Studies On The Structure And Micro-Mechanical Properties Of Carbon Fiber By Raman Sepctrum

The mechanical properties of carbon fibers are controlled by the structures. The strength andthe Young module of carbon fiber are much lower than theoretical figures. To recognize thestructure character of carbon fiber and to explore the relationship between microstructure andproperties are important to improve the properties of carbon fibers. At the same time, to obtainhigh-performance composite materials, it is important to further study the stress transfer efficiencyof fiber/composite and the stress distribution along the enhance fibers in the matrix.The microstructure and mechanical properties of PAN-based carbon fibers were studied byRaman spectroscopy, XRD technology and SEM. In addition, the stress transfer and stressdistribution in carbon fiber composite were explored by Raman spectroscopy technology. Theexperimental results showed that:1) From the results of SEM and Raman spectroscopy of surface of fiber, there are no obviouschanges of the structure, but there is a profound skin-core in the PAN-based carbon fiber with themore highly well-order structure in the "skin" and less in the "core". The values I_D/(I_D/I_G)show that dis-order structure holds 52.2% at the edge whereas 53.6% in the center of fiber'scross-section. Meanwhile the figure of distribution and relief map of values of R prove theasymmetry micro-structure existing on the cross-section.2) The size of microcrystallite of carbon fiber increases and the structure become perfect withthe increasing of carbonization temperature. Some important structure parameter, such as R =(I_D/I_G),I_D/(I_D/I_G),crystal size parameter (L_a) and crystal thickness parameter (L_C ) can be calculatedfrom Raman spectra and XRD curves.3) PAN-based carbon fibers with different crystallite sizes L_a were mainly characterized by Raman spectra at different excitation laser energies: 2.41 eV, 1.96eV and 1.58eV, respectively.We found that the wavenumber shifts in the D, D' and G' bands related to excitation laser energies,but the position of G band had no dispersion. Then we tried to explore the values ofR(R = I_D/I_G) and R'(R'=I_D/I_G) dependence on excitation energy and size of crystallitefor PAN-based carbon fibers. And we confirmed that both R and R' are inverse proportionalto E_l~4L_a. Finally, the proposed equations for the relationship among La and parameters of Ramanspectra were expected to be applicable to carbon fibers within a certain range of La.4) The ranges of Raman laser power from 10A to 16A were used to study carbon fiber to confirmthe laser heating effect. Changes of the Raman parameters show that the high power leads to the"ablation" phenomenon on carbon fiber surface. The changes of peak position and full-width at halfmaximum (FWHM) of the D, D' and G' indicate that the heating effect of laser power is obvious.Therefore, during the Raman spectroscopy experiments, we must pay attention to the choice of laserpower.5) In the Raman spectrum of carbon fibers, the G peak (1580cm-1) downshifts with the increasingof stress, and we found that the amount of displacement G and stress have a linear relationship. Thislinear relationship can be used to describe stress distribution along the fiber in the epoxy resin dropletmatrix. In the case of fiber with no stress, the stress distribution was "W" shaped, the fiber within thematrix endured residual stress and the value of maximum interfacial shear stress, 0.6GPa, existedon the edge of the microdroplet. In the case of fiber under the applying strains, the residual stressdispersed and stress distribution was "M" shape along the fiber in the matrix. The value ofmaximum interfacial shear stress was 0.8GPa. The stress transfer efficiency is relatively lower inthe case of fiber under stress than that without stress.