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⩽20 Oe, H a ← 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 vertices in a weak-link network of grain boundaries. Good agreement with the experimental data was found if J c( H)= J 0/(1+(| H|/ H 0) β ) where β ≅2 for our YBaCuO samples and gb=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.