Study On Preparation And Energy Storage Performance Of Sodium Bismuth Titanate Based Relaxor Ferroelectric Ceramic

With the gradual replacement of traditional fossil energy by clean energy,the development of new materials with more energy storage advantages is the key to improving the performance of energy storage devices.Energy density,energy efficiency,and power density are important parameters to measure the advantages and disadvantages of energy storage materials and are the key directions for developing excellent energy storage devices.Bismuth sodium titanate(Bi_0.5Na_0.5TiO₃,BNT)is a lead-free dielectric material with a perovskite structure with high saturation polarization,high Curie temperature,and high dielectric constant,which is very competitive in the field of energy storage applications in the future.However,BNT ceramics also have some disadvantages,such as a high coercive field,large polarization hysteresis,high remanent polarization,and many vacancy defects during sintering,which limit its potential application in the energy storage field.In this thesis,BNT-based ceramics were controlled using element doping,multiphase,and sintering process optimization,and the structure-property relationship between the crystal structure,domain structure,vacancy defects,and their energy storage properties was further explored.Finally,the ferroelectric and dielectric properties of BNT-based ceramics were effectively improved,and excellent energy storage properties were obtained.Firstly,（1-x）Bi_0.5Na_0.5TiO₃-xBi(Mg_0.3Zr_0.6)O₃ ceramics were prepared by the solid-state method.The T_m dielectric peak of BNT ceramics was increased from about320℃to about 400℃by the addition of Bi(Mg_0.3Zr_0.6)O₃（BMZ）.Meanwhile,the introduction of the BMZ also promoted the weak polarization phase-ferroelectric phase transition under the effect of electric field,non-ergodic state to ergodic relaxation state,and strengthened ceramic linear polarization and temperature stability.Finally,the（1-x）Bi_0.5Na_0.5TiO₃-xBi(Mg_0.3Zr_0.6)O₃ ceramic exhibits a clear waist-shaped hysteresis loop at 200℃.Under the electric field strength of 160 k V/cm,the discharge energy density of the components with x=0.1 and 0.08 exceeds 2.4 J/cm³.After the electric field strength of 80 k V/cm,the energy efficiency of the ceramic is always stable at 80%with the increase of the electric field strength,showing excellent energy storage characteristics.Secondly,BaTiO₃（BT）and NaNb O₃（NN）were introduced into0.92Bi_0.5Na_0.5TiO₃-0.08Bi(Mg_0.3Zr_0.6)O₃（BNTBMZ）.It was found that BT and NN have different effects on the ferroelectric and dielectric properties of BNTBMZ.In the0.85BNTBMZ-0.15BT ceramics,the dielectric constant near T_s temperature was significantly increased,and the frequency dispersion is stronger than that of the T_mdielectric peak,which promotes the transition from the non-ergodic state to the ergodic relaxation state of the ceramics near the room temperature,enhances the linear polarization of the ceramics near the room temperature,and thus exhibits a thin waist-shaped hysteresis loop at the room temperature.The dielectric loss and T_m dielectric peak intensity of BNTBMZ ceramics at room temperature were significantly suppressed by the introduction of NN,and the dielectric peak was flattened and the dielectric temperature stability was improved.Then,to coordinate the three key performance parameters of breakdown electric field strength,polarization strength,and energy efficiency,the BNTBMZ ceramic was co-optimized by BT and NN to prepare（1-x）0.92BNT-0.08BMZ-x[0.6BaTiO₃-0.4NaNb O₃]ceramics.The co-introduction of BT and NN significantly improves the energy storage performance of the ceramics at room temperature,wherein0.65BNTBMZ-0.35BTNN exhibits an ultra-high energy efficiency of nearly 96%and a high energy density of more than 4.11 J/cm³ under an electric field strength of 280k V/cm,which superimposes and couples the optimization effects of BT and NN on the phase structure,ferroelectricity and dielectric properties of BNTBMZ ceramics.At last,to solve the problem of positively charged oxygen vacancy defect caused by Bi and Navolatilization during sintering of BNT-based ceramics,Mn O₂ was introduced into[0.95Bi_0.5Na_0.5TiO₃-0.05Sr Zr O₃]-0.2NaNb O₃（BNTSZNN）ceramics.The Ti⁴⁺ions were prevented from being reduced by utilizing that association defect formed by the variable-valence Mn⁴⁺ions and oxygen vacancy.In addition,the heat preservation time at high temperatures was shortened by a Ramp-to-Spike sintering process,so that volatilization of Bi and Naelements was reduced,and the formation of oxygen vacancies was inhibited;and through optimization of the sintering process,the grain size of the ceramic was significantly reduced,and was reduced from 6μm of traditional sintering to 2μm.By optimizing the sintering process and controlling the defects,the dielectric constant of the ceramic was significantly improved,the dielectric loss at high temperature was significantly reduced,and the breakdown electric field intensity was increased to 387 k V/cm.Finally,an ultra-high discharge energy density of 7.05 J/cm³ was obtained.There are 43 figures,5 tables,and 109 references in this thesis.