If a wet gel is allowed to simply dry, it would shrink and crack.  Kistler correctly deduced 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 of the gel.  He then discovered the crucial 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 Next Step

           Following his discovery, Samuel Kistler continued his aerogel research at the University of Illinois (1931-1935).  Many

important early publications on the properties of aerogels came out of his work there, including studies on the thermal conductivity

of silica aerogel, and the catalytic properties of various oxide aerogels.

           In the early 1940s, Samuel Kistler completed a license agreement with the Monsanto Corporation for the production of

 silica aerogel.  Monsanto began production in a plant in Everett, Massachusetts and sold products for many years under the trade

names "Santocel", "Santocel-C", "Santocel-54", and "Santocel-Z".  Monsanto's aerogels were used as an additive or a thixotropic

agent in cosmetics and toothpastes and also as an insulator for freezers and oxygen-producing plants.  The aerogels were mainly

based on SiO₂ for production purposes but Kistler also did research by making aerogels using many other materials such as

alumina, tungsten oxide, ferric oxide, tin oxide, nickel tartarate, cellulose, cellulose nitrate, gelatin, agar, egg albumen, and rubber.

However, Kistler soon turned his attention to research in other areas and little new work was done on aerogels. Production of

aerogels ceased when cheaper alternatives made them unnecessary. Aerogels widely disappeared until their possibilities were

looked into again.

Diagram of Monsanto plant        

                                                            Monsanto Aerogel  Factory            Removing Aerogel from autoclave

The Return of the Aerogels

           In the late 1960s/early 1970s, scientists were looking for a medium in which to store oxygen and liquid rocket fuel.  The

French government approached Stanislaus Teichner of Universite Claud Bernard in Lyon, France. They wanted him to test the

possibilities of aerogels for this use.  Teichner delegated the task of creating of the aerogels for the test to one of his graduate

students.  Using Kistler's method, the first batch of aerogels took weeks to prepare.  Teichner then informed his hapless graduate

student that a vast number of aerogel samples would be needed for him to complete his dissertation.  Realizing this project could

take years, his student went home very distraught.  He returned after a rest determined to find a more efficient way to produce

aerogels.  A major advance in aerogel science followed shortly thereafter.  He applied sol-gel chemistry to the preparation of silica

aerogel and found a much quicker way of producing high quality aerogels.  His process involved replacing the sodium silicate used

by Kistler with an alkoxysilane, (tetramethyorthosilicate, TMOS).  An "alcogel" was produced in one step by hydrolyzing TMOS

in a solution of methanol.  This one-step process was much quicker and successfully eliminated two major downsides of Kistler's

method of production, the water-to-alcohol transfer and salts that were left in the gel.  The “alcogels” were then dried under

supercritical alcohol conditions, which produced high-quality silica aerogels.  In the years that followed, Teichner's group, and

others, extended this approach and prepared an array of different metal oxide aerogels.

The Aerogel Renaissance

            Following Teichner’s (Or more correctly, his student’s) discovery, interest in aerogels was heightened. Research was

begun and soon new discoveries were being made.  Safer and more efficient synthesis processes were created and new uses were

found for aerogels. Their potential also began to become evident and possibilities were delved into. For example, in the early 1980s

particle physicists realized that silica aerogels would serve as ideal mediums for the production and detection of Cherenkov

radiation. Two large detectors were created and put into use.  

           The first plant designed to implement the TMOS method of aerogel production was created in Sjobo, Sweden.  The plant

implemented a 3000-liter autoclave designed to endure the strain of containing the high temperatures and pressures needed for

supercritical methanol conditions (240 degrees C and 1600 p.s.i.). However, during a production run in 1984 the autoclave

developed a leak. The chamber housing the autoclave quickly filled with methanol vapors and consequently exploded. There were

no deaths in this accident, but the entire facility was completely destroyed by the massive explosion. The plant was later rebuilt and

actually it continues to produce silica aerogels to this very day using the same method.

             A revolutionary process modification was discovered in 1983 by Arlon Hunt and the Microstructured Materials Group at

Berkeley Lab. They found that the very toxic and potentially dangerous compound TMOS could be replaced with

tetraethylorthosilicate (TEOS). TEOS is non-toxic and much safer than TMOS. The Microstructured Materials Group was also

able to achieve this without degrading the quality of the aerogels produced. The Microstructured Materials Group also found that

they could replace the alcohol within a gel with liquid carbon dioxide before supercritical drying portion of the process without

harming the aerogel’s structure. This discovery made it so another portion of the process was safer. Since the critical point of CO₂

(31 degrees C and 1050 p.s.i.) is much lower than that of methanol (240 degrees C and 1600 p.s.i.), the conditions needed were

much safer. Carbon dioxide also doesn’t pose the explosion hazard that alcohol does. The German chemical giant BASF also

developed similar CO₂ substitution methods for the preparation of silica aerogel beads from sodium silicate around the same time.

Other notable achievements and uses that followed:

         Late 1980s - Researchers at Lawrence Livermore National Laboratory (LLNL) prepared the world's lowest  density silica aerogel (actually the lowest density of any solid material). The aerogel had a density of 0.003 g/cm³, which is only three times the density of air.

         LLNL also modified the techniques used to prepare inorganic aerogels for use in the preparation of aerogels of organic polymers. These polymers included resorcinol-formaldehyde and melamine-formaldehyde aerogels. Resorcinol-formaldehyde aerogels could also be pyrolyzed to give aerogels of pure carbon. This was an especially important discovery that opened a whole new vista of aerogel research.

         Very low-density silica aerogel, prepared at the Jet Propulsion Laboratory (JPL), was used to collect and return samples of high-velocity cosmic dust on several Space Shuttle missions.

         Researchers at the University of New Mexico managed to eliminate the supercritical drying step used in aerogel production by chemically modifying the surface of the gel prior to drying.

Discovery and History  //  Synthesizing Aerogels   //  Uses: Current and Future   //   Research and What Lies Ahead