Mechanical And Thermal Properties Of Graphene Under In-plane Torsion

As a two dimensional nanomaterial,graphene has attracted considerable attention because of its extraordinary thermal and mechanical characteristics.Strictly speaking,free graphene does not lie completely in a two-dimensional plane,having randomly distributed wrinkles on its surface.As an integral part of graphene,wrinkles have a great impact on various properties of this 2D material.However,due to the randomness of its morphology and distribution of position,the wrinkles cannot be utilized.Therefore,only by periodization and regularization,can wrinkle graphene have more practical application value.Spiral wrinkling patterns have been reported for graphene under circular shearing at the inner edge.This unusual wrinkling patterns has attracted the interest of scholars.But so far,relevant research is relatively lacking.Hence,by combining molecular dynamics method with the theory of continuum mechanics,this article investigated the tunable mechanical and thermal characteristics of graphene under in-plane torsion in three aspects: formation,regulation,and application of graphene wrinkle.The research achievements are listed as follows:(1)Combined with molecular dynamics simulation and continuum mechanics theory,we conducted a cross-scale analysis of the formation mechanism and initial configuration of graphene wrinkles.Firstly,by establishing the continuous medium model of the macro membrane under in-plane torsion and solving the von-Karman buckling equation,the analytical solutions of torque,torsion angle,wrinkle number and wrinkle spiral angle of the macro membrane in the critical buckling state are obtained.Secondly,the MD simulation was used to obtain corresponding data of graphene.And finally,the theory results were compared with the MD simulation results.Under the in-plane torsional loading,the wrinkle number and initial shape of graphene were consistent with that of macro membrane,indicating the cross-scale common characteristics of two-dimensional material torsional deformation.Further study found that the wrinkles of graphene and macro membrane were the result of local compression induced by shear.Although the critical torque and torsion angle of the two models are greatly different in numerical value,they are same in variation tendency,so this theoretical model can still be used to qualitatively predict the buckling of graphene,and the torsional deformation characteristics of graphene obtained by the MD simulation analysis can be applied to the torsional response of the two-dimensional materials under different scale.Besides,the influence of boundary size on the wrinkle number and other variables was also revealed.These results have important reference value for the accurate control of graphene nanodevices.(2)Using molecular dynamics simulation,the effect of outer boundary on the mechanical properties and wrinkle characteristic of graphene under circular shearing at inner edge was investigated.For circular annulus with fixed inner radius,the critical angle of rotation can be increased by several times without sacrificing its torque capacity by increasing outer boundary radius.Unlike circular annulus,the torque capacity of graphene with elliptical outer boundary anomalously decreases with the increase of aspect ratio,and a coupled effect of the boundary aspect ratio and the ratio of minor axis to inner radius on wrinkling is revealed.By studying the stress distribution and wrinkle characteristics,we find the decay of torque capacity is the result of circular stress concentration around the minor axis,and such nonuniform stress distribution is anomalously caused by the change of wrinkle profiles near the major axis.Surface hydrogenation can also tune the torsional strength and wrinkling characteristics of graphene.The ultimate torque of hydrogenated graphene shows a V-shaped variation with the increase of the hydrogenation ratio,which is related to the difference in the strength between sp2 and sp3 bonds and the local stress concentration caused by the non-uniform distribution of bonds.This phenomenon is applicable to other two-dimensional nano-materials such as graphyne.The tunable mechanism of surface hydrogenation on wrinkle amplitude is complex,affected by the material lattice and geometric size.Our results demonstrate that the topological and mechanical characteristics of graphene can be tuned with boundary properties and surface hydrogenation.The specific mechanism of out-of-plane deformation on in-plane strength opens up a straightforward means to develop novel graphene-based devices.(3)Using the non-equilibrium molecular dynamics method,the thermal properties of two dimensional nanomaterials are investigated by considering graphene nanosheets with circular boundaries.The thermal transport efficiency of graphene under heat flux from the inner boundary to outer boundary is revealed to be tunable by applying in-plane torsion at the inner boundary,and the tunable range of thermal conductivity for graphene could be up to 15%.With the increase of rotation angle,the thermal conductivity of graphene is found to increase at small rotation angles and then decrease after the occurrence of wrinkle deformation.The maximum thermal conductivity appears at the onset of wrinkling which depends on the lattice structure and stiffness of the nanosheets.By systematically investigating the morphological characteristics and the phonon spectra under different torsion angles,the tunable thermal conductivities of graphene are found to be controlled by three factors,namely,surface smoothness,stress concentration and lattice instability.The increase of thermal conductivity with small torsion angles is caused by the suppressed surface fluctuation which decreases the phonon scattering,while the wrinkling and lattice instability occurring under large torsion angles accounts for the deterioration of thermal conductivity.Such correlation between out-of-plane deformation and in-plane thermal conductivity is applicable to other two-dimensional nanomaterials,including graphyne.