Energy Storage Performance Of Barium Strontium Titanate-based Ceramics Prepared By Spark Plasma Sintering And Its Numerical Simulation

With the development of miniaturization,light-weight pulsed power system,dielectric materials with high energy storage density have been widely investigated.In this thesis,the effects of preparation process,microstructure and phase composition on the energy storage performance of Ba1-xSrxTiO3-based ceramics have been systematically investigated.Meanwhile,the dielectric breakdown process and energy storage densities of Ba1-xSrxTiO3-based ceramics were predicted by mumerical simulation.The results are as following:The microstructure was found to have a very important role on the dielectric breakdown strength.Ba0.4Sr0.6TiO3 ceramics prepared by the conventional solid-state reaction exhibit inhomogeneous microstructures with low relative density.In contrast,Ba0.4Sr0.6TiO3 ceramics prepared by the spark plasma sintering(SPS)show a high relative density of>99%with uniform grain size distribution.An enhanced dielectric breakdowm strength from 180 kV/cm to 260 kV/cm,an improved maximum energy storage density from 0.61 J/cm3 to 1.20 J/cm3 and an optimized energy storage efficiency from 72.6%to 91.6%were obtained by optimizing the microstructure in the SPS sample.To understand the effects of grain size on the dielectric breakdown strength and energy storage density,dense BaO.4SrO.6TiO3 ceramics with various grain size are prepared using sol-gel method and spark plasma sintering.Results reveal that the sample with an average grain size of about 200 nm shows the best energy storage performance,where a high dielectric breakdown strength of 240 kV/cm,a high energy storage density of 1.23 J/cm3 and a high energy storage efficiency of 94.52%were obtained.It was found that the dielectric breakdown strength of BaO.4SrO.6TiO3 ceramics can be improved by adding some high insulative MgO particles.The diffusion and reaction between BaO.4SrO.6TiO3 and MgO were suppressed by using SPS tecnique due to the low sintering temperature and very short sintering period.Significant improvement of energy storage performance with high breakdown strength of 300 kV/cm,maximum energy storage density of 1.50 J/cm3 and high energy storage efficiency of 88.5%were obtained in the 95 wt%Ba0.4Sr0.6TiO3 + 5 wt%MgO composite.The core-shell structure also helps to improve the energy storage performance in the BST composites.Ba0.4Sr0.6TiO3@SiO2 core-shell powders with various mole fraction of SiO2 were synthesized by wet-chemical method and dense Ba0.4Sr0.6TiO3@SiO2 composite ceramics were prepared by using SPS.SEM and TEM analysis indicate that the coating layer(SiO2)reacts with Ba0.4Sr0.6TiO3 at around 750℃,which results in the formation of Sr2TiSio8 phase.The formed Sr2TiSiO8 nano particles were observed as a shell coated on the BST grains in the sample with low contents of SiO2,which improves the dielectric breakdown strength.However,with the increase of the amount of SiO2,Sr2TiSi2O8 micron particles are formed at the grain boundaries of BST matrix and the dielectric breakdown strength decreases.The best energy storage performance was obtained in the 92 mol%Ba0.4Sr0.6TiO3 + 8 mol%SiO2 composite ceramics,which exhibits a high dielectric breakdown strength of 400 kV/cm,a maximun energy storage density of 1.60 J/cm3,and high energy storage efficiency of 90.9%.According to the electrostatic equilibrium equation,the large difference of the dielectric constant between Ba0.4Sr0.6TiO3(εr=950)and MgO(εr=10)in Ba0.4Sr0.6TiO3/MgO composite may lead to the nonuniform distribution of local electric field,which is supposed to affect the dielectric breakdown process.Here,the discharge channels in Ba0.4Sr0.6TiO3-based and MgO-based composites were predicted by using the stochastic model.The results reveal that in the Ba0.4Sr0.6TiO3-based ceramics,the dielectric breakdown channel always bypasses the MgO particles no matter what the shape of the added MgO particles is.In the MgO-based ceramics,the breakdown channel tends to go through the Ba0.4Sr0.6TiO3 particles.Finally,the ferroelectric properties of Ba1-xSrxTiO3 single crystal and polycrystalline have been simulated by phase field method based on the Landau-Devonshire phenomenological theory and time-dependent Ginzburg-Landau equation.The energy storage densities were calculated from the simulated hysteresis loops.The results show that with the increasing of x,the maximum energy storage density will first increases and then decreases.The most promising candidate for energy storage application is Ba0.4Sr0.6TiO3.