Enhanced thermal and mechanical properties of PAN-based carbon/epoxy composites reinforced with graphite sheets and stitched pitch-based carbon fibers

The low thermal conductivity of polymer matrices limits the applicability of carbon-fiber-reinforced plastics (CFRPs) in thermal management systems. To improve heat transfer characteristics while maintaining the excellent mechanical properties of CFRPs, we proposed a fabrication method for high-thermal-conductivity composites by stacking graphite sheets (GS) and stitching pitch-based carbon fibers into polyacrylonitrile (PAN)-based carbon/epoxy composites. Thermal conductivity was determined by measuring specific heat, density, and thermal diffusivity, and the through-thickness reinforcement was evaluated using double-cantilever beam (DCB) tests. Results showed that the proposed fabrication approach significantly improved both in-plane and through-thickness thermal conductivity compared to untreated PAN-based composites. Specifically, the in-plane thermal conductivity increased from 2.09 W/m·K (pristine) to 54.91 W/m·K after GS insertion, representing an improvement of approximately 2527 %. The through-thickness thermal conductivity was enhanced from 0.59 W/m·K (pristine) to 46.38 W/m K by pitch-based carbon fiber stitching, corresponding to an increase of approximately 7761 %. Dynamic heat transfer analysis using a heat element and thermocouples clarified the formation of efficient heat conduction pathways by the combined use of GS and stitching fibers, enabling effective heat dissipation in both directions. Additionally, DCB Mode I testing showed that GS insertion significantly reduced the failure load from 66.7 N to approximately 7.8 N (an 88.3 % decrease). However, the introduction of pitch-based carbon fiber stitching effectively restored the mechanical properties, increasing the failure load to 75.5 N, an improvement of 867.9 % compared to GS specimens. These improvements are attributed to the formation of continuous lateral heat transfer pathways by GS insertion, which enhances in-plane conductivity, and the establishment of aligned vertical thermal paths by pitch-based carbon fiber stitching, which enhances through-thickness conductivity. Furthermore, while GS insertion reduces interlaminar mechanical strength due to poor resin bonding, pitch-based carbon fiber stitching compensates for this by providing mechanical reinforcement across the laminate. Thus, the proposed method effectively enhances the thermal performance of conventional PAN-based carbon/epoxy composites without severely compromising mechanical integrity, demonstrating its promise for advanced thermal management applications.