Preparation And Energy Storage Properties Of High-temperaure Resistant And Low-loss Polyimide-based Dielectric Composite Films

As a physical energy storage device,polymer film capacitors have the advantages of high breakdown strength,low loss,easy processing and low cost.It plays an important role in the fields of new energy and flexible DC transmission systems,pulsed power supplies,electric vehicles,and electromagnetic energy equipment.However,polymer dielectric materials generally exhibit a sharp increase in dielectric loss and a sharp decrease in breakdown strength under high temperature conditions,which leads to a decrease in the reliability of capacitors.With the technological breakthrough and rapid development of the third-generation wide-bandgap and high-temperature power devices represented by Si C and Ga N,high-temperature-resistant film capacitors have become the main technical bottleneck on the road to compact and efficient electrical equipment.Therefore,the development of high temperature resistant polymer dielectric materials with low loss and high breakdown strength is of great significance for realizing high thermal stability,compactness and high efficiency of electrical equipment.In this paper,pyromellitic anhydride-4,4’-diaminodiphenyl ether（PMDA-ODA）type polyimide（PI）with excellent heat resistance was selected as the matrix material.By means of surface atomic layer deposition of wide-bandgap aluminum oxide（Al₂O₃）film,nano-doped wide-bandgap magnesium oxide（MgO）filler,and blending of wide-bandgap fluorine-containing PI,a series of PI dielectric composite film with low loss,high breakdown strength and high energy storage density at high temperature was successfully prepared.The main research contents and results are as follows:（1）Four Al₂O₃films with different thicknesses of 50 nm,100 nm,130 nm and150 nm were deposited on the surface of PI films by atomic layer deposition（Al₂O₃/PI/Al₂O₃）,and the microstructure and dielectric energy storage properties of the composite films were investigated.The results show that the deposited Al₂O₃film has an amorphous structure.Since the Al₂O₃thin film layer effectively inhibits the injection of electrons from the electrode,the conductivity loss of the Al₂O₃/PI/Al₂O₃nanocomposite thin film is suppressed,thereby achieving the effect of increasing the breakdown strength.For example,when the deposition thickness is 130 nm,the leakage current reaches a minimum value of 1.2×10^-10A cm^-2,which is 79.7%lower than that of pristine PI;and the breakdown strength is 440 MV m^-1at 150°C.The maximum discharge energy density is 2.25 J cm^-3,which is 2.5 times that of pristine PI film.（2）A series of nanocomposite films（MgO/PI）with different MgO contents were prepared by solution casting method,and the microstructure and dielectric energy storage properties of the composite films were studied.Through scanning electron microscope（SEM）analysis,it can be seen that the MgO filler has good dispersion in the matrix,and no obvious agglomeration is observed.Since lower XRD peak position corresponds to larger interchain spacing,it is suggested that incorporation of the MgO nanofiller can enlarge the interchain spacing of PI,especially at ultralow content.That’s because the extended interchain spacing can enable easier dipole reorientation to the applied field and leads to a higher dielectric constant while maintaining low dielectric loss.For example,when the MgO content is 0.1 vol%,the dielectric constant of the MgO/PI nanocomposite film reaches a maximum of 4.12,and its leakage current density is 1.6×10^-10A cm^-2,which is 72.9%lower than that of pristine PI;and its breakdown strength is 459 MV m^-1at 150℃,and the maximum discharge energy density is 2.6 J cm^-3,which is 3 times that of pristine PI.（3）A series of composite films（F-PI/PI）were prepared by adding different mass fractions of fluorine-containing polyimide（F-PI）into PI.X-ray diffraction（XRD）analysis showed that the introduction of F-PI into the composites significantly increased the free volume inside the PI,no obvious defects inside the composite films were observed from scanning electron microscopy（SEM）,and the two polymers had good compatibility.The experimental data show that when the F-PI content is 30wt%,the leakage current density of the F-PI/PI composite film is lower than 1.0×10^-10A cm^-2,which is 83%lower than that of pristine PI.The breakdown strength can reach 663 MV m^-1,and the maximum discharge energy density can reach 4.25 J cm^-3,which is 4.77 times that of pristine PI film.