| Film capacitors based on polymer dielectrics are widely used in high pulse electronic power systems for energy storage and conversion due to their lightweight,easy processing,and high power density.In recent years,heat resisting polymers have attracted increasing attention with the rapid development of high-power,compact electronic devices and electronic power systems,which place higher demands on the long-term stable operation of film capacitors under high temperatures and high electric fields.However,it is reported that only improving the thermal stability of polymers cannot obtain dielectric materials that meet the requirements of existing high-temperature capacitor applications.Conduction losses caused by various conductivity mechanisms are the main reason for reducing the high-temperature energy storage performance of polymer dielectrics at elevated temperatures.Therefore,how to effectively suppress the multiple conduction losses of polymer dielectrics and thereby improve their high-temperature dielectric energy storage performance is the urgent problem to be solved.In this paper,all-organic composite dielectrics with high temperature resistance and high energy storage performance are fabricated by introducing polymer dots with high electron affinity and wide band gap into heat resisting polymers,and combining with ultraviolet irradiation to improve the difficulty of valence band electrons transition and create deep trap levels in polymers to suppress multiple conduction losses under elevated temperatures and high electric fields.The main research contents and results are as follows.1.A type of polymer dots（PD）with high electron affinity are synthesized by hydrothermal method using citric acid and ethylenediamine as precursors and introduced into heat resisting polymers to prepare all-organic composite dielectrics.The energy level schematic and thermally stimulated depolarization currents curves have shown that the polymer dots trap charges injected from metal electrodes by building trap energy levels in polymer dielectrics,thus suppressing the leakage current density and conduction loss of the dielectrics.The addition of polymer dots increases the breakdown strength of the composite dielectrics by 20%at 150°C and increases the discharge energy density to three times that of pure polymers,indicating that the introduction of high electron affinity polymer dots to construct trap energy levels is a simple,effective and universal method to improve the high temperature energy storage performance of polymer-based dielectric materials.2.The all-organic composite dielectrics prepared in previous work are modified by ultraviolet irradiation to widen the band gap and optimize the energy level structure of the inside polymer dots.It is found that the widened band gap of the polymer dots can deepen the energy level traps inside the dielectrics after irradiation,which will more effectively trap the charges and suppress the multiple conduction losses.Compared with the composite dielectrics before irradiation,the breakdown strength and energy storage performance of the composite dielectrics at 150°C are significantly improved after ultraviolet irradiation.In particular,the discharge energy density of PEI/PD-UV at 150°C is increased by 30%compared with that before irradiation,reaching 4.07 J cm-3at 550 MV m-1and showing excellent charge/discharge cycling stability,and more importantly,it still achieves high discharge energy density of 3.27 J cm-3at the elevated temperature of 200°C.3.Photo-sensitive poly（arylene ether sulfone）（p PAES）is prepared by introducing photosensitive group,allyl,as the cross-linking unit into poly（ether sulfone）,and the ultraviolet cross-linking all-organic composite dielectric p PAES/PD is prepared by introducing polymer dots into the polymer.The cross-linked composite dielectric p PAES/PD-UV would be obtained by ultraviolet irradiation of p PAES/PD while the band gap of the polymer dots inside would be widened at the same time.The establishment of the cross-linked structure limits the relaxation of the polymer molecular chain segments at high temperatures,while the polymer dots with widened band gap suppress the conduction loss more effectively,and the synergistic effect of these two effectively improves the energy storage performance of p PAES/PD-UV at high temperatures and high fields.The breakdown strength and energy storage density of p PAES/PD-UV at 150°C are 587 MV m-1and 5.02 J cm-3,respectively.More importantly,even at 200°C,the breakdown strength still achieves as high as 569 MV m-1,and the maximum energy storage density can be increased to more than 8 times that of the pure p PAES,reaching 4.10 J cm-3.4.A type of energy level optimized polymer dots PAPD with high electron affinity and wide band gap can be synthesized using phthalic acid and ethylenediamine as precursors.The all-organic composite dielectrics can be obtained by introducing PAPD into polyetherimide（PEI）and modified by ultraviolet irradiation.Due to the higher electron affinity and wider band gap,the introduction of PAPD into PEI would construct deep trap energy levels and improve the transition difficulty of valence band electrons.Therefore,compared to PEI composite dielectrics filled with PD,composite dielectrics filled with PAPD exhibit better high-temperature energy storage performance.The maximum discharge energy density of PEI/0.3wt%PAPD-UV at 200°C is 3.39 J cm-3and the discharge energy density reaches 2.64 J cm-3when the charge and discharge efficiency is greater than 90%.This paper focuses on the problem of rapid decline in the energy storage performance of polymer dielectrics under high temperatures and high electric fields.By introducing polymer dots with high electron affinity and wide band gap into heat resisting polymers,a kind of all-organic composite dielectrics are prepared.Combined with UV irradiation,the difficulty of valence band electrons transition is improved and deep trap levels are constructed,thereby suppressing the multiple conductivity losses and achieving significant improvement in high temperature energy storage performance.This work provides a new strategy and new material system for the design and development of high temperature and high energy storage density polymer dielectrics. |
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