Studies On The Dielectronic And Breakdown Properties Of Polymer Thin Films For High Energy Density Capacitors Applications

High energy density capacitor is a hot research topic in recent years. It is widely used in the practical application. And it can not only be used to store energy, but also can be used as the parent flow line capacitors and so on. The film capacitors gradually begin to receive widespread attention because of its small volume, high energy storage density, simple preparation process and high reliability. Generally, ceramic or polymer materials are often selected as a dielectric layer due to they have a higher relative dielectric constant or breakdown voltage. In this paper, we select poly vinylidene fluoride(PVDF) as the main research object due to its excellent physical and chemical properties and high voltage-withstanding ability. A coating methods is utilized to prepare PVDF Film, and the dielectric and breakdown voltage properties of this PVDF film are investigated. Furthermore, scanning electron microscope(SEM), X-ray diffraction(XRD), Fourier transform infrared(FT-IR) spectrum and differential scanning calorimetry(DSC) methods are used to characterize film structure and morphology. The main research contents and conclusions are as follows:(1) The PVDF films with different crystal type are prepared by the controlling of thermal treatment parameters with different time and temperature. The dielectric property and voltage-withstanding property of α phase, β phase and γ phase are investigated. The microscopic analysis results show that the β phase can be transformed into α phase through a high temperature annealing process. The experimental results show that the free capacitance and loss are dependent on frequency with the evidence that free capacitance decreased with the increasing frequency, and dielectric loss increased with the frequency increasing. Due to the completely molecular polarization in α phase film, this film showed the highest relative dielectric constant. The voltage-withstanding test results confirm that the breakdown voltage of γ phase film is higher than other crystal films. The further investigation reveals that decreasing the crystalline degree of PVDF film can effectively improve the film breakdown voltage.(2) In order to improve the breakdown voltage performance of PVDF film, the poly(methyl methacrylate)(PMMA), possessing high breakdown voltage in a wide temperature, is added to pure PVDF. The high young’s modulus of PMMA can also improve the mechanical property of PVDF. During the mixing process, hydrogen bonds between PMMA and PVDF can increase the compatibility of two parts, avoiding the phase separation of PMMA and PVDF in composites. It has been found that the addition quality ratio of PMMA into composite showed distinct influence on crystalline phase, dielectric and breakdown voltage performance of PVDF. Below the 5wt% quality ratio, the addition of PMMA leads to proper crystallinity decrease and slight dielectric constant reducing. Moreover, an obvious increase of breakdown voltage(up to 153V/μm) of composite film was achieved, which result in construction of a film capacitor with high energy density about 0.7J/cc.