Structure And Thermal Conductivity Of Epoxy-based Composites With Modified Or Hybrid BN

Due to its excellent adhesion performance,robust chemical stability and easy processing,epoxy resin is widely used in electronic packaging.However,the common epoxy resin is difficult to meet the requirement of effective thermal management for the increasing power density of electronic devices.Therefore,the development of epoxy electronic packaging materials with high thermal conductivity,electrical insulation,low viscosity and low dielectric loss has become more essential for the smooth operation of high power electronic devices.With high thermal conductivity,electrical insulation and low expansion coefficient,boron nitride is an ideal thermal conductive filler,but the only high amount of boron nitride can significantly improve the thermal conductivity of epoxy matrix.However,the high content of thermal conductive filler not only breaks down the processing fluidity of epoxy resin but also reduces the dispersion uniformity of fillers in the matrix and affects the comprehensive performance of epoxy composites.In this paper,the hexahedral boron nitride?h-BN?modified with filler combination or polymer grafting before was used to prepare the epoxy composites.The synergistic effect of fillers and the interfacial capacitance enhancement of grafted polymers were used to improve the processing fluidity and dispersion of epoxy composite,thus improving the thermal conductivity of the composite materials.The main contents of this dissertation are as follows:?1?Silver nanowires?AgNWs?were used as the compound filler to prepare the epoxy/h-BN/AgNWs composites,the effects of aspect ratio?short-AgNWs,long-AgNWs and ultralong-AgNWs with aspect ratio of 75,150 and 1000?and filling content of AgNWs and h-BN,the synergistic effect of long-AgNWs and h-BN on shear viscosity,microstructure and thermal conductivity of epoxy composites were investigated.The results show that under the same filler amount,the more the improvement of the thermal conductivity of epoxy composites with the larger the aspect ratio of AgNWs is more obvious.The thermal conductivities of composites having 1.0 vol.%of short-AgNWs,long-AgNWs and ultralong-AgNWs,are 0.482 W/mK,0.591 W/mK and 1.035 W/mK respectively,which are 153.7%,211.1%and 444.7%higher than the thermal conductivity of pure epoxy.The shear viscosity and thermal conductivity of epoxy/h-BN composites are also enhanced with the increase of h-BN loading.When the loading of h-BN is 15 vol.%,the thermal conductivity of the composite is 1.03 W/mK,and the apparent viscosity at 25°C is less than20 Pa·s.Due to the synergistic effect of one-dimensional long-AgNWs and two-dimensional h-BN,long-AgNWs can significantly promote the dispersion of h-BN in the epoxy matrix,thus obviously improving the thermal conductivity of epoxy/h-BN/long-AgNWs composite and reducing the viscosity of composite materials.When h-BN and long-AgNWs were filled with 15 vol.%and 0.5 vol.%respectively,the thermal conductivity of composite was 1.121W/mK,which was 433.8%higher than that of pure epoxy,the volume resistivity was4.19×10¹¹?·cm,which was lower than the 2.15×10¹⁵?·cm of the epoxy,and maintained good electrical insulation properties.?2?Three types of anatase titanium dioxide?Ti O₂?coated with silver nanowires?AgNWs@TiO₂?were prepared by the sol-gel method and high temperature calcination process,and epoxy/AgNWs@TiO₂ composite were prepared.The effect of TiO₂ coating on the thermal conductivity of composites was investigated.The results show that since the elastic modulus?185 GPa?of anatase TiO₂ is larger than that of AgNWs?102 GPa?and epoxy matrix?3 GPa?,the modulus mismatch of the composite system leads to the interface phonon scattering,which affects the thermal conductivity of the composite.Therefore,at the same filler loading,the thermal conductivity of the epoxy/AgNWs@TiO₂ composite is lower than that of epoxy/AgNWs.For example,at the 4 vol.%filler loading,the thermal conductivity of the epoxy/long-AgNWs@TiO₂ composite is 0.673 W/mK,which is 14.7%lower than epoxy/long-AgNWs@TiO₂ composite?0.772 W/mK?.In contrast,the elastic modulus?70 GPa?of silica is between the AgNWs and the epoxy matrix,and the modulus matching makes the higher thermal conductivity of epoxy/AgNWs@SiO₂ composite comparing with epoxy/AgNWs composite with the same filler loading.Due to the synergistic effect of the long-AgNWs@SiO₂ and h-BN,long-AgNWs@SiO₂/h-BN uniformly disperse in the matrix,thus greatly improving the thermal conductivity of the composite.When the h-BN and long-AgNWs@SiO₂ are 15 vol.%and 0.5 vol.%loadings,the thermal conductivity of epoxy/h-BN/long-AgNWs@SiO₂ composite is 1.157 W/mK,which is 508.9%higher than epoxy resin.While,the thermal expansion coefficient of the epoxy/h-BN/long-AgNWs@SiO₂ composite material is 39.7 ppm/°C,which is 56.5%lower than that of epoxy resin;The volume resistivity is 4.37×10¹²?·cm,the dielectric constant is 5.1 at 10 KHz,and the dielectric loss is less than 0.07;the apparent viscosity at25°C is less than 10 Pa·s over the entire shear rate range,maintaining good fluidity.?3?Epoxy/Al₂O₃/h-BN composite materials were prepared with spherical Al₂O₃composite filling,and the effects of Al₂O₃ content and the synergistic effect of h-BN and Al₂O₃ on shear viscosity,dynamic mechanics and thermal conductivity of epoxy composite system were also studied.The results show that the thermal conductivity of epoxy/Al₂O₃composites increases with the increase of Al₂O₃ loading.When the Al₂O₃ content is 50 vol.%,the thermal conductivity of epoxy/Al₂O₃ is 0.37 W/mK,which increased by 94.7%compared with pure epoxy resin.The synergistic effect of spherical Al₂O₃ and lamellar h-BN can improve the dispersion of the composite,the fluidity of the system,and the thermal conductivity and dynamic mechanical properties of the composite.When the loading of h-BN and Al₂O₃ is 12 vol.%,the zero-cut viscosity of epoxy/Al₂O₃/h-BN composite is 23.7Pa·s;Under the shearing force,spherical Al₂O₃ can promote the fluidity of sheet h-BN in the epoxy resin matrix,showing a strong skateboard effect,the epoxy/Al₂O₃/h-BN composites exhibits strong shear thinning behavior,and the apparent viscosity of the system drops to 15.3 and 11.2 Pa·s when the shear rates are 200 s^-1and 600 s^-1,respectively.The hybrid filler h-BN/Al₂O₃ greatly improve the thermal conductivity of composite materials.When the loading of h-BN and Al₂O₃ are 12 vol.%respectively,the thermal conductivity of epoxy/Al₂O₃/h-BN was 1.24 W/mK,which was 552.6%higher than that of pure epoxy resin.The storage modulus of the composite reached 4.28 GPa,which was 130.1%and 36.7%higher than that of epoxy resin and epoxy/h-BN?12 vol.%?,respectively;the glass transition temperature of the composite was 160.4°C,it has an increase of 21.1%compared with pure epoxy resin;the coefficient of thermal expansion is 36.9 ppm/°C,which is 59.6%lower than that of pure epoxy resin.?4?Poly?glycidyl methacrylate??PGMA?was grafted onto the surface of h-BN by free radical polymerization,and the prepared PGMA-grafted h-BN?h-BN-PGMA?was incorporated into with epoxy resin,the effect of surface modification of h-BN on the dispersion,rheological properties,dynamic mechanics,thermal conductivity and dielectric properties of the composite were investigated.The results show that the PGMA grafted on the surface of h-BN contain more epoxy groups,which can enhance the interfacial compatibility between h-BN and epoxy resin matrix,improve the fluidity of h-BN and its shear-induced orientation ability.For example,at 25°C,the shear rate of the epoxy/h-BN-PGMA composite with a filler content of 15 vol.%is less than 20 Pa·s.Meanwhile,the epoxy group contained in h-BN-PGMA can participate in the crosslinking reaction of epoxy resin,improving the stress transmission ability and the storage modulus of the composite material,therefore,the storage modulus of the composite material reached 3.77 GPa,which was 16.4%higher than that of the same filling amount of epoxy/h-BN.The surface modification of h-BN is beneficial to build heat conduction network in epoxy composite material and improve the thermal conductivity of composite,at 15 vol.%filler loading,the thermal conductivity of the epoxy/h-BN-PGMA composite reached 1.198 W/mK,which was 15.5%and 505%higher than epoxy/h-BN composite and epoxy resin.The surface graft modification of h-BN also slightly increased the dielectric constant and dielectric loss of the composite but remained at a lower range,and the dielectric constant of the composite was 4.93,while the dielectric loss?10 kHz?was less than 0.07.