.A cheap, environmentally sound technique has been developed for producing nanotube ropes that could be used to make super-strong cables for such applications as a space elevator.
UK researchers Ya-Li Li, Ian Kinloch and Alan Windle from the University of Cambridge say that the "spinning" technique can generate ropes of any length and has already beaten the previous meter-long record.
"The present direct spinning process opens a novel way of one-step production of nanotube fibers, ribbons and coatings with potentially excellent properties," say the researchers.
Superior strength
Carbon nanotubes are tubular carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications.
They exhibit unusual strength and unique electrical properties, and are extremely efficient conductors of heat.
Of particular interest for the construction of super-strong cables is the high tensile strength of nanotubes—their ability to stretch without breaking or losing their shape.
Single nanotubes have been measured with a tensile strength 50 to 60 times greater than high-grade steel.
With bonds stronger than those in diamonds, nanotubes naturally align into threads that are held together by what's known as the van der Waals force, which is the same force that allows geckos to cling to smooth surfaces.
Under high pressures, nanotubes can merge, raising the possibility of creating strong, long ropes through high-pressure nanotube linking.
While such ropes likely wouldn't have the strength of a single nanotube, they would still be far stronger than anything available today.
They could be used for a wide range of applications, including for space elevators that require strong, light cables thousands of kilometers long.
Taking carbon for a spin
Many methods have been developed for creating short carbon nanotube fibers, but assembling carbon nanotubes into continuous ropes has been problematic.
While some researchers have successfully produced nanotube fibers by drawing or spinning them from gels and other solutions, such techniques have limited them to producing relatively short strands, and other techniques have involved such toxic chemicals as hexane—a common ingredient in gasoline.
Windle and colleagues have taken a different approach, winding freshly made nanotube fibers onto spinning rods straight out of a furnace.
First they inject ethanol mixed with ferrocene and thiophene into a hydrogen carrier gas in the hot zone of a furnace with temperatures between 1,050°C and 1,200°C.
Under these conditions, nanotubes form an aerogel that the researchers say looks like "elastic smoke." To produce ropes, they continuously draw the gel from the hot zone by winding it onto a rotating rod.
To switch from producing single- to multi-walled nanotubes—nanotubes nested in nanotubes—the researchers simply adjust the operating temperature and gas flow rates through the furnace.
"By mechanically drawing the carbon nanotubes directly from the gaseous reaction zone, we have found it possible to wind up continuous fiber without apparent limit to length," the researchers say. "If this new fiber can challenge conventional high performance fibers for properties, its vastly simpler method of production will commend it on both cost and environmental grounds."
In addition to spinning nanotube ropes, their technique can also be used to "spin coat" differently shaped scaffolds.
-By Gabe Romain
