INTERGRANULAR PROPERTIES OF YBCO AND BSCCO CERAMIC SUPERCONDUCTORS AT LOW FIELDS

Isothermal low-field AC susceptibility measurements have been used to analyze the intergranular critical current density J(c)(T) on sintered, non-oriented YBa2Cu3O7-delta and Bi2Sr2CaCu2O8+delta ceramic samples at zero field. Below the critical temperature, potential variations, J(c)(T) almost-equal-to (1 - t(J))m with t(J) = T/T(J), have been found, T(J) being the onset of grain's coherence, but with different exponents, supporting that different mechanisms limit the intergranular J(c) values. Moreover, the effect of texture has been also considered on Bi2Sr2CaCu2O8+delta ceramics grown by the laser floating zone method, which have stronger intergranular junctions. Their high-temperature behaviour is limited by intrinsic effects, while at low temperatures the quality of the junctions is the limiting factor. The temperature dependence of the chi'(h0) extrapolation at zero field has also been correlated with the evolution of the intergranular penetration depth, lambda(J)(T).