Study On Fabrication And Properties Of PVDF Matrix Composite Films With High Dielectric Properties

Pulsed-power capacitors will be needed dielectric materials which have high operating voltages, high dielectric constant, high energy density and short charging/discharging time in the future. But single kind of materials such as ceramics or polymers may not achieve this goal because of the complicated preparation process and frangibility of ceramics and the low dielectric constants of polymers. Recently, much attention had been paid on polymer-based on composite materials, especially nanocomposite materials. This material can be used to produce arbitrary shape multilayer chip capacitor which having extensive applications in electronic industry field.In this paper, different series of PVDF matrix composite films with different filler typies and contents were prepared by choosing three different properties of particles(core-shell structure of Ti C@Al OOH, hydroxylated graphite microsheets and metal Ag particles) as fillers. The microstructures, crystal phase transition, thermal stability and electrical properties of the composite films were characterized. The effects of different fillers and its’ contents on microstructure and properties of the composite films were analysized. Results showed that the Ti C@Al OOH particles prepared by heterogeneous nucleation have a typical core-shell structure, and the thickness of Al OOH shell is about 5 ~ 10 nm. Some slight agglomeration of Ti C@Al OOH particles existed in PVDF composite film. XRD analysis showed that the addition of Ti C@Al OOH particles promoted the transition of PVDF film from multiphase phase to excellent performance of ferroelectric ? phase. Thermal analysis results indicated that the introduced nanoparticles reduced initial thermal decomposition temperature of the composite films. Broadband dielectric spectroscopy analysis showed that dielectric property of the composite film both increases with the increase in filler contents, presenting a positive correlation between the dielectric constant and the dielectric loss. When the filler content added up to 27 wt%, the dielectric constant of the composite film increased sharply from 34.1(filler content 20 wt%) to 7.18?104, which indicated the filler content is at or near the percolation threshold. In the frequency range 10-1-104 Hz, the conductivity of the composite film keeps the value 4.33?10-7 S·cm-1 unchanged stably and increases with increasing in testing frequency from104 Hz to 107 Hz. Using the hydroxyl graphite micro-sheets as fillers, SEM showed that the diameter of prepared graphite sheets is micron level and they dispersed better in PVDF matrix. XRD spectrum indicated that the introduced of graphite micro-sheets have little effect on the phase change of PVDF films. Thermal analysis showed that the addition of graphite micro-sheets reduced the thermal stability of the composite films. Broadband dielectric spectroscopy test results showed that dielectric constant of composite films is as high as 36 under 100 Hz test frequency, when the doping amount of graphite micro-sheets is less than or equal to 3 wt%. When doping amount is up to 5 wt%, the dielectric constant of the composite film sharply increased to 6.5×105. The change of dielectric loss was similar to the change of dielectric constant. In the frequency range of 10-1-105 Hz, the conductivity of the composite film was 5.5×10-6 S·cm-1, the conductivity increased with increasing frequency at 105-107 Hz. The breakdown field strength of this composite film was 0.63 k V/mm, its specific energy storage was 1.29 J/cm3 which is about 75 times than that of pure film. Using silver particles as fillers, SEM analysis showed that most of the silver particles prepared by ethylene glycol reduction method were rob, the other were spherical, both of them dispersed in the PVDF matrix uniformly. With the increase of silver particles, the characteristic peak intensity of sliver increases. The sliver particles promoted the transition of PVDF film from multiphase phase to excellent performance of ferroelectric ? phase and increased the thermal stability of the composite films. Dielectric constant of PVDF was 9.2 at 100 Hz, dielectric constant of composite films which content of sliver is 20 wt% was 14 and stable relatively with the frequency change. The dielectric loss of composite film decreased with the frequency increases below 103 Hz, while it increased with the frequency increased over 103 Hz. The conductivity of the composite film increased with the frequency increases in the entire test frequency range. The breakdown field of composite film increased first and then decreased as the silver content increases. When the content of sliver is up to 10 wt%, the breakdown field strength is 22.39 k V/mm and the stored energy density is 0.026 J/cm3 which is a little higher than that of pure film.