MIT devises techniques to preserve carbon fibers' strength

The group then grew carbon nanotubes on the fibers, first covering the fibers with a special set of coatings, and then heating the fibers in a furnace. They then used chemical vapor deposition to grow a fuzzy layer of nanotubes along each fiber.

To get nanotubes to grow, the fiber typically needs to be coated with a metal catalyst like iron, but researchers have hypothesized that such catalysts might also be the source of fiber degradation. In their experiments, however, Steiner and Li found that the catalyst only contributed to about 15 percent of the fiber’s degradation.

“When we got to the nitty-gritty of it, we found that the metal catalyst, the perceived culprit, turned out to be more of an accomplice,” Steiner says. “We could see it did a little damage, but it wasn’t the thing really killing everything.”

Instead, the group found, after further experiments, that the majority of fiber degradation was due to a previously unidentified mechanochemical phenomenon arising from a lack of tension when carbon fibers are heated above a certain temperature.

Hair conditioner in reverse

After identifying the causes of fiber degradation, the researchers came up with two practical strategies for growing nanotubes on carbon fiber that preserve fiber strength.

First, the team coated the carbon fiber with a layer of alumina ceramic to “disguise” it, enabling the iron catalyst to stick to the fiber without degrading it. The solution, however, came with another challenge: the layer of alumina kept flaking off.

To keep the alumina in place, the team developed a polymer coating called K-PSMA — which, as Steiner describes it, works like hair conditioner in reverse. Hair conditioners have two seemingly opposite chemical features: a water-absorbent component that allows the conditioner to stick to hair, and a waterproof component that keeps hair from getting frizzy.

Likewise, K-PSMA has hydrophilic and hydrophobic components, but its waterproof feature sticks to the carbon fiber, while the water-absorbent component attracts the alumina and the metal catalyst.

In their experiments, the researchers found the coating allowed the alumina and metal catalyst to stick, without having to add other processes, like pre-etching the fiber surface. The team placed the coated fibers under tension, and successfully grew nanotubes without damaging the fiber.