Carbon Fibers Prepared by Isotropic Pitch with Different Subfraction Contents during Carbonization: Implications for Enhancing Mechanical Properties

Carbonization plays a crucial role in the production of pitch-based carbon fibers with enhanced mechanical properties. Understanding the carbonization behavior of these fibers with different compositions as a function of temperature is essential. In this study, four pitch fibers derived from isotropic pitches with varying pitch subfractions were stabilized in air at 270 °C and subsequently carbonized in an argon atmosphere over a temperature range of 400–1000 °C. A variety of advanced characterization techniques were employed to monitor the changes in chemical composition and crystalline structure at different stages of carbonization. The results indicate that carbonization is a complex process involving dehydrogenation, deoxygenation, polycondensation, and aromatization. Fibers with higher aliphatic content, such as GP-PF and H100-T0I-PF, exhibited greater oxidative reactivity, accelerating the stabilization process compared to H80-T20I-PF and H60-T40I-PF. This resulted in higher gas evolution (CO, CO2, and H2O) during carbonization. The crystallite size (La) of the carbon fibers increased monotonically with carbonization temperature. The interlayer spacing (d002) initially increased and then decreased with rising temperature, which attributed to interaction effects between gas evolution and microcrystallite rearrangement. The tensile strength of all fiber samples increased steadily with higher carbonization temperatures. Proper removal of soluble subfractions facilitated the development of the crystalline structure, thereby enhancing the mechanical properties of the resulting carbon fibers.