Research On The Preparation And Properties Of The Polyvinylidene Fluoride Based Composites With High Energy Density

The energy storage devices play an important role in the rapid development of modern electronic technology,especially in the field of pulsed power supply and hybrid power system.With the rapid development of science and technology,the requirement of material energy storage is increasing.Therefore,the polymerbased flexible materials with high energy storage have attracted more and more researchers' attention.In this paper,a novel design of dielectric composite with high energy density is proposed,that is the polyvinylidene fluoride(PVDF)works as the matrix and the high oriented inorganic nano-filler is introduced as second phase.In the study of the all-organic dielectric composite,the crystals of polyvinylidene fluoride and poly(vinylidene fluoride-trifluoroethylenechlorofluoroethylene)(P(VDF-Tr FE-CFE))are optimized by the quenching process.The sandwich-structured PVDF/P(VDF-Tr FE-CFE)film/PVDF composite was prepared with major gamma phase.Furthermore,the PVDF/P(VDF-Tr FE-CFE)fiber/PVDF film with better performance was obtained by exchanging the the central layer to the P(VDF-Tr FE-CFE)fiber.The results show that the dielectric and energy-storaged properties were improved by quenching.Moreover,the central P(VDF-Tr FE-CFE)fiber not only twisted the growth of electrical treeing,but also enhanced polarization intensity of the allorganic dielectric composite significantly.Therefore,the all-organic dielectric composite with excellent characteristics of great dielectric constant,enhanced breakdown strength and high storage energy was achieved.As for the inorganic nano-filler/PVDF dielectric composite,inorganic nano-filler with high storage energy influences the performance of dielectric composite.So it is important to select an appropriate inorganic phase to improve the performance of composite.In this paper,several typical ceramic particles and nano fibers were used as filling phase,and the PVDF composite was prepared.Meanwhile,the influence of filling type and size factor on the properties of the composite was systematically investigated.Study found that,the compound particles of copper calcium titanate(CCTO)with the surface growth of Fe3O4 particle(Fe3O4@CCTO)greatly improved the dielectric constant of PVDF-based composite comparing to the CCTO ceramic particles.However,due to the introduction of Fe3O4,the loss of the composite increased and the breakdown strength of the composite decreased,so that the composite with better energy storage could not be obtained.Nevertheless,the nano-fiber has become an ideal material with high orientation and high energy storage,because of its advantages such as large aspect ratio and high specific surface area.Accordingly,three kinds of typical nano-fiber filling phase(titanium dioxide,calcium copper titanate and calcium barium zirconate titanate nano-fiber)were prepared by electrospinning technique.Subsequently,the effect of nanofibers on the dielectric properties of PVDF-based composite was systematically investigated.In the results of this research,the energy density of CCTO/PVDF composite increased significantly under a lower electric field.But the breakdown strength of CCTO/PVDF composite reduced,because the dielectric difference between the CCTO and PVDF matrix caused serious distortion of the internal electric field of composite.However,the energy storage property of the Ti O2/PVDF composite was improved lightly due to the lower dielectric constant of Ti O2 nano-fibers,although dielectric difference between the Ti O2 and PVDF matrix was small.The BZT-BCT nano-fibers can be used as a high energy-storaged and an oriented inorganic filling phase,because of the moderate dielectric constant and the low conductivity of BZT-BCT in the morphotropic phase boundary region.For instance,the BZT-BCT/PVDF composite possesses an excellent energy storage property under the electric field of 250 k V/mm.On the basis of above results,the influence of different content of BZT-BCT nano-fiber on the properties of PVDF composite was systematically investigated.The experimental results show that,BZT-BCT/PVDF composite owned a higher dielectric constant(?~ 17.6 at 100 Hz)when the content of BZT-BCT nano-fiber of the central layer reached to 7 vol.%.And the BZT-BCT/PVDF composite had an excellent energy storage characteristic(Ue~ 7.9 J/cm3,??58.3%)when the content of BZT-BCT nano-fiber of the central layer was 3 vol.%.In order to optimize the electric field distribution and further improve the breakdown strength of composite,the sandwich-structured PVDF/BZT-BCT PVDF/PVDF composite was designed and prepared.Meanwhile,the probability of breakdown of sandwich-structured composite was reduced by twisting path of electrical treeing due to the nano-fiber in central layer.Based on the idea of compound particles to enhance the polarization characteristics of the dielectric composite,the CFO@BZT-BCT compound nano-fiber that Co Fe2O4 particles grew on the surface of BZT-BCT nano-fiber was prepared.The conductive Co Fe2O4 particles inject a small amount of electrons to composite,which increased the number of dipole polarization,and then the inner polarization of composite was enhanced.Therefore,the central layer of the inorganic-filler sandwich-structured composite was exchanged to the CFO@BZT-BCT composite non-fiber,which leaded a better performance of the sandwich-structured PVDF/CFO@BZT-BCT PVDF/PVDF composite.This dielectric composite showed excellent dielectric properties due to the redistribution of internal electric field in the sandwichstructured composite when the CFO@BZT-BCT nano-fiber content of the central layer is 5 vol.%(?~ 20.1 and tan??0.023 at 100 Hz,Ue~ 11.3 J/cm3 and ??55.5% at 350 k V/mm).