By Arlon Hunt and Michael Ayers
Many people assume that aerogels are recent products of modern technology. In reality, the first aerogels were prepared in 1931. At that time, Steven. S. Kistler of the College of the Pacific in Stockton, California set out to prove that a “gel” contained a continuous solid network of the same size and shape as the wet gel. The obvious way to prove this hypothesis was to remove the liquid from the wet gel without damaging the solid component. As is often the case, the obvious route included many obstacles. If a wet gel were simply allowed to dry on it own, the gel would shrink, often to a fraction of its original size. This shrinkage was often accompanied by severe cracking of the gel. Kistler surmised, correctly, that the solid component of the gel was microporous, and that the liquid-vapor interface of the evaporating liquid exerted strong surface tension forces that collapsed the pore structure. Kistler then discovered the key aspect of aerogel production:
- “Obviously, if one wishes to produce an aerogel [Kistler is credited with coining the term “aerogel”], he must replace the liquid with air by some means in which the surface of the liquid is never permitted to recede within the gel. 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.” (S. S. Kistler, J. Phys. Chem. 34, 52, 1932).
- The first gels studied by Kistler were silica gels prepared by the acidic condensation of aqueous sodium silicate. However, attempts to prepare aerogels by converting the water in these gels to a supercritical fluid failed. Instead of leaving a silica aerogel behind, the supercritical water redissolved the silica, which then precipitated as the water was vented. It was known at the time that water in aqueous gels could be exchanged with miscible 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, the first true aerogels were formed. Kistler’s aerogels were very similar to silica aerogels prepared today. They were transparent, low density, and highly porous materials that stimulated considerable academic interest. Over the next several years, Kistler thoroughly characterized his silica aerogels, and prepared aerogels from many other materials, including alumina, tungsten oxide, ferric oxide, tin oxide, nickel tartarate, cellulose, cellulose nitrate, gelatin, agar, egg albumen, and rubber.
- A few years later, Kistler left the College of the Pacific and took a position with Monsanto Corp. Shortly thereafter, Monsanto began marketing a product known simply as “aerogel”. Monsanto’s Aerogel was a granular silica material. Little is known about the processing conditions used to make this material, but it is assumed that its production followed Kistler’s procedures. Monsanto’s Aerogel was used as an additive or a thixotropic agent in cosmetics and toothpastes. Very little new work on aerogels occurred throughout the next three decades. Eventually, in the 1960s, the development of inexpensive “fumed” silica undercut the market for aerogel, and Monsanto ceased production.
- Aerogels had been largely forgotten when, in the late 1970s, the French government approached Stanislaus Teichner at Universite Claud Bernard, Lyon seeking a method for storing oxygen and rocket fuels in porous materials. There is a legend passed on between researchers in the aerogel community concerning what happened next. Teichner assigned one of his graduate students the task of preparing and studying aerogels for this application. 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, many years to accomplish, the student left Teichner’s lab with a nervous breakdown. Upon returning after a brief rest, he was strongly motivated to find a better synthetic process. This directly lead to one of the major advances in aerogel science, namely 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 aerogels. In subsequent years, Teichner’s group, and others extended this approach to prepare aerogels of a wide variety of metal oxide aerogels.
- After this discovery, new developments in aerogels science and technology occurred rapidly as an increasing number of researchers joined the field. Some of the more notable achievements are:
- In the early 1980s particle physics researchers realized that silica aerogels would be an ideal medium for the production and detection of Cherenkov radiation. These experiments required large transparent tiles of silica aerogel. Using the TMOS method, two large detectors were fabricated. One using 1700 liters of silica aerogel in the TASSO detector at the Deutsches Elektronen Synchrotron (DESY) in Hamburg, Germany, and another at CERN using 1000 liters of silica aerogel prepared at the University of Lund in Sweden.
- The first pilot plant for the production of silica aerogel monoliths using the TMOS method was established by members of the Lund group in Sjobo, Sweden. The plant included a 3000 liter autoclave designed to handle the high temperatures and pressures encountered for supercritical methanol (240 degrees C and 80 atmospheres). However, in 1984 the autoclave developed a leak during a production run. The room containing the vessel quickly filled with methanol vapors and subsequently exploded. Fortunately, there were no fatalities in this incident, but the facility was completely destroyed. The plant was later rebuilt and continues to produce silica aerogels using the TMOS process. The plant is currently operated by the Airglass Corp.
- In 1983 the Arlon Hunt and the Microstructured Materials Group at Berkeley Lab found that the very toxic compound TMOS could be replaced with tetraethylorthosilicate (TEOS), a much safer reagent. This did not lower the quality of the aerogels 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). Additionally, carbon dioxide does not pose an explosion hazard as does alcohol. This process was put to use in making transparent silica aerogel tiles from TEOS.
- BASF in Germany simultaneously developed CO2 substitution methods for the preparation of silica aerogel beads from sodium silicate. This material was in production until l996 and was marketed as “BASOGEL”.
- In 1985 Professor Jochen Fricke organized the first International Symposium on Aerogels in Wurzburg, Germany. Twenty-five papers were presented at this conference by researchers from around the world. Subsequent ISAs were held in 1988 (Montpellier, France), 1991 (Wurzburg), and 1994 (Berkeley, California, USA). The Fourth ISA set an attendance record with 151 participants, 10 invited papers, 51 contributed papers, and 35 poster presentations. The fifth ISA was recently held in Montpellier with almost 200 attendees.
- In the late 1980s, researchers at Lawrence Livermore National Laboratory (LLNL) lead by Larry Hrubesh prepared the worlds lowest density silica aerogel (and the lowest density solid material). This aerogel had a density of 0.003 g/cm3, only three times that of air.
- Shortly thereafter, Rick Pekala, also of LLNL, extended the techniques used to prepare inorganic aerogels to the preparation of aerogels of organic polymers. These included resorcinol-formaldehyde, melamine-formaldehyde aerogels. Resorcinol-formaldehyde aerogels could be pyrolyzed to give aerogels of pure carbon. This opened an completely new area in aerogel research.
- Thermalux, L.P. was founded in 1989 by Arlon Hunt, and others, in Richmond California. Thermalux operated a 300 liter autoclave for the production of silica aerogel monoliths from TEOS using the carbon dioxide substitution process. Thermalux prepared a large quantity of aerogels, but, unfortunately, ceased operations in 1992.
- Silica aerogel, prepared at the Jet Propulsion Laboratory, has flown on several Space Shuttle missions. On these flights very low density aerogel was used to collect and return samples of high-velocity cosmic dust.
- Researchers at the University of New Mexico, lead by C. Jeff Brinker and Doug Smith, and at other institutions have become increasingly successful at eliminating the supercritical drying step used in aerogel production by chemically modifying the surface of the gel prior to drying. This work lead to the founding of Nanopore to commercialize lower-cost aerogels.
- In 1992, Hoechst Corp. in Frankfurt, Germany aslo began a program in low cost granular aerogels.
- The Aerojet Corp. in Sacramento, California began a cooperative project with Berkeley Lab, LLNL, and others to commercialize aerogels using the carbon dioxide substitution process in 1994. Aerojet obtained the 300 liter autoclave formerly operated by Thermalux and produced various forms of silica, resorcinol-formaldehyde, and carbon aerogels. However, this program was abandoned in 1996.
With research and development proceeding at an ever increasing rate, it is likely that many more advances in aerogel technology and applications are imminent.