Structural Design Of Hollow Carbon Sphere And Study On The Mechanical Properties

In recent years,the hollow carbon sphere has become a research hotspot of new materials due to its advantages of light weight,high strength,controllable size,and large specific surface area.At present,there are many reports on the preparation methods of the hollow carbon sphere,but the theoretical analysis of its mechanical properties is limited,which is reflected in the difference between the theoretical model and the sample prepared in the laboratory.Therefore,in order to restore the configuration details of the samples prepared in the laboratory,and fully explore the mechanical properties and failure mechanism of the carbon sphere,this work designed and established graphene spliced hollow sphere and graphene assembled hollow sphere,based on C₆₀,the simple hollow carbon sphere which exists in nature.The molecular dynamics simulation was used to explore its mechanical properties,failure mechanism and strengthening method,and some conclusions were verified by in situ mechanical tests.Due to the limitation of the potential function accuracy,theoretical studies on the phase transition of C₆₀ molecules are rarely reported.The recently appeared machine learning potential function,GAP-20,has the accuracy which close to the density functional theory,so we use the molecular dynamics simulation based on machine learning potential to analyze the transformation process of C₆₀ clusters under different temperatures and pressures.According to the content of carbon atoms with different hybridization types,the structural transformation diagram of C₆₀ cluster was summarized.For the transformation process from amorphous carbon to diamond structure under high temperature and high pressure,we got the value of activation energy,2.42 e V,through theoretical calculation.The bulk modulus,Young’s modulus and tensile strength of different phase transition structures observed in the compression process were calculated,and the tensile mechanical properties of different structures were further analyzed.Based on the geometric characteristics of a series of fullerene conforming to the principle of independent pentagon rings,such as C₆₀,we designed and established a hollow carbon sphere structure based on graphene splicing,and analyzed the compressive mechanical properties of hollow and helium-filled states using the all-atom molecular dynamics method.The research results show that as the size of the structure continues to increase,the hollow graphene spliced carbon spheres are prone to collapse and failure.Filling the structure with helium can effectively improve the deformation stability of and enhance its anti-pressure properties.Finally,combined with simulation and geometric derivation,it is found that the helium-filled graphene spliced carbon sphere has ultra-light floating characteristics.In order to restore the multi-layer characteristics of hollow carbon spheres prepared in the laboratory and improve the accuracy of model construction and subsequent theoretical calculations,we used graphene as the basic unit to build a hollow carbon sphere model in an overlapping manner.With the help of molecular dynamics simulation,the compressive mechanical properties of graphene assembled hollow spheres are analyzed,and the configuration changes of carbon spheres under different strains are given,which is verified by in situ mechanical experiments.Theoretical and experimental results show that graphene assembled hollow spheres undergo recoverable elastic deformation under small compressive strains,and when the strain exceeds 90%,the structure shows irreversible plastic deformation due to the interlayer slip of graphene,which eventually occurs slip failure.In addition,simulations and experiments show that the shape of the compressing tip will affect the final failure mode of graphene assembled hollow spheres.When the tip changes from blunt to sharp,the failure mode changes from collapse failure to puncture failure.Finally,we introduced interlayer crosslinks within the graphene assembled hollow spheres to enhance the compressive mechanical properties and gas-carrying capacity.Firstly,the molecular dynamics stretching simulation is carried out on the graphene film,and for the graphene film containing large-area hole defects,interlayer crosslinks are introduced to fix the defect-free graphene sheet for supplementation.Calculations found that this method can strengthen the tensile mechanical strength of the damaged graphene film,which proves that crosslinks can improve the interlayer stress transfer of the structure.Based on the above conclusion,a graphene assembled hollow sphere containing interlayer crosslink interface was established.The molecular dynamics compression simulation was used to analyze the effect of the original van der Waals interface and the established crosslinking interface on the compressive mechanical properties of graphene assembled hollow spheres.The calculation results show that the interlayer crosslinks can effectively inhibit the interlayer slip of graphene sheets,so that the carbon spheres change from slip failure to buckling failure under compressive load,the compressive strength of the structure has been improved.In the air-tightness simulation experiment,the crosslinks interface can also improve the air-tightness and prevent the leakage of the gas at high temperature,and enhance its gas-carrying capacity.