Magneto-optical and electromagnetic studies of core connectivity and weak-link behaviour in Cu/MgB₂and Ni/MgB₂wires and tapes

Abstract

Critical current density, ac susceptibility (real part ) 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 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 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.

Effect of grain refinement on enhancing critical current density and upper critical field in undoped MgB2 ex situ tapes

“…16,17 3-5 mm long tapes were placed on the cooling finger of a continuous flow optical cryostat located on the X-Y stage of a polarized optical microscope in reflective mode. The Bi-doped garnet indicator film with in-plane magnetization was placed directly on the bare core of the MgB 2 tapes to register the normal component of magnetic flux at the tape surface.…”

“…The Bi-doped garnet indicator film with in-plane magnetization was placed directly on the bare core of the MgB 2 tapes to register the normal component of magnetic flux at the tape surface. 16,17 Short pieces of about 6 mm in length cut from the Nisheathed tapes were employed for the magnetization measurements in a 5.5 T Magnetic Properties Measurement System ͑MPMS͒ Quantum Design superconducting quantum interference device ͑SQUID͒ magnetometer. The magnetic field was applied perpendicular to the tape surface and critical current density values were extracted from the M-H loops applying the Bean model.…”

Intergrain connectivity of MgB2 ceramics studied by impedance analysis

“…Their results indicate that the low packing factor and poor connectivity strongly restrict J C in MgB 2 . Many experimental methods, such as Campbell's method, 10 magneto-optical and electromagnetic studies, 11 magnetoresistance and radio frequency penetration depth techniques 12 have been used to investigate the intergrain connectivity of MgB 2 .…”

Transport and magnetic critical currents of Cu-stabilized monofilamentary MgB2conductors

“…The measurement of magnetization hysteresis loops, M(B), is a non-destructive, reproducible and widespread method often used to estimate the critical current densities of superconducting materials. Nevertheless, other methods, such as Hall probe [6] and magneto-optical [7,8] imaging, ac susceptibility [6], the analysis of the reversal branch of the M(B) loops [9], or the analysis of the resistivity ρ(T ) curves in the case of bulk MgB 2 [9][10][11], are also required in order to rule out granularity or connectivity problems in MgB 2 materials.…”