The fabrication of composite reaction textured Bi2Sr2CaCu2O8+ deltasuperconductors

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Abstract
The application of high-temperature superconducting ceramics in magnet technology and power engineering is currently limited by the difficulty of fabricating wires and precision components with the required ceramic microstructure. The results of a detailed study of the reaction process for the fabrication of large bulk artefacts of Bi2Sr2CaCu2O8+ delta by composite reaction texturing are reported. A composite preform seeded with an aligned distribution of MgO fibres is prepared by elastomer processing or by tape casting, and reacted in three stages. The first stage is binder burn-off carried out in air; the optimum heating sequence depends on sample cross-section, and results in complete removal of the binder system without the introduction of macroscopic defects. The second stage involves densification in a pure oxygen atmosphere to >96% theoretical density, followed by partial melting and slow cooling. Finally, after the preparation of suitable current contacts, the oxygen stoichiometry, critical current density and irreversibility field are optimized by post-reaction annealing in a low partial pressure of oxygen. The process described enables the reproducible fabrication of textured composite conductors of arbitrary shape with characteristic dimensions ranging from 0.5 mm to 200 mm. The critical current density in optimized conductors is 5*104 A cm-2 at 5 K and 12 T, and at 77 K has a self-field limited value which is in excess of 3*103 A cm-2 in small cross-section samples.