Decoration of defect-free graphene nanoplatelets with alumina for thermally conductive and electrically insulating epoxy composites

Although graphene can significantly improve the thermal conductivity of polymers due to its high aspect ratio and excellent thermal conductance, it causes serious reduction in electrical insulation and thus limits the wide applications of its polymer composites in the thermal management of electronics and systems. To solve this problem, electrically insulating Al₂O₃ is used to decorate high quality (defect-free) graphene nanoplatelets (GNPs). Aided by supercritical carbon dioxide (scCO₂), numerous Al₂O₃ nanoparticles are formed on the inert GNP surfaces by fast nucleation and hydrolysis of Al(NO₃)₃ precursor followed by calcination at 600 °C. Alternatively, by controlling nucleation and hydrolysis of Al₂(SO₄)₃ precursor with a buffer solution, Al₂(SO₄)₃ slowly nucleates and hydrolyzes on GNPs to form aluminum hydroxide, which is then converted to Al₂O₃ nanolayers without phase separation by calcination. Compared to the Al₂O₃@GNP hybrid with the assistance of scCO₂, the hybrid prepared with the help of a buffer solution is highly efficient in conferring epoxy with excellent thermal conductivity while retaining its electrical insulation. Epoxy composite with 12 wt% of Al₂O₃@GNP hybrid exhibits a high thermal conductivity of 1.49 W/(mK), which is 677% higher than that of neat epoxy, indicating its high potential as thermally conductive and electrically insulating fillers for polymer-based functional composites.