Full of air

C&I Issue 2, 2013

Buy a bag of crisps that’s full of air, and you feel slightly cheated. But if you’re buying or making an aerogel, the air content should be as high as possible. Aerogels are solid to the touch – similar to expanded polystyrene – but can comprise more than 99% air, owing to their highly porous structure. This makes them incredibly light.

The first aerogels were made from silica gels in which the liquid had been replaced by air. Simply drying the material would result in the solid collapsing into a powder, so a supercritical fluid (SCF), typically CO2, was used to remove the liquid.

‘Silica aerogels are amazing materials,’ says Mary Ann Meador, senior research scientist at NASA’s Glenn Research Center in Cleveland, US. ‘They are low density – which is of great interest to NASA – as well as being good insulators and good dielectrics, with a high surface area. The pores in an aerogel are so small that gas does not travel through them easily,’ she explains. ‘It’s a better thermal break than a traditional foam, as solid phase conductivity is almost zero.’

NASA is already using silica aerogels in a few applications, such as to insulate the batteries and electronics in the Mars’ rovers – the two robots that are still crawling across the surface of this planet. Silica aerogels were also used on board NASA’s Stardust mission, which flew close to comet Wild2 to collect particles from it. An aerogel array, approximately the size of a tennis racquet, was unfolded during a fly-past of the comet. ‘The aerogel was able to “brake” these particles, from 6km/s down to zero,’ says John Bridges, reader in planetary science at the University of Leicester, UK.

The Stardust mission had to be careful not to get too close to Wild2 on its fly-past. ‘This could have led to larger particles destroying the collector,’ explains Bridges. ‘There’s a balance in getting close enough to collect samples, but staying far enough away so that the flux is not too high. The total mass of grains returned was less than a milligram, so these samples are incredibly precious and a considerable analytical challenge,’ says Bridges.

The particles were captured within the aerogel, forming micron-sized tracks. These ‘keystones’ were then cut out of the matrix, and analysed at the Diamond Light Source in Harwell, UK. ‘We used X-rays from the synchrotron to do X-ray fluorescence and X-ray spectroscopy, to try and characterise these grains,’ he says. The analysed samples showed the presence of iron particles, which he says could explain the reddish colour of the comet’s surface.

Aerogels are particularly good insulators in a light vacuum, such as on the surface of Mars. This property could potentially be exploited in MRI scanners. The superconducting magnets used in MRI scanners are usually insulated by multilayer insulation (MLI), which must be kept in a very strong vacuum. ‘Replacing the MLI with aerogels would mean that you wouldn’t need such a strong vacuum,’ says Meador.

Another promising application for aerogels is in clothing, because they offer the chance to make very thin but insulated garments. In 2010, sportswear company Hanesbrands produced an experimental ‘Supersuit’, a jacket that was used by a team climbing Mount Everest. Among the new materials being used was Element 21’s ZeroLoft, a silica-based aerogel. The suit was claimed to be four times as warm as a goose down jacket and much thinner – 3mm thick, compared with a typical 40mm for a goose down jacket. But insulation works both ways: runner Xy Weiss used aerogel insoles to prevent her feet from blistering when she ran across Death Valley.

But there are downsides to silica-based aerogels. They are naturally brittle and fragile. Meador and her team have tried to strengthen the silica matrix using a polymer reinforcement. While this boosts the mechanical properties by two orders of magnitude, allowing it to be used as a more robust insulation, it is not without drawbacks. ‘It’s still hard to fabricate these parts,’ she says. ‘You are still handling a fragile silica gel, which you have to crosslink with the polymer: it’s a long process.’

Meador’s team has recently developed an all-polymer aerogel, made from polyimide (PI) as part of NASA’s Inflatable Reentry Vehicle Experiment (IRVE-3), which is looking for better ways of dropping payloads onto the surface of planets. Polyimide was chosen because it is a high temperature thermoplastic widely used in engineering applications, such as semiconductor manufacturing and medical tubing. The polyimide aerogels are less brittle than silica-based ones because the chains between the cross links are longer.

Meador explains that the idea is to use a giant ‘airbag’ that unfolds and slows the descent of the craft onto the surface of the planet. Unlike a parachute, it deploys underneath the craft – so must take the full brunt of the heat of re-entry. A polymer-based aerogel could form the insulated coating on the outer surface. ‘An aerogel would be the best insulator for this type of application,’ says Meador. ‘It’s several times lighter than any other type of insulation, which means you can use less of it.’ Her team has developed a flexible film of the new polymer aerogel to act as insulation, she adds.

Electrical potential

While aerogels are best known as efficient insulators, Meador says another of its properties – a low dielectric constant – could lead to a more promising application, in the manufacture of lightweight antennas (Applied Materials & Interfaces, doi: 10.1021/am301985s). The aerogel would form the substrate on which an antenna is printed. According to Meador, the weight saving, and the ability to make the polyimide aerogel in film form, could potentially lead to the creation of conformal antennas – which can take the form of a curved surface. This project is currently at proof of concept stage, she says.

Meador is not the only researcher looking at the electrical potential of aerogels. Researchers in Germany have created an aerogel-like structure, using graphite as the matrix. The researchers claim that their material, which they call ‘aerographite’, is the lightest solid material known. The fact that it is based on graphite makes it highly conductive, so it might one day be used in batteries and other electrical and electronic applications.

The aerographite was discovered by accident. ‘We were interested in trying to develop and grow carbon nanotubes (CNTs) on a template,’ says Karl Schulte, head of the polymer composites group at the Technical University of Hamburg-Harburg, Germany. The template was zinc oxide, a ceramic material, and the reaction was carried out at >740°C in the presence of a catalyst, in an oxygen-rich atmosphere. No CNTs were formed, but when a small amount of hydrogen was added to the stream, the zinc oxide began to disappear – and was replaced with porous ‘aerographite’. ‘It was a graphite material with a 3D structure,’ he says. ‘We knew from our experience that it would have good properties.’

Aside from its electrical conductivity, the aerographite is lightweight and can be deformed. This robustness, Schulte says, makes it a potentially important material for lithium ion batteries. ‘These devices already use graphene or graphite structures, but there is a problem with battery lifetimes – and how often they can be recharged,’ he says. ‘Aerographite could be used many times. It could increase the lifetime – and maybe even the capacity – of the battery.’

Schulte stresses that the material is a long way from commercialisation; it is currently made in volumes of around 1cm3. Basic research will continue in order to understand everything from how it is formed to details of its atomic structure and thermodynamic properties.

Meanwhile, Schulte’s team has developed several new structures using aerographite. One comprises interconnected hollow tubes, while a second, which has a record low density of 0.2mg/cm3, is more like ‘spider threads’. A third structure is similar to the first, but the tubes are filled with a form of carbon called ‘pyrolytic graphite’. This allows the inner surface to be packed with conductive ions, which makes it the most promising of the structures for use in batteries.

According to Schulte, there are many other potential applications of aerographite. It might be used to filter air, or clean water, for example, or it might form the basis of gas sensors and fuel cells. But these applications, he stresses, are in the future. ‘We’re at the same stage with aerographite as scientists were with CNTs in the early 1990s,’ he says.

While aerogels are still highly specialised materials – that even NASA has not yet fully exploited – they are beginning to find their way into the mainstream. Let’s hope that it happens soon: if nothing else, the prospect of wearing a thin, aerogel-lined coat – rather than three or four separate layers – is really very appealing.

Energy savings

A four-year pan-European research project, Aerocoins, will try to develop new aerogel materials to insulate buildings and help Europe not only achieve a 20% cut in energy consumption by 2020 but also address the growing problem of climate change. According to Eunate Goiti, scientific and technical co-ordinator of Aerocoins, the construction industry accounts for around 40% of Europe’s energy consumption – and around one-third of its carbon dioxide emissions – so targeting energy savings here will have the largest effect.

Aerocoins is currently developing an aerogel-based board, which will be tested for its thermal properties and will eventually be produced in commercial quantities. The bulk of the research, though, focuses on fundamental chemistry, as researchers grapple with sol-gel chemistry in an attempt to produce more thermally efficient aerogels, more cheaply. ‘We are looking for processes that are cheaper than supercritical fluid (SCF) drying,’ says Goiti. ‘SCF uses high pressure: we need to find a continuous process that works at ambient pressure.’

Swiss research organisation Empa, one of the collaborating partners, has helped render manufacturer Fixit to develop an aerogel-based plaster that provides twice the insulation of conventional insulating renders. The secret is to incorporate the delicate aerogel into the plaster mixture so that it is not destroyed when fed through a high-pressure rendering machine. Thomas Stahl, a building physicist at Empa, says the render provides as much insulation as polystyrene board. ‘The render lies directly on the brickwork and does not leave gaps where moisture could condense,’ he says. The product is expected to hit the market in 2013.

Lou Reade is a freelance writer based in Kent, UK.

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