Preparation Of BNNS/CNF Thermally Conductive Film And Its Application In E Poxy Composites

As microelectronic devices and components are becoming increasingly smaller,lighter,and more versatile,their operating frequency increases,thus the amount of heat generated during operation also increases dramatically.Excessive heat accumulation will affect the reliability of electronic equipment,shorten its using time,and can even cause device failure.Achieving efficient heat dissipation is an urgent problem to be solved,which has a great impact on the performance,reliability and service life of electronic equipment.Polymer materials are widely used in the field of microelectronics due to their excellent processability,good electrical insulation,and low cost.However,the thermal conductivities of the polymers are generally low,and can not meet the heat dissipation requirements of microelectronic products.Therefore,in order to improve the thermal conductivities,it is necessary to adding heat conductive fillers into polymers.The thermal conductivity of polymer-based thermally conductive composites is determined by both the polymer matrix and the heat conductive fillers.The factors affecting the thermal conductivity of the thermally conductive composites mainly include filler type,shape,size and amount;the dispersion state of fillers in the matrix;the surface modification and orientation of fillers.Although high filling amount can lead to enhanced thermal conductivity,it also weakens the mechanical properties and processability of the composites.Despite that the surface modification of fillers can improve the interfacial interaction between fillers and the matrix,and reduce the interfacial thermal resistance,the surface modification process is generally complex and will weaken the inherent properties of fillers.In comparison,constructing heat conductive networks in the matrix by orientation treatment of fillers can effectively accelerate heat conduction and reduce phonon scattering,which is a more effective way for improving the thermal conductivity of composites.However,the current method commonly used for orientation treatment of fillers is not simple enough,and the construction of the thermally conductive networks is often focused on only one direction,rather than in both parallel and vertical direction.In order to solve the above problems,firstly,we exfoliated the hexagonal boron nitride to obtain boron nitride nanosheet by liquid phase ultrasonication,then TEMPO oxidized cellulose nanofiber was used as matrix,and the thermally conductive boron nitride nanosheet/cellulose nanofiber shear-oriented films with different filler contents were finally prepared by doctor blading method through shear-induced orientation.Boron nitride nanosheets are highly oriented in the matrix under the shearing force,forming a good heat conductive network,which effectively accelerate the heat conduction;After the ultrasonic exfoliation,hydroxyl groups were introduced onto the surface of boron nitride nanosheets,and they can interact with hydroxyl groups and carboxyl groups on TEMPO oxidized cellulose nanofiber.The hydrogen bonds between the filler and matrix were formed,which can effectively reduce the influence of interfacial thermal resistance.The in-plane thermal conductivity of the obtained boron nitride nanosheet/cellulose nanofiber thermally conductive film was as high as 24.66 W/(m·K)with filler content of 50 wt%.In addition,the thermally conductive film also possess high thermal stability,good flexibility,and low dielectric constant and dielectric loss.Then,the thermally conductive films(boron nitride nanosheets/cellulose nanofiber)with high in-plane thermal conductivities were employed as the heat conductive medium,while the platelet-shaped hexagonal boron nitride and the particle-shaped aluminum nitride were utilized as heat conductive fillers for enhancing the perfection of the thermally conductive network.The multilayered thermally conductive composite with ternary system of heat conductive fillers-thermally conductive films-epoxy matrix was prepared through a facile process of layer-by-layer stacking,pre-curing and hot-pressing.A horizontally heat conductive network was constructed within the epoxy matrix under the joint contribution of the horizontally aligned hexagonal boron nitride platelets and the parallelly spread thermally conductive films through hot-pressing.The aluminum nitride particles acted as linking points,filling the gaps between hexagonal boron nitride platelets,while constructing the thermally conductive path in the vertical direction.By changing the ratio of two fillers,the content of fillers in thermally conductive films,and the layer number of films,their impacts on thermal conductivities of the composite were investigated.The properties of the composites were analyzed by scanning electron microscopy(SEM),X-ray diffraction(XRD),thermogravimetric analysis(TGA),and dynamic thermomechanical analysis(DMA).The results showed the optimal filling ration of two fillers was 1:1,and when the ratio of two fillers and filler contents were fixed,the in-plane thermal conductivity of the composite increased with the increase of film layer number.With film layer number of 9,and filler content of 30 wt%(aluminum nitride/boron nitride(1:1)),the in-plane thermal conductivity of the composite was as high as 8.53 W/(m·K),which showed an enhancement of 4165%than that of the pure epoxy matrix,and an improvement of 613%compared to that of the composite without multilayered structure.Meanwhile,the out-of-plane thermal conductivity exhibited a slightly deceased tendency as film layer number increased,but it still reached 0.87 W/(m·K)with film layer number of 9,which was 335%higher than that of the pure epoxy.In addition,the multilayered composite also possessed good thermal stability,enhanced stiffness,as well as low dielectric constant and dielectric loss,which shows a potential application prospect in thermal management for packaging of integrated circuit and microelectronic devices.In this study,we firstly prepared a highly horizontally oriented film by sh ear-induced orientation treatment,and then used it as heat conductive medium to build a horizontal thermally conductive network in matrix through hot-pressing.Next,the aluminum nitride particles were introduced to form heat conductive pathway in the vertical direction.Due to the formation of heat conductive networks in both horizontal and vertical directions,a thermally conductive composite possessing high in-plane and out-of-plane thermal conductivities is finally obtained.The method of firstly constructing horizontally heat conductive paths by orientation treatment and then building vertically heat conductive channels by introducing another kind of heat conductive filler provides an effective guidance for the preparation of highly thermally conductive boron nitride/polymer composites.