Degradation by Mechanical Grinding, and Recovery by Annealing, in the Superconducting Phases of the Bi-Sr-Ca-Cu-O System

Abstract

It has been found that mechanical grinding may degrade phases from supercounducting to non-superconducting; i.e., a sample with 73vol% of the high-Tc phase (105K phase), 18vol% of the low-Tc phase (75K phase) and 8vol% of a non-superconducting phase resulted in a sample with 98vol% of non- superconducting phase after grinding for 62min. X-ray analysis showed that cleavage fracture of the high-Tc phase in the {001} planes occurred more easily than that of the low-Tc phase by mechanical grinding. The {001} fracture gradually changed to a more isotropic fracture on prolonged grinding. It was also found that the degraded sample did not recover easily to the initial conditions compared with the amorphous samples initially free of the high-Tc phase. A process which includes crystal grinding, causing the degradation of the superconducting phases, may not therefore be appropriate for fabricating practical superconducting wires or sheets.