Effect of nano-Al2O3 on the polarization of polyimide nanocomposite films with tri-layer structures

Polyimide (PI)/Aluminum oxide (Al₂O₃) nanocomposite films wtih tri-layer structures (PI-Al₂O₃/PI/PI-Al₂O₃) are manufactured from pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA) via in-situ polymerization method. Microstructure, dielectric property and electrical conductivity of the nanocomposite films are characterized in this paper. Transmission Electron Microscope (TEM) and X-Ray Diffraction (XRD) results show that nano-Al₂O₃ particles are successfully introduced into target tri-layered structure and are homogeneously dispersed in the polyimide matrix. Compared with pure films, the obtained composite films perform lower relative permittivity at low nano-Al₂O₃ doping content (0-8wt%) and higher relative permittivity at high nano-Al₂O₃ doping content (>8wt%). The dielectric loss (loss tangent) of the composite films is higher than that of the pure films at the same frequency, and increases consistently with the increase of nano-Al₂O₃ content, especially in low frequency range. Furthermore, at the same nano-Al₂O₃ concentration, the dielectric loss of the composite films reduces at frequencies <;103 Hz and increases at frequencies >103 Hz. The effect of nano-Al₂O₃ on electrical conductivity is only observed clearly at low frequency region (<;10Hz). The above-mentioned dielectric property depends on the interaction and compromise of two kinds of polarization, i.e., interfacial polarization and anti-orientation polarization, which are attributed to the interfacial defect effect and blocking effect of nano-Al₂O₃ particles, respectively.