Design,Fabrication And Energy Storage Property Of Poly(Vinylidene Fluoride)-based Dielectric Composites

With the flourishing of modern electrical and electronic industries,the demand for microelectronic devices with integration,miniaturization,flexibility and intelligence is ever-increasing,meanwhile the dielectric materials with more excellent properties are required,including light-weight,flexible,low-cost,multifunctional,large-specific-power and easy-processing/molding.Therefore,the design and development of novel dielectrics with high dielectric constant,high breakdown strength,low loss,easy processing,and operational reliability have become one of the frontiers and hotspots on the research of advanced energy storage technology.Although the traditional ferroelectric ceramic materials possess many advantages such as high dielectric constant and wide temperature range for use,their disadvantages are also obvious such as low breakdown strength,brittleness,poor mechanical properties and poor manufacturability,which seriously restrict their further improvement.In contrast with ceramics,polymer materials show advantages in flexibility,processability and excellent breakdown strength,while they are still trapped by low dielectric constant.Poly(vinylidene fluoride)(PVDF)and its copolymers are ferroelectric polymers,which possess high dielectric constant(?10),excellent breakdown strength(?500 kV/mm)and good processability,and suitable for the preparation of large-area or complex-shape film.Therefore,they are showing certain research significance and application value in the preparation of high-performance energy storage dielectrics.This paper focuses on the insufficiency of low energy density for PVDF and its copolymers.According to the polarization mechanism of polymers,a series of modification methods are used to prepare composite films in order to improve its energy storage performance.The research contents and conclusions of this paper are as follows:1.Poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene)(P(VDF-TrFE-CFE))with high dielectric constant and P(VDF-TrFE)with high breakdown strength are chosen to be mixed in different proportions to prepare composite films.The composites gradually change from a phase to ? phase as the P(VDF-TrFE)content.When the mixed ratio of P(VDF-TrFE-CFE)/P(VDF-TrFE)upto 90/10,the highest dielectric constant was achieved as?38 at 102 Hz,and its breakdown strength increases from 188.9 MV/m of P(VDF-TrFE-CFE)to 217.3 MV/m.Thereby,the highest released energy density is 6.4 J/cm3.2.P(VDF-TrFE)-g-PMMA was prepared by the pre-irradiation grafting method,and its degree of grafting could be adjusted by tailoring irradiation dosage and grafting reaction time.As the Poly(methyl methacrylate)(PMMA)content increases,the graft copolymers achieve the transition from normal ferroelectric performance to either antiferroelectric-like or linear behaviour.According to the nanoconfinement effect,amorphous PMMA can act as physical barrier to inhibit the thermal motion of P(VDF-TrFE)chain,which thereby restricting the nucleation and growth of the ferroelectric ?-phase,and resulting in the reduction of the crystallinity and grain size.The smaller grains are beneficial for the weakened coupling effect,and thus the reduced hysteresis loss.The dielectric constant and frequency dependence of P(VDF-TrFE)-g-PMMA gradually decreases with the PMMA content.In comparison with pure P(VDF-TrFE),the released energy density and efficiency of irradiated graft copolymers with 26 wt%PMMA increase upto 9.5 J/cm3 and?80%,respectively.3.Titanium dioxide nanofibers(TiO2 NFs)with high aspect ratio were synthesized by the combination of electrospinning and calcination,and the crystalline structure of which are the mixture of anatase and rutile phases(1:1).Subsequently,TiO2 NFs were functionalized by polydopamine(PDA)and incorporated into poly(vinylidene fluoride-hexafluoropropylene)(P(VDF-HFP))matrix by solution casting for fabricating TiO2@PDA NFs/P(VDF-HFP)nanocomposite films.In comparison with pure anatase/rutile TiO2,the mixed crystalline structure helps in increasing interfacial polarization of TiO2.In addition,by coating PDA on the surface of nanofibers,the dispersion and compatibility of TiO2@PDA NFs in P(VDF-HFP)matrix were significantly improved.With the increase of filler content,the interfacial polarization effect is greatly enhanced,the dielectric constant of the nanocomposite films is increased,while the dielectric loss in high electric fields is suppressed.When the filler content is 9.8 wt%,the highest energy density of the nanocomposite films is 9.34 J/cm3(216 kV/mm).4.Based on the third part,by controlling the pre-calcination temperature,TiO2/C NFs with different carbon content were synthesized in situ,and subsequently incorporated into P(VDF-HFP)matrix by solution casting for fabricating TiO2/C NFs/P(VDF-HFP).The characteristics of TiO2/C NFs with high aspect ratio bring good dispersibility in P(VDF-HFP)matrix,show a certain tendency of in-plane orientation,and could serve as crystallization sites to improve the crystallinity of the nanocomposites.Within TiO2/C NFs,any two adjacent carbon nanoparticles could be regarded as a "microcapacitor".And the existence of "microcapacitors" could further enhance the interfacial polarization inside TiO2/C NFs,so that the dielectric constant of nanocomposites can be increased,while the dielectric loss maintain at low level..Finally,by doping carbon nanoparticles,the nanocomposite films can achieve the higher energy density at smaller loading content.