Studies On The Microstructure And Properties Of Carbon Fibers By Raman Spectroscopy

Confocal Raman microscopy is a sophisticated optical measurement technology with higher spatial resolution, and can accurately analyze the microscopic structure of materials. Recognizing the structure character of carbon fibers is important for improving their properties. Meanwhile, interfacial mechanics of the carbon fiber reinforced composites have a great impact on the overall mechanical properties of composite materials.The carbon fiber microstructure and interfacial properties of carbon monofiber microdroplets were studied by confocal Raman microscopy. The main contents and conclusions show that:(1) The change of structure of PAN precursor fiber in preoxidation process (heat treating temperature arranging from 220～300℃) has been investigated by using confocal Raman microscopy and Infrared spectroscopy. Different excitation wavelengths may cause the fluorescence effect on the Raman spectra of PAN precursor fiber and preoxidized fibers, whereas this effect does not appear in the Raman spectra of carbon fibers. Meanwhile, the Raman spectrum of PAN precursor, with obvious Raman peaks emerging at 2240 cm-1 (-C≡N vibration) and 2930 cm-1 (-CH2-vibration), is totally different from those of carbon fibers, the later ones were characterized of D peak (1360 cm-1) and G peak (1575 cm-1). The results show that the most obvious change of PAN precursor fiber was due to the dehydrogenation cyclization reaction, which reacts at 240℃when the -C≡N vibration peak disappears. The C-C vibration peaks of sp3 hybrid (1360 cm-1) and sp2 hybrid (1575 cm-1) appear, namely aromatic ring takes place. During 250～300℃process, the reaction speed slows down, which is in accordance with Infrared spectroscopy results.(2) The changes of structure of PAN preoxidized fibers in carbonization and graphitization process have been also investigated by using confocal Raman microscopy coupled with XRD. The size of microcrystallite of carbon fiber increases with the increasing of treatment temperature. The downshifts of D and G peaks mean that the C-C bond length increases and the graphite plane gradually forms.(3) A systematic analysis of the dependence of the ratio between the integrated intensities of the D and G bands (R) on the crystallite size La and on the excitation laser energy is presented. The proposed equations for the relationship among La, parameter R of Raman spectra and the excitation laser energy are expected to be applicable to carbon fibers within a certain range of La. It is found La is inversely proportional to E-4R-1.(4) The circumferential microstructure of rayon based carbon fiber and rayon based active carbon fiber has been investigated by using Raman spectrum on the self-made rotating device. The results show that the degree of graphitization has been greatly improved with the increasing of activation temperature. But the Raman data exhibit some dispersion due to the heterogeneity of porous structure.(5) In the Raman spectrum of carbon fibers, the G peak downshift with stress increases, and the Raman shift of G peak and the stress possess a linear relationship. This linear relationship can be used to describe the distribution of interfacial mechanics along the carbon fiber/epoxy microdroplet. It is found that the stress distribution in the microdroplet is "M" shaped. With the interfacial edge angle increasing, the residual stress, the interfacial shear stress and the stress transfer efficiency increase. From the obtained experimental results, we can conclude that the geometrical characteristics affect significantly the stress distributions in the microdroplet。...