Molecular Simulation Studies On Low-dimensional Carbon Materials Coating On Micro/nano Patterned Surface And Their Promoting Effect In Methane Hydrate Dissociation

The unique geometric features of the low-dimentinal carbon materials(graphene and single-walled carbon nanotube) bring excellent physical and mechanical properties and obtain a very wide range of applications in nanocomposite materials and new energy industry. Considering the complexity of materials surface, the coating process of monolayer graphene in micro-nano surface has a very important practical significance in the filed of nano-composite materials. Methane hydrate is one of the green, environmentally friendly new energy alternatives. Therefore, the research of the releasing process of the methane from the methane hydrate can help to relieve the increasingly serous environmental and energy crisis.In the third part of this thesis, using the optimized Tersoff and AIREBO force filed, molecular dynamcics(MD) simulations are carried out to study the effects of different sizes of the nano pillars on the adhesion properties of graphene. Comparing the results of the preceding potentials, it is found that the adhesion energy got from the AIREBO force filed is larger than the one form the Tersoff force field because the former considers a long-range interaction correction. The simulation results show that, in the case of a fixed distance between pillars the adhesion energy increases with a decreasment of the hight of the nano pillar. When the height of the pillar is sufficiently hight, the graphene will be adhered to the top of the pillars and the adhesion energy reaches its minima. In addition, when the graphene begins to contact with the substrate, its configuration changes constantly until the system is stable. Analysis show that the overall tend of the potential energy is that the potential energy decreases with time, while there are some exceptions: the potential energy of the system reaches local minima, then it ‘jumps up’ through the saddle point and finally reaches its global minima. To investigate the graphene sheet which is adhered to the top of the pillars, the height of the pillar is set sufficiently large and the initial distance between the graphene and the top of the pillar is set to 3 ?, where the graphene can quickly achieve its stable configuration. On this basis, we consider the wave phenomenon on the edge of graphene with different temperatures and graphene size. Regardless the size of graphene and temperature, the wave exists on the armchair side while we cannot find obvious wave on the zigzag side. Temperature cannot obviously afftect the amplitude and wave length of the wave. When the size of graphene increases and the temperature is fixed, the amplitude and wave length of the wave increases as well. Consider the impact of the cross-secitonal area of the nano pillar on the adhesion energy, it is found that if the area is larger, the the adhesion energy is smaller.It is not easy to put graphene sheet horizontally on the top of the substrate in practice and some experimental results show that the tiled angle of graphene affects the final morphology of the graphene/membrane system. Based on the above considerations, in the fourth part of this thesis we investigate the effect of different tiled angles and different tiled ways of graphene on the final morphology. The results show that, in the scope of this study, if a parallel graphene cannot completely adhere to the substate, the tiled one can only get a partial adhesion. If one anlge alters, the final configurations of the graphene are mostly “I” shape. When two angles alter, the final configurations of the graphene are mostly “L” shape. Interestingly, the graphene get a “W” shape when the tiled angle equals 2° and the temperature is set to 50 K, while all configurations are “L” shape if the temperature increases to 300 K. The adhesion process can be treated as competition effect of the bending, stretching and termal fluctuation of the graphene. When the temperature is high, the effect of thermo fluctuation is obvious so that the system can overcome some energy barriers to reach a state of equilibrium as quickly as possible.The fifth part of this thesis investigates the dissociatation process of methane hydtate with the presence of a single-walled carbon nanotube and the accuracy of the force field is examined by the stability of the methane hyrate in the first stage of the simulation. This part dicusses the effects of different diameters of single-walled carbon nanotube on the dissocation rate of the hydrate, the diffusion rate of the methane and the concentration distribution of methane in the length direction of the carbon nanotube. The results show that if the diameter of nanotube is larger, the dissociation speed of hydrate in the stage of inserting and motionless process is faster because of the larger contact area of the carbon nanotube and the hydrate. In addition, when the diameter of carbon nanotubes is larger, the rate of diffusion of methane is faster: in the case of a larger diameter, i) the density differentce is higher, ii) the methane molecules can move with fewer constraints.In this thesis, we investigate the coating process of graphen on nono-pillars and the dissociation process of the methane hydrate with the presence of carbon nanotube. Our study provides some basic research achievements for the low-dimesional carbon materials.