| Film capacitors have the advantages of high withstand voltage,no inductance,low loss,and long service life,and play a key role in high-voltage flexible DC transmission,electric vehicles,new energy grid-connected,pulsed power supplies and portable electronic products.At present,the operating temperature of commercial film capacitor materials is low,which does not match the actual operating ambient temperature of the capacitor.As the operating temperature of related equipment further increases,the requirements for the temperature stability of films for dielectric capacitors are more stringent.Polyetherimide has excellent temperature stability and high temperature energy storage performance,and is considered to be the most potential material to replace biaxially oriented polypropylene,but there are still many problems in application.First,the high sensitivity of polyetherimide to humidity is not conducive to its industrial application.Secondly,the high-temperature carrier injection and conduction will cause a sharp increase in the conductivity loss,resulting in the deterioration of the hightemperature energy storage performance of the polyetherimide film.Reducing the conduction loss of the dielectric at high temperature is the fundamental method to improve the high temperature energy storage performance of the dielectric.Exploring the carrier transport mechanism and proposing methods to effectively reduce the high temperature loss of the dielectric are of great significance for improving the high temperature(≥100°C)energy storage performance.In order to solve the problems of polyetherimide,a series of researches have been carried out in this work.First,by controlling the environmental humidity of the closed electrospinning chamber,combined with drying and hot-pressing processes,polyetherimide films with different environmental humidity effecting are prepared.Influence laws of microstructure,polarization,breakdown strength and energy storage properties.The study found that with the increase of ambient humidity,the relative dielectric constant of the polyetherimide film first decreased and then increased,and the maximum energy storage density of the breakdown strength increased first and then decreased.X-ray photoelectron spectroscopy shows that water molecules in the air leave a certain number of hydroxyl groups inside the polyetherimide film,and these hydroxyl groups have a certain hindering effect on the inversion of some polar groups in polyetherimide.This affects the polarization properties of the film.The scanning results of electron microscope and atomic force microscope show that the environmental humidity of electrospinning can affect the morphology of polyetherimide fibers obtained by electrospinning,and affect the microstructure of the surface of the film after hot pressing,thereby affecting the breakdown performance of the dielectric film.Through the above studies,the preparation environment humidity most favorable for the application of polyimide film energy storage is determined.Secondly,on the basis of above research,based on the frontier molecular orbital energy level of the polymer,a design method of fiber-reinforced structural composite dielectric is proposed.Coaxial nanofibers are prepared by coaxial electrospinning technology.The polymer polyimide with high electron affinity and high glass transition temperature mixed with organic semiconductors is used as the inner layer of the fiber.Polyetherimide with low electron affinity and glass transition temperature serves as the outer layer.The fiber mat is hot-pressed at a suitable hotpressing temperature to obtain a composite dielectric with high glass transition temperature polymer fibers continuously distributed in the polyetherimide matrix.The composite dielectrics with different polyimide contents are prepared by controlling the thrust ratio of the internal and external precursors,and the effect of the polyimide fiber content on the high-temperature energy storage performance of the dielectric is explored.The study found that with the increase of polyimide content,the high-temperature breakdown strength of the dielectric increased first and then decreased,and the breakdown strength is greatly improved compared with pristine polyetherimide.The fiber-reinforced structure can realize the meandering movement of carriers along the fiber,and improve the dielectric breakdown strength.However,when the fiber content is too high,the tortuosity of the low-carrier moving path is reduced,and this enhancement effect is reduced.On the basis of the above research,different contents of organic semiconductors with high electron affinity are further added into the inner layer of the fibers to study the influence of organic semiconductor contents on the high temperature energy storage properties of the composite dielectric.Finally,a composite dielectric with improved high-temperature breakdown strength and energy storage density is obtained,and the comprehensive optimization mechanism of the dielectric’s high-temperature energy storage performance is clarified.Finally,in order to further improve the high-temperature energy storage performance of the polyetherimide-based composite dielectric,an organicsemiconductor blended multilayer structure is prepared by electrospinning and hotpressing techniques.A series of composite dielectrics are prepared by changing the thickness of the blended layer and the volume ratio of organic semiconductors in the layer while keeping the total amount of organic semiconductors unchanged.The effects of the density of organic semiconductors in the blend layer and the position of the blend layer on the energy storage performance of the composite dielectric are investigated respectively.The results show that with the increase of the semiconductor density and the position of the blended layer moving toward the center,the high-temperature breakdown strength and energy storage density of the composite dielectric both increase first and then decrease.When the volume content of organic semiconductor in the blended layer is 0.5 vol%,and the distance between the blended layer and the electrode accounts for 28% of the total thickness of the dielectric,the composite dielectric has the most excellent high-temperature energy storage properties.Using the finite element simulation and the current-electric field test curve,the internal mechanism of the influence of the density of organic semiconductors and the position of the blend layer on the high temperature energy storage characteristics of the dielectric is analyzed,and the improvement mechanism of the high-temperature energy storage characteristics of the dielectric is clarified.This work explores the influence law of environmental humidity on the energy storage performance of dielectrics during the preparation of electrostatic spinning,and provides strong support for the large-scale preparation of high-performance polyetherimide capacitive films.The high-temperature energy storage performance of the proposed two new structural composite dielectrics based on the frontline molecular orbital energy level design of organic materials is a leading level in the field of energy storage dielectric research,which provides a new idea for the development of energy storage polymer-based composite dielectrics and has important theoretical value and practical significance for the rapid development of new high temperature energy storage materials. |
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