Initial stages of deposition and film formation during spray pyrolysis — Nickel oxide, cerium gadolinium oxide and mixtures thereof

The formation of nickel oxide (NiO), cerium gadolinium oxide (CGO) and NiO–CGO thin films by air blast spray pyrolysis was studied at two scales. First, single droplets of precursor were deposited on sapphire substrates and the morphology of the formed residue was studied as a function of the substrate surface temperature, type of metal salt, salt saturation, and organic solvent in the precursor. Second, the synthesis of continuous films from repetitively deposited droplets and crack formation in the films were studied as a function of substrate temperature and salt decomposition kinetics. Nitrates, acetates, perchlorates and chlorides of nickel, cerium and gadolinium were the metal salts used, and mixtures of ethanol or water with di-, tri- and tetraethylene glycol were used as solvents.Regular ring- or disc-shaped deposits were formed from single droplets that evaporated without boiling and were mainly observed with metal acetate- and chloride-based spray solutions or at low substrate temperatures. Disc-shaped residues were obtained for saturated salt solutions and changed to rings with diminishing rim thickness with decreasing salt saturation. The formation of bubbles in the droplet from boiling or salt decomposition during evaporation resulted in the distortion of the circular shape and was predominantly observed for metal nitrate-based precursors and at high substrate surface temperatures.Continuous, dense and crack-free films of CGO and NiO–CGO with thicknesses up to 500 and 800 nm, respectively, were prepared from metal nitrate/chloride mixtures in a tetraethylene glycol-based solvent. The maximum crack-free thickness decreased with decreasing deposition temperature and was correlated to the metal salt decomposition kinetics.