Processing and characterization of high T/sub c/ magnetic shields and flux transformers

The use of high T/sub c/ SQUID (HTSQUID) magnetometers for magnetocardiography (MCG) and nondestructive testing (NDT) requires effective shielding from magnetic noise fields. The authors have investigated the shielding characteristics of high-T/sub c/ superconducting (HTS) material and developed a compact, portable system consisting of several HTS components. A process for fabricating these components out of Bi-Pb-Sr-Ca-Cu-O has been developed. Tubes and disks of various sizes have been formed by cold isostatic pressing (CIP) and sintered at reduced oxygen pressures. The HTS tubes exhibit shielding factors of up to 10/sup 3/ at low frequencies. A novel technique for fabricating superconducting flux transformers (specifically, first-order gradiometers) has also been developed. The flux transfer efficiencies of these first-order gradiometers have been evaluated. The tubes, gradiometers, and flux focusers have been incorporated into an HTSQUID setup. The authors find that the low-frequency noise rejection obtained in their setup is very promising for potential MCG and NDT applications in an unshielded environment.