Core-shell SiO2@RGO hybrids for epoxy composites with low percolation threshold and enhanced thermo-mechanical properties

Reduced graphene oxide (RGO)-encapsulated SiO2 hybrids (SiO2@RGO) were fabricated from the thermal reduction of electrostatically assembled SiO2@GO hybrids. Then, epoxy composites, filled with SiO2, SiO2@GO and SiO2@RGO hybrids, were prepared by a solvent-free curing process, and their thermal, dielectric and thermo-mechanical properties were investigated and compared. In the SiO2@RGO/epoxy composites, the mono-dispersed SiO2 nanoparticles are firmly embedded in the thin layer of RGO nanosheets, forming unique core-shell nanostructures that effectively prevent the aggregation of RGO nanosheets in the polymer matrix, construct conductive pathways at the particle-polymer interface and afford the epoxy composites with outstanding thermo-mechanical properties. The dielectric properties of the SiO2@RGO/epoxy composites exhibit a typical percolation transition near 0.174 vol% for RGO (20 wt% of SiO2@RGO hybrids), where the dielectric constant could reach 77.23 at 1 kHz, which is 22 times that of the neat epoxy resin. Upon further increase of the loading content, the gradual contact of the filler particles leads to the formation of interfacial continuous conductive networks, and both the thermal conductivity and dielectric constant of the composites show a dramatic increase. With a filler loading of 40 wt% SiO2@RGO (0.373 vol% for RGO), a thermal conductivity of 0.452 W m(-1) K-1 is obtained, which is two times larger than that of neat epoxy. In addition, SiO2@RGO/epoxy composites reveal significantly decreased coefficient of thermal expansion (CTE) and increased glass transition temperature (T-g). We believe this special core-shell SiO2@RGO structure, with its inner mechanically enhanced inorganic particles and outer interfacial conductive phase, could make full use of the enhancement effect of different components and thus endow the polymer composites with outstanding properties overall.