Aerial Oxidation of Tetraethyl Silicate and Effect on Ammonia Catalyzed Hydrolysis

Colloidal suspensions of SiO2 in ethanol prepared by the ammonia catalyzed hydrolysis of tetraethyl silicate (TEOS) in ethanol have been routinely used for over 10 years to prepare antireflective (AR) coatings on the fused silica transmissive optical components of high power fusion lasers. Very high purity coatings are required to avoid laser damage and these are obtained when the TEOS is fractionally distilled under N2 prior to use.Recently we found that products from aerial oxidation of distilled TEOS, had a significant effect on the particle size of our coating suspensions to the detriment of the optical performance. We require particle sizes less than 20 nm to avoid light loss due to scatter and contaminated TEOS gave suspensions with much higher particle sizes. Oxidation products were identified by GC mass spectroscopy and included acetaldehyde, acetic acid, silicon acetates and reaction products of these compounds with ethanol.Acetic acid and silicon acetates were found to be the major cause of large particle formation. These could be removed by careful redistillation preferably in the presence of a small quantity of magnesium ethoxide. Storage in sealed containers under N2 avoided further problems.