“Initially we were looking for high-efficiency methods of producing carbon nanotubes that could be scaled up commercially,” Alan Windle told nanotechweb.org. “To this end, we focused on a vapour-phase synthesis route instead of involving substrates and surfaces. We then found that by thinking in terms of polymer technology we had the opportunity of withdrawing carbon nanotubes from the CVD [chemical vapour deposition] furnace as a continuous fibre and winding this up.”

The starting material for generating the nanotubes was ethanol containing 0.23 to 2.3 wt% ferrocene, and between 1.0 and 4.0 wt% thiophene. Windle and colleagues injected this solution from the top of a furnace into a hydrogen carrier gas. Iron nanoparticles catalysed the reaction: nanotubes grew in the furnace’s hot zone at a temperature of 1050-1200 °C. The nanotubes formed an aerogel, which the scientists say appeared as “elastic smoke”.

The team found that they could wind the aerogel onto a rotating rod, creating either a single fibre or a continuous aligned film. The researchers were also able to control the proportions of multiwalled and single-walled nanotubes by altering the thiophene concentration and adjusting the hydrogen flow rate. Multiwalled nanotubes formed for a thiophene concentration of 1.5 to 4 wt% in ethanol, a hydrogen flow rate of 400-800 ml/min and a synthesis temperature of 1100-1180 °C. Single-walled nanotubes, on the other hand, formed when the thiophene concentration was around 0.5 wt%, the hydrogen flow rate was about 1200 ml/min and the temperature was up to 1200 °C.

The multiwalled nanotubes were about 30 nm in diameter and roughly 30 µm long, while the single-walled tubes had diameters of 1.6-3.5 nm and were grouped into bundles about 30 nm across. Windle and colleagues also coated a square-shaped wire frame with an aligned nanotube film by rotating it normal to the furnace axis. Then they infiltrated the nanotubes with a resin, making a composite shell that they were able to remove from the wire former.

The scientists believe the technique is an efficient way of making bulk quantities of carbon nanotubes. In fact they reckon nanotubes made in this way could ultimately become a low-cost substitute for carbon fibres. The method could also produce non-woven mat-type materials for gas filtering, or carbon nanotube-reinforced sheet material by back diffusion of molten polymers or polymer solutions. What’s more, the technique may be applicable to other fibres that can be synthesized directly from the gas phase.

Now, Windle and colleagues plan to collaborate with industry to scale up the process and develop materials applications. They also want to optimize the synthesis and properties of the product, as well as improving their basic understanding of the rapid synthesis reaction.

The researchers reported their work in Sciencexpress.