Study On Multi-layered Resin Matrix Composites With High Dielectric Constant And Low Dielectric Loss

High dielectric constant polymer matrix composites have important application value in advanced electronic and electrical power systems due to their excellent processing properties,mechanical properties and good plasticity.As an important class of dielectric materials,conductor/polymer composites have attracted a lot of attention from researchers because of their low percolation threshold.When the conductor content reaches the percolation threshold,the dielectric constant of the composite will be significantly improved,but this is accompanied by a large increase in dielectric loss and a severe decrease in breakdown strength,resulting in a lower energy density of the conductor/polymer composite,which limits its development in practical applications.Studies have shown that the incorporation of an insulating layer in a conductor/polymer composite is an effective method to solve the above problems.In this thesis two kinds of multilayered composites composed of conductor/polymer layers and insulating layers are reported.Firstly,the effects of the increase in the number of layers on the dielectric properties,breakdown strength and energy density of the multilayered composites are systematically explored.0.4wt%carbon nanotubes(CNT)/cyanate resin(CE)(0.4CNT/CE,C layer)and hexagonal boron nitride(BN)/CE(B layer)are uesd as the conductor layer and the insulating layer,respectively.Based on the three-layer structure CBC,a series of four-layer structure(CBCB)and five-layer structure(CBCBC and BCBCB)composites were prepared by designing different stacking structures of C layer and B layer as well as controlling different BN contents in B layer.The effects of changes in BN content and differences in structure and number of layers on dielectric properties,breakdown strength and energy density of composites were investigated.The results show that when the content of BN is 20wt%,the CBC20BC composite has the maximum dielectric constant(352,100Hz)and the highest energy density,which is 9%and 117.6%higher than those of CBC composite,respectively,and its breakdown strength is 1.4 times of that of CBC composite,indicating that when C layer and B layer are alternately arranged and C layer is located at both ends,the increase in the number of layers is advantageous for obtaining a higher dielectric constant,breakdown strength and energy density.This is because the increase in the number of layers not only enhances the interfacial polarization but also increases the length and tortuosity of the breakdown path.In addition,CBCB,CBCBC,and BCBCB multilayer composites have significantly reduced dielectric loss and obviously increased breakdown strength and energy density compared to 0.4CNT/CE single-layer composites.Dielectric constants of CBCB and CBCBC composites are greatly improved,especially the dielectric constant of CBCBC structure is greater than 240 at 100Hz and greater than 100 in the range of 1-6000Hz,which satisfies the requirement of high dielectric constant of embedded capacitors.In particular,the dielectric constant and breakdown strength of CBC20BC(352@100Hz,2.12 kV/mm)are 13 times and 2.2 times than that of 0.4 CNT/CE(27@100Hz,0.97 kV/mm),respectively.Thus the energy density of CBC20BC is increased by 6264%compared with that of 0.4CNT/CE,this is the maximum value of the improvement of energy density of the conductor/polymer multilayered composites reported so far.Moreover,the dielectric loss of CBC20BC is only 0.099(100Hz),about 0.0027 times of that of 0.4CNT/CE(36.5,100Hz).We have delved into the underlying mechanisms for improving the performance of multilayered composites.Secondly,we prepared an oriented CNT/polyethersulfone(PES)nanofiber membrane by electrospinning,and infiltrated it into an epoxy resin(EP)prepolymer to prqpare a pre-cured S layer.Then,the CSC three-layer composite was prepared with the S layer as the intermediate layer and 0.5 wt%CNT/EP(0.5CNT/EP,C layer)as the edge layer.The effect of the addition of S layer on dielectric properties of CSC composites was investigated.Results show that when the CNT contents in the S layer are 4wt%and 6wt%,the dielectric constants of CSC composite are as high as 519 and 1027(1Hz),about 2.8 and 5.6 times of that of 0.5CNT/EP(183,1Hz),respectively;meanwhile the dielectric losses are only 0.26 and 0.9(1 Hz),2.7×10-4 and 9.5×10-4 times of that of 0.5 CNT/EP(948,1Hz),respectively.