In this article, domain wall motion and the extrinsic contributions to the dielectric and piezoelectric responses in sol–gel derived lead zirconate titanate (PZT) films with compositions near the morphotropic phase boundary were investigated. It was found that although the films had different thicknesses, grain sizes, and preferred orientations, similar intrinsic dielectric constants were obtained for all films between 0.5 and 3.4 μm thick. It was estimated that about 25%–50% of the dielectric response at room temperature was from extrinsic sources. The extrinsic contribution to the dielectric constant of PZT films was mainly attributed to 180° domain wall motion, which increased with both film thickness and grain size. In studies on the direct and converse longitudinal piezoelectric coefficients of PZT films as a function of either stress or electric driving field, it was found that the ferroelastic non-180° domain wall motion was limited. Thus extrinsic contributions to the piezoelectric response were small in fine grain PZT films (especially those under 1.5 μm in thickness). However, as the films became thicker (>5μm), nonlinear behavior between the converse piezoelectric coefficient and the electric driving field was observed. This indicated that there was significant ferroelectric non-180° domain wall motion under high external excitation in thicker films. The activity of the non-180° domain walls was studied through non-180° domain switching. For fine grain films with film thicknesses less than 2 μm, non-180° switching was negligible. Transmission electron microscopy plan-view micrographs evidenced non-180° domain fringes in these films, where the vast majority of grains were 50–100 nm in diameter and showed a single set of domain fringes. Taken together, these measurements suggest that the pinning of non-180° domain walls is very strong in films with thickness less than 2 μm. In thicker films, non-180° domain switching was evidenced when the poling field exceeded a threshold field. The threshold field decreased with an increase in film thickness, suggesting more non-180° domain wall mobility in thicker films. Non-180° domain switching in large grained PZT films was found to be much easier and more significant than in the fine grained PZT films.
MXene for supercapacitor application which shows outstanding proton-induced pseudocapacitance in acidic aqueous electrolytes. [14,15] High electronic conductivity (up to 15 000 S cm −1), high packing density (up to 4 g cm −3), along with high pseudocapacitance endow Ti 3 C 2 T x ultrahigh volumetric capacitance (≈1500 F cm −3), which gives Ti 3 C 2 T x incomparable advantages over other electrode materials for supercapacitors. [16] However, Ti 3 C 2 T x electrodes suffer from long ion transport pathways due to the stacking nature of 2D materials, leading to ultra-low rate performance in a thick electrode. When used as a power supply for electronic devices where high areal energy densities at high rates are required, MXene electrodes need to be thick enough to ensure high charge storage capability. In this context, the ion transport issue becomes more critical because the low rate performance will deteriorate with the increase of film thickness. [17] Numerous efforts have been made to alleviate the restacking issue of Ti 3 C 2 T x film electrodes. Typical strategies include interlayer insertion of graphene, carbon nanotube or other nanomaterials, pillared structure design, template sacrifice method, vertical alignment, and etching holes. [17-27] However, by most of the reported approaches, the rate performances are increased at the expense of volumetric capacitance because of the introduction of inactive materials, excess spacing, or active materials with lower volumetric capacitance. For example, ≈15% decrease
We study invasion percolation in the presence of viscous forces, as a model of the drainage of a wetting fluid from a porous medium. Using concepts from gradient percolation, we consider two different cases, depending on the magnitude of the mobility ratio M . When M is sufficiently small, the displacement can be modeled by a form of gradient percolation in a stabilizing gradient, involving a particular percolation probability profile. We develop the scaling of the front width and the saturation profile, in terms of the capillary number. In the opposite case, the displacement is described by gradient percolation in a destabilizing gradient and leads to capillary-viscous fingering. This regime is identified in the context of viscous displacements and in general differs from diffusion-limited aggregation, which also describes displacements at large M . Constraints for the validity of the two regimes are developed. Limited experimental and numerical results support the theory of stabilized displacement. The effect of heterogeneity is also discussed. ͓S1063-651X͑97͒08812-0͔PACS number͑s͒: 47.55. Mh, 0.5.40.ϩj, 47.55.Kf
However, the unsatisfied moisture stability of the perovskite materials may become an obstacle for practical applications of PSCs. [5,6] Formamidinium lead iodide (FAPbI 3 ) is a widely used composition for high efficiency PSC owing to its superior properties such as optimal bandgap, high absorption coefficient and long carrier diffusion length, which is better than a prototype perovskite composition of methylammonium lead iodide (MAPbI 3 ). [7][8][9][10][11][12][13] FAPbI 3 has two phases, a nonperovskite heaxagonal phase (δ phase) and cubic perovskite phase (α-phase). [9] Normally, the phase transformation from δ-phase to α-phase FAPbI 3 requires the annealing temperature higher than 120 °C. [14] However, fabricating pure α-FAPbI 3 is a challenging due to the metastable α-FAPbI 3 , where the residual δ-phase forces α-phase to undergo a reverse phase transition back to δ-phase in the room temperature. [15][16][17][18] It has been known that α-phase FAPbI 3 can be stabilized by a partial substitution of FA and/or I with Cs, MA and Rb cation, and Br anion. Nevertheless, stabilizing the pure α-phase FAPbI 3 is crucial for achieving higher efficiency. [15,16,[19][20][21][22][23] Instead of substitution strategy, MACl has been proposed as an additive in perovskite precursor solution for inducing pure α-FAPbI 3 perovskite. It was found that addition of MACl increased crystallinity and grain size as well as formation of the pure black phase perovskite film. [12,24,25] However, an increase of bandgap cannot be excluded due to a potential incorporation of To achieve high efficiency perovskite solar cells (PSCs) based on α-phase formamidinium lead iodide (FAPbI 3 ), addition of methylammonium chloride (MACl) in the precursor solution is commonly used, mainly because of phase stability and improvement of grain size and crystallinity. However, the instability of MA in the perovskite limits the device long-term stability. In this report, n-propylammonium chloride (PACl) is proposed as an alternative to MACl for more stable and efficient FAPbI 3 -based PSCs. Perovskite grain size is increased after addition of PACl. Unlike the MA cation, the propylammonium cation passivates the grain boundary rather than being incorporated into the perovskite lattice due to larger ionic size, which minimizes the change in bandgap. Carrier lifetime is significantly increased by more than five times from 405 to 2110 ns with the PACl additive with negligible trap-mediated recombination, while only four times longer carrier lifetime is observed by MACl additive. As a result, a power conversion efficiency over 22.2% is achieved by 20 mol% PACl additive, which is one of the best efficiencies among the MA-free and Br-free PSCs. In addition, stability against moisture is much better for PACl than for MACl due to an in situ formed barrier at the bulk perovskite.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.202102538.
The nonlinear electromechanical behavior of cantilevered piezoelectric ceramic bimorph, unimorph, and reduced and internally biased oxide wafer actuators is studied in a wide electric field and frequency range. It is found that under quasistatic condition, linear relationships between actuator tip displacement-electric field, and blocking force-electric field are only valid under weak field driving. With increasing the driving field, electromechanical nonlinearity begins to contribute significantly to the actuator performance because of ferroelectric hysteresis behavior associated with piezoelectric lead zirconate titanate (PZT)-type ceramic materials. The bending resonance frequencies of all these actuators vary with the magnitude of the electric field. The decrease of resonance frequency with electric field is explained by the increase of elastic compliance of PZT ceramic due to elastic nonlinearity. Mechanical quality factors of the actuators also depend on the magnitude of electric field strength. No significant temperature increase is observed when actuators are driven near resonance frequency under high electric field.
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