A program of research has addressed key issues in the production of continuous carbon nanofibers and the utilization of carbon nanofibers as the reinforcing components in matrix/fiber composite materials. The goal of this research is to contribute to the development of advanced lightweight structural composites in which the exceptionally high strengths of carbon nanofibers are exploited to obtain mechanical strengths, delamination toughnesses, and fatigue lives greater than would otherwise be possible.

These Stress-vs.-Strain Plots from an edge-delamination tension test show that a specimen reinforced with continuous carbon nanofibers failed at higher stress and strain than did an otherwise equivalent specimen that was not so reinforced.

Continuous carbon nanofibers are related to, but not the same as, carbon nanotubes, which can also be considered superstrong nanofibers in their own right but are discontinuous (in other words, too short) and cannot be produced and aligned easily or economically in the quantity and quality needed for use as reinforcements in advanced structural composites. In this program, continuous carbon nanofibers were made in a process that began with electrospinning of nanofibers from commercial polyacrlyonitrile (PAN) precursors. The as-spun PAN nanofibers were stabilized by crosslinking of their molecules, then carbonized by heating to various high temperatures. Nanofibers at the as- spun, stabilized, and carbonized stages of manufacture were studied by scanning electron microscopy and x-ray diffraction. It was found that the nanofibers were uniform in diameter and that unlike in the production of carbon nanotubes, extensive purification was not necessary. Higher carbonization temperatures were found to result in better nanostructure.

In a series of experiments, panels were made, variously, from layers of as-spun PAN nanofibers, layers of stabilized PAN nanofibers, or layers of carbonized nanofibers inserted between plies of aerospace- grade composites consisting of epoxy matrices reinforced with conventional carbon microfibers. Panels were also made without nanofibers between the plies. The panels were subjected to fracture-mechanics and edge delamination tests. The results of the fracture toughness tests showed that the nanofibers at the different stages of manufacture afforded different degrees of toughening. The greatest toughening was found to occur in the panels containing carbonized nanofibers. The results of the edge delamination tests (for example, see figure) were interpreted as signifying that reinforcement by continuous carbon nanofibers yielded increases in delamination onset stress, ultimate strength, and fatigue life.

In other experiments, to demonstrate feasibility of manufacture, composite panels were made from sheets of continuous carbon nanofibers (without conventional carbon microfibers) stacked and impregnated with epoxy and cured at controlled temperature and pressure. Tests of the panels revealed substantial anisotropy of mechanical properties.

Continuous carbon nanofibers can be produced at a cost regarded in the art as reasonable in comparison with the costs of producing such other high-performance materials as carbon nanotubes. The continuity of these fibers reduces (relative to carbon nanotubes) the costs of handling and processing into nanocomposites. Newly developed methods of aligning nanofibers enable the fabrication of nanofiber products having superior anisotropic properties. In addition, these methods enable the fabrication of nanocomposites containing higher volume fractions of fibers and complex nanoreinforcement architectures.

This work was done by Y. A. Dzenis of the University of Nebraska for the Air Force Research Laboratory.

AFRL-0039


This Brief includes a Technical Support Package (TSP).
Continuous Carbon Nanofibers for Structural Composites

(reference AFRL-0039) is currently available for download from the TSP library.

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