OPTIMIZATION OF SUPERCRITICAL DRYING PARAMETERS FOR TRANSPARENT SILICA AEROGEL WINDOW APPLICATIONS

Silica aerogels have been prepared by hydrolysis and polymerisation of methanol (MeOH) diluted tetramethoxysilane (TMOS) in the presence of a basic catalyst (ammonia) at a pH of similar to 7. To obtain high opt ical quality silica aerogels for window applications, various supercri tical drying parameters such as volume of solvent (MeOH), nitrogen (N- 2) prepressure in the autoclave, alcogel heating rate, stabilisation p eriod, and solvent evacuation sate have been varied. It has been found that the volume of solvent and N-2 prepressure are to be adjusted in such a way that the solvent/vapour phase boundary should not be crosse d before reaching the critical point of the solvent (MeOH), to obtain crack free transparent (similar to 93% at wavelength 900 nm for 10 mm thickness sample) silica aerogels. Higher autoclave heating (> 40 K h( -1)) and evacuation rates (> 70 bar h(-1)) resulted in cracked and les s transparent (83% at 900 nm) aerogels. It has been observed that a st abilisation period of <0.5 h is required to avoid shrinkage of the aer ogels. The overall percentage of optical transmittance of the aerogels at 900 nm varied from 75 to 93%, depending upon the supercritical con ditions. To find out the reason behind the best optical transparency, scanning electron microscopic (SEM) observations and normal-hemispheri cal transmittance measurements have been performed. Finally, the therm al conductance of an aerogel filled window has been compared with that of the same thickness of air filled window in between two glass panes .