Study On Mechanical Properties And Micro-mechanism Of Graphene Foam Composed Of Single-and Bi-modal Flakes

In recent years,new nano-porous materials are macroscopic bulk porous materials composed of many carbon nanotubes or graphene sheets as basic units,which have received widespread attention.These new carbon nano-porous materials are carbon nanotube network materials,graphene foam materials and composite materials assembled from them.They have excellent mechanical,thermal,electrical and other properties,the same as the advantages of carbon nanotubes and graphene sheets.They are widely used in many frontier fields with broad application prospects such as flexible electronic devices,sensors,energy storage devices,environmental remediation and touch screens.Material scientists have used continuously improved preparation techniques to modify various parameters with an important impact on material properties to prepare various new carbon nano-porous materials with excellent properties.Among these porous carbon materials,graphene foam has been applied to daily life such as smartphone cooling,floor heating,artificial skin,heating clothing,etc.its stable properties,extremely low density,high specific surface area,and easy performance regulation is favored by many researchers.In terms of performance,deformation and conductive behavior of graphene foams composed of single-model sheet are preliminary studied.The mechanical and physical properties can be effectively regulated by changing the layer number of graphene sheets and applying pre-strain,however the effect of graphene sheets with different size on the mechanical properties of graphene foams composed of single-mode sheets remains to be revealed.In addition to the excellent mechanical properties,graphene foam also has viscoelastic properties that can be applied to noise reduction and shock absorption.Traditional damping materials such as rubber have a narrow temperature range and poor stability.As a damping material,graphene foam can be used in a wide temperature range and has good viscoelastic performance stability.Therefore,it is indispensable to study the viscoelastic properties of graphene foams.In daily life,there are many complex factors that affect the viscoelastic performance.The density of the material is one of the most important factors affecting the performance.The vibration frequency,vibration amplitude,pre-strain and other factors are also required for the viscoelastic performance of graphene foam to study.In terms of structure,it is not clear how to control the mechanical properties of graphene foams on the micro-structural level.The distribution and flow law of system energy cannot be obtained through experiments,which is not conducive to understanding the microscopic mechanism.Therefore,it is necessary to carry out systematic theoretical and simulation research on the basis of experiments.Whether all-atom simulation,finite element simulation or coarse-grained molecular dynamics simulation,the graphene foam system is mainly a distributed model composed of graphene sheets with the same layer number.The existing simulation research models are ideal.It is inconsistent with the actually prepared graphene foams composed of sheets with different layer number.Although there have some researches on the deformation and performance mechanism of graphene foams composed of single-model sheets,it lacks a full understanding of the deformation and performance mechanisms of graphene foams composed of different layers.As a result,the design modification and application of related materials are restricted.In this thesis,the graphene foam composed of single-model sheets including crosslinks and considering fractures is established to study the effects of graphene sheet size,number of layers,number of crosslinks on the deformation behavior of the material.The combination of coarse-grained molecular dynamics simulation and dynamic mechanical analysis experiments reveals the laws of viscoelastic properties and micro-mechanism under different temperatures,frequencies,vibration amplitudes,Taking advantage of simulation,effect of different densities,pre-strain on viscoelastic properties and micro-mechanism are studied.Furthermore the graphene foam in the experiment is composed of sheets with different layer numbers,without loss of generality,it is simplified that graphene foam is composed of sheets with two kind of different layer numbers,namely softer sheet and harder sheet.The graphene foam model composed of bi-modal sheets was established,which is qualitatively consistent with the material structure in the experiment.In the simulation,the total number of sheets was fixed at 100,and the number of soft sheets and hard sheets can be further adjusted,v(the number of hard sheets divided 100)is the proportion of hard sheet.The graphene foam model composed of bi-modal sheets has been systematically studied on the deformation behavior and microscopic mechanism of graphene foam materials under uniaxial tension and uniaxial compression loading,the different influences of softer and harder sheets are explored in materials,and the following four achievements have been obtained:(1)Deformation behavior and microscopic mechanism of graphene foam composed of single-model sheets.The size of graphene sheets,the layer number of graphene sheets,and the number of crosslinks regulate the properties of graphene foams.The single-model sheet lengths range from 25 nm to 125 nm,and the material densities range from 312 mg/cm~3 to 131 mg/cm~3,indicating that the basic unit size of the assembled graphene foam is an important factor in regulating the material density.Uniaxial tensile and compressive loads are applied to graphene foams with different characteristics.The higher density,the more crosslinks,and the larger the layer number of graphene sheets,the better the mechanical properties of the graphene foam.(2)Viscoelastic properties and microscopic mechanism of graphene foams composed of single-model sheets.The results of simulation and experimental studies show that,compared with the polymer,the viscoelastic properties of graphene foam remain unchanged and extremely stable in a wide range of temperature and frequency;the density of graphene foam is one of the most important factors affecting the viscoelastic properties.With the increase of density,both the storage modulus and loss modulus increase;the vibration amplitude can slightly affect the viscoelastic properties of the material,with the increase of the amplitude,the storage modulus decreases slightly,and the loss modulus and damping ratio increase slightly;compressive pre-strain and tensile pre-strain are two other parameters that can adjust the viscoelastic properties of graphene foams.During the compression process,the energy in the sheet keeps increasing.When the compression reaches the critical strain value of 0.4,the space between the sheets cannot freely slide to dissipate energy,so the loss modulus decreases.During the stretching process,the bonds are broken and the storage modulus decreases.When the tensile strain reaches the critical value of 0.5,the material is ruptured and the loss modulus decreases.(3)Macro-mechanical correspondence of graphene foams with bi-modal sheets.Materials with different proportion v show different structural and mechanical properties under tensile load.With the increase of the proportion v,the stiffness increases linearly,and the strength first decreases and then increases with the proportion v;when v=30%,the strength has a minimum value.The crosslinked material has four stress stages under tension:elasticity,yielding,hardening and fracture.The strength of the non-crosslinked material decreases with the increase of proportion under compression.The non-crosslinked material has four stages under compression.With the increase of strain,the phenomenon of small strain elasticity,slight increase of stress,sudden decrease of stress,and sudden increase of stress.The Poisson’s ratio curve of the crosslinked material changes drastically.The Poisson’s ratio in the two directions is an inverted"U"shape under tension,and a positive"U"shape under compression,and there is a near 0 or negative Poisson’s ratio phenomenon.The smooth Poisson’s ratio curve for the non-crosslinked material is due to the absence of abrupt flipping of the graphene sheets.Under stretching,the Poisson’s ratio quickly stabilizes.Under compression,the Poisson’s ratio increases with increasing strain because the hard sheets tend to be perpendicular to the loading direction.The elastic recovery ability of graphene foam increases with the increase of proportion v.(4)Microscopic deformation energy and local stress and strain concentration of graphene foams with bi-modal sheets.In initially equilibrated graphene foams,four basic microstructures are found:surface-surface,edge-surface,point-surface and edge-edge.For all materials,the ratio of surface-surface and edge-surface are larger.With the increasing of the proportion v,the areal density and the area of contact points decrease,indicating that the hard sheets are difficult to contact with the surrounding graphene sheets.The study found that under tension,the shear deformation energy of crosslinked materials remains unchanged with proportion v.The tensile bond energy and out-of-plane bending energy increase with the increase of strain.When the strain is less than 0.4,the bond energy dominates the deformation,and when it is greater than0.4,the out-of-plane bending deformation energy dominates.Three deformation energies of without crosslinked materials are positively correlated with the proportion v under tension and bending deformation dominates under any strain.The distribution of local von-Mises stress and local tensile bond strain/in-plane shear strain/out-of-plane bending strain of graphene foams bi-modal sheets is investigated.High-stress regions are unevenly distributed in the 8-layer hard sheet,and only a small portion of them appear in the 1-layer soft sheet.In contrast,whether local tensile bond strain,in-plane shear,or out-of-plane bending strain of graphene sheets,high-strain regions are non-uniformly distributed in 1-layer sheets.These results will help us to understand and further guide the design of graphene foams composed of soft and hard bi-modal sheets.