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Aerogels: Their History, Structure, and Applications

Summary

Aerogels are the lightest of all solid substances. Chemically, they are identical to silica glass, but they have a very porous internal structure, which leads to a number of interesting properties. They are excellent thermal insulators; they have a huge internal surface area; they are transparent; they can absorb a large amount of kinetic energy.

Aerogels were first produced in the late 1920s. They were first marketed as a paint additive, then as thermal insulation. Their production today is hindered by their cost.

Aerogels are expensive because of the time and energy required to produce them. Aerogel production involves two main steps: the preparation of a wet silica gel, and the removal of the wet matrix by supercritical fluid drying, a process requiring high temperatures and pressures.

Today, aerogels are sold as thermal insulation for specialized applications. They may also find use as temperature-resistant windows, and in the space industry. One of the most promising applications for aerogels is as a shock-absorbing medium in safety equipment. Their widespread application, however, is still dependent on a reduction in their production cost.

Definition: What are aerogels?

Aerogels are the lightest of all solid substances. In fact, they stretch the definition of the word solid. Aerogels consist of a fine network of bubbles, with cell walls just a few atoms thick. Inside these cells is simply air, or whatever gas the designer wishes to include. Aerogels can actually be made lighter than air by forming them in an atmosphere of helium, resulting in the world's only lighter-than-air solids.

History: The creation and evolution of aerogels

The first aerogels were produced in the late 1920s by Samuel Kistler, an undergraduate instructor at the College of the Pacific in Stockton, California. There is conflicting information regarding the precise timing of, and his motivation for, producing them, but throughout the twenties Kistler had been working with supercritical fluids (high-pressure fluids on the point of boiling). Whether by design or by accident, Kistler found a way to remove the fluid from a wet silica gel, leaving behind its solid structure. In the early 1930s, Kistler continued his experiments with aerogels, studying some of their thermal and catalytic properties. (Ayers, 3)

The first commercial aerogels were produced in 1942 by the Mosanto corporation, under the trade name Santocel. The process involved soaking a sodium silicate solution in sulfuric acid, then repeatedly washing it in alcohol before drying it at high pressure. Mosanto described the product as "a light, friable, slightly opalescent solid containing as much as 95 percent air volume. It is a very effective heat insulating material." (Ayers, 3) Mosanto claimed to have produced aerogels with densities of 1.8 pounds per cubic foot (29 kg/m3), but their regular output was between three and five pounds per cubic foot (48 to 80 kg/m3).

Mosanto marketed Santocel mainly as a flatting agent for paints and varnishes. Its applications, though not numerous, were as varied as thermal insulation in household freezers and an ingredient in Napalm. Because of its high manufacturing cost, however, Mosanto discontinued aerogel production in 1970. (Ayers, 3)

Interest in aerogels, and their very low thermal conductivity, increased in the 1980s as energy conservation became increasingly important. However, high production costs still prevent their widespread use. (Thermal Properties)

Structure and Properties

How aerogels are produced

The following information, unless specified otherwise, comes from How Silica Aerogels Are Made, published by the Berkeley Lab.

The production of an aerogel involves two main steps: the preparation of a wet gel, and the drying of that gel. The second step is the most critical, as the fluid must be removed without destroying the solid framework of the gel. This is achieved by the process of supercritical fluid drying. When a fluid is raised to its supercritical point (a specific combination of high pressure and high temperature), it exhibits some properties of a liquid, and some of a gas. This allows the fluid to boil out of the gel gently, without tearing the cell walls.

The first aerogels, as mentioned above, were produced by the supercritical drying of alcohol. This process, however, was quite dangerous, as the process needed a temperature of 550F (288C) and a pressure of 1150psi (7900kPa). (Ayers, 3) Modern aerogels are produced by substituting CO2 for the alcohol, then performing supercritical fluid drying. The Berkeley Lab's description of the process is as follows:

The alcogels [wet gels] 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.

Before the gel can be dried, of course, it must be prepared. The first aerogels were made by condensing sodium silicate as an aqueous solution. Unfortunately, the process produced salts that needed a great deal of time and effort to wash out. Modern aerogels are made from silicon alkoxide precursors: compounds of silicon and simple organic mers. Although many can be used, the most common are tetramethyl orthosilicate and tetraethyl orthosilicate (TEOS and TMOS, formulas: Si(OCH3)4 and Si(OCH2CH3)4 respectively).

Immersing either of these compounds in water and ethanol results in a bubbly structure of solid silicon in a matrix of an organic compound (HOCH2CH3) which is removed in the subsequent step. Because this process occurs slowly at room temperature, catalysts are used. These may be either acidic or basic, depending on the desired properties of the final product. Even with the use of catalysts, the wet gel (alcogel) must be soaked for up to 48 hours.

At this point, supercritical fluid drying, as described above, is performed.

Two typical recipes for aerogels can be found at the reference for this section.

Physical and chemical properties

The most obvious property of aerogels is their extremely low density. Aerogels have been produced with densities of 0.003g/cm3. At this density, a computer monitor made of aerogel would weigh about the same as a graphing calculator. Densities of around 0.1g/cm3, however, are more common; this is about 10% of the density of water. (Physical Properties)

The most studied property of aerogels is their thermal resistance. Aerogels can withstand temperatures up to 500C, above which they begin to shrink. Their melting point is around 1200C. (Physical Properties)

Another striking property of aerogels is their internal surface area. This is very difficult to measure, but can be estimated by the rate of adsorption and desorption of nitrogen. The internal surface area of an aerogel can be as high as 1000m2/g. For a sample of typical density, a cube an inch square would have as much internal surface area as ten copies of the Greater Vancouver Yellow Pages. Interestingly, the use that follows from this property is the same as the test used to measure it: under certain circumstances, aerogels can be used to absorb some gases and fluids. (Physical Properties)

The chemical makeup of aerogels strongly affects their adsorbent properties. Aerogels can be made hydrophilic or hydrophobic, depending on which drying process is used. Alcohol drying (now uncommon because of the high temperature and pressure required) results in pore surfaces covered with alkoxy-groups (an oxygen atom attached to a non-polar organic mer). These hydrophobic aerogels , if sealed, are completely impervious to water, and will float indefinitely.

Carbon dioxide drying results in pore surfaces covered with hydroxyl (OH) groups. When one of these hydrophilic aerogels is placed in a humid environment, it adsorbs water into its pores, up to 20% of its mass. The water may be released simply by heating the sample. These aerogels, however, cannot be used to adsorb liquid water. The high surface tension of water, upon entering the tiny pores, tears them apart. The aerogel seems to disappear; it has become a fine powder, less than 5% of the volume of the gel.

Another property that is fairly obvious is transparency. Aerogels have a very low index of refraction. While some have a milky appearance, others are as transparent as glass.

One of the disadvantages of aerogels is their brittleness. Being a form of solid silica, aerogels are basically an exotic form of glass. One of the authors of the Berkeley Lab's website gives this account of a typical first encounter with aerogels:

the piece is held between two fingers and gently squeezed. The aerogel gives a little, and springs back. Then a little more force is applied and...pffttt, the piece shatters into a thousand pieces, most of which find a home deep in the carpeting, never to be seen again.

This property can actually be turned into an advantage, due to the mode of brittle failure. Most brittle substances, including regular glass, fracture almost instantly. Aerogels, due to their low density and high porosity, fail much more slowly. This property will be discussed in more detail in the Applications section. (Kinetic Energy)

Applications: How aerogels are used

Aerogels are most sought after for their thermal resistance. They are such good insulators that a rose placed atop a relatively thin piece can be supported entirely by a bunsen burner flame (see Figure 1). The Berkeley Lab gives the thermal conductivity of aerogels as being in the range of 0.008 to 0.017W/mK, which corresponds to R10 to R20 per inch. By comparison, residential fiberglass insulation has an R value of around R3.5/inch (Owens Corning), while residential Styrofoam insulation has a value of about R5/inch. (Dow Chemical)

Unfortunately, the high cost of aerogels prevents their widespread use, but they have found some specialized applications as thermal insulation. Nanopore Incorporated markets vacuum sealed aerogels with R values as high as R57/inch as insulation for cryogenic fluids, and for the exhaust systems of military assault vehicles. (Nanopore)

Because of their transparency, aerogels may be incorporated into window panes (see Figure 2). Considering their thermal insulative properties, these window panes could be used as inspection ports in ovens, kilns, or furnaces. Because of their extremely low density, and the fact that they are not very transparent to frequencies above the visible spectrum, aerogel windows may find use in spacecraft.

An under-studied application for aerogels is as a shock-absorbing material. At first glance, a brittle material that is basically 'very light glass' would seem the worst choice for such an application, but, as stated above, the speed of brittle failure of aerogels is quite different from that of most brittle materials.

No material can fail instantaneously. Even a pane of tempered glass takes some time to fracture across its width. When a substance fails because of an impact, intermolecular bonds are broken one by one. Because glass is quite dense, the failure of one bond will very quickly strain the bond next to it, and failure occurs rapidly. When a bond inside an aerogel fails, it take much longer for the resultant stress to be transferred to the next bond because it is much further away.

Another reason for the slow failure of aerogels is their porous nature. As the gel fails, air must escape from the pore network. The air is pressurized as it is forced out, absorbing a large amount of the energy of impact.

Materials such as polystyrene are commonly used as shock absorbers. Such materials, however, have a disadvantage: they are highly elastic. When a foam bicycle helmet, for example, fails in an accident, there is a significant amount of elastic rebound, which can result in the injury the helmet was meant to protect against (although to a lesser degree). This effect is more pronounced when the impact is smaller. If the impact is not great enough to cause the failure of the helmet, all of the energy of the impact is returned to the wearer by the elastic rebound. Injuries from small impacts could therefore be worse than they would be if wearing a weaker helmet.

Aerogels would solve this problem. As the graph of deflection vs. time (Figure 3) shows, aerogels exhibit almost no elastic rebound when they fail, due to their brittleness. With a load distribution curve similar to expanded polystyrene (see Figure 4), aerogels become an attractive alternative for impact-absorbing applications such as high-end bicycle and motorcycle helmets.

Conclusions: Aerogels in the future

As stated above, the greatest obstacle to the widespread use of aerogels is cost. The cost of aerogels is not inherent in their composition; silica is one of the most common minerals on Earth. The cost results from the time and energy it takes to produce them. Advances have already been made on both fronts: using alkoxide precursors has limited the amount of washing before supercritical fluid drying can be performed, and using carbon dioxide instead of alcohol has reduced the time and energy required for the drying process. In the future, other ways may be found to make aerogel production more efficient.

Another way to reduce the cost of aerogels is to produce them in mass quantities. Before they can be mass produced, of course, they must find widespread application. This is a fundamental irony faced in the introduction of any new product. For example, in the 1980s a research team discovered a way of making plastic foam from cornstarch. The foam behaved almost exactly like polystyrene foam, but was fully biodegradable. This happened at the same time that McDonald's restaurants came under public criticism for their non-biodegradable hamburger containers. Unfortunately, no one managed to connect these two ideas. If they had, the mass production of cornstarch foam would have made it inexpensive enough to replace polystyrene in all of its disposable applications.

Aerogels need to find an application that will allow production on a large scale to commence quickly. Safety helmets could be the ideal product. Helmets are already relatively expensive due to their strict design requirements; their cost is already in excess of that of their component materials. The introduction of aerogels as part of their structure should not increase their price beyond a marketable range. And safety helmets are produce in large enough quantities that aerogel production for them would surely benefit from some economy of scale.

Aerogel helmets would still be more expensive than average, so they would not be marketed at the low-end consumer. But, for example, a marketing partnership between Bell, which makes a range of bicycle helmets, and Specialized, which makes high-end bicycles, could result in the market penetration required to make aerogel helmets feasible.

Original Articles | Main Page

References

Ayers, Michael. At Elevated Pressures: The Life and Science of Samuel S. Kistler. Berkeley: Ernest Orlando Lawrence Berkeley Laboratory, 2000.

Ayers, Michael and Arlon Hunt. A Brief History of Silica Aerogels. Berkeley: Ernest Orlando Lawrence Berkeley Laboratory, (no date, but after 1996).

(no author). Thermal Properties of Silica Aerogels. Berkeley: Ernest Orlando Lawrence Berkeley Laboratory, (no date, but after 1996).

(no author). How Silica Aerogels Are Made. Ernest Orlando Lawrence Berkeley Laboratory, (no date, but after 1996).

(no author). Physical Properties of Silica Aerogels. Ernest Orlando Lawrence Berkeley Laboratory, (no date, but after 1996).

(no author). Silica Aerogels for Absorbing Kinetic Energy. Ernest Orlando Lawrence Berkeley Laboratory, (no date, but after 1996).

Dow Chemical. Styrofoam Brand Insulation: Styrofoam Residential Sheathing.

Owens Corning. Pink Fiberglas Insulation.

Nanopore Incorporated. Nanogel-Based Vacuum Insulation.

Aerogels: Their History, Structure, and Applications, copyright 2001 by George Beckingham