Relationship Between Microstructure And Properties Of Theremal Conduction/Dielectric Polymer-matrix Composites

With the increasing miniaturization and densification of electronic appliances,it is urgent to design multifunctional thermal conductive composites to meet the increasing requirements for the thermal and electrical performance.However,traditional filled functional composites are usually filled with high-content single-component fillers,which not only increases the difficulty of material processing but also greatly reduces the mechanical properties of the material.This thesis aims to design a multi-layer,multi-dimensional new thermal structure and adjust the multi-component,multi-scale interface to improve the dispersion and compatibility of the filler in the polymer matrix.In the paper,the common polypropylene(PP),polyvinylidene fluoride(PVDF),polystyrene(PS),and polydimethylsiloxane(PDMS)are used as polymer matrices;boron nitride nanosheets(BN)is used as the thermal conductive filler;multiwalled carbon nanotubes(MWCNT)and barium titanate(BT)are used as high dielectric fillers.Polymer matrix composites of different structures are prepared with such techniques as melt blending,electrospinning,spin-coating,sol-gel etc.The influence of the new coupled heat-conducting structure on the electrical,mechanical and thermal properties of polymer-based composites was studied in depth,and its mechanism was analyzed from the microscopic-mesoscopic-macroscale.The main research progress is as follows:1)The boron nitride nanosheets are prepared with the ultrasonic peeling method.The incompatible SEBS and PP are selected as the polymer matrices.The bicontinuous structural design is employed to selectively control the dispersion of the BNNSs nano-filler.It is found that the introduction of BNNSs can not only effectively improve the thermal conductivity of composites,but also aggravate the positive charge migration and accumulation,effectively suppress the formation of space charge in composites under high DC field strength,and increase the breakdown strength from 190 kV/mm to 220 kV/mm,in addition,the addition of BNNSs has little effect on the dielectric constant and dielectric loss of the composite.2)Through the multilayer structure design,the effects of layer structure and outer filler content on the dielectric properties and thermal conductivity of the composites were investigated.The modified m-MWCNT and m-BN were selected as the filler,and the PVDF was used as the matrix.The composite film with multi-layer structure was prepared by spin coating process.The results show that the sandwich composite film containing 10 wt%m-BN filler in the outer layer and 5 wt%m-MWCNT in the intermediate layer has a large dielectric constant.Further,as the content of the outer layer of m-BN increases,the thermal conductivity and breakdown performance of the composite film are remarkably improved.3)Through the orientation structure design,the influence of filler dispersion and orientation on the thermal conductivity of the composite was investigated.PVDF is used as the matrix and m-BN is used as the filler.The electrospinning and hot pressing processes are combined to control the dispersion state of the filler in the polymer matrix,and the multilayer structure composite material with better orientation is obtained.The results show that the method can effectively improve the thermal conductivity and breakdown performance of the composite.4)Through the parallel orientation structure(Janus)design,the orientation and the double continuous advantage are integrated to further optimize the thermal conductivity and dielectric properties of the composite.The modified boron nitride(m-BN)and barium titanate(m-BT)are dispersed as heat conductive fillers and dielectric fillers in the corresponding PS and PVDF substrates,respectively,by parallel-electrospinning and hot press forming.The results show that the parallel alignment structure significantly improves its in-plane thermal conductivity and dielectric constant.The Janus structure design provides a new idea for the development of composite materials with excellent performance.5)The thermal conductivity of composites in the vertical direction is further effectively improved by the three-dimensional structure(3D)design.The aerogel prepared with this method is light with high porosity.The grid structure is not damaged after filling of polymer.The results show that the BN/BT(2:1)composites have ideal thermal and dielectric properties.The influence of filler dimensions is further explored.It is found that there is an excellent relationship between one-dimensional BT fibers and BN compared to zero-dimensional BT nanoparticles.This provides an effective way to prepare high performance composites.