NH2-functionalized carbon-coated Fe3O4 core–shell nanoparticles for in situ preparation of robust polyimide composite films with high dielectric constant, low dielectric loss, and high breakdown strength

Novel Fe3O4/polyimide (PI) composite films with extraordinary dielectric properties were prepared via in situ polymerization of PI in the presence of 4,4′-diaminodiphenyl ether (ODA), pyromellitic dianhydride (PMDA) and compatible amine-functionalized carbon-coated Fe3O4 nanoparticles (Fe3O4@C–NH2). Fe3O4 nanoparticles prepared via solvothermal reaction were coated with crosslinked glucose and then surface-grafted via carbodiimide coupling. The core–shell structure of Fe3O4@C–NH2 was thereafter obtained with amine functionalities on the carbonized-glucose shell. Homogeneous dispersion of Fe3O4@C–NH2 and strong interfacial covalent bonding on the shell surface contributed to high dielectric constants (e) and extremely low dielectric losses (tan δ) in the composite films. The highest dielectric constant of the composite films was 58.6 at 1 kHz when the Fe3O4@C–NH2 composition was 1.13 vol%, as well as a low dielectric loss of 0.0091 and a high breakdown strength of 126.5 ± 11.3 MV m−1 were observed without considerable sacrifice of the thermal stability and mechanical properties.