LIMITS TO THE CRITICAL-CURRENT IN HIGH-T(C) SUPERCONDUCTING TAPES

Within the framework of the ''brick-wall'' model, we review in detail how various physical mechanisms could act to limit the critical current in high-T(c) superconducting tapes. The brick-wall model attempts to mimic in a simplified manner the complicated tape microstructure. We generalized it to take into account a distribution of grains sizes and possible different strengths of weak links between the grains and the existence of weak links associated with intergrain boundaries perpendicular to the plane of the tape. We discuss what experiments can distinguish between a situation where weak links limit the critical current, and another plausible scenario where intragranular pinning is the limiting factor. Our conclusion is that according to the available experimental data only a small, well-connected fraction of the high-J(c) tapes is effective in providing high critical current densities. For this fraction of the tape, pinning turns out to be the limiting mechanism. Finally, several possibilities for increasing the critical current of the high-T(c) superconducting tapes are discussed.