The Research Of Dielectric And Energy Storage Properties Of Bismuth Ferrite Based Lead-free Ceramics

With the continuous growth of society and the continuous advancement of urban informatization,intelligence and greening,the inconsistency between energy scarcity and industrial innovation has become apparent.Industrial manufacturing and modern computer equipment have continually increased storage and conversion requirements.Stable and continuous construction of energy storage or conversion equipment is a required condition to support industrial upgrades.Among all kinds of electrical energy storage devices,the fast charge and discharge capabilities of ceramic capacitors make them play an important role in various filter integrated circuits,pulsed power supplies,electromagnetic weapons and other fields.However,the energy storage density of ceramics is limited,which is main obstacle for widely used,and how to break through this limitation has become the key to the further development of ceramic capacitors.Bismuth ferrite ceramics（BF）,which is lead-free and a multiferroic material,has shown great energy storage potential due to its excellent polarization ability,but BF ceramics do not have the ability to be directly used as capacitors due to its large remnant polarization and low breakdown strength.The transformation of ferroelectric into relaxor ferroelectric ceramics by forming a solid solution is an important method for preparation of both high energy storage density and high energy storage efficiency ceramics.In this study,this scheme is used to modify BF ceramics,combined with viscous polymer process（VPP）technology and finite element analysis to prepare or analyze the internal relationship between the structure and performance,and finally obtain ceramics with high energy storage behavior.The specific content includes:The Bismuth ferrite based ferroelectric solid solution formed by addition of barium titanate（BT）can effectively reduce the coercive field and leakage current of BF ceramics,and it will make the ceramics have a certain degree of relaxation behavior.The ceramics still exhibit the characteristics of typical ferroelectrics after forming solid solution.In the ceramic with a BT content of 35%（0.35BT）,the maximum polarization of 46.2μC/cm²is displayed.On the basis of the 0.35BT ferroelectric ceramics,the relaxor ferroelectric ceramics are obtained through forming solid solution with addition of composite perovskite structure.The composite perovskite selected in the experiment consists of Ba(Zn_1/3Nb_2/3)O₃（BZN）,Na_0.5Nd_0.5Ti O₃（NNT）and Sr_0.7Bi_0.2Ti O₃（SBT）.In the experiment,a series component of BZN,NNT and SBT were prepared and their structure,dielectric and energy storage behavior were characterized.Finally,15%-SBT ceramic had an energy storage density of 2.0 J/cm³with efficiency of 75%under an electric field of180 k V/cm.The 15SBT ceramic was further explored through the viscous polymer process（VPP）,and finally its dielectric breakdown strength（DBS）was increased to 330k V/cm,which effectively increased the recoverable energy storage density of the ceramic（4.95 J/cm³）.Research on charge and discharge shows that the discharge time t_0.9 is 134ns under 300 k V/cm for 15SBT ceramic.In addition,the result of finite element analysis shows that the ceramic with grain refinement brings the homogenization of the local electric field and electric potential distribution,which may be an internal factor for the improvement of the dielectric breakdown strength.Besides,the evenly distributed second phase may also increase the dielectric breakdown strength of the ceramic.On the basis of 15SBT relaxor ferroelectric ceramics,the rare earth elements La and Nd are further used for doping modification,with the purpose of compensating the A-site defects and partial oxygen vacancies of the perovskite structure.Studies have shown that both La and Nd enter the A site of the perovskite as a solid solution and have a defect compensation effect.The dielectric breakdown strength of the ceramic is increased to190～220 k V/cm with further enhancement of relaxation.In particular,an in-situ core-shell structure is observed in La-doped ceramics,the impedance of the ceramics is greatly increased,the concentration of oxygen vacancies is reduced,and the dielectric breakdown strength of the ceramics is increased.0.01La exhibits an energy storage density of 2.35J/cm³ under 220 k V/cm,and 0.005Nd exhibits an energy density of 2.43 J/cm³ under 190k V/cm.The 0.01La ceramic prepared by viscous polymer process has increased the withstand voltage to 480 k V/cm,the energy density of 6.15 J/cm³,and the energy storage efficiency increased to 81%;and it has good temperature stability under 350 k V/cm:W_recmaintains 4.05 J/cm³ under the temperature condition of 30～130℃,and the change rate is only 1.77%.The 0.005Nd ceramic prepared by viscous polymer process has a dielectric breakdown strength of 380 k V/cm,an energy storage density of 5.9 J/cm³,and an energy storage efficiency of 70%.Through this research,the energy storage performance of bismuth ferrite-based relaxor ferroelectric ceramics has been improved and the efficacy of the viscous polymer process has been confirmed.This research increases the application value of Bismuth ferrite-based ceramics as capacitors.The proposed ferroelectric-relaxor ferroelectric modification method can provide reference materials for research on other ferroelectric materials,which are useful for future research on energy storage ceramics and have a valuable reference.