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Carbon fibers are classified symbolically as CF. These fibers are of size about 5–50 μm in diameter and composed mostly of carbon atoms. They are also known as graphite fiber. Graphite is made of pure carbon arranged into big sheets of hexagonal aromatic rings (Figure 1.3X). The structure of carbon fibers is like the structure of graphite arranged in layers of long graphite sheets of length about 5–50 μm and each layer of graphite is stacked together forming a ribbon-like structure. Depending upon the precursors used to synthesize carbon fibers and the manufacturing processes, each layer planes may be either turbostratic graphitic, or a hybrid structure. In graphitic crystalline regions, the layer planes are stacked parallel to each other in a regular fashion. The atoms in each layer are covalently bonded through sp² bonding while the interaction between each layer is relatively weak van der Waals forces. In a single graphitic crystal, d-spacing between two graphene layers (d002) is about 0.335 nm (Figure 1.3Y).

Figure 1.3 (X) A sheet of graphite showing free edges A, which are reactive and can be functionalized, (Y) shows four layers of graphene sheet separated by van der Waals forces. In graphite this separation distance between two consecutive layers is 0.335 nm.

If carbon content in the fiber is 92 wt% then it is considered as carbon fiber. However, if carbon content is more than 99 wt% then the fiber is known as graphite fiber. This classification depends on the percentage content of carbon, process of synthesis and precursors; Carbon fibers can possess excellent tensile strength, low densities, high electrical conductivities, high temperature tolerance, low thermal expansion and chemical stabilities. High tensile strength and lightweight carbon fiber can be developed by making its composite with suitable polymers. Because of these properties, carbon fiber has found its place in aerospace, civil engineering, the military, and motorsports, along with other areas like sports.