Organically modified silica aerogels were prepared by
NH4OH-catalyzed hydrolysis and condensation of
RSi(OMe)3 / Si(OMe)4 mixtures, followed by
supercritical drying of the alcogels with methanol or CO2.
Terminal alkyl or aryl groups, bridging groups or functional organic
(methacryloxypropyl or glycidoxypropyl) groups were employed for R. By the
proper choice of the organic groups, the RSi(OMe)3 /
Si(OMe)4 ratio and the drying conditions, hydrophobic
aerogels, being insensitive towards moisture, were obtained with no residual
Si-OH or Si-OMe groups left. The transparency and porosity of the
organically modified aerogels was only slightly diminished relative to
unmodified silica aerogels. The elastic constant of the aerogels was
significantly influenced by the kind of organic groups. By pyrolysis of the
phenyl-substituted aerogels, nanometer-sized carbon structures were
generated. They partly coat the primary aerogel particles and provide a very
high mass specific extinction in the wavelengths interval critical for
thermal radiative transport.