Preparation And Research Of Thermal Conductive Composites Filled With Flaky And Fibrous Carbon Materials

With the rapid development of the microelectronics industry, electronic devices and equipment are becoming denser and more miniaturized as well as integrated. Thermal issues become a bottleneck that restricts the development of the microelectronics industry. Polymer composites play an increasingly important role in the thermal issues because of its good processibility, chemical stability and adhesive property. Therefore, the research on thermally conductive polymer composites with excellent integrated performance has attracted more and more researchers recently. In this paper, thermally conductive epoxy resin composites were prepared by using natural flake graphite and pitch-based carbon fiber as fillers, respectively. The effects of filler shape and orientation and other factors on the thermal conductivity were studied. Meanwhile, graphene fibers were prepared and a preliminary study on graphene fiber/epoxy composite was carried out.The natural flake graphite and pitch-based carbon fiber were used as fillers and the composites were prepared by cast molding process. The effects of filler content, particle size, surface modification and particle size gradation on the thermal property of the composites were discussed in detail. The results show that the thermal conductivity of the composites improves with the increased filler content. The thermal property increases with the growth of the particle size in the same filler content. Coupling agents can improve the thermal conductivity of the composites, and the silane coupling agent is more effective than titanate coupling agent. Particle size gradation has weak influence on the improvement of the thermal conductivity of the composites.Roll forming process can make fillers to arrange in the same orientation, therefore forming anisotropic thermally conductive composites. Moreover, the degree of anisotropy of the composites was determined by the size of roll gap. The wider the roll gap was, the higher the degree of anisotropy was. Natural flake graphite and pitch-based carbon fiber filled composites were prepared using roll forming process. With the mass fraction of50%, the thermal conductivity of32mesh graphite filled composites came up to11.52W/(m·K) along x-y plane, which was13.6times of that along z-axis. When the size of roll gap was0.5mm and1mm, respectively, for composites filled with50wt%long fiber, the ratios of thermal conductivity along x-y plane and thermal conductivity along z-axis were6.3and2.3, respectively. Furthermore, composite filled with fully oriented long fiber was designed. Its x-axis thermal conductivity reached up to151.8W/(m·K), while the z-axis thermal conductivity was only0.85W/(m·K).In addition, the graphene oxide solution was synthesized with an improved Hummers method and ultrasonic peeling. Graphene fibers were fabricated through chemical reduction of the graphene oxide fibers, which were obtained by wet-spinning process. After studying the impact of the spinning solution concentration, coagulation bath, reduction and drying conditions on the mechanical properties of the fibers, the optimal parameters were determined as the following: the graphene oxide concentration was15mg/mL;20wt%CaCl2in water/ethanol solution (whose volume ratio was3:1) was used as coagulation bath; the reducing agent was an80mg/mL aqueous dispersion of ascorbic acid and the reduction was performed under80°C for6h; the drying condition was a stressed drying for12h at100°C in vacuum oven. By cast molding process,10wt%graphene fiber filled epoxy composites were prepared, whose room temperature thermal conductivity was0.52W/(m·K). And the room temperature in-plane thermal conductivity of epoxy composites filled with20wt%graphene fiber by roll forming process was1.66W/(m·K). The results indicate that the prepared graphene fibers have good thermal conductivity and are potential candidates for thermal conductive fillers.