Optical properties of monolithic and granular SiO2-aerogels

摘要

The optical and thermal properties of aerogels play an important role for their application in transparent insulation (TI) or daylighting systems. In this paper the directional-directional transmission t$-dd$/ as well as the directional-hemispherical transmission t$-dh$/ are presented for 20 mm thick aerogel specimens. For optimized monolithic aerogels we obtained solar averaged values of $+$MIN$/t$-dh/sol$/ $EQ (91 $POOM 1/4)% and $+$MIN$/t$- dh,vis$/ $EQ (84 $POM 2)% in the solar and visible spectral region, respectively. The investigated granular aerogels show a visual transmission of $+$MIN$/t$-dh,vis$/ $EQ (58 $POM 4)% for waterglass as a precursor and of $+$MIN$/t$-dh,vis$/ $EQ (78 $POM 4)% for tetramethoxysilane. In order to correlate the structural build-up of the SiO$-2$/-network of the aerogels with their light scattering properties angular-dependent light and X-ray scattering measurements are performed. To characterize the visual image transmission the modulation transfer function (MTF) was measured for 20 mm thick monolithic layers. In order to explain the measured data a Monte Carlo based simulation model was developed. The analysis yields that the MTF is mainly effected by surface scattering.