Abstract
We fabricated a large Bi2223 hollow cylinder as a superconducting magnetic shield through plasma spraying in order to obtain a very low and calm magnetic-field environment where we can measure the weak pulsed magnetic fields caused by human brain activity. As a substrate, we used a pure-nickel hollow cylinder (323.2mm in O. D., 320mm in I. D. and 660mm in length), the outside of which was coated with a Ag buffer layer, as a diffusion barrier, and then a Bi-system oxide layer about 730μm thick, applied through plasma spraying. However the as-sprayed Bi-system oxide layer showed unclear broad X-ray diffraction patterns of amorphous-like crystal structure and no superconducting properties at 4.2K. Therefore, the sprayed Bi-system oxide hollow cylinder was heat treated at 838°C×100h to form a superconducting phase (Bi2223 phase) through solid-state diffusion reaction. The superconducting Bi2223 hollow cylinder showed a shielding factor of 103 at Z/D=0.98 at 79K in a dc field of 4×10-4T, where Z and D are the distance from the edge of the cylinder to the measuring position at the center line of the cylinder, and the inner diameter of the Bi2223 cylinder, respectively. The same shielding factor, of about 103, was obtained in ac fields of 3×10-5T with frequencies below 10Hz.
