TEM STUDY OF GRAIN-BOUNDARIES IN AG-CLAD BI2SR2CACU2O(X) TAPES

Grain boundaries in Ag-clad 2212 tapes have been characterized extensively by TEM and HREM. All tapes show a colonial microstructure. The grains in each colony are plate-like, and their c-axes are parallel to each other. Most of the grain boundaries in each colony are low-angle [001] twist boundaries. The equilibrium structure of the twist boundaries consists of the perfect crystal structure plus a square network of the primary grain boundary dislocations. A low-energy twist boundary in the high-angle region was also observed. Observation of a grid of the secondary grain boundary dislocations indicates that the twist boundary consists of the corresponding order structure and substantial periodic strain fields. The colony boundaries are in general high-angle mixed-grain boundaries. The c-axes between colonies are tilted by a large angle that ranges from 10 to 30-degrees. Most of the colony boundary planes are parallel to the (001) lattice plane in one of the grains that form the boundaries. Non-parallel boundaries are strongly faceted into a number of segments, which are parallel to the (001) plane. Large triangular holes were observed along a large part of the colonial boundaries in the cross-sectional samples, indicating that the impurity phases or voids could exist at colonial boundaries.