AC SCREENING MEASUREMENT FOR THE CHARACTERIZATION OF OXIDE SUPERCONDUCTORS .1. APPLICATION TO CERAMICS

Coils are placed coaxially on either side of a slab of superconductor of thickness 0.05-1 mm. The coil diameters are less than the lateral dimentions of the slab. The voltage in the secondary coil is measured as a function of excitation current and frequency in the primary, and of the temperature and the steady magnetic field provided by an external solenoid. A YBa2Cu3O7 ceramic cooled in zero field screens almost completely, but the greater part of this screening is suppressed by low magnetic fields. The remaining part is shown to be due to isolated superconducting grains. When intergrain superconductivity is partly suppressed the condition is reasonably well described by the Bean critical-state model. Hysteresis of the screening coefficient reflects the known hysteresis of the transport critical current. © 1990.