Scientists at the Department of Energy’s Oak Ridge National Laboratory have invented a roll-to-roll process to coat electrically conductive carbon fibres with semiconducting silicon carbide nanoparticles. The nanomaterial-embedded composite is stronger than fibre-reinforced composites and has a new feature – capability to monitor its own structural health.

The process has been developed by Chris Bowland, a Wigner Fellow at ORNL and Amit Naskar, leader of ORNL’s Carbon and Composites Group.

When enough coated fibre is embedded in a polymer, the fibres create an electrical network and the bulk composite becomes electrically conductive. The semiconducting nanoparticles can disrupt this electrical conductivity in response to applied forces, adding an electromechanical functionality to the composite.

If the composite is strained, the connectivity of the coated fibres is disrupted and the electrical resistance in the material changes. Should storm turbulence cause a composite airplane wing to flex, an electrical signal may warn the plane’s computer that the wing has endured excessive stress and prompt a recommendation for an inspection.

Carbon fibre composites—lightweight and strong—are great structural materials for automobiles, aircraft and other transportation vehicles. They consist of a polymer matrix, such as epoxy, into which reinforcing carbon fibres have been embedded. Because of differences in the mechanical properties of these two materials, the fibres can detach from the matrix under excessive stresses or fatigue. That means damage in carbon fibre composite structures can remain hidden below the surface, undetectable by visual inspection, potentially leading to catastrophic failure.

“Carbon fibre composites fail catastrophically, so you won’t see damage until the entire structure has failed,” said Bowland. “By knowing what’s going on within the composite, you can better judge its health and know if there is damage that needs to be repaired.”

ORNL’s roll-to-roll demonstration proved in principle that the method could be scaled up for high-volume production of coated fibres for next-generation composites. Self-sensing composites, perhaps made with a renewable polymer matrix and low-cost carbon fibres, could find themselves in ubiquitous products, even including 3D-printed vehicles and buildings.

To fabricate nanoparticle-embedded fibres, the researchers loaded spools of high-performance carbon fibre onto rollers that dipped the fibre in epoxy loaded with commercially available nanoparticles about the width of a virus (45–65 nanometers). The fibre was then dried in an oven to set its coating.

To test the strength with which nanoparticle-embedded fibres adhered to the polymer matrix, the researchers made fibre-reinforced composite beams with the fibres aligned in one direction. Bowland conducted stress tests in which both ends of this cantilever were fixed while a machine assessing mechanical performance pushed on the beam’s middle until it failed. To investigate the sensing capabilities of the composite, he affixed electrodes on both sides of the cantilever. In a machine called a “dynamic mechanical analyser,” he clamped one end to hold the cantilever stationary. The machine applied force at the other end to flex the beam while Bowland monitored the change in electrical resistance. ORNL postdoctoral fellow Ngoc Nguyen conducted additional tests in a Fourier-transform infrared spectrometer to study chemical bonds within the composites and improve understanding of the enhanced mechanical strength that was observed.

The researchers also tested composites made with different amounts of nanoparticles for the ability to dissipate energy—as measured by vibration-damping behaviour—a capability that would benefit structural materials subjected to impacts, shakes, and other sources of stress and strain. At every concentration, the nanoparticles enhanced energy dissipation (by 65 to 257 per cent).

Bowland and Naskar have applied for a patent for the process to make self-sensing carbon fibre composites.

“Dip coating offers a new route to utilise novel nanomaterials under development,” Bowland said.

ORNL's Laboratory Directed Research and Development Programme supported the research, which is published in ACS Applied Materials & Interfaces, a journal of the American Chemical Society. (SV)