Preparation And Properties Of Polymer-Based Thermally Conductive Composites Filled With Boron Nitride

With the advent of the 5G era,thermally conductive composites for electronic packaging have become a hot research topic.Based on the continuous development of electronic devices towards integration and miniaturization,the heat flow density per unit area is increasing,which leads to more and more prominent heat dissipation problems,and how to efficiently prepare composite materials with excellent thermal conductivity is the current goal pursued by researchers.Hexagonal boron nitride（h-BN）is used in various fields for its outstanding thermal conductivity and other excellent properties,so h-BN is chosen as the most important filler in this paper.To further improve the thermal conductivity of the composites significantly,a small amount of MWCNT is selected as the second thermally conductive filler in this paper.The introduction of carbon material does not degrade the insulation performance of the composite,which stems from the fact that the spatial distribution of MWCNT is controlled by fully considering the electrical conductivity of MWCNT in the structural design.The main research of this paper is as follows.（1）In this part,MWCNT and alumina（Al₂O₃）were introduced into the polystyrene（PS）matrix by melt mixing to make PS/MWCNT/Al₂O₃ complex spheres with a diameter of 500μm.MWCNT and Al₂O₃ established a three-dimensional thermal conductivity network inside the complex spheres through synergistic effects.The（PS/MWCNT/Al₂O₃）@h-BN composite was produced by hot pressing by wrapping different contents of h-BN on the surface of the complex spheres.The effect of filler content on the thermal conductivity of the composites was investigated by comparing and analyzing the microscopic morphology of the composites,and the effect of filler content on the properties of dielectric and thermal stability as well as mechanics was further analyzed.It was finally found that the thermal conductivity of the composites increased with the increase of both filler contents,and the thermal conductivity was elevated to 3.36W/m K when the h-BN content was 30 wt%and MWCNT was 0.5 wt%.With the increase of h-BN content,the dielectric properties,thermal stability and mechanical properties of the composites were gradually improved.The addition of MWCNT did not significantly affect the insulating properties of the material,and the conductivity value of the composites was only 6.44*10^-9 S/cm at 10 wt%of h-BN and 0.5 wt%of MWCNT.（2）In this part of the experiment,PS polymer was melt-mixed with quantitative MWCNT and Al₂O₃ to make PS/MWCNT/Al₂O₃ complex spheres with different diameters,and different contents of h-BN were introduced to the surface of these complex spheres,and the（PS/MWCNT/Al₂O₃）@h-BN composites with different densities of thermal conductivity networks were hot-pressed to investigate the effect of the denseness of the isolation network on the thermal conductivity of the composites.By comparison,it was found that the denser the isolation network,the higher the thermal conductivity of the composites.The thermal conductivity of the（PS/0.5%MWCNT/Al₂O₃）@30%h-BN composite was enhanced to 3.98 W/m K when the particle size of the complex spheres was 200μm.（3）In this part of the experiment,boron nitride nanosheets（BNNS）were prepared by the ball milling method and liquid phase exfoliation method.BNNS/polyvinyl alcohol（PVA）fiber film and MWCNT/PVA fiber film were obtained by electrospinning and wrapped with PS solution respectively.The composites with the sandwich structure were prepared by hot pressing in the sequence of BNNS/PVA/PS-MWCNT/PVA/PS-BNNS/PVA/PS,and the effects of filler content on the thermal conductivity,dielectric,thermal stability,and mechanical properties of the sandwich composites were investigated.It was found that when the filler content was 30 wt%（content in PVA）,the in-plane thermal conductivity reached 4.69 W/m K,which was 23 times higher than that of the pure polymer.The dielectric and thermal stability properties as well as the mechanical properties are gradually improved with the increase of filler.