Aerogels are a promising class of materials, with the potential to be used in various different applications, such as thermal insulation, and adsorption of chemicals.
True gels consist of mostly liquid, with a cross-linked solid structure present throughout the gel. An aerogel is created when the liquid is removed from the gel, and is replaced by air, without destroying the solid structure within the gel. This gel is responsible for the properties of the aerogel created.
The first aerogel was created in 1931 by Samuel Kistler in an attempt to replace the liquid in a gel with air without causing shrinkage of the structure. He used a process called supercritical drying to remove the liquid from a silica gel. This uses a supercritical fluid, which is a material with a temperature and pressure above its critical point, where the liquid and gaseous states are indistinguishable, leading to useful properties, such as being able to effuse into solids and gels, and act as a solvent. This means that it can be pumped into a gel to dissolve the liquid component, and then pumped out, leaving the structure intact.
Aerogels were first marketed by Monsanto, an agrochemical company. They used aerogels as thickeners in a variety of products, but most notoriously in manufacturing napalm, which was used against both soldiers and civilian populations in Vietnam, triggering widespread outrage. In addition, aerogels were used as cigarette filters, as their structure allowed most gas molecules to pass through it, but prevents larger particulate matter such as that produced by the burning of tar from reaching lungs (it is important to note that even with these filters, smoking is very detrimental to health). However, aerogels are very expensive to manufacture, and for that reason, Monsanto stopped using aerogels in their products
Currently, the largest commercial use of aerogels is in thermal insulation. This is because aerogels are both very poor conductors (as they are almost entirely composed of air, a poor conductor) and prevent convection within them, due to the very complex solid structure within them. One of the most well-publicised uses of an aerogel was in insulation for the Mars Exploration Rover, a rover made by NASA. Unfortunately, aerogels are a very poor insulator of thermal radiation, as they offer similar insulation as air in that regard, which is to say very little. In addition, while this is pure speculation, aerogel has been suggested as the "expensive, special" material used in the interstage section of thermonuclear weapons, although no government has offered information on this front.
Aerogels are also notable for their very low density; most aerogels have a density of between 0.0011g/cm^3 and 0.5g/cm^3, with the lowest being slightly less dense than air (at room temperature and atmospheric pressure, air has a density of 0.0013g/cm^3). While at the surface a striking property, this actually is not needed in most of the uses for aerogels, although it does give it an advantage over heavier materials.
Another use for aerogels is in the filtration from water of heavy metals such as mercury and cadmium. The aerogels used in this case must belong to a specific subclass called chalcogels, which are aerogels that contain chalcogens (elements from group 16 of the periodic table) such as oxygen and sulfur. The aerogel must also contain platinum, and the resultant aerogel is a very effective filter for heavy metals. In one study, a sulfur-germanium chalcogel was able to reduce the concentration of mercury in a water sample from 645ppm to 0.04ppm(though the EPA states that water must have a mercury concentration of below 2ppb, so it is still not advisable to drink this water). Aerogels are very effective filters as the cross-linked structure of the aerogel has a very large surface area, and therefore a much larger area for reactions to happen. The chalcogen contained in the aerogel (sulfur in this case) bonds with the heavy metal, removing it from the water.
Aerogels can also be used to clean up chemicals by adsorption, where the chemicals adhere to the material. Again, due to the very high surface area of the aerogel, it performs exceptionally when compared to other absorbers, with one cellulose-based aerogel capable of adsorbing roughly 100 times its weight, although due to the low density of aerogels, this is less impressive than it sounds. Aerogels are being investigated as a potential method of cleaning up chemical spills as the Wisconsin Institute for Discovery, based in Madison. The cellulose-based aerogel is water-repellant and very oil-adsorbing, meaning that potentially sheets could be cast into the ocean, and adsorb a large quantity of oil (and no water) before being reeled back in. The oil can then be removed, and while the aerogel is not as effective as before, it can still be used a few more times before a new sheet is used. The aerogel also has excellent flexibility, meaning that large sheets will not fracture on the open ocean, due to the waves.
One of the most pervasive engineering problems associated with aerogels is their friability, or tendency to shatter when a force is applied. While you may see sensationalist headlines which state that aerogels can hold objects 2000 times their weight, this is only possible under special circumstances, where the force is uniformly applied. One solution to this is the creation of x-aerogels, where plastic is added to the aerogel in a process known as doping. In the process, a gel is mixed with a cross-linking agent such as a diisocyanate, and then heated to fuse the agent to the gel structure. The liquid is then removed from the gel with the usual process of supercritical drying, creating a much more flexible aerogel than normal, which can bend, unlike normal aerogels, which fracture and break.
Another potential use of aerogels is in creating windows that are far more thermally insulative than regular glass windows. The company Origin has succeeded in creating windows that look like regular glass, without the typical "smokey" look of most aerogels. These windows are not true aerogel windows, as most of the window is composed of glass, with the inside cavities filled with aerogel. However, the cost of these windows currently prohibits them from being a mainstream option, and while it is difficult to find the cost of Origin windows (and installation fees vary), it is not uncommon to end up paying £500 for fitting a single small Origin window, significantly more costly than regular windows.
Overall, aerogels have many exciting new applications, and with further research could expand into a number of new engineering niches. However, the single biggest problem with aerogels is currently the extremely high cost of manufacture, and without significant cost reduction in this area, aerogels cannot compete with traditional materials.
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