Characterization of bulk yttrium barium(2) copper(3) oxygen(7-x) by trapped magnetic field and studies of enhanced flux pinning by high energy light ion irradiation

This dissertation describes work on the characterization of bulk high temperature superconductors (HTS) and on the improvement of magnetic flux pinning by ion radiation. These are part of a materials development program for the goal of high field magnets based on HTS. The bulk material characterization technique developed is based on the measurement of the trapped field distributions. When combined with a phenomenological two-current model, such measurements permit the calculation of the critical current density {dollar}Jsb{lcub}c{rcub}{dollar} of the measured sample, and the result agrees with standard magnetometer measurements to 1-10%. In addition, the method described here is more suitable for bulk characterization than standard methods, in that very large size specimens can be measured and more importantly, inhomogeneities of the material, common in ceramic HTS such as YBa{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}rm 7-x{rcub}{dollar} (Y123), are directly observable in the field measurement itself. The two-current model is a modification of the Bean critical-state model. It essentially agrees with standard models with field dependent currents, such as the Kim model, but is considerably easier to apply, because no iterations are required due to the use of constant currents.; Roughly a four-fold increase of the trapped field has been observed at 77 K after irradiating melt-textured Y123 with 200 MeV protons and {dollar}sp3{dollar}He{dollar}sp{lcub}++{rcub}{dollar}. The general behavior of the data as a function of ion fluence agrees qualitatively with a semi-phenomenological model based on the standard Anderson flux creep model, in the limited fluence range reached in the experiments, and is closely correlated to the nonionizing energy loss of the ions. The decay of the trapped field in both the unirradiated and irradiated samples is approximately logarithmic in time. The creep rate per order of magnitude of time at 77 K increases by about 15-35% as a result of irradiation. Lower pinning energy of the ion induced point defects is speculated to be the cause of this increase. However, there has been no attempt to analyze the apparent deviation from the Anderson theory in the creep data with theories such as different versions of extended Anderson model, the collective pinning theory, or the spin-glass model.