Spinnability Control and Nanoparticle Modification of Mesophase Pitch Carbon Fibers for High‐Performance Structural‐Thermal Composites

Mesophase pitch‐based carbon fibers (MPCFs) possess excellent graphitization, thermal conductivity, and mechanical strength but face challenges in precursor spinnability and structural integrity. Herein, an integrated optimization strategy was developed to improve fiber quality and composite performance. By analyzing MP precursors with varying softening points, quantitative spinnability criteria and optimal processing conditions were established for defect‐free fiber formation. The correlation between fiber diameter and preoxidation kinetics was clarified, enabling control of microstructural evolution and yielding MPCFs with high tensile strength (1.94 GPa) and modulus (178 GPa). Furthermore, Fe 3 O 4 nanoparticles were introduced into MPCFs to form anisotropic thermal networks in paraffin‐based phase change materials (PCMs). The Fe 3 O 4 @CFs/PCM composites achieved over 50% higher thermal conductivity with only 5 wt.% filler loading, exhibiting enhanced interfacial heat transfer and magnetic responsiveness. This dual optimization approach effectively improves MPCF fabrication and offers a rational pathway for designing multifunctional structural‐thermal composites for advanced energy storage and thermal management applications.