Study On Charge Characteristics Of Nanocomposite Polyethylene By Electrostatic Force Microscopy And Kelvin Force Microscopy

It is generally accepted that the interfacial area formed between nanoparticles and polymer matrix plays an important role in the performance of nano-dielectrics in the study of nano-dielectrics.The study of charge generation,movement and dissipation near the interface in nano-dielectrics is of great importance to reveal the dielectric mechanism of nano-dielectrics.At present,there is a paucity of methods to characterize the charge movement patterns on the microscopic scale of nano-dielectrics.Atomic Force Microscopy（AFM）and its various modes,including contact mode,Kelvin Force Microscopy（KPFM）mode and Electrostatic Force Microscopy（EFM）mode,are the only means of detection,but the technique still suffers from problems such as the scanning signal being covered by white noise.In order to systematically study the laws related to microscopic charges in nano-dielectric,this thesis improves the preparation method and surface treatment of nanocomposite polyethylene,observes the motion laws of charges in nano-dielectric,and performs finite element simulation.By the necessary dispersion and surface modification treatment of nanoparticles,the composite material with sufficient uniform dispersion of nanoparticles was obtained.By improving the sample surface treatment method,the problem of less exposure of nanoparticles was solved,and the dissipation of charge on the sample surface was accelerated to obtain a sample surface suitable for AFM observation.The surface morphology and surface charge motion of pure LDPE,nano-SiO₂/LDPE composites and nano-zeolite/LDPE composites with porous structures were studied by KPFM and EFM systems.The charge injection was performed on the sample surface by the contact mode of AFM,and the change pattern of the sample surface potential with time was subsequently observed using the KPFM mode.The study found that the surface potential of SiO₂/LDPE nanocomposites decayed significantly lower than that of pure LDPE,while the surface potential of nano zeolite/LDPE composites decayed more slowly.The effects of nanoparticles on the surface of different specimens on the distribution,movement and dissipation of naturally generated charges as well as injected charges were observed using EFM.It was found that both naturally occurring and injected charges in the SiO₂/LDPE and zeolite/LDPE nanocomposites are readily trapped by the interfacial region between the nanoparticles and the polymer substrate,and the surface charges are more difficult to dissipate compared to those of pure LDPE.Surface charges are trapped by traps in the interface region and are difficult to move and dissipate,and these accumulated charges tend to form barriers at high fields,further preventing charge injection,thereby improving the dielectric properties of the nano-dielectric.The above findings effectively prove the interface theory of nano-dielectric.In addition,3D modeling simulations were carried out using COMSOL finite element simulation software.The phase information of the EFM was analyzed by simulating the electric field distribution of nanoparticles in different states,and the dielectric and distance effects of the interface were verified.The inhibition of charge injection by the interfacial region was verified by simulating the surface potential during charge injection.Through the above study,a sample surface characterization method suitable for AFM observation was obtained,the connection between microscopic charge distribution and dielectric properties was established,the law of charge motion was revealed,and the mechanism of the interfacial region to suppress space charge injection in nano-dielectric at the microscopic scale was verified.It provides strong supporting evidence for the study of interfaces and their related theories,and also develops new characterization methods.