Aerogels come into their own: Rarefied substance is to end 60years of obscurity, writes Nuala Moran

AEROGELS, solids that can becomposed of up to 99 per cent air, are to become commercially available more than 60 years after their remarkable insulating, acoustic and optical properties were first recognised.

In a few years, aerogels will replace CFC-based insulation in refrigeratorsand other appliances. They will be used in building insulation and fireproofing. Aerogels do not burn, and only melt above 600C.

Transparent forms under development will be used as insulation in double glazing. And because sound travels through them at only 50-100 metres per second, aerogels will be used for acoustic insulation.

Aerogels are made by mixing silicon dioxide - the same material as sand - with alcohol to form a gel. This gel is heated at high pressure until the alcohol evaporates, leaving millions of tiny air pockets.

These materials - which have been described as frozen smoke - can be 90 to 99 per cent air. They have a very high surface area - one gram of a 99 per cent air aerogel has a surface area of about 100 square metres.

Despite these fascinating properties, aerogels have been little more than curiosities since they were invented in the early 1930s by Steve Kistler, at Stanford University in the United States. There have been some limited and applications, such as using them to trap sub-atomic particles at Cern, the European physics research centre in Geneva, and catching microsatellites - which burn up going through the atmosphere - by fixing aerogel tiles on the outside of spacecraft.

Applications have been limited because aerogels are difficult to produce. Evaporating the alcohol at high temperature and pressure is dangerous and time-consuming. At Airglass in Sweden, which produced the aerogel tiles used by Cern, an explosion during production destroyed an entire laboratory in 1984. The other shortcoming is that aerogels are extremely fragile.

Now, though, production has been made much safer by substituting alcohol with liquid carbondioxide, which is non- flammable and non-toxic. The German chemicals company BASF has overcome the fragility problem by producing aerogels in pellets rather than sheets.

But the real breakthrough has come from the US research organisation Battelle, which says it has a pilot plant for producing an aerogel called AMC. Battelle's aerogel expert, Sangeeta Ramamurthi, says the company is talking to potential partners about a full-size production plant.

AMC can be formed into sheets and has 'mechanical strength of an order of magnitude higher than aerogels produced by traditional methods' says Dr Ramamurthi. Battelle has also cut down processing time from up to 20 days to three to six hours.