HIGH CRITICAL CURRENT IN Y-Ba-Cu-O SUPERCONDUCTORS.

Melt-textured-growth of polycrystalline YBa sub 2 Cu sub 3 O sub (7-delta) superconductor using directional solidification created an essentially 100% dense structure consisting of long, needle- or plate-shaped crystals preferentially aligned parallel to the a-b conduction plane. The new microstructure, which completely replaces the previous granular and random structure in the sintered precursor, exhibits dramatically improved transport J sub c values at 77K of ~17,000 A/cm sup 2 in zero field and ~4,000 A/cm sup 2 at H = 1 tesla (as compared to ~500 A/cm sup 2 and ~1 A/cm sup 2 , respectively, for the as-sintered structure), with the severe field dependence of J sub c ("weak- link" problem) no longer evident in the new melt-textured material. The improvement in J sub c is attributed to the combined effects of densification, alignment of crystals, and formation of cleaner grain boundaries. Microstructure and distribution of various phases present in the melt-textured material are discussed in relation to the superconducting properties.