High‐Temperature Evolution of Nitrogen and Its Effect on the Carbon Fiber Structure

This study focused on the removal behavior of hetero‐element N and its influence on the reorganization of carbon structure in carbon fiber during the preparation of polyacrylonitrile‐based carbon fibers. The content, chemical forms, and evolution law of N element in the fibers were systematically analyzed by gradient‐density column, Raman spectroscopy, X‐ray diffraction, elemental analysis, and X‐ray photoelectron spectroscopy. To evaluate the potential effect of energy accumulation on nitrogen removal and carbon‐structure reorganization, multi‐step and two‐step high‐temperature treatments were designed to modulate the nitrogen‐removal rate and thus control carbon‐structure reorganization and crystallite growth. The results showed that the bulk density of carbon fibers first increased and then decreased with rising carbonization temperature, and the inflection point occurred at 1200°C. The nitrogen content continuously decreased with increasing treatment temperature. Around 1200°C, a pronounced conversion of graphitic‐N to pyridinic‐N occurred. At this stage, the nitrogen‐removal rate and the reconstruction rate of the carbon‐network planar structure reached a kinetic match, leading to densification of the internal structure. In light of the elucidated nitrogen evolution dynamics, this study innovatively developed a multi‐step heat treatment strategy to assess its efficacy in optimizing the fiber microstructure. The study demonstrated that, compared with the two‐step process, the multi‐stage process with incremental energy input was conducive to the preservation of surface activity and the enhancement of surface roughness. IFSS tests indicated that fibers treated via the multi‐stage process exhibited superior interfacial bonding performance.