The Study Of High Energy Density Organic/Inorganic Composites

Organic/inorganic composites were synthesized by high dielectric constant nano inorganic particle barium titanate (BaTiO3) and high polarity polymer polyvinylidene fluoride (PVDF). The surface modification on the BaTiO3 and structural design of the composites were utilized to improve the dielectric constant and breakdown strength, thus the energy density were improved. The mechanism of improving electric properties was explained and paves the road for material fabrication.100nm BaTiO3 particles were surface modified by titanate, dopamine, PEG and Silica in two approaches. Functionalized BaTiO3/PVDF composites were fabricated by liquid molding. The composites were characterized by Electron microscope, infrared spectroscopy and thermoanalysis, the dielectric constant was tested by precision impedance analyzer and the breakdown strength was tested by dielectric strength tester. The experiment results showed the dielectric constant of titanate functionalized BaTiO3/PVDF composites with 7 vol% BaTiO3 is 14.8, the breakdown strength is the highest reached 250 kV·mm-1, the composites has the max energy storage density, which is 8 times larger than the unmodified. Both dielectric constant and breakdown strength were enhanced and the energy storage density has a significant improving. PEG coating on BaTiO3 could effectively improve the compatibility and interfacial polarization effect, which lead to remarkable improvement on dielectric constant. Dielectric loss of composites was below 0.5 with 5 wt%PEG even the BaTiO3 content is high. At 60 wt% BaTiO3 content, the dielectric constant of PEG@BaTiO3/PVDF composites reached 44, which was double of BaTiO3/PVDF composites and 4 times of the pure PVDF, but the breakdown strength of this composites is poor, thus the energy density was not distinctly changed. The dielectric constant vs content of Silica@BaTiO3/PVDF composites has the same trend as uncoated BaTiO3/PVDF composites, both grow with the volume content of BaTiO3, but the nonpolar Silica-nH2O weaken the polar effect which lead to slightly lower of dielectric constant and dielectric loss than the unmodified composites. On the one hand, hydroxyl on Silica bonding with C-F on PVDF, which improved the compatibility, on the other hand, the rough surface of Silica increased specific surface area. Ample hydroxyl and large specific surface area enhanced the mechanical bonding of the particles and polymer matrix. When massive inorganic particles are mixed, crack will form due to the difference in shrinkage of the polymer and particles. The well bonding between Silica@BaTiO3 and polymer matrix reduces the crack and maintain the breakdown strength at a high level. This method hasn't been reported yet, and makes a break though of breakdown strength reach 190 kV·mm-1 when the BaTiO3 volume is over 10%. At BaTiO3:Silica= 8:1,33 vol% BaTiO3, breakdown strength of the composites performs well, and dielectric constant does not significantly change, dielectric loss is lower than uncoated composites. High breakdown strength and low dielectric loss of silica@BT/PVDF composites could result in an improved energy density (5.3J·cm-3) and high energy utilization efficiency compared with those of BaTiO3/PVDF.Multi-layer BaTiO3/PVDF composites with dielectric gradient were fabricated by two approaches. The dielectric constant was tested by precision impedance analyzer and the breakdown strength was tested by dielectric strength tester. The results showed the dry film method can obviously improve dielectric constant because of inter layer dielectric differences. PEP type doubled the dielectric constant to the single-layer, PB is 1.5 times that of single-layer composites. However, breakdown strength is lower than single-layer composites, mainly due to the excessively strong interfacial polarization and large polarization loss. The wet film method fabricated composites has a smaller increase than the dry film method as well as the dielectric loss. The disseverment at transition region weakens the interfacial polarization. The breakdown strength doesn't decrease, for high breakdown layer can protect the low breakdown layer by slowing down electron kinetic energy increase and decentralizing spatial charge distribution. Thus breakdown strength of composites fabricated by wet film method is higher than dry film method at same BaTiO3 volume content. Breakdown strength of alternate layer composites is improved significantly; the breakdown strength of five alternate layers composites at 20 vol% is 280 kV·mm-1. The optimal structure of composites was 5 layers with alternating structure. The energy density of the composites with lamellar structure was 12.5 J·cm-3, the discharged energy density was 3.8 J·cm-3 which is higher than that of monolayer composites and pure PVDF.