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).
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.
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