Aerogels Used in Construction - Silica - Assignment Example

AEROGELS USED IN CONSTRUCTION (SILICA) By Name Course Instructor Institution City/State Date Aerogels Used in Construction (Silica) Question One Silica 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: Pc = −γlv / (γp − δ) (1) Where, γlv = pore liquid surface tension γp = pore radius δ = surface adsorbed thickness The equation two below shows how to find the pore radius: γp = 2Vp / Sp (2) Where, Vp = pore volume Sp = surface area According to Thapliyal and Singh (2014, p.3), drying of the gels can be performed through several ways: one way is through the supercritical drying (SCD) technique, whereby the removal of the pore liquid takes place above the critical pressure as well as critical temperature of the concerned liquid to the extent that there is no capillary pressure. Another way is drying the gel at ambient pressure, whereby the vapour and liquid surface tension cannot be avoided. Basically, the stress in the gel is proportional to the pore liquid viscosity, the rate of drying as well as inversely proportional to the wet gel permeability. Drying of gel can also occur through freeze-drying, whereby the phase boundary between the gas and liquid phase are non-existent; therefore, the role of capillary pressure becomes irrelevant. In this method, the solvent have to be exchanged using a high sublimation pressure as well as a low expansion coefficient resulting in the freezing of the pore liquid. Question Two Aerogels are nowadays considered to be the most suitable thermal insulation materials for building construction applications. The thermal conductivity of aerogels is very low, and they exhibit extraordinary properties than the traditional materials used for thermal insulation. Additionally, the visible transparency of aerogels makes them suitable for insulation applications in skylights and windows; thus, offers engineers and architects the opportunity to reinvent architectural solutions. Aerogel properties such as a high solar energy, low thermal conductivity as well as daylight transmittance make the material suitable for windows, especially the highly energy efficient ones. Aerogel beads have been promising at soft vacuums thanks to its structural advantage since they can be installed and maintained easily. Because of their low density as well as porous structure, aerogels according to NASA have the ability to trap space projectiles that travel with hypervelocity speed. Aerogels are used by NASA to trap dust particles from the space as well as for space suits’ thermal insulation. Besides that, aerogels are considered to effective insulation material because their nano pores are extremely porous. These pores have made aerogel so effective at insulation. Aerogels can be used together with batting to generate insulating blankets, and can also be filled in between glass panes so as to generate translucent day-lighting panels. Recently aerogels have been used in highly advanced technologies such as Hypersonic Inflatable Aerodynamic Decelerator (HIAD). The HIAD is an expandable re-entry vehicle that is towed as well as folded in the launch vehicle and assist the spacecraft to decelerate, to descend and land on any planet with atmosphere safe and sound. In this case, aerogel is considered suitable for HIAD, especially in the construction of Flexible Thermal Protection Systems. In the test launches, NASA are utilising the new thin film aerogel as an upgrade of the baseline insulation. Aerogel are utilised widely because of the flexibility and foldable property, and because they are not messy or hazardous to handle. Basically, silica aerogels may be synthesised by utilisation of cheap precursors at ambient pressure; thus, improving aerogels ability of being commercialised. Aerogel facilitates higher values of insulation in less space as compared to the traditional insulation materials. Aerogel products for construction, like insulating plasters, rigid boards as well as thermal insulation coatings have the ability to deliver greater thermal performance that is almost three times better as compared to peer products. Besides that, aerogel-based solutions can help a person achieve energy efficiency requirements during a renovation with limited space. It also offers flexibility in modern buildings’ designs. Aerogel flexibility enables technological innovators to get a broad spectrum of properties in the creation of new high-performance insulation products. Furthermore, Aerogel particles may be added easily and designed into different forms, like composite boards, dispersions, slurries, coatings, plasters, dry mixtures, and more. A number of companies are focussing on developing new building fabric technologies that incorporates aerogels condensed in transparent polycarbonate sheets with the goal of improving the existing and new buildings’ thermal performance. One of the technologies includes polycarbonate panels retrofitting whereby the existing windows are filled with aerogel granules with the objective of improving their thermal performance. Another technology is retrofitting the translucent aerogel panel from the outside side of the concrete wall with the purpose of trapping solar energy, which may be utilised to passively warming the building. Question Three Currently, widespread application aerogel materials are limited largely because of their poor mechanical properties, high costs of production as well as health-related issues. In aerogel making process, supercritical drying according to Gurav et al. (2010, p.5) is the most risky and expensive aspect. In aerogel preparation, eliminating the process of supercritical drying is the main desired objective. Aerogel main weakness is its crack propagation, which according to Woignier et al. (2015, p.269) can ultimately destroy the whole aerogel lattice when the exposure is too long. The syneresis effect induces this weakness, and is an extension of the condensation and hydrolysis reactions following the gelling process, which results in the gel shrinkage. Besides that, silica aerogels are exceedingly fragile, and they can collapse when they come in contact with liquids if they are not adequately improved. Furthermore, aerogels as mentioned by Thapliyal and Singh (2014, p.7) are considered to be mechanical irritant to the digestive system, skin, eyes as well as respiratory tract. Aerogel particles when inhaled can possibly lead to silicosis and can as well induce dryness of the mucous membranes, eyes and skin. This creates the need for protective gear when handling aerogels. Most importantly, aerogels mechanical properties are exceedingly poor, and this mainly limits their commercialisation as well as practical application. For this reason, alumina aerogels are considered to have better chemical as well as thermal stability together with the aerogels basic properties, improving their application ability. For this reason, a number of studies have started examining how to prepare alumina aerogel as an alternative to silica aerogel. Still, alumina aerogels according to Cao et al. (2015, p.18025) are friable and their mechanical properties are still poor; thus, making their practical application largely limited. Attapulgite (ATP) is considered to be the best alternative for aerogels due to its intrinsic properties, like superficial character and special crystal structure. As mentioned by Cao et al. (2015, p.18025), ATP has many fine properties like being adiabatic and adsorptive and can be utilised as inorganic filler added into a rubber matrix and polymer leading to improved composites properties such as compressive strength and tensile strength. Besides that, Attapulgite properties make it naturally compatible with hydrophilic polymers. For that reason, adding ATP into the alumina aerogel structure can result in improved strength. References Cao, F., Ren, L. & Li, X., 2015. Synthesis of high strength monolithic alumina aerogels at ambient pressure. RSC Advances, vol. 5, pp.18025-28. Dorcheh, A.S. & Abbasi, M.H., 2008. Silica aerogel; synthesis, properties and characte. journal of materials processing technology, vol. 199, pp.10–26. Gurav, J.L. et al., 2010. Silica Aerogel: Synthesis and Applications. Journal of Nanomaterials, vol. 1, pp.1-11. Patel, R.P., Purohit, N.S. & Suthar, A.M., 2009. An Overview of Silica Aerogels. International Journal of ChemTech Research, vol. 1, no. 4, pp.1052-57. Thapliyal, P.C. & Singh, K., 2014. Aerogels as Promising Thermal Insulating Materials: An Overview. Journal of Materials, vol. 1, pp.1-10. Woignier, T. et al., 2015. Mechanical Properties and Brittle Behavior of Silica Aerogels. Gels, vol. 1, pp.256-75. Read More