Research On Conduction Mechanism And Control Method Of Fluoropolymer Dielectric Films

Plastic film capacitors exhibit ultra-high power density,high breakdown field strength,and good self-healing properties,which are widely used in fields such as new energy,aerospace,and intelligent devices.The core of Plastic film capacitors is the dielectric material.The dielectric,mechanical,and thermal properties of the dielectric materials directly determine the performance of capacitors.However,the low energy storage density of commercially available linear polymer dielectric materials currently limits the application of Plastic film capacitors in fields such as new energy vehicles,new concept weapons,and aerospace.Fluorinated polymer dielectric materials,represented by poly(vinylidene fluoride)(PVDF),have good mechanical properties,high breakdown field strength,and high dielectric constant,making them the preferred material for preparing high-energy Plastic film capacitors.Nevertheless,the leakage current loss of fluorinated polymer dielectric films is still relatively high,especially the sharp increase in leakage current loss under high field,which directly affects the lifespan of Plastic film capacitors.Therefore,this thesis focuses on fluorinated polymer dielectric films as the main research object,and carries out the following research work around the mechanism of leakage current loss and its control methods:(1)Selected several commonly used organic thin film capacitor dielectric materials such as polypropylene(PP)and poly(ethylene glycol terephthalate)(PET),tested their leakage current,and explored the main leakage mechanisms under different electric field ranges through data analysis.By fitting the leakage current data of PP,the main leakage mechanism under different electric field ranges were analyzed: under an electric field of0-50 k V/mm,the leakage current of PP follows Ohm’s law;under an electric field of150-390 k V/mm,the leakage current increases exponentially due to the electrons gaining enough energy to escape from the trap,and the main leakage mechanism follows the hopping conduction mechanism;under an ultra-high electric field exceeding 390 k V/mm,the leakage mechanism begins to transition from hopping conduction to ionic conduction.The leakage mechanism of PET under different electric field strengths was analyzed by data fitting of its leakage current: in the electric field range of 0-70 k V/mm,the leakage current of PET shows a linear slow increase,and the main mechanism follows Ohmic conduction;under an electric field of 70-210 k V/mm,the leakage current shows an irregular increasing trend,which is related to the establishment of the internal electric field;under an electric field of 220-300 k V/mm,the leakage current begins to exhibit an exponentially increasing trend,which is mainly influenced by the hopping conduction mechanism.The effect of coatings on leakage characteristics was preliminarily explored,and the results show through experiments that coatings can effectively increase the Schottky barrier between the electrode and the dielectric,reduce electron injection from the electrode,and decrease leakage current.Finally,the leakage conductivity flow of PVDF dielectric film is fitted,and the results show that PVDF mainly follows the space-limited conductivity,and the subsequent research direction on the leakage conductivity loss control method of PVDF dielectric film is proposed.(2)Starting from the micro-mechanism of electrode-limited conductivity,a polytetrafluoroethylene(PTFE)barrier layer was spray-coated onto a polyvinylidene fluoride(PVDF)substrate to prepare PTFE-c-PTFE composite dielectric film.The mapping relationship between the interlayer bonding strength of the composite dielectric film and temperature was explored under different processing temperatures.Meanwhile,the effective conductivity formula was used to plot the hysteresis and loss curves,and the relaxation loss and leakage loss of the PTFE-c-PVDF composite dielectric film were studied separately.Experimental results showed that the dielectric properties of the composite dielectric film were significantly improved after treatment at 80 ℃.The surface high-insulation PTFE layer effectively increased the height of the Schottky barrier between the electrode and dielectric in the composite dielectric film,reduced electron injection from the electrode under a high electric field,and significantly suppressed high-field leakage current.At 300 k V/mm,the leakage current decreased by an order of magnitude.The theoretical fitting results showed that the high-insulation and low-dielectric-constant PTFE layer made the electric field distribution more uniform,and the measured breakdown field strength of the composite dielectric film increased by 30.8%.(3)Starting from the microscopic mechanism of electrical conductivity arising from bulk-phase restrictions,a PVDF/MMT nanocomposite membrane was prepared by filling two-dimensional Montmorillonite(MMT)nanosheets into PVDF polymer.It was found that adding only 1 wt.% of MMT can effectively improve the leakage characteristics of the composite membrane while maintaining its energy storage properties.However,a higher MMT content can lead to phase separation with the PVDF matrix,resulting in a significant deterioration of the performance of the composite membrane.The study examined how thermal treatment affected the performance of the PVDF/MMT composite membrane,and found that it improved the membrane’s compatibility.