An investigation on high-angle grain boundaries in melt-textured YBa2Cu3O6+x superconductors

Six high-angle (range between 17 degrees and 77 degrees) grain boundaries in melt-textured YBa2Cu3O6+x superconducting materials, namely GB1, GB2, GB3, GB4, GB5 and GB6, have been investigated. Three of them GB1, GB2 and GB3 have high values of critical current, while the other three GB4, GB5 and GB6 possess law values of critical current. From TEM studies, GB1, GB2 and GB6 are found to be related to fully oxygenated CCSL boundary, while GB4 and GB5 are related to oxygen-deficient constraint coincidence site lattice (CCSL) boundary. GB3 is found to be a random grain boundary where its misorientation characteristics do not match any CCSL within the limitation of Sigma < 100. The analyses of the TEM studies along with the critical current measurements obtained on GB1, GB2, GB4 and GB5 indicate that oxygen distribution at the grain boundary plays an important role in the coupling of grain boundaries in YBa2Cu3O6+x superconductor. However, the behaviors of GB3 and GB6 suggest that the CCSL theory is not general in predicting the superconducting behavior of high-angle grain boundaries in YBa2Cu3O6+x superconductors. (C) 1998 Published by Elsevier Science B.V. All rights reserved.