Aerogel
Aerogels [ ae ro ː ˌ ɡe ː l] are highly porous solids, in which up to 99.98 % of the volume consists of pores. There are various types of aerogels with those being most widely silicate. Other materials, for example, plastic or carbon-based material, are placed in special cases. In principle all metal oxides, polymers and certain other materials can be used as a basis for the Aerogelsynthese by a sol -gel process.
- 3.1 storage medium
- 3.2 filtering
- 3.3 Thermal insulation
Properties and Structure
Aerogels have a highly dendritic structure, ie a branching of chains of particles with very many gaps in the form of open pores. These chains have contact points, so that ultimately leads to the image of a stable, sponge-like network. Meanwhile aggregates have a fractal dimension, ie they are self-similar to a certain extent.
The pore size is in the nanometer range and the inner surfaces are exceptionally large with up to 1000 m2 per gram. This aerogels can be used, inter alia, as an insulating or filter material. In addition, it is possible to store bio- active molecules, proteins, or even whole cells. Aerogels hold 14 entries in the Guinness Book of World Records for material properties, including the "best insulator " and " lightest solid ". The record holder in the category " solid with the lowest density " 2012 Aero graphite was developed with 99.99 % air and 0.01% graphitic carbon. It is pitch black, stable, electrically conductive, malleable and opaque.
Since especially silicate aerogels are comparatively well studied in all its diversity, can make quite specific for their spectrum. These properties enables us to match qualitatively as well as quantitatively for the most part quite well those of the other aerogels, but with some specific features. The exact material properties depend on the intended use and therefore can - depending on the starting material and manufacturing process - certainly vary greatly.
High optical transparency, together with a refractive index of about 1.007 to 1.24 and a typical value of 1.02, making aerogels interesting from an optical standpoint. A silica airgel appears milky - blue against a dark background because the silica the shorter wavelengths ( ie, the blue color from white light ) more scattered than the longer wavelength radiation. This effect can be observed in the form of Rayleigh scattering also at daylight in the Earth's atmosphere. Despite its appearance, the transparent airgel as hard plastic foam feels. Due to this property, they appear matte to transparent ( see figures at right) and therefore also carry the nickname " frozen smoke " or " blue smoke ". The name of silica airgel refers However, the structure and less on the chemical composition of the material. The latter corresponds approximately to SiO ( OH) y (OR) z, where y and z are as dependent on the manufacturing process parameters.
The individual particles of silica airgel are about one to ten nanometers in size and the distance between the chains is about 10 to 100 nm cylindrical mesopores are very easily accessible and have by definition a diameter of 2 nm to 50 nm wherein the porosity in is from 80 to 99.8%. The bulk density thus moves in the range from 0.003 to 0.5, with a typical value of 0.1, whereas the true density is 1.7 to 2.1. Accordingly silicate aerogels have a 100 to 1,600 and a typical value of 600 very high specific surface area.
The thermal conductivity in air at 300 Kelvin with 0.017 to 0.021 and a typical value of 0.02 is extremely low, giving the aerogels high temperature stability even under extreme conditions and it makes the best previous thermal insulators. Other sources give a range from 0.004 to 0.03.
Also, shows a very high density of states, which is associated with a greatly increased specific heat capacity at low temperatures.
Silica aerogels can not be wetted by liquid metals or chemically attacked, so they are against them chemically inert. Its melting point is about 1200 ° C. They are also non-flammable and non-toxic. However, they absorb humidity and prone to cracking during drying.
Another feature is the 20 to 800 and a typical value of 100 low sound speed, and thus also associated low acoustic impedance within field of aerogels.
The elastic modulus is moved in a range of 0.002 to 100 MPa, with a typical value of 1 MPa.
A phenomenon that could be observed in aerogels is that this can sound in the human audible range, ie represent resonance body. The frequency is dependent on the type of excitation. This effect is due to acoustic shear waves, which are excited when striking the gel.
Production
Aerogels are prepared by a gel is dried in a gelatinous material, usually silica, under extreme conditions. The first synthesis of silica aerogels succeeded Samuel Stephens Kistler in 1931 / 32nd He first developed a method for drying gels, without this thereby exhibited shrinkage.
Silica airgel by Kistler
Kistler used sodium silicate, which he mixed with water and so produce a solution (water glass). After the addition of acting as precipitation reagent hydrochloric acid coincide with those of silica from ( precipitation reaction ), which caused uncoordinated distributed by the Brownian motion in the solution and it crashed too.
Or:
The gradual liability these particles aggregated over time and within about one day resulted in a gel with a net-like structure. For this, the sodium chloride and the excess hydrochloric acid is flushed with water ( Aquagel ) and was followed by a displacement with alcohol ( alcogel ). This step is necessary because the water would otherwise destroy the gel structure further down the process again. The alcohol will evaporate slowly, formed from the basis of the surface forces acting menisci, which are in the gel " dig " and condition in this one vein-like structure. This would be associated shrinkage of the gel and as a result a porous structure with only about 50 % void fraction of what it should be avoided just was. Kistler therefore used for drying an autoclave, and increased the temperature and pressure above the critical point of alcohol, so that a supercritical fluid is formed. This procedure is referred to as supercritical drying. The phase boundary between gas and liquid was removed with it; Surface forces, which would have led in the other case to the formation of menisci, no longer existed. The supercritical fluid was then vented from the autoclave, making the product dried and was eventually become the airgel. In this case, the airgel had retained the size and shape of the original gel, manufactured by Kistler silicate aerogels had a density of about 30 to 300 kg/m3 and a porosity in the range 86-98 %. However, the production method according to Kistler had the disadvantage that it is long and expensive, which is particularly concerned the solvent exchange prior to the evaporation of the alcohol.
A method according to Teichner - the sol -gel process
Stanislas Teichner tried in the 1960s Kistler's method at the University of Lyon, reproduce, which, however, he took weeks to produce smaller airgel. As an alternative, he developed in 1968 as a standard procedure used today sol-gel process, which has also been further improved in 1986. Starting material here is the toxic tetramethylorthosilicate ( TMOS), is slowly hydrolyzed by the below equation using a defined amount of water after the addition of a catalyst to orthosilicic acid and methanol.
From the silicic acid water splits off as a result and there are SiO2 tetrahedra. This networking in the sequence to form a gel. The drying of the resulting alcogel is again equal to the method Kistler, the methanol critical values of 239.4 ° C and 80.9 bar has. The properties of the thus forming the airgel, particularly the structure and density can be controlled by the choice of the catalyst, the pH, or the amount ratio of the substances used, in particular methanol. Application, the procedures today at DESY and in Lund.
Other methods
In another method manufactures a research group Arlon Hunt at the University of California at Berkeley Aerogelstücke instead of the poisonous TMOS from tetraethylorthosilicate ( TEOS). In addition, replacing the ethanol by combustible carbon dioxide, but this is very time consuming. An advantage of the 31 ° C relatively low critical temperature of carbon dioxide, whereby the drying process is greatly facilitated.
Another method is by BASF in Ludwigshafen application where particular Aerogelkügelchen ( granules ) are produced with approximately one to six millimeters in diameter and a density of about 200 kg/m3. Bringing sulfuric acid and sodium silicate to react by being sprayed with a mixing nozzle in a flask. This leads to the formation of alkali salts which have to be washed out by a follow-up. The advantage of this process lies in the relatively lower cost of the disadvantage is the worse, and in particular the optical properties of the granules.
Carbon aerogels (CRF ) are generated mainly by pyrolysis of resorcinol -formaldehyde aerogels (RF). In the production of resorcinol -formaldehyde aerogels and the cheaper air drying may be used instead of the supercritical drying.
Applications
Since the refractive index of the aerogels is in a range that is accessible either by gases or liquids, or by conventional solid state, they play an important role as a so-called Cherenkov radiator material for detectors; Carbon aerogels also because of their high electrical conductivity and stability in materials research for electrode material in primary and fuel cells, automotive catalysts and supercapacitors.
Storage medium
Because of their high porosity is first developed aerogels with the intention to obtain storage for gases and solids. In the 1960s, aerogels were investigated for their suitability as storage media for liquid rocket fuel out.
Filtering
Due to their fine structure, aerogels can be used as a catch matrix for the smallest dust particles. Therefore, they were used aboard the " comet dust spacecraft " Stardust. The trapped dust particles and molecules are decelerated slowly in the airgel, so they are not thermally destroyed. Thus it succeeded, inter alia, also the first time, without prejudice material of a comet ( Wild 2 ) to bring back to Earth.
Thermal insulation
Especially silicate aerogels have a very low thermal conductivity and are therefore often used as an insulating material for special applications ( eg as transparent thermal insulation ); since the beginning of 2013 a corresponding special plaster with added airgel is distributed in Switzerland. He should come to Germany in 2014 to market.