Bulk and surface light scattering from transparent silica aerogel
Elastic light scattering has been used to study structural properties of different transparent aerogels, which may be used as filling materials in super-windows. With a goniometer having an angular resolution better than 0.6 degree and a He-Ne laser as the light source we investigated the angular distribution of scattered intensity from transparent silica aerogels and one xerogel. The densities ranged between 0.11 and 0.60 g cm-3. An exponential correlation function for the density fluctuations of a random porous medium has been utilised to analyse the large-angle scattering, which is dominated by bulk scattering, for different polarisation of the incident light. The determination of correlation lengths in the nanometre range was possible, because the absolute scattering intensities were determined. For relative angular dependence measurements, this range would have been accessible only to small angle X-ray scattering (SAXS). The resulting mean pore sizes between 8 nm and 50 nm and spe cific surface areas between 500 and 700 square m/g agree well with nitrogen-porosimetry data from the literature. The data compare quite well with correlation lengths calculated from specular transmittance data from an ordinary spectrophotometer. This method, which is not sensitive to the angular distribution of superposed forward scattering with large correlation lengths, has also been applied to a series of base-catalysed TMOS aerogels with different catalyst concentrations. The forward scattering peak of the signal may be attributed to correlation lengths in the micrometre range. Experimental results for aerogel surfaces with evaporated aluminium indicate that this might be due to the surface properties. A quantitative analysis, however, is not possible yet.