Flux pinning and twin boundaries in YBa₂Ca₃O7-x

The (110) twin boundary separation d has been varied from 1500 AA to 300 AA by annealing polycrystalline samples in air at different temperatures ranging from 400 to 700 degrees C. The intragrain critical current density J c and its in-field behaviour are strongly affected by the variation of the superconducting properties and the microstructures. J c (H) has been found to drop more rapidly with increasing field as the d value decreases. This decrease in J c with increasing field may be associated with the weak link effects created by the twin boundaries and a corresponding decrease in pinning effectiveness. The corresponding pinning mechanisms are discussed.