Improving the visible transparency of silica aerogels
Silica alcogels have been prepared in two steps, first in an acid and then in a base to control the relative rates of hydrolysis and condensation of TEOS. Increasing the H2O/TEOS ratio in the base-catalyzed step was the most important factor in reducing light scattering of the alcogel. The two-step gel could be practically made with H2O/TEOS = 30, achieving a scattering intensity less than 1/5 that of the best one-step gel. Lower scattering of the gel results in an aerogel with improved visible transparency and more homogeneous microstructure. TEM observation reveals that the two-step aerogel has a microstructure containing both linear polymers and colloidal particles that are cross-linked to a gel network.