Deposition parameters were varied systematically to obtain NiO/Gd-doped CeO2 composite films using the electrostatic spray deposition method. The microstructure and morphology of the obtained films were investigated using a scanning electron microscope and an x-ray diffractometer. The degree of wetting of aerosol droplets on substrates decreased in the order of silicon wafer > stainless steel > glass. For deposition on silicon wafers, a continuous, flat layer was formed first due to excellent wetting of aerosol droplets on the substrate. Isolated particles were then nucleated and precipitated from the subsequent landing droplets and scattered on the bottom layer due to the very limited wetting of aerosol droplets on the flat oxide layer. Preferential landing of aerosol droplets on the isolated particles and subsequent precipitation and agglomeration of the fine particles finally resulted in a cauliflower morphology. Higher deposition temperature and lower flow rate of precursor solution resulted in drier droplets and hence diminished wetting and favoured particle precipitation, preferential landing of aerosol droplets and subsequent particle agglomeration. Increasing substrate roughness also favoured preferential landing of aerosol droplets and particle agglomeration. The composition of the deposited films was in fairly good agreement with that of the starting solution as revealed by the inductively coupled plasma mass spectrometer.

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This study shows that there exist a pair of universal and unique phase diagrams to describe general development of the pore shape in solid, resulting from a bubble captured by a solidification front. Like thermodynamics, phase diagrams have advantages to generally identify the states and design processes of a system. Pore formation and its shape in solids influence not only microstructure of materials but also contemporary issues of biology, engineering, foods, geophysics, and climate change, etc. In this study, a pair of phase diagrams is thus found to be under dimensionless coordinate systems of dimensionless apex radius, contact angle, and base radius of the bubble cap, as well as solidification rate, contact angle, and growth rate of base radius. The computed results of the development of the pore shape agree with experimental data. The pore shape in solid thus can be optimistically predicted and controlled by choosing a desired path on phase diagrams.