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The main aspect of this report is to present a detailed explanation of AEROGELS. How it's been invented, processed for production, their properties (both physical and chemical) and various applications at present and in future. As it was discussed about aerogels in this report because of its advantages over remaining materials and their benefits which are used in respective applications.
Table of Contents
Aerogels, these are the lightest of all solid substances that are available in the world. It's a manufactured material with lowest bulk density of any known porous solid. It's been derived from a gel in which the liquid component is replaced by gas and the result is an extremely low density solid. Because of this, it got a feature of notable effectiveness as a thermal insulator. They are transparent and it's nickname as solid smoke, frozen smoked, solid air or blue smoke because of its translucent nature, the way light scatters through the material. It feels like expanded polystyrene by the time you touch it. They are identical to silica glass by chemically, but they have porous internal structure. This leads to a number of interesting properties and are discussed in following report.
Aerogels were invented in early 1900's itself. It's been invented by Samuel Stephen Kestler in late 1920's. Who is an undergraduate instructor at the College of the Pacific in Stockton, California. At that time, there is been a conflicting information regarding the precise timing of, and his motivation for, producing them, but Kistler had been working with supercritical fluids throughout 20's on high-pressure fluids on the point of boiling. And Kistler found a way to remove the fluid components from a wet silica gel, leaving behind its solid structure as it is, He found this by design or by accident. It was first created it was a brittle substance with very good insulation properties and through the years has been altered by several companies such as BASF, Thermolux, Aerojet, Airglass and Aspen Aerogels. These companies are responsible for the AeroGel that we have today, which is an AeroGel that is nanoporous, light weight material that exhibits extraordinarily low thermal and acoustic conductivity. AeroGel is an insulation that out ways all other general insulations used today and therefore will be a material that in the future might just change the way we live our lives.
And coming to the first commercial aerogels were produced in 1942 by the Mosanto Corporation, under the trade named as Santocel. The process involved of two steps, one is soaking a sodium silicate solution in sulphuric acid, then its repeatedly washed in alcohol before drying it at high pressure. Mosanto described the product as a light weight, friable, slightly opalescent solid containing as much as 95% of air volume. It is a very effective heat insulating material. Mosanto marketed this material Santocel, mainly as a flatting agent for paints and varnishes and because of its high manufacturing cost, however, Mosanto discontinued regal production in 1970.
TYPES OF AEROGELS:
There are a number of types of aerogels. But we discuss only three most common types of aerogels. Those are silica, carbon and metal oxides. In which silica is the most often used in experimentally and in practical application. If people are talking about aerogels, most chances they are talking about the silica aerogels.
Silica isn't to be confused with silicon which is a semiconductor and is used in microchips. Whereas silica is a glassy material used often used for insulation. Silica aerogels is the most common and extensively studied and used in applications. It's a silica based substances which is derived from silica gel. It's one of the lowest density solid in the world. These aerogels absorb infrared radiation also helps for the construction of materials hat let light into buildings but trap heat for solar heating. It has very remarkable thermal insulative properties.
Silica aerogels hold a total of 15 Guinness world records for material properties including best insulator and lowest density solid.
These aerogels are composed of particles with sizes in the nanometre range and are covalently bonded. They have a very high surface areas ranging between 400 - 1000m2/g. They are manufactured as composite paper like non-woven paper made of a carbon fibre impregnated with resorcinol-formaldehyde aerogel and pyrolyzed. Depending up on their density, carbon aerogels may be electrically conductive or making composite aerogel paper useful for electrodes in capacitors or deionization electrodes. These aerogels are also extremely "black" in the infrared spectrum and by reflecting only 0.3% of radiation between 250Â nm and 14.3Â Âµm which makes them efficient for solar energy collectors.
Aerogels made with metal oxides are known as metal oxide aerogels. Metal oxide aerogels are used as catalysts for chemical transformations. In those alumina is one of the metal, aerogels made of aluminum are known as alumina aerogels. These are used as catalyst especially when metal is doped with another metal. Nickel alumina aerogel is the most common combination for this type. These alumina aerogels are being considered by NASA for capturing of hypervelocity particles.
SEAgel is another type of aerogel similar to organic aerogel made of agar.
Chalcogel is another type of aerogels made of chalcogens such as selenium, sulfur and other elements. And aerogels made of cadmium, selenium, quantum dots in a porous 3-D network have recently been developed for use in the semi-conductor industry.
AEROGEL CRYSTAL STRUCTURE AND PROPERTIES:
In this, crystalline structure of silica aerogels are discussed. Silica aerogels usually contain primary particles of 2-5 nm in diameter. Silica particles of such a small size have an extraordinarily large surface-to-volume ratio and a corresponding high specific surface area. So the chemistry of the interior surface plays a dominant role in its chemical and physical behaviour of aerogels. This property that makes aerogels to attract materials, because of this its been used as catalysts, catalyst substrates, and adsorbents. The nature of the surface groups of a silica aerogel is strongly dependent on the conditions used in its preparation. The extent of hydroxyl coverage is ~5 -OH/nm2, a value consistent with other forms of silica. This value, combined with their high specific surface area, means that silica aerogels present an extremely large number of accessible hydroxyl groups. Silica aerogels are therefore a somewhat acidic material. A more important effect of the hydroxyl surface is seen in the physical behaviour of silica aerogels.
The surface of silica aerogels is a strong bonding effect, due to such kind of nature, silica aerogels are hygroscopic in nature. This leads the aerogels to absorb water from the atmosphere, as a result of this, their mass also increases. However, the process is reversible. The moisture can be removed from the gel by heating it up to 100-120 deg. C. This adsorption of water vapours by aerogels doesn't show any effect on it, but direct contact with water can harm it. When liquid water enters a nanometer-scale pore, the surface tension of water exerts capillary forces strong enough to fracture the solid silica backbone. The net effect is a complete collapse of the aerogel molecule. This results the aerogel loose its transparent solid property and definite shape to white powder. This powder has same mass and surface area but has lost its shape. And for this reason silica aerogels are classified as "Hydrophillic".
Schematic of hydrophilic aerogelSchematic of hydrophobic aerogel
Interaction of water with the pore structure and a solid backbone of silica aerogels
Because of this caution, arrogance can't be used in places of exposing to the environment. Even though the problem can be solved by conversion of surface -OH (hydroxide) groups to a non-polar (-OR) group when R is of any possible alphabetic groups. This can be done before are after super critical drying and this protects the aerogel from the attack of liquid water by converting the forces between water and silica surfaces.
MANUFACTURING PROCESS OF AEROGELS:
Typically Aerogels can be classified as:
Organic aerogels like Silica aerogel and Aluminium aerogel.
Inorganic aerogels like Carbon aerogels
Among them silica aerogels is the most common and are known as aerogels. Aerogels starts as silica dioxide gel. Then the liquid in the gel is removed by a process called supercritical dying in which the material doesn't collapse but remains its original size and shape.
In single step based catalysed silica aerogel, this will produce an aerogel with a density of approximately 0.08 g/cm3 but the gel time should be 60-120 min depending on the temperature. In this process we mix two solutions like silica solution containing 50mL of TEOS, 40mL of ethanol and a catalyst solution containing 35ml of ethanol, 70nl of water, and 0.275ml of 30% aqueous ammonia and 1.21ml of 0.5M ammonium fluoride. Slowly add the catalyst solution to silica solution while stirring and pour the mixture into an appropriate mould until gelation then allow settling till ages.
This is final and the most important process of making silica aerogels.Â In this process, the liquid within the gel is removed, and by leaving only the linked silica network. Â The process can be performed by venting the ethanol above its critical point at a very high temperature and dangerous point with a prior solvent exchange with co2 followed by supercritical venting. It's imperative that this process is only to be performed in an autoclave specially designed for this process.
Production of silica gels:
Silica aerogels are produced through sol-gel process in which nanoparticles suspended in a liquid solution are invoked to interconnect and form a continuous, porous, nanostructured network of particles across the volume of the liquid medium.
There are two major techniques for the production of silica aerogels are Silicon Alkoxide and water glass technique.
Silicon Alkoxide Gelation.
The most common technique used for the production of silica gels today involves the reaction of a silicon Alkoxide with water in a solvent such as ethanol or acetone, usually in the presence of basic, acidic, and/or fluoride-containing catalyst. In this technique, a silicon Alkoxide may be either tetramethoxysilane or tetraethoxysilane which serves as the source for the silica, water acts as a reactant to help join the alkoxide molecules together, and a catalyst helps the chemical reaction go fast. As silicon alkoxides are nonpolar liquids, they won't miscible with water. So a solvent such as ethanol or acetone, which is miscible with both silicon alkoxides and water, is then added to go everything in to same phases, as the chemical reaction is necessary. This type of process reaction can be done at ambient temperature and pressure but the mixing should be done in proper proportion where the liquid goes in gel comes out.
Drawbacks of the Alkoxide Method
Even the chemistry of silicon alkoxide gelation is very simple and straight forward from a chemical perspective they have their own drawbacks. Mainly alkoxides of silicon and alkoxides of metals are too hazardous for health. Inhaling their vapours will cause hydrolysis and thus results formation of nanoparticle to occur in wet tissues such as lungs and nasal membranes. Another drawback of this is alkoxides tend to be very expensive as they require several chemical reaction steps for preparing.
In this technique silica gels can be produced through a less expensive method involving an aqueous solution of sodium silicate which is an inexpensive white solid with a range of possible stoichiometries ( which have diff names like sodium orthosilicate Na4SiO4,Na2SiO3 or sodium metasilicate, sodium polysilicate or (Na2SiO3)n) . These are soluble in water and when dissolved the resulting solution is referred to as waterglass or liquid glass.
When compared with silicon alkoxides, sodium silicate molecules do not hydrolyze and condense together when placed in water. But sodium silicates are slightly basic and neutralizing with acid, hydrolysis will occur and silanol (Si-OH) groups will form. As the silanol groups form, the silicate molecules form siloxane bonds with other silicate molecules and bridge together to form nanoparticles, resulting in a sol. This sol is now ready to use in making of gels, just like in silicon alkoxide techniques. But in the waterglass technique, sometimes the sol is needed to be heated to 50Â°C or so to get gelation to occur in short period of time.
Drawbacks of the Waterglass Technique
This is a cheaper alternative to alkoxides. This technique is less expensive and easier than alkoxide pathway, where as the resulting gels are very fragile and requires purification before they can be used to make aerogels. Nonetheless, the significantly of lower cost technique has made most of the commercial producers of aerogel materials use waterglass technique.
AEROGEL COST AND MANUFACTURABILITY:
Even aerogels have been invented 1900's itself, but from only recent years the technology enabled the innovative product can be produced commercially and cheaper than ever before. In the past, Production and manufacturing of Aerogels cost were very high to produce commercially. Aspen Aerogels, Inc. Has become a worldwide leader in producing and manufacturing these aerogels and they invented a new technique for the extraction process. Through this process, Aspen cut the production costs and can now produce plentiful amounts of Aerogels.
This company Aspen has an agreement with Burton Snowboards, who sells extreme weather apparel, to insulate their clothing with AeroGel. The Burton extreme weather jacket is made with Spaceloft AeroGel AR3103, the thickest in comparison with AR3101 and AR3102. AR3103 runs about £2.48 - £3/ square foot, depending on quantity. The Burton jacket insulated with AeroGel retails for £340, when compared to their non-AeroGel insulated jacket, which retails for £170. AeroGel insulated jackets are still one of the most expensive on the market because of its manufacturing process. The past energy cost factors caused mainly them from being produced. Especially supercritical extraction process is the costliest step which included 3 factors results in high production cost.
The supercritical extraction of methanol,
The heating of the autoclave steel and
Heat loss due to surroundings in the manufacturing process.
Also the raw material for the production is also more, but due to technological advances its been decreased. To be specific, AeroGel costs, such as sodium silicate costs $0.39 per board foot and Resorcinol runs $0.83per board foot. As technology as been advanced and ingenious engineer troubleshooting through the years, commercial production of aerogels for consumers can be done at much lower cost than ever before.
The first thing most people do when they touch a piece of silica aerogel for the first time is shatter it into a million pieces. You may hear statements in the media like-"A new Space-Age material that will support up to 1000 times its own weight..." This may be true, but it is important to remember that since silica aerogel is a very low density material, "1000 times its own weight" isn't very much weight at all. The most obvious property of aerogels is their extremely low density. Aerogels have been produced with densities of 0.003g/cm3. At this density, a computer monitor made of aerogel would weigh about the same as a graphing calculator. Densities of around 0.1g/cm3, however, are more common; this is about 10% of the density of water. Also, remember that silica aerogel is just another form of glass. If aerogel is handled roughly, it will break just like glass. However, if care is taken, the material can be handled and shaped effectively. Pressing firmly enough will cause a catastrophic breakdown in the sparse structure, causing it to shatter like glass-a property known as friability.
Most common density is 0.1g/cm3 (air = 0.001g/cm3)
Inner surface area
As determined by nitrogen adsorption/desorption A cubic centimetre of an aerogel has about the same surface area as one soccer field)
A solid percentage of volume
Typically 5 % (95 % free space)
Mean pore diameter
As determined by nitrogen adsorption/desorption (varies with density)
Primary particle diameter
Determined by transmission electron microscopy
Index of refraction
Very low for solid material (nair= 1.004)
Up to 500 C
Shrinkage begins slowly at 500 C, increases with increasing temperature. Melting point is ~1200°C
Independent of density, similar to dense silica. Determined using ultrasonic methods.
Very small (<104) compared to dense silica
For density of 0.1 g/cm3
For density of 0.1 g/cm3. Determined by 3-point bending
For density of 0.1 g/cm3, very low for a solid material (kair= 1)
Determined using ultrasonic methods al KHz frequency.
Sound velocity through the medium
100 m/a is for a density of 0.07 g/cm3, one of the lowest velocities for a solid material
0.02 W/MK (20C)
Very low thermal conductivity. 2 cm slabs provides the same insulation as 30 panes of glass
The most studied property of aerogels is their thermal resistance. Aerogels can withstand temperatures up to 500°C, above which they begin to shrink. Their melting point is around 1200°C.
Coating the aerogels with chemicals, strongly affects their adsorbent properties. CO2 drying results in pore surfaces covered by OH (hydroxyl) groups. When one of these hydrophilic aerogels is placed in a humid environment, it will adsorb water into its pores, up to 20% of its mass. Later on the water can be removed simply by heating it up. Aerogels can be made hydrophilic or hydrophobic, depending up on the drying process is been used. These aerogels, however, cannot be used to adsorb liquid water. The high surface tension of water, upon entering the tiny pores, tears them apart. The aerogel seems to disappear; it has become a fine powder, less than 5% of the volume of the gel. Alcohol drying is uncommon now a days because of its high temperature and pressure required, results in pore surfaces covered with alkoxy-groups. These hydrophobic aerogels, if sealed, are completely impervious to water, and will float indefinitely.
Some of the common applications of the Aerogel
Being a brittle substance with light weight Aerogels is known to be a very good insulator.
Its been used in granular form to add insulation to skylights.
Silica aerogel in a weightless environment with more uniform size and reduced Rayleigh scattering, that aerogel is less blue and more transperant. This type of transparent aerogel is suitable as thermal insulation materials for windows, limiting thermal losses of building.
Aerogels having a high surface area is also used as chemical absorbents for cleaning up spills. This property of aerogels can also be used as catalysts or catalyst carriers.
It is also used as thickening agents in some paints and cosmetics.
Some Aerogel material can be used to trap space dust particles aboard the stardust spacecraft. It can also be used for thermal insulation of the Mars Rover and space suits. The low mass of aerogels is also advantageous for space emissions.
It can be used as radiators in Cherenkov effect detectors. The ACC system of the belle detector used in Belle Experiment at KEKB is a recent example of such use.
Resorcinol-formaldehyde aerogels are used as precursors for the manufacture of carbon regal.
The metal aerogel nanaocomposites can be used as catalysts, sensors, or electromagnetic shielding as well as in waste disposal. These metal aerogel nanocomposites can be prepared by impregnating the hydrogel with a solution containing ions of suitable metal or transition metal, these impregenates hydrogel is then irradiated with gamma rays leading to precipitation of nano particles of the metal.
Based on biocompatibility of aerogels, it can be used as drug delivery systems.
Carbon aerogels are used in the construction of small electrochemical double layer super capacitors.
Having a very low impedence compared to normal super capacitors aerogel can absorb or produce very high peak currents.
Dunlop has recently incorporated aerogel technology into a series of its tennis racquets, having used it earlier in squash racquets.
An aerogel material may be used to introduce disorder into the superfluid state of helium-three.
The present application of Aerogels
Porosity and Surface Area Applications
Having high porosity with very large inner surface area (easily accessible because of the open porosity), and the controllable dispersion of the active component, they are especially active catalysts or catalytic substrates.
Due to this they are widely used in filters absorbing media for desiccation, filters, reinforcement agents, pigments, gellifying agents, waste containment, encapsulation media and pesticides.
The carbon aerogels have been used as electrodes capacitors in energy storage devices known as double layer capacitor because they are electrically conductive with a very large surface area.
Transparent aerogels, having exceptional thermal insulation ability, have been considered for use as super-insulating sheets of double walled window systems because help considerably to reduce thermal losses in the windows of Homes, Buildings, etc.
Aerogels have also been used to prepare ultra-pure, full-density silica glass by sintering at temperatures below the melting temperature of silica
Silica aerogel, doped with radioactive tritium and phosphor, makes an efficient radio-luminescent light source.
Thermal Insulator Applications
Aerogel materials, having lowest thermal conductivity of the solid or porous materials, are excellent for applications requiring thermal insulation. This property of aerogel is used for development in electric automobiles equipped with batteries that operate at high temperatures and that need heat storage, and insulation for architectural purposes.
The dielectric constant of Aerogels is low, so they behave as insulators. The lower the dielectric constant, the higher the velocity. Therefore, thin aerogel films are almost ideal dielectrics for ultra-fast integrated circuits. The bulk aerogels can be used for the microwave electronics and high voltage insulators.
The pure carbon aerogels are electrically conductive, so they have applications as electrodes for batteries, fuel cells, and capacitors.
Other metal oxide aerogels have been made, which exhibit super-conducting behaviour, thermoelectric behaviour, and piezoelectric properties.
5.5 Mechanical Applications
Aerogels may also have acoustic and mechanical applications. Because of their unusual structure, aerogels have low sound velocities, as low as 30 meters per second.
Another important acoustic property of aerogel is its mechanical impedance. The impedance is the product of density and the sound velocity of the material. Since both are low, silica aerogel has the lowest impedance of all solid material. This allows the aerogels to be used for coupling sound waves in air to a transducer; this may be useful either for generating or detecting sound.
Lightweight silica aerogels have also been proposed as a contaminant collector, to protect space mirrors from volatile organic. Aerogels were used to insulate the Mars Rover
Future of Aerogels
GIA announces the release of a global report on Aerogels market. The world market for Aerogels is projected to exceed $1.0 billion by the year 2015. The market, still in its emerging phase, is driven by the rapid advancements in manufacturing processes and the growth in application markets. What we discussed till now is production practices for Aerogels. However, these are still on very higher cost and to commercialize them, a few newer technologies are being invented. Mechanically strong aerogels, aerogels of new materials and commercially available aerogels are few challenges of the future.
Mechanically Strong and Flexible Aerogels
Diffusing cross linking agents such as diisocyanates (used in making polyurethane) into the pores of a silica aerogel, he could use the hydroxyl groups on the surface of the particles that make up the aerogel framework to create polymer bridges between particles-the structural weak spots in the aerogel structure.
In the process the casting of the bimetallic alloys of gold and other metal over micron-diameter polystyrene spheres, which served as a template for a foam structure. After dissolving the template, the less-noble metal in the alloy was then etched away through a technique called deploying. The remaining structure was macro porous foam with nanoporous walls, closely approximating a "gold aerogel". The foams had two levels of porosity, micron-diameter pores leftover from the template and nanometer-size pores from dealloying. However, these materials did not exhibit significant mesoporosity, that is, pores between 2 and 50 nm in diameter, which is more typical of aerogels.
Carbon Nanotube Aerogels
Carbon nanotube is assembled into a gel structure using polyvinyl alcohol as a "binder. In the process, they super critically dried these gels to prepare carbon nanotube aerogels. These materials also showed higher electrical conductivity.
Aerogels which consist maximum potion of air like 90-98% is the lightest solid ever known to man. These compounds are ultra-light, low-density solids with remarkable optical, mechanical and thermal properties. Even number of industries are investing a lot of the projects to overcome the cost of production, but still it's in basic state. In future with the technology advancement and in the reduction of production costs, the global market of aerogels might include to over 800 products. Aerogels find applications in a number of industries like aerospace, medicine, apparel, automotive, pharmaceuticals, consumer goods, construction, electrical & electronics. Consumption of aerogels is to be improved in energy, apparel, architecture and electronics sectors. To overcome the disadvantages associated with high production costs, aerogel manufacturers are emphasizing on cost reduction and innovative methods of production. Over the years a number of methods be invented for the production of aerogels at lower costs, but it's not don't do. Still the experiments have to be done to find out the cheapest method of production.