Structure Design And Properties Investigation Of Pp-based High Energy Storage Materials

With the rapid development of electronic information technology,film capacitors play an important role in high energy density electric power and electronic devices.Compared to dielectric ceramics,polymer-based dielectric materials possess the superior advantages of high breakdown strength,great reliability,mechanical flexibility and low cost.The representative polymer-based dielectric materials are polypropylene(PP),and polyvinylidene fluoride(PVDF).Biaxially oriented polypropylene(BOPP)is in a dominant position in commercial applications because of its excellent mechanical properties,ultra-low dielectric loss and high breakdown strength.However,due to the low dielectric constant of BOPP,its energy storage density is only about 2 J/cm~3under the electric field of 350 MV/m,which seriously limits its application.Some researchers have made great efforts to improve the energy storage density of PP and achieved many valuable results,but there are still some problems to be solved:For example,as a polymer with low polarity,PP has a low dielectric constant,which limits the improvement of its energy storage density,so it is necessary to adjust the structure to improve the polarity;Inorganic particles are easy to aggregate when added into PP,so special interface design is needed to solve the interface compatibility between PP and inorganic particles.To address the existing problems,this paper regulates the chemical structure of PP by high-energy electron beam irradiation and the introduction of polar maleic anhydride(MAH)to increase the polarity of PP composites;and constructs different composite interfaces by surface organic modification of inorganic materials and designing core-shell structures to increase the interfacial compatibility between matrix and dispersed phases and reduce interfacial defects.The effects of matrix modulation and interface design on the comprehensive performance of PP-based dielectric materials such as dielectric properties and energy storage properties are investigated in depth.The specific studies are as follows.(1)Electron beam irradiation was used to modulate the polarity and chemical structure of PP to improve the dielectric and energy storage properties of PP films.Electron beam irradiation decreased the crystallinity and crystal size of PP.Besides,irradiation improved thermal stability of PP.Oxidation reaction between the reactive free radicals and oxygen formed the polar C=O,COOH,OH groups on the PP chains during the irradiation process,which enhanced the polarity of PP.The dielectric constant and breakdown strength of irradiated PP with 30 k Gy dose reached 3.98 and 430 MV/m,respectively,which are 1.4and 1.3 times higher than those of ordinary PP.The PP-30 k Gy film achieved a discharged energy density of 3.63 J/cm~3which was 279%higher than that of the ordinary PP(1.3J/cm~3)and the charge-discharge efficiency was higher than 95%.Therefore,electron beam irradiation technology provides positive results to obtain PP films with high energy density and the obtained PP dielectric films behave potential application prospects as thin film capacitor materials.Irradiated PP shows great potential in solving the contradiction between dielectric constant and breakdown strength.(2)Amino functionalized barium titanate(BT)was added to the polymeric matrix PP to increase the dielectric constant and energy storage properties of PP nanocomposites.Polypropylene-graft-maleic anhydride(PP-g-MAH)was introduced as an interfacial compatibilizer to increase the interfacial compatibility between PP and BT,reduce interfacial defects and promote the uniform dispersion of BT in PP matrix.PP/PP-g-MAH/BT nanocomposites were prepared by melt blending.The dielectric constant of PP/PP-g-MAH/BT nanocomposite with 10%BT content can reach 3.47 at 1000Hz frequency,and the discharge energy density of the nanocomposites is 1.81 J/cm~3at 300MV/m electric field strength.PP/PP-g-MAH/BT nanocomposites always maintain high charge discharge efficiency and good cycle stability.(3)Glycidyl methacrylate grafted ethylene-octene copolymer(POE-GMA)@BT/PP nanocomposites with excellent energy storage properties was prepared by melt blending method.The addition of POE-GMA@BT with soft-shell/hard-core core-shell structure to PP matrix improves the crystallinity and thermal stability of PP.The shell structure POE-GMA not only improves the dispersion and compatibility between BT nanoparticles and PP substrate,but also reduces the electric field concentration caused by interfacial defects.POE-GMA@BT/PP nanocomposite with an optimized BT content of 0.4 wt%offers a high dielectric constant of 2.94,a very low dielectric loss of 0.0005,a high breakdown strength of 460 MV/m,an excited discharged energy density of 3.31 J/cm~3,and excellent efficiency of 97.9%.(4)A one-step melt blending method was utilized to prepare PP-based nanocomposites to improve the energy storage performance of PP.PP-g-MAH was used as a polar compatibilizer to promote the uniform dispersion of organic nano-montmorillonite(org-MMT)in the PP matrix.Furthermore,the introduction of PP-g-MAH improved the dielectric constant of PP nanocomposites and increased charge traps.The org-MMT enhanced the crystallinity and reduced the crystal size of PP.On the other hand,the layered structure of org-MMT inhibited electric tree branching and growth.The PP-based nanocomposites possess a relatively high dielectric constant of 3.35 and an extremely low dielectric loss of 0.0012 at 1000 Hz.The PP/PP-g-MAH/org-MMT nanocomposite with an optimized org-MMT content(0.2wt%)exhibited a discharged energy density of 5.2 J/cm~3at the electric field of 500 MV/m with a superior charge-discharge efficiency of 93.5%,which were favorable for its application as film capacitor materials.(5)In order to simultaneously improve the dielectric constant and breakdown strength of PP,PP functionalized with polar maleic anhydride(PP-g-MAH)was used to replace PP as the matrix material.The org-MMT/PP-g-MAH nanocomposites were prepared to improve the energy storage performance.PP-g-MAH was chosen as the dielectric polymer matrix for the first time considering that it has the same main chain as polypropylene and higher polarity because of the grafted maleic anhydride side group.In addition,org-MMT was used as the 2D nanofiller to enhance the energy storage properties.On the one hand,the polar maleic anhydride groups on the side chain enhance the dielectric constant of PP due to the improved interfacial polarization;on the other hand,the polar anhydride group of PP-g-MAH can interact with org-MMT,leading to fewer interface defects,lower dielectric loss,deeper traps,and higher breakdown strength of PP-g-MAH nanocomposites.With the addition of only 0.4 wt%of organic montmorillonite,the discharge energy density of PP-g-MAH nanocomposites can reach 5.21 J/cm~3and the charge/discharge efficiency is over 94.9%,which is a PP-based nanocomposite with excellent comprehensive performance.