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General relations between the local atomic structure and properties of grain boundaries (GBs) in superconducting copper oxides are described. The results are based on the complex experimental and theoretical study, which includes: (1) the transmission electron microscopy characterization of GBs in the bulk materials, (2) GB atomic structure simulation, (3) first-principles calculations and modelling of the electron structure modification, crystal lattice relaxation and intergrain bond formation near the GBs simulated. The number of broken Cu-O bonds, the degree of electron state perturbation at the GB and the role of crystal lattice relaxation are found to be strongly dependent on the GB topology.

The critical current density of melt-textured YBa2Cu30^ superconductor has been enhanced by mechanical deformation at a high temperature. Hot deformation at 45°t o both the slip plane (001) and the slip directions [100]/[010] has resulted in a high density of dislocation loops and stacking faults. The deformed samples are found to exhibit a critical current density {Jc) at //||c-axis as high as that at H\\a-b plane at 1.5 T and 77 K. A Jc of 35300 A/cm2 has been achieved at H\\c (1.5 T and 77 K) which is twice as high as that observed in undeformed samples. The enhanced Jc in this magnetic field orientation is attributed to pinning by the defects created by mechanical deformation. This pinning mechanism is found to be effective over a wide angle between the magnetic field and the a-b plane and thus results in a marked reduction in the critical current anisotropy.

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