SHIELDING EFFECTS IN CERAMIC SUPERCONDUCTORS

The shielding properties of high-temperature superconductors in DC and AC magnetic fields are discussed using Bean's critical-state model an d models, which account for the field dependence of the critical curre nt density. Experimental work on the shielding properties of supercond ucting YBa2Cu3O7-delta and conductive (aluminum and copper) screens is presented. The maximum value of the shielded field for YBa2Cu3O7-delt a/Ag screens was measured at 3 mT. Shielding by superconducting screen s was found independent of frequency over the range 80 Hz-10(5) Hz. Th e limited shielding capability of ceramic superconductors results from the magnetic field dependence and low values of the intergrain curren t density in bulk materials. There is a practical limit to the externa l field that can be shielded by a superconductor, characteristic of th e material and size independent. Increasing of the shield thickness wi ll not increase the maximum value of the field which can be shielded.