Modeling of critical currents in granular high-Tc superconductors

The transport critical current density of several samples of bulk sintered high-Tc superconductors was measured at very low magnetic fields and fitted to a model which assumes that the impediments to current at such fields are Josephson weak links. A sample of particular interest was Y1Ba2Cu3Ox made from hydroxycarbonate precursors; the final bulk sintered sample was very fine-grained, having an average grain size of about 1.8 μm as determined by a linear intercept analysis. The fit to the model is excellent if the average linear dimension of the weak links is chosen to be 20 m. The authors conclude that this sample as well as the others has Josephson weak links at its grain boundaries, and that any intragrain defects which may be responsible for flux pinning are not the primary weak links limiting the transport Jc of bulk samples at very low magnetic fields