The fabrication of Bi-based superconductor tape by electrophoretic deposition and melt-texturing techniques

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Abstract
A processing technique has been developed for the fabrication of high-current Bi-2212 superconductor tapes. The technique comprises three steps: (i) treating the particulate ceramic superconductor with trichloroacetic acid in 2-butanol to generate a negative charge on the particle surface; (ii) electrophoretic deposition of the particulate ceramic onto an Ag-based substrate and (iii) melt-texturing of the deposit to form a superconductor coating on the substrate. It has been found that the 2212 superconductor can only be melt textured on an Ag substrate in a narrow temperature window around 885 degrees C. Slow cooling from the molten state not only allows the development of superconductor phase texture, but also suffers from the growth of Ca-Cu-O oxides. These oxides can be transformed into 2212 phase by annealing at 850 degrees C. However, the 2212 grains formed from the Ca-Cu-O precipitates are invariably oriented at right angles to the 2212 phase formed in the course of solidification. The presence of such large-angle grain boundaries is detrimental to the superconductor properties. An optimal processing procedure has been proposed and some important processing parameters discussed. A pilot plant has been set up and semi-continuous tape of Bi-2212 superconductor on Ag has been produced. Critical currents as high as 155 A have been measured over a length of 1 m of multilayer coil at 4.2 K. The performance corresponds to a critical current density of about 70000 A cm-2 in the superconductor layer in a self-field up to about 64 mT. The uniformity of Jc measured on adjacent 10 cm lengths in the multilayer coil is much better than that in a single layer coil, about 30% variation in the former compared with 80% in the latter. The results show that electrophoretic tapes have a promising future for integration into small magnet windings.