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

Results on the enhancement of critical current densities J(c) in melt-textured bulk YBa2Cu3O7-delta (Y-123) superconductors, which were doped with U-235 and exposed to thermal neutron irradiation to create fission tracks, are presented. These tracks, which ail originate from U-containing clusters of a stable UPtYBaO compound, form isotropic 'starlike' defects with diameters of approximately 10 nm and lengths of 2-5 mu m. However, if we consider projections onto the symmetry plane of the flux line lattice and allow for a certain tilt modulus C-44, 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 similar to 6 nm) introduced into undoped superconductors by fast neutron irradiation. We report on studies of J(c) and the irreversibility lines in superconductors containing various amounts (0.3-0.7 wt%) of uranium and various amounts of the fissionable isotope U-235. In the best case (0.3 wt% U, 4 x 10(20) m(-2) thermal neutrons), we find J(c) (77 K) to be around 3 x 10(9) A m(-2) (0 T) and 1 x 10(8) A m(-2) (5 T). Control experiments on samples containing (nominally) only U-238 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.