Tunable Poisson’s Ratio And Tension-compression Asymmetry Of Graphene-copper Nanolayered Composites

Graphene and its composites are research hotspots in the fields such as material physics,electrochemistry and mechanics.They have important applications in the fields of new energy vehicles,flexible screens,reinforced coatings due to the considerable economic prospects.The two-dimensional structure of graphene limits its application in engineering as structural materials,and thus the three-dimensional graphene composites with advanced functions attracted significant attention in recent years.Understanding the mechanical properties of graphene composites is important in the field of solid mechanics and materials science,as well as the engineering applications considering their further design,promotion and applications.In this thesis,a review of graphene and its composites including the fabrication,characterization,mechanical characteristics,and industrial applications is proposed.The theoretical methods and simulation methods used in this thesis are briefly introduced.Molecular dynamics simulation is a main method in following discussion,and the simulation details including the mechanical equations,potential functions,integration algorithms,ensembles,and atomistic modeling are discussed.In addition,the open source software LAMMPS is introduced.The analysis of dislocation and deformation by using free software OVITO is briefly described.The molecular dynamics model of graphene-copper nanolayered composites was established,and the theoretical loading conditions were designed.The effects of grain size,graphene chirality and repeating layer spacing on the Poisson’s ratio of graphene-copper nanolayered composites under tension and compression are investigated by combining theoretical mechanics and molecular dynamics simulations.Based on the mechanism of molecular dynamics simulation,the theoretical mechanical model of Poisson’s ratio of composite materials was established,and the molecular dynamics results were verified by comparison.The underlying mechanisms of elastic deformation and fracture failure of graphene-copper nanolayered composites are revealed,and a bottom-up mechanical model is proposed to describe related mechanical properties.Considering the Poisson’s ratios of graphene-copper nanolayered composites under tension and compression by molecular dynamics and theoretical analysis.It is found that the Poisson’s ratio of a graphene-copper nanolayered composite can be tuned by tailoring its repeat layer spacing without changing the topological structures.The effects of nanocrystalline Cu grain size on the Poisson’s ratio can be negligible.There are remarkable in-plane anisotropy and tension-compression asymmetry in the Poisson’s ratio due to the chiral difference in compressive stress in graphene layers.A mechanical model considering the chirality and repeat layer spacing is proposed,which can accurately predict the Poisson’s ratio of a graphene-copper nanolayered composite.For stable graphene-copper nanolayered composites,the repeat layer spacing should be larger than 2 nm,and their tunable range of Poisson’s ratio is 0.1-0.35.In summary,combining the microscopic mechanism and the macroscopic mechanical model,the simulation results greatly improve the research of the Poisson’s ratio of graphene-copper nanolayered composites under tensile and compressive loading.In this thesis,the microscopic mechanisms of the tensile-compression asymmetry of the composites are revealed.The understanding provides theoretical support for the realization of tunable Poisson’s ratio without changing the topological structure of materials.The results ensure the unique potential applications of graphene composites in engineering,and it is expected that this thesis could provide new insight into the further design of graphene composites and other materials with similar interface structures.