Study On Interface Strategy And Energy Storage Behavior Of Relaxed Ferroelectric Composites

Thin film capacitors possess irreplaceable advantages in the field of power electronic conversion technology.PVDF based polymers with polar dipoles are the next-generation dielectric materials for capacitor applications.It is an effective way to improve the energy density of dielectric materials by the addition of high-dielectric ceramics into polymer dielectric material matrix.However,there is poor compatibility between inorganic ferroelectric ceramic filler and polymer matrix in the nanocomposite system.High content of the ceramic nanofillers can result in the agglomeration in the matrix and forming conductive path,which leaded to a rapid decrease in the breakdown strength.Therefore,the surface-modification technology of ceramic nanofiller can improve its dispersibility in polymer matrix and reduce dielectric loss.Recently,the small-molecule layer,such as such as the silane coupling agent and the phosphoric acid ligand,cannot effectively suppress surface leakage current of ceramic nanofillers,and then the breakdown strength is not effectively improved.In the first section,fluorine containing aromatic polythiourea（Ar PFTU）with high breakdown strength and good compatibility was selected as an excellent shell to modify the ceramic nanofillers.Herein,BaTiO₃@fluorinated aromatic polythiourea（BaTiO3@Ar PFTU）nanoparticles（NPs）were successfully synthesized via microwave-initiated polymerization.Different shell thickness of Ar PFTU were synthesized by controlling the power and time of microwave polymerization.TEM characterization showed that three types of the shell thickness of 5,9 and 15 nm were prepared.Contact Angle of the nanofillers BaTiO₃@Ar PFTU changed from hydrophilic（17.2°）to hydrophobic（116.1°）.With the increase of thickness of Ar PFTU shell,impedance of BaTiO₃@Ar PFTU nanoparticles increased from 7.5×10⁴Ωto 9.3×10⁶Ω.In the second section,ultrahigh-insulation BaTiO₃@fluorinated aromatic polythiourea（BaTiO₃@Ar PFTU）nanoparticles,were served as novel nanofillers to prepare P（VDF-Tr FE-CTFE）/BaTiO₃@Ar PFTU nanocomposites for capacitor applications.The identification of interfacial morphology,interfacial polarization,and interfacial simulations,the dielectric and ferroelectric properties of the nanocomposites,and their relationship was well studied in this part.SEM images showed that the BaTiO₃@Ar PFTU NPs with good adhesion of Ar PFTU shell showed excellent compatibility with P（VDF-Tr FE-CTFE）matrix,and thus were well dispersed in the matrix.FTIR and XRD patterns showed that that the addition of BaTiO₃@Ar PFTU NPs cannot change the chain conformations of terpolymer P（VDF-Tr FE-CTFE）.Interestingly,there was a 30 nm thick interface between BaTiO₃NPs and the matrix.In PFM imaging,the amplitude value of the interface had higher than that of the matrix,which could be ascribed to nanoscale interfacial polarization enhancement.To further certify core@insulation-shell modulated electric field confinement,the electric distributions and current density distributions of nanocomposites with BaTiO₃ NPs and BaTiO₃@Ar PFTU NPs were well displaced at the same electric field by finite element simulation.Upon the existence of ultra-insulation Ar PFTU shell,local electric field distribution around BaTiO₃@Ar PFTU NPs decreased notably and could be confined in the interface layer.And the current density of BaTiO₃@Ar PFTU NPs decreased notably and was near to zero,which was consistence of high impedance.The insulating Ar PFTU shell on BaTiO₃ NPs was not only beneficial for improving dispersion quality,but also enabled to form an obvious interface,which might benefit the improvement in the dielectric permittivity and breakdown strength.At the same content of BaTiO₃@Ar PFTU NPs,the dielectric constant of P（VDF-Tr FE-CTFE）/BaTiO₃@Ar PFTU nanocomposite decreased with the increase of the thickness of the shell Ar PFTU.For instance,when the shell thickness was from 5 nm to 15 nm the corresponding dielectric constant decreased from 52 to 48,and the dielectric constant of BaTiO₃ was 46.The breakdown strength of P（VDF-Tr FE-CTFE）/6 wt%BaTiO₃@Ar PFTU reached 430 k V/mm.The breakdown strength of pure matrix was only 240 k V/mm.The polarization value of pure P（VDF-Tr FE-CTFE）was only 10.0μC/cm²,while the polarization value of P（VDF-Tr FE-CTFE）based nanocomposites was up to 14.2μC/cm²,which was 1.65 times that of pure matrix.High energy storage density can be achieved at low concentration filling.Under the applied electric field of380 k V/mm,the maximum discharge energy density of P（VDF-Tr FE-CTFE）/1 wt%BaTiO₃@Ar PFTU was 23.1 J/cm³ and the discharging efficiency was 65%.