History of Aerogol

Aerogel is one of the most fascinating solids ever known to man. It was first discovered in the late 1930s be Steven Kistler, and later improved on by many different scientist. As the worlds lightest solid it has many different properties engineers try to take advantage of by mixing it with various elements, and seeing if they get the results they need. One of the key aspects of Aerogel is its thermal properties which is good for insulation usage.

The history of Aerogel is that Steven Kistler a student at the College of the Pacific set out to prove that a gel without the liquid inside contained the same size and shape as a gel with the liquid. The only way for him to prove this was to prove his hypothesis, was to test his theory, and that’s what he did. The approach he took was to try and remove the liquid from the wet gel without damaging the solid structure of the object. Of course he tried a number of different procedures and none of them worked. If a wet gel is allowed to dry on its own the gel would shrink to a fraction of its original size, and shrinking usually was followed by the structure cracking. As Kistler gained more knowledge of the structure of gel he made a key discovery that would lead him to his answer. Kistler predicted that the solid component was microporous which mean that it had really small pores, and that the liquid vapor exerted strong surface tension forces that collapsed the pore structure when removed. So to counteract that he figured that if he were to replace the liquid with air by some means in which the surface of the liquid is never permitted to recede within the gel it could work. If a liquid is held under pressure always greater than the vapor pressure, and the temperature is raised, it will be transformed at the critical temperature into a gas without two phases having been present at any time. The first gels that Kistler studied were silica gels prepared in a acidic aqueous sodium silicate solution, but his attempts of trying to convert water into supercritical fluids from gels by this method didn’t work. As an alternative of leaving silica Aerogel behind, the supercritical water redissolved the silica, which then precipitated as the water was vented. With knowledge that water in aqueous gels could be exchanged with other organic liquids, Kistler then tried again by first thoroughly washing the silica gels with water to remove salts from the gel, and then exchanging the water for alcohol by converting the alcohol to a supercritical fluid and allowing it to escape and the first true Aerogel were formed. Kistler was the father of aeorgel but between the late 1930’s and the 1970’s there was not a lot of development with the use of Aerogel. In the late 1970’s the French government approached Stanislaus Teichner at University Claud Bernard, Lyon seeking a method for storing oxygen and rocket fuels in porous materials. They were honored and determine to use Areogel because of its properties, however using Kistler's method which included two time-consuming and laborious solvent exchange steps their first Aerogel took weeks to prepare. Teichner then informed his student that a large number of Aerogel samples would be needed for him to complete his dissertation. Realizing that this would take many years to accomplish, the student left Teichner's lab with a nervous breakdown. After a brief rest he was strongly determined to find a better synthetic process. This directly led to one of the major advances in Aerogel science which is the application of sol-gel chemistry to silica Aerogel preparation. This process replaced the sodium silicate used by Kistler with an alkoxysilane, (tetramethyorthosilicate, TMOS). Hydrolyzing TMOS in a solution of methanol produced a gel in one step called an "alcogel". This eliminated two of the drawbacks in Kistler's procedure, namely, the water-to-alcohol exchange step and the presence of inorganic salts in the gel. Drying these alcogels under supercritical alcohol conditions produced high-quality silica Aerogel. In following years, Teichner's group, and others, extended this approach to prepare a wide variety of metal oxide Aerogel. In 1983 Arlon Hunt and the Microstructured Materials Group at Berkeley Lab found that the very toxic compound tetramethyorthosilicate (TMOS) could be replaced with tetraethylorthosilicate (TEOS) a much safer reagent. This did not lower the quality of the Aerogel produced. At the same time the Microstructured Materials Group also found that the alcohol within a gel could be replaced by liquid carbon dioxide before supercritical drying without harming the Aerogel. This represented a major advance in safety as the critical point of CO2 (31 degrees C and 1050 psi) occurs at much less severe conditions than the critical point of methanol (240 degrees C and 1600 psi). Also carbon dioxide does not pose an explosion hazard as alcohol does this process was put to use in making transparent silica Aerogel tiles from TEOS.

The manufacturing process in forming silica Aerogel is a two-step process. The formation of a wet gel, and the drying of the wet gel. With a procedure called sol-gel chemistry, silica Aerogel are prepared using a silicon alkoxide precursor. The kinetics of the reaction is slow at room temperature, often this process requires several days to reach completion. For this reason, acid or base catalysts are added to the formulation. With acid catalysis the particles may appear less solid than the ones catalyzed in a base that is the first step is for an Aerogel to reach its gel point. From there hydrolysis and condensation must follow. Time must be given for the strengthening of the silica network, the gels are best left undisturbed for up to 48 hours. The time required for each processing step increases dramatically as the thickness of the gel increases. It is referred to as diffusion controlled because diffusion is affected by thickness. Immediately after an aging process takes place by removing all the water still contained within its pores. This is simply accomplished by soaking the gel in pure alcohol several times. The final, and most important process in making silica Aerogel is supercritical drying. Here the liquid within the gel is removed, leaving only the linked silica network. The process can be performed by venting the ethanol above its critical point or by prior solvent exchange with CO2 followed by supercritical venting.

Aerogel is the world lowest density solid with a lot of different properties that amazing the scientific community. Next to air which has a density of 1.2 mg/cm3, Aerogel has a density of 1.9 2 mg/cm3. Which means it is the lowest density solid know to man, .It is nicknamed frozen smoke, solid smoke or blue smoke due to its semi-transparent nature and the way light scatters in the material. Aerogel is made from the sol-gel polymerization of selected silica orresorcinol-formaldehyde monomers in solution. The sol-gel solution is cast into the desired shape and after the formation of a highly cross-linked gel, the solvent is removed from the pores of the gel. A common feature of Aerogel is that they have ultra small inter connected pores with a size of less than 100nm. This open cell structure provides access to the entire surface area for reaction of materials as either gas or ions in solution. Aerogel is the lightest of all solid substance, and give new meaning to the word solid. The unique design which is composed of a cell wall a few atoms thick, and air or whichever gas the design decides to choose they can actually be made to be lighter than air. The physical properties of Aerogel are controlled by the density and microstructure of the gel. They are studied with the hope of understanding the process of structure formation well enough to control it, so that particular properties can be optimized for specific applications. Aerogel it has good thermo insulative properties which can greatly the methods of heat transfer (convection, conduction, and radiation). There are three main types of Aerogels which are Silica, Carbon, and Alumina. Silica is the most commonly used, and the most studied. Carbon Aerogels are considered as very interesting materials for high-temperature thermal insulations in non-oxidizing atmospheres or vacuum. Aerogels are open porous solids consisting of a three-dimensional network of spherical interconnected primary particles. This means that the pore and particle size can be specifically adjusted to be in the range from several nanometers only to some microns by varying the synthesis conditions. Pore sites up to about 99% can be achieved. These properties make carbon Aerogel suitable for thermal insulation applications , in particular at high temperatures, but also for electrodes in super capacitors , and gas diffusion layers in fuel cells.

Over the past couple of years aeorgel has been in the eyes of a lot of different companies because of its unique properties, and researchers’ ability to make it evolve more and more. NASA is currently using Aerogel to capture high-velocity interplanetary and interstellar dust particles which normally range anywhere from 100 nm to 10 µm and traveling between 0.5 and 10 km/s. since Aerogel pore structure ranges in the nanometer range, and they are less dense then regular solids they have the ability to slow down the particles, and capture them. There is a company called Centerpoint which utilizes the thermal properties of Aerogel. They fill glass, or a polycarbonate with Aerogel, and this allow them to keep this light transmission qualities while providing good thermal insulation. Also it utilizes some of the strength aspects its give the glass or polycarbonate the ability to withstand wind gust up to 140 mph.

Results & Discussion
Some of the advantages are that it is environmentally safe. It is capable of reducing industrial waste by capturing waste and polluting gases before they reach the atmosphere. It is capable of supporting 100 times its own weight, and they provide exceptional mechanical integrity and optical clarity. The only restriction is its versatility, that depends is how clever you are, the more knowledge you have of materials, the more possibilities you have of discovering new uses for it with the right combination. Aerogels can produce more efficient fuel cells for pollution free energy among other things. It provides insulation five times better than fiberglass. It is a non-CFC (chlorofluorocarbons) using insulation. It retains its insulating qualities indefinitely. It doesn’t use any CFC gases or any type of vacuum that can lose pressure. An indirect advantage of Aerogel is its potential to play a role in keeping the atmospheric line clear. By reducing home heating costs, Aerogel could reduce global energy needs and minimize the pollutants that come with energy production. A long-term advantage of Aerogel is in reducing industrial waste, not only is Aerogel of scientific interest to reduce the energy load, but also to capture waste and polluting gasses before they reach the atmosphere. In principle, wherever alcohol and fossil fuels are used, Aerogel absorbents could capture waste gasses before they are emitted into the atmosphere.

Time for its disadvantages Aerogels are difficult and time consuming to manufacture, even after appearing in a few commercial products during the 70 plus years that it has been developed. The process of manufacturing it involves soaking it in hazardous solvents, such as super critical water or alcohol, and using it to high temperatures and pressure. This technique is not only dangerous, but also very expensive. Being that their weight is so close to that of air preparing Aerogel is a tedious task they can easily deflate when not careful. The wonderful properties have been expensive to provide, costing too much for architects chemist, and engineers to play with. Although in principle, making an Aerogel is simple; unfortunately, it is not as simple as it sounds. When the liquid evaporates, it tends to pull the gel and distort it leaving it in a state of uselessness.

Its versatility and flexibility make it a good contender for many applications that have a positive long-term effect, not only in the technological realm, but also in the environmental realm. The list of applications and benefits of this material is long and still growing. The positives outweigh the negatives. The biggest obstacle that researchers are going to have to overcome is the production, and cost once there is a more efficient way to produce Aerogel then it will become of great use to us in the future. But that what Aerogel is a material of the future, and like many thing that have been made and improved upon it just takes time, money, and knowledge.

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