Ferroelectric nanocomposite networks with high energy storage capacitance and low ferroelectric loss by designing a hierarchical interface architecture.

Nanoscale design of nanofillers and interfacial architecture are vital to achieve high-capacity and high-energy-conversion efficiency poly(vinylidene fluoride) [(PVDF)-based] nanocomposite materials for vast potential applications in modern electronic devices and electric power systems. Using traditional methods, the addition of ceramic nanoparticles can only produce one type of interface between the nanoparticles and this matrix, achieving an enhanced dielectric constant and energy density at the expense of the charge-discharge efficiency. Herein, we demonstrate a novel class of cross-linking nanofiller system, poly(vinylidene fluoride-chlorotrifluoroethylene)/γ-methacryloylpropyl trimethoxysilane@BaTiO3 [P(VDF-CTFE)/MPS@BT]. This novel approach can not only provide the interfaces between the nanoparticle and the matrix, but also scale down the size of crystalline domains, which results in producing more additional interfaces between the crystalline and amorphous phases to achieve an improved discharged energy density. Remarkably, the smaller crystalline domains, which were characterized by XRD and FTIR spectroscopy, could be beneficial for improving the dipole switchability from the polar phases to non-polar phases during the charge-discharge cycles, leading to unprecedented charge-discharge efficiency. Furthermore, the addition of MPS@BT NPs can regulate two stages of the discharge rate. The early discharge process can be accelerated, while the following stage is obviously delayed. The simplicity of the hierarchical interfacial engineering method provides a promising path to design ferroelectric polymer nanocomposites for dielectric capacitor applications.

Large Flexoelectriclike Response from the Spontaneously Polarized Surfaces in Ferroelectric Ceramics.

Nonpoled ferroelectric ceramics are thought to be nonpolar because of randomly oriented grains and the formation of ferroelectric domains in the grains. Here, we discover the surfaces (∼several µm thick) of ferroelectric ceramics are spontaneously polarized. Because the orientations of ferroelectric polarization of the opposite surfaces are antiparallel, ferroelectric ceramics are nonpolar as a whole. However, the ceramics exhibit a strong flexoelectriclike electromechanical response from the piezoelectric response of the polarized surfaces if they are asymmetrically strained (such as bending). Our results reveal a major mechanism to resolve one important but largely unresolved issue: the experimentally measured flexoelectric effect is typically orders of magnitude larger than the theoretically predicted value in ferroelectrics.

Polyimide Films Impregnated with Epoxy Resin Demonstrating Superior Self-Healing Properties for Thermally Stable Energy Storage Capacitors.

Metallized polymer films (MPFs) with superior self-healing properties are extremely attractive for application in energy storage capacitors. Self-healing behaviors allow MPFs to keep insulating between the local electrical breakdown region and the electrode, thereby reserving long-term operational viability of the capacitors. Polyimide (PI) is a type of well-developed polymer material with excellent mechanical and thermal stabilities, but it is deficient in intrinsic self-healing capabilities. This work reports a facile surface engineering strategy to endow metalized PI films with self-healing capabilities. By simple immersion of bare PI films in the solution of epoxy resin (ER) accompanied by curing of ER, PI films impregnated with ER (P-E films) not only show enhanced dielectric characteristics but also obtain excellent self-healing abilities upon multiple cycles of electrical breakdowns, even at a high temperature. For example, in comparison to bare PI films, PI films impregnated in ER solution with a solid content of 1 wt % (P-1%E) display improved initial Weibull breakdown strength (αb1 of 353.0 versus 310.9 kV/mm), maximum discharging energy density (Ud of 2.1836 versus 0.8254 J/cm3), and charging/discharging efficiency (η of 95.72 versus 55.19%) at 150 °C. After 5 breakdown cycles, P-1%E films could maintain a much higher breakdown strength (αb5 of 338.1 versus 21.3 kV/mm). When subjected to a constant electrical strength of 350 kV/mm at 150 °C, P-1%E films show merely <6% decline in both Ud and η values after 5 breakdown cycles. On the contrary, bare PI films would undergo dramatic performance decay after 1 or 2 breakdowns under similar conditions. In view of their outstanding self-healing properties at a high temperature, P-E films can serve as a promising candidate to fabricate thermally stable MPF capacitors for long-term operation.

Effect of Dielectric Barrier Discharge (DBD) Treatment on the Dielectric Properties of Poly(vinylidene fluoride)(PVDF)-Based Copolymer.

Understanding the mechanism of dielectric breakdown is important for improving the breakdown field of a polymer. In this work, dielectric barrier discharge (DBD) treatment was applied to one surface of P(VDF-CTFE) (vinylidene fluoride-chlorotrifluoroethylene) film, and the dielectric properties of the film were studied. When the treated surface was connected to the high potential side of the power source for the breakdown test, the breakdown field of the treated film was significantly reduced compared to that of the pristine film. Based on the characterization results for the surface chemistry and morphology, it was proposed that the phenomenon was caused by the combined effects of hole injection from the metal electrode and the damage of polymer chains near the surface of the polymer film after the DBD treatment process.

Impact-induced solidlike behavior and elasticity in concentrated colloidal suspensions.

Modified drop weight impact tests were performed on SiO_{2}-ethylene glycol concentrated suspensions. Counterintuitive impact-induced solidlike behavior and elasticity, causing significant deceleration and rebound of the impactor, were observed. We provide evidence that the observed large deceleration force on the impactor mainly originates from the hydrodynamic force, and that the elasticity arises from the short-range repulsive force of a solvation layer on the particle surface. This study presents key experimental results to help understand the mechanisms underlying various stress-induced solidification phenomena.

Complex notation for the dielectric response of ferroelectric materials beyond the small sinusoidal fields.

For the polarization response beyond the small field range, Rayleigh's law has been introduced in the past to describe the field-dependent behavior (with loss) of ferroelectric materials with some success. We examine the relationship between Rayleigh's law and the complex dielectric constant notation that has been used widely in the scientific and engineering community; and we show that a modified complex notation can describe the field-dependent dielectric response with loss in the small and medium field range quite well. In addition, the modified complex notation easily can include a field independent dielectric loss that is, in fact, present in all the dielectric materials. The results also show that the alternating current (AC) field response is still predominantly linear with the amplitude and phase of the complex coefficient changing with the applied field amplitude.

Lead-Free Metamaterials with Enormous Apparent Piezoelectric Response.

Lead-free flexoelectric piezoelectric metamaterials are created by applying an asymmetric chemical reduction to Na1/2 Bi1/2 TiO3 -BaTiO3 ceramics. The reduction induces two gradient-generating mechanisms, curvature structure and chemical inhomogeneity, and enhances the flexoelectric effect. The ceramics behave like piezoelectric materials, exhibiting an enormous and high-temperature stable apparent piezoelectric response, outperforming existing lead-oxide-based piezoelectrics.

Large electrocaloric effect in ferroelectric polymers near room temperature.

Applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the electrocaloric effect (ECE), it was determined that a large ECE can be realized in the ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer at temperatures above the ferroelectric-paraelectric transition (above 70 degrees C), where an isothermal entropy change of more than 55 joules per kilogram per kelvin degree and adiabatic temperature change of more than 12 degrees C were observed. We further showed that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene).

A dielectric polymer with high electric energy density and fast discharge speed.

Dielectric polymers with high dipole density have the potential to achieve very high energy density, which is required in many modern electronics and electric systems. We demonstrate that a very high energy density with fast discharge speed and low loss can be obtained in defect-modified poly(vinylidene fluoride) polymers. This is achieved by combining nonpolar and polar molecular structural changes of the polymer with the proper dielectric constants, to avoid the electric displacement saturation at electric fields well below the breakdown field. The results indicate that a very high dielectric constant may not be desirable to reach a very high energy density.