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The interfaces and microstructural defects in the broad face and long'tudinal cross-sectional specimens of Agsheathed (Bi, Pb)2Sr2Ca2Cu30,h,, (2223) tapes have been studied by TEM. Most of the twist boundaries in the colonies have a small rotation angle, accommodated by a screw dislocation network. The twist boundaries are normally atomically flat without a second phase. Most of the colony boundaries are of mixed type with a misorientation angle up to 3 0 ' . A thin amorphous layer is commonly formed at colony boundaries with adjacent basal planes on both sides of the boundary. Most of the cdirection colony boundaries are mixed-type ones, and second phases were sometimes observed at these boundaries. lilt boundaries, consisting of a wall of edge dislocat:ons. are clean and the lattices from both sides of the boundary are well matched. At the triple colony junction, second phases were ofien found. Around a large isolated particle of a second phase, small colonies were found to be randomly orientated and seriously distorted. The [OOl] edge dislocations, and the bend and interruption of the (001) planes were often observed. A systematic study on lhe correlation between Jc, and 2212 lraction in numerous tapes shows that tapes with higher J, always contain a certain fraction of 2212 phase, while the tapes with a very low fraction of 2212 did not show high Jc. These results seem to indicate that the predominant weak links in the tape are not 2212 layers at twist boundaries in the low 2212 fraction regime. The predominant weak links in tapes are colony boundaries and Jc is assumed to be controlled by colony boundaries in low fields.

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