Theoretical and experimental considerations of the critical current density of DI-BiSCCO superconducting tapes as a function of magnetic field, field orientation, temperature and strain

Abstract

The critical current density (Jc) of DI-BiSCCO superconducting tapes was measured as a function of magnetic field (B), field orientation (θ), temperature (T) and strain (e) in a 15 T split-pair horizontal superconducting magnet using probes designed and built in-house. Strain was applied to samples using a modified bending beam apparatus with a copper beryllium springboard-shaped sample holder, which is capable of applying uniaxial strains of -1.4% < e < 1.0%. The temperature of the sample was controlled with the use of an inverted insulating cup with a temperature stability of +/-80 mK to +/-200 mK. The vapour-cooled brass critical-current leads (incorporating high-temperature superconducting tapes) were optimised to minimise helium consumption. Optimisation includes consideration of the maximum safe temperature of the current leads and the effects of duty cycle and static helium boil-off. The optimised helium consumption of the leads is a factor of two lower than standard current leads optimised for magnets. Jc(B,T,θ,e) data of the DI-BiSCCO tapes were characterised based on the superconducting-normal-superconducting Josephson junction model where Jc is determined by flux flow along the grain boundaries (or the normal junctions). It was found that grain boundaries in the DI-BiSCCO tapes are thick (several tens of nanometre) and exhibit semiconducting behaviour. The degree of misalignment has been included into the anisotropy analysis of Jc and the correlation between the effective anisotropy and texturing of the sample obtained. Analysis of three different samples (Nb3Sn, YBCO and BiSCCO) is presented where the average local properties of the grain boundaries were extracted from magnetisation and the transport Jc data.