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Details of Aerogels

 Aerogel is a lightweight, advanced material that consists of more than 96 percent air. The remaining four percent is a matrix of silica (silicon dioxide), a principal raw material for glass. This material is one of the lightest weight solids ever developed.

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

                                              A 2.5 kg brick is supported by a piece of aerogel weighing only 2.38 grams.

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.

Possible uses include:

                                                       The Stardust dust collector with aerogel blocks. (NASA)             

• Environmentally friendly, energy-efficient, recyclable alternatives for polyurethane foam in freezers, refrigerators, refrigerated vehicles and freezer display cases.
• Alternative insulators in appliances such as water heaters and ovens.
• Aircraft and aerospace industry applications.
• Luminescent composites with potential opto-electronic applications.
• Magnetic composites that may be useful for paramagnetic cooling at ambient temperatures.
• High surface area carbon monoliths for electrochemical applications
• 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.
• 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.

Properties
To the touch, aerogels feel like a light but rigid foam, something between a Styrofoam peanut and the green floral foam used for potting artificial flowers. Despite what their name may suggest, aerogels are dry materials and do not resemble a gel in their physical properties (the name comes from the fact that they are derived from gels). Pressing softly on an aerogel typically does not leave a mark; pressing more firmly will leave a permanent dimple. Pressing firmly enough will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass--a property known as friability. Despite the fact that it is prone to shattering, it is very strong structurally, able to hold over 2000 times its own weight. Its impressive load bearing abilities are due to the dendritic microstructure, in which spherical particles of average size 2-5 nm are fused together into clusters. These clusters form a three-dimensional highly porous structure of almost fractal chains, with pores smaller than 100 nm. The average size and density of the pores can be controlled during the manufacturing process.

Aerogels are remarkable thermal insulators because they almost nullify three methods of heat transfer (convection, conduction, and radiation). They are good convective inhibitors because air cannot circulate throughout the lattice. Silica aerogel is an especially good conductive insulator because silica is a poor conductor of heat (a metallic aerogel, on the other hand, would be a less effective insulator). Carbon aerogel is a good radiative insulator because carbon absorbs the infrared radiation that transfers heat. The most insulative aerogel is silica aerogel with carbon added to it.

Due to its hygroscopic nature, aerogel feels dry and acts as a strong desiccant. Persons handling aerogel for extended periods of time should wear gloves to prevent the appearance of dry brittle spots on the hands.

Since it is mostly air, it appears semi-transparent. The color it does have is due to Rayleigh scattering of the shorter wavelengths of visible light by the nanosized dendritic structure. This causes it to appear bluish against dark backgrounds and whitish against bright backgrounds.

Aerogels by themselves are hydrophilic, but chemical treatment can make them hydrophobic. If moisture is absorbed, they will usually cause a structural change of contraction etc. and deteriorate; however, degradation can be prevented by turning them hydrophobic. The aerogel which has hydrophobicity to the interior can prevent degradation, even if a crack reaches deeper than its surface, compared with an aerogel that is only hydrophobic on its surface. Hydrophobic treatment makes processing easy because it allows the use of a water jet cutter.

Types

Silica aerogels

                                                    A demonstration of aerogel's insulation properties.
Silica aerogel is the most common type of aerogel and the most extensively studied and used. It is a silica-based substance, derived from silica gel. The world's lowest-density solid is a silica aerogel (the latest and lightest versions of this substance have a density 1 mg/cm³, 1/1000 as dense as water), produced by the Lawrence Livermore National Laboratory.

Silica aerogel strongly absorbs infrared radiation. It allows the construction of materials that let light into buildings but trap heat for solar heating.

It has extremely low thermal conductivity (0.003 W/(m·K)),[4] which gives it remarkable insulative properties. Its melting point is 1,473 K (1,200 °C or 2,192 °F).

Silica aerogel holds 15 entries in the Guinness Book of Records for material properties, including best insulator and lowest-density solid.

Silica aerogel can protect the human hand from the heat of a blowtorch at point blank range.

Carbon aerogels

                                                                          Aerogel and Peter Tsou

Carbon aerogels can be electrically conductive, depending heavily on the density of the aerogel. They are composed of particles with sizes in the nanometer range, covalently bonded together. They have very high porosity (over 50%, with pore diameter under 100 nm) and surface areas ranging between 400-1000 m²/g. They are often manufactured as composite paper - non-woven paper made of carbon fibers, impregnated with resorcinol-formaldehyde aerogel, and pyrolyzed. The composite aerogel paper is frequently used for electrodes in capacitors, or deionization electrodes. Due to their extremely high surface area (about 800 m²/g), carbon aerogels are used to create supercapacitors, with values ranging up to thousands of farads based on a capacitance of 104 F/g and 77 F/cm³. Carbon aerogels are also extremely "black" in the infrared spectrum, reflecting only 0.3% of radiation between 250 nm and 14.3 µm, making them efficient for solar energy collectors. The term "aerogel" has been incorrectly used to describe airy masses of carbon nanotubes produced through certain chemical vapor deposition techniques--such materials can be spun into fibers with strength greater than aramid (twaron / kevlar) and unique electrical properties. These materials are not aerogels, however, since they have no internal structure giving them monolithicity (structural continuity) and do not have the regular pore structure characteristic of aerogels.

Alumina aerogels
Aerogels made with aluminum oxide are known as alumina aerogels. These aerogels are used as catalysts, especially when "metal-doped" with another metal. Nickel-alumina aerogel is the most common combination. Alumina aerogels are also examined by NASA for capturing of hypervelocity particles; a formulation doped with gadolinium and terbium could fluoresce at the particle impact site, with amount of fluorescence dependent on impact velocity.

Other aerogels
SEAgel is a material similar to organic aerogel, made of agar
 

Production
Silica aerogel is made by drying a hydrogel composed of colloidal silica in an extreme environment. Specifically, the process starts with a liquid alcohol like ethanol which is mixed with a silicon alkoxide precursor to form a silicon dioxide sol gel (silica gel). Then, through a process called supercritical drying, the alcohol is removed from the gel. This is typically done by exchanging the ethanol for liquid acetone, allowing a better miscibility gradient, and then onto liquid carbon dioxide and then bringing the carbon dioxide above its critical point. A variant on this process involves the direct injection of supercritical carbon dioxide into the pressure vessel containing the aerogel. The end result removes all liquid from the gel and replaces it with gas, without allowing the gel structure to collapse or lose volume.

Aerogel composites have been made using a variety of continuous and discontinuous reinforcements. The high aspect ratio of fibers such as fiberglass have been used to reinforce aerogel composites with significantly improved mechanical properties.

Resorcinol-formaldehyde aerogel (RF aerogel) is made in a way similar to production of silica aerogel.

Carbon aerogel is made from a resorcinol-formaldehyde aerogel by its pyrolysis in inert gas atmosphere, leaving a matrix of carbon. It is commercially available as solid shapes, powders, or composite paper.

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

Aerogel Links on the Internet :

The following web pages give a brief history of aerogels and describe the different ways that aerogels are produced. Each page also gives an overview of the many different applications of aerogels -

The sites below hold more information about silica aerogel, its properties, and its applications.

Other sites that are strictly related to the chemistry and the production of aerogels can be found below.

This website provides photographs of aerogels and equipment that is related to its production and use.

http://eande.lbl.gov/ECS/Aerogels/saphoto.htm

For information on purchasing aerogels and on the different manufacturers around the world, check the addresses below. Some of these sites contain prices, types of aerogels, and ordering information.