Processing and characterization of YBCO grain boundaries obtained by the liquid phase removal method

Using the recently developed liquid phase removal method, melt textured polycrystalline YBCO bars with clean grain boundaries are processed. These samples exhibit a high current transport capacity required for applications such as current leads. Microstructure as well as current density studies are performed on the grain boundaries of these samples in order to determine mechanisms involved in the current transport across such boundaries. Using optical techniques, the misorientation of the a-b planes across each boundary (the kink angle) is measured in several samples and the measurements reveal the presence of a predominantly large number of high angle grain boundaries in these samples. These results, along with the high current transport capacity of these samples, suggest the possibility of strong grain boundary coupling in spite of high misorientation angles. In addition, using transmission electron microscopy (TEM), weakly coupled (low Jc) and strongly coupled (high Jc) grain boundaries with high misorientation angles are studied. The strongly coupled boundary is found to be related to the constrained coincidence site lattice (CCSL) misorientation of fully oxygenated YBCO, while the weakly coupled boundary is related to the CCSL misorientations of oxygen deficient YBCO. These results imply that the strength of the boundary depends not only on the misorientation angle, but also on the extent of oxygen distribution along the boundary. The results also indicate that the liquid phase removal method has the potential to produce polycrystalline YBCO with clean grain boundaries capable of high current transport.