Comparative studies on “sandwich” rolling and flat rolling in processing Ag/Bi-2223 tapes

The normal rolling method is commonly used for intermediate mechanical deformation in the powder-in-tube process to make Ag-clad Bi-based superconducting tapes. To compare the effects of different reduction processing on the superconducting properties of Ag/Bi-2223 tapes, the reduction rate dependence of the critical current densities J c of tapes processed by three methods, including normal rolling, pressing and “sandwich” rolling, has been systematically studied. Experimental results show that Jc dependence on the reduction rate for the three processes follows the same law. There is an optimal reduction rate in each case, which gives the highest physical density and the highest Jc. When the reduction rate increases, Jc initially increases, reaches a maximum and then diminishes. The density-reduction rate follows a corresponding trend. However, the maximum Jc of the Ag/Bi-2223 tapes made using “sandwich” rolling is 35% higher than is the case for normal rolling even though the tapes in both cases have the same density of the oxide core. XRD analysis shows that the significant difference in Jc of tapes processed using the three different intermediate deformation procedures cannot be attributed to the phase composition. SEM images show that normally rolled tapes have much higher density of microcracks than “sandwich” rolled tapes. It is evident that the Jc of a tape is strongly dependent on oxide core density and deformation procedure.