Phase-field method is a density-based computational method at the mesoscale for modeling and predicting the temporal microstructure and property evolution during materials processes. The focus of ...this article is on connecting the most common phase-field equations to the very basic first and second laws of classical thermodynamics through rudimentary irreversible thermodynamics. It briefly discusses the relations of the continuum phase-field equations to their counter parts at the microscopic and atomic levels. It attempts to clarify the contributions of long-range elastic, electrostatic, and magnetic interactions to domain structure evolution during structural, ferroelectric, and ferromagnetic phase transformations by separating order parameter changes due to the presence of quasi-static fields and those arising from phase transformations. A few examples are presented to demonstrate the possibility of employing the phase-field method to provide guidance to designing materials for optimum properties or discovering novel mesoscale phenomena or new materials functionalities. The article ends with a brief perspective on a number of potential future directions on the development and applications of phase-field method beyond its traditional applications to structural alloys.
Long‐range ordering of dipoles is a key microscopic signature of ferroelectrics. These ordered dipoles form ferroelectric domains, which can be reoriented by electric fields. Relaxor ferroelectrics ...are a type of ferroelectric where the long‐range ordering of dipoles is disrupted by cation disorder, exhibiting complex polar states with a significant amount of local structural heterogeneity at the nanoscale. They are the materials of choice for numerous devices such as capacitors, nonlinear optical devices, and piezoelectric transducers, owing to their extraordinary dielectric, electro‐optic, and electromechanical properties. However, despite their extensive applications in these devices, the origins of their unique properties are yet to be fully understood, hindering the design and exploration of new relaxor ferroelectric‐based materials. Herein, the complex polar states and applications of relaxor ferroelectrics are first introduced. Attention is then focused on their electromechanical properties, where the relationship between local structural heterogeneity and the extraordinary electromechanical properties is discussed. Based on the understanding of relaxor ferroelectrics, potential strategies to exploit the local structural heterogeneity to design ferroelectrics for drastically enhancing their electromechanical performances are also discussed. It is expected that this article will stimulate future studies on the important roles of local structural heterogeneity in improving the properties of various functional materials.
Relaxor ferroelectrics are a type of ferroelectrics where long‐range ordering of dipoles is disrupted by cation disorder, and thus exhibiting complex polar states at the nanoscale. Due to the outstanding electromechanical properties, relaxor ferroelectrics have received continued interest during the last decades. This review attempts to clarify the relationship between local structural heterogeneity and extraordinary electromechanical properties in relaxor ferroelectrics.
Next‐generation microelectronics and electrical power systems call for high‐energy‐density dielectric polymeric materials that can operate efficiently under elevated temperatures. However, the ...currently available polymer dielectrics are limited to relatively low working temperatures. Here, the solution‐processable polymer nanocomposites consisting of readily prepared Al2O3 fillers with systematically varied morphologies including nanoparticles, nanowires, and nanoplates are reported. The field‐dependent electrical conduction of the polymer nanocomposites at elevated temperatures is investigated. A strong dependence of the conduction behavior and breakdown strength of the polymer composites on the filler morphology is revealed experimentally and is further rationalized via computations. The polymer composites containing Al2O3 nanoplates display a record capacitive performance, e.g., a discharged energy density of 3.31 J cm−3 and a charge–discharge efficiency of >90% measured at 450 MV m−1 and 150 °C, significantly outperforming the state‐of‐the‐art dielectric polymers and nanocomposites that are typically prepared via tedious, low‐yield approaches.
High‐temperature dielectric polymer nanocomposites with facilely prepared nanostructured Al2O3 fillers exhibit remarkable electrical energy storage and discharge capabilities at elevated temperatures and high electric fields, outperforming state‐of‐the‐art polymer dielectrics. The significant impact of the filler morphology on conduction behavior and capacitive performance of the composites is revealed.
Piezoelectricity of ferroelectric crystals is widely utilized in electromechanical devices such as sensors and actuators. It is broadly believed that the smaller the ferroelectric domain size, the ...higher the piezoelectricity, arising from the commonly assumed larger contributions from the domain walls. Herein, the domain‐size dependence of piezoelectric coefficients of prototypical ferroelectric crystals is theoretically studied based on thermodynamic analysis and phase‐field simulations. It is revealed that the inverse domain‐size effect, i.e., the larger the domain size, the higher the piezoelectricity, is entirely possible and can be just as common. The nature of the domain‐size dependence of piezoelectricity is shown to be determined by the propensity of polarization rotation inside the domains instead of the domain wall contributions. A simple, unified, analytical model for predicting the domain‐size dependence of piezoelectricity is established, which is valid regardless of the crystalline symmetry, the materials chemistry, and the domain structures of a ferroelectric crystal, and thus can serve as a guiding tool for optimizing piezoelectricity of ferroelectric materials beyond the “nanodomain” engineering. In addition, the theoretical approach can be extended to understand the microstructural size effect of multifunctional properties in ferroic and multiferroic materials.
Theoretical simulation reveals that the piezoelectricity of ferroelectric crystals does not necessarily increase with smaller domain size as conventionally believed. A simple thermodynamic model applicable to ferroelectrics of arbitrary symmetry is proposed to guide the design of high‐performance piezoelectrics.
The existence of polar nanoregions is the most important characteristic of relaxor‐based ferroelectric materials. Recently, the contributions of polar nanoregions to the shear piezoelectric property ...of relaxor‐PbTiO3 (PT) crystals are confirmed in a single domain state, accounting for 50%–80% of room temperature values. For electromechanical applications, however, the outstanding longitudinal piezoelectricity in domain‐engineered relaxor‐PT crystals is of the most significance. In this paper, the contributions of polar nanoregions to the longitudinal properties in 001‐poled Pb(Mg1/3Nb2/3)O3‐0.30PbTiO3 and 110‐poled Pb(Zn1/3Nb2/3)O3‐0.15PbTiO3 (PZN‐0.15PT) domain‐engineered crystals are studied. Taking the 110‐poled tetragonal PZN‐0.15PT crystal as an example, phase‐field simulations of the domain structures and the longitudinal dielectric/piezoelectric responses are performed. According to the experimental results and phase‐field simulations, the contributions of polar nanoregions (PNRs) to the longitudinal properties of relaxor‐PT crystals are successfully explained on the mesoscale, where the PNRs behave as “seeds” to facilitate macroscopic polarization rotation and enhance electric‐field‐induced strain. The results reveal the importance of local structures to the macroscopic properties, where a modest structural variation on the nanoscale greatly impacts the macroscopic properties.
The contributions of polar nanoregions to piezoelectric responses are successfully quantified for domain‐engineered relaxor‐ferroelectric crystals. A mesoscale mechanism is proposed to explain the ultrahigh longitudinal piezoelectric responses and strain behaviors of relaxor‐PbTiO3 ferroelectrics, where the polar nanoregions act as “seeds” to facilitate polarization rotation and enhance the piezoelectric response.
Ferroelectric polymers have been regarded as the preferred matrix for high‐energy‐density dielectric polymer nanocomposites because of their highest dielectric constants among the known polymers. ...Despite a library of ferroelectric polymer‐based composites having been demonstrated as highly efficient in enhancing the energy density, the charge–discharge efficiency remains moderate because of the high intrinsic loss of ferroelectric polymers. Herein, a systematic study of the oxide nanofillers is presented with varied dielectric constants and the vital role of the dielectric match between the filler and the polymer matrix on the capacitive performance of the ferroelectric polymer composites is revealed. A combined experimental and simulation study is further performed to specifically investigate the effect of the nanofiller morphology on the electrica properties of the polymer nanocomposites. The solution‐processed ferroelectric polymer nanocomposite embedded with Al2O3 nanoplates exhibits markedly improved breakdown strength and discharged energy density along with an exceptional charge–discharge efficiency of 83.4% at 700 MV m−1, which outperforms the ferroelectric polymers and nanocomposites reported to date. This work establishes a facile approach to high‐performance ferroelectric polymer composites through capitalizing on the synergistic effect of the dielectric properties and morphology of the oxide fillers.
This work reports the dielectric properties and capacitive performance of ferroelectric polymer composites containing a series of inorganic fillers with varied dielectric constants and morphologies. Solution‐processed ferroelectric polymer nanocomposites incorporated with 2D oxide fillers with a dielectric constant comparable to that of the matrix exhibit markedly improves energy densities along with exceptional charge–discharge efficiency.
Increases in ambient temperatures have been a severe threat to crop production in many countries around the world under climate change. Chloroplasts serve as metabolic centers and play a key role in ...physiological adaptive processes to heat stress. In addition to expressing heat shock proteins that protect proteins from heat-induced damage, metabolic reprogramming occurs during adaptive physiological processes in chloroplasts. Heat stress leads to inhibition of plant photosynthetic activity by damaging key components functioning in a variety of metabolic processes, with concomitant reductions in biomass production and crop yield. In this review article, we will focus on events through extensive and transient metabolic reprogramming in response to heat stress, which included chlorophyll breakdown, generation of reactive oxygen species (ROS), antioxidant defense, protein turnover, and metabolic alterations with carbon assimilation. Such diverse metabolic reprogramming in chloroplasts is required for systemic acquired acclimation to heat stress in plants.
Energy harvesting from human motion is regarded as a promising protocol for powering portable electronics, biomedical devices, and smart objects of the Internet of things. However, state‐of‐the‐art ...mechanical‐energy‐harvesting devices generally operate at frequencies (>10 Hz) well beyond human activity frequencies. Here, a hydrogel ionic diode formed by the layered structures of anionic and cationic ionomers in hydrogels is presented. As confirmed by finite element analysis, the underlying mechanism of the hydrogel ionic diode involves the formation of the depletion region by mobile cations and anions and the subsequent increase of the built‐in potential across the depletion region in response to mechanical pressure. Owing to the enhanced ionic rectification ratio by the embedded carbon nanotube and silver nanowire electrodes, the hydrogel ionic diode exhibits a power density of ≈5 mW cm−2 and a charge density of ≈4 mC cm−2 at 0.01 Hz, outperforming the current energy‐harvesting devices by several orders of magnitude. The applications of the self‐powered hydrogel ionic diode to tactile sensing, pressure imaging, and touchpads are demonstrated, with sensing limitation is as low as 0.01 kPa. This work is expected to open up new opportunities for ionic‐current‐based ionotronics in electronics and energy devices.
A hydrogel ionic diode for ultralow‐frequency mechanical‐energy harvesting is designed based on a sandwiched structure of anion and cation ionomers in hydrogels embedded with carbon nanotubes and silver nanowire electrodes. The ionic diode exhibits power density, charge density, and harvested energy that outperform current energy‐harvesting devices by several orders of magnitude at 0.01 Hz.
According to Gibbs, “for the purposes of defining chemical potential, any combination of elements in a given proportion may be considered a substance, regardless of whether it exists as a homogeneous ...body.” In Equation 8, G = μN can be understood as the total Gibbs free energy or chemical energy of the substance, while μiNi can be considered the Gibbs free energy or chemical energy possessed by chemical component i in the substance. ...the chemical potential μ of a homogeneous n-component system can be written in terms of chemical potentials for the n individual components, μ1, μ2, … μn,9\\mu = {\mu _1}{x_1} + {\mu _2}{x_2} + ...{\mu _n}{x_n},\where xi (= \({{N_i } \over N}\), where N = N1 + N2 +…+ Nn) are mole fractions. To be consistent with the units for the other potentials, it is useful to introduce a unique unit for the chemical potential. Since all the familiar potentials are associated with the names of the scientist who invented them and since Gibbs introduced this important concept of chemical potential, it is only natural and appropriate to adopt the unit “Gibbs” or “G” (Table I) as the unit of chemical potential to replace the unit of J/mol. ...while chemical potential is measured in Gibbs (=J/mol), Gibbs free energy is measured in Joules (J). ...the molar Gibbs free energy or partial molar Gibbs free energy, which has the unit of J/mol (= Gibbs), should be identified as a chemical potential.
This article briefly reviews recent applications of phase‐field method to ferroelectric phase transitions and domain structures in thin films. It starts with a brief introduction to the ...thermodynamics of coupled electromechanical systems and the Landau description of ferroelectric transitions in homogeneous ferroelectric single crystals. The thermodynamic potentials of a homogeneous crystal under different mechanical boundary conditions are presented, including the thin‐film boundary conditions. The phase‐field approach to inhomogeneous systems containing domain structures is then outlined. It describes a domain structure using the spatial distribution of spontaneous polarization. The evolution of a domain structure towards equilibrium is driven by the reduction in the total‐free energy of an inhomogeneous domain structure including the chemical driving force, domain wall energy, electrostatic energy as well as elastic energy. A number of examples are discussed, including phase transitions and domain stability in ferroelectric thin films and superlattices. It is demonstrated that using a set of independently measured thermodynamic parameters for the corresponding bulk single crystals, the phase‐field approach is able to quantitatively predict not only the strain effect on phase transition temperatures but also the correct ferroelectric domain structures for a given strain and temperature.
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