INVESTIGATION OF FLUX PINNING AND CRITICAL CURRENT DENSITIES IN TYPEⅡ SUPERCONDUCTORS

摘要

The relation between flux-pinning and microstructure has been established for Nb-25%Zr superconducting wires, cold worked and annealed between 600 and 700C. This treatment produces a recovery of the dislocation structure and the development of a cell structure but does not produce any detectable beta Zr precipitation. Optimum pinning as measured by the flux-pinning force density maximum results from a fine cell structure with the greatest structural difference between wall and interior. There is some indication that stronger pinning result from a higher initial level of cold work. Techniques for preparation of thin foils from longitudinal and transverse sections of multifilament superconductors for transmission electron microscopy have been developed. Longitudinal sections are prepared by first grinding the wire into a thin ribbon followed by electropolishing. Transverse sections are obtained after increasing the diameter at the conductor by copper plating. Large areas suitable for microscopy have been produced in multifilament niobium conductors with filament diameter of only 7 micrometers (0.00028 in).