Investigation Of Energy Storage Mechanism And Performance In Sodium Bismuth Titanate-based Relaxor Ferroelectric Ceramic Materials

Dielectric capacitors,one of the core electronic components,play a vital role in modern electronic systems and pulse power technology.To meet the demand of integration,miniaturization,and diversification,higher requirements including high energy storage density and stability are proposed for dielectric capacitors.In this dissertation,the performance of sodium bismuth titanate ceramic was optimized based on its strong polarization ability.The relaxation induction,reduction of oxygen vacancy content,and variation of permittivity peak temperature were explored to overcome the shortcomings of high remanent polarization,low field strength,and sharp changes in permittivity near room temperature.The mechanism of the influence of its microstructure on the properties was systematically investigated.The contents of this dissertation are listed as follows:1.By adding Ba_0.3Sr_0.7TiO₃（BST）into Bi_0.5Na_0.5TiO₃（BNT）,the polarization response of BNT ceramic transformed from ferroelectric to relaxor ferroelectric characteristics with a high polarizability.And then,the Na_0.4Bi_0.4Ba_0.06Sr_0.14TiO₃ ceramics were modified with antiferroelectric Na Nb O₃（NN）.The lower polarizability of the ceramic is ascribed to the phase transition of rhombohedral（R3c）to tetragonal（P4bm）and the random occupation of cations with different radii and valences,which enhanced relaxor behavior and delayed the state of polarization saturation to eventually enhanced the energy storage capacity.2.To compensate for oxygen vacancies in BNT-based ceramics,a strategy of donor substitution was designed.The leakage current in the ceramics was reduced and the microstructure of the ceramics was modified to enhance its breakdown strength.Firstly,the Ti⁴⁺in 0.8Na_0.4Bi_0.4Ba_0.06Sr_0.14TiO₃-0.2Na Nb O₃ ceramic was substituted with Ta⁵⁺ions.The Electron paramagnetic resonance results showed that a small amount of Ta⁵⁺ions substituted for Ti⁴⁺ions could significantly reduce the oxygen vacancy content in ceramics.The lowest leakage current density and the smallest grain size were observed when the substitution of Ta was 0.01 mol.Therefore,the breakdown strength of the ceramics was effectively enhanced because of the smaller grain size and lower leakage current density（from 180 k V/cm to 270 k V/cm）.Similarly,the donor substitution of Nb⁵⁺for Ti⁴⁺in Na_0.3Bi_0.3Ba_0.04Sr_0.36TiO₃ ceramic also reduced the content of oxygen vacancy in the ceramics and enhanced its breakdown strength.Meanwhile,the disorderly distribution of dipoles disturbed the orderly arrangement of dipoles in the ceramics with A-site ions substituted by Ca²⁺.This phenomenon reduced the size of ferroelectric domains,improved the relaxation behavior of the ceramics,and subsequently increased energy storage efficiency.Simultaneously increased the breakdown field strength and enhanced the relaxation behavior of ceramics to improve the energy storage properties of ceramics.The results indicated that reducing the oxygen vacancy content with donor substitution is valid for enhancing the breakdown strength of BNT-based ceramics.3.ZnTaO₆（ZT）was added into Na_0.3Bi_0.3Ba_0.04Sr_0.36TiO₃ ceramic to form the disorder local composition and build random fields to improve the relaxation behavior of the ceramics.Furthermore,the grain size of ceramic was reduced and the breakdown strength was modified.Meanwhile,the temperature stability region of the permittivity shifted from high temperature to room temperature with ZT additive,and the change of the permittivity relative to 25°C satisfied the R characteristic(Δε/ε_25°C≤±15%)in the temperature range of-59°C to 139°C.The decrease of the bond energy of the B-O bond is mainly responsible for the decreased energy of the phase transformation,leading to the shift in the position of the permittivity peak towards lower temperatures shown.Likewise,in the sample composed of BNT ceramic with Ca TiO₃（BNT-x CT）,there was a simultaneous decrease in the bond energy of the B-O bond and a shift in the position of the permittivity peak toward lower temperatures.Besides,the doping of CT promoted the transformation of the polarization response from the ferroelectric characteristics of BNT ceramics to relaxor ferroelectrics.At x=0.25,excellent dielectric temperature stability(-100°C～136°C,Δε/ε_25°C≤±15%)was also obtained with a high recoverable energy density of 2.74 J/cm³.Bi_0.425Na_0.425Ca_0.15TiO₃（BNCT）which met the EIA X7R standard(-55°C～125°C,Δε/ε_25°C≤±15%)was used as the matrix,and K_1.94Zn_1.06Ta_5.19O₁₅（KZT）was chosen for further doping modification.The breakdown strength and energy storage density of the designed ceramics were significantly enhanced.When the KZT content was0.01,the ceramic showed excellent temperature stability,a high recoverable energy density of 4.34 J/cm³,a high power density of 79.41 MW/cm³ and excellent fatigue resistance with only about 5%degradation of energy storage density after 100,000charge/discharge cycles,which guaranteed a large potential for application in the field of dielectric energy storage.It is proposed that the temperature stability region of the permittivity of BNT-based ceramics would move towards low temperature after decreasing the bond energy of the B-O bond in this dissertation to overcome the shortcomings of sharp changes in permittivity near room temperature.The performance of BNT-based energy storage ceramic in this dissertation met the EIA X7R standard,showing the application potential.