Study On Preparation,Properties And Mechanisms Of Ferroelectric/Relaxor Ferroelectric Energy-storage Ceramics

This dissertation focuses on the study of ferroelectric/relaxor ferroelectric energy-storage ceramics.A novel method was developed to effectively modify the structure of barium titanate-based energy-storage ceramics,and its mechanism was investigated.A type of weakly coupled relaxor ferroelectric energy-storage ceramics was fabricated,and the relationship between composition-structure-property as well as the origin of the good energy-storage characteristics was studied.A mathematical model for the core-shell ferroelectric energy-storage ceramics was constructed,and the key parameters influencing the energy-storage performance was analyzed.Finally,the dynamic electric breakdown process of ferroelectric ceramics was simulated via a phase-field model.A novel method for fabricating local compositionally graded core-shell barium titanate-based energy-storage ceramics via chemical coating was developed.BaTiO3@SrTiO3 and BaTiO3@BiScO3 particles were synthesized via a sol-based and a precipitation-based method,respectively.Results demonstrated that the core-shell structured ceramics showed wide dielectric-temperature features and excellent energy-storage properties,which verified the feasibility and universality of the method proposed to improve the performance of barium titanate-based energy-storage ceramics.A type of(1-x)BaTiO3-xBi(Zn2/3Nb1/3)O3 weakly coupled relaxor ferroelectric energy-storage ceramics was fabricated.The ferroelectric-relaxor transition and the composition-structure-property relationship were systematically studied.The advantages and mechanisms of weakly coupled relaxor ceramics for energy storage were demonstrated.Through composition optimization,the ceramics with x=0.13 showed good energy-storage characteristics with energy charge density of 1.57J/cm3,energy discharge density of 1.27J/cm3,and energy efficiency of 80.5%at room temperature.Through further Ta substitution of Nb in the ceramics,an optimal energy-storage feature was achieved in the composition with Ta content of 15%.At room temperature,the composition showed energy charge density of 1.55J/cm3,energy discharge density of 1.44J/cm3,and energy efficiency of 92.5%.Moreover,good temperature and frequency stabilities of energy and power density were also achieved in the same composition,with the possibility of working at a high temperature of 150?.A mathematical model for the core-shell ferroelectric energy-storage ceramics was constructed.Virtual core-shell ceramic samples were generated via a voronoi tessellation random construction routine.Assuming that the shell was linear dielectric,and depicting the polarization response of the ferroelectric core with a classical and a modified hyperbolic tangent model,the inner electric field distribution,dielectric nonlinearity and polarization behavior were systematically analyzed.The most effective way to achieve high energy density was by enhancing the saturation polarization and lowering the remnant polarization of the core,as well as modulating the shell fraction within a favorable range.The dynamic electric breakdown process of ferroelectric ceramics was simulated via a phase-field model.The influence of critical material parameters on the breakdown behavior was investigated.Results showed that increasing the grain boundary thickness,the nonlinear factor and the breakdown energy ratio of grain to grain boundary,and decreasing the permittivity ratio of grain to grain boundary would benefit the improvement of dielectric breakdown strength.The underpinning mechanisms were explained by the variation of the inner permittivity and the electric field of the ceramics.