Research On Structural Design And Energy Storage Performance Of BaTiO₃/Polymer Dielectric Composites

Electrostatic capacitors have been widely applied in power electronics field,such as high-power pulse weapons,flexible power transmission and transformation engineering,new energy vehicles and 5G communication,due to its unique advantages of high-power density,fast charging speed and long service life.With the development trend of electronic devices towards light weight,flexibility and intelligence,the relative low capacitance and energy storage density of electrostatic capacitors have become the main bottlenecks and restrict their applications.In this research,the multi-level structure of dielectric composites is exquisitely designed,including crystal structure of polyvinylidene fluoride PVDF polymer in micro scale,nanocrystal shape factor and orientation distribution regulation of Ba Ti O₃ and layered topology layered structure in mesoscale,etc.The correlation between multi-scale structure and electrical properties is analyzed and discussed from the perspective of interface polarization and local electric field distribution for the collaborative optimization of dielectric and breakdown properties.Accordingly,this article mainly carried out the following work:（1）The Ba Ti O₃/epoxy dielectric composites with oriented distribution of ceramic particles are prepared by freeze casting and impregnation process.The effects of different ceramic phase content and orientation structure on dielectric properties,breakdown strength and energy storage properties are systematically studied.The electric field distribution of ceramic phase with different orientation structure is simulated by finite element theory,and the enhancement mechanism of polarization effect of directional arrangement of ceramic particles under electric field is especially studied.The ultrahigh dielectric constants of“BT-//-59”and“BT-⊥-59”composites are 999 and 1408 at 1k Hz,respectively,which fills the blank area between the dielectric parallel mode and logarithmic model of ceramic/resin dielectric composite.The discharge energy densities of“BT-//-59”and“BT-⊥-59”composites are 19.6×10^-2 J/cm³,18.1×10^-2 J/cm³ at electric field of 6 k V/mm,which is much higher than the 3.0×10^-2J/cm³ of random distribution composites,indicating that constructing the oriented structure of dielectric materials is an effective method to improve the dielectric and energy storage properties.（2）High perpendicularity Ba Ti O₃ nanowire arrays with length and diameter of 3μm and 200 nm are synthesized by hydrothermal method,and transformation mechanism of crystal structure is clear clarified.The thickness of nanocomposite is optimized to 12.5μm by spin coating.The permittivity at 1 k Hz of Ba Ti O₃/PVDF nanocomposite is significantly increased from 9.1 to 33.9 by the coupling effect of interface polarization and orientation polarization from nanowire array.An appropriate amount of BN nanosheets is introduced into the upper layer of PVDF to effectively prevent the growth of electrical tree,reduce leakage current density and improve breakdown strength.The Ba Ti O₃-3wt.%BN/PVDF composites reach 12.1μC/cm² at 500.3 k V/mm,yielding a high discharge energy density and efficiency of 16.1 J/cm³ and 57%,respectively.（3）The high aspect ratio Ba Ti O₃ nanowires are synthesized by hydrothermal reaction,and then Ba Ti O₃/PVDF nanocomposites are prepared by solution casting method.The effects of different loading nanowires on dielectric and breakdown properties were systematically studied.The optimal filler content of Ba Ti O₃ nanowires was 3wt.%.The addition of an appropriate amount of Ba Ti O₃ nanowires can introduce some weak field regions,which effectively hinder the extension of electron tree.The electric polarization and breakdown strength of sandwich structured Ba Ti O₃/PVDF nanocomposites are significantly enhanced due to the interlayer interface polarization and barrier effect.The changes of dielectric and electric field distribution of sandwich composites with different configurations are discussed for the further optimization of dielectric/electrode interface.Finally,0-3-0 nanocomposites achieve D_maxof 12.1μC/cm² at 519.7 k V/mm,leading to a high discharge energy density of 19.1 J/cm³ and energy efficiency of 68.5%,respectively.（4）The aggregation structure of PVDF polymer is regulated by uniaxial drawing process and the evolution law of micro-crystalline under different stretch ratios is systematically studied.With the increase of stretch ratio to R=5,PVDF polymer presents polarβphase transformation,decreased crystallinity and crystal size,and obvious crystal orientation.The crystal morphology changed from spherulite to fiber strip.The influence mechanism of microstructure on the dielectric,polarization,breakdown and other electrical properties of PVDF polymers is deeply explored.The oriented amorphous caused by stretching makes the dipole easy to flip under the electric field,which is conducive to the improvement of the dielectric constant of stretched PVDF polymer.The enhancement of Young’s modulus,crystal orientation and crystal morphology fibrosis significantly increased the breakdown strength of PVDF polymer from440.3 k V/mm for R=1 to 795.4 k V/mm for R=5.The D_max of PVDF polymer reaches 16.10μC/cm²,leading to an ultrahigh U_dis andηof 34.9J/cm³ and 68.2%,respectively.（5）The energy storage performance of Ba Ti O₃/PVDF composites is further improved by uniaxial stretch process.The orientation angle of Ba Ti O₃ nanowires in the plane under different stretch ratios is studied,and the effect of orientation angle on the dielectric and breakdown properties is mainly explored.With the increase of stretch ratio from R=1 to R=5,the orientation angle of nanowires decreased from 44.2°to 3.7°.The decrease of nanowire orientation angle reduces the contact probability at the top of nanowire,which alleviates concentration of electric field and reduces the formation of conductive path.Benefitting from the enhanced polarization and breakdown strength,the U_dis andηof 0-3-0 composite at R=5 is 40.9J/cm³ and 69.7%at 843 k V/mm.The charge-discharge performance shows the power density and discharge time of the composite are 63.2 MW/cm³and 0.368μs,respectively.Therefore,excellent energy storage density,ultrahigh power density and ultra-fast discharge speed make it a promising candidate in the application of capacitor devices and pulse power systems.