Aerogel
This allows the liquid to be slowly dried off without causing the solid matrix in the gel to collapse from capillary action, as would happen with conventional evaporation.
[7] The first documented example of an aerogel was created by Samuel Stephens Kistler in 1931,[8] as a result of a bet[9] with Charles Learned over who could replace the liquid in "jellies" with gas without causing shrinkage.
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.
People handling aerogel for extended periods should wear gloves to prevent the appearance of dry brittle spots on their skin.
The slight colour it does have is due to Rayleigh scattering of the shorter wavelengths of visible light by the nano-sized dendritic structure.
Aerogels by themselves are hydrophilic, and if they absorb moisture they usually suffer a structural change, such as contraction, and deteriorate, but degradation can be prevented by making them hydrophobic, via a chemical treatment.
The result of the polymerization and critical heating is the creation of a material that has a porous strong structure classified as aerogel.
The volume of the gas adsorbed is measured by using the Brunauer, Emmit and Teller formula (BET), which gives the specific surface area of the sample.
[18][19] This is caused by the Knudsen effect, a reduction of thermal conductivity in gases when the size of the cavity encompassing the gas becomes comparable to the mean free path.
Effectively, the cavity restricts the movement of the gas particles, decreasing the thermal conductivity in addition to eliminating convection.
For example, thermal conductivity of air is about 25 mW·m−1·K−1 at standard temperature and pressure (STP) and in a large container, but decreases to about 5 mW·m−1·K−1 in a pore 30 nanometers in diameter.
The precursors are a liquid alcohol such as ethanol which is mixed with a silicon alkoxide, such as tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and polyethoxydisiloxane (PEDS) (earlier work used sodium silicates).
[24] The solution of silica is mixed with a catalyst and allowed to gel during a hydrolysis reaction which forms particles of silicon dioxide.
[27] For others, crosslinking materials are added to the dispersion to promote the strong interaction of the solid particles in order to form the gel.
[28][34][35][36][37][38] Once the gelation is completed, the liquid surrounding the silica network is carefully removed and replaced with air, while keeping the aerogel intact.
As the liquid evaporates in such manner, forces caused by surface tensions of the liquid-solid interfaces are enough to destroy the fragile gel network.
[39] To avoid the collapse of fibers during slow solvent evaporation and reduce surface tensions of the liquid-solid interfaces, aerogels can be formed by lyophilization (freeze-drying).
Depending on the concentration of the fibers and the temperature to freeze the material, the properties such as porosity of the final aerogel will be affected.
[42] A variant on this process involves the direct injection of supercritical carbon dioxide into the pressure vessel containing the aerogel.
This method is proven to be excellent at preserving the highly porous nature of the solid network without significant shrinkage or cracking.
[44] While being a highly effective method for producing aerogels, supercritical drying takes several days, requires specialized equipment, and presents significant safety hazards due to its high-pressure operation.
Freeze-drying, also known as freeze-casting or ice-templating, offers an alternative to the high temperature and high-pressure requirements of supercritical drying.
The high aspect ratio of fibers such as fiberglass have been used to reinforce aerogel composites with significantly improved mechanical properties.
[43] It is very robust with respect to high power input beam in continuous wave regime and does not show any boiling or melting phenomena.
It is also worth noting that even lower conductivities have been reported for experimentally produced monolithic samples in the literature, reaching 0.009 W·m−1·K−1 at 1atm.
Due to their extremely high surface area, carbon aerogels are used to create supercapacitors, with values ranging up to thousands of farads based on a capacitance density 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" to describe airy masses of carbon nanotubes produced through certain chemical vapor deposition techniques is incorrect.
Aerogels made of cadmium selenide quantum dots in a porous 3-D network have been developed for use in the semiconductor industry.
[73] Aerogel performance may be augmented for a specific application by the addition of dopants, reinforcing structures, and hybridizing compounds.
