Study On Preparation,modification And Dielectric Properties Of BaTiO₃-Based Dielectric Ceramics With "Core-Shell" Structure

BaTiO₃?BT?,a corner-stone dielectric material with perovskite structure,was widely applied in electronic industries,especially in the multilayer ceramic capacitors?MLCC?due to its high dielectric constant and superior ferroelectric properties.The dielectric constant of BT based ceramics would undergo a dramatic increase in Curie temperature?130 ^oC?,which cannot meet the application requirements in harsh and extreme working environments.The“core-shell”structure within ceramics is thought to be efficient approach to enhance dielectric temperature stability of BT based dielectric ceramics.By constructing and modifying this structure,the dielectric properties of ceramics can be optimized.To date,the formation and stabilization mechanism of this structure were not well understood.To gain a deep understanding about“core-shell”structure in BT based ceramics,high-quality BT powders with various sizes were synthesized by hydrothermal method.The effects of core size,shell materials,sintering temperature and dwelling time on the“core-shell”structure as well as dielectric properties of ceramics were systematically investigated.The high-purity,high-crystallinity and narrow size-distributions BT ultrafine powders were prepared by hydrothermal method successfully at various ratios of Ba/Ti,reaction temperatures,reaction times and contents of solvents,and the effects of these influencing factors on the phase structures,morphology and particle size of BT powders were studied.The particle size of BT powders could be controlled by adjusting reaction conditions.The synthesized powders with various size were utilized as core materials to coat with shell materials to fabricate“core-shell”structure in the following work.BT powders with average size of 80 nm,120 nm and 200 nm abbreviated as BT-80,BT-120,BT-200,respectively,were all coated with Nb2O5,MgO and0.3Bi(Zn_1/2Ti_1/2)O₃-0.7BT?0.3BZT-0.7BT?,to investigate the influence of starting particle size and shell materials properties on the structures and properties of ceramics.The results indicated that using large size BT powders,limited solubility and diffusion rate and suppressed grain growth played a critical role on the formation of“core-shell”inhomogeneous structure.Compared with Nb2O5 coating materials,MgO and BZT-BT exhibited lower diffusivity in BT matrix and inhibted grain growth more significantly,making it easier to induce composition fluctuation and form“core-shell”structure.The dielectric properties of BT-120@x?0.3BZT-0.7BT?ceramics fabricated by using chemical coating method as a function of sintering systems were studied.When x=1.0,the ceramics sintered for 0.5-2 h exhibited enhanced dielectric properties,which owned room-temperature dielectric constant of around 2000 and could meet the X8R speciation at high temperature end.Then,the compositions,microstructures and dielectric properties of BT-120@1.0?0.3BZT-0.7BT?ceramics sintered for various dwelling times from 0.5-6.0 h were investigated,and the relationship between“core-shell”structure and dielectric properties was established by employing TEM-EDS analysis techniques.Both the extended sintering temperature and prolonged dwelling time provided large driving force for the ion diffusion between core and shell,local compositional gradients and heterogeneity tended to homogenize.And the number of grains with“core-shell”structure decreased gradually,leading to the decrease of core dielectric peak.The“double dielectric peak”become single diffused peak with the disappearance of core BT ferroelectric domain structure and the transformation of inhomogeneity“core-shell”structure into homogeneous solid solutions eventually.When sintering in relatively lower temperatures and for shorter dwelling times?0.5-2.0h?,the“core-shell”structure could be formed and retained owing to the limited diffusion and grain growth process,enabling superior dielectric temperature stability of BT based dielectric ceramics.