The Fabrication And Dielectric Property Study On High Energy Density Dielectrics

The dielectric materials store charge by polarization under an electric field.It not only can be used in capacitors to achieve ultra fast storage and release of electrical energy but also be used in gate dielectric to control the migration of carriers in semiconductor active materials.Therefore,dielectric materials are widely used in the electronics and energy industries.With the continuous miniaturization of electronics and power devices and the desire for higher charge storage density,the demand for high energy density dielectric is increasingly urgent,which has stimulated the research and development of high-energy storage dielectric material systems.A core issue in developing this material system is how simultaneously to obtain the high energy density and excellent dielectric properties.For most dielectric materials,the two main factors affecting the energy density are dielectric constant and dielectric strength(breakdown voltage),which have a zero-sum game.To this end,this study develops corresponding high-energy dielectric preparation methods for applications at different scales.Through dielectric performance characterization to systematic study the structure-function relationship between their structure and dielectric failure mechanism,to explore solutions for dielectric strengthening and optimization mechanisms based on application considerations,and provide a basis for the application of new principles and new methods at different scales.Firstly,for the applications of ceramic energy storage capacitors,this study is based on the colossal permittivity Ba Mn₃Ti₄O_14.25(BMT-134)nano powder and spark plasma sintering(SPS)technology to prepare micron-thick dense polycrystalline ceramic dielectric and study their dielectric and breakdown properties.The results show that the dielectric properties of BMT-134 ceramics obtained by SPS are better than those obtained by conventional sintering.And their giant dielectric constant are caused by the internal barrier layer capacitance(IBLC)effect and the grain boundary layer capacitance(GBLC)effect formed by the semiconductor grains and the insulating grain boundary.Oxygen vacancy defects introduced during the SPS process will enhance the GBLC effect,thereby increasing the dielectric constant.However,they can cause losses peak around 20 k Hz.Moreover,the trap brought by the oxygen vacancy also determined that its leakage current mainly comes from the space charge limited current(SCLC),and its breakdown mechanism is mainly a soft breakdown of time dependent dielectric breakdown(TDDB).Annealing helps to remove the effects of oxygen vacancies and restore their intrinsic semiconductor ceramic properties,resulting in higher losses and breakdown voltages.However,it does not change the mechanism of leakage and breakdown type.Therefore,a denser ceramic dielectirc can obtained by SPS technology.Moreover,the oxygen vacancy can effectively suppress the rapid increase of the conductivity to increase the dielectric constant and suppress the corresponding high loss.This has important guiding significance for the development of giant dielectric ceramic materials based on the introduction of elements with varying valences.Sencondly,for the applications of thin-film energy storage capacitors,this study developed a method for preparing ultra-thin polymer dielectric films based on heating control spin coating(HCSC)and Langmuir-Blodgett deposition(LBD)technology.This method is the first to obtain high dielectric constant and high breakdown voltage simultaneously on the nanoscale,which make it possible to manufacture high energy density implanted capacitors on the chip.The results show that LBD technology can effectively fill the nanopores brought by the HCSC process and increase the concentration of?phase in the PVDF films,thereby improving the dielectric strength and dielectric constant of the film.After further optimization,the film with a thickness of 76 nm has better comprehensive dielectric properties.their dielectric constant are higher than that of?-phase PVDF,dielectric loss are less than 0.1,breakdown strength are up to 171 MV/m,that is,energy density are up to 2 J/cm³.This result is not only the first high-performance PVDF dielectric film below 100 nanometers,but also obtained better dielectric properties than most reported micron-level PVDF membranes.In addition,the breakdown type of the as-fabricated film is a soft breakdown of the TDDB,so that it can used repeatedly around its breakdown voltage.Finally,to further studying the influence of scale effect on dielectric properties and laying a solid foundation for further research and development of two-dimensional(2D)dielectric materials in the future.The direct tunneling behaviors through Ti_1-?O₂^4?-nanosheets assembled ultra-thin(0.7-3.5 nm)2D dielectric materiasl are studied by a modified WKB approximation and conformed by ab-initio calculation.The Results showed that in the metal-oxide-metal(MOM)structures,the current density caused by direct tunneling decreases exponentially as the number of 2D material stacks increases.,and decreases by 3-4 orders of magnitude for each Ti_1-?O₂^4?-nanosheets layer addition.the localized intercalation charges naturally embedded in chemically-derived Ti_1-?O₂^4?-nanosheets could reduce tunneling current by 3 orders of magnitude and that a thickness threshold of three layers(?2 nm)exists as to the titania nanosheets not to exceed a gate current density of 1 A/cm² at 1 V.The results can be used to understand the dielectric properties of most two-dimensional oxide nanosheet dielectric films,and provide basic data for the design and selection of dielectric materials in atoms and devices.