Local texture and grain boundary misorientations in high H(sub C) oxide superconductors

摘要

The orientations of hundreds of contiguous grains in high J(sub C) TlBa(sub 2)Ca(sub 2)Cu(sub 3)O(sub x) deposits and (Bi, Pb)(sub 2) Sr(sub 2)Ca(sub 2)Cu(sub 3)O(sub y) powder-in-tube tapes have been determined from electron back scatter diffraction patterns (EBSP). The misorientation angles and axes of rotation (angle/axis pairs) for grain boundaries connecting these grains were calculated. For both materials the population of low angle boundaries was found to be much larger than expected from calculations based on the macroscopic texture. The TlBa(sub 2)Ca(sub 2)Cu(sub 3)O(sub x) deposits exhibit pronounced local texture which has been defined by EBSP and x-ray diffraction. Locally grains show significant in-plane (a-axis) alignment even though macroscopically a-axes are random, indicating the presence of colonies of grains with similar a-axis orientations. In (Bi, Pb)(sub 2) Sr(sub 2)Ca(sub 2)Cu(sub 3)O(sub x) tapes no local texture was observed. In both materials the existence of connected networks of small angle grain boundaries can be inferred. Coincident site lattice (CSL) grain boundaries are also present in higher than expected numbers. Grain boundary energy thus appears to play a significant role in enhancing the population of potentially strongly-linked boundaries. We propose that long range strongly-linked conduction occurs through a percolative network small angle (and perhaps CSL) grain boundaries.