Structure and Properties of Mesophase-Pitch-Based Carbon Fibers

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The mesophase-pitch-based carbon fibers with different textures of radial with wedge(I-1), radia(I-2), random (I-3) and onion(I-4) types were supplied to all the members of this project after carbonization and graphitization at high temperatures. The structure and microtexture of the precursor pitch and rheological behavior of mesophase pitches were also studied.From optical and high-resolution transmission electron microscopic observations, the precursor pitch was found to be bi-phasic, consisting of optically isotropic drops and anisotropic matrix. Even after spinning, carbonization, both phases remained individualized and after graphitization the former gave microporous non-graphitizing carbon and the latter yielded well-graphitized lamellae.X-ray diffraction and magnetoresistance measurement agreed with the observations under optical and electron microscopes. The fiber I-1 having radial with wedge texture showed the highest degree of graphitization and largest crystallite sizes. Also it had the highest degree of orientation of carbon layers along the fiber axis.Tensile breaking strength and Young's modulus, strain to break and also Weibull modulus were determined on four fibers carbonized and graphitized. The fiber I-1 had the highest strength and Young's modulus after graphitization. This results suggest hat the wedge in the fiber does not act as a crack for fracture and high degree of orientation of carbon layers along the fiber axis plays an important role to develop the strength. Weibull modulus decreased after graphitization, related to the large fluctuation in the strain to break.Thermal expansion coefficients for graphitized carbon fibers were nearly the same at temperatures up to 1000 C. Cross-sectional microtexture of the fibers had only a little effect on the coefficent.