Enhancement of the critical current density of Ag-sheathed Bi-2223 tapes by optimized pre-annealing and pre-rolling process

The effect of pre-annealing and pre-rolling (an initial rolling deformation prior to pre-annealing) on the final engineering critical current density Je of Ag-sheathed Bi-2223 tapes has been investigated. The optimization of pre-annealing process was found to be dependent on annealing temperature much more than on oxygen partial pressure or soaking time. The highest Je has been achieved for pre-annealing at 790 °C for 20 h in a flowing 8.5% O2-balance N2 atmosphere. Optical microscope and XRD rocking curve analysis show that pre-rolling improved the homogeneity of the tapes, reduced the sausaging effect and improved the texturing of the superconducting phase. All these effects strengthen the grain connectivity and further enhance the Je value of the tapes. Too large a pre-rolling reduction, however, decreased the Je owing to the introduction of more cracks that cannot be completely healed in subsequent heat treatment.