Evolution Of Nitrogen Structure In Pan Fibers

Nitrogen is critical structural element during preparation of polyacrinitril(PAN)-based carbon fibers, and its content is greatly reduced in high temperature carbonization stage. Nitrogen atom participated in cyclization, intermolecular crosslinking, and pyrolysis reactions during the condition of heat treatment, which lead to change of the bonding forms and their relative content. To study the evolution of the nitrogen chemical structures during pre-oxidation and carbonization is helpful for further understanding the chemical reaction mechanisms of PAN fiber during heat treatment.In this thesis, the evolution of nitrogen chemical structures during pre-oxidation and low temperature carbonization stages was studied. Characterizaion methods, such as FTIR,13C-NMR, XPS were used for analyzing the nitrogen chemical structure and their relative content of PAN fiber. Efforts were made to establish correlations between the nitrogen chemical structure and the density, graphitization degree of the heat treated PAN fibers. The result showed that:1. Nitrogen atoms exist in the forms of-C≡N,-C=C-C=N-, and -C=C-NH-in the pre-oxidation PAN fibers, and the enamine structure is the foundation of oxygen atoms combining with PAN macromolecules.2. The bonding forms and their relative contents of nitrogen in the PAN fiber are strongly relied on temperature and time during pre-oxidation process. Raising temperature and prolonging time are helpful for accumulation of the enamine content. The generation of oxygen containing pyridine rings is advantageous for transformation of the nitrogen structures from imine to enamine.3. Different bonding forms of nitrogen in fiber transforms reciprocally during low temperature carbonization. The relative content of-C=C-NH-decreased, while relative content of-C=C-C=N-increased with increasing of treatment temperature. Graphite-like nitrogen appears at 500 ℃.4. The formation and increasing content of graphite-like nitrogen promote the increasing of the density and graphite degree for the fiber during low temperature carbonization.5. The nitrogen chemical structure of the pre-oxidation fiber affects the transformation of nitrogen in the low temperature carbonization. Higher content of oxygen containing pyridine rings in the pre-oxidation fiber results higher content of graphite-like nitrogen in the carbonized fiber.