MAGNETIC SHIELDING USING HIGH-TEMPERATURE SUPERCONDUCTORS

Magnetic shields of various high-temperature superconductors, YBa2Cu3O 7-x (YBCO), YBa2Cu3O7-x-Ag composites (random inclusions as well as no n-random coatings) and Bi2Sr2Ca2Cu3O(x) (BSCCO) were prepared by uniax ial as well as isostatic compression with various dimensions. The shie lding properties were measured at 77 K for dc and ac magnetic fields i n the range of frequencies from 100 Hz to 10 kHz. The critical penetra tion field (CPF), defined as the value of the applied magnetic field a t which a detectable field was observed inside the cylinder, varied fr om cylinder to cylinder and also with the ageing of the cylinders in t he case of YBCO shields. The highest value of CPF was 16 G at 77 K for YBCO shield prepared by isostatic compression. Even though the stabil ity of BSCCO shields with respect to ageing is good, the CPF values ar e very low compared to those for YBCO. Detailed studies were performed in the case of YBCO shields. The CPF decreased as a function of time over a period of 90 days. The CPF decreased as the frequency of the ap plied field was increased. The wave form of the field inside the pot f or a sinusoidal applied field was highly distorted and showed the pres ence of higher harmonics with appreciable amplitude. The wave form was Fourier-analysed to yield the field inside the shield along with the harmonics. The shields with Ag addition seem to give better performanc e at high fields.