Preparation And Characterization Of PA11-based Segregated Structure Dielectric Composites

In recent years,with the development of electronic devices toward high performance and miniaturization,the demand for high-performance dielectric materials has become more urgent.Traditional ceramic dielectric materials with low toughness and great processing difficulty,which is difficult to meet the performance requirements of dielectric materials.In contrast,polymer-based dielectric composites have the advantages of good mechanical properties,high breakdown strength,and low cost.Therefore,it has a very high research value and broad application prospects in the field of electronic appliances.Directly introduced ceramic fillers such as silicon carbide(SiC)and barium titanate(BT)or conductive fillers,such as graphene nanosheets(GNPs)and carbon nanotubes(CNTs)into the polymer matrix can produce composite materials with high dielectric constant,but a large amount of fillers are required so that the mechanical properties would be impaired.At present,many studies having indicated tailoring microstructure of composites is effective methods to prepare composite material with excellent dielectric performance.Tailoring the microstructure of the composite material can effectively reduce the threshold of the filler in the composite material,improve the mechanical and processing properties of the composite material,and thereby obtain a composite material with excellent dielectric performance.This thesis mainly studied the mechanisms of the segregated network and filler dispersion on the dielectric properties of composites.Firstly,the effect of CNT segregated network on the dielectric properties of polyamide 11(PA11)-based composites was investigated.Polypropylene(PP)was introduced to improve the dispersion of CNTs in the segregated network,and multiple interfacial polarization structures were constructed.The mechanism of segregated structure and filler dispersion on enhancement of the dielectric properties in the composites was explored.Then,PA11 powder of different sizes were used to prepare segregated composites with different filler network densities.The CNT network density and CNT dispersion state were adjusted by tailoring the particle size of PA11 powder.The effects of CNT network density and CNT dispersion on the dielectric properties of the composites were analyzed in detail.Finally,GNPs and boron nitride nanoplates(BNNPs)were introduced into the polymer to prepare the segregated composites.The mechanism of the hybrid filler network on the dielectric properties and thermal conductivity of the composites was studied.The main research results are as follows:(1)PA11/CNT composites and PA11/PP/CNT composites with segregated structure were prepared by blending and forming.The results showed that the composite with segregated structure could achieve high dielectric properties at ultralow filler content.It was found that the segregated structure composite had a lower percolation threshold,so the dielectric properties could be improved at a low filler content.Introduced PP into the composite structure of the segregated structure could improve the dispersion of CNTs at the interface,and the percolation threshold of the composite material reduced to around 0.05 wt%,so that the composite material could achieve high dielectric properties at a lower filler content.Further research found that the introduced PP could diffuse into the agglomerates of CNTs and successfully block the overlap between CNTs,thus constructed microcapacitor networks in the segregated structure materials.The PA11/PP/CNT composite could achieve more interfacial polarization near the percolation threshold,reduced conductance polarization,and limited dielectric loss.(2)The ethylene-vinyl acetate copolymer(EVA)was blended with CNTs and coated on the surface of PA11 powder with different particle sizes by solution blending.And then PA11/EVA/CNT segregated structure composites having different filler network densities were prepared by molding.Through microscopic observation,it was found that PA11 powders with different particle sizes could be used to prepare composite materials with different filler network densities.The smaller the particle size of the PA11 powder was,the greater the density of the filler network and the greater the uniform dispersion of the CNTs were.In this work,the method for quantifying the density of filler network was proposed.The effects of filler network density and filler content on the dielectric properties of composites were analyzed.The microscopic image of the composite was processed using software to obtain values indicative of the density of the composite filler network.As the particle size of the PA11 powder decreased,the density of the filler network was increasing.It was found that the dielectric constant of the composite had a significant dependence on the density of the filler network.Before the filler content reached the percolation threshold,the smaller the filler network density value was,the more uneven the CNT dispersion and the denser CNT microcapacitor network could be formed,which can increase the dielectric constant more effectively.The dielectric loss of the composite material had no significant dependence on the density of the filler network.Only when the CNTs formed a conductive path,the dielectric loss was significantly increased.(3)GNPs and BNNPs were introduced into EVA by solution blending method,and EVA/GNP/BNNP was wrapped on the surface of PA11 particles,and PA11/EVA/GNP/ BNNP segregated structure composites were obtained by forming.By controlling the hybrid filler network,a composite material with excellent dielectric properties was obtained,and the thermal conductivity of the material was improved.Observing and analyzing the microstructure of the composites,it was found that the introduction of EVA phase could improve the dispersion of the filler,improve the dielectric properties of the composite,and at the same time reduced the defects in the composite and stabilized mechanical behavior of the composite.The dielectric properties of the composites were studied.When the GNP content was near the threshold,the introduction of BNNPs could block the GNP network,inhibit the conductance polarization of the composites,and reduce the dielectric loss of the composites.At the same time,the introduction of BNNPs could construct more interfaces to generate interfacial polarization,so that the composite material maintained a relatively high dielectric constant.When the GNP content was much larger than the threshold,the introduction of BNNPs would eliminate the volume of GNPs,making the GNP network more dense,and the conductivity of the composites would increase significantly.As the GNP and BNNP content increase,the thermal conductivity of the composites tends to increase.Heat conduction network would appear inside the composite material,which significantly improved the thermal conductivity of the composite material when the GNPs content was 2 wt% and the BN content was 1 wt%.