Improvement of critical current densities by fission tracks in U-doped high temperature superconductors

Abstract

Results on the enhancement of critical current densities in melt-textured bulk (Y-123) superconductors, which were doped with and exposed to thermal neutron irradiation to create fission tracks, are presented. These tracks, which all originate from U-containing clusters of a stable UPtYBaO compound, form isotropic `starlike' defects with diameters of approximately 10 nm and lengths of . However, if we consider projections onto the symmetry plane of the flux line lattice and allow for a certain tilt modulus , their pinning-effective size is mostly (90%) between 10 and 20 nm, i.e. only twice as large as the characteristic defect structure (amorphous collision cascades with diameters of nm) introduced into undoped superconductors by fast neutron irradiation. We report on studies of and the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope . In the best case (0.3 wt% U, thermal neutrons), we find (77 K) to be around (0 T) and (5 T). Control experiments on samples containing (nominally) only and exposure to thermal or to thermal plus fast neutrons confirm the above flux pinning considerations. As reported before for undoped melt-textured Y-123, the irreversibility lines remain practically unchanged due to the presence of strongly pinning Y-211 precipitates.