Decoupling crystallinity from thermal stability: revisiting thermal resistance of PAN-based high modulus carbon fibers

Abstract

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.