The Study On The Energy Storage Properties And Modification Research Of Sr_xBa_1-xNb₂O₆ Ceramics

With the development toward miniaturization,lightweight,integration in pulse power system,high energy-storage dielectric materials have attracted more and more attentions,which puts forward higher requirements of dielectric constant and breakdown strength.Due to the unique properties of suitable dielectric constant,breakdown strength,low remnant polarization,et.al,the key to obtaining the high energy storage,lots of researches have been focused on relaxor ferroelectric.Sr_xBa_1-xNb₂O₆（SBN）ceramics,relaxor ferroelectric with tungsten bronze structure,are chosen as research object in this paper because of high dielectric constant,low dielectric loss,low remnant polarization and the flexible structure.SBN ceramics with different compositions were prepared by conventional solid-state method,whose energy storage properties were studied.MgO additive is promised to achieve higher breakdown strength in order to optimizing the energy stroge density of SBN ceramics.The structure,dielectric and energy storage properties were investigated by the testing techniques,such as X-ray diffraction,scanning electron microscope and so on.The influences of holding time were further explored on the structures and properties for the modified ceramic with the highest energy storage density.Sr_xBa_1-xNb₂O₆（x=0.4,0.5,0.6,0.7,abbreviated by SBN40,SBN50,SBN60and SBN70）ceramics were synthesized.By the examination of XRD,all the peaks are ascribed to the single tetragonal tungsten bronze structure.The sintering behavior and SEM image showed sintering temperature and holding time increased as the Sr content increased,accompanying with the lager grains.The dielectric peaks were all observed evidently with the dielectric dependence on temperature in SBN ceramics.Curie temperature shifts toward lower temperature and dielectric constant largely increases with x increasing.The SBN ceramics with larger Sr/Ba ratio exhibit stronger relaxation as the slim P-E illustrates obvious relaxor behavior.However,the low breakdown strength of approximately 130 kV/cm limits the obtainment of high energy storage density.Thus SBN60 and SBN70 ceramics were adopted to be modified with addition.The sintering temperature lowered down as MgO was added into SBN60 and SBN70 powders.The XRD pattern of SBN60-xwt%MgO and SBN70-xwt%MgO ceramics displays that Mg²⁺ion exists as two forms.Part of Mg²⁺incorporates into the lattice of B site,part of that reacts with the matrixs.It turned out that rod-like grains and the grain size reduced with the increase of MgO amount,leading to denser and more uniform microstructure.Energy dispersive X-ray spectroscopy indicated that Mg-rich grains were very fine grains distributing around grain boundary and gains.From the relationship between dielectric and temperature,Tc moved to low temperature,even below the room temperature,as MgO content increased.Both dielectric constant and dielectric loss reduced greatly.Owing to the uniform and dense microstructure,maximum breakdown strength of SBN60 and SBN70 ceramics modified by MgO could be up to 216 kV/cm and 226 kV/cm.The energy storage efficiency of samples was also improved.For SBN60-xwt%MgO ceramics,the greatest value of energy storage density was 0.73 J/cm³ under the electrical field of 174 kV/cm and the energy storage efficiency was 90.3%as x=2.SBN70-0.5wt%MgO ceramics possessing the best energy storage property had the discharge energy storage of 0.93 J/cm³,the breakdown strength and energy storage efficiency were 157 kV/cm and 89.4%,respectively.SBN70-0.5wt%MgO ceramics,which was pre-sintered at different holding time（3h,6h,9h）,were measured through XRD testing.Secondary phases were detected for SBN70-0.5wt%MgO ceramics with 3h-holding time and 9h-holding time.The dielectric constant decreased and the grain size increased a little due to the secondary phase.Thus,energy storage density was not promoted because of both lowering maximum polarization and breakdown strength.Keep the pre-sintering holding time6h for SBN70 powder,then sinter with MgO addition at different holding time（2h,4h,6h,8h）.XRD pattern showed SBN70-0.5wt%MgO ceramics was all prepared with the single tetragonal tungsten bronze structure.However,the grain size became larger with the abnormal grain growth as the holding time increased,leading to the reduction in breakdown strength.While the dielectric constant showed the tendency of increasement.Energy storage density enhanced for SBN70-0.5wt%MgO ceramics with more holding time at the same given electrical field,but lowered down at the critical breakdown strength.As a result,changing the holding time didn’t effectively improve the energy storage properties for SBN70-0.5wt%MgO ceramics in this paper.