Role of fractal structure on thin-film processing of YBa2Cu3O7 –x using alkoxide sols

Abstract

An alkoxide sol–gel route has been developed for producing YBa₂Cu₃O_{7 –x} superconducting powder and oriented thin films. Cu^II methoxyethoxide, Y methoxyethoxide and Ba methoxyethoxide precursors were synthesized and processed into stable sols with methoxyethanol (MOE), methyl ethyl ketone (MEK), toluene and diisopropyl ketone (DIK) solvents. Pyridine (C₅H₅N), lutidine (C₁₃H₂₁N) and toluene-2,4-disulphonic acid were used as organic additives to modify the polymeric network structure and corresponding sol behaviour. Small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) were used to study the structural modifications in sols and gels with changes in solution chemistry parameters, viz. acidity/basicity, moisture exposure and solvent system. Mass-fractal dimensions in the range 2.0–2.5 were obtained for 0.01–0.1 mol kg^–1 YBa₂Cu₃ sols in methoxyethanol–methyl ethyl ketone–toluene solvent. Significant effects of two organic base additives, pyridine and lutidine, on the sol structure were observed as a function of sol concentration. The ability of the different solutions to cover SrTiO₃, ZrO₂, sapphire, SiO₂ and silicon substrates was measured by contact-angle measurements. Thin films produced via spin casting, dip coating and spray coating were characterized using optical, scanning electron microscopy (SEM) and X-ray diffraction techniques, and the results are discussed in terms of the sol structures.