Magnetic properties of Y-Ba-Cu-O superconductors

We report on the superconducting magnetic properties of two polycrystalline YBa2Cu3Ox samples which have high densities and show current densities of 7×104 A/cm2. In addition, two single crystals are studied, one of which can support a current density of 3×106 A/cm2. The diamagnetic shielding in these samples in fields of 14 Oe is almost 100%, but the Meissner effect in applied fields above 1000 Oe is less than 1%.