TEM AND CRITICAL-CURRENT DENSITY STUDIES OF MELT-TEXTURED YBA2CU3OX WITH SILVER AND Y2BACUO5 ADDITIONS

TEM studies have been conducted on oriented domains of melt-textured YBa2Cu3Ox/15 wt.% silver (123/Ag) composites with various amounts of Y2BaCuO5 (211) additions. The observed microstructural defects are compared with the transport critical current densities, J(c), measured on the same domains in an effort to ascertain the likely flux pinning centers responsible for J(c) and its increase with 211 addition in melt-textured 123. In the TEM studies, defects such as dislocations, pile-ups and loops, Ag and 211 inclusions are observed. The characteristics and densities of dislocations, pile-ups and loops within the bulk HTS do not appear to vary with Ag and 211 additions, and dislocation densities remain on the order of 10(9) cm-2 for all specimens. In addition to the typically observed large 20 to 60 mum Ag inclusions, we found that much smaller Ag particles of 2 to 200 nm are dispersed throughout the bulk in all specimens. Even though the small dimensions of these particles may allow them to be potential pinning sites, they do not appear to be responsible for the increase in J(c) of the specimens examined. With 211 additions, the amount of 211 particles ranging in size from 0.2 to 2 mum is increased. It is observed that while large 211 inclusions possess defect-free interfaces, the interfaces of micron-size 211 particles with their high surface curvature are associated with numerous dislocations, where the localized dislocation density is on the order of 10(10) cm-2 . These interface-associated defects are believed to be responsible for the enhancement in J(c) with an increasing amount of 211 addition.