The Preparation Of Flake-like Magnesium-aluminum Hydrotalcite And Application Research In Dielectric Composites

This thesis mainly focuses on the experimental and simulation study of dielectric composites and their dielectric performances for the application at room-temperature and high-temperature requirements.Based on the preparation of flake-like magnesium-aluminum hydrotalcite(MgAl LDH)and interface design of composites,firstly,the dielectric properties of MgAl LDH/PVDF and Ni-MgAl LDH/PVDF at room-temperature are systematically investigated.The high-temperature dielectric properties of MgAl LDH/PEI dielectric composites with polyetherimide(PEI)as the matrix were further studied.Finally,the improvement mechanism of the dielectric properties of the composites is simulated and analyzed by using the finite element method.The MgAl LDH with different lateral dimensions were prepared by the hydrothermal method.It is found that when the hydrothermal temperature increases from 100? to 140?,the lateral dimension of the MgAl LDH increases gradually,which is about 90 nm,150 nm and 200 nm,respectively.Further increasing the temperature,the particle size is basically unchanged.The results show that the room-temperature breakdown strength of MgAl LDH/PVDF composites increases with the increase of particle size.The effect of 200 nm MgAl LDH content on the dielectric properties of the composites at room-temperature was systematically studied.The results show that when the content of MgAl LDH is 0.6 wt%,the breakdown strength and energy density of the composites reach 573 kV/mm and 13.6 J/cm3,respectively,which are 15%and 34.7%higher than pure PVDF.The increase of energy density is mainly attributed to the flake-like morphology,good insulation and medium dielectric constant of MgAl LDH.In addition,the ultra-low content of MgAl LDH results in fewer defects in the composites,which is beneficial to reduce the loss and increase the breakdown strength.Based on the design idea of making the interface of composites produce conductivity difference,the Ni-loaded MgAl LDH(Ni-MgAl LDH)was prepared by the impregnation reduction method,and the effect of Ni-MgAl LDH content on the energy storage performance of PVDF-based composites was studied.When the content is 0.6 wt%,the energy density of the composites is further increased,reaching 23.9 J/cm3,which is 1.76 times that of composites without Ni loading.The enhancement of energy storage performance is attributed to the Ni nanoparticles loaded on the surface of MgAl LDH.There is a large electrical conductivity difference between the Ni-MgAl LDH and the PVDF matrix,resulting in high interfacial polarization and high dielectric constant.The ultra-small Ni nanoparticles(1-2 nm)can produce local Coulomb blockade effect,which further improves the breakdown strength.The dielectric properties of MgAl LDH/PEI composites at 25?200?were studied.It was found that the dielectric constant is basically stable,while the breakdown strength decreases slightly at high temperatures.This may be due to the decrease of interfacial interaction force at high temperatures,which leads to the deterioration of interface compatibility and more likely to cause electric breakdown.Therefore,MgAl LDH was modified by polyvinylpyrrolidone(PVP).The results show that the dielectric constant and breakdown strength of the modified composites are improved simultaneously,and the decreasing amplitude of breakdown strength at high temperatures is also effectively controlled.At 150?,the highest energy density of the composite is 6.18 J/cm3,which is 16%and 114%higher than that of the unmodified composites and pure PEI,respectively.The finite element method is employed to simulate the dielectric properties of the MgAl LDH/PVDF composite system by establishing a geometric model of the parallel plate capacitor in the current field.It is found that adding larger size MgAl LDH to the composites is more conducive to homogenizing the internal electric field,which leads to the improvement of the breakdown performance.The charge density at the interface of the Ni-MgAl LDH/PVDF composite is significantly higher than that of the composite without Ni loading,so it has a higher polarization.