| As a functional material with wide application prospect,energy storage material has a good function of storing electric charge.Film capacitors are an important energy storage device and have been widely used in aviation,aerospace,home appliances,transportation,communication and other fields.In order to obtain excellent energy storage performance,the performance of the dielectric material used for film capacitors is crucial.The low energy density of dielectric films is the limitation to improve the energy storage density of film capacitors.Therefore,the development and research of high energy density thin film dielectric materials is a hot spot and difficult area in this field.It is well known that the dielectric property of the material is the main factor affecting the energy storage density.The electric polarization is the most fundamental property of dielectric.An electric field can induce dielectric polarization and store electrical energy in the material to reach a charged state.Upon removal of the electric field,the dielectric can be completely or partially restored to its original state,thus releasing the stored electrical energy.Therefore,the polarization ability and polarization recovery ability of the dielectric determine the energy storage density of the capacitor.In this thesis,we use PVDF as the matrix material to enhance the energy storage performance of the dielectric material through the influence of the processing process and the two aspects of all-organic composite for industrial application,and the main research contents as well as the results are as follows.1.In this chapter,the PVDF powder is treated by environmental induction,and the crystalline structure of PVDF is significantly changed by treatment at different environmental temperatures.In the experimental design,aqueous and methanol environments were used to treat PVDF at different temperatures,respectively.FTIR and XRD test results proved that when treated in aqueous environment,PVDF was mainly in non-polarα-phase and no significant crystalline transformation occurred.However,when PVDF was treated by methanol environment at temperatures above40°C,the crystalline shape shifted to the polarβ-phase.This is the main reason for the difference in discharge energy density between the two environmental treatments.In addition,the crystalline properties of PVDF received due to the interaction of water molecules or methanol molecules with PVDF molecular chains during the environmental treatment.The DSC test revealed that the treated PVDF was limited in crystallization behavior,the crystallization temperature was shifted toward the low temperature region,and the grain size was significantly reduced compared to the untreated PVDF.In addition,the dielectric behavior as well as the energy storage properties of PVDF films were significantly improved by receiving the effect of grain size change.The frequency dependence of the untreated PVDF is weakened by the environmental treatment and the dielectric loss is significantly reduced.The reduction in grain size leads to easy PVDF dipole flipping,resulting in significant improvement in discharge energy density as well as efficiency of the treated PVDF films.2.In this chapter,we prepared linear methyl methacrylate-co-glycidyl methacrylate(MG)copolymers to improve the dielectric and energy storage properties of PVDF.FTIR spectroscopy and XRD results showed that the introduction of MG induced the crystallographic transition of PVDF.Polarized light microscopy and DSC tests demonstrated that the PVDF grain size in the composites was reduced and the crystallinity was decreased.the introduction of MG lowered the dielectric constant of PVDF and suppressed the dielectric and conductive losses.The composites show higher breakdown strength,which increases from 300 MV/m for PVDF to 540 MV/m for PVDF/MG-40%composites.the complementary breakdown strength confers good discharge energy density and efficiency to the PVDF/MG composites,despite the weaker polarity of MG.The discharge energy density increased from 3.75 J/cm~3 for PVDF to 9.2 J/cm~3 for the PVDF/MG-40%composite,while the PVDF/MG-40%composite maintained a high efficiency of 84%even under an electric field of 540MV/m.This study provides a feasible design idea for all-organic composite membranes.More importantly,the reactivity of the epoxy groups on the MG copolymer can be exploited for later performance Optimization of PVDF/MG dielectric membranes by tuning strategies.3.In this paper,PVDF composite dielectric materials with high U_e andηwere prepared by the synergistic effect of MMA-co-GMA(MG)copolymer and cyanoethylated cellulose.According to the previous section,the linear structure of MG and the hydrogen bonding with the epoxy group after MG was blended with PVDF reduced the tanδand increased theηof PVDF.to further improve the Ue of the dielectric films,cyanoethyl-cellulose(CR-C)was added to the PVDF composite matrix as a third component to improve the U_e as well as the breakdown strength.PVDF/MG and CR-C In addition,the high polarization of cyano significantly increases the U_e of the three-phase composite dielectric film(24.43 J/cm~3)and the efficiency can be maintained above 75%at 640 MV/m.The deep trapping effect of the hydrogen bonding between PVDF/MG and CR-C increases the breakdown strength of the three-phase composite film from 440 MV/m to 640 MV/m.This study provides a new idea for the fabrication of uniform and stable all-organic PVDF dielectric composite films based on the hydrogen bonding structure strategy.4.In this chapter,polyfunctional DHTA small molecules are used as the third component of the composites.The modulus of PVDF/MG composites is increased due to the benzene ring and the multifunctional groups,and the formation ofβ-crystals is induced in the three-phase composites,and the formation ofβ-crystals improves the loss and efficiency of the three-phase composites.However,hydrogen bonds are formed between the hydroxyl groups of DHTA and GMA in the composites,and the hydrogen bonds have the advantages of trapping carriers all the way to the loss of the composites and enhancing the efficiency and breakdown strength.Because DHTA carries strong polar carboxyl functional group,the dielectric constant of the three-phase composites gradually increases with the increase of DHTA content,but the loss enhancement of the composites is not obvious due to the effect of hydrogen bonding inhibition.the synergistic effect of the high modulus of DHTA and the inhibition effect of hydrogen bonding contributes to the breakdown strength of the composites,and the breakdown strength of the composites after adding DHTA is generally higher than that of PVDF/MG composites,but at too high content,the breakdown strength decreases instead due to the high brittleness of the film,and the highest breakdown strength(640MV/m)was obtained at DHTA addition of ah 0.4%.Similarly,the high polarity of DHTA also enhances the discharge energy density of the composites,which reaches23.32 J/cm~3 when DHTA is added at 0.4%,and the efficiency can be maintained at 71%. |
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