Designing the structure of carbon fibers for optimal mechanical properties

The formulation of carbon structures from various polymer precursors is reviewed along with the resultant fiber properties. The final structures and end properties of the carbon fiber can differ significantly depending on both the precursor chemistry and the associated processing. Polyacrylonitrile (PAN) and mesophase pitch are the predominant precursors. PAN-based carbon fibers consist of nanocrystalline graphitic domains typically 1.5-5 nm in size surrounded by amorphous carbon. With PAN based carbon fibers, the skin-core structure plays a significant role in their mechanical properties and a more homogenous carbon fiber micro structure offers the possibility of a new set of tensile strength and elastic moduli. Pitch-based carbon fibers are 10-50 nm crystallites with the graphitic (002) planes mostly aligned parallel to the fiber axis. Here we show that microstructural defects distribution (0.1-200 nm) measured by small angle X-ray scattering are directly related to the tensile strength of carbon fibers. Ultimately a comprehensive understanding of carbon fiber structure, defects and processing science offers the opportunity to design carbon fiber microstructures with improved properties and from alternative precursor at reduced cost.