Search

The radial structure of polyacrylonitrile fibers oxidized before carbonization and its distribution directly affect the performance of the resulting carbon fibers. Optimizing the radial distribution of the oxidized structure and establishing a relationship between this structure and the mechanical properties of the final carbon fibers will help optimize the oxidation conditions for the preparation of high-performance carbon fibers. Solid-state nuclear magnetic resonance spectroscopy, optical microscopy, thermogravimetric analysis, and mechanical tests were used to investigate the effect of the oxidation reaction rate on the radial distribution of the structure of the oxidized fibers and the mechanical properties of the resulting carbon fibers. The oxidation reaction rates were controlled by regulating the oxidation temperature gradient. Results show that the degree of oxidation increases with both the average and initial oxidation rates. By increasing the average oxidation reaction rate, the oxidized structure penetrates deeper into the core region of the fibers, the content of oxygen-containing functional groups increases, the thermal stability of the fibers decreases, and the degree of graphitization of the final carbon fibers increases, but the density of the fibers is decreased and their mechanical properties are degraded. Compared with sample obtained with the lower initial oxidation rate, the number of oxygen-containing functional groups, thermal stability, degree of graphitization and density of the final carbon fibers of the sample with the higher initial oxidation rate are higher, and its tensile strength and modulus are respectively 4.2% and 2.2% higher. A new type of carbon fiber with high strength, medium modulus and a relatively large diameter is obtained under the optimized oxidation conditions.

The radial microstructural in carbon fibers is important for their mechanical properties. Microstructures at different locations in the radial direction produced by heat treatments from 1350 to 2400 °C and their relationship to the mechanical properties were investigated by Raman spectroscopy, elemental analysis, X-ray diffraction, high-resolution transmission electron microscopy and mechanical testing. Results indicated that the differences of the shift and full widths at half maxima (FWHM) of the Raman G and D bands at different points in the radial direction from the skin to the core, showed minima while the tensile strength of the fibers was a maximum at 1700 °C heat treatment temperature. The maximum tensile strength of carbon fibers at 1700°C was ascribed to the smallest skin-core variation at this temperature. The FWHM of the G and D bands from the same points in the radial direction decreased with heat treatment temperature, indicating an increased uniformity of the vibration modes of both G and D bands The sizes and the orientation degree of the crystallites increased with heat treatment temperature, leading to an increase of tensile modulus. Non-carbon elements were preferentially released, and crystallites were preferentially grown and orientated in the skin region compared to those in the core due to the preferential heat absorption in the skin, leading to the lower values of FWHM of G and D bands in the skin than in the core. The Raman shift of the G band had a maximum at 1700 °C while that of D band decreased with heat treatment temperature at the same radial points of the fibers. The fact that the release rate of non-carbon elements increased with heat treatment temperature below 1700 °C could be responsible for the increased G band shift below 1700 °C. With a continuous decrease in the non-carbon element content, this effect was lessened and the improvement of the perfection of the graphite structure became dominant above 1700 °C.

The mechanism for the change in the size of the geometry of graphite crystallites in polyacrylonitrile-based carbon fibers heat treated at 1500–2500 °C is proposed. The study indicated that the growth transition temperature of graphite crystallites in carbon fibers is 2000 °C. Above 2000 °C, the interlayer space (d002) decreases slowly and an ordered graphitic structure forms. Below 2000 °C, the variation in the size of the crystallites along the directions of the axis (La‖) and the radius (La⊥) of the carbon fibers that were heat treated at various temperatures was almost the same and the crystallite form factor, La‖/La⊥, which is nearly 1. Above 2000 °C, La‖/La⊥ increases rapidly because the anisotropy of the arrangements of the crystallites in the carbon fibers and the geometry of the crystallites change from being square to becoming rectangular. The crystallite thickness (Lc) also increases with an increasing temperature and below 2000 °C the crystallite form factor (La‖/Lc) increases rapidly. Above 2000 °C, the La‖/Lc stabilized at 3.1. The different growth models in the different directions are the intrinsic root for the variations in the crystallite geometries that underwent high-temperature heat treatments.

Pyrolysis mechanism of nitrogen in carbon fibers, heat-treated at various temperatures, was investigated, in relation to the nitrogen chemical bonding states. Nitrogen content shows exponential decreasing with increased temperature and extended time. The pyrolysis of nitrogen exhibited time-temperature equivalence. Nitrogen atoms containing hydrogen in the amorphous region and at the edge of microcrystalline, and the double-bonded nitrogen in pyridine rings are easily pyrolyzed, while nitrogen atoms in the six-member ring of the graphite sheets are difficult to be pyrolyzed.

The microstructure of the polyacrylonitrile (PAN) precursors can strongly affect the structure of the carbon fibers which determine the final mechanical properties of carbon fibers. The orientation factors ( n f ) of PAN precursors under different draw ratios were characterized by the birefringence ( n f ), which was obtained through polarized microscopy. The results show that the increasing of the draw ratio may enhance the degree of orientation, which can correspondingly improve the mechanical properties. However the increasing speed of the orientation factor slows down as the draw ratio reached to 8.

Non-woven hemp fiber mat has been used to reinforce unsaturated polyester to make natural fiber composites. Thermal properties of the hemp fiber mat were investigated to discover the range of heat treatment temperatures suitable for the hemp fiber mat. Loss of weight during heat treatment and absorption of moisture from the environment during storage of the hemp fiber mat were also studied. Both hand lay-up technique and compression molding were used to make hemp mat composites. Due to the low fiber fraction, no significant reinforcing effect was found in the composite made by the hand lay-up technique. The effects of heat treatment of fibers, water content in the fibers, fiber fraction, and manufacture methods on tensile properties of the resulted composites were investigated. Hemp mat composites with tensile strength and modulus comparable to those of [±45°]4 glass fiber reinforced polyester were achieved by compression molding at a molding pressure of 2MPa.

The rheological properties and viscoelasticity of PAN/DMSO solutions were studied by means of revolving rheometer under the scan modes of steady shear rate and dynamic frequency. The effects of shear rate and temperature of PAN/DMSO solution on its Non-Newtonian viscous index, zero shear viscosity, activation energy of viscous flow and modulus of the solutions were studied. It was found that PAN/DMSO solution is a kind of pseudoplastic fluid. And the more temperature of solution, the less Non-Newtonian viscous index at steady experiment curve, the less the loss modulus and storage modulus of solution at dynamics experiment curve. In addition, the liquidity and spinnability of the solution also increased when the temperature went up.