To truly understand what an aerogel is you must first make sure you know what a regular gel is. What to us is a regular gel is in fact called a wet gel. A wet gel is composed of a continuous solid network of the same size and shape of the wet gel with a liquid that fills the empty space within.
An aerogel is a gel in which the liquid has been replaced by air, hence the name “aerogel” due to the Greek for air being “aero.” The process of replacing the liquid with air is an intricate and delicate set of procedures. However, though the process may be difficult and somewhat costly, the results are nothing short of amazing. Aerogels possess the lowest density, highest thermal insulation, lowest refractive index, and highest surface area per unit volume of any solid. These are truly remarkable properties with possibilities just as astounding.
The pages below reveal how aerogels were initially created, how the process has been refined, their current uses, their great potential, and the future that lies before them.
Discovery and History // Synthesizing Aerogels // Uses: Current and Future // Research and What Lies Ahead
Due to the relative obscurity of aerogels until lately, it is generally assumed that they are a fairly recent discovery. However, the first aerogels were created in 1931 by Samuel S. Kistler at the College of the Pacific in Stockton, California. It was he that hypothesized that a “gel” is composed of a continuous solid network of the same size and shape of the wet gel and that it would be possible to remove the liquid without damaging the solid network.
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 first attempts at aerogels were by using silica gels created via acidic concentration of aqueous sodium silicate. Kistler converted the water in the silica gels into supercritical fluid in an attempt to remove the water. These endeavors failed though because the supercritical water dissolved the silica and therefore left no solid structure. Kistler then tried again by first thoroughly washing the silica gels with water (to remove any salts from the gel), and then exchanging the water for alcohol via an organic process. Converting the alcohol to a supercritical fluid and allowing it to escape allowed Kistler to form the first true aerogels.
In the early 1940s, Samuel Kistler completed a license agreement with the Monsanto Corp. 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".
SiO2 for production purposes 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.
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 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 new vistas 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.
A solution of various reactants that are undergoing hydrolysis and condensation reactions. The molecular weight of the oxide species produced continuously increases. As these species grow, they may begin to link together in a three-dimensional network.
The point in time at which the network of linked oxide particles spans the container holding the Sol. At the gel point the Sol becomes an Alcogel.
The kinetics of the above reaction are impracticably slow at room temperature, often requiring several days to reach completion. For this reason, acid or base catalysts are added to the formulation. The amount and type of catalyst used play key roles in the microstructural, physical and optical properties of the final aerogel product.
Acid catalysts can be any protic acid; such as HCl. Basic catalysis usually uses ammonia, or, more commonly, ammonia and ammonium fluoride. Aerogels prepared with acid catalysts often show more shrinkage during supercritical drying and may be less transparent than base catalyzed aerogels. The microstructural effects of various catalysts are harder to describe accurately, as the substructure of the primary particles of aerogels can be difficult to image with electron microscopy. All show small (2-5 nm diameter) particles that are generally spherical or egg-shaped. With acid catalysis, however, these particles may appear "less solid" (looking something like a ball of string) than those in base-catalyzed gels.
As condensation reactions progress the sol will set into a rigid gel. At this point, the gel is usually removed from its mold. However, the gel must be kept covered by alcohol to prevent evaporation of the liquid contained in the pores of the gel. Evaporation causes severe damage to the gel and will lead to poor quality aerogels
When a sol reaches the gel point, it is often assumed that the hydrolysis and condensation reactions of the silicon alkoxide reactant are complete. This is far from the case. The gel point simply represents the time when the polymerizing silica species span the container containing the sol. At this point the silica backbone of the gel contains a significant number of unreacted alkoxide groups. In fact, hydrolysis and condensation can continue for several times the time needed for gelation. Failure to realize, and to accommodate this fact is one of the most common mistakes made in preparing silica aerogels. The solution is simple--patience. Sufficient time must be given for the strengthening of the silica network. This can be enhanced by controlling the pH and water content of the covering solution. Common aging procedures for base catalyzed gels typically involve soaking the gel in an alcohol/water mixture of equal proportions to the original sol at a pH of 8-9 (ammonia). The gels are best left undisturbed in this solution for up to 48 hours.
This step, and all subsequent processing steps, are diffusion controlled. That is, transport of material into, and out of, the gel is unaffected by convection or mixing (due to the solid silica network). Diffusion, in turn, is affected by the thickness of the gel. In short, the time required for each processing step increases dramatically as the thickness of the gel increases. This limits the practical production of aerogels to 1-2 cm-thick pieces.
After aging the gel, all water still contained within its pores must be removed prior to drying. This is simply accomplished by soaking the gel in pure alcohol several times until all the water is removed. Again, the length of time required for this process is dependent on the thickness of the gel. Any water left in the gel will not be removed by supercritical drying, and will lead to an opaque, white, and very dense aerogel.
The final, and most important, process in making silica aerogels is supercritical drying. This is where the liquid within the gel is removed, leaving only the linked silica network. The process can be performed by venting the ethanol above its critical point (high temperature-very dangerous) or by prior solvent exchange with CO2 followed by supercritical venting (lower temperatures-less dangerous) It is imperative that this process only be performed in an autoclave specially designed for this purpose (small autoclaves used by electron microscopists to prepare biological samples are acceptable for CO2 drying). The process is as follows. The alcogels are placed in the autoclave (which has been filled with ethanol). The system is pressurized to at least 750-850 psi with CO2 and cooled to 5-10 degrees C. Liquid CO2 is then flushed through the vessel until all the ethanol has been removed from the vessel and from within the gels. When the gels are ethanol-free the vessel is heated to a temperature above the critical temperature of CO2 (31 degrees C). As the vessel is heated the pressure of the system rises. CO2 is carefully released to maintain a pressure slightly above the critical pressure of CO2 (1050 psi). The system is held at these conditions for a short time, followed by the slow, controlled release of CO2 to ambient pressure. As with previous steps, the length of time required for this process is dependent on the thickness of the gels. The process may last anywhere from 12 hours to 6 days.
0 = 0.935-0.993
Aerogels have also been used to conduct experiment with lasers to help explain the phenomena accompanying supernova explosions. In the experiments, the aerogel served as a skeleton for holding liquid or solid deuterium-tritium that was used as the target for the lasers. Aerogels were extremely useful due to the dimensions of the pores in the gels being so small. This made it so they could detect only short-range nuclear radiation. The aerogel also had the advantage of having a high radiation resistance and meeting the "wettability" (no I didn't make up the word) requirements of the liquid filler. In the laser experiments, the phenomena which scientists were wishing to detect were more pronounced when the difference between the density of the skeleton and that ofthe filler was greater. Since aerogels possess the lowest density of any solid, the benefit is obvious.
Carbon aerogels are obtained by pyrolyzing an organic compound in an inert medium. For example, ethanol can be pyrolyzed at 1,000–1,200 °C in an argon flow. This process removes everything except the carbon and leaves behind so-called solid smoke.
One simple property is extreme blackness, caused by internal scattering and absorption of light by the graphite molecules that make up the aerogel.
Capacitance increases as the distance between conductors decreases and the surface area of the conductors increases. Because carbon aerogels have huge surface areas per unit mass or volume and tiny pores, researchers have achieved capacitances as high as 104 F/g and 77 F/c m3.
Carbon aerogels show promise in many different areas and new applications continue to turn up for carbon aerogels. Physicists recently announced that they had produced a carbon aerogel made not of graphite particles but of nanotubes. The aerogel they produced supposedly has great promise. Supposedly, it is highly elastic rather than rigid, so it can be spun into pure nanotube fibers with unique electrical properties and with strength greater than that of Kevlar.
The deep blue aerogel contains nickel; the pale green, copper; the black, carbon and iron; the orange, iron oxide; and the remaining aerogels, organic compounds.
There is always a need for strong lightweight materials in our modern world. Silica aerogels would be extremely attractive if they were not fragile when made with very low density. However, recently, the strength of silica aerogels has been improved by a factor of over 100 through cross-linking the nanoparticle building blocks of preformed silica wet gels with polyhexamethylene diisocyanate. These composites do not absorb liquid as silica aerogels do and they do not collapse when in contact with liquids.