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 µm−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.
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