Structural Characteristics And Mechanical Behavior Of Three-dimentional Graphene Networks

The novel physical properties of two-dimensional graphene,such as in-plane ultrafast electron transport,high thermal conductivity,superconductivity under certain conditions,and super strong in-plane mechanical properties,enable it a potential material for important applications in catalysis,adsorption,electrode,sensor,and composites.However,due to the large specific surface area and interlayer adsorption,the aggregation of the few-layer graphene can be harmful to its performance in devices.Constructing three-dimensional graphene networks with good connectivity is one strategy to solve this problem.The three-dimensional graphene networks,prepared experimentally or designed theoretically,are found to inherit the properties and potential applications of graphene.The structural and mechanical properties of graphene networks are foundations to support their application in devices,where deformation usually happens.To ensure the stability and durability of performance,it is necessary to have a full understanding of its mechanical properties and deformation mechanism.At present,there are many reports on the mechanical behavior of non-covalently connected graphene networks in the literature,but the understanding of the structure and mechanical behavior of covalently connected graphene networks is still lacking.The covalently connected graphene networks have better connections compared to the non-covalently ones.However,their microstructure is still far from being fully understood by experiments due to inconvenience for real-time observation and limited resolution,which hinders the prediction of mechanical and other properties.A study on their microstructure and mechanical behavior is of great importance to predict the material properties and future applications.In this paper,the molecular dynamics simulation method is used to construct the full-atom model of disordered graphene networks and study the mechanical behavior of three-dimentional graphene networks.The structural evolution of disordered graphene networks was revealed by annealing simulations,and the structural features,including connections,defects,and stacking behavior,were studied by characterizing the atomic coordinates.We found three typical stages during the formation of disordered graphene networks;that is,the formation of polyaromatic fragments,the formation of a disordered framework,and graphitization,respectively.From the perspective of the final morphology,graphene is spliced in the layer through grain boundaries and defect clusters,and connected by screw dislocation,Y/T junction,and single C-C bonds.Besides,it is found that density and reaction temperature affect the local curvature,stacking behavior,connections,and porosity of the structure.Temperature also has a great influence on the graphitization level.The mechanical behavior of disordered graphene networks under tension,compression,and shearing is investigated in this work.The deformation mechanism and the scaling law of mechanical properties to density are analyzed.It is found that the disordered networks show linear elastic deformation under different loadings when the strain is small.The local deformation includes the bending and rotation of graphene,and the average rotation angle depends on the density.Under large deformation,the fractures are restricted to the local sites because of the disordered connections,resulting in a wide plastic range in the stress-strain curve.The modulus and strength change near linearly with the decrease of density,which is close to the theoretical limits of low-density materials.The disordered graphene networks overcome the shortcomings of the rapid reduction of mechanical properties of common materials at low densities.It highlights the advantages in mechanical properties of the covalent networks compared with the non-covalent networks.Graphene and carbon nanotubes can be connected by heterojunction to form ordered pillared layers.In this paper,the out-of-plane mechanical behavior of pillared graphene networks under uniaxial compression and tensile deformation is studied.All structures exhibit excellent elasticity with a large compressive/tensile elastic strain limit.In all cases,the small deformation is dominated by the bending of graphene layers.At larger strain,graphene layers form a corrugated pattern and shrink laterally in both the compression and tension processes,while the structure remains elastic behavior.Under compressive loading,the failure mode depends on the pillar distance:damage on the junctions when pillar distance is small and interlayer aggregation when pillar distance is larger than a critical value.The elastic compression strain limit exhibits a local maximum at the critical point between the two failure modes.Under tensile loading,failure is due to structural damage both in small and large pillar distance cases and elasticity increases with the pillar distance increasing.In conclusion,the structural morphology of three-dimensional graphene networks,the deformation mechanism under various loadings,and the influence of structural parameters on mechanical properties are investigated from a molecular sight.These results will help to understand the formation process of flash graphene,pyrolytic carbons and carbide-derived carbons,give additional insight into the outstanding properties of three-dimensional graphene networks,and will guide further designing and application of high-performance graphene materials.