THE NATURE OF GRAIN-BOUNDARIES IN THE HIGH-T-C SUPERCONDUCTORS

Grain-boundary weak links limit the critical current density of polycr ystalline high-T-c superconductors. Themes that have evolved from rese arch on these grain boundaries include (a) an apparent general and dra matic distinction between the behavior of low-angle and high-angle gra in boundaries, (b) electrical heterogeneity along as well as among bou ndaries, (c) indications that the macroscopic current path may or may not cross certain high-angle grain boundaries, (d) a key role for TEM- EELS in studies of the electronic structure near grain boundaries, and (e) evidence that current models for grain boundary structure pertain to the high-T-c superconductors and may eventually help us to underst and their electromagnetic character. Results regarding each of these t opics are reviewed in this paper.