Preparation And High-Temperature Energy Storage Properties Of Core-Shell Structured Al₂O₃@ZrO₂/Polyetherimide Composite Films

As a high-power-density energy storage device,polymer-based dielectric capacitors have a wide range of applications in industries such as new energy,pulse power,power electronics and high-voltage direct current transmission.The current commercial polymer capacitor films have relatively small dielectric constants and low energy densities,which often occupy a large volume in the systems.On the other hand,with the development of electrical insulation technology towards high temperatures,the operating temperature requirements for dielectric capacitors are becoming higher and higher.Therefore,the study of high temperature,high energy density dielectric has crucial scientific research significance and practical application value to realize the miniaturization,light weight and integration of energy-storage capacitors in pulse power and power electronic systems.This thesis focuses on high temperature and high energy density nanocomposite dielectrics.Using high temperature-resistant thermoplastic polyetherimide（PEI）as the matrix,a series of PEI-based nanocomposites are prepared through the solution casting method.Firstly,the influence of the bandgap and dielectric constant of the nanofillers on the thermal,mechanical and electrical properties of the PEI-based nanocomposites is studied.Then,the mechanism of introducing nanofillers on the thermal stability of dielectric energy storage properties for the PEI-based nanocomposite is analyzed.Afterward,the core-shell structure design of the nanofiller is applied to study its regulating effect on the organic-inorganic interface in the PEI-based nanocomposite,as well as the mechanism of suppressing high-temperature conductance and improving electrical strength and energy density.Finally,based on the finite element calculation method and the bipolar charge carrier transport model,the electric field distribution and conduction process of the core-shell nanocomposite are verified theoretically.The main conclusions and achievements of the thesis are as follows:（1）To study the influence of the bandgap and dielectric constant of nanofillers on the energy storage characteristics of PEI-based nanocomposite,SiO₂,ZrO₂ and TiO₂nanoparticles with significant differences in bandgap and dielectric constant are selected as fillers.A series of uniform and dense PEI-based nanocomposites is obtained by the solution casting method.The dielectric constant of the PEI-based nanocomposite is in sequence of PEI/SiO₂<PEI/ZrO₂<PEI/TiO₂,which is consistent with the trend of the dielectric constant of the nanofillers.The breakdown strength of the nanocomposite is relevant to the properties of the nanofiller,and the breakdown strength of the three kinds of PEI-based nanocomposites are higher than that of neat PEI.It is found that the ZrO₂ nanofiller combines a wide band gap and a moderate dielectric constant,favoring the resultant nanocomposite with excellent energy storage performance.The maximum discharged energy density of PEI/3 vol%ZrO₂nanocomposite is 6.15 J/cm³ at room temperature,which is 57.7%higher than that of neat PEI.（2）From the three kinds of PEI-based nanocomposite discussed above,the optimal nanosystem（PEI/ZrO₂）is selected as the research object.The influence of temperature on the performances,including dielectric property,breakdown strength,discharged energy density and charge–discharge efficiency,of the PEI-based nanocomposites are investigated.As the temperature increases,the dielectric constant of the nanocomposite remains stable,and the dielectric loss increases slightly.The breakdown strength of the nanocomposite gradually decreases but is always higher than that of neat PEI.The addition of ZrO₂ nanoparticles introduces deep traps with an activation energy of 1.60e V at the organic-inorganic interface,which plays the role in preventing carrier transport and inhibiting electrical conduction under high temperatures and high electric fields.Therefore,the deterioration rates of the discharged energy density and charge–discharge efficiency are decreased with increasing temperature.The maximum discharged energy density of PEI/3 vol%ZrO₂nanocomposite is 3.30 J/cm³ at a high temperature of 150°C,which is superior to commercial high-temperature dielectric polymers.（3）The ZrO₂ nanoparticle is wrapped with a wide-bandgap Al₂O₃ shell layer,which regulates the organic-inorganic interface in the PEI-based nanocomposites.As a result,a dielectric constant gradient transition in the nanocomposite is established.The dielectric mismatch between the ZrO₂ nanoparticles and the PEI matrix is alleviated;thus further enhancement in the high-temperature energy storage characteristics of the PEI-based nanocomposite is achieved.The highly insulating Al₂O₃ coating effectively suppresses electric conduction of the PEI-based nanocomposite at high electric fields,making the PEI/Al₂O₃@ZrO₂ nanocomposite superior to uncoated PEI/ZrO₂nanocomposite in terms of dielectric property,breakdown strength,electrical resistivity,discharged energy density and charge–discharge efficiency,and far surpasses neat PEI.When the Al₂O₃@ZrO₂ filler content is 11 vol%,the breakdown strength of the nanocomposite is 585 MV/m at 150°C,and the maximum discharged energy density can reach 5.19 J/cm³.（4）Combining the finite element simulation method and the bipolar carrier transport model,the electric field distribution and charge transport process in the PEI/Al₂O₃@ZrO₂ nanocomposites are studied,revealing the functions of the core-shell structure on regulating the breakdown strength,high field conductivity,charge trapping behavior and capacitive energy storage characteristics of the nanocomposites.The establishment of a dielectric constant gradient from 25 of ZrO₂ to 8.3 of Al₂O₃ to 3.2 of PEI alleviates the excessive concentration of the electric field around the nanofiller.The maximum electric field strength in the dielectric is reduced by 15.5%compared with the uncoated PEI/ZrO₂ nanocomposite,which improves electric field tolerance of the PEI/Al₂O₃@ZrO₂ nanocomposite.The wide bandgap and high insulating characteristics of the inorganic Al₂O₃ shell make the charge carrier mobility of PEI/Al₂O₃@ZrO₂nanocomposite dropped by 74.1%compared to uncoated PEI/ZrO₂ nanocomposite.The core-shell structured nanofiller introduces more and deeper traps at the organic-inorganic interface.Accordingly,the charge injected from the electrode is limited to a small area near the electrode,which prevents the charges from dispersing deeply into the dielectric film.