Structure and Properties of Mesophase-Pitch-Based Carbon Fibers

经高温炭化、石墨化处理后的中间相沥青基碳纤维，具有楔形径向（I-1）、径向（I-2）、无规径向（I-3）和洋葱状（I-4）等不同的织构，可供本项目各成员使用。通过光学显微镜和高分辨透射电子显微镜的观察，发现前驱体沥青是由光学各向同性的液滴和各向异性的基质组成的两相体系。经纺丝、炭化后，两相仍保持独立，石墨化后，前者得到微孔非石墨化碳，后者得到石墨化良好的薄片。X射线衍射和磁电阻测量结果与光学显微镜和电子显微镜下的观察一致。具有径向楔形织构的纤维I-1表现出最高的石墨化程度和最大的微晶尺寸。此外，它有最高程度的碳层沿纤维轴沿着的取向。拉伸断裂强度和杨氏模量，断裂应变，也Weibull模量测定四个纤维的拉伸和石墨化。纤维I-1在石墨化后具有最高的强度和杨氏模量。这一结果表明，纤维中的楔形物并不作为断裂的裂纹，碳层沿纤维轴沿着的高度取向对提高强度起着重要作用。Weibull模量在石墨化后下降，这与断裂应变的大波动有关。石墨化碳纤维的热膨胀系数在高达1000 ℃的温度下几乎相同。纤维的横截面微观结构对该系数的影响很小。

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.