Correlating microstructure-property relationships in carbon fiber-expanded graphite hybrid composites for synergistic improvements in thermal and mechanical properties

Abstract

Thermally conductive and structurally reinforcing polymer composites are in high demand for applications such as electric vehicles, rapid prototyping, and thermal interface materials. Including a single geometry (platelet or rod) filler is a common strategy to improve the performance of polymers. However, the thermal and mechanical properties follow an inverse dependence and often require high filler loadings that compromise the overall properties. Hybridization of filler geometries has effectively improved the thermal conductive network while retaining mechanical performance. The focus of this work is to understand the effect of microstructure and interactions of hybrid fillers in high strength thermally conductive composites. For this study, expanded graphite, and carbon fibers (CFs) were used as the hybrid filler system to improve the thermal conductivity and mechanical properties of polyamide-based thermoplastic composites. The detailed microstructural study revealed that at a lower normalized weight ratio of CFs, the fibers bridge the expanded graphite particles to create a filler-filler network which leads to synergistic effect on both the tensile strength (by 29%) and thermal conductivity (by 290%) compared to a single-filler expanded graphite-based system. Above a critical mass fraction of CF, the planar graphitic orientation led to a loss in thermal conductivity which is attributed to reduced exfoliation and packing density of fillers.