The superconducting properties of Y 1 Ba 2 (Cu 1−x Al x) 3 O 7 /Y 2 BaCuO 5 bulk superconductors with different Al contents were studied (x = 0.0025-0.05). A peak effect was observed at intermediate magnetic fields over a wide temperature range below T c. The influence of standard oxygenation (SO) at 400 • C and high pressure oxygenation (HPO) at 750 • C on the critical current density J c was investigated. We show that the critical current density is higher by a factor of 3 in samples oxygenated at high pressure. The magnetization relaxation rates were investigated for an undoped HPO sample as well as for the lowest Al concentration for HPO and SO in the temperature range from 15 to 77 K and at magnetic fields of 0.5 and 1 T. The normalized relaxation rates, S, and the pinning potentials, U 0 , have been calculated from the time-dependent magnetization curves.

As-grown YBa 2 Cu 3 O 7 bulk superconductors have low oxygen content, YBa 2 Cu 3 O 6.3 , are not superconducting, and must be oxygenated to form YBa 2 Cu 3 O 7 . During standard oxygenation at 400 • C the shortening of crystal lattice parameters causes intensive cracking. The created cracks allow oxygen penetration into the bulk and cause the oxygenation time to be technologically acceptable but reduce significantly the superconducting properties. Here we show that it is possible to eliminate the formation of oxygenation cracks and to reach a critical current density 2.5 times higher than in bulk oxygenated in a standard way. The oxygenated crack-free samples were obtained by high pressure oxygenation with progressively increasing oxygen partial pressure.

No abstract