The Microstructure Evolution Of Pan Based Carbon Fiber During Heat Treatment

Carbonization is one of the key processes during the manufacturing of high-performance Polyacrylonitrile (PAN) based carbon fiber, in this stage, PAN fiber was changed from organic into inorganic fiber. Along with the solid phase reaction, the chemical and micro-physical structures within fiber were under severe change during that period. The micro-physical structures in carbon fiber were mainly containing high level aggregation and microvoid defect structures. By now, there was few related research about the high level aggregation structure and no systematically research about the microvoid structure within carbon fiber, furthermore, no efficacious evolution model of the microstructure during carbonization was put forward.In this thesis, by the methods of small angle X-ray scattering (SAXS) and high-resolution transmission electron microscopy (HRTEM), the periodical structure within carbon fiber was detected. The evolution of the micro structures during carbonization and graphitization were also discussed. The results indicated that:1. From the HRTEM photograph, it was clearly shows that the inner structure of carbon fiber contains the ordered and disordered stacking of graphite crystallite structure, between those two structures was the transition region, the three kinds of structures were arranged orderly as well. According to SAXS analysis, the scattering curves along the equatorial direction for carbon fiber showed a shoulder peak at around q=0.15nm-1(q indicated the scattering vector, q=4πsin0/λ), which indicated that the periodical structure was alternately arranged perpendicular to the fiber axis.2. The generation and evolution of this periodical structure during carbonization was also investigated. The initial heat treatment temperature that the periodical structure started to appear was700℃. The length of long period was about40nm and increased up to about1350℃and then decreased.3. The electronic density correlation function analysis was also applied to get the detail parameters of the periodical structure, such as the length of the ordered stacking of graphite crystallite structure (Lo), disordered stacking of graphite crystallite structure (Ld) and transition region (Ltr). During carbonization, Lo and Ld were increased, Ltr was decreased before1350℃and after that, all of those three parameters were decreased.4. The scattering curves along the equatorial direction for carbon fiber show broad peak at q=2nm-1after heat treatment at1200℃which were due to the scattering of the pseudo-graphite structure. The result of calculation indicated that Lg(calculated by SAXS data) was bigger than Lc (calculated by WAXD data), but they have similar variation during heat treatment, the increasing rate was increased after2000℃.5. The SAXS patterns of the carbon fiber showed fan-like shape, and by2θ integration, the one dimensional scattering profiles were all smooth curves, meaning that the scattering signals were resulted from the microvoids structure, which belongs to dilute system within fiber. This result indicated that the shapes of the microvoids were mainly ellipsoid.6. According to the SAXS theory, the parameters of the micro-voids structure:mean square radiuses of gyration, relative volume of the microvoids, porosity and specific surface area were calculated. The results indicated that the development of the micro-voids structure during carbonization could be divided into three stages:from400℃to700℃, the volume change was faster than the shape change; between700℃and1800℃, the volume change was keeps pace with the shape change; from1800℃to2400℃, the volume change was slower than the shape change.Form the discussion of the research, the periodical structure of the PAN based carbon fiber during carbonization and graphitization was detected from reciprocal and real space, the formation mechanism and evolution during heat treatment processes was further discussed. At the same time, the change of the microvoids structure during carbonization and graphitization was also studied. The model of the evolution structure of the periodical structure and microvoids structure during heat treatment was constructed preliminary.