| The characteristic scale of microelectronic circuits is approaching the physical limit with their high integration and miniaturization.In this trend,the heat flux density of electronic devices increases sharply,while the heterojunctions of three-dimensional chips have high thermal contact resistance.The thermal conductivity of the chip has become one of the key bottlenecks restricting its development.Epoxy is a thermosetting material with excellent mechanical properties,bonding properties and curing properties.Epoxy has been used as the thermal interface material to fill the space of heterogeneous integrated structures,which can improve the bonding strength between the heterojunctions and improve the thermal transport efficiency of the chip.However,the intrinsic thermal conductivity of epoxy is very low,which cannot meet the industrial demand.In this paper,epoxy-matrix composites with graphene added have been studied.The influence mechanism and regulation method of thermal transport of the epoxy-matrix composites has been investigated by molecular dynamics simulation.Firstly,the molecular dynamics modeling and simulation methods are verified by the calculation of the thermal conductivities of graphene and epoxy.While the effect of vacancy defects on the thermal conductivity of graphene is obtained.Then,three factors affecting the heat transport of epoxy-matrix composites are studied,which include the graphene-epoxy interfacial thermal resistance,graphene arrangement and concentration.It shows that the important way to regulate the thermal conductivity of composites could be by reducing the interfacial thermal resistance of graphene edge-epoxy and improving the heat transfer efficiency of the graphene network.Therefore,works on the reinforcement and regulation methods of thermal conductivity of composites are carried out in this paper:1)Graphene network.Based on the relative position relationship between two graphene sheets,the composite material with an inter-graphene network is classified as two representative volume elements:the stacked model and the interested model.The heat transfer mechanism of stacked and intersected graphene and its enhancement and regulation methods of thermal conductivity of corresponding composites are studied respectively;2)Heat transport of graphene edge-epoxy interface.For the composites without an inter-graphene network,the graphene sheets are not in contact with each other,and the composites can be described by a representative volume element:the contactless model.The effect mechanism of amino groups on the graphene edgeepoxy interfacial heat transfer as well as the strengthening and regulation of the composites heat transport are studied.The stacking of graphene is an important part of the graphene network,and it is very important to explore the mechanism of enhanced heat transfer between layers to improve the heat transfer efficiency of the graphene network.By constructing the partially stacked graphene model,it is found that the interfacial thermal resistance of the stacking region is positively corrected with the stacking area.After adding a vacancy defect with the same size,shape and in-plane coordinates to the two graphene sheets respectively,adaptive interlayer bonding occurs and strong covalent interactions reduce the interfacial thermal resistance by more than one order of magnitude.The number of unsaturated carbon atoms on the edge of vacancy increases as the vacancy size increases.Among those unsaturated carbon atoms,all zigzag unsaturated carbon atoms can form interlayer bonding,while only half of armchair unsaturated carbon atoms occur interlayer bonding and may inhibit the interlayer bonding of adjacent zigzag unsaturated carbon atoms.Therefore,with the increase of vacancy size,the zigzag edge and mixed edge appear alternately,and the interfacial thermal resistance fluctuates down.Furthermore,based on the thermoelectric analogy theory,the mathematical expression describing the relationship between interfacial thermal relationship and Van der Walls interactions as well as the number of interlayer bonds is obtained.Finally,the effect of vacancy dislocations on interlayer bonding and interfacial thermal transport is proved.Based on the interlayer bonding mechanism and heat transfer law above,random defects are constructed to the stacking region of the stacked graphene,and the effects of vacancy coverage rate,the coupling between stacking length and vacancy coverage,stacking forms on the heat transfer efficient are studied.The results of the vacancy coverage rate show that the interfacial thermal resistance is inversely proportional to the vacancy coverage rate,and the inplane thermal resistance is directly proportional to the vacancy coverage rate.The synthesis effect of interfacial thermal resistance and the in-plane thermal resistance leads to the effective thermal conductivity increasing firstly and then decreasing.The study on the coupling between stacking length and vacancy coverage shows that the larger the stacking length,the larger the effective thermal conductivity’s peak value and the smaller the vacancy coverage rate corresponding to the peak value.The study on the stacking forms shows that when there are no vacancy defects,the effective thermal conductivities of AA stacking and ABZZ stacking are similar,which are both larger than that of ABZA stacking.After the vacancy is constructed,there is little difference between the effective thermal conductivity of different stacking forms.The intersecting of graphene is another important part of graphene networks.By constructing the intersected graphene model,the effects of nodal gap width,the number of covalent bond and the intersecting angle on the heat transfer coefficient of intersected graphene are investigated.The results show that with the decreases of the nodal gap width,the heat transfer coefficient in the intersecting node increases due to the stronger Van der Waals interactions.When the nodal gap width decreases to 1.8 ?,two graphene sheets are crosslinked with covalent bonds in the intersecting node,and the strong covalent interactions strengthen the heat transfer coefficient in the intersecting node by two orders of magnitude.After the covalent crosslinking is formed in the intersecting node,the heat transfer coefficient is independent of the nodal gap width and proportional to the number of covalent bonds.In addition,the study on the intersecting angle shows that the heat transfer coefficient increases with the decrease of the intersecting angle under the Van der Waals interactions.But the law is the opposite under the covalent interactions.Based on the above research,the regulation methods of thermal conductivity of composites are studied from two aspects:the graphene network and the interface between graphene edge and epoxy.Firstly,the effect of amino activity and coverage rate on the graphene-epoxy interfacial heat transport is investigated.The results show that the active amino group significantly reduces the interfacial thermal resistance,while the effect of the inactive amino group depends on its coverage rate and distribution.Based on this,the graphene-epoxy interfacial heat transfer coefficient is improved by 303%,and the thermal conductivity of composites is improved by 45.3%when the active amino groups are added to the graphene edge in the contactless model.Secondly,based on the strengthening mechanism of vacancy defects on the heat transport of stacked graphene,the thermal conductivity of composites increased by 130%when 4%vacancy defects are added to the stacked graphene region.Thirdly,based on the heat transfer enhancement mechanism of the intersected graphene by covalently crosslinking in the intersecting node,the thermal conductivity of composites increased by 590%after the two graphene sheets in the intersected model are crosslinked covalently.Finally,a graphene network consisting of stacked graphene and intersected graphene is constructed inside the composite to strengthen and regulate the thermal conductivity of composites. |
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