Magnetization Studies in Nb 3Sn and Ti–Nb Alloys

Abstract

Magnetization measurements were performed on commercial Nb3Sn and Ti–Nb alloy samples to determine hysteretic loss (magnetization vs magnetic field) and flux-jump stability at 4.2°K in magnetic fields up to 9.3 Wb/m2. The aforementioned superconducting properties were also observed and recorded with various geometries, field sweep rates and thermal environments. The samples consisted of ribbons and/or wires ranging in size from 1.0 to 0.1 cm wide and 0.1 to 0.0001 cm thick. The transport currents ranged from a few amperes to greater than a thousand amperes. These data were obtained utilizing an experimental setup similar to that described by Fietz. The shape and magnitude of the magnetization curves with and without the presence of transport current was not a function of frequency in the range employed, 0.012 to 3.0 Wb/m2·min, except for the multifilament composites. The magnetization and flux-jump stability was found to be a function of the sample dimensions, as would be expected with the earlier expressions given by Wipf, Hancox, Chester, and Stekly. The transport current density was not a function of the dimensions. These data on magnetization and transport current effects are in accord with earlier work by LeBlanc and co-workers on Nb–Zr. There is a first principle derivation of the flux-pinning effects that uses a Josephson junction tunneling model and results in the Kim-Anderson expression. The internal field distribution could be characterized by Hint(x)=(Ha−Hs exp(−x/λ)+Hs, λ=(Hs−Ha)/Jc(Ha). The exponential expression for Jc(Ha) vs Ha of Fietz et al. fits the sample data; however, the Kim-Anderson expression that would be the first term in the expansion of the exponential would also fit the observed data within the experimental error. The data concerning transport current and magnetization can be accurately accounted for (in the infinite sheet configuration) by expressions derived by Yasukochi. The thin plate geometry, with the proper demagnetization factor, is in agreement as well.