The Research On Composition Design And Dielectric Energy Storage Performance Of Na_0.5Bi_0.5TiO₃ Based Lead-free Ferroelectric Ceramics

Dielectric capacitors are the core components suitable for high power pulse systems,which can adapt to the development requirements of device integration and miniaturization.At present,the dielectric capacitors in reality are mainly lead-containing ceramics,which are easy to cause environmental pollution and threaten human health.Therefore,the development of environmental friendly lead-free high energy storage density dielectric ceramics has become a research hotspot.Sodium bismuth titanate has good ferroelectric properties and is expected to replace lead-containing ceramics.In this paper,sodium bismuth titanate（NBT）is the main component,and different amounts of barium titanate（Ba Ti O₃,BT）and bismuth ferrite（Bi Fe O₃,BFO）are doped to increase the saturation polarization and destroy the long-range order of ferroelectric domains.Then select the best components for further optimization.By introducing different compounds to improve its energy storage performance,lead-free dielectric ceramics suitable for applications in the pulse field are prepared.（1-2x）NBT-x BT-x BFO ceramics are prepared by traditional solid-state method.Ba²⁺entered A-site and Fe³⁺entered B-site,forming the phenomenon that Na⁺,Bi³⁺,Ba²⁺competed for A site lattice point and Ti⁴⁺,Fe³⁺competed for B-site lattice point,which led to the destruction of ferroelectric domain and the formation of polar nano-region（PNRs）.Temperature dependent dielectric curves show that the obvious relaxor peak gradually appears at about 160℃,and the coexistence of a tripartite phase and a tetragonal phase appears at 330℃.All these provide more polarization direction for the polar nano-region.The addition of BT and BFO can refine the grain size and help increase the breakdown field strength.When x=0.01 mol,the optimal energy storage density of0.3 J/cm³ is obtained;the variable temperature hysteresis loop and current curve analysis show that this component forms two-phase coexistence at 130℃,and the hysteresis loop also appears"beam waist".This phenomenon is conducive to energy storage.In this paper,x=0.01 mol（hereinafter referred to as NBB）is selected as the further optimized matrix.On this basis,A-site single ion compounds（Ca Ti O₃,CT;Sr Ti O₃,ST）,A/B site double ion compounds(Bi(Mg_2/3Nb_1/3)O₃,BMN;(K_1/2Na_1/2)Nb O₃,KNN),and A-site defect compounds((Sr_0.7Bi_0.2)Ti O₃,SBT;(Sr_0.7La_0.2)Ti O₃,SLT)are doped to improve the energy storage performance.Both CT and ST have obvious peak shifting effects on NBB,and CT also has peak pressing effects;NBB-CT ceramics can achieve 3.14 J/cm³ energy storage density and75%energy storage efficiency under 250 k V/cm electric field,while showing good temperature and frequency stability;NBB-ST ceramics can only achieve 1.81 J/cm³energy storage density under 190 k V/cm electric field.There is a second phase in NBB-BMN ceramics,which can effectively fill the gaps between grains and increase the density of the sample.Under the electric field of 250k V/cm,the energy storage density reaches 2.39 J/cm³,and the energy storage efficiency is 75%.NBB-KNN ceramics have small grain size,but show obvious"square"shape.Therefore,it is easy to form large defects in the sample,resulting in low density and high dielectric loss.When the breakdown field reaches 220 k V/cm,the energy storage density is only 1.98 J/cm³.SBT and SLT have obvious peak shifting effects,and can also significantly enhance the relaxation,while reducing dielectric loss and improving energy storage performance.However,because Bi³⁺in SBT can improve the saturation polarization,the energy storage density of NBB-SBT ceramics is 3.2 J/cm³ and the energy storage efficiency is 82%under240 k V/cm electric field;the energy storage density of NBB-SLT ceramics only reaches2.13 J/cm³.In addition,NBB-SBT ceramics have excellent temperature stability and frequency stability.In a word,the thesis modifies the matrix through a variety of component methods.It is found that NBB-CT ceramics and NBB-SBT ceramics have excellent energy storage density,as well as good temperature and frequency stability.Therefore,they are expected to develop into high energy storage density ceramic media.The doping methods involved in this paper can also provide new ideas for the development of new lead-free energy storage media,while promoting the development of high power pulse capacitors.