Structural Design And Energy Storage Performance Of BST/PVDF Dielectric Composite Materials

Under the background of emerging energy development,dielectric capacitors have gradually become an important part of electrical systems such as pulsed lasers and inverters due to their high power density.As the core of dielectric capacitors,the development of dielectric materials is of great significance to realize the integration,intelligence,miniaturization and lightweight of power storage and conversion devices.Polymer-based dielectric composites can effectively combine the advantages of high dielectric constant of inorganic dielectrics and high breakdown electric field strength of organic dielectrics.However,the large difference in electrical properties between the inorganic phase and the organic phase leads to the low breakdown electric field strength of the difficult to obtain high energy storage density.In this thesis,based on the design and optimization of the microstructure and macrostructure of polymer-based dielectric composites,the effects of filler microstructure and layered macrostructure on the dielectric properties and energy storage properties of composites were discussed.(1)In terms of the effect of the microstructure of the filler on the energy storage performance of the composites,polyvinylidene fluoride(PVDF)matrix composites(BaxSr1-xTiO3/PVDF(0.1≤x≤0.9))containing different phase structures of barium strontium titanate nanopowders(BST,BaxSr1-xTiO3(0.1≤x≤0.9))were prepared by tape casting method,and structural characterization,dielectric and energy storage properties were analyzed.The results show that the dielectric constant of 1 vol.%BaxSr1-xTiO3/PVDF(0.1≤x≤0.9))increases first and then decreases with the increase of x,and the breakdown field strength and energy storage density increase first and then decrease.BaxSr1-xTiO3(x=0.6)has high dielectric constant and paraelectric cubic perovskite structure.The discharge energy density of 1 vol.%BarSr1-xTiO3/PVDF(x=0.6)is 7.8 J/cm3.BaxSr1xTiO3/PVDF(x=0.6)not only has a more uniform internal electric field,but also inhibits the migration of carriers,resulting in higher conductivity activation energy.(2)In order to further improve the breakdown field strength of Ba0.6Sr0.4TiO3/PVDF composite dielectric materials,the effects of sandwich structure and five-layer structure on the breakdown field strength of Ba0.6Sr0.4TiO3/PVDF composite dielectric materials were discussed.Ba0.6Sr0.4TiO3/PVDF composites with sandwich structure were prepared by layerby-layer casting method using inorganic powder(Ba0.6Sr0.4TiO3)and boron nitride nanosheets(BNNS)as fillers and PVDF as matrix.Among them,5 wt.%BNNS/PVDF was used as the charge blocking outer layer of sandwich structure,and Ba0.6Sr0.4TiO3/PVDF with different volume fractions(0 vol.%,1 vol.%,2 vol.%and 3 vol.%)was used as the intermediate layer.The results show that the dielectric constant of the sandwich structure BNNS/PVDF-Ba0.6Sr0.4TiO3/PVDFBNNS/PVDF increases with the increase of Ba0.6Sr0.4TiO3 volume fraction,and the breakdown field strength and energy storage density increase first and then decrease.The breakdown field strength of the sandwich structure composite with 1 vol.%Ba0.6Sr0.4TiO3/PVDF interlayer reaches 310 kV/mm,and the maximum discharge energy density is 8.9 J/cm3.The charge blocking outer layer in the sandwich structure can inhibit the charge injection from the electrode and reduce the conductivity of the composite,thereby increasing the breakdown field strength.Based on the sandwich structure,the effect of introducing charge blocking inner and outer layers to form a five-layer structure on the breakdown performance of Ba0.6Sr0.4TiO3/PVDF composites was further discussed.Five-layer BNNS/PVDFBa0.6Sr0.4TiO3/PVDF-BNNS/PVDF-Ba0.6Sr0.4TiO3/PVDF-BNNS/PVDF composites were prepared by the same method.The results show that the dielectric constant of the five-layer structure BNNS/PVDF-Ba0.6Sr0.4TiO3/PVDFBNNS/PVDF-Ba0.6Sr0.4TiO3/PVDF-BNNS/PVDF increases with the increase of Ba0.6Sr0.4TiO3 volume fraction,and the breakdown field strength and energy storage density first increase and then decrease.The breakdown field strength of 2 vol.%five-layer structure BNNS/PVDF-Ba0.6Sr0.4TiO3/PVDF-BNNS/PVDFBa0.6Sr0.4TiO3/PVDF-BNNS/PVDF composite is 420 kV/mm,and the discharge energy density is 14.89 J/cm3.The five-layer composite material simultaneously suppresses the injection of charge from the electrode and the movement of charge inside the composite material,thereby reducing the electrode conductivity and bulk conductivity of the composite material,and finally significantly improving the breakdown field strength.(3)In order to improve the polarization strength of Ba0.6Sr0.4TiO3/PVDF composites and increase the energy storage density,the effects of Ba0.6Sr0.4TiO3 content gradient(content gradient structure)and BaxSr1-xTiO3/PVDF(x=0.7;x=0.8;x=0.9)composition gradient(composition gradient structure)on the energy storage performance of the composites were studied.The results show that the dielectric constant,breakdown field strength and energy storage density of the content gradient structure Ba0.6Sr0.4TiO3/PVDF are larger than those of the nongradient structure samples.When the content of Ba0.6Sr0.4TiO3 increases from the outer layer to the inner layer(positive content gradient structure),the saturation polarization of the composite reaches 10.1 μC/cm2,and the discharge energy density is 9.3 J/cm3.The positive content gradient structure Ba0.6Sr0.4TiO3/PVDF composites effectively increase the polarization intensity by increasing the interfacial polarization.The dielectric constant,breakdown field strength and energy storage density of BaxSr1-xTiO3/PVDF(x=0.7;x=0.8;x=0.9)composites with sequential composition gradient structure are larger than those of nonsequential composition gradient structure samples.The saturated polarization strength of BaxSr1-xTiO3/PVDF composites with composition gradient changes in turn reaches 10.51 μC/cm2,and the discharge energy density is 12.9 J/cm3.The composition gradient structure composites not only increase the interfacial polarization intensity in the layer but also increase the interfacial polarization intensity between the layers,thus increasing the polarization intensity of the composition gradient structure BaxSr1-xTiO3/PVDF composites.(4)Inspired by the charge barrier layer structure and gradient layered structure,BaTiO3@BaxSr1-xTiO3/PVDF powder with gradient core-shell structure was constructed and its effect on the energy storage performance of the composite was studied.The breakdown field strength and polarization strength were improved simultaneously.BaTiO3@BaxSr1-xTiO3 nanopowders with gradient core-shell structure were synthesized by hydrothermal method.Five kinds of 5 wt.%BaTiO3@BaxSr1-xTiO3/PVDF(0≤x≤0.9)composites with different shell compositions were prepared by tape casting method.The results show that the dielectric constant of BaTiO3@BaxSr1-xTiO3/PVDF increases first and then decreases with the increase of powder shell thickness,and the breakdown field strength and energy storage density increase first and then decrease.The dispersion layer in BaTiO3@BaxSr1-xTiO3/PVDF composites is beneficial to improve the interfacial bonding strength.The buffer layer can promote the uniform distribution of electric field,and the shielding layer limits the migration of free charge,thus improving the breakdown field strength of the composites.The polarization layer in the composite is beneficial to improve the polarization strength.The breakdown field strength of BaTiO3@BaxSr1-xTiO3/PVDF reached 320 kV/mm and the saturation polarization reached 11.2 μC/cm2 after hydrothermal reaction for 3 h.In order to further explore the effect of gradient core-shell structure fillers on the energy storage performance of composites,BaTiO3@BaxSr1-xTiO3/PVDF(0≤x≤0.9)composites with different mass fractions(0 wt.%,1 wt.%,2 wt.%,3 wt.%,4 wt.%and 5 wt.%)were prepared.The results show that the dielectric constant of the composites increases with the increase of powder mass fraction,and the breakdown field strength and energy storage density increase first and then decrease.On the one hand,the gradient core-shell structure nanopowders can effectively reduce the difference in dielectric constant and conductivity between the filler and the polymer matrix,thereby reducing the space charge density.On the other hand,it can form more gradient interfaces,thereby increasing the polarization intensity.The breakdown field strength of 2 wt.%BaTiO3@BaxSr1-xTiO3/PVDF composites reached 385 kV/mm,the saturation polarization reached 11.0 μC/cm2,and the discharge energy density of 13.75 J/cm3 was obtained,which was better than that of BaTiO3/PVDF composites with the same mass percentage.