Direct evidence for residual, preferentially-oriented cracks in rolled and pressed Ag-clad BSCCO-2223 tapes and their effect on the critical current density

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Abstract
We have studied the connectivity of rolled and pressed Ag-sheathed tapes through three deformation and heat treatment cycles using magneto-optical imaging. In pressed samples, a transport critical current density of (77 K, 0 T) was reached after two heat treatment steps, and increased to after a third heat treatment. Critical current density values in rolled samples also reached after two heat treatments, but decreased to after the subsequent deformation and third heat treatment. Magneto-optical imaging using fields applied perpendicular to the rolling plane revealed that flux penetrated the superconducting core mainly through defects oriented perpendicular to the direction of current flow in rolled samples, and parallel to the direction of current flow in pressed samples. In those samples which had received more than one heat treatment, the flux is believed to penetrate through cracks produced during the deformation steps which do not heal during the subsequent heat treatments due to lack of sufficient residual liquid phase.