Dielectric Properties Of Crosslinked Polyetherimide And The Corresponding Nanocomposites

Dielectric materials play a crucial role in the modern electronic industry and advanced energy storage applications.In comparison with ceramic dielectric materials,polymer dielectric materials are more applicable for energy storage capacitor applications owning to their unique features,including high electric breakdown strength,low dielectric loss,excellent flexibility and easy-processing.Hence,more and more researchers are focus on polymer dielectric materials.However,most polymer dielectric materials are limited to their relatively low operate temperatures,and fail to meet the further development of numerous applications such as hybrid electric vehicles and aerospace power conditioning,in which the devices always have to work at elevated temperatures.Therefore,it is urgent to develop high temperature polymer dielectric materials with good thermal stability,dielectric properties and energy storage properties in order to meet the demand for high-temperature applications.Polyetherimide（PEI）is supposed to be the most promising candidate of polymer dielectric materials for high-temperature energy storage applications.However,the movement of local molecular segments of polymer will causeβ-relaxation,which is detrimental to the dielectric properties of PEI.In this thesis,a high-performance crosslinked polyetherimide dielectric film was obtained by introducing phenylethynyl group into PEI molecular chain,where the phenylethynyl group was used as the crosslinking point for crosslinking reaction.The influences of crosslinking conditions and the degree of crosslinking on the dielectric energy storage performance of the crosslinked polyetherimide dielectric film were systematically studied.Further,aluminium oxide（Al₂O₃）nanoparticles were introduced into polymer to prepare nanocomposite dielectric films and the effects of filler content on dielectric properties of composite films were studied.The main research contents are as follows:1.A series of self-crosslinkable oligomers were prepared by combining the advantages of PEI and phenylethynyl group and the crosslinked polyetherimide dielectric films were obtained by further heat treatment.Crosslinked polymer dielectric films with enhanced voltage resistance and low leakage current density are obtained by crosslinking under oxygen.The occurrence ofβ-relaxation is diminished by crosslinking that makes polymer dielectrics exhibit the desirable dielectric stability over a broad temperature and frequency range.Particularly,the dielectric loss of c-10%PEPA-PEI at 1000 Hz is 0.0037 and 0.0043 at room temperature and 150°C,respectively.Simultaneously,the polymer dielectric maintains a still low dielectric loss（<0.01）between 10² Hz and 10⁶ Hz.Furthermore,c-10%PEPA-PEI possesses excellent high-temperature energy storage performance owing to the best crosslinking structure originating from proper chain length and exhibits ultrahigh charge-discharge efficiency（>95%）and improved energy density（3.60 J/cm³）at 150°C.2.By in-situ polymerization,nanocomposite films including Al₂O₃nanoparticles with different volume fractions were prepared.The c-0.3 vol%NPs@10%PEPA-PEI has the highest dielectric constant（3.49）and low dielectric loss（0.0043）.The increase of dielectric constant is not only because of the introduction of nanofillers,but also due to the fact that the nanoparticles with proper contents caused a slight increase in the interchain spacing,thereby reducing the barriers for polarization of dipoles in the polymer chains along the applied electric field.In addition,c-0.3 vol%NPs@10%PEPA-PEI displayed the enhanced discharge energy density（4.23 J/cm³）and high charge and discharge efficiency（93.4%）at 150^oC.In conclusion,both the crosslinked polyetherimide films and the nanocomposite films process improved energy storage density and ultrahigh charge-discharge efficiency at 150^oC.Moreover,they exhibit the excellent dielectric stability at 200^oC.Hence,these crosslinked dielectric polymer films with excellent dielectric performance have a promising future in the development of high-temperature and energy storage applications.