DEPENDENCE OF CRITICAL-CURRENT DENSITY ON MICROSTRUCTURE AND PROCESSING OF HIGH-T-C SUPERCONDUCTORS

Microstructural origins for reduced weak-link behavior in high-J(c) me lt-processed YBCO, spray pyrolyzed thick films of Tl-1223, metallic pr ecursor Y-124 polycrystalline powder-in-tube (PIT) wires and PIT Bi-22 12/2223 are discussed. Since the materials studied are the highest J(c ), polycrystalline, high-T-c superconductors fabricated worldwide, the results provide important guidelines for further improvements in supe rconducting properties, thereby enabling practical applications of the se materials. It is found that strongly linked current flow within dom ains of melt-processed 123 occurs effectively through a single crystal path. In c-axis oriented, polycrystalline Tl-1223 thick films, local in-plane texture has been found to play a crucial role in the reduced weak-link behavior. Formation of ''colonies'' of grains with a common c-axis and modest in-plane misorientation was observed. Furthermore, a colony boundary in general has a varying misorientation along the bou ndary. Large regions comprised primarily of low angle boundaries were observed. Percolative transport through a network of such small angle boundaries appears to provide the nonweak-linked current path. Althoug h powder-in-tube BSCCO 2212 and 2223 also appear to have a ''colony'' microstructure, there are some important differences. Colonies in BSCC O consist of stacks of grains with similar c-axis orientation in contr ast to colonies in Tl-1223 films where few grains are stacked on top o f one another. Furthermore, most grains within a colony in BSCCO have the same lateral dimensions as that of the colony, resulting largely i n ''twist'' boundaries. Further microstructural characterization of hi gh-J(c) PIT 2212 and 2223 is currently underway. In the case of Y-124 wires, weak macroscopic in-plane texture is found. Additional measurem ents are underway to determine if a sharper, local in-plane texture al so exists. It is found that in three of the four types of superconduct ors studied, reduced weak-link behavior can be ascribed to some degree of biaxial alignment between grains, either on a ''local'' or a ''glo bal'' scale.