Preparation And Properties Of Bismuth Sodium Titanate-based Energy Storage Ceramics

With the rapid development of electronic information technology and the continuous consumption of non-renewable energy,the problem of energy crisis and environmental pollution is becoming more and more serious.Finding and developing high-performance energy storage devices consistent with sustainable development requirements has become one of the current research hotspots.Ceramic dielectric capacitors have received increasing attention in pulsed power applications due to their high power density and fast charge-discharge rate.However,these materials are limited by relatively low energy storage density compared with energy storage devices such as batteries and electrochemical capacitors.Solving the above problems is crucial for the application of ceramic dielectric capacitors.In this thesis,Bi_0.5Na_0.5Ti O₃（BNT）ferroelectric is selected as the matrix material,through controlling the phase structure and microstructure of the material by ion doping,multi-component compounding and changing the sintering process,the breakdown strength and storage performance of the material are expected to be improved.The main research contents are as follows:（1）（0.93-x）Bi_0.5Na_0.5Ti O₃-0.07Ba Ti O₃-x Na Nb O₃（abbreviated as BNT-0.07BT-x NN）energy storage ceramics were prepared by solid-phase method.The results showed that a single perovskite structure formed in all samples.The introduction of NN destroyed the ferroelectricity of the BNT-0.07BT material,which induces the formation of polar nano-regions（PNRs）and promotes improvement in the energy storage poformance.At x=0.20,sample exhibted a thin P-E hysteresis loop at an electric field of 160 k V/cm,where an energy storage density(W_rec)of 2.08 J/cm³ and efficiency（η）of 81%were obtained.In addition,under such a electric-field,sample also had a high discharge energy density（W_d=1.44 J/cm³）and a short discharge time(t_0.9=153 ns).（2）（0.93-x）(Bi_0.5Na_0.5Ti O₃)-0.07Ba Ti O₃-x(Sr_0.7Bi_0.2)Ti O₃（abbreviated as BNT-0.07BT-x SBT）energy storage ceramics were prepared by solid-phase method.The results showed that the introduction of SBT does not significantly change the phase structure of BNT-0.07BT ceramics,and all samples have a single pseudocubic perovskite structure.The introduction of SBT improved the relaxor behavior of BNT-0.07BT ceramics and destroyed the long-range ferroelectric order,showing a broad dielectric peak and slender P-E hysteresis loops.As a result,the energy storage performance of the material was greatly improved.When the SBT content is 0.40,the sample achieved a large energy storage density(W_rec)of 3.26 J/cm³ and a high efficiency（η）of 90.3%at an electric field of 260 k V/cm.In addition,the charge-discharge performance of the sample showed that the BNT-0.4SBT ceramic had a current density of 509 A/cm² and a power density of 40.8MW/cm³ at an electric field of 260 k V/cm.Meanwhile,sample also had a faster discharge time(t_0.9=143 ns).（3）[(Bi_0.5Na_0.5)_0.94Ba_0.06]_（1-1.5x）La_xTi O₃（abbreviated as BNBT-x La）ceramics were prepared by solid-phase method.The influence of the La³⁺ion doping on the phase structure and energy storage properties of the BNBT ceramics was studied in detail.Through controlling the phase structure and domain structure of the BNBT by La³⁺ion doping,the nomal ferroelectric state with long-term order transformed into relaxor ferroelectrics with long-range disorder,and thereby improved the energy storage performance of the materials.The results showed that La³⁺ion doping improved the crystal structure symmetry of the BNBT matrix material,destroyed the ferroelectric order with the formation of PNRs and greatly improved the breakdown electric field resistance of the material.At x=0.12,large energy storage density of 5.93 J/cm³ and efficiency of 77.6%were obtained at a high breakdown electric field of 440 k V/cm.The sample also exhibited excellent frequency stability（0.5-500 Hz）,good temperartue stability（25-180°C）,and high fatigue resistance（10⁵ cycles）.Transmission electron microscopy（TEM）and piezoelectric force microscopy（PFM）results verified that the high energy storage performance of the samples was related to the dynamic PNRs driven by the electric field.In addition,the sample exhibits an ultrashort discharge time(t_0.9=89 ns)and high power density（P_D=112MW/cm³）at 240 k V/cm.Furthremore,Si O₂ was doped into the BNBT-0.12La materials,which effectively improved the comprehensive energy storage performance:under 410k V/cm,a large energy storage density(W_rec=5.33 J/cm³)and a high efficiency（η=82.6%）were achived in samples doped with 0.5wt.%Si O₂.（4）The microstructure of BNBT-0.12La ceramics was further optimized using a two-step sintering method.It was found that the two-step sintering method effectively inhibited the growth of grains:the average grain size decreased from 0.78μm for conventionally sintered samples to 0.47μm for two-step sintered sample.As a result,the energy storage properties of the material were effectively improved.Under the electric field of 440 k V/cm,the energy storage density(W_rec)reached 6.69 J/cm³,and the energy storage efficiency was 87.0%,which are all about 13%higher than that of conventionally sintered samples.The improvement of energy storage performance was related to the decrease of grain size which promoted the reduction of the size of PNRs and improved the response ability of PNRs to external electric field.In addtion,the two-step sintered samples also maintained excellent frequency stability,temperature stability and fatigue resistance,rendering them promising in pulsed power systems.