Frozen smoke, the world’s lightest solid material, is hard at work powering supercapacitors, insulating space ships, firefighters, surfers and rockets, thickening paints and cosmetics, performing classified roles in nuclear weapons, collecting interstellar dust… It is one of the lightest, most expensive substances on Earth, and we are surrounded by it.
Aerogel, also known as frozen smoke, solid smoke or solid air, is an ultralight synthetic material produced from a gel. Composed of 99.98% air, it looks and feels like very light polystyrene, with a slight blueish tinge. First developed in the 1930s as the result of a bet, aerogels are incredibly light, strong and flexible, and are being applied to everything from home decor to aerospace engineering.
Gels, such as jelly (Jell-O, Gelatin, whatever you like to call it) and superglue, are quite remarkable substances themselves. They exhibit some properties, such as density, that are characteristic of liquids; other properties, such as their fixed shape, are usually only found in solids. The cause of their identity crisis lies in their internal structure. Gels are formed from a network of microscopic pores called nanopores. Through these pores, the liquid component of the gel flows. The pores are so small that capillary forces exerted by the liquid are strong enough to hold the whole thing together, and prevent the liquid from flowing out.
The only problem for a gel is evaporation. The water inside the nanopores is essential to maintain the structure of the gel, but if it is left exposed to the air, the water will begin to evaporate out of it. As the water evaporates the gel begins to shrink, the internal structure collapses in on itself, and eventually the gel is reduced to xerogel – a hard, dense substance less than a tenth of it’s original size. An aerogel is a gel which has had the liquid removed without the collapse of the nanopore structure; the solid internal framework is maintained, but the liquid interior replaced with air.
“An aerogel is the solid skeleton of a gel”
Aerogels can be produced from several different chemical compounds including carbon, silica, alumina, chromia and tin dioxide. Depending on which materials you construct your aerogel from, they can have radically different properties; some aerogels are highly effective insulators, others are extremely conductive. Some aerogels are exceptionally absorbent, others remarkably water resistant.
Aerogels are produced by replacing the liquid component of a gel with gas in a process known as supercritical drying, producing a solid with a very low density. Supercritical drying allows the liquid in the gel to be slowly dried away without the solid matrix of the pores contracting and the gel collapsing, as would normally happen if the liquid simply evaporated. An aerogel begins as an alcogel – a silica gel with alcohol in it’s pores. Solvents like ethanol, methanol and acetone are used to purify the starting gel, removing impurities held in the gel’s original liquid. Purification can take weeks for large batches, but the resulting alcogel is ready for supercritical drying.
During supercritical drying, the alcogel is put under high pressure and temperature, past the critical point at which the distinction between liquid and gas is gone. At this point, the alcogel behaves like a gas, but has the density and thermal conductivity of a liquid. It is a supercritical fluid. Next, the pressure is reduced whilst the temperature remains high, and both remain past the critical point. This allows some molecules of alcohol to condense as liquid and be released, making the remaining liquid inside the alcogel less dense. Finally, the gel is cooled, and the remaining alcohol, not dense enough to condense as water, turns into a gas. The result is a solid with an internal stricture of nanopores filled with gas. Only between 1% and 50% of the original volume of the alcogel is left in the finished aerogel.
One problem aerogels have is that they are hydrophilic – they love water. But it’s a love-hate relationship, because contact with water can cause the structure to degrade and cause the aerogel to fall apart. They can be treated, however, to become hydrophobic, and can then be extremely water resistant.
Supercritical drying can do remarkable things. Unfortunately, the process is very expensive and time-consuming, and involves the use of organic solvents that are dangerous at high temperatures and pressures. Because of these difficulties, scientists have also developed alternative methods to produce aerogels. One of the first improvements to supercritical drying came in the 1980s in the form of liquid carbon dioxide exchange. In this method, also known as subcritical drying, the alcogel is prepared as usual, and then soaked in liquid carbon dioxide, replacing the liquid inside the gel with liquid CO2. CO2 is non-flammable and it’s critical point is very low, making this process far safer. However, carbon dioxide does not naturally occur as a liquid – dry ice, the solid form of CO2, evaporates straight into a gas. It needs to be highly pressurised in order to form a liquid, and then added to the pressurised alcogel. It can take several days for the CO2 to fully diffuse into the gel to form the aerogel. Another subcritical method produces something called an ambigel, which tends to be slightly less dense than aerogels because their nanopores contain tiny holes called porosities. In these subcritical drying techniques, the gel often has be be modified in order to strengthen the nanopore structure inside and reduce the stress caused as the liquid evaporates at subcritical temperatures and pressures.
Grow You Own
One alternative to supercritical drying is to simply grow an aerogel. Aerographite, for example, is a carbon-based aerogel that can be can be grown on a lattice of zinc oxide crystals which are later burned off in an oven. Aerographite is amazingly absorbant, and it is now being applied to oil spills, where it can absorb up to 900 times it’s own weight. Another option is to simply fabricate the aerogel yourself. Recently, researchers at HRL Laboratories in Miami succeeded in creating a microlattice by essentially micro-sculpting a lattice structure from a template.
Strong and Lightweight
Aerogels are best known for being mind-bendingly light. Silica aerogels have a density of about 0.0019 g/c3, almost as light as air. That’s 500 times lighter than polystyrene. Aerographite has a density of 0.3g/c3, while the microlattice has a density of 0.0009 g/c3. Aerogels are around 1,000 times less dense than a far more well-known silicon-based material; glass. While light, aerogels are also incredibly strong, carrying up to 4000 times their own weight!
What Are Aerogels Useful for?
The uses of aerogels appear to be almost endless, owing to their complex and varied set of extreme characteristics. Their first applications were as thickening agents in paint and cosmetics. Now they are being applied to a diverse set of challenges from space exploration to medicine. Aerogels have the lowest thermal conductivity of any solid material, making them excellent insulators. They have been used for insulating houses and buildings, and insulating people in the form of wetsuits and fire fighter’s fire-proof uniforms, to name a few. While some aerogels are excellent great insulators, others are fantastic at conducting electricity, meaning they could be used to produce extremely light, high-energy batteries.
Some aerogels are very absorptive, mopping up 100 to 900 times their own weight at up to 70 grams per second! They may therefore be useful for cleaning up oil spills – they could even be ‘squeezed’ afterwards to reclaim the oil and then reused. A group in Wisconsin has recently developed a ‘green’ aerogel; formed from cellulose and treated with silane and freeze-dried, this new aerogel can absorb up to 100 times it’s own volume, and provides a more ecofriendly method for dealing with pollution.
More recently aerogels have been applied to a new frontier: medicine. Aerogels have high surface areas and high porosities (they’re full of pores!), and their internal pore structure can be manipulated during their production. This makes them an excellent choice for a drug delivery system. That is, the carrier of any medicine you take, which makes sure the medicine arrives at the right location to be effective. Researchers have manipulated aerogels in all sorts of ways, producing biocompatible (so they don’t elicit an immune response) and biodegradable organic aerogels that will dissolve after use, and even mixed and layered aerogels. Aerogels are particularly useful for delivering drugs that are not soluble in water.
The stardust mission aims to collect samples of comets and interstellar dust. However, these particles are moving through space at extremely high velocities, and a big challenge is slowing them down sufficiently without damaging them. If the particles are caught at full speed, there is a risk of changing their shape and composition, or just vaporising them entirely. That’s where aerogels come in. A silicon aerogel can capture the particles within it’s nanopore structure, slowing them down gently. Researchers can then dig the particles out of the aerogel for testing.
Unfortunately, aerogels are extremely expensive and time-consuming to produce – aerogels cost around $1 per cm3, or over $10,000 per kg, aerogels are more expensive than gold. It is not easy to produce aerogels in large quantities, either. As the volume of a cuboid aerogel increases, the time needed for liquid to diffuse out of the aerogel increases exponentially. Large, flat structures are easier to achieve, although if you’re using a supercritical drying technique, you’re ultimately limited by the size of your pressurised container. Subcritical methods mentioned above tend to be a little cheaper and can potentially produce larger volumes.
The New Lightest Material on Earth
Last year, Chinese material scientists developed a new and improved aerogel, even lighter than it’s predecessors. Graphene aerogel is seven times lighter than air, and 12% lighter than the next lightest material, aerographite. A single cubic centimeter of graphene aeorgel weights a measly 0.16 milligrams. It is light enough that blocks of it can be balanced on a blade of grass. Graphene aerogel was developed using another novel method; freeze-drying. By pouring a solution of grapheme and carbon nanotubes into a mold and freeze-drying it, it is possible to dehydrate the solution leaving an aerogel composed of layers of graphene supported by carbon nanotubes. This method produces extremely light aerogels because each graphene layer is a single atom thick. Graphene aerogels are also highly elastic – you can compress them to 90% of their original size and they will spring right back.
Aerogels are a remarkable feat of human engineering, and they boast some incredible features to boot. They are light, strong, insulating, conducting, absorbent, and biocompatible. Too bad they cost so much, or we’d be using them in everything.
Want to Know More?
- Wisconsin Institute for Discovery (2014)“Greener” Aerogel Technology Holds Potential for Oil and Chemical Clean-Up [VIDEO]
- Ulker & Erkey (2014)An emerging platform for drug delivery: Aerogel based systems Journal of Controlled Release
- Graphene aerogel is seven times lighter than air, can balance on a blade of grass
- Popular Science (2013)This Is Now The World’s Lightest Material
- How Stuff Works – How Aerogels Work
- Popular Science (2011)Invented: World’s Lightest Material, 99.99 Percent Air
- Schaedler et al (2011)Ultralight Metallic Microlattices