How the Aerogels Is Synthesised and Processed and How This Impacts Its Structure - Assignment Example

Aerogels Name: Course: Institution: Instructor: Date: Overview of Aerogels Aerogel is a collective names commonly used to refer a group of materials extensively used in space navigation and travel. The term does not therefore refer to a single or specific mineral element or material with a distinct chemical formula. Aerogels therefore collectively comprises of all materials with a unique geometric structure which makes them extremely porous in solid foam thereby establishing extremely high connective bonds linking the branched structure of a few nanometers across the material structure. Aerogels initially found applications in space navigation but have soon been exploited and now find broad uses in a wide range of industrial plants. Aerogel is basically foam in nature. However, it takes a number of different shapes and forms even as foam. The major constituent structural components of aerogel include silica, semiconductor nanostructures, various polymers of carbon, oxides of iron with traces of gold and copper. A unique feature of aerogel is that its structure composes of 99.9% air and very little traces of solid component. Due to this composition, aerogels have been often described as having a ghostly appearance, commonly referred to as “Frozen smoke”. There are three commonly used types of aerogels. These include the inorganic aerogel, organic aerogel and carbon aerogels. Synthesis and production of Aerogel The synthesis and production of aerogel can be summarized as drying of gels at extremely high temperatures through the sol-gel process. a gel, a material with both soft and weak as well as hard and tough properties and capable of flowing in its steady state, is first produced in a solution state. This is then followed by careful extraction of any present liquids from the gel to yield an aerogel intact. A sol, which refers to a colloidal suspension of tiny solid particles, is produced by creation of colloidal silica. In this process, liquid alcohol, especially ethanol, is thoroughly mixed with a preferred silicon alkoxide. The subsequent hydrolysis reaction of the mixture yields silicon dioxide particles which eventually form a sol. The sol is then subjected to various condensation processes thereby creating metal oxide bonds, (Two metal particles linked with oxide bonds from an oxygen particle between them) which creates strong links between the colloidal particles. When all the links have been formed, the resulting structure is referred to as a gel. The process of producing a gel is known as gelation. Special processes are required to completely remove the liquids from the gel. For instance, xerogels are produced by allowing the liquids to evaporate from the gel. However, such evaporation has a tendency of causing surface tensions in the liquid-solid interfaces sufficient enough to destroy the extremely fragile gel network. Therefore, xerogels cannot attain sufficiently high porosities but rather peak at slightly lower porosities as exhibited through the numerous shrinkages after drying. Initially, superficial drying was used to develop aerogels. This was done by increasing the temperature and pressure and thereby driving the liquid into a superficial fluid state. When pressure is dropped with instant gasification, the liquid inside the aerogel could be easily removed without necessarily tampering with the fragile three dimensional networks within the aerogel. However, high pressure and temperature conditions are not advisable for reactions involving ethanol. To counter the danger posed by experimenting with ethanol at high pressure and temperature, solvent exchange process is often used. Alternatively, superficial carbon dioxide can be directly injected into the aerogel pressure vessel. In both process, the aim is to replace the initial liquid present in the aerogel with carbon dioxide while avoiding damage to the gel structure. A number of reinforcements, both continuous and discontinuous, can be successfully be used in production of aerogel composites. The high aspect ratio of fibers is capitalized on for use as reinforcements which results in improved mechanical properties of the aerogel composites. Inorganic aerogels synthesized through supercritical drying in which extremely highly cross-linked are produced hydrogels from continuous poly-condensation of metal alkoxides. A common example of inorganic aerogels is the silica aerogels. Organic aerogels on the other hand are produced from the sol-gel poly-condensation process which involves a reaction of a chosen resorcinol such as ethanol with formaldehyde in a hydroxide solution to initiate solvent exchange. The final type of aerogels, carbon aerogels are produced from pyrolyzation of organic aerogels in inert conditions of temperature and pressure. Properties of Aerogels emanating from their synthesis and production Several properties of Aerogels resulting from the synthesis and production procedures often make them best suited for their applications as the best insulator. For instance, Aerogels are known for their extremely incredible light weight with a density of between 0.0011 to 0.5 g cm-3. Aerogels are therefore typically about 15 times heavier than air. Aerogels have for a long time been in the Guinness books of records as the ‘solid with the lowest density’, before it was recently replaced by the metallic microlattice and then aerographite. i. Low mean free path of diffusion ii. High specific surface area (for a non-powder material) Specific surface area is defined as the total surface area of a material per unit mass. Aerogels usually exist in foam form which is almost weightless. The amount of foam of an aerogel required to make up a unit mass is therefore extremely huge compared to other forms of mater. The high specific surface area of aerogels have been hugely exploited in space science and astronomy where huge volumes of aerogels can be effectively attached to the spacesuit without adding any extra weight. Astronauts can therefore attach as much volume of the aerogel as they can without necessarily worrying of any increasing mass and thereby affecting takeoff iii. Low thermal conductivity The low thermal conductivity of aerogels can be explained using the Knudsen effect. Knudsen effect is defined as the reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the mean free path. Aerogels as basically used as thermal insulators. Low thermal conductivity therefore serves to improve their insulating properties. The structure of the aerogels allows them to clearly cut out at least two of the three heat transfer methods; convection, conduction and radiation. Since air cannot percolate and circulate within their lattice structure, they easily act as perfect convective inhibitors thus nullify convectional heat transfer in their lattice. Silica, a constituent of the aerogel lattice is permeable to infrared radiations. As a result, aerogels subsequently are poor radiative insulators. iv. Low sound speed Besides serving as heat insulators, Aerogels are also used as sound insulators too. The porous structure of aerogels reduces the speed of travelling sound through two major processes, absorption and reflection. Low sound speed thus implies the high resistance to movement of sound within its structure and thus making them preferred for production of sound proof environments. Aerogels have extremely good impedance match with sound waves and therefore it absorbs most sound reaching its surface rather than reflect it away. The structure of aerogels sis such that whenever a sound wave strikes the surface of the aerogel, a torturous path is created for the sound to travel through once the waves get inside the aerogel surface. The torturous path eventually yields a reduction in the speed of travelling sound waves from approximately 350 meters per second to around 100 meters per second. The reduction in speed also affects the amplitude of the sound waves by way of reducing their peak values. v. Low refractive index The refractive index determines the extent to which light rays bend at interface when entering a transparent medium or material. The porous structure of aerogels offers little resistance to the penetration of light waves. In fact, light waves travel within the aerogel structure irrespective of the composite constituents that make up the aerogel. This therefore implies an extremely low refractive index of most aerogel, whether organic, inorganic or carbon aerogels. The low refractive index increases viscosity in aerogels. This is the major logic behind the extensive use of aerogels in capture of dust particles. The highly viscosity as a result of low refractive index provides a perfect grip between the aerogel and the dust particles once they get into the structure of the aerogel. vi. Low dielectric constant The dielectric constant of a material is often used to determine the extent of the material’s effect on a capacitor. It is defined as the ratio of the capacitance of a capacitor containing the dielectric to that of an identical capacitor in a vacuum without the dielectric material. The fact that aerogels have low dielectric constant therefore implies their subsequent high insulating properties. Uses of Aerosols basing on synthesis and structure Due to their wide range of chemical and physical characteristics, aerogels often find diverse applications. Since its inception in around 1960, aerogels have been dominantly used as insulating material in spacesuits during space navigation by astronauts. This is due to the fact that aerogels are almost weightless and therefore will add no extra weight to the space suit when used as insulators. The strong bond within the structure lattice of aerogel makes it string enough to withstand the severe take off conditions despite its wispy physical appearance. Aerogel has also been used in creation of dust free environments. For instance, t was widely used by the NASA astronauts on their mission to capture space dust in the early 21st century. The mission to capture space dust has been widely exploited and currently, experiments are underway to effectively use aerogel on the “Stardust” mission aimed at successfully bringing back particles from space far beyond the moon for the first time in the astronomy history. Aerogel has ability to trap small particles within its structure without altering the same particles. For this reason, it has been widely used in capture and collection of comet dust. The logic behind this is that when a moving particle bombards aerogel, the speed of such particle is thought to be approximately 6 times that of a bullet coming from the nozzle of a gun. No substance can therefore be used to slow down such a particle at that incredibly high speed without necessarily heating up and thereby altering the particle. However, within aerogel, the moving dust particle buries itself or embeds within the porous structure of the aerogel, gradually losses momentum and is thus safely carried within the lattice of aerogel without any alteration or whatsoever on its structure. In the insulation industry, aerogel find extensive applications in cavity injected wall insulation as well as production of insulating boards. The recent breakthrough in insulation was successfully insulating buildings in Switzerland by use of an aerogel-based plaster. The EMPA (Swiss Federal Laboratories for Materials Science and Technology) aligned to Fixit AG labs successfully developed a render (insulating material) based on aerogel, which according to the information provided, has ability to provide twice the insulation of normal renders. The initial structure and physical characteristics of aerogel have been modified through careful selection of constituent materials as well as altering the manufacturing conditions to give yield to composite aerogels. In the same case aerogel composites can therefore be manipulated to depict certain desired thermal characteristics as well as acoustic and chemical properties. Aerogel composites exist in form of powder, micro-spheres, thin films or even monoliths depending on the required applications. Carbon aerogels find extensive uses in construction of small electrochemical double layer super capacitors owing to their high surface area. For instance, carbon aerogels can effectively be used to miniaturize the size of super capacitors to almost 1/2000th to 1/5000th the normal size of similarly rated electrolytic capacitors. The presence of carbon aerogel in the structures can effectively enable the same capacitors to absorb or produce extremely high peak to peak currents as well as low impedance compared to normal capacitors of similar rating and functionality. References AEGERTER, M. A. (2011). Aerogels Handbook. Aerogels Handbook. New York, NY, Springer FRICKE, J. (1986). Aerogels: proceedings of the 1. internat. symposium, Würzburg, FRG, September 23 - 25, 1985. Berlin, Springer. SVINGALA, F. R. (2009). Alkali activated aerogels. Thesis (M.S.)--Rochester Institute of Technology, 2009. YAO, C. (2006). Nanoparticles in aerogels and cellulose composites. Thesis (Ph. D.)--Brown University, 2006. Read More

Special processes are required to completely remove the liquids from the gel. For instance, xerogels are produced by allowing the liquids to evaporate from the gel. However, such evaporation has a tendency of causing surface tensions in the liquid-solid interfaces sufficient enough to destroy the extremely fragile gel network. Therefore, xerogels cannot attain sufficiently high porosities but rather peak at slightly lower porosities as exhibited through the numerous shrinkages after drying. Initially, superficial drying was used to develop aerogels.

This was done by increasing the temperature and pressure and thereby driving the liquid into a superficial fluid state. When pressure is dropped with instant gasification, the liquid inside the aerogel could be easily removed without necessarily tampering with the fragile three dimensional networks within the aerogel. However, high pressure and temperature conditions are not advisable for reactions involving ethanol. To counter the danger posed by experimenting with ethanol at high pressure and temperature, solvent exchange process is often used.

Alternatively, superficial carbon dioxide can be directly injected into the aerogel pressure vessel. In both process, the aim is to replace the initial liquid present in the aerogel with carbon dioxide while avoiding damage to the gel structure. A number of reinforcements, both continuous and discontinuous, can be successfully be used in production of aerogel composites. The high aspect ratio of fibers is capitalized on for use as reinforcements which results in improved mechanical properties of the aerogel composites.

Inorganic aerogels synthesized through supercritical drying in which extremely highly cross-linked are produced hydrogels from continuous poly-condensation of metal alkoxides. A common example of inorganic aerogels is the silica aerogels. Organic aerogels on the other hand are produced from the sol-gel poly-condensation process which involves a reaction of a chosen resorcinol such as ethanol with formaldehyde in a hydroxide solution to initiate solvent exchange. The final type of aerogels, carbon aerogels are produced from pyrolyzation of organic aerogels in inert conditions of temperature and pressure.

Properties of Aerogels emanating from their synthesis and production Several properties of Aerogels resulting from the synthesis and production procedures often make them best suited for their applications as the best insulator. For instance, Aerogels are known for their extremely incredible light weight with a density of between 0.0011 to 0.5 g cm-3. Aerogels are therefore typically about 15 times heavier than air. Aerogels have for a long time been in the Guinness books of records as the ‘solid with the lowest density’, before it was recently replaced by the metallic microlattice and then aerographite. i. Low mean free path of diffusion ii.

High specific surface area (for a non-powder material) Specific surface area is defined as the total surface area of a material per unit mass. Aerogels usually exist in foam form which is almost weightless. The amount of foam of an aerogel required to make up a unit mass is therefore extremely huge compared to other forms of mater. The high specific surface area of aerogels have been hugely exploited in space science and astronomy where huge volumes of aerogels can be effectively attached to the spacesuit without adding any extra weight.

Astronauts can therefore attach as much volume of the aerogel as they can without necessarily worrying of any increasing mass and thereby affecting takeoff iii. Low thermal conductivity The low thermal conductivity of aerogels can be explained using the Knudsen effect. Knudsen effect is defined as the reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the mean free path. Aerogels as basically used as thermal insulators. Low thermal conductivity therefore serves to improve their insulating properties.

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