Effects of spatial distribution of fullerenes on the mechanical behaviour of graphene fullerene composites

Carbon based nanostructures such as carbon nanotubes (CNTs), graphene and fullerenes have attracted great attention due to their remarkable thermal, mechanical and electrical properties. In the last years, hybrid carbon nanomaterials, which enable to construct higher scale, tailorable materials consisting of coupled nanostructures such as graphene-CNT, fullerene-CNT and graphene-fullerene, are in the focus of researchers. In this respect, this study examines the mechanical characteristics of a hybrid nanostructured material that consists of fullerenes covalently sandwiched between parallel graphene sheets. After checking stability of the covalent junctions and thermodynamic feasibility of the overall nanostructure by monitoring the free energy profiles over a sufficiently long period through molecular dynamics simulations (MD),as the main objective of this study, the effects of layerwise spatial distribution of fullerenes on the compressive mechanical properties are investigated. For this purpose, atomistic models for the proposed fullerene-graphene composite structures are generated by the use of C180 fullerene with different spatial arrangements between graphene sheets. Random and ordered type fullerene dispersions are considered as two main fullerene distribution schemes employed in the atomistic modelling process. Comparisons are performed between fullerene-graphene composite structures with randomly and evenly distributed fullerenes in terms of elastic mechanical properties and energy absorbing characteristics. In this regard, compressive loading tests at different strain rates and various temperatures are performed via MD simulations to capture the mechanical response of sandwiched fullerene-graphene structures with different fullerene arrangements. In the MD simulations, the four nanostructures were assumed to be different spatial fullerene arrangement between graphene layers and results were compared. It was found that spatial distribution of fullerenes has remarkable influence on both compressive stress level and stress-strain characteristic of the novel fullerene –graphene foams. Mechanical response of the hexagonal and square fullerene arrangement models are in good agreement with both each other and conventional foam materials while rotated hexagonal and randomly fullerene distributed models are exhibited totally unique mechanical behaviours due to their special structures. In addition to investigation of spatial fullerene distribution effects on mechanical properties of the fullerene-graphene specimens, temperature and strain rate sensitivity of nano foams are studied in this thesis. As a consequence of certain MD simulation results, applied temperatures have no major impact on stress-strain curve tendency; however, young modulus of the materials tend to decrease with higher employed temperature level. In parallel that mechanical tests of fullerene-graphene foams under uniaxial compression show that the form of the stress-strain diagram does not depend on the applied strain rates. However, higher strain rates in general lead to higher stresses under compression at the beginning of the plateau regime and densification phase.