Evolution of dielectric relaxation under elevated electric field of polypropylene-based films

Extensive research has been performed on polypropylene (PP) based films focusing on the improvement of energy storage performance for power capacitor application. The increase of energy storage density was often accompanied with the rising of dielectric losses, especially under high electric field. The gap of electric field between the laboratorial level (0.006 ∼ 0.02 kV mm⁻¹) and practical working condition of capacitor (more than 50 kV mm⁻¹) could be thousands of times, which was often neglected in most research. In the present paper, the systematical study of the dielectric relaxation processes on elevated electric field was carried out on neat PP and three typical kinds of PP-based films, i.e. PP/ZrO₂ nanocomposite, PP/maleic anhydride grafted polypropylene (PP-g-MAH) blends, and PP/PP-g-MAH/ZrO₂ ternary nanocomposite with the energy storage density 17.3% higher than neat PP. The frequency dependence of dielectric losses under elevated field showed that even under 20 kV mm⁻¹, the loss of ternary PP increased little, while the losses of the binary sample increased much more. The space charge test as well as the DC conductivity test revealed the origin of increased dielectric loss, which was the consequence of extrinsic ion hopping stimulated by the elevated field. Meanwhile, in the ternary PP/PP-g-MAH/ZrO₂ nanocomposite, the activation energy of ion hopping was greatly improved due to the enhanced interaction between molecules, so the dielectric loss under high field, the space charge accumulation and the DC conductivity of the ternary sample all showed an extremely low level. This study has directive significance for the application of nanocompositse. (paper)