SAMPLE SIZE DEPENDENCE AND IRREVERSIBILITY OF THE CRITICAL CURRENT DENSITY OF CERAMIC HIGH-TEMPERATURE SUPERCONDUCTORS

The intergranular critical state model is employed to calculate the sample-size dependence of the transport critical current density, < J(c) >, of polycrystalline high-temperature superconductors. Good agreement with experimental data has been found. The generalized London equation is used to determine the Josephson current across grain boundaries when magnetic flux is trapped inside grains. The field at which < J(c) > peaks when the applied field is decreased under ZFC conditions is predicted and agrees well with experimental observations.