Preparation And Properties Of Novel "Sandwich Structure" Insulating High Thermal Conductivity PVDF Multilayer Composite Films

With the continuous miniaturization,integration and intelligent development of electronic equipment and devices,in the face of the popularization of flexible wearable devices and thermal management textiles,it is inevitable to introduce high-power devices and limited space in the working environment.In order to solve the problem of heat accumulation,new requirements and challenges are put forward for traditional heat dissipation materials,and the development of high-performance,multi-functional and insulating thermally conductive composite films has become an urgent need at present.Inspired by the sandwich structure,based on the layered property control strategy of multilayer films,the conductive and highly thermally conductive polyvinylidene fluoride（PVDF）fiber film and the insulating and thermally conductive PVDF fiber film are designed with a layered structure and then thermally melted into one.The purpose is to prepare PVDF composite films with high thermal conductivity,electrical insulation and flexibility.The main preparation process is electrospinning PVDF fiber membrane.On this basis,alumina（Al₂O₃）and silver（Ag）are separated by filler-polymer matrix co-spinning,nanoparticle spraying,cyclic infiltration self-assembly,in-situ reduction and other technologies.,boron nitride nanosheets（BNNS）and carbon nanotubes（CNT）multi-dimensional nanoparticles are combined with fiber membranes to build an efficient three-dimensional thermal conductivity network in the PVDF matrix,effectively improving the thermal conductivity of PVDF composite films,and is committed to breakthroughs The technical bottleneck for90 the preparation of high thermal conductivity composites at low loadings.In this work,based on the principle of sandwich structure design,the mechanism of three different synergistic strategies of thermally conductive fillers on enhancing the in-plane and out-of-plane thermal conductivity of PVDF composite films was studied,and the three researches were explained in depth.Strategies Specific mechanisms for constructing thermally conductive networks in PVDF composite films.The main research contents and conclusions are as follows:（1）Research on BNNS/CNT/BNNS multilayer PVDF thermally conductive composite films:in order to enhance the in-plane thermal conductivity of the film and improve the spacing defects of fillers between electrospun fibers.Based on the hybrid synergy and interlayer orientation enhancement strategy of BNNS-CNT nanoparticles,this paper is devoted to designing an oriented BNNS structure in PVDF electrospun films,and further enhancing the packing density of the oriented network by hybridizing CNT particles.The BNNS/CNT/BNNS/PVDF composite film（B-C-B/PVDF）with high in-plane thermal conductivity and insulating properties was prepared through the process of electrospinning BNNS/PVDF film,spraying CNT on the surface and hot pressing of multilayer film successively.Electrospinning technology enables BNNS to achieve better in-plane orientation in PVDF films,and the sprayed CNTs build an efficient heat transfer path between film layers,which has a bridging effect on BNNS in adjacent fiber films,thereby promoting the in-plane orientation of BNNS.The internal heat conduction network can be built,but it can still maintain good insulation performance.As a result of the optimization of film preparation conditions,the in-plane thermal conductivity of B-C-B/PVDF with a filling amount of 33.9 wt%（mass fraction,the same below）reaches 3.38W·m^-1·k^-1,which is 1152%higher than that of pure PVDF.,the out-of-plane conductivity is as low as 2.09×10^-12 S·cm^-1,and good flexibility is maintained,and the tensile strength reaches 31.03 MPa.The film is expected to have good application prospects in thermal management and flexible electronic devices.（2）Research on BNNS/CNT&BNNS/BNNS multilayer PVDF thermally conductive composite films:In order to get rid of the spacing effect of polymer matrix on thermally conductive fillers,high-efficiency thermally conductive paths are constructed.In this work,based on the hybrid synergy of CNT&BNNS nanoparticles and the enhancement strategy of line-plane structure,the electrospun film was modified by polyethyleneimine（PEI）for the first time with positive charge.Thereby,the gap between the polymer matrix and the filler is eliminated,and a high-efficiency thermal conduction network of CNT&BN is built.Then,the multi-layer films are compounded together by hot pressing technology.On this basis,the insulating property control strategy of the multi-layer films is introduced.BN/PVDF is set as the upper and lower insulating layers,and CNT&BN/PVDF is set as the middle conductive and high thermal conductivity layer.BNNS/CNT&BNNS/BNNS/PVDF thermally conductive composite films（B-C&B-B/PVDF）with excellent performance were prepared.The results show that the thermal conductivity of B-C&B-B/PVDF under the loading of 3wt%CNT and 13wt%BN reaches 2.68 W·m^-1·k^-1,which is 1016%higher than that of pure PVDF,and the out-of-plane conductivity is as low as 3.41×10^-12 S·cm^-1,the tensile strength reaches 28.06 MPa,and it has good mechanical flexibility.This research achieves the research goal of combining low filling,high thermal conductivity,and flexibility.（3）Research on BNNS/Al₂O₃@Ag/BNNS multilayer PVDF thermally conductive composite film:In order to construct an isotropic thermally conductive network in the multilayer composite film,this work is based on the hybrid synergy and point of BNNS-Al₂O₃@Ag nanoparticles.The surface structure enhancement strategy is dedicated to constructing a three-dimensional thermal conductive network structure in PVDF films,and at the same time improving the in-plane and out-plane thermal conductivity of the film.Through electrospun PVDF fiber membrane,electrostatic self-assembly of BNNS and Al2O3 on the surface of the fiber membrane,in-situ synthesis of Ag particles on the surface of the fiber,and hot pressing of the multilayer film,high in-plane and out-of-plane thermal conductivity were successfully prepared.Insulating BNNS/Al₂O₃@Ag/BNNS/PVDF composite film（B-A@A-B/PVDF）.At the same time,the morphology structure and insulating property control strategy of the multilayer film was introduced.First,the upper and lower layers in the composite film were set as two-dimensional filler BNNS with in-plane orientation,and the middle layer was set as isotropic zero-dimensional Al₂O₃.After the welding and reinforcement of silver particles,Al₂O₃@Ag and BNNS build a high-efficiency and isotropic thermally conductive network macroscopically.The internal and external thermal conductivities of B-A@A-B/PVDF reach 4.13 and 1.05,respectively,at a filling amount of 32.1wt%.Compared with pure PVDF,W m^-1k^-1 is increased by 1620%and452%respectively.In addition,the out-of-plane conductivity is as low as 5.38×10^-11 S·cm^-1,which is much higher than the application standard of insulating materials.At the same time,the tensile strength of 22.71 MPa basically meets the application requirements,and has great application prospects in the application field of advanced thermally conductive composite materials.