Aerogels Used in Construction (Silica)Question OneSilica aerogels synthesis can be grouped into three general steps: The first step is the preparation of gel, whereby the silica gel is acquired through sol–gel process. According to Dorcheh and Abbasi (2008, p. 11), the preparation of sol happens through a silica source solution as well as by adding a catalyst, resulting in gelation. Usually, the gels are grouped based on the dispersion medium utilised, such as aquagel or hydrogel, aerogel as well as alcogel (for water, air, and alcohol respectively). The second step is the aging of the gel, whereby the mother solution is used to age the prepared gel with the goal of strengthening the gel.
The final step is the drying of the gel, wherein the gel’s pore liquid is freed. In order to ensure that the gel structure does not collapse, the drying process is performed under special conditions. Therefore, all aerogel production methods involve these general steps, but more procedures may be carried out to influence the structure of the final product. The first step in Silica aerogels synthesis is the preparation of the gel, which is often achieved through the sol-gel process.
In this case, a ‘sol’ as mentioned by Thapliyal and Singh (2014, p. 3) is a colloidal liquid character system wherein particles that have been dispersed are either large or solid molecules with a colloidal range dimensions. On the other hand, a gel has been described as a colloidal solid character system wherein the substance that has been dispersed creates a continuous, coherent framework, which is normally interpenetrated by a system whose kinetic units are smaller as compared to the colloidal entities.
Therefore, gels are seen as three-dimensional network that are filled with solvent. Patel et al. (2009, p. 1053) assert that the dispersed phase of gels is normally very small (1 to 3 per cent) and some measure of elasticity and rigidity is exhibited. Classification of the gels normally depends on the dispersion medium utilised; for instance aquagel or hydrogel is used for water, aerogel is used for air respectively and alcogel is used for alcohol. Gelatin is an example of gel. Different chemical reactions may be utilised to achieve a Gel phase, but the choice of reaction relies on the anticipated properties of the final product.
For instance, Silicate gels are synthesised by trifunctional silicon alkoxide and hydrolysing monomeric tetrafunctional precursors that employ mineral base such as Ammonia or, Ammonium hydroxide solution or acid such Hydrogen chloride and Oxalic acid as a catalyst. The second step of the synthesis is the aging of the gel using the mother. According to Dorcheh and Abbasi (2008, p. 15) effect of the aging time and aging solution concentration on the silica aerogels porosity characteristics were examined by Smitha et al.
(2006), and they established a decrease in the linear shrinkage and bulk density while an increase in the surface area, pore volume as well as pore size in the concentration of Tetraethoxysilane in the aging solution. They further established that the effect of aging time on surface area, bulk density as well as pore volume was similar. Therefore, long period of gel aging leads to increased shrinkage, density and also the optical transmittance. To obtain transparent silica aerogels with larger size having no cracks and low density, the period of gel aging according to Dorcheh and Abbasi (2008, p. 22) have to be sufficiently long in order that the wet gel modulus increases thereby resulting in the monolithic aerogels. The final step of the synthesis is drying of the gel, which is governed by capillary pressure.
Gels shrinkage at the drying stage is steered by the capillary pressure (Pc), as shown in the equation one below: