"Throughout the relatively brief history of carbon nanotube research, the creation of a usable nanotube fibre has been one of the ultimate goals," said John Fischer of the University of Pennsylvania. "Its applications are nearly limitless, from replacing copper wiring to creating super-strong fabrics to, as some have suggested, building the cable tethers that will allow space elevators to travel from the earth to orbit."

To make the fibres, Fischer and colleagues dispersed single-walled carbon nanotubes in 102% sulphuric acid. This created an aligned phase of positively charged nanotubes surrounded by acid anions, with around 8% nanotubes by weight. The team then extruded this dispersion through a capillary tube less than 125 µm in diameter into a coagulant bath containing either diethyl ether, 5 wt% aqueous sulphuric acid or water. Finally, the researchers heat-treated the fibres to remove water and residual acid.

The structure of the resulting carbon nanotube fibres depended on the coagulation conditions. Fibres spun into diethyl ether had a collapsed "dogbone" structure, while fibres spun into dilute sulphuric acid or water retained their circular shape and were more dense. All the fibres contained nanotube super-ropes about 200 to 600 nm in diameter, each composed of smaller ropes around 20 nm in diameter.

"We've got no impurities, our densities [for the water-coagulated fibres] are about 77% of what's theoretically possible, and we're confident that the strength and conductance will improve as we refine the heat treatment, spinning and other elements of production," said Richard Smalley of Rice University.

The fibres had a Young's modulus of 120 GPa, a tensile strength of 116 MPa, an electrical resistivity of around 0.2 milliohm cm and, for ether-coagulated fibres, a thermal conductivity of 21 W/K m. A high-temperature annealing treatment increased the electrical resistivity of the fibres by an order of magnitude.

The thermal and electrical conductivities of the fibres were similar to those of randomly oriented and aligned mats of single-walled nanotubes. They were, however, higher than the values for single-walled carbon nanotube fibres made by production processes involving polymers.

"Single-walled carbon nanotubes are predicted to have tensile strengths many times that of Zylon or Kevlar, based upon the much greater theoretical strength of the single molecule carbon nanotube," said Wade Adams, director of Rice's Center for Nanoscale Science and Technology. "However, unlike Zylon and Kevlar, single-walled carbon nanotubes are also excellent conductors of electricity and heat. This unique multifunctionality makes them candidates for many critical applications beyond structural ones."

The researchers reported their work in Science.