Carbon fibers coated with carbon nanotubes have been made twice as strong

MIT researchers have produced carbon fibers coated in carbon nanotubes without degrading the underlying fiber’s strength. The engineered fibers may be woven into composites to make stronger, lighter airplane parts.

The researchers coated carbon fibers with nanotubes without causing fiber degradation, making the fibers twice as strong as previous nanotube-coated fibers — paving the way for carbon-fiber composites that are not only stronger, but also more electrically conductive. The researchers say the techniques can easily be integrated into current fiber-manufacturing processes.

Applied Materials and Interfaces – Circumventing the Mechanochemical Origins of Strength Loss in the Synthesis of Hierarchical Carbon Fibers

Hierarchical carbon fibers (CFs) sheathed with radial arrays of carbon nanotubes (CNTs) are promising candidates for improving the intra- and interlaminar properties of advanced fiber-reinforced composites (e.g., graphite/epoxy) and for high-surface-area electrodes for battery and supercapacitor architectures. While CVD growth of CNTs on CFs has been previously shown to improve the apparent shear strength between fibers and polymer matrices (up to 60%), this has to date been achieved only at the expense of significant reductions in tensile strength (30–50%) and stiffness (10–20%) of the underlying fiber. Here we demonstrate two approaches for growing aligned and unaligned CNTs on CFs that enable preservation of fiber strength and stiffness. We observe that CVD-induced reduction of fiber strength and stiffness is primarily attributable to mechanochemical reorganization of the underlying fiber when heated untensioned above 550 °C in both hydrocarbon-containing and inert atmospheres. We show that tensioning fibers to ≥12% of tensile strength during CVD enables aligned CNT growth while simultaneously preserving fiber strength and stiffness even at growth temperatures over 700 °C. We also show that CNT growth employing CO2/acetylene at 480 °C without tensioning—below the identified critical strength-loss temperature—preserves fiber strength. These results highlight previously unidentified mechanisms underlying synthesis of hierarchical CFs and demonstrate scalable, facile methods for doing so.

The researchers strung individual carbon fibers — each 10 times thinner than a human hair — across the device, much like the strings of a guitar, and hung tiny weights on either end of each fiber, pulling them taut. 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.

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.

For the group’s second strategy, Steiner observed that it may be possible to eliminate the need for tension by reducing the temperature of nanotube growth. Using a recently discovered nanotube-growth process together with K-PSMA, the team demonstrated it is possible to grow nanotubes at a much lower temperature — nearly 300 degrees Celsius cooler than is typically used — avoiding damage to the underlying fiber,.

“This process reduces not only the amount of energy and volume of gas required, but the amount of extraneous substances you have to put on the fiber,” Steiner says. “It’s actually pretty simple and cost-effective.”

Milo Shaffer, a professor of materials chemistry at Imperial College, London, says the group’s carbon-fiber techniques may be useful in designing composites for use in electrodes and air filters. A next step toward this goal, he says, is to make sure the fiber’s various layers and coatings stay in place.

“This result indicates an important factor to be incorporated in future ‘hairy carbon fiber’ developments,” says Shaffer, who did not contribute to the research. “The effect of the various coating combinations on [nanotube] attachment, and the eventual — and critical — fiber-matrix adhesion in composites, remains to be explored.”

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