Heat Resisting Polyetherimide Dielectric And The Corresponding High Temperature Energy Storage Performance

Polymer dielectric materials,as energy storage and conversion devices are widely used in high pulse electronic power systems,owing to light weight,easy processing,high power density,and high reliability.However,the state-of-the-art commercial polymer dielectric films,biaxially oriented polypropylene(BOPP),exhibit excellent room-temperature breakdown strength,but the operating temperature is only 105 °C,as the temperature continues to rise,the internal leakage current density of the material rises rapidly,resulting in a sharp drop in charge-discharge efficiency and discharged energy density.With the development of aerospace,electric vehicles,oil and gas exploration,etc.,higher energy storage density and temperature resistance of capacitors are required.In this thesis,we are aiming to developing high temperature resistant dielectrics.The relationship between polymer cross-linked structure and capacitive performance and the synergistic optimization of the nanometer-size effect and the cross-linked confinement effect on capacitive performance are investigated.Besides,two-dimensional high thermal conductivity material is used to regulate the breakdown mechanism.A series of universal strategies for improving high temperature energy storage performance by modulating the crosslinked structure,dipole polarization and thermal conductivity are summarized.The main research contents are as follows:1.Energy storage performance optimization of cross-linkable polyetherimide films.Analysis of the gel content and molecular weight of soluble(linear or uncrosslinked)portions of the polymer after heat treatment(cross-linking)is conducive to obtain the relative contents of linear and three-dimensional network segments in the cross-linked polymer with different heat treatment conditions.In the initial stage of heat treatment,the molecular weight of the linear chain and the crosslinking degree of the crosslinked network increase simultaneously until the molecular weight reaches the maximum(c PEI320-2h),this process is beneficial to increasing the breakdown strength of polymer dielectrics.When the heat treatment time continues to be extended or the heat treatment temperature rises above a critical point,the molecular weight decreases due to chain scission,which may produce free radicals in the small molecular weight chain segments and thus destroying the high temperature energy storage properties.By controlling the heat treatment time and temperature to obtain similar gel contents and molecular weights of the soluble molecular chain,resulting in similar energy storage properties and exhibiting the " time-temperature superposition" characteristics.The most excellent discharge energy density is achieved when the gel content of the cross-linked system reaches about 35% and the molecular weight of the soluble molecular chain is about 6 × 104,and even at 150 °C,the discharged energy density exceeds 3.60 J/cm3 and the charge-discharge efficiency remains above95%.2.Optimization of dielectric properties of low content alumina/cross-linked polyetherimide composites.By studying the effect of different particle sizes(5 nm,20 nm,50 nm)of alumina with low filler content(≤1 vol%)on the dielectric properties of the cross-linked system,it was found that the dielectric constants of three particle sizes of alumina composites increased and then decreased with the increase in filler content,but the larger size of alumina,the higher amount of alumina filling was needed to reach the peak of the dielectric constant.The corresponding contents of maximum permittivity for three particle sizes(5 nm,20 nm and 50 nm)were 0.22 vol%,0.3vol% and 0.8 vol%,respectively.In addition,the smaller the size of the alumina nanoparticles,the larger the specific surface area,which is more conducive to dipole rotational polarization,and the more significant increase in the dielectric constant of the composite.In particular,c PEI5nm-0.22 achieves a maximum dielectric constant of3.67 at room temperature,increased by 26% compared with c PEI,with a dielectric loss of only 0.003.The introduction of nanoparticles could enhance the polarization response of polymer chain segments in their vicinity,which was first confirmed by AFM-IR,and the results combined with X-ray diffraction studies indicate that the nano-effect can effectively enhance the dielectric constant of cross-linked polymer composites.3.Effects of alumina nanoparticle content and size on the energy storage performance of composites.The high temperature energy storage performance of cross-linked polyetherimide composites with different particle sizes of alumina(5 nm,20 nm and 50 nm)at low filler levels(≤1 vol%)were investigated and found that the energy storage density of three composites increased and then decreased with increasing fillers,which was consistent with the change in dielectric constant.The addition of a small number of nanoparticles can improve the dielectric constant by reducing limitations on dipole rotation without causing defects,thus achieving superior high temperature energy storage performance.The smaller the size of nanoalumina and the larger the specific surface area lead to the higher the polarization of the composite under the electric field,thus c PEI5nm-0.22 composite can achieve a higher dielectric constant and maintain a lower leakage current density at a lower filling content,leading to a higher energy storage density and charge-discharge efficiency.Its energy srorage density and charge-discharge efficiency at 150 °C and500 MV/m are 4.58 J/cm3 and 94% respectively.And even at 200 °C and 300 MV/m,the energy storage density still reaches 1.70 J/cm3,with a charge-discharge efficiency of over 87%.4.High temperature energy storage performance of boron nitride nanosheet/cross-linked polyetherimide composites.Ultra-thin boron nitride nanosheets(BNNs)were prepared using liquid phase exfoliation technique and a series of highly thermally conductive boron nitride nanosheet/cross-linked polyetherimide composites were prepared.The effects of different contents of boron nitride on the thermal,dielectric and energy storage properties of the composites were systematically investigated.Benefiting from the good physical properties(high thermal conductivity,wide band gap)and special two-dimensional structure,ultra-thin boron nitride can prevent electrical breakdown and reduce the formation of conductive pathways,combined with timely heat conduction and diffusion,a increased breakdown strength and energy storage density can be obtained of the composites.c PEI-10 BNNs nanocomposite has the lowest leakage current density at150 °C,with a maximum discharged energy density of 4.57 J/cm3 and a chargedischarge efficiency of 94%.