Nanotubes hang toughTiny
nanotubes form super-tough material when glued
make super-strong web (scale: 5µm
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
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 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.