Preparation And Properties Of NFC/M_xO@rGO Composite Films With Highly Thermal Conductivity And Electrical Insulation

Recent decades have witnessed the unprecedented development of the electronic industry due to miniaturization and integration technology of electronic devices,which subsequently bring about a serious problem on accumulation of heat.Thus,it generates the higher demands for thermally conductive materials.Polymer-based heat conducting materials have received wide attention on the field of heat dissipation due to their easily processing and excellent electrical insulation properties,especially in electronic packaging.However,on account of the polymer’s low thermal conductivities,they often need to be modified before being used as heat conductive materials.In this paper,reduced graphene oxide（r GO）with excellent thermal conductivity was chosen as the thermally conductive filler,an oxide（M_xO）insulating layer was decorated on the surface of r GO to prepare thermally conductive hybrid fillers and the obtained hybrid fillers were mixed with nanofibrillated cellulose（NFC）to prepare the composite films with high thermal conductivity and electrical insulation via the vacuum-assisted filtration and mechanical compression.The detailed research contents and conclusions are summarized as follows:（1）The graphene oxide（GO）decorated by magnesium hydroxide（Mg（OH）₂）particles was synthesized by chemical co-precipitation and the r GO hybrid filler（Mg O@r GO）coated with magnesium oxide（Mg O）was obtained after thermal reduction treatment.Then the NFC was mixed with Mg O@r GO to prepare NFC/Mg O@r GO composite films by a simple compounding method.The results showed that Mg O nanoparticles can not only cut off the r GO conductive pathways,but also improve the dispersion of r GO in NFC and reduce the interface thermal resistance of the composite films.When the addition amount of filler was 20 wt%,the in-plane thermal conductivity（λ_x）of the NFC/Mg O@r GO composite film was significantly improved to 7.45 W/（m·K）,which was 7.3 times larger than that of the pure NFC film and 1.3 times of the NFC/Mg O/r GO film.However,the cross-plane thermal conductivity（λ_z）of 80NFC/20Mg O@r GO film was only 0.32 W/（m·K）,indicating highly anisotropic heat diffusivity.LED lamp simulation test confirmed that the NFC/Mg O@r GO composite film had stronger ability of heat dissipation than pure NFC film.Meanwhile,the surface electrical resistivity and volume electrical resistivity of the film were above 10¹⁰Ωand 10¹¹Ω·m respectively,which was in the range of electrical insulation.（2）The r GO hybrid fillers loaded with alumina（Al₂O₃）particles of different sizes were synthesized by different drying methods and then they were mixed with NFC to prepare a series of NFC composite films.The effect of Al₂O₃ particle sizes on the thermal conductivity of NFC composite films was studied.The results showed that the surface areas of l-Al₂O₃ particles were smaller than that of s-Al₂O₃ particles,therefore,the interface resistance of the film containing l-Al₂O₃ particles was lower,resulting in the smaller interface thermal resistance and superior thermal conductivity.When the Al₂O₃@r GO was at a loading of 40 wt%,theλ_x of the NFC composite film containing l-Al₂O₃ particles reached 11.74 W/（m·K）while theλ_x of the film containing s-Al₂O₃particles was 10.76 W/（m·K）,accompanying with the lowλ_z of both films.LED lamp simulation test confirmed that the film containing l-Al2O3 particles owned a better heat dissipation ability.（3）In order to increase theλ_z value of the NFC composite films,silicon dioxide（Si O₂）particles with high thermal conductivity at a high temperature when they grow into silicon carbide（Si C）whiskers were chosen as the insulating layer on r GO.The composite film was prepared by blending NFC with hybrid fillers and the effect of molding behavior on the stratified structure and performance of the composite film were discussed.The results showed that the compatibility between NFC and Si O₂@r GO filler was outstanding,and the addition amount of Si O₂@r GO fillers could reach a high value.The higher filler loading brought about more pores in the film,and the molding treatment could increase the opportunities of contact between the fillers,thereby improving the thermal and mechanical performance of the composite films.Theλ_x of the NFC composite film with 60 wt%Si O₂@r GO filler was 8.97 W/（m·K）and increased to 10.32 W/（m·K）after the molding treatment.In addition,the mechanical properties of the composite film after molding had also been improved.