Aerogels are unique materials. Regarded as the most useful substance ever created, aerogels are the lightest solids produced. They consist of a fine network of bubbles, containing over 95% air, or whatever gas the designer wishes to include, with cell walls just a few atoms thick. Both their pores and their properties are smaller than the wavelength of light (less than 100 billionths of a metre). Aerogels’ capacity to insulate is unmatched. A six-foot block of aerogels would weigh less than a pound, yet can support 1,500 times its own weight – enough to support a compact car. Chemically, they are identical to silica glass (but are 1000 times lighter than glass), and can absorb a large amount of kinetic energy
Aerogels were first discovered in 1931 by two scientists, Dr Steven Kistler and Dr Charles Learned, at universities in California. They were competing to replace the liquid inside a jelly jar without causing any shrinkage. Dr. Kistler won the bet and published his findings in that year. When they were discovered, aerogels were brittle and difficult to use, but insulated better than any other known material. Their production involves two main steps: the preparation of a wet silica gel, and the removal of the wet matrix by supercritical fluid drying, a process requiring high temperatures and pressures. Aerogels were rather expensive because of the time and energy required to produce them. . Being a form of solid silica, aerogels are an exotic form of glass. Too much force and it shatters into many pieces.
Types of Aerogels
There are three different kinds of aerogels: silica aerogels, carbon aerogels and resorcinol-formaldehyde (RF) aerogels.
Silica aerogels feature extremely light weight , excellent thermal insulating properties, high temperature stability and extremely large surface areas. They are very useful in the construction industry. Electrically conductive carbon aerogels are the newest form of aerogel materials. They are available in block, thin film and powder forms. Carbon aerogels are composed of covalently bonded, nanometer-sized particles that are arranged in a three-dimensional network. These materials have high porosity (in excess of 50 percent) and are available in solid shapes, powders and composite paper. RF Aerogels are primarily used in association with the carbon ones when an organic, electrically insulating material is desired. Beginning with this type of aerogel which has a higher density and larger surface area allows the researcher to control the temperature, atmosphere, time, etc. during the processing of a carbon aerogel.
The most studied property of aerogels is their thermal resistance. Aerogels can withstand temperatures up to 500ºC, above which they begin to shrink. Their melting point is around 1200ºC. Another property that is fairly obvious is transparency. Aerogels have a very low index of refraction. While some have a milky appearance, others are as transparent as glass.
The first commercial aerogels were produced in 1942 by the Mosanto corporation and marketed as a flatting agent for paints and varnishes as well as an additive in cosmetics and toothpastes. Today, aerogels are sold as thermal insulation for specialised applications and have also found use as temperature-resistant windows. Their widespread application, however, is still dependent on a reduction in their production cost which is not due to their actual composition because silica is one of the most common minerals on earth. The cost is due to the time and energy it takes to produce them. However, advances have already been made in its production by using carbon dioxide instead of alcohol, for example, which has reduced the time and energy required for the drying process. Gradually other ways are being found to make aerogels production more efficient, like producing them in mass quantities. In fact, demand for aerogels has soared in recent years, primarily because of developments in process technology that increase output and lower cost. So far, the biggest end-use leader is thermal insulation, which accounted for 57.6% of market value in 2006, according to a study from BCC Research, Conn., USA.
Possible uses and applications of Aerogels
Many potential applications are already being developed in the consumer market. Benefits for aerogels can be seen as heat-insulating, sound-dampening, and vibration-dampening materials, though their high price compared with those of current technologies may be a problem. However, the current main usages are as follows:
a. They are excellent thermal insulators because they have a huge internal surface area. They are such good insulators that a rose was once placed on top of a relatively thin piece which was above a bunsen burner flame, yet the rose was not affected by the heat at all.
b. Due to their transparency, aerogels may be incorporated into window panes which could be used as inspection ports in ovens, kilns, or furnaces.
c. Their extremely low density, and the fact that they are not very transparent to frequencies above the visible spectrum, mean that aerogel windows have been used in spacecraft too. Recently, NASA’s Stardust mission used a block of aerogel to catch high-speed comet particles and specks of interstellar dust without damaging them. This amazing accomplishment, bringing space particles back to earth, was made possible by the equally amazing properties of aerogel.
d. One of the most promising applications for aerogels is as a shock-absorbing medium in safety equipment. Due to their brittleness aerogels exhibit almost no elastic rebound when they fail. They have a load distribution curve similar to expanded polystyrene which makes aerogels an attractive alternative for impact-absorbing applications such as bicycle and motorcycle helmets.
e. A unique benefit from aerogels is “daylighting” – translucent window and roof systems that let filtered light into a room with no increase in heating or cooling costs. The nanosize structure of the aerogel provides insulation while also letting light through the windows. Experts say this type of application, where aerogels not only enhance the performance of a product but also permit it to be redesigned for more efficient use, will drive growth and, possibly, offset the material’s relatively high price.
f. Researchers have already successfully created aerogel films – made mostly of air.
g. Aerogels are also finding application as filters for seawater desalination and subatomic particle detectors.
Other experts foresee aerogels having uses “as diverse as food packaging, synthetic textiles, including shoes and boots, acoustic panels, and as dispersants or opacifiers in polymers.” As a result, the global market for aerogels, which BCC analysts say was worth US$62 million in 2006, “is projected to be worth $951 million by 2011”.
Sources
http://findarticles.com/p/articles/mi_hb6619/is_3_64/ai_n29422256/pg_2/
http://www.connectexpress.com/~ips/aerogel/faq.html
http://www.sti.nasa.gov/tto/Spinoff2008/ch_9.html
