Magneto-optical and electromagnetic studies of core connectivity and weak-link behaviour in Cu/MgB₂ and Ni/MgB₂ wires and tapes

Critical current density, ac susceptibility (real part χ′) and magneto-optical (MO) imaging were used to characterize round wire and tapes made from pre-reacted MgB₂ powder by 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 showed a two-step transition in χ′ versus T traces as evidence of weak links in the tape core. The first heat treatment (HT) of as-deformed tapes led to a sharp susceptibility transition, very strong connection in the tape core as seen in MO images, and high critical current. A second rolling of an as-sintered tape induced a network of defects in the brittle core and gave rise to χ′ curve broadening, but did not result in a two-step transition. These data show that deformation-induced cracks are not the primary source of weak-link behaviour. A large decrease in transport current was also registered in this sample. The second HT, for 30 min, restored the sharp magnetic transition and high transport current. Alternating areas of strong and weak connection in the tape core have been observed on MO 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 HT, no mechanical defects were observed in MO images. Such fast transformation could be explained by rapid sintering due to highly strained MgB₂ grains.