By using different preparation and processing methods, poly(vinylidene fluoride-co-hexafluoropropylene) [P(VDF−HFP)] films with different crystal orientations were fabricated. Anisotropic dielectric properties and different electric energy storages were observed in these films. When the PVDF crystals in a film oriented with their c-axes perpendicular to the applied electric field, they exhibited large polarizability because the CF2 dipole moments were randomly distributed in a plane parallel to the electric field. As a result, high dielectric constant and high electric energy density were achieved. On the contrary, when the crystal c-axes in a film oriented parallel to the electric field (or the CF2 dipole moments perpendicular to the electric field), polarization became difficult. Consequently, low dielectric constant and low electric energy density resulted. The anisotropic polarizability was also displayed at high electric fields as evidenced by the difference in the remnant/maximum polarization and the dipole switching field for different crystal orientations. These results provide us a guidance to achieve optimal crystalline morphology in PVDF random copolymer films for high electric energy storage applications.
Poly(vinylidene fluoride) (PVDF) and poly(VDF-co-hexafluoropropylene) [P(VDF-HFP)] films having different polymorphisms and crystallite sizes but a similar crystal orientation (i.e., c-axes parallel to the film surface) were prepared by different film processing methods. Effects of polymorphism and crystallite size on the dipole reorientation behavior and electric energy storage/release were studied by electric displacement-electric field (D-E) loop measurements. Experimental results suggested that coupling interactions among ferroelectric domains, which could be adjusted by different polymorphisms and/or crystallite sizes, determined dipole reorientation/switching behaviors. Note that the ferroelectric domain coupling is realized via induced compensation polarizations from the media (either amorphous or crystalline PVDF) between aligned ferroelectric domains. A high β rather than R/δ content and a large crystallite size facilitated the coupling interactions among ferroelectric domains, and thus dipoles in highly coupled ferroelectric domains could be easily polarized, resulting in a high dielectric constant and a high stored energy density. However, strong coupling interactions impeded an easy dipole reversal to the so-called antiferroelectric-like (or random) state and thus reduced the discharged electric energy due to a high remanent polarization. Instead, the film with a high β content and a small crystallite size showed the highest discharged electric energy density, suggesting that the ferroelectric domain coupling could be weakened by confining them in nanoscale crystallites. These findings provide us useful guidance to achieve optimal crystalline morphology in PVDF copolymer films for high electric energy storage applications.
The synthesis and characterization of poly(phthalazinone ether ketone) (PPEK) for high-temperature electric energy storage applications is described. It was found that PPEK displayed excellent stability of the dielectric properties over a broad frequency and temperature range. Little change in the breakdown field and discharge time has been observed in PPEK with the increase of temperature up to 190 degrees C. A linear correlation between the AC conductance and the angular frequency implied that the hopping as a dominant conduction process contributed to the dielectric loss. Superior energy densities, remarkable breakdown strengths, and fast discharge speeds have been demonstrated in PPEK at various temperatures.
The dielectric constant and loss of poly(ether ketone ketone) have been investigated over a range of frequency and temperature. A linear correlation between the ac conductance and the angular frequency implies that the hopping as a dominant conduction process contributes to the dielectric loss. The energy density and discharged efficiency of the polymer were measured at various temperatures. High dielectric strengths (>400 MV/m) at temperatures up to 150 °C have been achieved. The increase of the dielectric loss and reduction in the breakdown field with increasing temperature could be attributed to thermally enhanced conduction of charge carriers in the polymer.
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