Nucleation of YBa₂Cu₃O_7-x on buffered metallic substrates in thick precursor films made by the BaF₂ process

We present experimental data on direct measurements of area densities of YBa₂Cu₃O_7-x, YBCO, nuclei in fluorinated precursor films which are on CeO₂ buffered metallic and single-crystalline SrTiO₃ substrates. The area density of the YBCO nuclei was measured by polarized-light microscopy after the nuclei had grown to the surface of the film. The density was found to depend strongly not only on processing conditions, but also on the type of the substrate. We also established a correlation between the area density of the nuclei and the nucleation of the randomly and c-axis oriented YBCO grains in the films. A model which was based on classical nucleation theory predicted that the nucleation was a result of collective interactions among the existing nuclei. Also, it predicted qualitatively some aspects of the functional dependence of the nucleus density on processing parameters. It was found that J_c values of very-large-grained c-axis-oriented YBCO films were low due to weak connectivity at grain boundaries.