Optimized synthesis of ambient pressure dried thermal insulating silica aerogel powder from non-ion exchanged water glass

Silica aerogels are considered as promising materials for future energy saving buildings, however, their reliability remains questionable as commercially available aerogels can show relatively high standard deviations e.g. 22.2 +/- 1.4 mW m(-1)K(-1). Therefore a synthesis protocol for silica aerogel powder with thermal conductivities having extremely reduced standard deviations was designed, compatible with mass production, i.e. reduced use of solvents and hydrophobization agents. Silica sols were prepared from non-ion exchanged water glass, while a combined solvent exchange, silylation and washing out of sodium ions was carried out using a hexane/trimethylchlorosilane/isopropyl alcohol solution with a molar ratio trimethylchlorosilane/pore water of only 0.11. The hexane amount was reduced 35 times compared to typical processes in literature. The aerogel powder was finally dried at ambient pressure at 150 degrees C. No notable shrinkage was observed for 8 wt% silica aerogel samples, having a thermal conductivity of 25.4 mW m(-1) K-1 with a standard deviation of only 0.1. The 6 wt% silica aerogels had a thermal conductivity of 23.4 mW m(-1) K-1 with a standard deviation of 0.3, but had shrunken 36%. By replacing trimethylchlorosilane with hexamethyldisiloxane, aerogels having a thermal conductivity of 22.4 mW m(-1) K-1 with a standard deviation of 0.3 were obtained.