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Carbon fiber is a continuous filamentary material of roughly 5–10 µm diameter whose composition is predominantly carbon (typically >92 % up to ≈99 %), arranged in a turbostratic graphitic microstructure where stacked graphene layers exhibit strong axial covalent bonding and weak transverse cohesion. Industrial grades derive mainly from polyacrylonitrile (PAN) or mesophase pitch precursors, with legacy rayon-based grades largely displaced; PAN-based fibers dominate high-strength applications, while mesophase-pitch fibers provide very high axial modulus and thermal conductivity. Fibers are supplied as tows designated by filament count (e.g., 1K–24K for aerospace, 24K–200K for industrial) and converted into prepregs, unidirectional tapes, woven/NCF fabrics, chopped or milled forms with a surface sizing compatible with thermoset epoxies, vinyl esters, polyesters, or high-temperature thermoplastics such as PEEK, PEKK, PPS, and PA. Typical properties span tensile strengths on the order of several gigapascals, axial tensile moduli from standard (~230 GPa) through intermediate (~300 GPa) to high and ultra-high (>350 GPa), densities near 1.75–1.95 g·cm⁻³, near-zero to slightly negative axial CTE, high specific stiffness and strength, pronounced anisotropy of electrical/thermal conduction, and compressive and shear response governed by fiber morphology and interfacial coupling. Manufacture from PAN involves solution spinning (wet or dry-jet wet) of acrylonitrile copolymers to precursor fibers, oxidative stabilization in air (~200–300 °C) with cyclization and dehydrogenation to form a ladder polymer, high-temperature carbonization in inert atmosphere (~1000–1600 °C) to expel non-carbon elements, and optional graphitization (~2500–3000 °C) to raise modulus. Surface treatment—typically controlled electrochemical oxidation—creates oxygen-bearing functional groups that promote adhesion, after which a thin polymer sizing is applied and the tow is dried, wound, and qualified. Mesophase-pitch routes start from a liquid-crystalline pitch refined and melt-spun, followed by oxidative stabilization to lock in molecular orientation and extended carbonization/graphitization to achieve very high axial order; pitch-based fibers cover isotropic, intermediate, and high-modulus families, with the highest-modulus grades trading off compressive strain and handling robustness. Process controls address filament uniformity, defect and flaw population, tow flatness, residual twist, moisture and volatile content, surface oxygen level, and sizing weight, because these set translation efficiency into composite performance and dictate drape, tack, and consolidation in downstream layup. Applications are centered on fiber-reinforced composites where axial properties are exploited through laminate architecture: primary and secondary structures in aircraft, spacecraft, and launch vehicles; wind-turbine spar caps and shells; Type III/IV pressure vessels for CNG/H₂; automotive body-in-white members, monocoques, and crash structures; sporting goods and instrumentation; civil engineering strengthening using plates, fabrics, and strands; marine hulls and masts; and electronics thermal-management elements and EMI/ESD structures. At higher temperatures carbon/carbon composites made from carbon fiber preforms densified with pyrolytic carbon serve in aircraft and industrial brakes, re-entry thermal protection, and furnace fixtures. Selection among PAN and pitch families, tow size, modulus class, and sizing chemistry is dictated by required stiffness, strength, damage tolerance, processing route, and operating environment, with laminate design and matrix choice determining compression, interlaminar, impact, and durability performance in service.
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