Critical currents and flux creep in melt processed high T_c oxide superconductors

YBa₂Cu₃O₇ crystals fabricated by a quench and melt growth process contain fine Y₂BaCuO₅ particles. These fine precipitates are considered to have three beneficial effects: they suppress crack formation; they promote oxygen diffusion; and they act as pinning centres. Such crystals exhibit larger flux pinning than single crystals and their magnetic behaviour can be understood in terms of the critical state model which has been developed for conventional type II superconductors. Both transport and magnetization measurements gave J_c values exceeding 30 000 A cm^-2 at 77 K and 1 T. Flux creep rate is also much smaller than that of single crystals and a value of pinning energy, U, comparable to that of conventional superconductors was obtained. A theoretical estimation of the contribution of 211 precipitates to flux pinning is also presented.