Summary
This chapter reports an approach to enhance the mechanical strength of silica aerogels via densification. Although the loss of porosity and consequently the increase of thermal conductivity of silica aerogels represent drawbacks related to the densification process, a combination of enhanced mechanical performance and optical transparency indicates that the densificated silica aerogels may be used as new glass material for window glazing application. Preliminary experimental results indicate lightweight (density 1.8 g/cm3, compared to 2.5 g/cm3 for float glass) and thermal insulating (thermal conductivity k ≈ 0.18 W/(mK), compared to about 0.92 W/(mK) for float glass) aerogel glass materials with high visible transparency (Tvis ≈ 95.4% at 500 nm, compared to 92.0% for float glass) can be achieved by annealing an acid-catalyzed silica aerogel precursor at 700 °C. Typical elastic modulus Er of the obtained aerogel glass materials is about 6.42 GPa, which can be further enhanced by, e.g., increasing the annealing temperatures.
Abstract
Glass represents an important and widely used building material, and crucial aspects to be addressed include thermal conductivity, visible light transmittance, and weight for windows with improved energy efficiency. In this work, by sintering monolithic silica aerogel precursors at elevated temperatures, aerogel glass materials were successfully prepared, which were characterized by low thermal conductivity [k ≈ 0.17–0.18 W/(mK)], high visible transparency (Tvis ≈ 91–96 % at 500 nm), low density (ρ ≈ 1.60–1.79 g/cm3), and enhanced mechanical strength (typical elastic modulus Er ≈ 2.0–6.4 GPa). These improved properties were derived from a series of successive gelation and aging steps during the desiccation of silica aerogels. The involved sol → gel → glass transformation was investigated by means of thermo-gravimetric analysis, scanning electron microscopy, nanoindentation, and Fourier transform infrared spectroscopy. Strategies of improving further the mechanical strength of the obtained aerogel glass materials are also discussed.