KINETICS OF GROWTH OF 2212 PHASE IN MELT-QUENCHED BI2SR2CACU2OX

A kinetic study of growth of the superconducting phase in melt-quenched Bi2Sr2CaCu2Ox has been performed by coloured optical microscopy. The pellets of Bi2Sr2CaCu2Ox with the critical temperature at zero resistance (T(c, zero)) of 66.8 K were melted and quenched by pouring on a steel plate. Upon annealing, the plate-like structure of the superconducting phase (2212) grew and showed a distinct colour in the polarized light of an optical microscope. The volume fraction of 2212 phase, as determined from the coloured optical micrographs, was analysed with the help of the Avrami transformation relation. The thickness of 2212 plates, measured by a scanning electron microscope, followed a power law of time with the exponent ranging between 3 and 5, and had an activation energy of 80 kJ mol-1. The superconducting properties of the melt-processed samples were better than those sintered; the highest T(c, zero) and transport critical current density (J(c)) obtained were 85.3 K and 54.2 A cm-2 at 77 K, compared with 66.8 K and 15.1 A cm-2 at 55 K respectively for the sintered material. The optimum annealing temperature was found to be 820-degrees-C. Annealing at temperatures higher than 820-degrees-C decreased both T(c) and J(c) of melt-quenched Bi2Sr2CaCu2Ox.