Preparation And Properties Of Potassium Sodium Niobate/PVDF Composite Films With Different Structures

Among many polymer materials,PVDF-based composites have received widespread attention in high-energy-density capacitors due to their high breakdown strength and ease of processing.K0.5Na0.5NbO3(KNN)ceramic is a lead-free ferroelectric material with excellent ferroelectric performance,piezoelectric performance,and environmental friendliness.In this study,KNN ceramics with different structures and morphologies were synthesized by different methods.A series of single-layer and sandwich-structured PVDF-based composite films were prepared through the casting process.The energy storage characteristics of KNN ceramics with different structures and PVDF-based composites were systematically studied.By coating the surface of zero-dimensional KNN particles with dopamine(PDA),the dispersibility of the filler in PVDF was optimized,which was beneficial to improve the energy storage performance of the polymer matrix.The study found that the zero-dimensional KNN@PDA particles significantly increased the dielectric constant of the PVDF matrix.When the zero-dimensional KNN@PDA particle loading was 9vol%,the sample achieved a high dielectric constant of 22.9,which was nearly three times higher than that of pure PVDF.In addition,with the increasing of the content for zero-dimensional KNN@PDA particles,the polarization of the composite film increased significantly.The sample that composite film with 6vol%of zero-dimensional KNN@PDA particles,achieved an energy storage density of 6.5 J/cm3.One-dimensional KNN nanofibers(KNN-nfs)with large aspect ratio were synthesized by the molten salt method.A single-layer one-dimensional KNN-nfs/PVDF composite film was prepared by the solution blending method and the casting process.The study found that the introduction of the high-dielectric constant one-dimensional KNN-nfs into the PVDF matrix caused the Maxwell-Wagner-Sillars interface effect in the sample,which significantly increased the dielectric constant of the sample.By analyzing the energy storage performance of the sample,it was found that the one-dimensional KNN-nfs effectively improved the breakdown performance of the sample.When the one-dimensional KNN-nfs content in the composite film was 3vol%,the breakdown strength reached 350 MV/m.And the sample achieved an excellent discharge energy density of 7.95 J/cm3 and a high discharge efficiency of 72.8%.Two-dimensional KNN sheets with a large aspect ratio were synthesized by a two-step molten salt method.A single-layer two-dimensional KNN/PVDF composite film was prepared by the solution blending method and the casting process.The study found that the two-dimensional KNN sheet effectively blocked the charge migration in the composite film,thereby improving the breakdown strength of the sample.When the loading of the two-dimensional KNN sheet in the composite film was 3vol%,the breakdown strength of the sample reached 350 MV/m.At the same time,with the increasing of the amount of two-dimensional KNN sheet,the maximum polarization of the sample increased significantly.Thus,a high discharge energy density of 7.44 J/cm3 and a discharge efficiency of 72.1%were achieved in a 3vol%sample.KNN-nfs/PVDF composite film with sandwich structure was prepared by a simple layer-by-layer casting process.It was found through the electric field distribution simulation and breakdown simulation that the electric tree always grew along the high electric field region inside the sample in composite film.The one-dimensional KNN-nfs effectively prolonged the electric tree growth path,and formed a weak field in the middle layer,which effectively consumed the energy of electric tree growth,thereby improving the breakdown strength of the sample.In addition,the introduction of one-dimensional KNN-nfs with high dielectric constant significantly increased the maximum polarization of the composite film.In the 0-3-0 composite film,an excellent discharge energy density of 14.2 J/cm3 and a discharge efficiency of 78.5%were achieved.Through the leakage current simulation,it was found that the sandwich structure effectively improved the leakage current density in the composite film and effectively reduced the conductivity of the composite film.The influence of the introduction of two-dimensional KNN sheet and sandwich structure on the energy storage performance of composite films in PVDF matrix was studied.Through the calculation of the electric field distribution theory and the simulation of the electric field distribution,it was found that the both two-dimensional KNN sheet and the sandwich structure reduced the electric field strength of the middle layer.Furthermore,the breakdown simulation exhibited that the two-dimensional KNN sheet increased tortuosity of the electrical tree growth,and the weak field in the middle layer effectively consumed the energy of electrical tree growth,thereby improving the breakdown strength of the sample.The analysis of energy storage performance showed that the introduction of the two-dimensional KNN sheet significantly increased the maximum polarization of the composite film without significantly changing the residual polarization.In the 0-3-0 sample,an excellent discharge energy density of 14.5 J/cm3 and an ultra-high charge and discharge efficiency of 80.2%were achieved.The leakage current simulation proved that the reasonable two-dimensional KNN sheet content and sandwich structure effectively improved the leakage current density in the composite film.