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Mechanical Properties of Graphene and Graphene-based Nanocomposites
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
A detailed review has been undertaken upon graphene and graphene reinforced epoxy nanocomposites. In addition, the literature on Raman spectroscopy and its application in graphene and graphene nanocomposites has also been reviewed. The deformation and fracture behaviour of one-atom-thick monolayer graphene has been studied in detail. Monolayer graphene flakes with different widths and shapes were successfully prepared by mechanical exfoliation and pressed onto PMMA beams. It has been possible to follow the fracture behaviour of one-atom-thick monolayer graphene by deforming the PMMA beams. Through in-situ Raman mapping at different strain levels, the strains over the whole flake were determined from the shift of the graphene Raman 2D band. The fracture of the flakes was observed from the development of zero strain lines that prefer to form in a direction perpendicular to the strain axis but it still tends to follow the zig-zag or armchair crystallographic direction in graphene single crystal if the angle is close. It was found that the strength of the monolayer graphene flakes with no clear edge defects decreases with an increase in the flake width. The strength dropped to only 5 GPa for some large flakes, much less than the reported 130 GPa. This was thought to be due to the presence of topological defects that may form pseudo cracks. It was also found that the strength for small monolayer graphene flakes with pre-existing cracks and notched edges is only about 4 to 5 GPa. Seven different kinds of graphene materials with different microstructural parameters including flake width, thickness, defect concentration and chemical functionalization have been studied and used to fabricate the nanocomposites with an epoxy matrix. Graphene materials with higher aspect ratios have been found to give better reinforcement than those with lower aspect ratios. Oxidative functional groups on graphene flakes can help achieve a better dispersion but they also can compromise the mechanical properties of the graphene materials. The strength of pristine graphene reinforced epoxy nanocomposites is dominated by the width of the graphene flakes and the dispersion quality of graphene materials within epoxy matrix. The narrower the graphene flakes or the better the dispersion, the higher the strength. Defects in the functionalised graphene flakes have potential to nucleate cracks within the nanocomposites and cause premature fracture that reduces the strength.