Nanofibers yield stronger, tougher carbon fiber composites

Researchers at the U.S. Department of Energy (DOE)’s Oak Ridge National Laboratory (ORNL) have developed an innovative new technique using carbon nanofibers to enhance binding in carbon fiber and other fiber-reinforced polymer composites—an advance likely to improve structural materials for automobiles, airplanes and other applications that require lightweight and strong materials.

The results, published in the journal Advanced Functional Materials, show promise for making products that are stronger and more affordable, opening new options for U.S. manufacturers to use carbon fiber in applications such as energy and national security.

“The challenge of improving adhesion between carbon fibers and the polymer matrix that surrounds them has been a concern in industry for some time, and a lot of research has gone into different approaches,” said Sumit Gupta, the ORNL researcher who led the project. “What we found is that a hybrid technique using carbon nanofibers to create chemical and mechanical bonding yields excellent results.”

Carbon fiber is a type of composite in which strands of pure carbon are embedded in a polymer matrix, much like rebar is embedded in concrete, making the resulting material stronger and lighter than steel. The challenge is that the matrix polymer does not cling strongly enough to the carbon fiber, reducing the performance of the composite material. To improve the fiber-matrix interfacial bond, industry has tried texturing the exterior of the fibers or injecting chemicals into the process—with limited success.

The ORNL approach combines both mechanical and chemical bonding to yield a 50% improvement in tensile strength and a nearly two-fold increase in toughness, essentially the durability of the material, through use of carefully tailored nanofibers.

“We developed this process in 2023 but have been focused lately on optimizing it and fully understanding the physical processes that enable these improvements,” said ORNL researcher Chris Bowland. “We found that by carefully controlling multiple variables, we can create nanofibers that greatly enhance the performance of carbon fiber composites and potentially other types of composites.”

The key to the improvements is an innovative technique known as electrospinning in which a carbon fiber precursor, polyacrylonitrile, is extruded into fibers, much like a spider extrudes silk from its abdomen. The polyacrylonitrile is extruded through a strong electric field to produce strands about 200 nanometers wide, or one-hundredth the width of a typical human hair. The strands land on a spinning metal drum overwrapped with carbon fiber fabric.

By varying the strength of the electric field, the speed of the drum and other factors, the researchers can create fibers that chemically bond to the matrix and mechanically bond to other carbon fibers, essentially creating “bridges” between the two dissimilar materials. The researchers were also able to control the types of chemical bonding and the orientation of the fibers by tweaking the electrospinning conditions.

The research team has applied for a patent on the technique and plans to seek out industrial partners to license the approach in hopes of improving the competitiveness of commercial carbon fiber composites, which are already used extensively in applications such as automobiles, aerospace and energy. They see potential for the reinforcing technique to open new applications for the use of carbon fiber, such as civil infrastructure or defense and security.

A key limiting factor to broader carbon fiber deployment is cost. By improving fiber adhesion, manufacturers can use less of the material and even use shorter carbon fibers, known as discontinuous fibers, that might otherwise have been discarded.

To ensure the new technique is as impactful and flexible as possible, the team wanted to deeply understand the forces at play at the most fundamental levels. They first turned to ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science user facility, and its vast array of characterization and imaging tools.

These tools allowed the researchers to see what was happening at the sub-micron level. They also used techniques such as X-ray scattering and nuclear magnetic resonance (NMR) imaging to understand how the fibers and matrix interact.

Finally, they accessed the Frontier supercomputer at the Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility, to fully model and simulate how the fibers form and interact with the matrix.

“The characterization and computational science really required the resources of a place like ORNL,” Gupta said. “By accessing expertise and capabilities from across the lab, we gained a deeper understanding of this technique, along with the ability to improve it and make it more flexible for industry to use in multiple applications.”

The research team plans to continue refining the electrospinning technique to provide greater control and better results while exploring potential applications for other fiber-reinforced composites. Ongoing research is looking into integrating the new technique with prior research on developing self-sensing composites that can monitor their own health through embedded particles of semiconducting or piezoelectric materials.