ENHANCING THERMAL CONDUCTIVITY OF EPOXY/GRAPHENE COMPOSITES THROUGH FILLER DISPERSION, EXPANDED GRAPHITE NETWORKS, AND 3D PRINTING

Abstract

Modern thermal management systems require materials which are low weight, flexible in processing, durable, cost effective and highly thermally conductive. Polymers like epoxy are a good candidate by having all the essential properties except one – high thermal conductivity. The low intrinsic thermal conductivity of epoxy limits its direct use in modern thermal management applications. Adding highly thermally conductive fillers to polymers is considered as a strong method to prepare composite materials. Thermally conductive composites, if processed right, can have high thermal conductivity while also retaining the intrinsic properties of polymers to make them suitable for modern thermal management systems. Among multiple filler materials studied, graphene nanoplatelets are particularly promising. Although, translating the high thermal conductivity of graphene into a high thermal conductivity composite material is extremely challenging. The presented work addresses three of these challenges – graphene agglomeration at low concentrations, high filler-filler contact resistance and filler-matrix interfacial resistance, and anisotropic thermal conductivity of graphene with multifold higher in-plane thermal conductivity than through-plane. This dissertation investigates three processing-based strategies to enhance the thermal conductivity of epoxy/graphene-based composites by controlling filler dispersion, graphitic network connectivity, and filler alignment. The first study addresses the first challenge of efficient graphene dispersion in epoxy matrix by comparing two solvents – dimethylformamide (DMF) and acetone, used as processing solvents to mix epoxy and the graphene fillers. To focus on the dispersion aspect, this study was performed at low graphene loadings of 3, 5 and 7wt%. Results showed that the composite samples made with DMF as a solvent showed 40% and 44% higher thermal conductivity than samples made with acetone at 5wt% and 7wt% respectively. Laser Scanning Confocal Microscopy (LSCM) was used to analyze and compare the graphene dispersion in these samples. LSCM images showed that acetone-based samples contained large graphene-rich agglomerates, whereas DMF-based samples exhibited uniform distribution of the graphene nanoplatelets. Further analysis of these images showed that the maximum agglomerate volumes in acetone-based samples were 211% and 93% higher than DMF-based samples at 5wt% and 7wt% respectively. Using effective medium theory, the apparent interfacial thermal resistance of DMF-based samples was found to be approximately 45% lower than the acetone-based samples, thus confirming the improved dispersion of graphene nanoplatelets in the composite. The second study addresses the second challenge of filler connectivity within the composite material by using expanded graphite (EG) as a filler. To prepare the expanded graphite filler, -10 mesh graphite flakes were first intercalated using hydrogen peroxide and sulfuric acid as intercalating agents to obtain graphite intercalated compounds (GICs). These GICs were then thermally expanded around 900 C to obtain expanded graphite. A reaction time optimization was conducted to find the highest epoxy/EG composite through-plane thermal conductivity at 10wt% EG loading. The optimized H₂O₂-derived expanded graphite produced a substantial increase in epoxy thermal conductivity, reaching a maximum value of 5.15 W m⁻¹ K⁻¹ at 10wt% filler loading. This enhancement is attributed to the ability of expanded graphite to form more continuous graphitic pathways within the epoxy matrix, thereby reducing the dependence on many discrete filler-filler contacts. However, higher filler loadings of 12.5 and 15wt% showed reduced thermal conductivity due to poor composite structural integrity, insufficient epoxy bonding, and weaker filler-filler contact quality. Raman, XPS, XRD analysis were performed to characterize the EG fillers. FE-SEM imaging was used to check the structural morphology of the fillers. The third study focuses on the third challenge of filler alignment. Paste extrusion 3D printing was investigated as a processing route to improve the in-plane directional thermal conductivity of epoxy/graphene composites. An 80 mm long rectangular opening nozzle was used to print rectangular shaped epoxy composite samples with 9wt% graphene loading and the effect of printing speed on thermal and electrical conductivity along the printing direction was evaluated. Angstrom method and four-probe method were used for thermal conductivity and electrical conductivity measurements respectively. Increasing the printing speed from 750 mm/min to 1500 mm/min resulted in approximately 27% higher thermal conductivity and 106% higher electrical conductivity along the print direction. These results indicate that printing speed can influence the effectiveness of the graphitic network, likely through changes in filler orientation and filler connectivity. Overall, this dissertation demonstrates that thermal conductivity enhancement in epoxy/graphene-based composites is controlled not only by filler loading, but also by the internal organization of graphitic fillers within the epoxy matrix. Improved dispersion, expanded graphite network formation, and 3D-printing-induced alignment provide complementary approaches for improving thermal transport in lightweight epoxy-based composites for thermal management applications.