Effects of film processing conditions on electric energy storage for pulsed power applications

Processing conditions have significant influence on the crystalline morphology in poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF-HFP)] copolymer films and in turn determine the electric energy storage in the final products. The electric energy storage and discharge behaviors in these P(VDF-HFP) films obtained from different processing conditions were studied by electric displacement (D) - electric field (E) loop measurements. Under the same mechanical stretching conditions in terms of stretching ratio and stretching rate, more α-crystals could be transformed into β-crystals when stretching at a lower temperature than at a higher temperature. As a result, strong coupling interactions among ferroelectric domains were induced in the films stretched at low temperatures due to a high β-crystal content. The strong coupling interactions facilitated the dipole switching behavior under the forward poling field and prevented the oriented dipoles from switching to the random (or antiferroelectric-like) state upon the reverse poling. Therefore, more electric energy was stored but less was discharged for films with a high β-crystal content. If the crystalline morphology with a high β-crystal content can be further modified to help the discharge process, a polymer capacitor film with high energy densities can be achieved.