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Posted: Jun 08, 2012

Ultrastrong, foldable, and highly conductive carbon nanotube film

(Nanowerk Spotlight) Carbon nanotubes (CNTs) show extraordinary multifunctionality, including excellent mechanical, electrical and thermal properties. To be used in practical applications, the CNTs need to be assembled into macroscale structures such as films, fibers and composites. Unfortunately, so far the properties of these macro-structures have been very poor, usually one to three orders of magnitude lower than that of individual CNTs.
"Low CNT structural perfection, alignment and intertube interaction are key issues that are responsible for such disappointing properties," Qingwen Li, a professor in the Key Laboratory of Nanodevices and Applications at Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, tells Nanowerk. "In our recent work we demonstrate a simple – free from dispersion limitation – efficient and scalable strategy to make CNT films ultra-strong and conductive, in which the CNTs are well aligned and densely packed by exploiting drawable CNT arrays as precursors."
Reporting their findings in the May 16, 2012 online edition of ACS Nano ("Ultrastrong, Foldable, and Highly Conductive Carbon Nanotube Film"), Li and her team fabricated ultrastrong, fold-crack-free CNT films by solid-state layer-by-layer assembly of few-walled CNT sheets.
aligned carbon nanotube film
(A) Setup for the preparation of the aligned CNT film. A CNT sheet from a spinnable nanotube array on a 4 in. silicon wafer was continuously wound on a rotating spindle and collected. (B) As-prepared CNT film on a spindle. (C) Flexible CNT film strip. (D) Photographs of a shining CNT strip and (E) strip coated with a 12 nm gold layer. Inset in (D) is the shape of a water droplet on the strip. (F) SEM image of a woven fabric consisting of CNT strips. (Reprinted with permission from American Chemical Society)
To prepare their aligned CNT film, the team drew an aligned and transparent CNT sheet from a spinnable nanotube array (grown by catalytic chemical vapor deposition) and then continuously wound onto a rotating spindle layer by layer to produce a seamless film roll.
"A nanotube array with a length of 1 centimeter can be continuously converted into a 10 meter long nanotube sheet at a drawing rate of 1 meter per minute," explains Li. "The thickness of the CNT film depends on the number of windings, and the width of the film is determined by the initial width of the nanotube sheet."
After the CNT film is taken off the spindle, it can simply be cut into strips of desired size with a sharp knife.
Compared to previous work, the researchers observed that:
– Fewer-walled CNTs outperform thick multi-walled CNTs for making the film stronger;
– CNT alignment and condensation work together to increase the strength;
– Aligned CNT films help provide efficient charge transport paths and therefore can be used as high-performance electrode under high-rate charging and discharging.
Li notes that such high-performance CNT films possess four essential features of CNTs: good alignment; long length; small diameter (few walls); and high packing density.
Also, with drawable CNT arrays, the CNTs can be manipulated free from dispersion and surface contamination by surfactants.
"Due to the unique structure, the films are very promising for many advanced applications, such as strong but lightweight composites, heat-conducting foils, flexible electrodes, or high-temperature abrasive and sealing materials," she says. "Moreover, considering the easy manipulation and scalability, we believe that this method can shed new light on the fabrication of novel carbon-based functional materials."
Compared with other CNT or graphene based assemblies, the super-mechanical properties of the team's CNT films indicate that the organization of CNTs with good alignment, less defects, and strong intertube interaction is critical to the best utilization of carbon nanotube's intrinsic properties at the macroscale.
In separate work, the researchers have proposed a similar fabrication strategy using drawable CNT arrays to make strong and conductive CNT/PVA and CNT/BMI composites (see papers in Composites Science and Technology: "Mechanical and electrical property improvement in CNT/Nylon composites through drawing and stretching" and in Carbon: "Producing superior composites by winding carbon nanotubes onto a mandrel under a poly(vinyl alcohol) spray").
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