Investigation On The Structural Design And Energy Storage Characteristics Of High Breakdown Strength Na_0.5Bi_0.5TiO₃-based Ceramics

The rapid development of miniaturisation and integration of electronic power systems has led to an urgent need for a dielectric material with high power density,fast charging and discharging capability,and high energy storage density in the application fields of controlled nuclear fusion,new energy electric vehicles,and high-end medical devices.Na_0.5Bi_0.5TiO₃（NBT）-based ceramic dielectrics have been widely studied for their high value of-polarization and excellent temperature stability,but the low breakdown strength may limit its application.In this thesis,the effects of the compositional alteration of NBT-based ceramics and its deriving composite structures into 2-2 and 0-3 type composite on the phase,microstructure,dielectric properties,breakdown strength and energy storage properties are systematically investigated,and the relationship between material structure and properties is explored.The main studies and conclusions are as follows:Firstly,（1-x）Na_0.5Bi_0.5TiO₃-xSr_0.7Bi_0.2TiO₃（x=0.35,0.45,0.55 and 0.65）ceramics were prepared by the conventional solid-state reaction,and their dielectric energy storage properties were investigated.The results show that with increasing amount of SBT,the large size grains decrease and the grain size becomes more uniform.The temperature T_m corresponding to the dielectric peak in the dielectric temperature spectrum gradually decreases,and the temperature stability of the dielectric constant improves.The ferroelectricity is weakened,while relaxation is enhanced and the macroscopic ferroelectric domains are refined,resulting in improved energy storage properties.The NBT-SBT ceramics exhibits the best recoverable energy storage density of 1.34 J/cm³ and an energy storage efficiency of 96.03%at x=0.55 under an electric field of 100 k V/cm.Next,0.65Na_0.5Bi_0.5TiO₃-0.35SrTiO₃（NBT-ST）with large polarization and0.45Na_0.5Bi_0.5TiO₃-0.55Sr_0.7Bi_0.2TiO₃（NBT-SBT）with high efficiency were selected to construct 2-2 type composites.The effect of the numbers of interfaces on the breakdown behaviour was investigated by controlling the way of stacking.The results show that the increasing number of the interlayer interface favours the increase of the breakdown strength.The single-layer NBT-ST/NBT-SBT alternately stacked 2-2 type composite achieves a maximum breakdown strength of 242 k V/cm,resulting in high recoverable energy storage density（4.48 J/cm³）and energy storage efficiency（～90%）.Meanwhile,the energy storage performance of NBT-ST/NBT-SBT composites also exhibit excellent frequency（10～200 Hz）and temperature（25～125℃）stability.Finally,as the interface diffusion of NBT-ST/NBT-SBT 2-2 type composite is relatively serious leading to a certain impact on the energy storage performance.Due to the excellent chemical stability and high breakdown strength of AlN,it was selected for preparing the composite,and 0.45Na_0.5Bi_0.5TiO₃-0.55Sr_0.7Bi_0.2TiO₃（NBT-SBT）/xAlN 0-3 type composite were prepared.The results show that the addition of AlN significantly reduces the grain size and dielectric constant of the NBT-SBT ceramics and increases the breakdown strength of the composite.At x=6.0 wt.%,high breakdown strength of 360 k V/cm,large energy storage efficiency of 90.12%and excellent recoverable energy storage density of 5.53 J/cm³ were obtained.