A carbon nanotube is tough—by some measures, more than 30 times more robust than Kevlar. As they're only a few atoms thick, however, that toughness isn't especially useful. Attempts have been made to bundle them together, but nothing has worked out especially well; the individual nanotubes are typically short, and it's difficult to get them to all line up in the same direction. As a result, these attempts have resulted in bundles that are filled with structural defects, often perform worse than Kevlar, and are only a few micrometers long.

Now, a group at Beijing's Tsinghua University seems to have found a way around many of these problems. It was able to synthesize nanotubes that are centimeters long and bundle them together to make a fiber that's nearly as strong as an individual nanotube. It's not quite time to start booking rides on a space elevator, but this work at least hints that nanotubes might eventually break out of the realm of the microscopic.

Go long

The biggest problem with assembling nanotubes into a useful fiber is the length of the individual nanotubes. It's what keeps the fibers short, and the loose ends probably contribute to the defects that weaken the end product. So the first step in building better ones was finding a way to make longer carbon nanotubes in the first place. This was accomplished through a variant of a standard technique called chemical vapor deposition, in which the reactants that generate the nanotube are present in the atmosphere of the reaction chamber. In this case, the researchers flow the reactants through the chamber in a single direction, and the nanotubes grow along the same direction as that flow.

This process produced a population of carbon nanotubes that could extend up to several centimeters in length. Tests showed a tensile strength of 120 GigaPascals, indicating the nanotubes were free of imperfections.

Measuring toughness

"Strong" isn't exactly a well-defined physical quantity. And there can be different types of toughness—something that holds up to a strong blow might deform easily when placed under constant strain. When talking about things like fibers and cables, the relevant quantity is called tensile strength, which is a measure of how much force you can apply to stretch something before it snaps. "Strong" isn't exactly a well-defined physical quantity. And there can be different types of toughness—something that holds up to a strong blow might deform easily when placed under constant strain. When talking about things like fibers and cables, the relevant quantity is called tensile strength, which is a measure of how much force you can apply to stretch something before it snaps. Tensile strength is measured in Pascals, the same unit that is used to quantify pressure. High-strength steel cables can reach roughly 2,000 MegaPascals. But a carbon nanotube has a tensile strength of 100 GigaPascals, or 50 times tougher. These figures are maximum values; there's also an engineering tensile strength that registers a more typical performance, usually somewhat lower. For example, one sample measured in this paper had a tensile strength of 80 GigaPascals, but its engineering tensile strength was 43 GigaPascals.

The next issue was bundling the tubes up, but the researchers were able to use a similar approach to solving this problem. They continued to flow gas over the nanotubes but narrowed the chamber on the downwind side, creating a channel that forced the nanotubes together. Once pressed together, basic chemical interactions called Van der Waals forces held them in place as a bundle.

Unfortunately, they were also noticeably weaker than individual nanotubes. As more nanotubes got incorporated into the bundle, the tensile stress at failure dropped, bottoming out at somewhere around 50 GigaPascals, or less than half the strength of an individual nanotube. What went wrong?

The authors got a hint by tracking the strain of individual bundles. In a single nanotube, the strain would build up until the tube snapped, at which point the strain dropped to zero. But for the bundles, the strain would build, drop to some intermediate level, and start building again. The authors concluded that the nanotubes in the bundles weren't aligned along their lengths, so there were some that bulged out a bit and others that were shorter. As a result, putting the bundle under stress put strain on the shorter ones, while the longer ones just sat in reserve. When the short ones snapped, some of the longer ones took up the strain. There was never a point where the entire bundle was distributing the stress.

Fortunately, this experiment also showed them how to fix the problem. The forces holding the bundle together aren't especially strong, and it should be possible to shift individual nanotubes around within the bundle without breaking anything. To do so, the researchers simply put the bundle through a cyclic stress-relaxation cycle, which they reasoned should cause some internal rearrangements. This process got the tensile strength back up to 80 GigaPascals—not the full strength of an individual nanotube but much better than it had been. And it's about 25 times the strength of Kevlar and five times higher than the best existing engineering fiber.

While the authors note that this work could find a home in "sports equipment, ballistic armour, aeronautics, astronautics and even space elevators," we're still a long way from any of that. Ideally, rather than synthesizing the nanotubes in centimeter-long chunks, we'd like to have some sort of continual production process. Still, the work is important in that it hints that there is a world beyond micrometer-scale nanotube fragments.

Nature Nanotechnology, 2018. DOI: 10.1038/s41565-018-0141-z (About DOIs).

Correction: use of the term toughness de-emphasized to avoid confusion with the engineering term.