Critical current density and microstructure of Nb Ti/Nb/Cu superconducting multilayer sheet

The correlation between the microstructure and critical current density (J(c)) 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 240 nm and a minor axis of about 160 nm, and is precipitated at the grain boundaries. The other is fine, 20-30 nm in length, a few nanometers in thickness and is precipitated in crystal grains and at 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 J(c) attributed to the dislocation pinning agree well with the measured J(c) of the specimens. The J(c) of transverse specimens subjected to the parallel 5 T magnetic field was about 1.2 x 10(3) A/mm(2), and the J(c) of longitudinal specimens subjected to, the parallel 5 T magnetic field was 40% lower. The J(c) of specimens subjected to the perpendicular magnetic field was about 40% of that of the specimens subjected to a parallel magnetic field. The dependence of J(c) on the rolling and magnetic field directions can be qualitatively explained by the shape of Ti precipitates elongated in the rolling direction. (C) 1998 Elsevier Science Ltd. All rights reserved.