Study On Some Mechanical Behaviors Of Graphene

Graphene was discovered in2004by A. Geim and K. Novoselov, and hasattracted numerous attentions. Graphene has outstanding mechanical, thermal,electromagnetic, and optical properties. Graphene is regarded as another wonderfulcarbon based nano-materials after carbon nanotubes and fullerene, which hassuperior performance and plenty of application values. In particular, A. Geim and K.Novoselov were awarded the Nobel prize in physics in2010.The mechanical behaviors of graphene are a hot topic in both scientific researchand commercial application. In this dissertation, some mechanical behaviors ofgraphene are studied by molecular dynamics simulations and theoretical modelanalysis. Parts (1-2) are about the strength of graphene, part (3) is about the rigidityof graphene, and part (4) is about the structure stability of graphene. The maincontents are as below:(1) Based on molecular dynamic simulations, we investigate the effects oftemperature and strain rate on the strength of single layer graphene with tilt grainboundaries under tension. We show that temperature can play an important role inthe strength of grain boundaries in graphene. With increasing temperature, thestrength of graphene decreases. In particular, we show that the previously reportedanomalous strength characteristic [Grantab et al, Science,330,(2010)946-948] thatgraphene with large-angle tilt boundaries (which has a high density of defects) ismuch stronger than those with low-angle boundaries holds true at various temperatures ranging from10K to1800K. We show also the strength of grapheneincreases with increasing strain rate. Our results may provide guidelines for thestrength of grain boundaries in graphene.(2) The mechanical behavior of graphene under in-plane shear is studied usingmolecular dynamics simulations. We show that the shear behavior of chiral grapheneis dependent on the loading direction due to its structural asymmetry. The maximumshear failure strain of graphene in one direction may be1.7times higher than that inthe opposite direction. We discuss also the influence of the cut-off parameters on thecalculations. Our findings are useful for the understanding of mechanical behavior ofgraphene and the potential applications of graphene in nanodevices.(3) Although the bending rigidity is a key parameter of graphene, thetemperature effects on it is still standing theoretical debate, and the mechanism ofthis remain unclear. Using molecular dynamics and density function simulations, wefind that the bending rigidity of graphene increase with increasing of temperature.The configurational entropy plays a crucial factor, and this phenomenon is mainlydecided by the configurational status of atoms in real atoms membranes, which iscompletely different of that of macro-and micro-scale membranes theory. We alsofirst report that the Gaussian bending stiffness of graphene decrease with theincreasing of temperature.(4) Different morphologies of graphene can provide great potential for theapplication of graphene-based nano-devices and functional nano-materials. Using molecular dynamic simulations, we show that by altering the temperature, one caninduce unfolding of short (length less than~50nm) scrolled or folded graphene to aplanar state. The mechanism of these phenomena is that temperature modifies thestability of these open structures. We show in particular that morphologytransformation of graphene is not explained by the change of the potential energy ofthe system, but rather it can be explained by a free energy analysis based on thermaldynamics.