Heterogeneous and microporous structure evolution of heated/radiated polyacrylonitrile fibers revealed by micro X-ray computed tomography and scattering

Heat treatment, a critical pre-oxidation process, plays an essential role in the preparation of carbon fibers. Most defects arising from this stage are likely to be transferred to the final carbon fibers. Electron beam irradiation is an effective modification method to optimize the structure of polyacrylonitrile (PAN) pre-oxidized fibers. In order to investigate the microstructural evolution of irradiated PAN fibers during the heat treatment process, synchrotron radiation X-ray micro-computed tomography (micro-CT) was used in this work and a 3D four layer structure of pre-oxidized PAN fibers was obtained. The heat treatment process of irradiated PAN fibers was also characterized by synchrotron radiation in situ small-angle scattering (SAXS) and wide-angle scattering (WAXS). The results indicate that electron beam irradiation enhances the cross-linking and cyclization of PAN fibers during heat treatment, while reducing the initial cyclization temperature. The micro-CT results show that irradiation treatment makes the ratio of each layer of PAN fibers change during the heat treatment process. The proportion of both outer and inner skin increased significantly, while that of the sub-skin decreased. Additionally, there is a slight decline in the core-skin proportion. A gradual transition from a loose to dense state occurs between the layers, and irradiation reduces the heterogeneity among them. Further in situ scattering experiments showed that the microporous size of irradiated PAN fibers decreased during heat treatment relative to untreated fibers. Irradiation enables oxygen to diffuse deeper into the fiber, enhances oxidation reactions, and facilitates the cross-linking and cyclization of PAN.