A team of MIT researchers have found a way to predict the shapes into which carbon nanotubes form.

Up until recently, when carbon nanotubes (CNTs) were synthesized, it was impossible to know for certain which shape they would take. This is a problem because industry needs high-quality CNTs that are straight and stiff, and current production methods often left suppliers with wavy, flexible tubes. The breakthrough from MIT will make carbon nanotube production a more reliable science, reducing costs, improving quality, and increasing CNTs industrial versatility.

One of the most economical and reliable ways to synthesize CNTs is by chemical vapor deposition (CVD). But while this process provides affordable CNTs in sufficient quantities, the products are often too sparsely placed and/or too compliant. This reduces their value, as they are of insufficient quality for most applications.

Applying and then evaporating drops of liquid (such as acetone) on the tubes is a simple and cost-effective way to pack the CNTs together more tightly and simultaneously increase their stiffness.

Aligned carbon nanotubes grown by chemical vapor deposition are typically wavy, as seen in side view at center of illustration, rather than straight, as illustrated in a single nanotube at right. They also settle into somewhat random patterns, as shown in box at upper left. Waviness reduces the stiffness of CNT arrays by up to 100,000 times, but their stiffness can be increased by densifying, or compressing, the nanotube clusters from two different directions

This latest study has revealed what the MIT website describes as, “a systematic method to predict the two-dimensional patterns CNT arrays form after they are packed together, or densified, by evaporating drops of either acetone or ethanol. CNT cell size and wall stiffness grow proportionally with cell height.”

Put in layman’s terms the scientific journal Phys.org suggests that, “One way to think of this CNT behavior is to imagine how entangled fibers such as wet hair or spaghetti collectively reinforce each other. The larger this entangled region is, the higher its resistance to bending will be. Similarly, longer CNTs can better reinforce one another in a cell wall. The researchers also find that CNT binding strength to the base on which they are produced, in this case, silicon, makes an important contribution to predicting the cellular patterns that these CNTs will form.”

The team have named this method ‘capillary-mediated densification’ and have published a full analysis of the technique in the journal Physical Chemistry Chemical Physics. Here they also explain how the process, “could be used to inexpensively, quickly, and accurately design bulk nanofiber systems with tuned topologies that could enable next-generation processors, batteries, and lightweight structures.”

Effect of CNT–substrate adhesion, either baseline not cemented or cemented via post-processing, on CNT cell network formation. (a) Side view illustrations and top view SEM images of cell networks formed from ∼35 μm tall CNT arrays

“These findings are directly applicable to industry because when you use CVD, you get nanotubes that have curvature, randomness, and are wavy, and there is a great need for a method that can easily mitigate these defects without breaking the bank,” says Itai Stein, one of the study’s authors from MIT’s Department of Aeronautics and Astronautics.

“From our previous work on aligned carbon nanotubes and their composites, we learned that more tightly packing the CNTs is a highly effective way to engineer their properties,” adds senior author Prof. Brian Wardle. “The challenging part is to develop a facile way of doing this at scales that are relevant to commercial aircraft (hundreds of meters), and the predictive capabilities that we developed here are a large step in that direction.”

The significance of the researchers' progress was highlighted by the journal ScitechDaily, which reported how, “Earlier work in Wardle’s lab demonstrated that waviness reduces the stiffness of CNT arrays by as little as 100 times, and up to 100,000 times. The technical term for this stiffness, or ability to bend without breaking, is elastic modulus. Carbon nanotubes are from 1,000 to 10,000 times longer than they are thick, so they deform principally along their length.”

Capillary-mediated densification of aligned carbon nanotube arrays into 2D cellular networks. (a) Illustrations and scanning electron microscopy (SEM) images demonstrating the formation of cells from CNT arrays
Side view illustrations and top view SEM images of CNT cell networks showing that the height (h) of CNT arrays governs the width (w) and wall thickness (t) of the resulting CNT cell, all other factors being the same

Carbon nanotubes are highly sought after because of their electrical, thermal, and mechanical properties, which are directionally dependent. By allowing CNT producers to predict the shape, stiffness, and density of their product, the researchers have given a greater level of control to the nanotube industry. Lowering costs, improving quality, and increasing efficiency. It may also lead to more detailed command of the way that nanotubes are synthesized, removing volatility from the production process.

As co-author and MIT engineering student Ashley Kaiser says, “I think there is an underlying beauty in this nanofiber self-assembly and densification process, in addition to its practical applications. The CNTs densify so easily and quickly into patterns after simply being wet by a liquid. Being able to accurately quantify this behavior is exciting, as it may enable the design and manufacture of scalable nanomaterials.”

Photo credit: MIT