ELECTRON-MICROSCOPY STUDY OF GRAIN-BOUNDARIES IN 1-2-3 SUPERCONDUCTORS

High resolution electron microscopy and analytical electron microscopy have been carried out on grain boundaries in polycrystalline YBa2Cu3O7-δ and LaCaBaCu3O7-δ, both with randomly orientated grains. Grain boundaries in YBa2Cu3O7-δ are mostly parallel to the (OO1) plane of one of the adjacent grains, while in LaCaBaCu3O7-δ they are almost random. In general, no amorphous material or other phases are observed at the grain boundaries of YBa2Cu3O7-δ. The grain boundaries in LaCaBaCu3O7-δ can be clean or can contain a thick amorphous layer, depending on the annealing conditions. Annealing in dry oxygen resulted in clean boundaries. Both materials show the intercalation of extra CuO layers at the grain boundaries. It is proposed that the low critical currents in polycrystalline YBa2Cu3O7-δ are partly caused by a non-superconducting layer due to interruption of the structure by frequently occurring (OO1) interfaces at grain boundaries. © 1990.