Production of hydrophobic silica aerogels

Since aerogels having the required low refractive indices were not commercially available, we decided to produce aerogels by our own efforts. We adopted a new production method for the preparation of aerogels, in which methylalkoxide oligomer is used as a precursor [45]. This oligomer is hydrolyzed and polymerized in a basic catalyst ($NH_4OH$) in a solution of methyl or ethyl alcohol. The average size of alcogels is $120 \times 120 \times 24~ mm^3$ and they are formed in aluminum molds coated with a thin PTFE film. The gelation time ranges from a few minutes to 10 min depending on densities.
Silica aerogels have been used in several experiments, but their transparencies became worse within a few years of use, which was our great worry to adopt an aerogel Cerenkov counter as a particle identification device. This phenomenon may be attributed to the hydrophilic property of silica aerogels. In order to prevent such effects, we have made our silica aerogels highly hydrophobic by changing the surface hydroxyl groups into trimethylsilyl groups [46]. This modification is applied before the drying process. As a result of this treatment, our silica aerogels remain transparent even four years after they were produced.
After three weeks of aging including the surface modification, the alcogels were dried by a super critical drying method of $CO_2$. This drying process took 48 h. The volume of extractor is 140 $l$ and we could produce about 38 $l$ of silica aerogel in one batch. After seven months of operation (two batches/week), we produced about 2 $m^3$ of silica aerogel. Details of the production method can be found in [45,47].