The Out-of-plane Mechanical Properties Of Graphene And The Design Of Graphene Composites

Graphene is a two-dimensional membrane material with a single layer carbon atoms. Due to the fascinating properties such as extremely high strength, high thermal conductivity, quantum electronic transport, optical properties etc., studies on graphene are widely spreaded in material science, physics, chemistry and mechanics. The deformation of graphene can change its electronic, magnetic and optical properties. It is important to understand the mechanical properties of graphene for the applications to nanomaterials, nanodevices and nanoelectromechanical systems. Graphene can be designed at atomistic scale. For example, inducing defects and functionalized groups may significantly improve the mechanical and electronic properties of graphene. To break the limitation of applications in engineering, three-dimensional graphene composites become a study hotspot. Understanding the mechanical behavior of graphene in composites is important for the design of graphene composites.The mechanical properties of graphene can be classified as in-plane and out-of-plane. Graphene generally exists in composites as interface, and its out-of-plane mechanical behavior plays an important role in the mechanical properties of composites. In this work, the out-of-plane mechanical properties of graphene and the influence of graphene on composites is studied systematically.The static and dynamic out-of-plane mechanical behavior of graphene is studied. Buckling is the main static out-of-plane mechanical behavior of graphene. It is found that the buckling is highly influenced by the size and the chirality of graphene. Synthesizing the electron density, potential energy analysis and continuum plate model, the underlying mechanism of the size- and chirality-dependent buckling in graphene monolayer is revealed. Further, the transverse waves in graphene monolayer as typical dynamical out-of-plane mechanical behavior is investigated. It is found that there are remarkable chiral differences of the propagation velocity of transverse waves in graphene monolayer when the vibrational frequency is over 3 THz. In addition, there is an upper limitation of frequency for transverse waves in graphene monolayer, and the critical frequency is also chirality-dependent. Based on the continuum plate model, it is found that the chirality-dependent propagation velocity of transverse waves is resulted by the bending stiffness of graphene monolayer; the upper limitation of frequency is resulted due to the non-continuum effect when the wavelength is close to the carbon-carbon bond length. Based on the out-of-plane mechanical properties of graphene monolayer, high quality, stable ripple patterns are generated and maintained using interferencefrom transverse waves in the graphene monolayer at room temperature. Furthermore, the ballistic impact response of multilayer graphene membrane is studied. It is found that the energy delocalization effect in multilayer graphene is much stronger than other common materials.Inducing defects in graphene is the foundation of atomistic design for graphene and its composites. The bombardment of a suspended monolayer graphene sheet via different energetic atoms is investigated. It is found that, with increase of the bond energy between carbon and incident atom, the probability of single-vacancy defect decreases, and that of direct substituting carbon atom increases. As a special kind of defects, grain boundary can also influence the mechanical properties of graphene. According to the results of study on buckling behavior of graphene, it is found that the grain boundary can improve the twisting strength and the electron transport properties of graphene nanoribbon.The out-of-plane shock and in-plane shear response of graphene-copper nanolayered composites is investigated in this work. It is found that the graphene interfaces show strong/weak duality in composites under shock loading due to the in-plane and out-of-plane difference of strength:on one hand, the weak bending stiffness of graphene leads to interlayer reflections and weakening the shock wave; on the other hand, the strong in-plane sp2-bonded structures constrain the dislocations and heal the material. In graphene-copper nanolayered composites, the copper layers impede the out-of-plane displacement of graphene monolayers, which highly improve the shear strength and toughness of composites. Furthermore, it is found that there are remarkable self-healing effects in graphene-copper nanolayered composites during releasing process.The out-of-plane mechanical properties and the influence of defects on the out-of-plane mechanical properties of graphene are investigated in this work. Furthermore, the influence of graphene on the mechanical properties of graphene-copper nanolayered composites is investigated. It is expected that this work would provide a valuable guideline for design and application of graphene and its composites.