Preparation of alumina aerogel films by low temperature CO2 supercritical drying process

Alumina aerogel thin films were formed by a new synthesis route. Sols were prepared by the Yoldas process. Gels were formed by sol evaporation in a few hours. Films were prepared by dip coating glass or alumina substrates into both the sols and the gels. Aerogel films with special morphology were produced for the first time by exchanging the film solvent with acetone after the dip coating, followed by supercritical drying. The morphology of the films, studied by SEM and TEM, consists of fiber-like network of round chains (similar to 0.1 mu m thick), and pores (0.1-0.5 mu m in diameter). It is shown that the fibers contain a homogeneous arrangement of sol particles, 2-4 nm in size. Formation of this microstructure can be attributed to phase separation in the alumina-water-acetone system in a 2D him geometry. A conceptual model for the film development is proposed.