Coming of age for 'future fibres'

By Paul Rincon
Science reporter, BBC News

Juan sits beneath the hot furnace to monitor the fibre-spinning process Super-strong body armour

In a subterranean laboratory in the centre of Cambridge, PhD student Juan Vilatela crouches beneath a furnace that is raging at a temperature of 1,300C.

Wearing white protective coveralls with a breathing mask, and sealed off behind a plexiglass-walled enclosure, he watches intently as a motorised wheel turning at several centimetres per second drags a fine black thread from the furnace and winds it around a reel.

Afterwards, Juan crawls out from the enclosure, pulls off his mask, takes a few gulps of air and grins.

"What we are dealing with is a single filament, with a diameter of around five microns. So being able to catch it and see it is quite a challenge," Juan explains over the whirr of machinery.

What we're all hoping to do is create the strongest fibre made yet Dr Marcello Motta, University of Cambridge

He jokes: "I think that's why I got this PhD, because I'm the only one who can see the fibre sometimes." This experimental rig built at the University of Cambridge's Department of Materials Science and Metallurgy is manufacturing a form of carbon fibre with remarkable properties and enormous potential.

One of the principal applications for this new material is in super-strong bullet-proof vests.

The material is already up to several times stronger, tougher and stiffer than fibres currently used to make protective armour.

The UK Ministry of Defence and the US army have already shown an interest in the work.

Varied applications

"There are a lot of military applications, uses in speciality products such as sports equipment, in transparent conductive films and in energy. You can go in many different directions," says Dr Anna Moisala, a member of the team working on the new fibre.

The experimental rig produces kilometres of fibre each day

Each fibre contains millions of carbon nanotubes entangled in a network.

Nanotubes are graphite - a common natural form of carbon. In graphite, carbon atoms are bonded in hexagonal structures to form flat layers stacked on top of one another like sheets of paper.

To make nanotubes, scientists essentially take individual graphite layers and fold them over so they join at either edge. This forms cylinders measuring just a few billionths of a metre across.

When pulled along their axes, individual nanotubes have extraordinary strength: "Probably 10 times greater than the strongest fibre we know of," explains Professor Alan Windle, head of the research group at Cambridge. In addition, the fibre is nearly as stiff as diamond.

"As building blocks, they are very promising indeed," he says.

Previously, scientists were only able to turn carbon nanotubes into fibres using post-processing techniques. Here, the nanotubes are created and spun into filaments in a single process.

"It is what is called in the trade a 'disruptive technology', meaning that it is not an extension of a process that is already used, rather, it is something that is totally 'left field'," says Alan Windle.

Keep it simple

The method is very simple but ingenious. A hydrocarbon feedstock, such as ethanol, hexane, methane or diesel, is injected into the furnace along with a small amount of iron-based catalyst called ferrocene.

The nanotube fibre is lightweight but very, very strong.

Inside the furnace, this feedstock is broken down into hydrogen and carbon. The carbon is then chemically "re-built" on particles of iron catalyst as long, thin-walled nanotubes.

"It makes particles of carbon that are like smoke. But because the nanotubes are entangled, the smoke we make is elastic," explains Professor Windle.

To the eye, this "elastic smoke" looks a bit like an ever-expanding dark "sock". To begin winding it up, a rod is inserted into the furnace from below to grab the nanotube sock and yank it down. This stretches the sock into a fine thread that can be wound up continuously.

The strength of the filament is improved by pulling on it, because this aligns the nanotubes in parallel along the fibre axis.

Gradual improvement

Further close packing of the nanotubes is encouraged by swathing it in a faint mist of acetone as it emerges from the furnace.

The team says they are excited about the material's potential

Team member Dr Marcello Motta told me: "What we're all hoping to do is create the strongest fibre made yet.

"It starts with a high temperature gas phase reaction, which gives you perfect control over the building blocks. That's a major advantage. If you have the need to improve your material, all you have to do is adjust the process, its balance, or chemistry to end up with better properties."

Alan Windle adds: "It's up there with the existing high performance fibres such as carbon fibre and Kevlar.

"We've seen bits that are much better than Kevlar in all respects. But this is a laboratory-scale fibre, and laboratory-scale fibre will always have good bits and bad bits because you have to work to get the defects out."

Easily woven

Unlike ordinary carbon fibre, it can be easily woven into fabric. For body armour, the strength of fibres in a fabric is a critical parameter. Strain-to-failure - in other words, how much the material can extend before it breaks - is another.

This fibre is strong yet lightweight and good at absorbing energy in the form of fragments travelling at very high velocity.

Pulling on the elastic "smoke" stretches it out into a fibre

Related applications include stronger containment surrounds for the fan blades of jet engines. In the event that a blade becomes detached, these prevent it from flying out of the engine and damaging the aircraft.

It could also be used to make bomb-proof refuse bins and in blast protection for the engines of tanks and other armoured vehicles.

Hi-tech "smart" fabrics may also be in the pipeline. Because the fibre conducts electricity, it could lead to the development of clothes that, for example, shimmer, light up, or even generate moving images.

The researchers can also wind the nanotube sock horizontally to make a film. This can then be embedded in plastic to create a composite. The transparent conductive films created in this way could be used for flat-panel displays and solar cells.

The nanotubes can also be wound horizontally into a thin film

The researchers are even hopeful their product could eventually provide an alternative to copper wire. Though they admit this is some way off yet.

Professor Windle explains: "Our fibre is beginning to show conductivities which, although still a long way short of copper, are showing potential for the transmission of electrical power and signals.

"If one could control the particular type of nanotube one is making, then the sky is the limit. Then one really could rival copper for power transmission. About 9-10% of all electrical energy is lost in transmission, so that would make huge global energy savings."

High performance

Dr Krzysztof Koziol, a member of the Cambridge team, explained: "We are looking for high mechanical performance, but also quite high electrical conductivity and high thermal conductivity.

"Some materials are only good for their mechanical properties, others for their electrical properties. We are trying to put all three together to make a material that is good in many different respects."

The films can be dipped in plastic to make a composite material.

One inevitable question being asked of this material is what its cost will be: "The answer to that is there is no way of knowing until someone has built a plant and the accountants have got to work and decided what they can charge for their product," Professor Windle says.

He adds: "Our process is intrinsically very cheap. The interesting thing about the process is that it has much more in common with that used to make carbon black - which is a very cheap form of carbon - than it has in common with the current process for making carbon fibre. Carbon black is about 100 times cheaper than carbon fibre."

The researchers are seeking funds to investigate whether the method can be upgraded from a laboratory to an industrial process.

"Although we might be making several kilometres a day, it's a very thin fibre, weighing much less than a gram," says Alan Windle.

"We have to scale this up to make kilograms. Before anyone will commit the huge investment needed to build a full-scale plant, they need to know that it will make a good bullet-proof vest."

[e-mail]