Yoshihiro Sakurai

Cu ion plating as a technique for enhancing the mechanical, electrical and thermal bonding between Cu stabilizer and the RHQT-processed Nb/sub 3/Al conductors

The surface of RHQT-processed Nb/sub 3/Al wires with pure Nb matrix are covered by a strong stable Nb oxide surface layer. In order to obtain good mechanical, electrical and thermal bonding between the Cu stabilizer and RHQT-processed Nb/sub 3/Al wires, it is required to destroy the Nb oxide layer on the surface of the wire. We tried to fabricate a thin Cu layer on the surface of the RHQT-processed Nb/sub 3/Al wires through the Cu ion-plating technique. Before electroplating of thick Cu stabilizers, Cu was ion-plated to a thickness of about 1 /spl mu/m. The Cu ion-plated wire showed no folded projections, cracks, or exfoliation of the Cu stabilizer even when the wire was bent through 180 degrees, showing that Cu and the wire were tightly bonded mechanically. This tight bonding between Cu and the wires should be due to the removal of stable Nb oxide layers from the surface of the wire. The V-I characteristic of wires that were ion-plated and then applied with an appropriate amount of Cu by an electroplating showed no quenching, and the wires were able to carry current up to the normal critical current transition. The measured values of recovery current were almost equivalent to the calculated values, showing that the ion-plated Cu/Nb interface had a very tight bond, allowing good electrical and thermal conductivity. In addition, a reel to reel Cu-ion plating apparatus for the long length wires is demonstrated, and Cu ion plating has been successfully carried out on 100 m long RHQT-processed Nb/sub 3/Al wire at present.

Trial Manufacture of a km Class Length of Cu Cladding RHQT

We have produced so far about a 400 m length of Cu stabilized RHQT (rapid-heating, quenching and transformation) Nb3Al flat-wire using a small-sized multi-billet (10 kg). Although our goal is to make a 2.5 km unit length of Cu cladding flat-wire for high-field NMR uses, an attempt was made to fabricate a 1 km class unit-length of conductor as a milestone of commercialization. A long-length of precursor wire over 2.5 km was prepared by using a large-sized multi-billet (50 kg). A 1.3 km length RHQ operation by using a newly installed large-scale RHQ apparatus was performed and then a km-class length Cu cladded flat-wire was fabricated to evaluate the uniformity of resultant superconducting characteristics of wire

rm Nb_3rm Al

Transmission electron microscopy and electron backscatter diffraction were used to observe the grain morphologies of transformation-processed superconductors to identify pinning centers. Their grain sizes and stacking fault densities were analysed to determine whether they are correlated with the critical current density . The stacking faults can be seen as planar defects stacked in three orthogonal directions, when the electron beam was parallel with the direction. was not inversely proportional to the grain size, whereas and appear to be correlated with the stacking fault density.

Flat-Wire

Since the RHQT-Nb/sub 3/Al conductor shows excellent superconducting characteristics in high magnetic fields, its application to a 1 GHz NMR insert coil is in particular expected. For such an application, the development of a long-length of RHQT-Nb/sub 3/Al wire is indispensable. However, the unit length of wire achieved was no more than 400 m so far, due to a restricted size of the multi-billet, while about a 2.5 km-length of wire is necessary for the NMR insert coil application. Thus, an attempt was made to produce a 2.5 km-length of precursor wire by using a large-sized billet, and we have succeeded in producing such a long-length of precursor wire (2.6 km) without any breaking of wire during hydrostatic extrusion and subsequent drawing processes.