NbTi/Nb/Cu multilayer composite materials for superconducting magnetic shielding: Superconducting performances and microstructure of NbTi layers

NbTi alloy is popularlized for practical uses of superconducting multifilamentary wires, and is utilized for MRI systems in large amounts. Those wires are effective as current conductors, but a superconductor consisting of a two-dimensional plane has been often referred to as a possibly better shield against electromagnetic fields than wires. In recent years, Nippon Steel Corp. has developed the superconducting multilayer composite sheet and cylinder consisting of NbTi alloy as a superconductor being clad with copper for superconducting stabilization. A sample consisting of four cup-like cylinders with a thickness of lmm, which are concentrically stacked, can stably reduce the applied magnetic field of 2.6 tesla to less than 10 gauss. This paper describes their characteristics of the multilayer structure and fabricating processes, and as important superconducting properties, critical current densities(J c ), shielding properties of DC and AC magnetic fields, magnetic field homogenizing effects and new techniques for extremley increasing J c in consideration of the internal microstructure of the NbTi layer. Finally, a few test results of the performances of the material provided by domestic and foreign organizations are also touched.