In this paper, poly­(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)/polyimide (abbreviated as P­(VDF-TrFE-CFE)/PI) bilayer films are prepared by a casting method, where the PI with linear polarization characteristics is used as the bottom layer and P­(VDF-TrFE-CFE) with nonlinear ferroelectric polarization characteristics is used as the top layer. The microstructure, polarization behavior, and energy storage performances of the bilayer composite films have been systematically investigated. The results show that the interfaces between the P­(VDF-TrFE-CFE) layer and PI layer are uniform and continuous. On comparing with individual films, it is observed that the dielectric constant and breakdown field strength of bilayer films are higher than those of PI films, leading to the improved discharge energy density, and the residual polarization of bilayer films is effectively suppressed compared to that of P­(VDF-TrFE-CFE), resulting in the enhanced energy storage efficiency. Excellent energy storage performances have been obtained by regulating the volume content of PI in P­(VDF-TrFE-CFE)/PI bilayer films, which possess a discharge energy density of 9.6 J/cm3 and an energy storage efficiency of 58% with a PI content of 50 vol %. The results of this work indicate that constructing the ferroelectric/linear bilayer films is an effective way to improve the energy storage performances of flexible polymer-based capacitors.

In this work, polypropylene (PP) film samples doped with an organic phosphorus nucleating agent under three cooling processes are examined for the effects of regulating the crystallization. The conductivity and DC breakdown strength of the film samples were tested at 25, 55 and 85 °C. The average breakdown strength with 0.01 wt% nucleating agent increased by approximately 25% compared to un-nucleating samples and the DC conductivity decreased slightly. For the three cooling methods in these tests, the slow process increased the crystallinity of the film samples and stabilized the electrical properties of the PP samples. It is concluded that improving the insulation performance through crystallization control is feasible, and this method shows great potential for the modification of PP films.

No abstract

No abstract

In this paper, an improved method for the electric performance of polypropylene (PP) film was proposed to promote the safety and stability of power capacitors. Modified PP films containing three different polycyclic compounds were prepared, which showed good thermal properties and decreased DC conductivity. The DC breakdown strength of the modified PP films under both positive and negative voltage is increased compared with that of the original film. The deep traps introduced by polycyclic compounds and the decreased carrier mobility give an explanation of the decreased DC conductivity. A quantum chemistry calculation was further performed to clarify the mechanism for improving electrical performance, presenting that polycyclic compounds with a high electron affinity and low ionization energy can capture high-energy electrons, protecting the PP molecular chain from attack, and then increase the breakdown strength. It is concluded that the modified PP films by polycyclic compounds have great potential in improving the insulating properties of power capacitors.