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

Abstract

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.