CRITICAL CURRENT-DENSITY OF CERAMIC HIGH-TEMPERATURE SUPERCONDUCTORS IN A LOW MAGNETIC-FIELD

The transport critical current densities, <J(c)>, of bulk sintered YBaCuO and Bi(Pb)SrCaCuO samples in the shape of slabs have been measured at 77 K as a function of the slab thickness, d, and of a weak applied magnetic field, H(a), (H(a) less-than-or-equal-to 20 Oe, H(a) perpendicular-to <J(c)>). It was found that the transport critical current density <J(c)> decreases rapidly with increasing applied field, H(a), and that the self-field generated by the transport current causes <J(c)> to increase significantly with decreasing slab thickness. A critical state model which describes the intergranular field distribution inside the slab samples is used to calculate the transport critical current density <J(c)> (d, H(a)). Different local critical current densities J(c)(H) are assumed to account for the pinning of Josephson vortices in a weak-link network of grain boundaries. Good agreement with the experimental data was found if J(c)(H) = J0/(1 + (\H\/H0)(beta)) where beta congruent-to 2 for our YBaCuO samples and beta = 1.3 for our Bi(Pb)SrCaCuO sample. A poorer agreement is obtained if an exponential form for J(c)(H) is assumed. It is speculated that the high <J(c)>-value found in thin films is partly due to the small thickness of the films.