The Preparation And High-Temperature Capacitive Energy Storage Properties Of Polyimide-based Nanocomposites

The polymer-based dielectric capacitors exhibit high power density,fast charge and discharge speed,low loss,good flexibility and stability.It has become an extremely important energy storage device and plays a key role in power electronics and pulse power systems.However,the polymer-based dielectric materials deliver relatively lower energy density,and a large number of capacitors are often needed to meet the demand for high energy storage density of the capacitor in the energy storage field,which is not conducive to the miniaturization of energy storage devices.In addition,next-generation microelectronics and electrical power systems call for high-energy-density dielectric polymeric materials that can operate efficiently under elevated temperatures.However,the currently available polymer dielectrics are limited to relatively low working temperatures.Therefore,the development of dielectric materials with high breakdown,low loss,high energy storage density and high operating temperature is of great significance for improving the stability of dielectric capacitors and expanding their applications.This paper focuses on polymer-based dielectric nanocomposites with high temperature resistance and high energy storage density.Polyimide（PI）,which possesses the inherent advantages including much better processability,considerably low cost,great mechanical strength and flexibility,and high glass transition temperature,was selected as the polymer matrix.The high-temperature PI based dielectric thin films with high energy storage density were fabricated by adjusting the dielectric constant,band gap and morphology of the nanofillers.And the influence of nanofillers on the high-temperature capacitive performance of the polymer composites was systematically investigated.The main research conclusions are as follows:（1）Pyromellitic dianhydride（PMDA）,and 4,4-diaminodiphenyl ether（4,4’-ODA）were condensed by a two-step process to obtain flexible neat PI films with excellent thermal and dielectric stability.Ultra-thin boron nitride nanosheets（BNNS）were prepared by liquid phase exfoliation.The thermal stability,mechanical strength,dielectric properties and high-temperature capacitive performance of the PI composites were enhanced by the addition of BNNS.The much reduced leakage current and the increased activation energy account for the enhancement in charge-discharge efficiency and discharged energy density observed in the PI/BNNS nanocomposites.The PI nanocomposites with 5 vol%BNNS exhibits a breakdown strength of 418 MV/m and a discharged energy density of 1.93 J/cm³ at 150℃.Compared with commercial biaxially oriented polypropylene（BOPP）,the discharged energy density and power density at 200 MV/m of PI/BNNS at 150℃are 1.75 and 1.95 times higher than that of BOPP at 70℃,respectively.And the charge-discharge efficiency of PI//BNNS is up to94.4%,showing its huge application prospects.（2）A series of PI based dielectric composites were prepared from in situ polymerization in the presence of inorganic nanofillers with systematically varied dielectric constants and bandgap,including aluminium oxide（Al₂O₃）,hafnium dioxide（Hf O₂）and titanium dioxide（Ti O₂）.The investigation of high-temperature capacitive performance of the PI composites reveals the dominant role of the bandgap of the fillers in determining and improving the high-temperature capacitive performance of the polymer composites,and further its rationality was validated via the analysis of the conductivity mechanism.It’s found that the addition of nanofillers significantly enhances the breakdown strength（E_b）of the PI nanocomposites,and the systematic enhancement in the E_b of the PI nanocomposites coincides with the increasing trend of band gap of the nanofillers.The addition of the inorganic fillers with largeΔE significantly reduces conduction current,which yields the dielectric composites with high discharged energy denssity and great charge-discharge efficiency at high fields and elevated temperatures.The synergistic combination of a large band gap and a moderate dielectric constant in Al₂O₃ and Hf O₂ gives rise to the corresponding composites with superior capacitive performance.The maximum discharged energy densities of PI/Al₂O₃ and PI/Hf O₂are 2.48 J/cm³ and 2.21 J/cm³,respectively,which are enhanced by 202%and 168%relative to neat PI.（3）The one-dimensional Al₂O₃ nanowires and two-dimensional Al₂O₃ nanoplates were prepared by hydrothermal method.The systematic manipulation of the morphology of the Al₂O₃ from nanoparticles,nanowires to nanoplates has been specifically exploited to boost the elevated-temperature capacitive performance of the PI nanocomposites.A strong dependence of the conduction behavior and breakdown strength of the polymer composites on the filler morphology is thus revealed.The Al₂O₃ nanowires and nanoplates are more effective in hindering the dendrite growth of the breakdown phase due to the high aspect ratio,thus yielding the higher breakdown strength of the composites loaded with Al₂O₃ nanowires and nanplates when compared to the Al₂O₃ nanoparticles-based composites.The PI composites containing Al₂O₃nanoplates display the highest breakdown strength（436 MV/m）and discharged energy density（2.67 J/cm³）at 150℃,outperforming the state-of-the-art dielectric nanocomposites c-BCB/BNNS（421 MV/m and 2.42 J/cm³）.It is conclusively demonstrated that nanofillers have a significant influence on the high-temperature capacitive performance of the polymer composites at high fields.This work sheds light on the rational design of scalable high-performance high-temperature dielectric polymer composites and reveals great potential for the practical use of dielectric polymeric materials in power electronics and electrical systems under extreme environments.