High-dielectric-constant graphite oxide-polyimide composites as embedded dielectrics

Microstructure and dielectric performance of graphite oxide (GO)/polyimide (PI) composites were investigated. Graphite oxide was prepared by hummer method^[1] through which graphite flakes were oxidized by concentrated sulfuric acid, potassium permanganate and hydrogen peroxide. Polyimide was synthesized with pyromellitic dianhydride (PMDA) and 4,4-diamino-diphenyl ether (ODA) in N,N-dimethylformamide (DMF) solution. GO/PI composite film was fabricated via coating process. The experimental results showed that the dielectric constant ε_r increased gradually with GO content before reaching the percolation threshold. The value of dielectric constant ε_r was 68 at 1000 Hz for the composite containing 1wt% of GO, which was 19 times larger than that of the pure PI. The dielectric loss tanδ was 0.6 for the composite containing 1 wt% GO. When the GO content reached 2 wt%, both of the ε_r and tanδ values increased substantially, indicating formation of conducting pathway between GO particles. The results implied that when GO content was at a low level, the insulating PI served as electrons barrier layer between GO and prevented electrons from transferring from one GO to another under an external field. As a result, the measured ε_r value increased with GO content, while the tanδ remained low. However, when GO filler loading reached a higher level and was in the vicinity of a critical concentration, GO was so close to each other and the conducting pathway could be developed in PI, leading to remarkable growth of ε_r and tanδ.