Creating longer nanotubes

Materials World magazine
,
1 Jun 2007
Carbon nanotube array

The world’s longest array of aligned carbon nanotubes has been created, say researchers from the University of Cincinnati, USA.

Using a novel composite catalyst and optimal synthesis conditions, the team was able to grow the nanotubes to a length of 18mm – a full four millimeters longer than previous attempts.

Carbon nanotubes offer high mechanical, electrical and optical properties which, coupled with their excellent strength, has made them an intriguing area of research. However, attempts to grow them uniformly have had limited success.

‘Long [carbon nanotubes] are difficult to produce because the catalysts used to promote their synthesis [usually nickel, cobalt or iron] become deactivated and “poisonous” during growth,’ explains researcher Dr Vesselin Shanov. Deactivation occurs because excess carbon starts to accumulate on the catalyst during chemical vapour deposition (CVD). This acts like a curtain, preventing the surrounding carbon from reaching the growth zone.'

‘That is why the [nanotube] array grows fast initially, but with time, the growth rate slows exponentially,’ says Shanov.

But despite these inherent problems, the desire to grow elongated nanotubes remains strong. ‘Being in the millimetre or centimetre length [would mean] they can be handled easily without special tools such as powerful and expensive microscopes,’ he adds.

Shanov’s team found that ‘by using a novel catalyst to modify the tubes, the growth rate was dramatically increased and maintained over a long period of time’. Shanov says, ‘It took us about a year to develop and patent the new catalyst.’

Until this patenting process has been completed, the team is tight lipped about the catalyst’s structure and composition.

The process involves a multilayered substrate in which a catalyst made of alternating layers of metal and ceramic is formed on top of an oxidised silicon wafer. During CVD in a 750ºC furnace that maintains the optimum hydrogen/hydrocarbon/water/argon environment, the new catalyst is able to survive longer than other alternatives, thus promoting the growth of longer nanotubes.

These long nanotubes can exhibit different properties to their smaller counterparts, as their top layers have been exposed to the growth conditions for a greater period of time than the base layers. The research team practiced post-treatment of the arrays to equalise the quality, but will need to further monitor the progress as lengthier nanotubes are developed.

There are several possible applications for this technology. Spinning the nanotubes into threads could create fibres with extremely low-weight, high strength, and excellent thermal and electrical conductivity. These could replace carbon macrofibres in materials reinforcement.

Shanov also sees potential use in the hypothetical space elevator, where the spun carbon nanotubes could act as the tether connecting the Earth to the lift in space.