Preparation And Energy Storage Performance Of Barium Titanate-based Lead-free Ceramics And Their Composites

With the increasing depletion of fossil energy,the development of new energy sources and energy storage has become a critical global issue.Among them,electrical energy storage is of great significance.Compared with fuel cells and electrochemical capacitors,dielectric capacitors have the advantages of fast charging and discharging speed and high-power density,and are widely used in such fields as new energy vehicles,power pulsers and electromagnetic guns.However,the low energy storage density limits its application in above areas.Barium titanate（BT）lead-free ceramics have attracted much attention over the past decade due to their good ferroelectricity and rich phase transition.In this study,BT-based ceramics was taken as the prototype of lead-free relaxation energy storage ceramics to obtain good dielectric energy storage properties by doping modification in order to reduce the residual polarization and enhance the material relaxation.Based on this idea,the viscous polymer processing（VPP）process was used to further reduce the internal defects,improve the densities and optimize the ceramic properties,and analyze the mechanism of the process on the structure and properties of ceramics.Finally,the ceramic/resin composites were prepared based on the freeze-casting process to inverstigate the effects of slurry composition and other effects on the structure of the composites.Synergistical regulations among the mechanical properties,dielectric properties and energy storage properties of the composites were performed to verify the possibility of this method for constructing a composite dielectric material with excellent performance.The details are as follows.Based on BT lead-free ceramics,（1-x）(0.65BaTiO₃-0.35Sr_0.7Bi_0.2TiO₃)-xCeO₂（BT-SBT-xCe）series ceramic samples were prepared and the effect of Ce elements on the energy storage performance of the ceramics was investigated.Ce elements mostly exist in ceramics as the valence state of Ce³⁺and enter the A-site,resulting in the reduction of cell parameters.The doping of appropriate amount of CeO₂ helps to improve the relaxation behavior,enhance the impedance of ceramics,reduce the dielectric loss of ceramics and promote the storage and release of electrical energy.In particular,the BT-SBT-0.02Ce ceramic has an electrical breakdown strength of 330 k V/cm,and the effective energy storage density and energy storage efficiency are 2.57 J/cm³ and 81.30%,respectively.In addition,the ceramic exhibits good stability in a certain frequency range（1～300 Hz）and temperature range（30～150°C）,and has high current density and power density.The lead-free relaxor ferroelectric ceramics with different polyvinyl alcohol contents were prepared using the VPP process by selecting the composition-optimized BT-SBT-0.02Ce component.The effects of polyvinyl alcohol content on the microstructure and dielectric energy storage properties of the ceramics were systematically investigated.Under the premise of film formation,the lower the polyvinyl alcohol content is,the less internal defects is formed and the higher densities are obtained after porcelain formation.The finite element simulation results show that when the polyvinyl alcohol content is 5wt.%,the local electric field distribution inside the sample is more uniform and has excellent electrical properties.The Weibull breakdown strength reaches 420 k V/cm and the effective energy storage density approches 4.41 J/cm³,which corresponds to an energy storage efficiency of 84.21%.Meanwhile the change rate of energy storage efficiency is less than 5%in a certain temperature range（30～150℃）and frequency range（1～300Hz）,showing excellent stability.Porous BT-SBT-0.02Ce ceramics with lamellar structure were fabricated by freeze-casting process and then backfilled with epoxy resin to obtain ceramic/epoxy composites.The rheological properties of the slurry and the influence of the slurry composition on the structure were investigated in depth.When the ceramic powder solid phase content was45 wt.%,the ceramic/epoxy resin composites were successfully obtained in parallel arrangement.The modulus and hardness of the composites were～20 times higher than that of the epoxy resin result in significantly improved mechanical properties.However,the interaction between the highly coupled dipoles in the polymer reduced the electrical breakdown strength of the composites.Therefore the final obtained composites with lamellar structure has an electrical breakdown strength of 220 k V/cm,corresponding to an effective energy storage density of 0.62 J/cm³ and an energy storage efficiency of～80%.In conclusion,this study has effectively improved the energy storage performance of barium titanate-based lead-free ceramics by doping modification.The electrical breakdown strength of the ceramics was further improved through the regulation of the VPP process parameters.Besides,a method for fabricating laminated ceramic/polymer composites was designed,which provides a reference strategy for composite capacitors with high dielectric constant and high energy density.Overall,the different strategies mentioned in this thesis can provide case studies for other ferroelectrics,which is an important guideline for the research and development of new dielectric energy storage ceramics or composites.