Microstructure Modification And Its Mechanism Study On The Energy Storage Properties For (Ba_0.4Sr_0.6)TiO₃ Ceramics

Energy storage dielectric capacitors are widely used in strong high-current and high-power electrical devices,with their properties mainly dominated by the interior energy storage dielectrics.Ba_0.4Sr_0.6TiO₃（BST）is the solid solution of BaTiO₃ and SrTiO₃,showing paraelectric phase structure at room temperature（Curie temperature≈-65?C）and relatively high permittivity（ε_r≈1000）,low dielectric loss（tgδ≈0.005）,high dielectric breakdown strength（¹50 kV/cm）,as well as low residual polarization and hysteresis loss,being potential for electrical energy storage applications.Great attempts has been performed on the enhancement of electric breakdown strength for energy storage dielectrics,with compromising their high dielectric constant.Compared to the other techniques,processing modification was considered beneficial for improving the energy storage performance more directly and effectively,accompanied by slight change in chemical compositions and intrinsic properties for the dielectric materials.The effects of processing on the performance are mainly due to the variations in microstructure including the grain size,grain boundary,oxygen vacancies and pores,etc.However,the study on the microstructure effects and the corresponding mechanisms is still insufficient and controversial.In addition,multiple mechanisms would coexist for materials with different processing conditions,making it difficult to research on the specific mechanism.In this study,Ba_0.4Sr_0.6TiO₃ was chosen as the research object.The ceramic powders were synthesized by oxalate co-precipitation and conventional solid state reaction methods.Dense BST ceramics with different microstructure characteristics were fabricated by fast firing techniques（such as plasma activated sintering and microwave sintering）as well as conventionally sintering method.The microstructure modifications and energy storage enhancement were achieved by adjusting the process conditions.Comprehensive study of microstructure effects on the energy storage properties of Ba_0.4Sr_0.6TiO₃ and the corresponding mechanisms were investigated,as following:Microstructure effects on the dielectric properties:Dense BST ceramics with various grain sizes（0.5⁵.6μm）were fabricated by conventional solid state reaction method with different sintering temperatures.With decreasing grain size,the room temperature permittivity was found to gradually increase and then reduce.A clear tendency toward diffuse phase transition induced by the enhanced internal stress among grains was observed.With the refinement of grains,the dielectric nonlinearity is gradually depressed under DC bias electric field,driven by the increase amount of insulated grain boundaries with low permittivity.The grain size-dependent nonlinearity mechanisms were also discussed in this study.Based on the multi-polarization mechanism model,the intrinsic polarization response(anharmonic interactions among Ti⁴⁺ions)and the extrinsic polarization contribution（polarization of polar nano-regions,PNRs）can be separated from the dielectric nonlinearity.The polarization behavior of PNRs was found to make a unique contribution to the nonlinear characteristics of BST,where the polarization tends to be reduced and the equivalent sizes gradually increase with increasing grain size.At microwave frequency,the polarization of dielectrics will be dominated by ionic polarization and electronic displacement polarization,making the dielectric behavior and the corresponding mechanism different from the low frequency.BST ceramics with grain size from 8.0⁶5.0μm were prepared by varying the sintering process during the conventionally sintering.Sintering process effects on the dielectric properties of BST ceramics at high frequency and the corresponding mechanisms were discussed in this study,focusing on the varying microstructure.With higher sintering temperature or longer dwell time,the decreased grain boundary density and less confined phonons induced by the grain growth were believed to contribute to the improved dielectric properties at high frequency,as the intrinsic effect.However,for coarser grained BST samples（grain size?25μm）,the permittivity and dielectric loss at high frequency tend to saturate（or deteriorate）,being thought to be dominated by the abundant defects within large grains（regarded as extrinsic factors）,as confirmed by the SEM observation.Grain boundary effect on the energy storage properties of BST ceramics:BST nanopowders（average powder size≈50 nm）were synthesized by the oxalate co-precipitation method.Dense BST ceramics with various grain sizes（0.3³.4μm）were prepared by plasma activated sintering and conventionally sintering process.Due to the dielectric breakdown strength variation,the energy density is obviously improved from0.49 J/cm³ to 1.30 J/cm³,and then deteriorated to 0.63 J/cm³ with decreasing grain size,which was considered to be affected by grain boundaries.With the refinement of grains,the increased grain boundary density was thought to play a significant role on the obvious improvement of the dielectric breakdown strength,which was confirmed by the impedance spectroscopy analysis based on the double layered dielectric model.However,the dielectric breakdown strength was found to be reduced with further decreased grain size≤0.7μm,being believed to be dominated by the enhancement of interfacial polarization at grain boundary layers.In addition,a sharp decrease of dielectric breakdown strength was observed for sample with 1.8μm grain size,being considered to be induced by the combination effect of lower grain boundary density and more intensive interfacial polarization.To reveal the effect of grain boundary on the energy storage performance of BST samples more qualitatively.Microwave sintering（MWS）and conventionally sintering（CS）techniques were employed to fabricate BST ceramics with different electrical characteristics at grain boundaries.Thermal annealing process（1100^oC-10 h）was performed to eliminate the oxygen vacancy concentrations after the sintering process.Considering the role of modified grain boundaries,the annealing effects on the energy storage properties and interfacial polarization behavior of BST samples were investigated.Accompanied by the suppressed dielectric loss,the energy storage performance for annealed MWS BST is optimized,with increasing energy storage efficiency from 60%to82%,and improved discharged energy density from 0.77 J/cm³ to 1.15 J/cm³.The development of reoxidation was confirmed to occur primarily at grain boundaries,other than grains during the annealing process,controlled by the oxygen diffusion behavior,leading to enhanced insulation of grain boundaries as compared to the grains.The recovery and improvement of insulating characteristics of grain boundaries were believed to contribute to the increased dielectric breakdown strength after annealing,consistent with the greater electrical difference between grains and grain boundaries,accounting for the improved energy storage properties and enhanced interfacial polarization.In addition,as a result of the improved insulating characteristic of grain boundaries,the accumulation of defect dipoles and consequently higher leakage current induced by enhanced interfacial polarization were regarded to contribute to the insulation deterioration at high temperature for annealed samples.Effect of oxygen vacancies on the energy storage properties of BST ceramics:The energy storage properties of BST ceramics were obviously dependent on the thermal annealing conditions（atmosphere）.In this study,as-sintered BST samples were annealed in air,oxygen and reducing atmosphere,respectively,where the interior oxygen vacancies were qualitatively and quantitatively charactered by thermally stimulated depolarization current analysis.The concentration of oxygen vacancies was found significantly dependent on the annealing conditions,being believed to contribute to the significantly changed insulation and energy storage properties for BST ceramics.