Graphene is a single-layer carbon sheet with a hexagonal packed lattice structure. The long-range π-conjugation in graphene yields remarkable and unique properties, such as high Young’s modulus (~1.0 TPa), large theoretical specific surface area (2630m2g-1), excellent thermal conductivity (~5000 Wm-1K-1), high charge carrier mobility (200 000 cm2 V-1s-1), and excellent optical transmittance (~97.7%). These properties support the use of graphene as an ideal building block in nanocomposites. Nanocomposites are multiphase materials, in which one phase (dispersed phase) in the nanoscale regime is dispersed in a second phase (matrix/ continuous phase), resulting in a combination of the individual properties of the component materials.
However, no scalable method exists to product large quantities of defect-free graphene. For example, although oxidative exfoliation of graphite can potentially give large quantities of graphene-like nanosheets, graphene oxide is typically defective and insulate. Admittedly graphene oxide has proved very useful in applications from composites to catalysis, but it is very likely that an equally wide range of applications will require graphene that is free of basal-plane defects. Alternatively, sonication of graphite, or indeed other layered compounds, in certain stabilizing solvents or aqueous surfactant solutions gives defect-free nanosheets. However, the scalability of the latter process is limited by the use of sonication as an energy source.
In new work conducted by Professor Lianjun Wang’s group at Donghua University, not only graphene but also graphene nanosheet (GNS)/Al2O3 composites were prepared via a scalable method. The researchers employed planetary ball milling to guarantee that their method can be scaled up. Graphene is exfoliated from expanded graphite and mixed with alumina powders simultaneously by ball milled expanded graphite and Al2O3. The added oxide powders acted as nano-balls to exfoliate the expanded graphite (EG), and were also loaded on the surface of delaminated graphene to prevent the aggregation of graphene. After spark plasma sintering , fully dense composites with homogenous dispersion of GNS were obtained. Compared with monolithic Al2O3, the flexural strength, fracture toughness and Vickers hardness of as-prepared GNSs/Al2O3 samples have been improved by 103%, 25%, 26%, respectively, which are attributed to the grain refinement, strong interfacial bonding and pull-out of GNSs from the matrix.