Carbon fibers, with a diameter of only 5 micrometers, equivalent to one-tenth to one-twelfth the thickness of a human hair, are more than four times stronger than aluminum alloys. Carbon fibers are a polycrystalline material composed of incompletely crystallized graphite arranged along the fiber axis. Graphite has a layered structure; within each layer, carbon atoms are arranged in hexagons, with each atom forming sp2 hybrid orbitals with three neighboring carbon atoms and one unhybridized orbital. The unhybridized orbitals give graphite high thermal and electrical conductivity on its parallel basal plane. These layers stack together to form the three-dimensional crystalline structure of graphite, with an interlayer spacing of 0.334 nm. The interlayer bonding is through van der Waals forces. This gives it a high degree of anisotropy. Elastic modulus: 1060 GPa under load parallel to the basal plane, 36.5 CPa under load perpendicular to the basal plane, and a shear load of 4 GPa on the basal plane.
The graphite plane can be rotated about an axis, thus losing the ABAB stacking sequence of three-dimensional crystalline graphite and producing the well-known two-dimensional crystalline structure of helical layered carbon. The interlayer spacing in helical layered carbon is larger than that of graphite in general. High-modulus carbon fibers prepared from PAN (poly(alkylene oxide)) are helical layer crystals with a preferential orientation of graphite layers parallel to the fiber axis. The crystal structure is determined by two parameters: width (parallel to the fiber axis) and height (perpendicular to the fiber axis). The fiber modulus strongly depends on the degree of layer orientation, which increases with increasing heat treatment temperature and stretching during fiber fabrication.
Another structural feature of the carbon fibers is the presence of 15%–20% porosity between the grains. These micropores are elongated and preferentially parallel to the fiber axis. The diameter of the micropores is 1–2 nm, and their length is at least 20–30 nm. The presence of pores and the greater layer separation of the helical carbon layers result in a fiber density lower than the theoretical density of graphite. X-ray diffraction and electron diffraction studies of the structure of PAN-based carbon fibers reveal a branched microfiber structure, with the basic structural unit being a 6 nm wide, thousands of nanometers long ribbon-like layer. Several ribbons combine to form bonded microfibers. The microfibers are highly parallel to the fiber axis and branch to form elongated pores with a diameter of 1–2 nm.
