Structure and properties of melt-textured YBa2Cu3O7−δ, high pressure–high temperature treated and oxygenated under evaluated oxygen pressure

MT-YBCO samples oxygenated under controlled oxygen pressure exhibited at 77 K a critical current density jc = 85 kA cm-2 in zero field and more than 10 kA cm-2 up to 5 T field when the external magnetic field was perpendicular to the ab-plane of Y123, and a jc = 23 kA cm-2 in zero field and jc close to 1 kA cm-2 in 10 T field when the magnetic field was perpendicular to the c-axis of Y123. The microstructure of these samples contained an unusually high density of twins (about 30 twins mum-1) as well as a lot of stacking faults around Y211 inclusions. Using quasi-hydrostatic high pressure-high temperature (HP-HT) treatment we may vary the twin and dislocation densities in the material by changing the sample orientation in high pressure apparatus, while the oxygen content of Y123 phase as well as the lattice parameters remain unchanged. The microstructure of the material in the case where the highest pressure has been applied in the direction perpendicular to the ab-plane of Y123 is characterized by a very low twin density, perfect dislocations stepped along directions and small faulted loops corresponding to CuO intercalating in the matrix. For this material jc = 10 and 8 kA cm-2 in zero field were observed (when the external magnetic field was perpendicular to the ab-plane and perpendicular to the c-axis of Y123, respectively). High pressure-high temperature treatment causes an increase in the material density (up to near the theoretical one), microhardness and fracture toughness.