Crystallinity has long been regarded as the hallmark of carbon fiber thermal stability; however, our findings reveal that increased structural order does not invariably translate to enhanced thermal resistance. In this study, we graphitized PAN-based carbon fibers up to 2700 °C and performed a comprehensive multiscale analysis of their structure and oxidation behavior, challenging the conventional assumption that greater crystallinity guarantees better thermal stability. Heat treatment did improve the graphitic alignment, microvoid evolution, and tensile modulus across all samples. Yet under oxidative conditions, a surprising reversal was observed: among T300B, T700S, and T800H, the least graphitized fiber (T300B) exhibited the highest thermal resistance, outperforming its high-modulus counterparts. This unexpected behavior is attributed to a dual mechanism: once thermal conductivity exceeds a critical threshold it accelerates oxidative degradation, while pronounced radial heterogeneity (skin–core transition zones) in the fiber structure impedes heat and oxygen penetration. These findings reshape the design paradigm for high-performance carbon fibers. They suggest that maximizing crystallinity alone is insufficient; instead, controlling thermal transport properties and internal structural gradients in tandem is crucial for engineering fibers capable of withstanding extreme oxidative environments.

Decoupling crystallinity from thermal stability: revisiting thermal resistance of PAN-based high modulus carbon fibers
    Abstract
    1 Introduction
    2 Materials and methods
        2.1 Thermal treatment of PAN-based CFs
        2.2 Characterization
        2.3 Model fitting for SAXS data
    3 Result and discussion
        3.1 Structural crystallization and its impact on mechanical properties
        3.2 Evolution of microvoids during graphitization
        3.3 Thermal resistance of graphitized fibers
    4 Conclusion
    References

• Sora Le(Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-ro Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea)
• Yang Ki Chae(Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-ro Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea, Department of Quantum System Engineering, Jeonbuk National University, 567 Baekje-ro, Deokjin- gu, Jeonju-si, Jeonbuk 54896, Republic of Korea)
• Tae-Hwan Kim(Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-ro Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea, Department of Quantum System Engineering, Jeonbuk National University, 567 Baekje-ro, Deokjin- gu, Jeonju-si, Jeonbuk 54896, Republic of Korea)
• Sungho Lee(Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-ro Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea, Department of Nano Material Engineering, KIST School, University of Science and Technology, 217 Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea) Corresponding author
• Jiho Choi(Carbon Composite Materials Research Center, Korea Institute of Science and Technology, 92 Chudong-ro Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea, Department of Biological and Chemical Engineering, Hongik University, 2639 Sejong-ro, Jochiwon- eup, Sejong 30016, Republic of Korea)
• Jinhyeong Lee(Department of Biological and Chemical Engineering, Hongik University, 2639 Sejong-ro, Jochiwon- eup, Sejong 30016, Republic of Korea)
• Jongbok Lee(Department of Biological and Chemical Engineering, Hongik University, 2639 Sejong-ro, Jochiwon- eup, Sejong 30016, Republic of Korea)