Cross-linked nanotubes make super-strong web (scale: 5µm across).© Nature Materials

By sandwiching tiny but super-tough carbon nanotubes between layers of polymer, researchers have created a revolutionary material that is six times stronger than conventional carbon-fibre composites and as hard as some ultrahard ceramic materials used in engineering¹.

An international team led by Nicholas Kotov of Oklahoma State University in Stillwater say their new material could be used in space engineering or for long-lasting medical implants. Because the composite is completely organic (carbon-based), it is as lightweight as traditional carbon-fibre materials.

Carbon nanotubes are single molecules: hollow cylindrical tubes made of a web of carbon atoms, just a few nanometres (millionths of a millimetre) across and several thousand nanometres long.

Ever since their discovery in 1991, researchers hoped that carbon nanotubes would become the ultimate carbon fibres. Tests on individual tubes show that they are far stronger and stiffer than those used to make carbon-fibre tennis-rackets or racing cars. But incorporating them into fibre-composite materials has proved tricky.

Usually, fibres are embedded within a solid 'matrix', such as the polymer resins used for fibreglass. But when nanotubes are mixed with polymers they tend to form useless clumps. If fewer nanotubes are used, the mixing is better but the composites are weak.

Kotov's nanotube composites are built by stacking single-molecule layers of nanotubes and polymer on top of each other. By dipping alternately into nanotubes dispersed in water and into a solution of the polymer, one layer of nanotubes or of polymer molecules sticks to the surface.

The layered composite is made even stronger by attaching chemical groups to the nanotubes; these form bonds with the polymer when the material is heated, or treated chemically. If the strengthening step is done after each layer is added, the composite's components become crosslinked into a robust, three-dimensional web, Kotov's team show.

The final composites contain 50% nanotubes. Tests indicate that they are about as strong as materials such as silicon carbide and tantalum carbide, which are used to make super-strong components for cutting-tools, jet engines and aerospace applications.

Though laborious, the dipping method is cheap-so it wouldn't be hard to do on an industrial scale. But carbon nanotubes are still expensive to produce, and several teams are looking for production methods that would be viable on a commercial scale. 

• References

  1. Mamedov, A. A. et al. Molecular design of strong single-wall carbon nanotube/polyelectrolyte multilayer composites. Nature Materials, advance online publication (2002). | PubMed |