STRUCTURAL AND ELECTRICAL-PROPERTIES OF THE Y-BA-CU-O SUPERCONDUCTING FILMS PREPARED BY SPRAY PYROLYSIS

High-T(c) superconducting YBa2Cu3O7-delta thin films have been grown on (100) MgO substrates by a chemical spray pyrolysis method. The crystal structure and surface morphology have been studied by X-ray diffraction and scanning electron microscopy, respectively. The assprayed films were amorphous and insulating, but upon annealing the films became superconducting and show a textured surface morphology with an average grain size of the order of 5-15 mum. The films were highly oriented with the c-axis being perpendicular to the substrate surface. Three different microstructures were recorded: long rod-shaped grains, platelet or rounded-shape grains, and a melting-like growth. Electrical measurements were carried out in a low-temperature cryostat using a standard d.c. four-probe technique. The onset transition temperature was around 83-86 K, and the completion of the transition to zero resistance was in the range 73-78 K. The magnitude of the measured critical current density was in the range 750-3750 A/cm2 at 30 K. A correlation between the resistance of the tunnelling junctions and the critical current density was found from the theoretical models.