Influences Of Ferric Ions On The Microstructure And Mechanical Properties Of Polyacrylonitrile-Based Carbon Fibers

Polyacrylonitrile based carbon fibers have attracted worldwide interests as high performance materials. Their high strength and modulus, combined with their low density, make these fibers widely used in every aspect of the society. The characteristics of high performance carbon fiber include high tensile strength, high Young's modulus, and high carbon content, less porosity, less structure defects. However, in addition to improving the technology of pre-oxidation, carbonization and graphitization, the influence of metal ions of carbon fibers is also important.A polyacrylonitrile fiber was dipped with ferric chloride solution, and then was oxidized, carbonized and graphitized successively. The structure and performance of the precursor fiber, the derived oxidized fibers, carbon fibers were studied on the characterizations of inductively coupled plasma atomic emission spectrometry (ICP-AES), fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetry (TG), as well as fiber density and tensile strength tests. The results showed through impregnation, the ferric ions could enter the interior of the precursor, but was only physically adsorbed without chemical reactions. The adsorption dynamics of ferric ions to the precursor were fitted by the Bangham equation based on the experimental data. The densities of stabilized fibers with treatment were decreased for the existence of Fe, compared with the ones without treatment. Both the decrease in released heat calculated by DSC spectral of the precursors with treatment and the variation of the peaks of functional groups during stabilization indicates that the present of Fe reduces the cyclization degree during stabilization. The increasing of weight loss rate during the stimulant carbonization through TG analysis also indicates that stabilization degree slows down by the effect of Fe and the properties of carbon fibers would be influenced ultimately.The influence of ferric ions on graphitization of polyacrylonitrile-based carbon fibers was investigated by dipping fibers in FeCl3 solutions with different concentrations before graphitization. The crystallite parameters and morphology of resulting carbon fibers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The effect of ferric ions on mechanical properties and microstructure of carbon fibers was investigated. The results show that the treatment by FeCl3 solutions has a catalytic-graphitization effect on fibers by increasing the interlayer spacing, stack height, modulus and changing the surface state of carbon fibers during high-heat treatment, which means ferric ions accelerate the microstructure of carbon fibers transforming from turbostratic to graphitic structure. That carbon loss increases and specific density of the derived fibers decrease with additional amount of ferric ions, results in lower tensile strength.