Nanotube sheets come of age
Clear, conductive sheets produced at high speed.
They're soft, strong, and very, very long.
Large, transparent sheets of carbon nanotubes can now be produced at lightning speed. The new technique should allow the nanotubes to be used in commercial devices from heated car windows to flexible television screens.
"Rarely is a processing advance so elegantly simple that rapid commercialization seems possible," says Ray Baughman, a chemist from the University of Texas at Dallas, whose team unveils the ribbon in this week's Science1.
Nanotubes are tiny cylinders of carbon atoms measuring just billionths of a metre across. They are light, strong, and conductive. But for years their promise has outweighed their utility, because the complicated processes involved in making devices from nanotubes were too slow and expensive to be used in large-scale manufacturing.
But now, nanotubes have gone into warp drive. Baughman's team can churn out up to ten metres of nanoribbon every minute, as easily as pulling a strip of sticky tape from a reel ( see video ). This ribbon can be up to five centimetres wide, and after a simple wash in ethanol compacts to just 50 nanometres thick, making it 2,000 times thinner than a piece of paper.
The ribbons are transparent, flexible, and conduct electricity. Weight for weight, they are stronger than steel sheets, yet a square kilometre of the material would weigh only 30 kilograms. "This is basically a new material," says Baughman.
Scientists have been weaving carbon nanotubes into fibres and sheets for several years (see ' Yarn spun from nanotubes' ). But until now, the most common way of making large sheets of nanotubes relied on a labour-intensive technique much the same as that used by the ancient Egyptians to make papyrus. Nanotubes suspended in a solvent were slowly filtered to create a mat, which was then dried and peeled off the filter.
Baughman's team instead start with a 'forest' of half-millimetre-long nanotubes sticking upright on an iron-based platform. Pulling gently from the edge of the forest with an adhesive strip, such as a Post-It note, uproots a row containing millions of nanotubes. As these nanotubes pull out, they tangle with the next row, and so on.
The nanotubes tangle together just enough to keep a ribbon growing, without jumbling up into a huge ball. "They've found the magic spot," says Ian Kinloch, a materials scientist at the University of Cambridge. "A lot of people will now try this out with a Post-It in their own labs." The team says a one-centimetre-long forest of nanotubes can produce three metres of nanoribbon.
The researchers had previously used a similar method to draw strings of nanotubes from a forest2. Getting them to knit into a wider fabric is a bit trickier, but Baughman says that scaling the work up to produce large sheets will now be "easily do-able".
Nanotubes are already replacing graphite in certain commercial devices such as batteries. But this technique could now propel many more nanotube products into the marketplace, agrees Kinloch.
The team has already proved the sheets' usefulness in several applications, filing patents as they go. They have sandwiched a nanoribbon between two Plexiglass plates, for example, using the heat of a domestic microwave oven to weld the layers. This forms a transparent, conductive sheet ideal for a heated car window, they say.
And since bending does not change the electrical properties of the nanotubes they could be used to carry current in a 'rollable TV screen', something that has long been promised by nanotechnologists.
"Things move quickly if you can prove that the supply of the material is good," says Baughman.
- Zhang M., et al. Science, 309. 1215 - 1219 (2005).
- Zhang M., Atkinson K. R. & Baughman R. H. Science, 306. 1358 - 1361 (2004).
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