Molecular Dynamics Study On The Relationship Between Graphite Microcrystal Structure And Thermal Conductivity In Mesophase Pitch-based Carbon Fibers

Mesophase pitch-based carbon fiber（MPCF）is a kind of carbon fiber material prepared from mesophase pitch.It is considered as one of the most potential heat management materials due to its excellent thermal conductivity and stability.The high thermal conductivity of MPCF is closely related to its unique graphite microcrystal structure.Therefore,in order to prepare high thermal conductivity MPCF,it is necessary to explore the relationship between its graphite microcrystal structure and thermal conductivity to guide the optimization of the preparation process.In view of the difficulty of practical research on the microstructure of carbon fibers and the difficulty of accurately describing the relationship between graphite microcrystal structure and performance,this thesis employs the non-equilibrium molecular dynamics（NEMD）method to study the correlation and the micro-mechanism between microcrystal structure and thermal conductivity by separating various structural parameters of graphite microcrystal in MPCF and modeling variables separately.The main conclusions are as follows:1.The effects of structural parameters such as the number of microcrystal layers,d₀₀₂ value,length,width,and the coupling effect between different parameters on thermal conductivity were studied.The results show that changing the size parameters of microcrystals will change the free path of phonons or the average number of phonons per unit volume in the system,thus affecting the thermal conductivity of microcrystals.However,different structural parameters cause various trends of change.Increasing the number of microcrystal layers or the d₀₀₂ value will reduce the thermal conductivity,but the d₀₀₂ value has a more severe impact;Increasing the length or width of microcrystals will enhance the thermal conductivity,and increasing the length has the most pronounced effect.In terms of effect coupling,the layer number effect and the in-plane thermal conductivity size effect can only form a superposition relationship,but the in-plane thermal conductivity size effect can create a coupling effect;High temperatures can reduce the thermal conductivity of microcrystals and weaken the thermal conductivity length advantage of microcrystals but have no significant impact on the width effect or layer number effect.2.The influence of the defect structure of microcrystals on their thermal conductivity and system stability was studied.The results show that defect structures can reduce the system stability and cause phonon defect scattering to reduce the thermal conductivity and thermal conductivity length advantages of microcrystals.Increasing the size or dispersion of vacancy defects in the body will reduce the microcrystal thermal conductivity and stability,but the harm of distribution is more significant.The damage of doped defects to microcrystals is relatively mild,and doped N atoms better match the structure of graphite microcrystals.Increasing the amplitude or disorder degree of edge defects will reduce the thermal conductivity and stability of microcrystals,but the influence of disorder degree is relatively small,and its impact trend is related to the defect amplitude.The layer distribution of defects does not affect stability,but a more concentrated layer distribution is beneficial for preserving thermal conductivity.Increasing the angle between sheets or bending the sheets will reduce thermal conductivity and stability.The gap between microcrystals significantly impairs thermal conductivity.However,dangling bond edges and card stack structures will facilitate intergranular bonding to retain thermal conductivity.The effect of Umklapp scattering is related to the intensity of defect scattering.When the defect scattering is strong,the thermal conductivity of microcrystals is not sensitive to temperature.3.The micro change process of graphite microcrystal structure during heat treatment and its influence on the properties of graphite microcrystals were studied.The results show that higher heat treatment temperatures or longer treatment times will cause deformation of the edge structure of intact microcrystals,thus reducing performance.However,from the perspective of microcrystal clusters,such deformation will facilitate the bonding and growth between microcrystals,thus improving the overall thermal conductivity.During heat treatment,single vacancy（SV）defects will undergo random migration behavior,and the migration speed is positively correlated with temperature;Simultaneous migration of multiple SV defects may produce four different results,but the final state and property repair performance of migration is related to defect concentration;Large vacancy defects are hard to migrate during heat treatment,but it’s structural deformation which affects the properties of microcrystals can still occur when the number of missing atoms is below 5.During heat treatment,edge atoms will spontaneously fill in edge blocky defects to repair microcrystal properties,but this is related to heat treatment temperature,the size of blocky defects,and the degree of confusion of edge defects.