Effects of high-temperature thermal reduction on thermal conductivity of reduced graphene oxide polymer composites

The graphitic crystalline structure of reduced graphene oxide (rGO) can be improved by high-temperature thermal reduction at various heat-treatment temperatures ranging from 1000 to 2500 °C. The crystallinity significantly increased with increasing heat-treatment temperature. The electrical conductivities of the rGOs heat-treated at 2000 and 2500 °C (h-rGO-2000 and h-rGO-2500, respectively) were similar to those of commercial graphite. The isotropic thermal conductivity of rGO/epoxy composite with 10 wt% h-rGO-2500 (2.56 W/mK) is 11.6 times higher than that of pristine rGO (p-rGO; 0.22 W/mK) and significantly superior to those of epoxy composites with commercial graphite (0.82 W/mK) and mesophase pitch-based carbon fibers (MPCFs; 1.29 W/mK). Moreover, owing to the synergistic effect operating in the MPCF–h-rGO hybrid filler in epoxy composites, this combination of fillers increases the thermal conductivity to a greater extent than the MPCF–p-rGO hybrid filler. Optimum synergistic effects on the isotropic and in-plane thermal conductivities were achieved with an MPCF:h-rGO-2000 weight ratio of 49:1 (17.93 W/mK, 1.85 times higher than 9.69 W/mK for MPCF–p-rGO). Finally, a machine learning method that could predict and optimize the properties of rGOs based on their HT temperatures and material compositions was developed.