FLUX PINNING MECHANISMS IN SUPERCONDUCTING A15 MATERIALS AND THE OPTIMIZATION OF THEIR TRANSPORT PROPERTIES

摘要

The superconducting properties and microscopic characterization are presented for Nb_3Sn, (Nb,Ta)_3Sn and (Nb,Ti)_3Sn conductors made using several different fabrication routes. The element concentration profiles at the grain boundaries, determined using STEM, leads to a pinning model that includes the pinning by normal boundaries and by the grain boundary flux shear. The use of the shear modulus C_(66) for all the range of field and temperature does not fit the Kramer theory. Use of a flux pinning effective grain size, D_(effective), in the model leads to good agreement. D_(effective) depends on the magnetic field (flux line spacing), has a smooth behavior at high fields and increases almost exponentially at low fields following the flux line spacing behavior, as expected. The determination and optimization of D_(effective) leads to the best superconductor current carrying characteristics.