The 1970’s: The Aerogel Renaissance

In the decade following their discovery in 1929, Samuel Stephens Kistler produced many different types of aerogels, including transparent silica aerogels with densities as low as 0.030 g cm-3, alumina aerogels, tungsten oxide aerogels, iron oxide aerogels, and tin oxide aerogels. And as you’ll read in the article The Early Days of Aerogels, aerogels were even commercialized as early as the 1940’s by Monsanto Corporation, who produced a powdered silica aerogel product called “Santocel” for use as a thickening agent for paints, makeup, and napalm; as cigarette filters; and as insulation for a national line of freezers. Monsanto produced Santocel until the 1970’s, when it was abandoned most likely due to manufacturing costs and competition from other types of silica and thermal insulation.

But not long after Monsanto abandoned its commercial production of Santocel, interest in aerogel was rekindled thanks to the discovery of safer and more efficient processing techniques. Simultaneously, the birth of Materials Science and Engineering as a discipline in the late 1970’s provided a unified framework for scientists to understand and characterize aerogels as nanostructured architectures and, subsequently, how to engineer them at the nanoscale.

Faster, Clearer, Better: Aerogels in The 1970’s

In article published in the New Scientist on January 30, 1993, Jochen Fricke, professor of experimental physics at the University of Wurzburg in Germany, describes the rebirth of aerogels in the late 1970’s (reproduced with permission from New Scientist):

Twenty years later Stanislas Teichner and one of his students at the Claude Bernard University in Lyons were asked by the French government to develop porous storage materials for liquid fuels and tried to repeat Kistler’s pioneering work. But it took weeks to prepare just two aerogel samples; at this rate, the student would never finish his PhD thesis. So the French group decided to look for a faster way to make aerogels. Kistler had mixed water glass (sodium silicate) with hydrochloric acid to produce a gel which he then saturated with methanol; this was the most time-consuming part of the process. Teichner’s group mixed tetramethoxysilane Si(OCH3)4 with alcohol, water and a catalyst. In this mixture the -OH groups from the water replace -OCH3, and methanol is released in the process. The resulting Si-OH groups then pair up, giving Si-O-Si bonds and water. Finally, clusters of Si-O-Si aggregate to give the characteristic silica skeleton, immersed in a mixture of water and methanol.

With this new technique, alcogels could be made within a few hours, but aerogel production remained difficult because of the delicate drying procedure. Not until the early 1980s did aerogels find their first useful application, in the field of high-energy physics to detect fast subatomic particles. Gunter Poelz at DESY, Germany’s national accelerator laboratory in Hamburg and Sten Henning at a Swedish company, Airglass, used the French method to produce hundreds of highly transparent flawless silica aerogel tiles, each 20 centimetres square by 2 centimetres thick.

At DESY and at CERN (the European Laboratory for Particle Physics) in Geneva the tiles were used by particle physicists as an alternative to compressed gases or low-density liquids in so-called Cherenkov detectors, to detect pions, muons and protons moving at close to the speed of light. Such particles give off light in a cone-shaped electromagnetic shock front which surrounds their path and resembles the acoustic shock wave emitted by a supersonic airplane. Just as a sound shock wave can be detected as it moves through air by the bang it makes, so a shock wave created by subatomic particles can be detected by the light it emits when it moves through a compressed gas, low-density liquid or aerogel. A particle’s velocity can then be deduced from the angle between the shock front and the particle’s flight path. Aerogels bridge the gap between compressed gases and low-density liquids in the spectrum of materials that can be used to detect fast-moving subatomic particles. With this complete spectrum, these experiments can now be performed with even greater accuracy.

Dr. Arlon Hunt and Dr. Mike Ayers from Lawrence Berkeley National Laboratory offer a slightly different persepctive:

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 a wide variety of metal oxide aerogels.

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.

In any case, aerogels had been reborn thanks to new, faster, better techniques, paving the way for a bright future of new possibilities.

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