Simulations Of The Mechanical And Thermal Properties Of Graphene/epoxy Nanocomposites

Owing to its good bonding properties and chemical stability,epoxies have been widely used in many engineering fileds.However,their low toughness,poor impact resistance and low thermal conductivity(⁰.2W/mK)have limited their high-end application in key fields such as aerospace and electronic packaging.The incorporation of fillers is a common way to improve the their strength and thermal conductivity.among various fillers,graphene(Gr)is expected as an ideal one due to its unique two-dimensional structure and excellent physics properties.However,the experimental values of the strength and thermal conductivity of Gr/epoxy composites are much lower than the expected theoretical values.Existing researches suggest that the main reasons may be related to the intrinsic defects in Gr and the interfacial mechanical/thermal properties between Gr and epoxy.Due to the lack of effective experimental methods for verifing the above speculations,the mechanisms affecting the mechanical/thermal properties of Gr/epoxy composites are not clear.Aming at the above problems,by carring out the equilibrium and non-equilibrium molecular dynamic simulations,this thesis investigates the intrinsic defects in Gr,the interfacial properties between Gr/epoxy and the mechanical/thermal properties of the composites.The specific work and results are listed as follows:First,the molecular models of Gr with different defect tyeps/concentratios were established,and the effects of temperature,strain rate,size and defect on its mechanical and thermal properties were studied.The results show that the mechanical and thermal conductivity of Gr decrease with increasing temperature.By calculating the strain rate sensitivity index,it is found that the mechanical properties are more sensitive with strain rate in the zigzag direction than that along the armchair direction.Besides,the thermal conductivity follows a power lawer of ?^L0.28,the increase of length can excite more phonon vibrations and contribute to the improvement of thermal conductivity.When the defects are introduced,the mechanical properties,including Young's modulus,fracture strength,fracture strain,and the thermal conductivity are significantly reduced even at a low defect concentration.The existence of defects also reduces the sensitivity of thermal conductivity to temperature.The interfacial mechanical properties between Gr and epoxy resin are studied by conducting pull-out simulations.Two typical separation modes i.e.,the normal separation and shear separation were considered,and the effects of defects are investigated.It is found that the vacancy defect will reduce the interfacial cohesive and shear strength,while the introduction of Stone-Wales(SW)defects can effectively improve the interfacial strength.The main enhancement mechanism is that the adsorption energy between the Gr containing SW defects and epoxy resin is larger,the existence of defects can increase the?-?interaction force,thus leading to a better interfacial load transfer efficiency.In addition,the agglomeration of Gr reduces the interfacial shear strength compared to the well dispersed state.The interfacial thermal properties between Gr and epoxy resin are studied by non-equilibrium molecular dynamics simulations,and the effects of system size and typical defects on the interfacial thermal properties of the Gr/epoxy are investigated.The results show that both the SW and multi-vacancy defects can effectively enhance the interfacial thermal conductance.The mechanisms can be explained by the fact that the introduction of defects facilitates better overlaps between the phonon vibration spectrum of Gr and epoxy especially at the low-frequency modes(i.e.,the out-of-plane phonon vibration of Gr),leading to better interfacial thermal transport.Based on the above results,the mechanical and thermal properties of Gr/epoxy composites were investigated.The stress-strain curves are obtained by conducting uniaxial stretching and shearing simulations.The effects of temperature,dispersion state,volume fraction and defects of Gr on the mechanical properties of the composites are discussed.The results show that the mechanical properties of composites decrease with the increase of temperature due to the temperature-dependent behavior of Gr,while the sensitivity to temperature increases with the increase of volume fraction.When the SW defect is introduced,the tensile modulus and in-plane shear modulus of the composites will decrease due to the degradation of mechanical properties of Gr,however,the shear modulus along the longitudinal direction will increase with the increase of SW defect concentration.The main enhancement mechanisms are that,on the one hand,the SW defect will increase the wrinkles of the Gr,and the roughness becomes larger,effectively preventing the slip between the Gr and the epoxy resin during the longitudinal shear deformation.On the other hand,by calculating the interfacial energy,it shows that the SW defect can effectively increase the interaction energy between the Gr and matrix,leading to improve the mechanical properties of the composites.By the calculation of the effective medium theoretical model,it showed that increasing the size,volume fraction of Gr and improving the interfacial thermal conductance could increase the thermal conductivity of the composites,and the volume fraction is the dominant factor.Combined with the calculation results of interface thermal conductance,it is found that although SW defects can reduce the thermal conductivity of Gr,the thermal conductivity of nanocomposites containing defective Gr outperforms the ones without defects due to the enhancement of interfacial thermal conductance between Gr and epoxy when the filler size is smaller than a critical value.The thermal conductivity of composites will change from“interface dominant”to“filler dominant”as the increasing size of the Gr.In this paper,the microstructure of Gr and the interfacial properties between Gr and epoxy are investigated by using simulations,the factors affecting the mechanical/thermal properties of Gr/epoxy composites and the related mechanisms are discussed,which are of great significance for guiding the intrinsic defects in Gr and the interfacial properties,as well providing new ideas for the development and design of Gr/epoxy nanocomposites.