Study Of PVDF-based Dielectric Composite Films With High Energy Density

Recently, to address energy shortage and low energy efficiency problems,much work has been focused on developing new energy storage and conversion devices. In this context, dielectric capacitors have become a research hotspot due to their high power density, cyclic aging resistance and stable performance. Currently, traditional ceramic dielectric materials possess high dielectric constant. However, their application for high-energy capacitors is limited by their low breakdown field, poor flexibility and complicated processing conditions. Polymers typically have high breakdown field,excellent mechanical properties and easy-processing conditions but very small dielectric constant. Therefore, it is critical to study and prepare polymer-based composites with high dielectric constant and breakdown field as well as low dielectric loss.In this dissertation, Poly(vinylidene fluoride) (PVDF)-based were chosen to be polymer matrix. The influences of different fillers with different size and morphology, interfaces between organic matrix and inorganic fillers and organic fillers on the dielectric behaviors of composite films were studied.The mechanisms were investigated and structure-function relationships were established. With the guidance of the above research, novel dielectric composites with high energy density and low loss were designed and synthesized. The detail research contents are as follows:1. SnO2 nanoparticles with size of 5-7 nm were prepared by hydrothermal method. SnO2/PVDF nanocomposites were designed based on percolation theory. The dielectric constant of the nanocomposites reached at 320, which was 40 times higher than that of PVDF matrix, and the dielectric loss was only 0.8 at 103 Hz. According to the results of linear simulation, the dielectric behavior of nanocomposites fit the percolation theory well. The steadily decreased breakdown field may be related to the wide band gap of SnO2, which could prevent charge carriers from penetrate the nanocomposites.The interfaces created by nanosized SnO2 particles and polymer matrix can bring effective electron scatterers and trapping centers, which would also prevent quick decrease of breakdown field.2. Sodium titanate nanotubes (TNT) with diameter of 8-10 nm and length of 300-500 nm were synthesized by a hydrothermal method and incorporated into poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)(PVDF-TrFE-CTFE). Bipolar and unipolar electric displacement electric field(D-E) loop tests were used to explore mechanism of dielectric loss (both linear and nonlinear). Both external and internal electronic conduction loss were contributed to dielectric loss of the nanocomposites. Under the frequency of 10 Hz, bipolar D-E loops showed upshift, indicating the external electronic conduction loss between fillers as well as fillers and polymer matrix. Under the frequency of 100 Hz and above, the external electronic conduction loss was negligible, but the significant losses were still observed using dipolar D-E loops tests, suggesting the existence of internal electronic conduction loss.3. Ag@SiO2 nanoparticles were prepared by a simple Stober method. The Ag cores were about 40-55 nm in diameter and the thickness of the shell was 5-20 nm. Compared to the Poly(vinylidene fluoride-trifluoroethylene)(PVDF-TrFE) matrix, the dielectric constant of the nanocomposites can reach up to 31 due to the enhanced interfacial polarization originated from the core-shell filler structure. Meanwhile, the dielectric loss of the nanocomposites remained relatively low (less than 0.05). The dielectric constant decreased with increasing shell thickness while the breakdown field increased. At low filler contents, the breakdown field could be improved and reach up to as high as 346 MV/m. The results proved the insulating SiO2 shell could improve the breakdown field by suppressing leakage current. The dielectric properties of the nanocomposites could be controlled by changing the thickness of SiO2 shell of core-shell nanoparticles.4. BN nanosheets (BNNS) with lower layers were prepared by chemical exfoliation method and incorporated into PVDF-TrFE-CTFE. The optimal filler contents of BNNS were studied. It is found that BNNS/PVDF-TrFE-CTFE nanocomposite with 12 wt % of BNNS showed best dielectric properties with discharged energy of 7.1 J/cm3. The breakdown field of BNNS/PVDF-TrFE-CTFE nanocomposites could reach up to 610 MV/m due to the wide bandgap and high breakdown field of BNNS. Adding BNNS could decrease the crystallite size and thus improve the charged-discharged efficiency (84 %). Based on the above results, barium titanate, which was modified with dopamine (BT@DA), was added to prepare three-phase BT@DA/BNNS/PVDF-TrFE-CTFE nanocomposites. By utilizing high dielectric constant of BT and high breakdown field of BNNS, the dielectric properties of three-phase nanocomposites were improved with dielectric constant of 76. With 20 wt % of BT@DA and 12 wt % of BNNS,the discharged energy density and charged-discharged efficiency of three-phase nanocomposites were 13.3 J/cm3 and 72 %, respectively.5. Aromatic polythiourea (ArPTU) was prepared by a conventional polycondensation method. ArPTU had low dielectric loss (0.0067), high breakdown field (746 MV/m) and high charged-discharged efficiency (more than 90 %) due to its amorphous phase polymer structure and quick response of dipoles in the polymer chain to the applied electric field. By incorporating ArPTU into PVDF-TrFE-CTFE, the electric resistivity increased and the crystalline structure changed. The crystallite size of the composites decreased and the interchain distance increased, providing extra room for the friction-free rotation of dipoles, leading to low hysteresis loss and high discharged energy density as well as high charged-discharged efficiency. The discharged energy density of composites could reach up to 19.2 J/cm3 under the electric field of 700 MV/m, and the efficiency remained higher than 85 %.