Defect Behavior And Mechanism Of BaTiO₃ Based Dielectric Materials Under External Field

As the degradation properties of MLCC is mostly dependent on dielectric materials,the mechanism of defects formation and properties evolution for the dielectric materials are still unclear.In this study,we focus on the dielectric materials based on BaTiO3 including undoped BaTiO3 single crystals,undoped BaTiO3 ceramics,Ca-doped ceramics,Mg-doped ceramics,ceramics codoped with Ca and Mg and ceramics codoped with Ca,Mg and Dy.Thermally stimulated depolarization current(TSDC),AC impedance spectra and other dielectric characterization methods were carried out on dielectric materials based on BaTiO3.The effect of different electric field,temperature field,ceramic grain and grain boundary on degradation was studied.The main content and conclusions of this thesis are as follows:(1)defect behavior of undoped BaTiO3 single crystalsConcentrations of point defects like the oxygen vacancies in undoped BaTiO3 single crystals were simply controlled by thermal treatment under different atmospheres.Defect behavior of single crystals with different orientations was studied in different electric field or temperature field.The results show that:1)The<111>crystalline face of BaTiO3 is more stable under external field than others;2)The main defects of single crystals after thermal treatment are defect dipoles(Ti’Ti-VO")and oxygen vacancies VO";3)The formation of oxygen vacancy clusters in BaTiO3 single crystals leads to a high dipole activation energy.(2)defect behavior of undoped and doped BaTiO3 ceramicsUndoped BaTiO3 ceramics,Ca-doped ceramics,Mg-doped ceramics,ceramics codoped with Ca and Mg and ceramics codoped with Ca,Mg and Dy were prepared by solid-state reaction method.Defect behavior of undoped and doped BaTiO3 ceramics was studied by TSDC,AC impedance spectra and other dielectric characterization methods.Finally,the defect types and their production mechanism were analyzed:i)VNa" and VO" are the main defects in undoped BaTiO3 ceramics;CaBa× and VO"are the main defects in Ca-doped BaTiO3 ceramics;VBa"、VO" and MgTi" are the main defects in Mg-doped BaTiO3 ceramics;VO",MgTi" and CaBa× are the main defects in BaTiO3 ceramics codoped with Ca and Mg;VO",MgTi",CaBa× and DyBa’ are the main defects in BaTiO3 ceramics codoped with Ca,Mg and Dy.Oxygen vacancies generated for charge equilibrium and were constrained with the formation of dipole(MgTi"-VO")and(VBa"-VO’);ii)The dipole oriented along the electric field and the oxygen vacancies started to get rid of the dipole with an enhanced polarization condition.Oxygen vacancies across the grain boundary to form long-distance migration and result in the degradation.But the long-distance migration of oxygen vacancies was difficult due to the core-shell structure.The dipole oriented along the electric field and the oxygen vacancies started to get rid of the dipole with enhanced polarization condition.