However, while chemical vapor deposition (CVD) growth of CNTs on CFs can improve the apparent shear strength between fibers and polymer matrices by 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.

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 >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.

—Steiner et al.

Applying their discoveries, the researchers coated carbon fibers with nanotubes without causing fiber degradation, making the fibers twice as strong as previous nanotube-coated fibers. The researchers say the techniques can easily be integrated into current fiber-manufacturing processes.

To understand how carbon fibers are manufactured, the group visited carbon-fiber production plants in Japan, Germany and Tennessee. One aspect of the fiber-manufacturing process stood out: During manufacturing, fibers are stretched to near their breaking point as they are heated to high temperatures. In contrast, researchers who have tried to grow nanotubes on carbon fibers in the lab typically do not use tension in their fabrication processes.

Using a small-scale apparatus made of graphite, the researchers strung individual carbon fibers across the device 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 such as iron, but researchers have hypothesized that such catalysts might also be the source of fiber degradation. In their experiments, however, they found that the catalyst only contributed to about 15% of the fiber’s degradation.

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. They then devised 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 has hydrophilic and hydrophobic components. The hydrophobic feature sticks to the carbon fiber, while the hydrophilic component attracts the alumina and the metal catalyst.

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

• For the second strategy, 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 °C cooler than is typically used—avoiding damage to the underlying fiber.

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.

The researchers have filed a patent for the two strategies, and envision advanced fiber composites incorporating their techniques for a whole range of applications.

This study provides, for the first time, viable pathways for growing CNTs on carbon fibers suitable for advanced composites applications without compromising in-plane properties. Hierarchical carbon fibers produced through these approaches may also find application as electrodes for batteries, supercapacitors, and structures that double as energy-storing devices.

We note that while unsized fibers such as those used in this study may provide a more ideal surface for the application of functional coatings than sized fibers, current commercial manufacture of carbon fibers relies on sizings for many aspects of processing and handling, and many resin systems leverage carbon fiber sizings for fiber-matrix bonding. This said, the approaches demonstrated in this work could easily be extended to sized fibers by appropriately tailoring the polyelectrolyte and sizing chemistries to allow for noncovalent functionalization of sized carbon fiber surfaces. Alternatively, sized fibers could be de-sized (i.e., the sizing could be removed) prior to CNT growth via solvent treatment or thermal treatment in air or inert atmosphere.

Hierarchical carbon fibers offer numerous processing advantages over sized carbon fibers, however, including the ability to wick resins into the fiber via capillarity-driven wetting as well as greatly enhanced interfacial area for bond formation, suggesting that hierarchical carbon fibers such as those produced in this work may ultimately displace the need for sizings altogether.

—Steiner et al.

Resources

• Stephen A. Steiner, III, Richard Li, and Brian L. Wardle (2013) Circumventing the Mechanochemical Origins of Strength Loss in the Synthesis of Hierarchical Carbon Fibers. ACS Applied Materials & Interfaces doi: 10.1021/am4006385