Thick silica foam films through combined catalytic decomposition of H2O2 and sol–gel processes

Abstract

A novel method for the preparation of thick silica foam films is reported that combines an alkoxide-based hydrolytic sol–gel process and in situ catalytic decomposition of hydrogen peroxide on a catalyst-coated support. A hydrogen peroxide/nitric acid aqueous solution was used to carry out acid-catalysed hydrolysis of tetramethoxysilane. Complete stability of H2O2 in the resulting silica sols over extended periods (>120min) was demonstrated by in situ Raman spectroscopy. The H2O2-loaded sols were sprayed on MnO2-coated substrates, resulting in heterogeneous catalytic decomposition of H2O2 and effective foaming and simultaneous gel formation due to oxygen gas and water formation. Silica foam films with a well-defined closed-cell porosity were annealed at 600°C without any damage to the closed-cell porous film morphology. Up to 530μm thick films were prepared with macropore sizes in the range of 29–47μm, exceptionally thin macropore wall thicknesses of 16–50nm and a bulk density as low as 64kg/m3, comparable to that of the aerogels. The lowest measured thermal conductivity of the prepared foams was 0.018±0.001W/(m∗K), which is also similar to silica aerogels, enabling the prepared foams to be used as efficient thermal insulation materials.