The Leidenfrost effect during spray pyrolysis of nickel oxide-gadolinia doped ceria composite thin films

NiO-Ce_0.8Gd_0.2O_{1.9 - x} (CGO) thin films were prepared by air blast spray pyrolysis with precursors containing nickel nitrate, cerium nitrate and gadolinium chloride in ethanol and a high boiling point organic solvent. Precursors containing solvents with boiling points between 120 and 314 °C were sprayed on sapphire, silicon, Foturan^®, yttria stabilized zirconia and CGO at different substrate surface temperatures. A maximum deposition temperature, above which film deposition ceased completely, was observed. The limiting temperature for film formation was correlated with the Leidenfrost phenomenon. At temperatures above the Leidenfrost point of the precursor, the sprayed droplets do not impact and spread on the substrate surface but levitate on a vapour cushion above the substrate and are swept away by the air stream. The Leidenfrost point of a precursor was found to depend on the solvent boiling point, the metal salt concentration and the thermal properties of the substrate expressed as the product of density, thermal conductivity and heat capacity. The maximum deposition temperature increased with increasing solvent boiling point or metal salt concentration and with decreasing product of density, thermal conductivity and heat capacity of the substrate.