Generally, carbon fiber refers to fibers heat-treated to 1000-1500℃, while graphite fiber refers to fibers heated to 2000-3000℃. The term "graphite fiber" does not mean the fiber's internal structure is entirely graphite; it simply indicates a higher heat treatment temperature. Carbon fiber and graphite fiber are often collectively referred to as carbon fiber. In the initial stage of graphitization, residual non-carbon atoms such as N and H in the fiber are further removed, increasing the aromatized carbon in the polymer and transforming it into a graphite-like layered structure. The disordered graphite layers within the fiber further converge during graphitization, transforming into a graphite-like crystalline state. At this point, crystallinity increases, the proportion of crystalline carbon increases, and the orientation along the fiber axis also increases. With further increases in temperature, the fiber modulus increases, strength decreases, elongation at break decreases, and it completely transforms into a brittle material.
After carbonization, polyacrylonitrile fiber already has a relatively regular structure, so the graphitization time is very short, generally only tens of seconds or a few minutes. At extremely high temperatures, the vapor pressures of graphite and carbon are very high. Under these conditions, carbon on the surface of carbon fibers may evaporate, reducing their mass and creating defects on the fiber surface, thus lowering their strength. Graphitization under pressure yields high-strength graphite fibers. Scanning electron microscopy reveals significant differences in the surface morphology of graphite fibers obtained under pressure and without pressure. Graphite fibers obtained under normal pressure have a rougher surface with more voids and defects. Nitrogen reacts with carbon above 2000℃, therefore argon is often used as a heat transfer medium and protective gas during graphitization of carbon fibers above 2000℃. The high-purity argon gas used must undergo special treatment before entering the graphitization furnace to prevent trace amounts of oxygen from being introduced. Trace amounts of oxygen at high temperatures are extremely harmful to the fibers and can significantly shorten the lifespan of the graphite furnace tubes.
