Aerogels belongs to a class of solid porous materials that shows a long array of extreme material properties. Aerogels are so amazing that almost everyone who see this material for the first time will simply be astonished by it. The name “aerogel” would indicate towards its character. The adjective “Aero” mean air and the “Gel” is used to introduce semisolid like substance that doesn’t drip. Aerogel is basically a gel like substance that almost made of air. Unlike most of the other gels that we are familiar with, aerogels are effectively a solid structure. Their inner structure is made up of numerous small pores that are filled with air or some other gas. Some aerogels even contain pores that contain a vacuum or gas at very low pressure. In the crudest form, it can be viewed as a sponge with nano sized pores.
Silica (silicone dioxide or SiO2) aerogels are the most common. They are the most extensively used type and therefore the most explored among others. The structure of the silica aerogels is made of intertwined network of silica clusters that has solidified to make a unique three dimensional arrangement. These clusters typically owe to only 3% of the total volume of the aerogel material. This unique arrangement of silica clusters entraps air in the middle of the solid, confining it in to very small nano sized pores. Therefore, if you observe an aerogel material under a powerful microscope, you would see lot of vacated areas inside its structure. Most of the silica aerogels are translucent with a slight bluish hue.
How aerogels are made
Most of the silica aerogels are produced using a method called sol-gel, where metal alkoxides, for example tetraethyl orthosilicate or tetramethyl orthosilicate are subjected to hydrolysis in an aqueous solution that may contain alcohols and other catalysts. These hydrolyzed precursors then undergo a process called polycondensation where they are connected through metal oxide bonds ( ….Metal – Oxigen – Metal – Oxigen …..) to make polymer like chains that are interlinked across its backbone. This would substantially increase the viscosity of the solution, ultimately resulting a gel.
What begins next is the most critical for the making of an aerogel. In this so called solvent extraction process, water and other water soluble components in the gel is slowly taken out, leaving the interconnected network of the silica cells. However, if the gel is allowed to sit for evaporation under normal conditions, the surface tension forces excreted by the liquid would cause the fragile silica network to collapse. This would make a silica network with very little porosity(a xerogels). Aerogels however, are made with a cleaver liquid extraction method called supercritical drying. In this process, the pore liquid is first exchanged with suitable liquid such as ethanol. This is then exchanged with supercritical carbon dioxide; a fluid made by increasing the pressure and temperature of gaseous carbon dioxide, followed by instant gasification through dropping the pressure once the ethanol is extracted. This would avoid the damage to the three dimensional network while vacating the small pores inside the aerogel.
Some other aerogel types
There are also aerogels made with metals and some metal oxides. The latter type of aerogels include, aluminium oxide, iron oxide, chromium oxide and vanadium oxide. The most common is aluminium oxide. Typically, these metal oxide aerogels are doped with some other metal for mechanical improvements or to be used as catalysts.
Carbon or organic aerogels are the most spoken about in aerogel family these days. Unlike other aerogels where the internal structure is made of a ceramic or metal oxide network, carbon aerogels comprise of network made of organic polymers. Scientists have fabricated organic aerogels with number of different resins. Among them, resorcinol formaldehyde, phenol formaldehyde and melamine formaldehyde are the most common. Organic aerogels are made primarily using polymer chemistry procedures followed by heating them at several hundred degrees Celsius under an inert atmosphere. This leaves a porous network of carbon that are interconnected. Carbon aerogels are more economically viable because it don’t need expensive supercritical drying procedure to fabricate. Also they can be made in large quantities. Carbon aerogels are completely black and unlike other aerogels, it’s elastic.
Aerographene are a special class of organic aerogels. They are made of graphene and carbon nanotubes. Carbon nanotubes makes the internal structure of aerographene like steel bars in a big stadium while graphene makes the wall that supports the structure externally. Aerographene is the lightest material made by man so far.
Among other things, aerogels are known for their outstandingly high heat insulating property. This means that if you completely wrap an object at a given temperature with an aerogel, its temperature tend to stay at the same value for extremely long durations. Aerogel does this by almost completely stopping the heat transfer in all three possible methods; convection, conduction and radiation. Gas trapped inside the tiny pores of greatly restricts the mobility of the air, thus stopping the convection. This is known as the Knudsen effect. Heat loss through conduction is also greatly inhibited by having a high percentage of air in its internal structure. This is because air is a very good insulator. Three dimensional network of ceramic or carbonous material in the aerogel doesn’t help conduction either because heat conduction through numerous little interconnections is very inefficient. Aerogel also acts as a good radiative insulator. This is because it can scatter infrared radiation back in to its source rendering very low transmission across the material.
Aerogel insulation is the most widely used application of the aerogel materials so far. Both silica aerogel and carbon aerogels are used commercially for aerogel home insulation, high temperature insulation and industrial insulation. Easy to use forms of aerogel composites such as aerogel blankets and thin insulation sheets also exist. Especially fiber reinforced aerogel blankets from aspen aerogel company is a good example. Aspen aerogel product line also includes specific products for subsea oil pipelines, reactors, refineries and even aerogel clothing items such as winter apparel and shoe insoles.
Light but strong
Aerogels are known for their exceptionally low densities. Some of the latest aerogels such as graphene aerogels (aerographene) is as light as 160 g/cubic meter which is only 13% of the density of air (1225 g/cubic meter). This means that if you can hold an aerographene block having a size of an average passenger car, you would only feel as holding a half a brick.
However, most of the aerogels are made of silica and have densities in the range of 1000 – 2000 g/cubic meter. These values are still impressive compared to the common materials such as steel (8,050,000 g/cubic meter), wood (510,000 g/cubic meter) or even expanded polystyrene foam (22000 g/cubic meter) which is one of the lightest materials that we encounter in our daily life. The specific compression strength of aerogels are quite astonishing too. Specific compression strength is the maximum compressive load a kilogram of a material can withstand without breaking or buckling. Steel has a specific compressive strength of 140,000 Nm/kg while some special aerogels can reach to 389,000 Nm/kg, a value almost 2.5 times higher.
Aerogels are very versatile and scientists can tune their properties for specific applications. There are number of starting materials to choose from and each would provide unique set of properties that can unlock many potential applications. There’s a long list of already existing and potential applications of aerogels. Following are some of them
Aerogels as chemical catalysts: high surface area induced by the pore structure of aerogels is ideal as catalytic surfaces. Surface modified or doped, aerogels typically made from metals or metal oxides are used for this purpose
Electrodes in supercapaciters: Some carbon and metal aerogels are ideal for electrodes in supercapaciters due to its high surface area electrical conductivity
Aerogels as storing media : Partially sintered aerogels can be used as a storage material for liquids. Their extremely low weight is particularly advantageous for this application
Aerogels as a template: Porous structure of aerogel can be used as a template to make certain reactions. Template assisted synthesis can be used to make nanoparticles with unique sizes and shapes
Aerogels as an absorbing media: Lot of research is conducted to make lipophilic (oil loving) carbon aerogels to suck up oil from major oil spills. Aerographene can absorb oil 900 time of its own weight
In space applications : Aerogels were already used to capture comet dust from the space. They are currently used as insulators in space suits and space crafts.
- Patel, R. P., Purohit, N. S., & Suthar, A. M. (2009). An overview of silica aerogels. International Journal of ChemTech Research, 1(4), 1052-1057.