CRITICAL AND IRREVERSIBILITY FIELDS OF GRANULAR SOL-GEL Y-BA-CU-O SUPERCONDUCTORS

We have measured the absorptive and dispersive parts of the a.c. susce ptibility for YBa2Cu3O7-x samples, obtained by the sol-gel method, as functions of magnetic field and temperature. The bulk samples with var ious grain diameters from 3 to 16 mu m and pressurized under pressures from 5 to 800 MPa have been measured. A few characteristic fields H1- H5 in chi ''(H) have been determined. We have argued that the field H1 is the lower critical field H-c1J for the intergranular junctions sys tem, whereas the fields H2 and H4 are the irreversibility fields for t he inter- and intragranular junctions respectively. We have shown the importance of the difference H5-H2, which has been correlated with the pinning force of the intergranular junctions. The magnetic field posi tions of all these features are strongly dependent on the granularity and porosity of samples. (C) 1997 Academic Press Limited.