Small angle X-ray scattering (SAXS) is one of the most important methods in the study of the internal void structures of carbon materials. When X-ray irradiates, the scattering phenomenon will essentially appears around the incident X-ray beam if there was any detectable density differences within any nanoscale area of the samples. Based on this, by using SAXS we can obtain not only the pore structure information of carbon materials, but also other structural information including the microfibular structure, the local density fluctuation, etc., through appropriate methods. In recent years, the SAXS analytical theories for carbon fiber, etc. have gradually developed. Classical theories such as Debye's correlation theory have emerged, and the development of the Unified fit model and the "Ruland streak" method has also enabled researchers to get a comprehensive understanding of the carbon structures. Among them, a breakthrough was made in the analysis of carbon fiber microstructure as quasi-two-phase system by SAXS. Significant differences on both microscopic and mesoscopic scale were observed when a scattering system analysis was adopted to carbon fibers and their graphitization fibers according to Debye theory. The amorphous structure was considered to be the main cause of the differences, and the structure of scattering information can be captured by SAXS and then become a component of total scat-tering intensity. In this case, the Unified fit model or the "double Debye" model can be used to analyze the structural characteristics of the microvoids and amorphous structure within the so-called "quasi-two-phase system". Furthermore, the scattering orientation analysis based on "Ruland streak" method was successfully applied to the microvoids analysis of carbon fibers. The method assumed that the preferred-oriented scatterers all had a large aspect ratio, and the scattering intensity was mainly concentrated on the normal direction of the principal axis. A single scatterer would produce a scattering fringe along the normal direction, so the preferred orientation of the scatterers could be obtained according to the distribution of the signals in the receiver plane. In addition, the application of Porod theory to the analysis of microelectronic density fluctuations in carbon structures, and the application of Maxwell function to the analysis of pore size distribution are also new theories or technologies emerging in recent years. The paper hereby reviews the progress on SAXS methodologies centred on the microstructure of carbon fibers. The details of the experiments and data analysis about the structure features including density fluctuation, fractal phenomenon, microvoids, anisotropy and amorphous structure, etc. are elaborated. © 2021, Materials Review Magazine. All right reserved.
