THE FABRICATION OF COMPOSITE REACTION TEXTURED BI2SR2CACU2O8+DELTA SUPERCONDUCTORS

The application of high-temperature superconducting ceramics in magnet technology and power engineering is currently limited by the difficul ty of fabricating wires and precision components with the required cer amic microstructure. The results of a detailed study of the reaction p rocess for the fabrication of large bulk artefacts of Bi2Sr2CaCu2O8+de lta by composite reaction texturing are reported. A composite preform seeded with an aligned distribution of MgO fibres is prepared by elast omer processing or by tape casting, and reacted in three stages. The f irst stage is binder burn-off carried out in air; the optimum heating sequence depends on sample cross-section, and results in complete remo val of the binder system without the introduction of macroscopic defec ts. The second stage involves densification in a pure oxygen atmospher e to > 96% theoretical density, followed by partial melting and slow c ooling. Finally, after the preparation of suitable current contacts, t he oxygen stoichiometry, critical current density and irreversibility field are optimized by post-reaction annealing in a low partial pressu re of oxygen. The process described enables the reproducible fabricati on of textured composite conductors of arbitrary shape with characteri stic dimensions ranging from 0.5 mm to 200 mm. The critical current de nsity in optimized conductors is 5 x 10(4) 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 x 10(3 ) A cm-2 in small cross-section samples.