高强高模型碳纤维与高模型碳纤维微观结构分析

Microstructures of High-strength High-modulus Carbon Fibers and High-modulus Carbon Fibers

利用拉曼光谱(Raman)、X射线衍射(XRD)、X射线光电子能谱(XPS)、元素分析(EA)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)和小角X射线散射(SAXS)表征和分析了高强高模型碳纤维和高模型碳纤维的微观结构和化学组成.结果表明,高强高模型碳纤维的石墨微晶晶粒尺寸细小,组成微晶的石墨片层间距较大,片层间和片层边缘保留了非共轭碳构成的晶体缺陷,石墨化程度较低,微晶间缝隙较小,分布均匀,路径曲折;高模型碳纤维具有更加完善的石墨微晶结构,石墨化程度较高,微晶尺寸更大,组成微晶的石墨层堆砌整齐有序,非碳元素较少,共轭态碳明显增多,但其微晶间裂纹和孔隙结构也较大.高强高模型碳纤维因具有多层次的微观结构和缺陷,相比高模型碳纤维,有更多的应力扩散和能量存储及耗散路径,是其拉伸应变和强度保持的关键所在.

The differences between the microstructure and composition of high-strength high-modulus carbon fibers and those of high-modulus carbon fibers, both domestic products, are presented by laser micro-Raman scattering（Raman）, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, elemental analysis（EA）, scanning electron microscopy（SEM）, transmission electron microscopy（TEM） and small-angle X-ray scattering（SAXS）. The results show that the graphite crystallites in high-strength high-modulus carbon fibers are fine（L_a = 0.543 nm, L_c = 0.301 nm）. There are many defects in the plane and at the edge of the graphite sheets. On one hand, the small graphite crystallites are linked to each other by sp3 amorphous carbons, which are shown as the wave-like and winkled ribbons in TEM images. The defects in the micro-crystallites, mainly composed of non-conjugated carbon atoms and oxygenated/nitrous carbon, cause a relative larger space between graphite layers（d_（002） = 0.343 nm） with a lower degree of graphitization（R = 1.15）. The fine crystallites and defect structures have a larger orientation angle（Z = 10.33°） of（002） crystal face of the graphite crystallites in the high-strength high-modulus carbon fibers. In addition, the microvoids in the high-strength high-modulus carbon fibers are smaller in size（L = 74.7 nm） and larger in orientation angle（B_（eq） = 9.97o） than those in high-modulus carbon fibers, which attributes to the mergence and development of the microvoids at the interface of graphite crystallites resulted from exhausting of the non-carbon elements during the heat treatment. The high-modulus carbon fibers have a higher degree of graphitization（R = 0.29）, and the graphite crystallites are bulky and stacked orderly（L_a = 0.687 nm, L_c = 0.484 nm, d_（002） = 0.337 nm, Z = 9.77o）. Moreover, the cracks and microvoids in high-modulus carbon fibers are larger in sizes（L = 102.4 nm） and smaller in orientation angle（B_（eq） = 8.11o） than those in the high-strength high-modulus carbon fibers. The hierarchical structure in the high-strength high-modulus carbon fibers, including the microcrystalline structure, the microvoids and the nitrogen/oxygen doped graphite sheets, offers various paths to dispersing the stress during the stretching process of the carbon fibers, which leads to large elongation and high tensile strength for the carbon fibers.