Comparative Study On Different Preparation Processes And Energy Storage Characteristics Of BNT-based Ferroelectric Ceramics

Energy storage dielectric capacitor has the advantages of ultra-high power density,ultra-fast discharge rate of microsecond,good stability,excellent reliability,etc.So they have a wide range of application prospects in particle cutting,electromagnetic emission,petroleum exploration and other pulse technology fields.The performance of energy storage dielectric capacitor is mainly determined by its internal dielectric.PZT-based antiferroelectric energy storage dielectric has become the first choice for energy storage dielectric capacitor due to its advantages of high saturation polarization intensity,low residual polarization intensity and high breakdown electric field.However,PZT-based materials contain a large amount of lead,which is contrary to the current international concept of green environmental protection.Therefore,it is urgent to find environmentally friendly lead-free dielectric energy storage materials.Bi_0.5Na_0.5TiO₃（BNT）is a lead-free dielectric material with ABO₃perovskite structure.Because of its high dielectric constant and high saturation polarization intensity,it is a potential lead-free dielectric capacitor.However,its large residual polarization intensity and low breakdown electric field lead to low energy storage density and efficiency of Bi_0.5Na_0.5TiO₃dielectric capacitor,which limits the practical application of Bi_0.5Na_0.5TiO₃dielectric capacitor.In this paper,by introducing HfO₂into 0.75Bi_0.5Na_0.5TiO₃-0.24Na Nb O₃-0.01Sr TiO₃（0.75BNT-0.24NN-0.1ST）dielectric,the effects of HfO₂on the microstructure,breakdown electric field,saturation polarization strength and residual polarization strength of 0.75BNT-0.24NN-0.1ST dielectric in two different preparation processes were studied,and the enhancement of the energy storage characteristics of0.75BNT-0.24NN-0.1ST dielectric by two different preparation processes was compared.Specific research contents are as follows:（1）The 0.75BNT-0.24NN-0.1SH_xT_（1-x）solid solution ceramics were prepared by introducing HfO₂into 0.75BNT-0.24NN-0.1ST dielectric by solid solution method.The effects of different content of Hf⁴⁺on the phase structure,microscopic morphology,polarization characteristics,breakdown electric field and energy storage characteristics of0.75BNT-0.24NN-0.1SH_xT_（1-x）ceramics were studied.The experimental results show that Hf⁴⁺penetrates into the 0.75BNT-0.24NN-0.1ST ceramic lattice,and the 0.75BNT-0.24NN-0.1SH_xT_（1-x）solid solution were still pure perovskite phase.The grain size of the sample decreases with the increase of Hf⁴⁺content,and the compactness of the ceramic is enhanced.At the same time,the introduction of Hf⁴⁺enhances the relaxation of the material,improves the saturation polarization strength of the 0.75BNT-0.24NN-0.1SH_xT_（1-x）solid solution,and then improves the energy storage characteristics of the material.When x=0.8,the polarization intensity reaches the maximum of 57.24μC cm^-2,the effective energy storage density reaches3.84 J cm^-3,and the energy storage efficiency is 69%.At the same time,the sample also has excellent charging and discharging performance,the discharge rate can reach 34.5 ns,and the discharge power density can reach 208.98 MW cm^-3.（2）The 0.75BNT-0.24NN-0.1ST/x HfO₂composite ceramics were prepared by solid phase sintering method by introducing HfO₂into 0.75BNT-0.24NN-0.1ST dielectric.The effects of different contents of HfO₂on the phase structure,microscopic morphology,polarization characteristics,breakdown electric field and energy storage characteristics of the0.75BNT-0.24NN-0.1ST composite ceramics were studied.The experimental results show that HfO₂is distributed at the grain boundaries of 0.75BNT-0.24NN-0.1ST dielectric ceramics to form 0-3 type composite ceramic dielectrics.The presence of HfO₂effectively inhibits the growth of electric branches and increases the breakdown electric field of0.75BNT-0.24NN-0.1ST/x HfO₂composite ceramics.At the same time,the trace Hf⁴⁺infiltration into the crystal lattice of 0.75BNT-0.24NN-0.1ST ceramics leads to the increase of Ti-O bond length,which is conducive to the flip of electric dipole,so that it still maintains a high saturation polarization intensity.When x=7 wt%,the 0.75BNT-0.24NN-0.1ST/x HfO₂composite ceramic achieves an electric field strength of 270 k V cm^-1,a saturation polarization strength of 48.39μC cm^-2,an effective energy storage density of 5.3 J cm^-3,and an energy storage efficiency of 85%.The effective energy storage density and efficiency change rate are less than 5%in the range of room temperture to 120℃,and it has excellent energy storage characteristics and temperature stability.At the same time,the discharge rate of the ceramic material can reach 22 ns,and the discharge power density can reach 284.5 MW cm^-3.The final analysis shows that the 0.75BNT-0.24NN-0.1ST/x HfO₂composite ceramic shows better energy storage and charge-discharge performance.