Preparation And Properties Of Poly(vinylidene Fluoride)-based Polymer Dielectrics And Novel Layered Dielectric Composites

Due to their extremely high discharge rate,dielectric capacitors play an irreplaceable role in high-power energy storage devices.Among them,Poly(vinylidene fluoride)(PVDF)-based ferroelectric polymers received tremendous attention because of their high dielectric constant and good processability.However,their charge-discharge efficiency and energy density are restrained by their high dielectric loss and inferior breakdown strength.Some fruitful outcomes about improving their energy storage performance have been achieved in previous research.However,high dopant or tedious approach is requisite to ensure a good energy storage performance of dielectric composites.To address this issue,in this dissertation,highly insulated polymer and boron nitride nanosheets(BNNS)are utilized to suppress the dielectric loss of composites.Aiming to explore a facile and efficient route for improving energy storage properties of composites,composites with aligned BNNS are designed and fabricated.The relationship between structure and properties of composites and its mechanism are systematically studied.First,polymer blend composites with improved energy storage capability are fabricated from poly(vinylidene fluoride-trifluoroethylene-chlorofluroethlyene)(P(VDF-Tr FE-CFE))and poly(methyl methacrylate)(PMMA).Experimental results reveal that the addition of PMMA can significantly reduce the dielectric loss of the composite dielectric film,remarkablely suppress the leakage current,and effectively improving the discharge energy density and energy storage efficiency of composites.The maximum discharge energy density of the composites reaches 9.3 J/cm~3,which is 172%of pure P(VDF-Tr FE-CFE)and 258%of commercial biaxially oriented polypropylene,while maintaining a high charge-discharge efficiency(73%).It provides a simple and feasible strategy for the preparation of high-performance dielectric energy storage materials.Secondly,PVDF composite films with a nanosized interlayer of assembled BNNS aligned along the in-plane direction are prepared through a facile layer-by-layer solution-casting process.Compared with PVDF,the composite film exhibits significantly improved breakdown strength and energy density,which are 136%and 275%of PVDF,respectively.Experiments and computational simulation results show that the compact and continuous BNNS layer can more effectively suppress the propogation of electrical tree,compared to uniformly dispersed BNNS.In addition,unlike traditional dielectric composite materials(filler addition amount is 5-10 vol%),such a high lifting of energy storage performance is accomplished by 0.16 vol%BNNS.This provides a novel,facile,and efficient paradigm for fabricating high performance polymer dielectric composites.Then,in order to further improve the energy storage properties of the composite material,BNNS/PVDF dielectric composite films with a five-layer structure are designed and fabricated.Experimental results show that the BNNS layer can effectively suppress the migration of carriers,significantly reduce the dielectric loss,increase the Young's modulus,and hence remarkably improve the breakdown strength and charge-discharge efficiency of the dielectric films.The highest breakdown strength and charge-discharge efficiency reaches 611 MV/m and 75%,respectively.The discharge energy density of composites films are also greatly improved,which is 340%of pure PVDF.It provides an experimental foundation for designing and manufacturing high performance multilayer dielectric composite materials.Finally,in order to improve the interfacial binding force between BNNS and polymer matrix,fluorine-containing silane coupling agent are attached onto BNNS.Then the sandwich structured BNNS-F/PVDF dielectric films are prepared by layer-by-layer solution casting process.Experimental results show that the modified BNNS-F is tightly bound to the PVDF layer and the BNNS-F layer still possesses a highly oriented and continuous structure.The presence of the BNNS-F layer can significantly suppress the dielectric loss caused by direct current(DC)conductance,and significantly improve the breakdown resistance of the dielectric film.With only 0.59 vol%BNNS-F,the breakdown electric field of BNNS-F/PVDF dielectric film is as high as 660 MV/m with a discharged energy density of 14.2 J/cm~3,which is 3.37 times to that of PVDF.This provides a significant research basis for the development of high breakdown strength sandwich structure dielectric films.