Modulation And Design Mechanism Of Energy Storage In Ferroelectric Films

The dielectric capacitors,as energy storage devices,are full of attractive in the energy storage field based on their advantages of extremely fast charge and discharge rate and good stability.However,with the rapid development of dielectric capacitors in the current electronic field,some existing shortcomings of the dielectric capacitors become obvious such as:(1)low energy storage density and energy storage efficiency,(2)the temperature stability,frequency stability and fatigue stability of the material need to be optimized,and(3)the bending performance of flexible capacitors need to be improved.With the electronic products gradually trend to functionalization and integration,the shortcomings of traditional dielectric capacitors are becoming more and more obvious.In order to further improve the performance advantages and avoid the performance deficiencies of dielectric capacitors,the film capacitor devices should be produced with the support of existing film technology,which makes the research on the film energy storage performance become a hotspot.The relaxor ferroelectrics have been the focus of attention due to its internal polar nanometer micro-region,which can maintain a low residual polarization intensity and a high dielectric breakdown intensity in the process of charge and discharge.In this thesis,based on the research and preparation of relaxor ferroelectrics,the scheme and strategy of enhancing the energy storage performance of dielectric are proposed.Monolayer perovskite ferroelectric materials of barium titanate(Ba Ti O₃),sodium bismuth titanate(Bi_0.5Na_0.5Ti O₃)and potassium sodium niobate(K_0.5Na_0.5Nb O₃)with excellent piezoelectric properties were synthesized by the chemical sol-gel deposition method.By modifying the internal lattice,constructing the solid solution,and forming the interfaces and the core-shell structures,the material was designed to illustrate the mechanism of improving the dielectric energy storage.In view of the above,the specific research contents of this thesis include the following aspects:1.The lead-free Bi Mn O₃doped Ba Ti O₃solid solution films were successfully fabricated via chemical solution deposition.In the 0.92Ba Ti O₃-0.08Bi Mn O₃solid solution films,the segregation particles were prepared and the"Jujube-cake"structure was formed.This structure improves the electrical insulation,breakdown strength and maximum polarization of the film by providing a dielectric differential interface that blocks the passage of free electrons.The 0.92Ba Ti O₃-0.08Bi Mn O₃solid solution film achieved a high energy storage density of 94.1 J/cm³and energy storage efficiency of 84.51%,with excellent frequency stability and thermal stability.2.A PN-like junction was designed by combination between Ag₂O nanoparticles and lead-free 0.92K_0.5Na_0.5Nb O₃-0.08Bi Mn O₃solid solution films.And the relevant research for PN-like junction effect on energy storage performance was discussed.Compared the film with that without PN-like junction structure,the energy density increased from 20.1 J/cm³to 65.1 J/cm³and the energy storage efficiency increased from 50.7%to 62.6%after the introduction of Ag₂O nanoparticles.The improved energy storage performance can be attributed to the formation of a depletion layer with high resistance at the interface between the Ag₂O nanoparticles and the matrix.The rectifying effect induced by the depletion layer improves the insulation.Besides,the local random field generated by the high insulation interface strengthens the relaxor characteristics of materials,intensifing the energy storage efficiency.This design method of the PN-like junction structure presents an alternative method to enhance the dielectric energy storage performance.3.The(1-x)Na_0.5Bi_0.5Ti O₃-x Bi Mn O₃(NBT-BMO)finite solid solution films were prepared to investigate the energy storage performance via lattice modification.The introduction of BMO solute modified the original lattice for this material,which has undergone a transition from pure phase to solid solution,then solubility finite,and finally to precipitation.Correspondingly,the polarization state evolved from macroscopic ferroelectric to relaxor ferroelectric and finally to compound ferroelectric.The modified NBT matrix produced local lattice fluctuation and large lattice stretching,improving the energy storage performance.As the BMO proportion in the BNT-BMO finite solid solution films increased from 0 to 2%and then to 4%,the energy storage efficiency increased from 39.2%to 51.7%and 53.2%,and the energy density increased from 33.1 J/cm³to 76.5 J/cm³and 83.8 J/cm³,respectively.4.The high quality flexible 0.94Na_0.5Bi_0.5Ti O₃-0.06Eu Ti O₃solid solution thin film was prepared by magnetron sputtering,chemical solution deposition and mechanical peeling.A high energy storage density of 65.4 J/cm³and energy storage efficiency of 52%were obtained due to its good insulation and relaxation behavior.In addition,the 0.94Na_0.5Bi_0.5Ti O₃-0.06Eu Ti O₃flexible thin film possesses excellent temperature stability and frequency stability,especially in the bending endurance stability,where the bending cycle can reach 10⁵times.And the energy storage performance barely changed after placement for one month.This result indicates that the 0.94Na_0.5Bi_0.5Ti O₃-0.06Eu Ti O₃flexible thin film capacitors have great applicatable potential in wearable electronic devices.5.The theory study shows the electric displacement(D),which includes three parts contributions of electronic conductivity(D1),dielectric displacement(D2)and domain switching(P),can be distinguished by current-electric field(I-E)curve.In this thesis,the appropriate model was established to calculate more accurately the three contribution values.Compared with the previous model,the present model is closer to the actual contribution.Moreover,the accuracy of the proposed model was verified by fabricating the traditional Ba Ti O₃ferroelectric film.The present model not only provides theoretical guidance for distinguishing electric displacement contributions of each part,but also provides a method to study the leakage proportion in ferroelectrics.