Silica aerogels; a review of synthesis, applications and fabrication of hybrid composites

Abstract

Silica aerogels are gaining significant importance and have attracted considerable interest due to their extraordinary properties and numerous applications. Silica aerogels are highly porous with high surface area and very low density and thermal conductivity. Usually they are prepared and synthesized via sol-gel technique, which involves making a sol containing a precursor, a solvent and a catalyst. The properties possessed by the final product depend upon numerous factors such as ratio of precursor to solvent and the drying method employed. Due to the flexibility of synthesis methods and the production of aerogels with tailored properties, silica aerogels have found numerous commercial applications and are being investigated of their suitability in several areas, such biomedical and aerospace engineering. Despite having exceptional properties, silica aerogels come with drawbacks such as brittleness and low mechanical strength, which can be resolved by fabricating composites extending the potential applications. This paper reviews the synthesis of silica aerogels via sol-gel technique, and the applications where this extraordinary material has shown promising results. The different materials used for the fabrication of composites to improve and enhance the physical and chemical properties of silica aerogels are also presented.

The Role and Applications of Aerogels in Textiles

“…Supercritical Drying. Kistler first dried the silica aerogels using high-temperature supercritical drying in 1931 [83]. Ethanol or methanol was used as organic solvent, and drying was carried out in an autoclave at high temperature and pressure.…”

Recent Progresses in Eco-Friendly Fabrication and Applications of Sustainable Aerogels from Various Waste Materials

“…Freeze-drying does not require solvent modifications; thus, no waste chemicals are generated. It is noted that generally the resultant gels that forms out of ambient pressure drying is termed xerogels while gels that are formed out of freeze-drying is termed cryogels [ 24 ]. However, in this review, the term aerogel will be used throughout as an indication to the porous material obtained from each of the gel drying method.…”

Pectin Nanoporous Structures Prepared via Salt-Induced Phase Separation and Ambient Azeotropic Evaporation Processes

“…To obtain dry pectin with a 3D nanoporous structure in the next step, it is critical to maintain the porous morphology during solvent removal. This is because the porous pectin structures are mechanically weak, and the evaporation of water molecules tends to induce the aggregation of sugar chains via hydrogen bonding, thereby destroying the porous structure. ,, To overcome this problem, the 3D porous gel structure is formed using different drying methods, including freeze-drying (to form cryogels) and supercritical CO 2 drying (for aerogels). − Unfortunately, the sublimation process during freeze-drying requires a long time and the network structure is easily damaged. Supercritical CO 2 drying methods ,− are preferred for preparing natural polymer aerogels because no menisci are present at the liquid–vapor interface above the critical point, thus avoiding capillary stresses, and the 3D structure is not compromised.…”