Study On PVDF Based High Dielectric Composite Films

With the rapid development of electronic and power energy systems,traditional dielectric materials are increasingly unable to meet the demand.Flexible dielectric materials with high dielectric constant,high breakdown field strength,high energy storage density and low dielectric loss are the focus of current research.Ceramic materials have high dielectric constant,but they are limited because of their relatively low breakdown strength,high brittleness and difficult to process.Organic polymers usually have high breakdown strength,but the dielectric constant is usually small.The traditional way to improve the dielectric constant of polymer composite is to add ceramic materials to polymer.However,due to the poor interface compatibility of composite materials,it can not be dispersed evenly,agglomeration or even porosity occurs,which makes the composite materials have low breakdown resistance field strength,low energy storage density,large dielectric loss and large mechanical properties.In this paper,different methods are explored to improve the dielectric constant and energy storage density of PVDF and PVDF based composites(1)The phase structure of biaxial oriented polyvinylidene fluoride(BOPVDF)was adjusted by high electric field polarization.When the frequency is 100 Hz,the dielectric constant of unpolarized BOPVDF film is only 9.0,and the charge discharge energy density is 9.8J/cm~3 and 7.9J/cm~3,respectively.After the field intensity polarization of 73MV/m for 1h,the dielectric constant of bopvdf can reach 13.2,and the charge discharge energy density can reach 33.3J/cm~3 and 25.4J/cm~3,respectively,which is 20 times of commercial BOPP film.(2)PVDF polymer(MD-PVDF)was prepared by eliminating reaction.Different MD-PVDF were prepared by controlling the time of HF removal.It is found that the dielectric constant of MD-PVDF is 17.2(the dielectric constant of PVDF prepared under the same conditions is 7.1),and the discharge energy density is 9.8 J/cm~3 at the field strength of 400MV/m.PVDF/MD-PVDF films were prepared by solution casting and hot pressing.It was found that when the amount of MD-PVDF was 5wt%,the dielectric constant of the polymer was 9.1,and the energy storage density could reach 12.2J/cm~3 at the field strength of 400MV/m.(3)1-methyl-3-butylimonium bromide ionic liquid(IL)was prepared.PVDF/IL films with different amount of IL were prepared by solution casting method and hot pressing method.It was found that the ? phase content of PVDF increased from36.4% to 89.8%;the dielectric constant was 325,46 times higher than that of pure PVDF,but the breakdown strength decreased significantly;the elongation at break increased by 300%.