WEAK-LINK NATURE OF AC SUSCEPTIBILITY IN THE GRAIN-ORIENTED YBA2CU3O7-DELTA SUPERCONDUCTING CERAMICS

Weak-link behavior of magnetic susceptibility and its dependence on the grain boundary phase and grain alignment in the granular YBa2Cu3O7-delta superconductors are investigated. The grain-oriented specimens with uniaxial symmetry of preferred c-axis are prepared by tape casting the mixture slip of large grain powder and polymer binder. As an intergranular material, molten flux of BaCuO2 and CuO is added into the stoichiometric compound. Two superconducting components are observed in both real (chi') and imaginary (chi") parts of the ac susceptibility. The intrinsic signal (just below the transition temperature) which is induced by the shielding currents inside the grains is insensitive to the liquid phase at the grain boundaries. The coupling signal which is induced by the macroscopic shielding currents through grains and grain boundaries is sensitive to it. The shifting of the coupling peak in chi" to lower temperature with additional grain boundary phase is attributed to the more weakened superconducting coupling across the grain boundary. No anisotropy in the position and transition width of the coupling signal is observed in the grain-oriented sample with strong preferred c-axis. It can thus be suggested that the weak-links between the aligned superconducting grains with parallel c-axis are still remained in those samples. Possible causes for this weak-coupling behavior observed on the basal planes of the grain-oriented sample are discussed.