Novel stiff aerogel-aerogel composites for thermal insulation applications

Abstract

Aerogels are suitable materials for ultralight thermal insulation due to their properties of very low thermal conductivity <0.02 W/mK and low density <0.1 g/cm³ as a result of a high porosity (up to 99%) and their nanostructure. However, they are usually fragile and tend to release dust. We investigated aerogel-aerogel composites to develop new materials with optimal properties adapted to the application. In our study we embedded two different kinds of highly insulating granular aerogels in cellulosic aerogel: hydrophilic silica aerogel (SiO2) and resorcinol-formaldehyde (RF) aerogel which were prepared by ourselves. In the first step a salt hydrate melt of 59 wt.-% Ca(SCN)2 with 2 wt.-% of cellulose from cotton linters is prepared at 110 °C. Between 0.8-3.8 wt.% of granular aerogel resulting in ≤36 vol.% of granular material in the aerogel-aerogel composite is directly mixed into the melt. The hot melt is filled into a vessel after the cellulose is completely dissolved. A wet cellulose gel forms while cooling and builds a nanofibrillar felt structure. The composite gels are aged and washed in ethanol and are dried supercritically with CO2. We obtained non-fragile materials with density about 0.06-0.17 g/cm³ and thermal conductivity about 0.036-0.044 W/mK. Compared to pure aerogels, embedding granular aerogel material into the structure of a cellulosic aerogel has a strong stiffening effect regarding the Young’s modulus.