Effect Of Molecular Structure Modification And Nanoparticles On Characteristics Of Surface And Space Charge Of Polyimide Films

With the advantages of high temperature resistance, low temperature tolerance,radiation resistance and excellent dielectric properties, Polyimide that serves as atypical kind of engineering polymer material has been widely used in different fields,such as the aerial, nuclear, microelectronic industry，turn to turn insulation and turn toground insulation of inverter-fed motors. However, its usage as an insulating materialfor inverter-fed motor, and extra-high voltage electric reactor is not successful up tonow. Insulation early failures occur frequently. The existence of surface charge andspace charge have a great effect on breakdown characteristic and is the main reasonleading to dielectric breakdown. So it is important to investe the characteristics ofsurface and space charge in polyimde films. In this paper, the dynamic behavior ofsurface and space charge of polyimide films modified by nanoparticles and molecularstructure were studied. The main research work and achievements are as follows:(1) The polyimide nanocomposite films were prepared using in-sutepolymerization method with the nanoparticls content1wt%、3wt%、5wt%、7wt%.The pure polyimide films were also prepared. A Nova NanoSEM430(FEI Company)Scanning Electron Microscope (SEM) was used for observating the distribution ofnanoparticles in polyimide matrix. The relative permittivity of the samples weremeasured by using a Concept40(Novocontrol Company) Broadband DielectricSpectrometer at20oC.(2) The polyimide films and the polyimide nanocomposites films wererespectively fluorinated under various fluorination time. Surface chemicalcompostions of the fluorinated films together with the original (untreated) films werecharacterized by attenuated total reflection infrared (ATR-IR) analysis. The SEM wasusing to observe the cross sections morphologies of the samples under variousfluorination times to determine the thickness of the fluorinated layer. The relativepermittivity of the samples were also measured.(3) Effects of the nanoparticles and surface fluorination on surface chargeaccumulation and charge decay of polyimide films were investigated by using DCcorona charging method. The dynamic behaviors of surface charge accumulation anddecay were associated with the characteristics of localized surface states that arechanged with the fluorinated layer of samples. The effects of fluorination and nano-Al2O3concentration on the charge deep were understood. The mechanism forthe influence on the surface charge characteristics were analyzed.(4) Effects of the direct fluorination on space charge behavior of multilayeredpolyimide films were investigated. The space charge distributions in multilayeredpolyimide films were measured using the pulsed electro-acoustic (PEA) method. Theconductivity of fluorinated films were studied. And the mechanism for the influenceon the space charge characteristics were also analyzed.