Influence of Graphitization Conditions at 3000°C on Structural and Mechanical Properties of High-Modulus Polyacrylonitrile-Based Carbon Fibers

The effect of the graphitization conditions at 3000°C on the crystal structure and properties of high-modulus polyacrylonitrile-based carbon fibers (CFs) is studied. It is shown that an increase in the temperature of CF processing from 1400 to 3000°C leads to a decrease in the tensile strength and an increase in Young’s modulus. However, an increase in the winding rate from 10 to 300 m/h during the synthesis of CFs at a fixed graphitization temperature of 3000°C leads to decreases in both the tensile strength and Young’s modulus. The crystal structure of the synthesized CFs is studied by the X-ray diffraction and Raman spectroscopy methods. It is shown that an increase in the thermal processing temperature from 1400 to 3000°C leads to a decrease in the d₀₀₂ interlayer spacing and an increase in the crystallite size L_c. It is established by the Raman spectroscopy method that the I_D/I_G parameter (the ratio of the integral intensities of the D and G spectral bands) decreases at the same time, which is also characteristic of an increase in the degree of perfection of the CF crystal structure. On the contrary, an insignificant increase in the d₀₀₂ interlayer spacing and a decrease in the L_c value are observed with an increase in the winding rate for CFs synthesized at a fixed graphitization temperature of 3000°C, while the I_D/I_G parameter in this case hardly changes. A detailed analysis of the shape of the (002) diffraction peak shows that, unlike CFs obtained at a winding rate of 10 m/h, CFs obtained at higher winding rates consist of at least two distinct structures of different degrees of graphitization. Further thermal treatment of these CFs at 2650°C under steady-state conditions leads to a significant decrease in the d₀₀₂ spacing parameter and an increase in the L_c value. It is established from the measurements of the I_D/I_G ratio that radial inhomogeneity of the CF crystal structure over the cross section increases at higher winding rates, which is associated with inhomogeneous heat transfer in the filaments. It is concluded that high rates of heating to 3000°C have a negative effect on the structural and mechanical properties of CFs.