The CNTs wound onto a round bar – the black ribbon at the top is combed so that it reflects light better.
The CNTs wound onto a round bar – the black ribbon at the top is combed so that it reflects light better.

Scientists from North Carolina State University and the Suzhou Institute of Nano-Science and Nano-Biotics in China have developed a new and relatively inexpensive way of aligning carbon nanotubes (CNTs) based on a technique called ‘microcombing’. Their simple process produces large and pure CNT films (or ‘buckypaper’) that are also stronger and more conductive than previously achieved. The novel approach could be scaled-up to offer superstrong and conductive CNT films with potential applications in aerospace and electronics.

While working on superstrong CNT composites, the team found that CNT waviness and misalignment is a critical factor limiting the strength of CNT films. This led them to try a combing processing similar to that used on cotton before it is spun in traditional textile industries. As reported in the journal Small [Zhang et al. Small (2015) DOI: 10.1002/smll.201500111], they grew CNTs on a conventional substrate in a closely packed array. As the CNTs become tangled, an end of the array was pulled to attain a continuous ribbon of only a few nanometers thickness, before a spool wound the CNT ribbon between a couple of surgical blades with tiny fissures on the blade edges. It is these micrometer-scale fissures that create a microcomb that pulls the CNTs into alignment in the same way as combing straightens out unruly hair, and which works to reduce any structural defects within the film and enhance the nanotube packing density.

"this proves that it is important to straighten CNTs at micrometer scale to produce high strength and high conductivity."Yuntian Zhu

An alcohol solution was applied as the ribbon is wound onto the spool, drawing the CNTs closer together and making the bonds between them even stronger. The CNT ribbon also wraps around itself as it goes onto the spool, producing layered films of pure CNT the thickness of which can be controlled through altering the number of layers. The research demonstrated that films from this technique, which are a meter wide, have more than double the tensile strength of uncombed CNT films, as well as 80% higher electrical conductivity.

The work proves how vital it is to straighten CNTs at the micrometer scale to produce high strength and conductivity. However, as corresponding author, Yuntian Zhu, points out, “This is a significant advance, but we want to find ways to make CNT alignment even straighter”. They now intend to incorporate a polymer matrix such as epoxy and BMI to produce CNT composites that could be even stronger, as well as looking to design special micro-combs that allow for more effective combing.