Research On Thermal Conductivity Of Boron Nitride/epoxy Composites Modified By Functional Group

Epoxy resin has the advantages of easy processing,strong formula operability,excellent insulation performance,chemical stability and low cost,so it has been widely used in the areas of electrical equipment and electronic packaging.But the intrinsic thermal conductivity of epoxy resin is low,and it is not conducive to the effective heat dissipation of system,which severely limits the application of epoxy resin.Hexagonal boron nitride(h-BN)having graphite-like crystal structure,has excellent thermal and insulation properties,strong chemical stability,and low thermal expansion coefficient.It has been an ideal thermally conductive filler.At present,the preparation of BN/epoxy resin composites by filling BN filler particles into the epoxy resin matrix has become one of the effective methods for improving the thermal conductivity of epoxy resin.However,the pristine BN agglomerates seriously in the epoxy resin matrix,so it is difficult to achieve uniform dispersion,and impossible to form effective heat conduction paths.At the same time,the interface compatibility between BN and epoxy resin is poor,and the bonding at the interface is not tight enough,which caused large interface thermal resistance.All of these factors seriously restrict the improvement of thermal conductivity of BN/epoxy resin composites.On the other hand,BN nanosheets(BNNS)can be obtained by peeling off the BN bulks.As a typical two-dimensional sheet-liked thermally conductive filler,the thermal conductivity of BNNS has obvious anisotropy.There is a significant difference between the in-plane thermal conductivity and out-plane thermal conductivity of BNNS.As for the calculation of thermal conductivity of BNNS/epoxy resin composite,most of the reported theoretical models often ignore the influence of the anisotropic thermal conductivity of BNNS.Regarding above practical problems,modified BN/epoxy resin composite material has been prepared and characterized experimentally.The theoretical formula for predicting the macroscopic thermal conductivity of general geometric size BNNS/epoxy resin composite material was derived.The thermal conductivity of BNNS/epoxy resin composite was regulated and analyzed.The main content of this thesis includes:(1)BN has been treated with sucrose-assisted ball milling modification method to prepare hydroxyl functionalized BNNS.The BN before and after modification has been characterized by SEM,FTIR,and XRD.As the characterization results show,the h-BN bulks has been successfully exfoliated into BNNS.At the same time,hydroxyl functional groups were successfully grafted on the surface of BNNS.The epoxy resin was used as the matrix,and the modified BN and pristine BN have been used as thermally conductive fillers to prepare BN/epoxy composites.The test results of thermal conductivity showed that,at low filler content,compared with pristine BN bulks,the modified BNNS showed more significant effects on improving thermal conductivity.When the mass fraction of fillers reached 15%,the thermal conductivity of the composite reached 0.51W/m·K,which is 3.2 times higher than that of pure epoxy resin.Interface thermal resistance calculation showed that the modified BNNS/epoxy resin composite had a lower interface thermal resistance,which is about five points of that of the pristine BN/epoxy resin composite.(2)A two-step method has been used to derive the theoretical formula for the thermal conductivity prediction of BNNS/epoxy resin composite.Firstly,the unit cell model of the BNNS wrapped by the interface layer has been established,and the model was decomposed along the x-axis and z-axis,respectively.The equivalent thermal conductivity expression of the model along the x-axis and z-axis were derived,and then substituted into the theoretical formula proposed by Nan et al.Considering the thermal conductivity anisotropy of the BNNS and the thickness of the interface layer,the effective thermal conductivity calculation expression of the BNNS/epoxy resin composite has been derived.(3)By changing parameters such as the radius and thickness of BNNS,the thickness of the interface layer,and the spatial orientation angle,we calculated the corresponding thermal conductivity according to the derived thermal conductivity expression of BNNS/epoxy resin composite.The thermal conductivity of BNNS/epoxy resin composite was regulated and analyzed.Calculation results showed that the thermal conductivity of BNNS/epoxy resin composite increased with the increase of the radius of the BNNS,when the fillers content was fixed.With the thickness of BNNS increased,the thermal conductivity of composite decreased gradually.When the BNNS aligned parallel to the heat flow direction,the BNNS/epoxy resin composite showed a higher thermal conductivity along the heat flow direction.