COMPARISON OF Ag/Bi-2223 TAPES PROCESSED WITH AND WITHOUT HOT-PRESSING AND POST-ANNEALING

摘要

Comparative studies on the Ag/Bi-2223 powder-in-tube tapes processed with or without hot pressing and post-annealing were carried out in terms of J_c, J_c-H, and magneto-optical image (MOI). It was found that hot-pressing significantly improved the J_c in zero field. SEM showed that the hot-pressed tapes were highly textured and very dense compared to the tapes without hot pressing. MOI revealed that the current distribution in the hot-pressed tape was more homogeneous than in the cold-pressed tape in the same field. However, there are more pronounced flux penetrations in the former than the latter, suggesting that the hot pressing under a large pressure only improved grain connectivity but not the flux pinning. The pronounced flux penetrations in the hot-pressed tape are attributable to highly stressed grains induced under hot pressing. Although these imperfect grains can carry large critical current in zero field, they broke down and J_c dropped rapidly as soon as a field was applied. By optimising the hot pressing conditions the strain on the oxide core can be largely relieved and J_c-H dependence improved. A post-annealing relieved the strains but created voids, which degraded the connectivity. J_c-H behaviour for the post-annealed tape showed a clear enhancement in weak links in low fields, but an improvement in high fields.