Preparation And Properties Of Fluoropolymer/BaTiO₃ Dielectric Composites

Dielectric composites with high breakdown strength（E_b）and high dielectric constant have received great attention due to their wide range of applications especially in energy storage.In this paper,barium titanate（BT）and polyvinylidene fluoride（PVDF）based polymer are used as ceramic filler and matrix respectively.The chemical structure of shells,shape of fillers,type of interaction between fillers and polymer matrix and the effect of cross-linking of polymer matrix on the dielectric properties and E_b of nanocomposite are systematically investigated.（1）The effect of polymer shell on the dielectric properties and E_b of dielectric composites is investigated.Thiol-terminated polyvinylidene fluoride（PVDF-SH）and polystyrene（PS-SH）were used as shell materials by“grafting to”method to modify BT nanoparticles.Then the core-shell structured hybrid nanoparticles are used as ceramic fillers to prepare nanocomposite.The results showed that the compatibility of the core-shell structured hybrid nanoparticles with polymer matrix is better than that of unmodified one,and BT@PVDF fillers is better than BT@PS.The E_b of BT@PVDF/PVDF and BT@PS/PVDF is increased by 101%and 88.2%respectively compared with BT/PVDF at 30 vol%loading.The dielectric loss is also effectively suppressed.（2）Insulating silica is used as shell structure to prepare core-shell structured hybrid particles（BT@Si O₂）via a facile method.Two kinds of nanocomposites are prepared using BT and BT@Si O₂ as fillers with poly（vinylidene fluoride-chlorotrifluoroethylene）（P（VDF-CTFE））,respectively.The Si O₂ shell has been proved to show an obvious positive effect on the dielectric properties and E_b.At 10² Hz,the dielectric loss of BT@Si O₂/P（VDF-CTFE）and BT/P（VDF-CTFE）at 20 vol%fillers loading are 0.0427 and 0.0594,respectively.It is indicated that the presence of the silica shell layer can suppress the dielectric loss.In addition,BT@Si O₂/P（VDF-CTFE）has an E_b of 189.2 MV/m at 20 vol%fillers loading,while BT/P（VDF-CTFE）has an E_b of only 127.2 MV/m.The significant increase in the E_b is beneficial to increase the energy density.（3）A ternary nanocomposite with high dielectric constant and E_b is prepared by adding two kinds of fillers.Although the BT-NH₂ fillers can significantly increase the dielectric constant of the nanocomposites,the E_b decreased sharply.Boron nitride sheet（BNNS）with high E_b is further added to the high dielectric constant nanocomposites to achieve high E_b.The E_b of P（VDF-CTFE）matrix is 279.1 MV/m.The E_b of BNNS/P（VDF-CTFE）with 3 wt%BNNS increased to 325.9 MV/m.The SEM characterization results showed that the two-dimensional sheet structured BNNS can promote the uniform dispersion of the two kind of fillers and achieve high E_b.The energy density of the nanocomposite with 5 wt%BT-NH₂ and3 wt%BNNS reached to 5.55 J/cm³,which is 49%higher than that of P（VDF-CTFE）.（4）BT-c-P（VDF-CTFE-DB）nanocomposites with chemical bonds between fillers and polymer matrix at the interface is prepared,and it is compared with BT/P（VDF-CTFE-DB）and BT-SH/P（VDF-CTFE-DB）nanocomposites which only have physical interaction at the interface.It is found that the presence of chemical bonds between the two components can significantly improve the dispersion of the filler in matrix,resulting superb mechanical properties.When the fillers loading is 20 vol%,the elongation of BT-c-P（VDF-CTFE-DB）can reach up to 510%,while the elongation of BT/P（VDF-CTFE-DB）and BT-SH/P（VDF-CTFE-DB）is only 150%and 420%.It provides an opportunity to prepare stretchable nanocomposites.Owing to the significant increase in the E_b of BT-c-P（VDF-CTFE-DB）,a discharged energy density of 6.20 J/cm³ is successfully achieved.（5）In order to achieve high energy storage density at low fillers loading,barium titanate nanofibers（BT NFs）are used instead of barium titanate nanoparticles（BT NPs）.The effects of modification of the fillers and the cross-linking of polymer matrix on the properties of the nanocomposites are investigated.The results proved that the cross-linked nanocomposite film shows the highest Young’s modulus and interfacial interaction.After the stretching of the nanocomposites,the orientation of BT-SH NFs is more clearly perpendicular to the electric field,which further increases the path of electric trees and reduces the deformation under mechanical pressure.The increased electric tree path and anti-deformation ability both help to improve the E_b.Compared with the unstretched nanocomposite,the E_b of s-BT-SH NFs/c-P（VDF-CTFE-DB）with 5 vol%BT-SH NFs increased by 27%,reaching 447.8 MV/m.Finally,a high discharge energy density of 10.2 J/cm³ was achieved.