a superconducting characteristic as well as the 870°C annealed film, in spite of their crystal structures being entirely different. Besides annealing temperature, cooling rate from annealing temperature, film thickness and substrate dependences of superconductivity in the BiSrCaCu2Ox thin film were also investigated.