The Preparation Of Flake-like Alumina Nanoparticles And Application Research In Dielectric Composites

Based on the application needs of organic-inorganic dielectric composites,this thesis starting from the synthesis of flake alumina nanopowders（Al₂O₃ NSs）as inorganic fillers and using polyetherimide（PEI）as an organic matrix,systematically carries out experimental studies and finite element simulations to improve the breakdown strength and energy density of Al₂O₃ NSs/PEI composites and explains the performance enhancement mechanism of the composite.The specific research work in this thesis is as follows.The effect of hydrothermal temperature on the particle morphology and hydrothermal time on the breakdown field strength of the composites was investigated.When the hydrothermal temperature was increased from 140°C to 180°C,the shape of the Al₂O₃ powder gradually changed to a regular flake shape with a transverse dimension of about 50 nm and a thickness of about 10 nm,and when the temperature was increased to200°C,the synthesized Al₂O₃ powder agglomerated severely.When the hydrothermal time was increased from 16 h to 28 h,the synthesized Al₂O₃powder was added to PEI to prepare a composite material and the breakdown field was studied.It was found that when the hydrothermal time of the powder was increased from 16 h to 24 h,the breakdown field of the composite material at 150°C increased continuously and continued to increase to 28 h,the breakdown strength decreased.The Al₂O₃ NSs/PEI composite was prepared by solution casting using the above-mentioned Al₂O₃ NSs as the inorganic phase and PEI as the organic phase,and the effect of the addition of Al₂O₃ NSs on the breakdown strength and energy density of the composite was investigated.The breakdown strength and discharge energy density of the composites reached 693 k V/mm and 7.33 J/cm³ at 25°C,which were26.2%and 133%higher than those of pure PEI,and the breakdown field strength and discharge energy density at 150°C reached 565 k V/mm and5.2 J/cm³,which were 29.8%and 168%higher than those of pure PEI.The increase in breakdown strength and discharge energy density of the composites can be attributed to the wide band gap and lamellar morphology of the Al₂O₃ NSs being able to create deep traps within the composites,trapping free charges and blocking the development of electrical dendrites.Based on these findings,surface modification of Al₂O₃ NSs was carried out to improve the interfacial compatibility of the composites in order to further increase the energy density of the composites at high temperatures.Three modifiers,3-aminopropyltriethoxysilane（KH550）,sodium dodecylbenzene sulfonate（SDBS）and polyvinylpyrrolidone（PVP）,were investigated separately.It was found that PVP had a better effect on the modification of Al₂O₃ NSs,and the energy storage density of the composites reached 6.33 J/cm³ at 150°C,compared to the composites with unmodified Al₂O₃ NSs added.The energy storage density of the composite at 150°C reached 6.33 J/cm³,which is 20%higher than that of the composite without Al₂O₃ NSs.The increase in energy storage density at high temperature was attributed to the improved interfacial compatibility between Al₂O₃ NSs and PEI through PVP,which enhanced the interfacial bonding force at high temperature.Finally,finite element method simulations were used to investigate the effects of electric field strength and charge density distribution inside the Al₂O₃ NSs/PEI composites.The simulation results show that the addition of Al₂O₃ NSs can effectively reduce the electric field strength inside the composite and thus increase its breakdown strength,and that the electric field strength inside the composite can be minimized when the Al₂O₃ NSs spacing is kept within a certain range.By coupling the current field with the temperature field and simulating the temperature gradient inside the composite,it was found that the Al₂O₃ NSs acted as a heat scattering centre in the composite and could transfer the Joule heat generated during the charging and discharging process in time.