Ferroelectric behavior in the high temperature paraelectric phase in a poly(vinylidene fluoride-co-trifluoroethylene) random copolymer

Suppressing the Loss of Polymer-Based Dielectrics for High Power Energy Storage

Polymer-based dielectrics have received intensive interest from academic community in the field of high-power energy storage owing to their superior flexibility and fast charge-discharge ability. Recently, how to suppress the loss of polymer-based dielectrics has been increasingly recognized as a critical point to attain a high charge-discharge efficiency in the film capacitors. Some achievements are made in analyzing the source of loss and suppressing loss via Edison's trial and error method. In this review, the significance of suppressing loss in polymer-based dielectrics is firstly emphasized. Then, different sources of loss are discussed carefully and an in-depth analysis of the related measurements is presented. Next, recent research results in suppressing loss are summarized and discussed in detail according to different strategies. Finally, the challenges and opportunities in the loss suppression research for the rational design of high-efficiency polymer-based dielectrics are proposed.

Multi-scale characterisation of a ferroelectric polymer reveals the emergence of a morphological phase transition driven by temperature

Ferroelectric materials exhibit a phase transition to a paraelectric state driven by temperature - called the Curie transition. In conventional ferroelectrics, the Curie transition is caused by a change in crystal symmetry, while the material itself remains a continuous three-dimensional solid crystal. However, ferroelectric polymers behave differently. Polymeric materials are typically of semi-crystalline nature, meaning that they are an intermixture of crystalline and amorphous regions. Here, we demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) strongly affects its Curie transition, as not only a change in crystal symmetry but also in morphology occurs. We demonstrate, by high-resolution nanomechanical measurements, that the semi-crystalline microstructure in the paraelectric state is formed by crystalline domains embedded into a softer amorphous phase. Using in situ X-ray diffraction measurements, we show that the local electromechanical response of the crystalline domains is counterbalanced by the amorphous phase, effectively masking its macroscopic effect. Our quantitative multi-scale characterisations unite the nano- and macroscopic material properties of the ferroelectric polymer P(VDF-TrFE) through its semi-crystalline nature.

Ferroelectric polymeric materials possess intermixture of crystalline and amorphous regions with complex Curie transition. Here, the authors demonstrate that the semi-crystalline morphology of the ferroelectric copolymer of P(VDF-TrFE) strongly affects its Curie transition.

Structural Tailing and Pyroelectric Energy Harvesting of P(VDF-TrFE) and P(VDF-TrFE-CTFE) Ferroelectric Polymer Blends

The copolymer P(VDF-TrFE) is a normal ferroelectric because the bulky TrFE monomer improves its crystalline chain structure, while the terpolymer P(VDF-TrFE-CTFE) is a relaxor ferroelectric because the third monomer CTFE makes it amorphous. Herein, in order to induce a crystalline beta phase in the terpolymer, we blended a small amount of crystalline P(VDF-TrFE) into P(VDF-TrFE-CTFE) and investigated the effect of blending on the pyroelectric energy harvesting (PyEH) properties. The polarization-electric field hysteresis loops at different temperatures and energy densities were investigated. The PyEH energy density (N _D) is compared with the electrical energy density (U _E). The U _E and N _D at the ferroelectric-paraelectric transition temperature for the χ = 0.1 blend are reported as 3.18 and 5.04 J/cm³, respectively, which are higher than the other polymer blends. Interestingly, the N _D of the χ = 0.9 blend is found to be 3.44 J/cm³ when operated at lower and higher temperatures, that is, at T _L = 25 °C and T _H = 40 °C, respectively, which is the highest possible energy density at the lowest possible transition temperature for the polymer blends.

Enhanced piezoelectric and acoustic performances of poly(vinylidene fluoride-trifluoroethylene) films for hydroacoustic applications

Concerning the study of flexible piezoelectric devices, both scholars and engineers propose that poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) shows more merits than oriented polyvinylidene fluoride (OPVDF) in terms of dielectric, piezoelectric, mechanic-electric, acoustic emission reception performances, etc. Thus, in this study, to clarify the differences between the two types of polymers on their ferroelectric and piezoelectric behaviors, we systematically investigated samples to analyze their molecular structures and phase structures, and to compare their dielectric properties and acoustic emission reception performances. It was found that the wedge effect of TrFE, P(VDF-TrFE), possesses higher regular β phase crystal grains, which are easier to order along the electric field and possess more ordered static charge distribution than that of OPVDF. Consequently, a considerable saturated electric polarization (Pm ∼ 15 μC cm-2 under 225 MV m-1), a large piezoelectric coefficient (d33 ∼ -21.5 pC N-1) and a low coercive electric field (Ec ∼ 50 MV m-1) were obtained in the P(VDF-TrFE) films. It is worth noting that P(VDF-TrFE) shows a more stable d33 piezoelectric response (up to 120 °C) than that of the OPVDF. Additionally, the P(VDF-TrFE) piezoelectric films exhibit a sensitive acoustic emission reception property at approximately 70 dB and an extensive response frequency range from 10 to 100 kHz. These combined properties demonstrate that P(VDF-TrFE) piezoelectric films are a promising material for flexible and easily shaped electronic devices, including hydroacoustic sensors, actuators, and energy transfer units.

Ferroelectric relaxation dependence of poly(vinylidene fluoride-co-trifluoroethylene) on frequency and temperature after grafting with poly(methyl methacrylate)

By monitoring the D–E curves of P(VDF-TrFE-CTFE)-g-PMMAs at different frequency and temperature, the dependence of the ferroelectric relaxation of grafted copolymers onto testing conditions and composition has been finely illustrated.

Understanding crystallization features of P(VDF-TrFE) copolymers under confinement to optimize ferroelectricity in nanostructures

The successful development of ferroelectric polymer devices depends on the effective fabrication of polar ferroelectric crystalline nanostructures. We demonstrate, by scanning X-ray microdiffraction using synchrotron light, the heterogeneous character of high aspect ratio one-dimensional nanoarrays of poly(vinylidene fluoride-co-trifluoroethylene) copolymers supported by a residual polymer film. They were prepared by melt and solution template wetting, using porous anodic aluminum oxide as a template. The spatial evolution of different polymorphs from the mixture of paraelectric and ferroelectric crystal forms (residual film) to the pure ferroelectric form (nanoarray) is evidenced for the samples prepared by solution wetting. However, for samples prepared by melt wetting the ferroelectric phase is exclusively obtained in both the residual film and nanoarray. The crystal nuclei formed in the polymer film connected to the nanoarray play a key role in determining the formation of a crystallinity distribution gradient, where the crystallinity decreases along the first 5-10 microns in the nanorods reaching a steady value afterwards. The minimum decrease in crystallinity is revealed for samples prepared by melt wetting. The results reported in this work endeavour to enhance the understanding of crystallization under confinement for ferroelectric copolymers and reveal the parameters for improving the ferroelectric character of polymer nanostructures.