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An aerogel is a gel in which the liquid has been replaced by air, hence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
Discovery and History
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 solgel 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. Resorcinolformaldehyde 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.
Synthesizing Aerogels
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 threedimensional 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.
safety glasses that are worn when working with lasers
Leventis, N.; Sotiriou-Leventis C.; Zhang G.; Rawashdeh A.M.M.
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 socalled 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.
Leventis, N.; Sotiriou-Leventis C.; Zhang G.; Rawashdeh A.M.M.
Nanoengineering Strong Silica Aerogels
Nano Letters