Evolution of microstructure within carbon fiber during pre-carbonization revealed by mean field theory

Microstructure dominating the conductivity in carbon fiber (CF) was investigated to construct an initial model for the interpretation of the sudden shift in conductivity of pre-oxidized polyacrylonitrile precursor fibers (POF) during pre-carbonization. The electrical conductivity of POF treated at different pre-carbonization temperatures was measured by four-point probe meter. According to the result of high-resolution transmission electron microscopy and X-ray diffraction, the microstructure of CF was approximately regarded as nanocomposite consisting of pseudo-graphite sheet (PGS) and an amorphous carbon matrix. Based on the analysing of this nanocomposite microstructure model with mean-field theory and applying effective-medium approximation, the evolution of electrical conductivity and microstructure of POF was investigated. The microstructure of CF during pre-carbonization process, including the volume fraction and geometric factors of PGS, was determined by X-ray diffraction and Raman spectroscopy. The existence of percolation effect was determined by applying a combination of simulation calculating and experiment. The results indicated that a carbon skeleton consisting of connected PGSs was formed within CF during the pre-carbonization process. In addition, the carbon skeleton provides efficient pathways for the lattice vibrations and phonon propagation required for heat conduction and has a significant impact on the subsequent crystallization of the final CF and its properties.