Theoretical Researches On The Impact Mechanical Behavior Of Graphene

As known to all, graphene shows good structure stability, electrical conductivity, thermal conductivity and other properties, due to its unique structural characteristics, and graphene has broad application prospects in electronics, energy, biological medicine, sensors, composite materials and other fields. Current research on the mechanical properties of graphene mainly concentrates on the elastic properties and intrinsic strength. The young’s modulus, poisson’s ratio, fracture strength, etc. have always been studied and the effect of size, chirality and defect on properties also attracts more and more attention. However, study on mechanical properties of graphene under impact has been always ignored. As the material of the highest intensity in the world, graphene has potential applications in a lot of areas. So it is of great significance to figure out the deformation mechanism and destruction mechanism of graphene under impact.In this thesis, we have carried out systematic molecular dynamics simulations on the elastic deformation behavior and fracture behavior of monolayer and multilayer graphene sheets under impact. Our work reveals the details of forming process of wavelike deformation and atomic vacancy defects resulted of fracture of graghene. The primary coverage and results of this work are given as follows:1) It has been found that hexagonal C-C bond stretching bands caused by impact appear in the two-dimentional (2D) plane of graphene. When the graphene is impacted, in the impact area, propagable vibration ripples form in the three-dimensional (3D) space, and in the surrounding area, propagable hexagonal C-C bond stretching bands form in the2D plane. The shape of vibration ripples is affected by the shape of impactors at beginning of its formation. Finally, vibration ripples evolve into circular ripples while they are always polygonal ripples before. The formation of hexagonal C-C bond stretching bands is decided by its structure characteristics. Parallel C-C bonds in the graphene tend to stretch together when graphene is impacted. And this kind of stretching can propagate in graphene plane. The propagation velocity is higher in the direction of parallel C-C bonds than other directions. Finally, the stretching encounters to form the hexagonal C-C bond stretching bands.2) Because of its propagable property, hexagonal C-C bond stretching bands have some characteristics of general physical wave. The bands could reflect when they meet the edge or barrier in its propagation direction. And the reflected bands still show hexagonal shape. Interference will appear when the reflected bands meet the original bands. However, we have proved that there is no diffraction in the propagation of hexagonal C-C bond stretching bands. In addition, we also study the effect of atomic vacancy defects on the propagation of hexagonal C-C bond stretching bands. The results show that the propagation velocity and the hexagonal shape are independent on the presence of small atomic vacancy defects.3) We proposed that hexagonal structural fragment is the most stable unit of graphene. Under the sharp impact, the parallel C-C bonds surrounding the hexagonal structural unit tend to fracture together. Then the hexagonal structural unit divorces from the graphene structure and hexagonal vacancy defect forms. Sometimes metastable triangular vacancy defect could also be created. The shape and size of vacancy defects can be well-defined by the impact energy.The results of this article greatly enrich the research on the mechanical properties of graphene and confirm its great application potential. This study not only reveals the deformation theory of the graphene under impact but also opens up a new way to machine graphene on a nanoscale.