Aerogel

Aerogels are a diverse //class// of porous, solid materials that exhibit an uncanny array of extreme materials properties. Most notably aerogels are known for their extreme low densities (which range from 0.0011 to ~0.5 g cm-3). In fact, the lowest density solid materials that have ever been produced are all aerogels, including a silica aerogel that as produced was only three times heavier than air, and could be made //lighter// than air by evacuating the air out of its pores. That said, aerogels usually have densities of 0.020 g cm-3 or higher (about 15 times heavier than air). But even at those densities, it would take 150 brick-sized pieces of aerogel to weigh as much as a single gallon of water! And if Michaelangelo’s //David// were made out of an aerogel with a density of 0.020 g cm-3, it would only weigh about 4 pounds (2 kg)! Typically aerogels are 95-99% air (or other gas) in volume, with the lowest-density aerogel ever produced being 99.98% air in volume. Essentially an aerogel is the dry, low-density, porous, solid framework of a gel (the part of a gel that gives the gel its solid-like cohesiveness) isolated in-tact from the gel’s liquid component (the part that makes up most of the volume of the gel). Aerogels are open-porous (that is, the gas in the aerogel is not trapped inside solid pockets) and have pores in the range of <1 to 100 nanometers (billionths of a meter) in diameter and usually <20 nm. Aerogels are //dry// materials (unlike “regular” gels you might think of, which are usually wet like gelatin dessert). The word aerogel refers to the fact that aerogels are derived from gels–effectively the solid structure of a wet gel, only with a gas or vacuum in its pores instead of liquid. Learn more about gels, aerogels , and how aerogels are made.

Technical Definition
By definition, > An aerogel is an open-celled, mesoporous, solid foam that is composed of a network of interconnected nanostructures and that exhibits a porosity (non-solid volume) of no less than 50%. The term “mesoporous” refers to a material that contains pores ranging from 2 to 50 nm in diameter. Generally speaking, most of the pores in an aerogel fall within this size range. In practice, most aerogels exhibit somewhere between 90 to 99.8+% porosity and also contain a significant amount of microporosity (pores less than 2 nm in diameter).

What Are Aerogels Made Of?
The term aerogel does not refer to a particular substance, but rather to a geometry which a substance can take on–the same way a sculpture can be made out of clay, plastic, papier-mâché, etc., aerogels can be made of a wide variety of substances, including: and Aerogel composites, for example aerogels reinforced with polymer coatings or aerogels embedded with magnetic nanoparticles, are also routinely prepared. Learn more about the different [|flavors of aerogels] that exist.
 * Silica
 * Most of the transition metal oxides (for example, iron oxide )
 * Most of the lanthanide and actinide metal oxides (for example, praseodymium oxide)
 * Several main group metal oxides (for example, tin oxide)
 * Organic polymers (such as resorcinol-formaldehyde, phenol-formaldehyde, polyacrylates, polystyrenes, polyurethanes, and epoxies)
 * Biological polymers (such as gelatin, pectin, and agar agar)
 * Semiconductor nanostructures (such as cadmium selenide quantum dots)
 * Carbon
 * Carbon nanotubes
 * Metals (such as copper and gold)

Special Properties of Aerogels
Many aerogels boast a combination of impressive materials properties that no other materials possess simultaneously. Specific formulations of aerogels hold records for the lowest bulk density of any known material (as low as 0.0011 g cm-3), the lowest mean free path of diffusion of any solid material, the highest specific surface area of any monolithic (non-powder) material (up to 3200 m2 g-1), the lowest dielectric constant of any solid material, and the slowest speed of sound through any solid material. It is important to note that not all aerogels have record properties (in fact most don’t, although they may have very good values for many properties)! By tailoring the production process, many of the properties of an aerogel can be adjusted. Bulk density is a good example of this, adjusted simply by making a more or less concentrated precursor gel. The thermal conductivity of an aerogel can be also be adjusted this way, since thermal conductivity is related to density. Typically, aerogels exhibit bulk densities ranging from 0.5 to 0.01 g cm-3 and surface areas ranging from 100 to 1000 m2 g-1, depending of course on the composition of the aerogel and the density of the precursor gel used to make the aerogel. Other properties such as transparency, color, mechanical strength, and susceptibility to water depend primarily on the composition of the aerogel. For example, silica aerogels, which are the most widely researched type of aerogel (and the type people typically see in photographs), are usually transparent with a characteristic blue cast due to Rayleigh scattering of the short wavelengths of light off of nanoparticles that make up the aerogel’s framework. Carbon aerogels, on the other hand, are totally opaque and black. Furthermore, iron oxide aerogels are just barely translucent and can be either rust-colored or yellow. As another example, low-density (<0.1 g cm-3) inorganic aerogels are both excellent thermal insulators and excellent dielectric materials (electrical insulators), whereas most carbon aerogels are both good thermal insulators and electrical conductors. Thus it can be seen that by adjusting processing parameters and exploring new compositions, we can make materials with a versatile range of properties and abilities. [|137921main_image_feature_449_ys_4.jpg]

"What Is Aerogel?" //Aerogel.org Â»//. Web. 08 May 2012. .