Structural and compositional defects in high-J/sub c/ Bi-2223 tapes

High critical current density (J/sub c/) Bi-2223 multifilamentary tapes (55 kA/cm/sup 2//spl les/J/sub c//spl les/70 kA/cm/sup 2/, 77 K and self-field) have been examined by transmission electron microscopy. Filament microstructures within the tapes consist of two distinct regions: a highly aligned, dense colony structure (brick-wall) near the silver interface and a porous, poorly-textured interior region (impurity channel) that contains significantly more secondary phases. Quantitative compositional analysis clearly showed a heterogeneity in the Bi-2223 composition throughout the tapes. Spatial variations in composition were especially prevalent in the vicinity of secondary phases. Potential current limiting microstructures or defects were categorized depending upon the length scale at which they would operate. Macroscale defects were defined as potentially limiting current flow between regions of a filament. Mesoscale defects affect current flow between adjoining colonies. And finally, microscale defects were classified as potentially limiting current flow within a colony structure.