Critical current density, alternating-current susceptibility (real part χ’) and magneto-optical imaging were used to characterize round wire and tapes made from pre-reacted MgB2 powder by using the powder-in-tube method. Magnetic susceptibility measurements indicated the existence of large-scale weak-link networks in the cores of as-deformed Ni/MgB2 and Cu/MgB2 wires and tapes. As-deformed samples exhibited a 2-step transition in χ‘ versus T traces as evidence of weak links in the tape core. The first heat treatment of as-deformed tapes led to a sharp susceptibility transition, very strong connection in the tape core as seen in magneto-optical images, and a high critical current. A second rolling of an as-sintered tape introduced a network of defects into the brittle core and gave rise to χ’ curve broadening, but did not result in a 2-step transition. These data showed that deformation-induced cracks were not the primary source of weak-link behavior. A large decrease in transport current was also found for this sample. The second heat treatment, for 0.5h, restored the sharp magnetic transition and high transport current. Alternating areas of strong and weak connection in the tape core were observed in magneto-optical images. Fast heating and cooling of the as-deformed tape, with no dwell time at the maximum temperature, resulted in a strongly linked core with a sharp transition and increased transport current. After fast heat treatment, no mechanical defects were observed in magneto-optical images. Such rapid transformation could be explained by rapid sintering due to highly strained MgB2 grains.

Magneto-Optical and Electromagnetic Studies of Core Connectivity and Weak-Link Behaviour in Cu/MgB2 and Ni/MgB2 Wires and Tapes. A.Polyanskii, V.Beilin, I.Felner, M.I.Tsindlekht, E.Yashchin, E.Dulkin, E.Galstyan, M.Roth, B.Senkowicz, E.Hellstrom: Superconductor Science and Technology, 2004, 17, 363-70