Oxygenation of bulk and thin-walled MT-YBCO under controllable oxygen pressure

The possibility to saturate MT-YBCO with oxygen up to about 7 oxygen atoms per one unit cell of YBa2Cu3O7-δ at 800 °C under controllable oxygen pressure (from 0.5 kPa to 16 MPa) has been established. On condition that the material heating to high temperatures occurs in the nitrogen atmosphere with gradual substitution of it for oxygen and further increase of the oxygen pressure, the saturation of MT-YBCO by oxygen takes place practically without the macrocrack formation and results in the decrease of microcrack density parallel to ab-planes, that leads to the increase of critical current density and mechanical characteristics. It has been shown that in the case of a higher twin density, the material demonstrates higher critical currents, jc. Bulk and thin-walled MT-YBCO oxygenated at high controllable pressure and 800 °C contained a large amount of twins (4-11 and 20-22 μm-1, respectively) and were practically free from dislocations and stacking faults, but demonstrated very high values of jc at 77 K: jc higher than 10 kA/cm2 was observed in the fields up to 5T in ab-planes and up to 2 T in the direction of the c-axis. Specially detwinned MT-YBCO containing a large amount of dislocations (up to 1012 cm-2) demonstrated critical currents lower by about one order of magnitude, which gives us ground to conclude that the presence of twins is extremely important for attaining high critical currents in MT-YBCO. The high-pressure oxygenation allows one to essentially reduce the duration of the oxygenation process as compared to that under atmospheric pressure.