BULK SUPERCONDUCTIVITY ON OZONE ANNEALED YBCO SAMPLES BY AC SCREENING CURRENT MEASUREMENTS

Sintered samples of Y1Ba2Cu3O7-δ prepared with a modified citrate pyrolysis and annealed in an ozone atmosphere are tested with a.c. screening current in a geometrical configuration able to detect a true bulk superconductivity. A picture of the main features of diamagnetic behaviour of sintered high Tc superconductors is given to explain the a.c. susceptibility measurements in such materials. Comparisons among samples respectively annealed with and without ozone atmosphere have been performed as a function of the amplitude of the a.c. magnetic field. At small fields ozone samples just 2K below the superconducting transition temperature have a complete shielding while, with the same a.c. magnetic field, oxygen samples show the complete shielding at lower temperature. For samples with simple geometrical shape the absolute reference, corresponding to the 100 % shielding is obtained by using Nb or Pb superconducting samples with identical shapes. In the absence of an absolute reference for arbitrary shapes, the complete diamagnetic shielding can be inferred by the absence of the frequency dependence in the a.c. susceptibility. Such analysis clearly shows the better quality of the ozone samples. © 1990.