Fangxiao Guan

Fast discharge speed in poly(vinylidene fluoride) graft copolymer dielectric films achieved by confined ferroelectricity

Although a high electric energy density was recently reported for defect-modified poly(vinylidene fluoride) (PVDF) copolymers at millisecond discharge, they usually deliver less energy at a high pulse rate. In this letter, a low loss polystyrene (PS) was grafted as side chains onto the P(VDF-chlorotrifluoroethylene) main chain. After PVDF crystallization, dielectric PS side chains were segregated to the crystalline-amorphous interface, forming a finite confinement layer for ferroelectric PVDF crystals. We speculated that less space charge was induced during electric poling because of the nanoscale confinement effect. Consequently, a fast discharge speed and a relatively high energy density were achieved.

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.