多层石墨烯纳米压痕和划痕的分子动力学研究
发布时间：2018-10-15 11:30
【摘要】：石墨烯具有块体材料无法比拟的优良力学性质,被认为是具有战略意义的新材料,可望在纳电子器件、能量存储、生物医药及场发射材料等领域获得广泛应用,具有非常广阔的应用前景。由于独特的二维平面特性,研究石墨烯力学、摩擦等性质对石墨烯产品的制备有着非常重要的意义。基于分子动力学理论,本文首先模拟了双层石墨烯的纳米压痕过程,讨论了Lernnard-Jones势函数的截断半径最佳值以及得出了典型的载荷-位移曲线。接着以压头数目、温度、尺寸效应等因素为主要考察指标分析了其对石墨烯力学性能的影响。得出双层完美石墨烯薄膜弹性模量和强度分别为0.897 TPa和197.3 GPa。石墨烯的力学性能对温度以及压头尺寸有较强的依赖性。目前生产的石墨烯含有各种缺陷,相较于完美石墨烯,其仍有较大应用价值。因此有必要研究和掌握缺陷对石墨烯性能的影响,以便在目前的生产技术下,推动其工业化应用。在完美石墨烯压痕模型的基础上,建立含缺陷石墨烯压痕模型,重点探讨Stone-Thrower-Wales、空位以及圆孔缺陷对石墨烯力学性能的影响。得出结论:薄膜中心存在STW和空位缺陷时,石墨烯的弹性模量略有降低,而破坏强度下降幅度特别明显。空位缺陷在压头半径范围内存在时,临界载荷与缺陷到薄膜中心的距离呈线性增长比例关系;缺陷数目越多,其杨氏模量、破坏强度等就越低。薄膜中心存在的圆孔缺陷半径越大,则其对薄膜的影响范围越大。在压头范围外存在圆孔缺陷的数目多或半径达到一定尺度后,将会使石墨烯的力学性质显著降低。本文结论也说明石墨烯结构稳定,对小缺陷不敏感,缺陷石墨烯仍具有较好的性能和使用价值。最后模拟了四层石墨烯的纳米划痕过程,特别对划痕深度,速度以及划痕方向等因素与石墨烯摩擦特性之间的关系进行了研究。得出结论:划痕深度大小不同,石墨烯表现出的摩擦特性也不同。当划痕深度较小时摩擦力曲线非常光滑并具有正弦周期性,波动周期约为2.45?,基本等于石墨烯的晶格常数2.46?,石墨烯处于超低摩擦状态。当划痕深度较大时会导致石墨烯层间表现出交联现象,交叉连接原子数目的增多导致了摩擦系数的增高。在一定划动速度范围内,石墨烯薄膜的摩擦力并不随着速度的改变而变化,只是由基体的晶格常数大小来决定。不同划痕方向的摩擦力振幅不一致且波动周期也有一定的差别,摩擦系数表现出明显的各向异性。
[Abstract]:Graphene is considered to be a new material of strategic significance because of its excellent mechanical properties compared with bulk materials. It is expected to be widely used in nanoelectronic devices, energy storage, biomedicine and field emission materials. It has a very broad application prospect. Because of the unique two-dimensional plane characteristics, it is very important to study the mechanical and frictional properties of graphene for the preparation of graphene products. Based on the theory of molecular dynamics, the nanocrystalline indentation process of bilayer graphene is simulated, the optimal truncation radius of Lernnard-Jones potential function and the typical load-displacement curve are discussed. Then, the influence of pressure head number, temperature and size effect on the mechanical properties of graphene was analyzed. The elastic modulus and strength of the bilayer perfect graphene film are 0.897 TPa and 197.3 GPa., respectively. The mechanical properties of graphene have a strong dependence on temperature and pressure head size. The graphene produced at present contains all kinds of defects, and it still has great application value compared with perfect graphene. Therefore, it is necessary to study and master the effect of defects on the properties of graphene in order to promote its industrial application under the present production technology. Based on the perfect graphene indentation model, a graphene indentation model with defects was established, and the effects of Stone-Thrower-Wales, vacancies and circular hole defects on the mechanical properties of graphene were discussed. It is concluded that the elastic modulus of graphene decreases slightly with the existence of STW and vacancy defects in the center of the film, but the decrease of the fracture strength is especially obvious. The critical load increases linearly with the distance from the defect to the center of the film when the vacancy defect exists within the radius of the head, and the more the number of defects, the lower the Young's modulus and the failure strength. The larger the radius of circular hole defect in the center of the film is, the greater the influence range is on the film. The mechanical properties of graphene will be significantly reduced when the number of circular hole defects outside the pressure head range or the radius reaches a certain scale. The conclusion also shows that graphene is stable in structure, insensitive to small defects, and still has good performance and use value. Finally, the nano-scratch process of four layers of graphene is simulated, especially the relationship between the scratch depth, velocity and scratch direction and the friction characteristics of graphene is studied. It is concluded that the friction characteristics of graphene are different with different scratch depth. When the scratch depth is small, the friction curve is very smooth and has sinusoidal periodicity, the fluctuation period is about 2.45, which is basically equal to the lattice constant of graphene 2.46, and graphene is in the ultra-low friction state. When the scratching depth is high, the cross-linking between graphene layers will occur, and the increase of the number of cross-linked atoms will lead to the increase of friction coefficient. The friction force of graphene film does not change with the change of velocity, but only depends on the lattice constant of the substrate. The amplitude of friction force in different scratch directions is different and the fluctuation period is different. The friction coefficient shows obvious anisotropy.
【学位授予单位】：西安建筑科技大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O613.71
，
本文编号：2272413
[Abstract]:Graphene is considered to be a new material of strategic significance because of its excellent mechanical properties compared with bulk materials. It is expected to be widely used in nanoelectronic devices, energy storage, biomedicine and field emission materials. It has a very broad application prospect. Because of the unique two-dimensional plane characteristics, it is very important to study the mechanical and frictional properties of graphene for the preparation of graphene products. Based on the theory of molecular dynamics, the nanocrystalline indentation process of bilayer graphene is simulated, the optimal truncation radius of Lernnard-Jones potential function and the typical load-displacement curve are discussed. Then, the influence of pressure head number, temperature and size effect on the mechanical properties of graphene was analyzed. The elastic modulus and strength of the bilayer perfect graphene film are 0.897 TPa and 197.3 GPa., respectively. The mechanical properties of graphene have a strong dependence on temperature and pressure head size. The graphene produced at present contains all kinds of defects, and it still has great application value compared with perfect graphene. Therefore, it is necessary to study and master the effect of defects on the properties of graphene in order to promote its industrial application under the present production technology. Based on the perfect graphene indentation model, a graphene indentation model with defects was established, and the effects of Stone-Thrower-Wales, vacancies and circular hole defects on the mechanical properties of graphene were discussed. It is concluded that the elastic modulus of graphene decreases slightly with the existence of STW and vacancy defects in the center of the film, but the decrease of the fracture strength is especially obvious. The critical load increases linearly with the distance from the defect to the center of the film when the vacancy defect exists within the radius of the head, and the more the number of defects, the lower the Young's modulus and the failure strength. The larger the radius of circular hole defect in the center of the film is, the greater the influence range is on the film. The mechanical properties of graphene will be significantly reduced when the number of circular hole defects outside the pressure head range or the radius reaches a certain scale. The conclusion also shows that graphene is stable in structure, insensitive to small defects, and still has good performance and use value. Finally, the nano-scratch process of four layers of graphene is simulated, especially the relationship between the scratch depth, velocity and scratch direction and the friction characteristics of graphene is studied. It is concluded that the friction characteristics of graphene are different with different scratch depth. When the scratch depth is small, the friction curve is very smooth and has sinusoidal periodicity, the fluctuation period is about 2.45, which is basically equal to the lattice constant of graphene 2.46, and graphene is in the ultra-low friction state. When the scratching depth is high, the cross-linking between graphene layers will occur, and the increase of the number of cross-linked atoms will lead to the increase of friction coefficient. The friction force of graphene film does not change with the change of velocity, but only depends on the lattice constant of the substrate. The amplitude of friction force in different scratch directions is different and the fluctuation period is different. The friction coefficient shows obvious anisotropy.
【学位授予单位】：西安建筑科技大学
【学位级别】：硕士
【学位授予年份】：2016
【分类号】：O613.71
，
本文编号：2272413
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