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Strong, Stretchable Carbon Nanotube Films Surpass Kevlar And Carbon Fiber

Nanomaterials: Simple new method yields films of highly aligned, densely packed nanotubes

by Prachi Patel
January 29, 2016

Credit: Nano Lett.
Spooling a cylinder of blown carbon nanotubes onto a rolling drum, researchers create a black film containing aligned, densely packed nanotubes (left). After being passed through a roller several times, the film becomes flatter and the nanotubes more densely packed (right). The film is exceptionally strong and ductile.

Carbon nanotubes are exceptionally strong and stretchy. To take advantage of these properties, scientists have been trying to make thin sheets from nanotubes that could be used as structural coatings for vehicle or aerospace parts or for protective military and sports gear. But nanotube films’ mechanical properties have so far come nowhere close to those of individual nanotubes. Researchers now report a simple fabrication method to make carbon nanotube films that are five times as strong as those made before—and stronger than films made from Kevlar or carbon fiber (Nano Lett. 2016, DOI: 10.1021/acs.nanolett.5b03863).

Blowing an ethanol solution through a heated tube with nitrogen gas results in a hollow cylinder made of carbon nanotubes floating out from the other end.
Credit: Nano Lett.

The new films have densely packed nanotubes, nearly all oriented parallel to each other, which give the films their superior strength, says Jian Nong Wang, a professor of mechanical and power engineering at East China University of Science & Technology. Many groups have tried to align and assemble nanotubes into films, typically by spraying or filtering suspensions of nanotubes onto a surface. But these techniques use short nanotubes and do not align the tubes well, so the films are weak.

Wang and his colleagues made nanotubes with a process akin to glass blowing: Using a stream of nitrogen gas, they injected ethanol, with a small amount of ferrocene and thiophene added as catalysts, into a 50-mm-wide horizontal tube placed in furnace at 1,150–1,130 °C. A hollow cylinder with walls made of aligned carbon nanotubes forms in the furnace and emerges from the other end of the tube, driven by the nitrogen. As the tube emerges, the researchers wind the floating carbon nanotube cylinder onto a rotating drum. As the drum spins, the hollow cylinder condenses and flattens into a two-layered, black, carbon nanotube film. Faster winding resulted in better nanotube alignment, the researchers found. Finally, they packed the nanotubes even more densely by pressing the film repeatedly between two rollers.

The resulting films had an average strength of 9.6 gigapascals. By comparison, the strength of nanotube films made so far has been around 2 GPa, while that for Kevlar fibers and commercially used carbon fibers is around 3.7 and 7 GPa, respectively. The films are four times as pliable as conventional carbon fibers, and can elongate by 8% on average as opposed to 2% for carbon fibers.

Wang says that in addition to their useful mechanical properties, the films have high electrical conductivity, which could make them useful as electrodes for wearable devices and as artificial muscles.

Yutaka Matsuo, a professor of chemistry at the University of Tokyo, says that the simplicity of this mechanical winding technique to align nanotubes and make ultra-strong films is notable. The technique also results in pure carbon films, whereas earlier, solution-based methods that press premade nanotubes into films require surfactants that contaminate the films.

This article has been translated into Chinese and can be found here.

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Frank Laiza (February 4, 2016 9:38 AM)
Functional concerns would be; is there any residual ethanol in the fiber (material needs to be flame retardant), is the strength reduced by exposure to water (current Kevlar strength is reduced by water or sweat), and what effect will elongation (8%) have on use in ballistic vests (sagging equipment?)?
Eric (February 9, 2016 12:22 PM)
Frank, as the ethanol is injected in a 1150C furnace with nitrogen stream it should be completely released in the process and merely used as a carrier processing solvent.

The article states that the resultant film is a pure carbon film, which should be hydrophobic and repeal water/sweat so no issues there and I would imagine the elongation of the material would be utilized for forming to shapes/supports and not result in sagging equipment.

As for flame retardant, without modification I wouldn't expect this to be retardant as pure carbon is essentially charcoal, which works very well on the BBQ.

Sounds pretty awesome though!
Sanford Kirksey (June 7, 2016 10:25 PM)
Is the material bio-compatible? I noted the potential use as an "artificial muscle" but I took this to refer to a robotics application. Also, is it potentially susceptible to oxidation (by, for instance, singlet oxygen or ozone)?

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