Fabrication And Energy-storage Properties Of Poly(vinylidene Fluoride) Based Dielectric Films

Plastic film capacitors are widely used in military,scientific research and civil applications for their stability in superhigh voltage,superhigh current,superhigh power applications.This makes it a basic component of electric vehicles and renewable energy power generation systems.However,the widely used polypropylene(PP)dielectric film is close to its use limit,and it is impossible to continue to make the plastic film capacitor smaller and lighter using PP as dielectrics.It is essential to develop new type high energy storage density polymer dielectric films.With regard to the above problem,a variety of poly(vinylidene fluoride)(PVDF)based polymer(composite)dielectric films with different structures were designed and prepared in this dissertation.To achieve this objective,several technical routes were carried out in this thesis,including the selection of materials,the optimization of solution-cast preparation process,the exploration of PVDF-based bulk composite films,and the design of PVDF-based interlayer composite films.Meanwhile,a series of research work on dielectric film preparation,microstructure characterization,measurement of dielectric and energy-storage properties,was conducted and the relevant mechanisms were also analyzed.The main content of this dissertation is summarized as follows.1.The effects of PVDF molecular weight,solvent type,solution concentration,drying temperature and ambient humidity on the microstructure and energy storage characteristics of solution-cast PVDF dielectric films were systematically studied.It is found that these variables have little impact on the dielectric loss of PVDF films,but have a significant influence on the charge-discharge efficiency and resistivity of PVDF films.Except for PVDF molecular weight and solvent type,other solution-cast process parameters have little effect on the relative dielectric constant of PVDF films.The results show that the performance of PVDF films will be better if the films are cast from a high concentration solution(15 wt.%)of low-molecular-weight PVDF in highvapor-pressure solvent(DMF),at a low drying temperature(60 °C)and low ambient humidity.The predominant crystal form of the obtained PVDF film is ? phase,with higher charge-discharge efficiency(1000 kV/cm,~81.77%)and higher resistivity(1000 kV/cm,~17.38 T??cm),which means a promising dielectric material of plastic film capacitors.2.The microstructure and energy-storage properties of solution-cast PVDF-based copolymer and terpolymer dielectric films were systematically studied.For PVDFbased copolymer,the effects of solution casting process parameters,such as solvent type,solution concentration and drying temperature,on the microstructure and energystorage performance of P(VDF-HFP)dielectric films were studied.Overall,these parameters have little effect on the dielectric characteristics of P(VDF-HFP)films,but have a great influence on their charge-discharge efficiency and insulation resistivity.It is better to prepare P(VDF-HFP)film from a high concentration solution(~20 wt.%),at a lower drying temperature(~80 °C),using a high vapor pressure solvent(DMF).The obtained P(VDF-HFP)film is a mixture of ? phase and ? phase,with higher charge-discharge efficiency and higher resistivity,which is more suitable for plastic film capacitor dielectric material.For PVDF-based terpolymer,two kinds of dielectric films,P(VDF-TrFE-CTFE)films and P(VDF-TrFE-CFE)films,were prepared.The crystallinity of these dielectric films was really high.At 1 kHz,the relative dielectric constant of these films is as high as 20.78 and 31.74,while the dielectric loss is 7.18% and 5.74%,respectively.The charge-discharge efficiency of these films increases with the applied electric field.The resistivity shows a trend of decreasing first and then rising with the increase of the applied electric field.3.The effects of PMMA molecular weight and ratio on the microstructure and energy-storage properties of PVDF-based copolymer and terpolymer composite dielectric films were systematically studied.When high-molecular-weight PMMA is compounded into P(VDF-HFP)or P(VDF-TrFE-CTFE),obvious phase separation was observed,resulting in interfacial polarization which increases the relative dielectric constant and causes a drop in the breakdown field.However,low-molecular-weight PMMA improves the performance of PMMA/P(VDF-HFP)and PMMA/P(VDF-TrFECTFE)composite films: although the relative dielectric constant is slightly decreased,both the charge-discharge efficiency and the resistivity have been significantly improved.With the increase of PMMA composite ratio,the content of ? phase in PMMA/P(VDF-HFP)composite films increases gradually,and the crystallinity of PMMA/P(VDF-TrFE-CTFE)composite films decreases gradually.Compared with the pure PVDF-based copolymer and terpolymer dielectric films,the charge-discharge efficiency of composite films decreases much more slowly with the increase of the applied electric field,achieving a high charge-discharge efficiency at high electric field.The PMMA/P(VDF-HFP)film with a composite ratio of 25/75 has a relative dielectric constant of about 5.21 at 1 kHz,an intrinsic breakdown field of 372.96 V/?m with a shape factor ? of 10.56,a charge-discharge efficiency as high as 86.22% under the electric field of 2000 kV/cm,and a resistivity as high as 86.1 T??cm under the electric field of 1000 kV/cm.When the PMMA mass ratio exceeds a certain threshold,the microstructure of the PVDF-based terpolymer composite film changes substantially,leading to a sudden drop in relative dielectric constant and dielectric loss,resulting in a linear polarization-electric field(P-E)loop(no antiferroelectricity).The unipolar P-E loop of the PMMA/P(VDF-TrFE-CTFE)film with a composite ratio of 30/70 is completely linear,and the relative dielectric constant at 1 kHz is about 5.17,and under an electric field of 1000 kV/cm the charge-discharge efficiency and the resistivity are as high as 85.65% and 29.96 T??cm,respectively.The unipolar P-E loop of the PMMA/P(VDF-TrFE-CFE)film with a composite ratio of 40/60 is completely linear,and the relative dielectric constant(at 1 kHz),the intrinsic breakdown field,the shape factor ? value,the charge-discharge efficiency(under 1000 kV/cm)and the resistivity(under 1000 kV/cm),are as high as 4.09,396.85 V/?m,9.95,89.76% and 62.54 T??cm,respectively.4.A variety of PVDF-based polymer/PP composite dielectric films were prepared by introducing interlayer recombination method,and their dielectric and energystorage properties were studied.PVDF-based polymer/PP interlayer composite films with different structures were prepared,and a high charge-discharge efficiency and resistivity were achieved.In order to verify the role PP played in interlayer composite films,the multilayer composite dielectric film with sandwich structure was designed and prepared.The interlayer composite structure combines the advantages of the two materials,resulting in an excellent dielectric and energy-storage characteristics.The dielectric and energy-storage performance of P(VDF-TrFE-CFE)/PP films are better than those of P(VDF-TrFE-CTFE)/PP films.As the thickness of P(VDF-TrFE-CFE)layer increases,the relative dielectric constant,the dielectric loss,the polarization and the leakage current of the interlayer composite films are gradually increased,while the charge-discharge efficiency and the resistivity are decreased.The sandwich structure avoids the damage of the PP film from the metallization process and improves the charge-discharge efficiency of the dielectric films.The relative dielectric constant and dielectric loss of the sandwich structure P(VDF-TrFE-CFE)/PP/P(VDF-TrFE-CFE)composite film at 1 kHz are 5.73 and 6.02 ‰,respectively.The charge-discharge efficiency and the resistivity under a 1000 kV/cm electric field are as high as 97.45% and 367.9 T??cm,respectively.