Optical investigations of granular aerogel fills

Publisher Website
Google Scholar
Abstract
The light scattering behaviour of granular silica aerogels is measured at a fixed wavelength of 633 nm. Two different correlation lengths ac,bulk and ac,sur, which characterize the bulk and surface scattering properties, are derived. A value of ac,bulk,=(45+or-5) nm is typical for the bulk scattering, whereas surface imperfections, such as dust grains and scratches as well as bubbles and streaks, cause a strongly forward-peaked scattering distribution with ac,sur=(600+or-50) nm. The spectral scattering coefficient in the visible region is also measured. By modelling the data, the contributions of large and small inhomogeneities to the scattering coefficient S can be separated. Typical values are Sbulk=(65+or-10) m-1 at a wavelength of 633 nm and Sbulk=(115+or-20) m-1 at 543 nm. For the large inhomogeneities a wavelength-independent scattering coefficient of Ssur=(100+or-30) m-1 is obtained. We have also measured the directional-hemispherical transmission tdh of a loose granular fill as a function of granule size. The influence of the aerogel nanostructure on tdh is determined from radiation transport calculations. Because of the forward-peaked scattering distribution of the large inhomogeneities, tdh is predominantly affected by the bulk.

This publication has 4 references indexed in Scilit: