Critical Current Density and Microstructure of NbTi/Nb/Cu Multilayer Superconducting Sheet.

The correlation between the microstructure and critical current density of NbTi/Nb/Cu multilayer superconducting sheet has been studied. Two types of Ti precipitates have been found in the NbTi layers of the sheet. One type is elongated in the rolling direction with a major axis of about 240nm and a minor axis of about 160nm, and is precipitated at the grain boundaries. The other is fine, 20 to 30nm in length, a few nm in thickness and is precipitated in the crystal grains and at the grain boundaries. The sum of the critical current densities calculated from the size and volume fraction of the two types of Ti precipitates and the critical current density attributed to the dislocation pinning agree well with the measured critical current density of the specimens. The critical current density of transverse specimens subjected to a parallel 5T magnetic field was about 1.2×103A/mm2, and the critical current density of longitudinal specimens subjected to the same 5T parallel magnetic field was 40% lower. The critical current density of specimens subjected to a perpendicular magnetic field was about 40% of that of the specimens subjected to the parallel magnetic field. The dependence of critical current density on the rolling and magnetic-field directions can be qualitatively explained by the shape of Ti precipitates elongated in the rolling direction.