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Aerogel FAQs

What are aerogels?

Aerogels are the lightest weight of the known solid materials. They are extremely porous and have extraordinary thermal insulation abilities.

The lightest aerogels have a density only about three times that of air. A block the size of a person weighs less than one pound, and looks like it would blow away in a slight breeze. Yet it is able to support the weight of a subcompact car.

Aerogels are formed by dispersion of a gas in a solid or liquid. Aerogels can be thought of as the reverse of aerosols.

What are the properties of aerogels?

Aerogels feature outstanding thermal, electrical and acoustic insulation properties. Their high porosity gives them an extremely high surface area (including the surfaces of the pores). Although they have very a high compression strength relative to mass, they do not have much value as a structural material.

What are their optical properties?

Silica aerogels are made from the same material as glass, and, like glass, they are transparent. However, currently available aerogels are slightly less transparent than window glass and have a slight bluish tinge.

Because the intrinsic light absorbency of silica is low in the visible region of the spectrum, transmittance in this region is primarily attenuated by scattering effects resulting from particles in the aerogel. Scattering increases as wavelengths become progressively shorter, and transmission is eventually cut off near 300nm. Absorbency also begins to weaken in the near-infrared region, and it is cut off for light around 2700nm to 3200nm.

Who invented aerogels?

Aerogels were discovered in 1931 by two scientists at universities in California, Dr. Steven Kistler and Dr. Charles Learned. They were competing to see if they could replace the liquid inside of a jelly jar without causing any shrinkage. Dr. Kistler won the bet and published his findings in a 1931 edition of Nature.

These first gels were silica gels. They were prepared by the acidic condensation of aqueous sodium silicate. Attempts to prepare aerogels by converting the water in these gels to a supercritical fluid failed, but the scientists succeeded by converting the alcohol to a supercritical fluid and allowing it to escape. These aerogels were very similar to the silica aerogels produced today.

They attracted considerable academic interest. Kistler continued to study silica aerogels. He also produced aerogels from numerous other materials, including alumina, tungsten oxide, cellulose, cellulose nitrate, tin oxide, gelatin, egg albumen, rubber and agar.

A few years later Kistler joined Monsanto Corporation, and that company began marketing a granular silica aerogel, which was used as an additive in cosmetics and toothpastes.

Has aerogel technology progressed since that time?

Not much happened again until the late 1970s. At that time the French government asked Stanislaus Teichner at Universite Claud Bernard in France to help develop a method for storing oxygen and rocket fuels in porous materials. Frustration with the time-consuming (several weeks) and laborious solvent exchange process invented by Kistler led to the idea of using a sol-gel process.

This process replaced the sodium silicate with an alkoxysilane. Hydrolyzing this in a solution of methanol produced a gel in a single step. This eliminated both the water-to-alcohol exchange step and the presence of inorganic salts in the gel. Drying these alcogels under supercritical alcohol conditions resulted in high-quality silica aerogels.

This approach was subsequently extended by Teichner's group and others to producing a wide variety of metal oxide aerogels. Thereafter, the number of researchers in the field increased and progress occurred at a much more rapid rate.

What is the physical structure of aerogels?

Aerogels feature an extremely high density of interconnected pores. These pores are extremely small with diameters of less than 100 nanometers, and there is considerable variation in pore size below this level.

This porous nature results in a very large total surface area (i.e., including the surfaces in each pore), and the interconnected structure of these open pores provides access to this entire surface area for gasses and liquids for use in chemical reactions and for making composites.

There is an inverse relationship between the surface area and density. These and other properties can be controlled during production according to the intended application.

Silica aerogels contain primary particles of 2nm to 5nm in diameter. This very small size provides an extremely great surface-to-volume ratio and a corresponding high specific surface area.

Consequently, the chemistry of the interior surface of aerogels plays a dominant role in its chemical and physical behavior. It is this property that makes aerogels well suited for use as catalysts, catalyst substrates and adsorbents.

Are there different kinds of aerogels?

Yes. They include silica aerogels and carbon aerogels and resorcinol-formaldehyde (RF) aerogels.

Silica aerogels feature extremely light weight (down to under 0.005 grams per cubic centimeter), excellent thermal insulating properties, high temperature stability, very low dielectric constants and extremely large surface areas. The standard density is nominally 0.1 grams per cubic centimeter with a surface area of about 800 square meters per gram.

Electrically conductive carbon aerogels are the newest form of aerogel materials. They are available in block, thin film and powders forms.

Carbon aerogels are composed of covalently bonded, nanometer-sized particles that are arranged in a three-dimensional network. These materials have high porosity (in excess of 50 percent), pores that are less than 100nm in diameter and surface areas ranging from 400 to 1000 square meters per gram. They are available in solid shapes, powders and composite paper.

How are aerogels produced today?

They can be produced by the sol-gel polymerization of selected silica or resorcinol-formaldehyde (RF) monomers in solution. This solution is cast into the desired shape and forms into a highly crosslinked gel. The solvent is then removed from the pores of the gel.

The remaining rigid monolithic shape consists of covalently bonded, nanometer-sized particles that are arranged in a three-dimensional network. RF aerogels are carbonized to form pure carbon aerogels or can be applied to carbon paper to form sheets of RF aerogels that can be carbonized.

Carbon aerogels are produced by the sol-gel polymerization of selected organic monomers in solution. After the solvent is removed, the resultant organic aerogel is pyrolyzed in an inert atmosphere to form a carbon aerogel.

What are RF aerogels used for?

Monolithic RF Aerogels are primarily used as the precursor for carbon aerogels or when an organic, electrically insulating material is desired. The standard RF aerogel is offered as a high density material of 0.6 grams per cubic centimeter and a surface area of 600 square meters per gram. Beginning with this type of aerogel allows the researcher to control the pyrolysis conditions (temperature, atmosphere, time, etc.) during the processing of a carbon aerogel.

RF Aerogel Composite Sheets are formed by impregnating non-woven carbon paper with the RF gel.

What is aerogel composite paper?

It is a non-woven carbon paper than has been impregnated with a carbon aerogel. Applications include dielectric (insulation) materials for high performance capacitors and deionization electrodes.

How can porosity be determined?

The most commonly used method is gas/vapor adsorption. The amount of nitrogen or another gas is adsorbed at its boiling point on a solid sample is determined by both the size of the pores and the partial pressure of the gas relative to its saturation pressure.

Other methods include scattering (visible light, x-ray and neutron), nuclear magnetic resonancing, electron microscopy, atomic force microscopy and mercury porosimetry. It is very important when interpreting porosity data to indicate the method used because the results can differ according to the technique.

What are open pores?

They are pores which have holes in their walls that allow gasses or liquids to flow to and from other pores. This is similar to the structure of sponges. An example of closed pores is bubble wrap packaging: gasses or liquids can not enter or exit the pores (i.e., the bubbles) without breaking them.

How are aerogel composites produced?

There are three basic ways to produce aerogel nanocomposites. One is the addition of the second material during the sol-gel processing of the aerogel. A second is addition of the second material through the vapor phase (after supercritical drying). The third is chemical modification of the aerogel backbone through reactive gas treatment.

Have there been other applications in the space program?

It was used as insulation in the Sojourner Mars rover in 1997. The temperature at night dropped to minus 67 degrees(C), but it remained a comfortable 21 degrees inside of the vehicle, thus protecting sensitive electronic devices.

Could aerogel be used for consumer applications?

Yes. There are many possibilities. For example, a single window with a thickness of one inch would have the same insulation value as about 20 sheets of ordinary glass. This could result in a large reduction in heating bills and energy consumption because so much heat is lost through windows.

So why isn't aerogel already being used for windows?

One reason is the still high production cost. A second reason is that it is not fully transparent. It usually has a hazy appearance with a slight bluish tinge. Most consumers would not find this satisfactory.

Would it be possible to make aerogel fully transparent?

Work is being carried out towards this goal. The lack of complete transparency is a result of the variation in the sizes of the particles in the aerogel. The smaller ones are too small to affect the passage of light, but the larger ones cause it to scatter, thus reducing transparency. Therefore, the trick would be to develop a technique that results in pores which are of uniformly small size.

One idea which is being studied by NASA is to produce aerogels in space. Tests in low gravity environments have already shown that this results in increased uniformity of pores. However, it will be necessary to find lower cost processes.

What are the applications for carbon aerogels?

Carbon aerogels are the first electrically conductive aerogels. This, in combination with the extremely high surface area, controllable pore size and high purity makes them very useful for electrochemical applications.

One of the most important uses is electrodes in double layer capacitors. These "supercapacitors" can store electrical energy and discharge it faster than conventional batteries. Among their diverse applications are telecommunications systems and electric vehicles.

Can aerogels be used in computers?

Research conducted at Rensselaer Polytechnic Institute in New York state suggests that aerogel insulators could make it possible to double computer speeds.

Aerogels can have low dielectric constants, thus making them excellent electrical insulators. This makes it possible to put circuit lines closer together without slowing the electrical signals.

Chips incorporating aerogels can be made extremely thin and porous, and they consist of mostly air. Some of the test-produced chips consist of as much as 90 percent air. The advantages of using aerogels increases as circuit line widths decrease.

There are also other potential semiconductor applications for aerogels. For example, semiconductor materials could be deposited inside of them to make sensors and other devices.

What are some other applications for aerogels?

There are hundreds of potential uses. They include thermal insulation for a wide variety of products, as catalysts, catalyst, substrates and adsorbents. There are numerous possible uses for composites with other materials.

One example is capacitive deionization. The application of an electric field to electrodes formed from this material could be used for water purification.

Other examples of applications for aerogels (including composites made from them) include a medium for detecting radiation, cosmetics, toothpaste, composite structures, reinforcing agents for organic rubbers, pigments for ink jet printers, electro-chemical storage devices, media for gas separations or storage, carriers for controlled release agents, electrodes for fuel cells, noble metal catalyst supports, atomic particle detectors, sensors for oxygen and other gasses and even surfboards.

An aerosol is a suspension of liquid or solid particles in a gas. Examples include fog, smoke and fine spays (including hair spray and sprayed paint) from aerosol spray cans.

Pyrolysis is the transformation of one substance into one or more other substances using heat in the absence of oxygen.



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