Microstructure Design And Energy Storage Performance （NiO/FeO）@BaTiO₃ Ceramic

With the rapid development of energy and electronic technology,dielectric materials with high energy storage density have great research value for the development and application of modern information technology.The main way to improve the energy storage density of the material is increased the polarization and breakdown strength of the material.The polarization and breakdown strength of the material are mainly related to the quality of the raw material powder and the structure of the material.Due to the problems of uneven dispersion and easy introduction of impurities in the mixing process of the traditional solid-phase method,the energy storage density of the ceramics prepared by the solid-phase method still has room for improvement.Compared with the traditional solid-phase method,the chemical coating technology can obtain more uniform high-quality ceramic powders,so it has been paid attention by researchers in recent years.In addition,the interface composition,defect structure and microstructure of ceramic materials can be adjusted by chemical coating method,which makes this method possess broader application prospects.In this paper,（NiO/FeO）@BaTiO₃ceramic materials were prepared by chemical coating method.The preparation method,crystal structure,defect structure and the relationship between the microstructure,the dielectric and energy storage properties of ceramics were studied.X-ray diffraction,photoelectron spectroscopy,transmission electron microscopy,and ferroelectric workstations are used to study the energy storage mechanism of ceramics,and establish the relationship between ceramic structure and dielectric and energy storage properties.By adjusting the sintering atmosphere and microstructure design,the defect composition and microstructure of the ceramics are optimized,and the dielectric and energy storage properties of the ceramics are improved.The specific research contents and main conclusions are as follows:Firstly,NiO@BaTiO₃ceramic powders were successfully synthesized by chemical coating method,and ceramic materials with dense structure were prepared.The relationship between the crystal structure,microstructure and electrical properties of ceramics was studied.An antiferroelectric-like ceramic material characterized with the double hysteresis loop of the bulk is obtained.The remanent polarization of ceramic materials with double hysteresis loops decreases while maintaining a higher polarization.NiO@BaTiO₃ceramics showes good frequency stability,and the dielectric constant changed less than±1%with the increasement of frequency.In addition,under the electric field of 210 k V/cm,the discharge energy storage density of BTN1 ceramics reaches 1.88 J/cm³,and the energy storage efficiency increases to70%.At the same time,the ceramic has fast charge-discharge capability(τ_0.9<2μs)and good cycle stability（10⁵cycles）and temperature stability（20～80^oC）.Secondly,the defect structure of NiO@BaTiO₃ceramics is controlled by atmosphere sintering,the mechanism of double hysteresis loop is analyzed,and the energy storage performance of ceramics is optimized.The formation of defect dipoles(Ni’’_Ti-V_o^··)formed by low-valent Ni ions replacing titanium ions is the main reason for the double hysteresis loops in NiO@BaTiO₃ceramics.By controlling the content of(Ni’’_Ti-V_o^··)defect dipoles in the ceramics by atmosphere sintering,the oxygen atmosphere sintered ceramics showed the most obvious double hysteresis loop characteristics,which confirmed the appearance of double hysteresis loops is related to the defect dipoles.These charged defect dipoles can fix the domain wall,and the existence of oxygen vacancies restricts the movement of the domain wall,making the material appear a shrinking hysteresis loop,showing a double hysteresis loop like an antiferroelectric.The energy storage performance of NiO@BaTiO₃ceramics sintered in oxygen atmosphere is also improved.Under the electric field of 340 k V/cm,the discharge energy storage density of NiO@BaTiO₃ceramics sintered in oxygen atmosphere increases to 2.48 J/cm³,and the energy storage efficiency is 72%.Thirdly,FeO@BaTiO₃ceramic powders are successfully synthesized by chemical coating method,and ceramic materials with nano-scale grains are prepared.The relationship between the electrical properties and the structure of the ceramics is studied by means of XRD,SEM and XPS.The second phase formed at the interface suppresses the grain boundary migration,so that the grains of the ceramics hardly grow,and the grain size remains at the nanometer scale.Nanoscale ceramic materials have a multilayer structure comprising 1)an internal tetragonal core;2)a gradient lattice strained layer;and 3)a surface insulating layer.The inner tetragonal BaTiO₃core provides excellent dielectric properties,resulting in the high dielectric constant of the ceramic.The high dielectric constant gradient lattice strain layer broadens the dielectric constant peaks.The multi-layered nano-ceramics improve the dielectric temperature stability and breakdown strength of the material.In the temperature range of-55～125^oC,the change rate of the dielectric constant of the ceramic is less than15%,which is in line with the EIA X7R standard.The DC breakdown strength of the ceramic reaches 383.2k V/cm.Meanwhile,under the electric field of 300 k V/cm,the discharge energy density of BTF3 nanoceramics reaches 1.50 J/cm³,and the energy storage efficiency reaches 88%.In addition,FeO@BaTiO₃nanoceramics have the characteristics of fast discharge rate(τ_0.9<1.5μs),good thermal stability（25～120 ^oC）,and good cycle stability（1×10⁵cycles）.Finally,the energy storage performance of the ceramics is optimized by introducing highly polarized micron-scale grains into the nanoscale FeO@BaTiO₃ceramics.The structure of the ceramics is analyzed by XRD and SEM,and it is found that the nano-scale grains surrounded the micro-scale grains,forming a multi-level structure.The introduction of micron-sized grains improves the polarization of FeO@BaTiO₃ceramics,but reduces the breakdown strength of the ceramics.By coordinating the proportion of micron-sized grains,the energy storage performance of the ceramics is optimized.When the content of micron grains is 10%,the discharge energy storage density is increased to 2.17 J/cm³under the electric field of 340k V/cm³,and the energy storage efficiency is 80%.Due to the addition of micron-sized ferroelectric grains,the dielectric temperature stability of the ceramic is improved from EIA X7R to X9R.