Authors: Venkateswara Rao, A.; Haranath, D.; Pajonk, G. M.; Wagh, P. B.

Source: Materials Science and Technology, Volume 14, Number 7, July 1998 , pp. 1194-1199(6)

Publisher: Maney Publishing

Abstract:

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 ∼7. To obtain high optical quality silica aerogels for window applications, various supercritical drying parameters such as volume of solvent (MeOH), nitrogen (N₂) prepressure in the autoclave, alcogel heating rate, stabilisation period, and solvent evacuation rate have been varied. It has been found that the volume of solvent and N₂ prepressure are to be adjusted in such a way that the solvent/vapour phase boundary should not be crossed before reaching the critical point of the solvent (Me0H), to obtain crack free transparent (≲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 less transparent (83% at 900 nm) aerogels. It has been observed that a stabilisation period of <0.5 h is required to avoid shrinkage of the aerogels. The overall percentage of optical transmittance of the aerogels at 900 nm varied from 75 to 93%, depending upon the supercritical conditions. To find out the reason behind the best optical transparency, scanning electron microscopic (SEM) observations and normal-hemispherical transmittance measurements have been performed. Finally, the thermal conductance of an aerogel filled window has been compared with that of the same thickness of air filled window in between two glass panes.

Document Type: Research article

Publication date: 1998-07-01

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