Research On Thermal Conductivity Regulatory Mechanism Of Polymer Composites

Thermal conductive polymer composite refers to a composite system with a certain thermal conductivity,which is composed of polymer as matrix and thermal conductive material as filler.It possesses both the advantages of polymer and good thermal properties,having broad application prospects in energy,electronics,information,aerospace and other industries.Until now,the thermal conductivity of polymer composites is generally lower than 5W/mK,which cannot meet the needs of production.In this paper,thermal properties of polymer composites were studied from the perspective of engineering thermophysics.On the one hand,theoretical modeling methods were used to study the variation of thermal conductivity of composites with filler networks.Two thermal conductive models for composites containing oriented two-dimensional fillers and hybrid fillers were respectively established.The quantitative rules for the effect of filler network on the thermal conductivity of composites were obtained.On the other hand,the effect of the chemical modification degree and morphology of fillers on the thermal conductivity of composites was investigated through the experiments of graphene/epoxy composites,and the MD models of graphene-epoxy interface were established.The variation laws of interfacial interaction energy with the chemical modification of the filler and the interfacial gaps were analyzed.Through these works,the key influencing factors and their influence laws that affect the thermal conductivity of polymer composites were explored,and the techniques and methods for improving the thermal conductivity of composites were refined,thus providing theoretical basis and method guidance for the preparation of thermal conductive polymer composites.The main work and research results of the paper are as follows.For the single-filled h-BN/polymer composites,a thermal conductive model synthesizing the structural features of most components in polymer composites was proposed to calculate the anisotropic thermal conductivity of hexagonal-BN/polymer composites,which contains fillers with a oriented distribution.The computational results are consistent with the experimental data,and have a regulatory relationship with the key parameters of the model.The results show that the in-plane and out-of-plane thermal conductivity of the composites change inversely with the increase of oriented slope of hexagonal-BN;both the in-plane and out-of-plane thermal conductivity of composites can be enhanced by reducing the interfacial thermal resistance or increasing the filler geometry.For the hybrid-filled GNP-SWCNT/polymer composites,a thermal conductive model based on series-parallel thermal resistance model and Maxwell-Garnett equivalent medium method is proposed.The computational results are consistent with the experimental data,show obvious synergy effects,and have a regulatory relationship with the key parameters of the model.The results show that the equivalent thermal conductivity and the optimal filler ratio(GNP:SWCNT)of the model are influenced by the dispersion of SWCNT;with the increase of the thermal conductivity of SWCNT,the optimal filler ratio keeps constant,while the corresponding highest equivalent thermal conductivity increases;when the total content of hybrid filler increases,the equivalent thermal conductivity increases,while the optimal filler ratio decreases.The experimental thermal conductivity results of epoxy composites filled with SRGO show that when the reduction degree of SRGO increases,the functional groups on its surface decrease(more than 6 wt%in this paper),its own thermal conductivity increases,and the out-of-plane thermal conductivity of SRGO/epoxy composites increases,which indicates that the SRGO with a higher reduction degree is more suitable as the filler of polymer composites and expected to improve the thermal conductivity of polymer composites at a lower filler content;however,when the reduction degree of SRGO decreases,the interfacial gaps between SRGO and epoxy resin increase,which increases the interfacial thermal resistance,and hence slowes down the trend that the thermal conductivity of composites increases with the reduction degree.The thermal conductivity experimental results of epoxy composites filled with different forms of graphene show that graphene nanoplates have better dispersion in epoxy resin matrix than graphite oxide,and when the weight fraction of GNP increases,their edge overlaps,and an effective thermal conductive network is formed in the composites;the GF forms a better thermal conductive network in composites,and the thermal conductivity of 3wt%GF/epoxy composites is nearly 7 times that of pure epoxy resin,and 2 times that of 5.4 wt%GNP/epoxy composites.The MD simulation results of the thermal conductivity of graphene show that the thermal conductivity of monolayer and bilayer graphene both increases firstly and then declines with the increase of the functional group content.There is an optimal modification degree(1～2 wt%in this paper).The interface simulation results of graphene/epoxy composites show that the interaction energy between graphene and epoxy resin decreases with the increase of interfacial gap and increases with the increase of modification degree.When the weight fraction of oxygen-containing functional groups is appropriate(2～4 wt%in this paper),the performance of graphene/epoxy composites can be greatly improved by chemical modification on the surface of graphene;in the graphene/epoxy composites,the interaction energy between the two modified graphene sheets is greater than that between the graphene and epoxy resin,which indicates that the modified graphene tends to agglomerate in the epoxy resin matrix.The chemical modification can reduce the interaction energy between the two modified graphene sheets while increase that between the graphene and epoxy resin,which is helpful for the dispersion of graphene in the composites.