Chemical engineering of aerogel morphology formed under nonsupercritical conditions for thermal insulation

Aerogels are among the best thermal insulating materials known. This property arises from the fact that in these materials the gaseous phase, which may occupy over 90% of the volume, is compartmentalized to a size smaller than the mean free path of air, thus severely restricting its thermal conductivity. The solid framework, which compartmentalizes the gaseous space with a minimum amount of solid, is fragile and thus must be assembled in a liquid phase that is later extracted under supercritical conditions to create the porosity. This requirement for supercritical liquid extraction, to eliminate surface tension, makes the process costly and often unsuitable for large-scale commercial applications. By computer-designed experiments, an investigation was conducted in order to produce aerogels under non-supercritical conditions. Silica aerogels having approximately 80% porosity were produced under atmospheric conditions. Moreover, the resulting pore morphology is more restrictive to thermal conductivity than that produced under supercritical conditions. This aerogel was also transformed into a micronized powder form without affecting its pore morphology, by interruption of its gelling state, thus eliminating solid-state grinding. In this paper the design of this aerogel, its pore morphology, micronization, hydrophobizing, and its use in thermal insulation are discussed.