Material of the month - Silica aerogel
Maria Felice examines the extraordinary properties of aerogels.
Whether in pretty pictures where it looks like solid sky, or having its properties scrutinised by aerospace engineers, aerogel is receiving a lot of attention at the moment. This material, made by drying out a gel, is ultra lightweight and has fantastic energy damping and weight bearing properties. If the average man were made of silica aerogel – the most popular aerogel – he would weigh just 450g and be capable of supporting almost half a tonne.
Silica aerogel was first produced in 1931 by Samuel Stephens Kistler as the result of a bet with Charles Learned over who could remove the liquid from so-called jellies and put a gas in its place, without causing shrinkage. Other types of aerogels have since been developed, for example carbon aerogels in the late 1980s.
If a gel is dried quickly and normal evaporation takes place, the solid matrix (made of, for example, silica) will collapse from capillary action. However, if the liquid is extracted slowly by supercritical drying, the solid matrix does not collapse – this is how aerogels are produced. Therefore, despite having gel in its name aerogel is not actually a gel. Aerogels typically retain more than 90% of the gel framework’s original volume.
It is interesting to note that the silica gel that one finds in little packets with new shoes and consumer electronics is silica xerogel. A xerogel is a solid made from drying a gel with unhindered shrinkage. Despite shrinking, the solid still remains highly porous and is therefore good at absorbing moisture.
Silica aerogel is one of the lowest density solid materials that exists, making it particularly attractive for aerospace and outer-space applications. The lightest kind of silica aerogel has a density of 1.9kg/ m3 and 1kg/m3 when evacuated. To the touch, silica aerogel feels like expanded polystyrene. If pressed lightly, the material will not be altered. Pressing more firmly leaves a permanent depression, and pressing more firmly can shatter the material. However, it has very good weight-bearing properties due to its microstructure. Spherical particles of diameter ranging from 2–50nm are fused into clusters of diameter ranging from 50–2,000 nm. These clusters are connected together in strands, forming a 3D highly porous structure. Aerogel’s porosity gives it an extremely high specific surface area (600m2/g), making it useful for sensor and absorption applications.
Silica aerogel has a thermal conductivity of 0.03W/ mK or lower (the value for rubber is in the order of 0.1W/mK) and because of the ceramic silica, has a high melting temperature of approximately 1,200°C. The low conductivity is due to the high proportion of air in the material together with the low conductivity of silica itself. Silica aerogel is also a good convective insulator because air cannot circulate through the lattice and it absorbs infra-red radiation. This makes silica aerogel an ideal material for thermal insulation in high temperature applications, for example protecting electronics from thermal transients within an aerospace engine.
Silica aerogel resembles the sky in the sense that it is up to 99% transparent and has a bluish hue caused by Rayleigh scattering of shorter wavelengths of visible light by the nanosized microstructure. The combination of transparency and thermal properties, make silica aerogel an ideal candidate for window material. However, it is a challenge to manufacture a monolithic window-sized piece of aerogel. Also, if the manufacturing process is not carefully controlled, the aerogel’s transparency suffers. Hydrophobic silica aerogels repel water but attract other liquids such as oils. Special powders and blankets made of silica aerogel have been proposed for use in oil-spill cleanups and are being developed commercially.
Silica aerogel was used on NASA’s Stardust spacecraft because of its excellent energy damping properties, which are brought about by its intricate nanostructure. Despite each being the size of a grain of sand, the particles ejected from comets are extremely difficult to capture because they travel at over 20,000km/h. They vaporise on contact with a solid and pass through a gas. Special blocks of silica aerogel with varying density were used to dampen the particles’ energy and trap them. Scientists at Imperial College London and the Natural History Museum have developed an X-ray technique to examine these information-packed stardust particles while they are trapped in the aerogel. Silica aerogel has also been used for its thermal insulation properties in various Mars rovers, including Curiosity.
This article has not even touched upon the production process of aerogel and has only given a taster of the many applications of this fantastic material.