Ambient-dried thermal superinsulating monolithic silica-based aerogels with short cellulosic fibers

Short (<2.5 mm) cellulose fiber–silica composite aerogels were synthesized by dispersing cellulose fibers in polyethoxydisiloxane-based sol. After in situ gelation, silica phase was hydrophobized with hexamethyldisilazane, and the composites were dried either at ambient pressure or with supercritical (sc) CO₂. Fiber concentration was varied from 0 to 25 wt% (corresponding to 0–2.1 vol%) of the final dried composite. Preformed cellulosic fiber network preserved the monolithic shape of the silica-based composites during ambient drying. At room conditions, thermal conductivities were 0.015 ± 0.001 W/(m K) for sc-dried aerogels and 0.017 ± 0.001 W/(m K) for their ambient-dried counterparts. Materials dried with either method exhibited large specific surface areas, from 570 to 730 m²/g, and SEM analysis did not show significant differences in the global structure of the silica network. Composite aerogels were hydrophobic with water contact angles around 138°. Based on this proof of concept, the same approach was used with a variety of natural and recycled cellulosic fibers also resulting in silica-based monoliths with low thermal conductivities in the 0.016–0.023 W/(m K) range, all produced via ambient drying.