SILICA AEROGEL FILMS AT AMBIENT-PRESSURE

Silica films with refractive indices in the range of 1.006-1.036 (equivalent porosity 98.5-91%) have been prepared at ambient pressure by a process wherein organo-siloxane polymers are deposited on a silicon substrate by conventional dip-coating at 25 degrees C and 0.85 bar (atmospheric pressure in Albuquerque) and heating to 450 degrees C. The film thicknesses (from scanning electron microscopy) vary from 0.1 to 3.5 mu m, depending upon the dip-coating rate (0.05-1.9 cm/s) and concentration of the sol. The process was optimized by varying the dilution, aging, organic modification, heat treatment and dip-coating conditions, allowing control of film porosity in the range similar to 30-99%. Imaging ellipsometry has been used to study the evolution of film porosity and thickness in situ. It is observed that the high porosity in these films is mainly attributable to dilation or 'springback' of the film during the final stage of drying.