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Spongy carbon tubes take the strain

Somewhere between carbon fibres and carbon nanotubes, a new material shows its strengths.

The colossal carbon tube is stronger and lighter than a conventional carbon fibre. Credit: PRL/ H. Peng et al

A new form of carbon material, potentially lighter and stronger than conventional carbon fibres, has been discovered by researchers in China and the United States.

Huisheng Peng of Tongji University in Shanghai and his colleagues have found that a carbon vapour, made by heating ethylene and paraffin oil, will condense into tubes of pure carbon tens of micrometres wide and up to several centimetres long1.

Individual tubes have a tensile strength greater than that of conventional carbon fibres. What's more, the tubes don't simply snap when pulled, but show ductile behaviour, similar to metal wires.

Bulletproof theories

The team says that the carbon fibres might be used for making high-strength fabrics for body armour and engineering. Their measurements suggest that it may perform better than Kevlar fibres currently used for super-strong cables and bulletproof jackets.

"This is a new form of carbon that was unexpected to me," says Mildred Dresselhaus, a specialist on carbon technology at the Massachusetts Institute of Technology, Cambridge.

The walls of the tubes, typically about 1.4 micrometres thick, have a foam-like structure, consisting of hollow, rectangular compartments sandwiched between smooth inner and outer cylindrical shells of graphite-like carbon. It's a surprising structure for a carbon fibre, says Dresselhaus.

The spongy structure, coupled to the intrinsic lightness of carbon, gives the tubes an extremely low density. A sugar-cube-sized lump (1 cubic centimetre) has a mass of just 0.1 gram, compared with a typical mass of about 2 grams for a comparable amount of conventional carbon fibres.

Colossal strength

Porous forms of carbon have been made before, but not in this fibrous form. And they have previously all been rather weak, says Dresselhaus. The porous tubes also conduct electricity rather well, suggesting that they could find uses in flexible 'textile electronics'.

The team call these structures colossal carbon tubes, because they are immense compared with carbon nanotubes. Since their discovery in 1991, carbon nanotubes have been lauded for their potential strength.

But attempts to turn nanotubes into fibres and composites have so far failed to capitalize on the superior properties of individual tubes: large-scale fibres show only a fraction of the expected strength. Yet single colossal carbon tubes already show tensile strengths slightly greater than the best traditional carbon fibres.

The researchers don't yet know how the carbon atoms organize themselves into such complicated structures. But they suggest that the spongy walls form first as flat sheets, which then gradually curl up and ultimately meet edge to edge to form cylinders.

References

  1. Peng, H. et al. Phys. Rev. Lett. 101, 145501 (2008).

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Ball, P. Spongy carbon tubes take the strain. Nature (2008). https://doi.org/10.1038/news.2008.1164

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