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| Fig. 1: A flower on a piece of aerogel suspended over a bunsen burner. (Courtesy of NASA. Source: Wikimedia Commons) |
An aerogel is a gel in which the liquid part has been replaced by a gaseous maintaining the same solid structure. First aerogels were made from silica back in 1931 and had outstanding physical properties in terms of low density, and thermal and acoustical insulation. [1] Aerogels can be made of many different materials as its defined as a state of the material, not as a material itself. Some of the first made aerogels were made from sodium silicate (water glass, Na2SiO3). [1] The aerogel seen in Fig 1. is a silica-based aerogel with so low density that a flower is suspended on it over the flame of a Bunsen burner. It also has so extreme thermal insulation properties that the flower is unharmed by the flame.
Even though aerogels have been around for a long time and have promising applications its production process has not made viable its wide commercialization. Initially aerogels where produced through a sol-gel process in which the solution was slowly extracted by dilutions that enable the gel to slowly dry. This technique is very time and labor consuming. In the later days a technique in which the aerogel is dried by getting the solution in super-critical state and replacing it with a gas was developed. [1] In both production processes a great expense comes from the precursors utilized to form the solution.
Many different types of aerogels can be produced now a days and depending on the material in which it is based, its precursors and the production process followed aerogels will exhibit different properties. For example, silica-based aerogels may have hydrophobic properties, carbon-based and silica-based aerogels may have electric conductivity properties, some oxide-based aerogels can be used as catalysts of very high temperature processes, and polysaccharide-based aerogels as drug carriers. [1-3] This are only some type of aerogels and some of their potential applications. In general aerogels are most useful in applications that require insulation properties, and high contact area given their high porosity.
With a thermal conductivity down to 13 mW m-1 °K-1 for commercial products aerogels show remarkable characteristics compared to traditional thermal insulation materials. [4] Some of the largest energy savings that aerogels can bring are by insulation of buildings with panes and even windows through a highly-insulating translucent glazing. [5] Other current applications are in industrial settings as insulation for pipes, high temperature reactors, and cryogenic uses. In electric generation, aerogels can serve in solar trough power plants insulating the heated material to increase the energy conversion of the plant by reducing its losses to the ambient.
Even though initially most of the applications were for scientific purposes some are immigrating into commercial areas, and such will increase as its production costs diminish which may come from either techniques utilizing commercially available precursors or commercially viable processes. Given aerogels' thermal insulating properties the commercial availability of aerogels can have a great impact in many industries reducing energy use, and/or making solar energy available.
© Luis Lascurain. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] M. A. Aegerter, N. Leventis, and M. M. Koebel, eds., Aerogels Handbook (Springer, 2011).
[2] G. M. Pajonk, "Aerogel Catalysts," Appl. Catal. 72, 217 (1991).
[3] T. Mehling et al., "Polysaccharide-Based Aerogels as Drug Carriers," J. Non-Cryst. Solids 355, 2472 (2009).
[4] R. Baetens, B. P. Jellea, and A. Gustavsen, "Aerogel Insulation for Building Applications: A State-of-the-Art Review," Energy Buildings 43, 761 (2011).
[5] M. Reim et al., "Highly Insulating Aerogel Glazing for Solar Energy Usage," Sol. Energy 72, 21 (2002).
