Critical currents in conductors-exploring the limiting mechanisms

The success of the BiSrCaCuO phase conductors in overcoming the "weak-link" problems that plague polycrystalline YBa/sub 2/Cu/sub 3/O/sub 7/ has been outstanding. In order to improve further their performance, we need to determine whether the critical current density J/sub c/ in these high current conductors is limited by "weak links" at the grain boundaries, or if flux pinning is the controlling factor. Magnetisation studies and current-voltage transport measurements made on the same sample allow this issue to be explored in detail. None of the classical "weak link" signatures appear in the behaviour of well-processed BiSrCaCuO conductors, but there is strong evidence of inhomogeneity that is field- and temperature-dependent. At high fields, the pattern of current flow appears to fragment into macroscopic islands, connected by "narrow bridges". The transport current is dominated by dissipation within these "narrow bridges", but this dissipation is similar in form to that found within grains, and is to be associated more closely with our pinning than with "weak links". These conclusions are reinforced by evidence on the anisotropy of the critical current with respect to field orientation, and also by the results of irradiation experiments.<>