Structuring 3D-printed polypropylene composites with vertically aligned mesophase pitch-based carbon fibers for enhanced through-plane thermal conductivity and mechanical properties

Vertically aligned structures in thermally conductive polymer-based composites (TPMCs) present an efficient tool for managing heat dissipation in battery packs and the central processing unit (CPU). Although there is significant progress in developing vertically aligned structures for thermal management using two-dimensional thermally conductive fillers (<i>e.g.</i>, boron nitride and graphene) in TPMCs, their practical applications are limited by the compromised mechanical properties. In this study, carbon fiber (CF) reinforced polypropylene (PP) composites with vertically aligned structures were successfully fabricated using 3D printing. The CFs exhibited exceptional alignment along the printing direction in the PP matrix, attributed to the shear and compression effect during printing. Additionally, the incorporation of CFs and the use of a hot-pressed PP substrate instead of the original platform effectively mitigated shrinkage and warping of PP. The vertically printed samples achieved a superior through-plane thermal conductivity (TC) of 3.61 W m^-1 K^-1 at 21 vol% CF loading, representing an improvement of 5.56 and 15.41 times over that of horizontally printed parts and neat PP, respectively. Meanwhile, the as-printed vertically aligned parts also demonstrate excellent tensile strength (40.16 MPa) and impact strength (28.17 kJ m^-2), which are around 1.70 and 11.45 times that of horizontally printed parts. Notably, the surface temperature of the vertically printed heat sink was comparable to commercial parts, underscoring the superior thermal dissipation performance of the composite material. Simulations verified the anisotropic design's effectiveness in enhancing thermal conductivity. This work provides a facile and cost-effective method to simultaneously enhance through-plane TC and mechanical properties, with promising application in electronic packaging and battery thermal management.