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Electric field-induced antiferroelectric-ferroelectric phase transition and the associated nonlinear dielectric behavior in particulate composites are investigated for achieving a high dielectric capacity. A phenomenological thermodynamic model based on the Landau theory is first developed to discuss the generic phenomena of a temperature-electric field phase diagram, coexistence of antiferroelectric and ferroelectric phases, field-induced antiferroelectric-ferroelectric phase transition, and nonlinear dielectric behavior. The model is then used to carry out the phase field simulation of particulate nonlinear dielectric composites. It is found that the composites exhibit nonlinear dielectric behaviors, and the depolarization field in the composites helps reduce the dielectric hysteresis and enhance the reversibility of antiferroelectric-ferroelectric phase transitions, which are desired for energy storage applications. The simulations also reveal the underlying domain-level mechanisms for nucleation and growth processes of the phase transitions. It is shown that the macroscopic properties of the composites sensitively depend on the directional alignment of the antiferroelectric filler particles, and thus the filler morphology is an effective control variable in designing nonlinear dielectric composites. [ABSTRACT FROM AUTHOR]

Mechanisms of nucleation and growth of domains during ferroelectric phase transformation are investigated by using theoretical and computational approaches. It is shown that ferroelectric phase transformation exhibits some peculiar behaviors due to strong long-range dipole-dipole-like interactions involved in the system. Incorporation of electrostatic and elastostatic energies into the classical Landau-Ginzburg-Devonshire theory effectively modifies the coefficients of the polynomial free energy function and introduces extra energy barrier for ferroelectric phase transformation. It is found that independent nucleation of ferroelectric phase in the context of classical nucleation theory is prohibited, because electrostatic interaction generates an insurmountable energy barrier to isolated nucleus. Phase field modeling and computer simulation reveals that, in order to circumvent such an energy barrier, ferroelectric nucleation exhibits strong spatial correlation and self-organization behaviors from the very beginning, and ferroelectric phase transformation proceeds via spatial and temporal evolution of self-accommodating domains that provide a low-energy kinetic pathway throughout the phase transformation process. Theoretical analysis of the critical wavelength of correlated nucleation agrees with the computer simulation. Heterogeneous nucleation as induced by externally applied local electric field and subsequent polarization evolution process is also simulated to further demonstrate such self-organized pattern formation behaviors. [ABSTRACT FROM AUTHOR]

Large, reversible and anhysteretic strain induced by external field is desired for multifunctional materials used in sensors, actuators and transducers. However, these desired attributes usually compromise each other, leading to trade-offs in materials’ properties and limiting their applicability. This paper focuses on field-induced inter-ferroelectric phase transformations. Some fundamental principles and domain mechanisms are systematized based on insights learned from phase field modeling and simulations, whose synergistic operations are expected to provide unique combination of large, reversible, and anhysteretic strain attributes. These working principles are: (i) Field-induced inter-ferroelectric structural phase transformation to achieve large strain; (ii) Field-induced stable-metastable phase transformation to maximize reversibility; (iii) Heterogeneous nucleation-and-growth process at domain walls to enhance low-field responses; (iv) Deactivation of domain wall motion by applying external fields along nonpolar axes to minimize hysteresis; (v) Domain wall broadening mechanism and domain size effect to exploit nanoscale engineered domain microstructures; and (vi) Bridging domain mechanism and phase coexistence to promote ferroelectric shape memory effects. It is shown that special initial domain microstructures and preferred evolution kinetic pathways can be achieved by crystallographic domain engineering technique, which allow multiple principles to work together without compromising one another. Due to the commonalities and fundamental interrelations among ferroelectric, ferromagnetic and ferroelastic materials, the gained understanding of thermodynamic and kinetic principles has general implications to displacive phase transformations in ferroic materials and is helpful for design of new functional materials with advanced field-induced strain properties. [ABSTRACT FROM AUTHOR]

The phase field microelasticity approach to coherent phase transformations is extended to the case of a finite sample with free surfaces. The 3D simulation model is based on numerical solution of exact elasticity equation. No ad hoc assumptions on the microstructure morphology along evolution path are made. It is used to investigate the effects of free surfaces on multi-variant martensitic transformations in AuCd and FeNi alloys. The simulation shows that formation of free surfaces changes the martensite microstructures mostly in the surface layers, the thickness of these layers being commensurate with the size of martensite domains. It also shows that the free surfaces affect the microstructures of the high-symmetry FeNi more significantly than that of the low-symmetry AuCd because the latter has more orientation variants and can achieve better stress accommodation. [Copyright &y& Elsevier]

Computer simulation is performed to study the mechanisms for controlling domain configurations and sizes in crystallographically engineered ferroelectric crystals. It is found that minimal domain sizes and highest domain wall densities are obtained with intermediate electric field applied along nonpolar axis of ferroelectric crystals, while lower and higher fields produce coarser domains, and temperature also plays important role in domain size control. The simulation shows that selection of polar domain variants by external electric field during nucleation stage of ferroelectric phase transition significantly affects subsequent domain growth and evolution kinetics, controlling the formation and sizes of lamellar domains. [ABSTRACT FROM AUTHOR]

Computer modeling and simulation reveals a grain size- and composition-dependent behavior of phase coexistence around the morphotropic phase boundaries in polycrystals of ferroelectric solid solutions. It shows that the width of phase coexistence composition range increases with decreasing grain sizes; phase-coexisting domain microstructures effectively reduce elastostatic, electrostatic, and domain wall energies; and grain boundaries impose internal mechanical and electric boundary conditions, which affect the phase-coexisting domain microstructures in the grains and give rise to the grain size effect of phase coexistence. [ABSTRACT FROM AUTHOR]

Abstract: A phase field model was developed to study the relationships between particulate microstructure and the magnetoelectric properties of magnetostrictive–piezoelectric composites. The model explicitly treats grain structures and domain processes in two-phase composites and automatically takes into account boundary conditions at inter-phase interfaces and grain boundaries. The model was used to investigate certain fundamental issues important for both rational design and practical application of magnetoelectric composites, such as connectivity of constituent phases, electrical conductivity of the magnetostrictive phase, in situ electric poling of ferroelectric phase within composite and the bias magnetic field during sensing. The evolution of magnetic and electric domains and their associated magnetostriction and electrostriction were simulated to study the strain-mediated domain-level coupling mechanisms that exist between magnetization and polarization. The simulations show that in situ poling of the ferroelectric phase within the composite microstructure is essential for piezoelectricity and thus the magnetoelectric effect, which requires 0–3 connectivity of the magnetostrictive–ferroelectric microstructure. Contrary to conventional wisdom that regards the conductivity of the magnetostrictive phase as detrimental to the magnetoelectric effect, it was found that a finite electrical conductivity of an isolated magnetostrictive phase can significantly enhance the magnetoelectric coefficient by both assisting effective poling and helping to stabilize the poled ferroelectric state. The simulations also show that the polarization response markedly lags behind the magnetization response, and a bias magnetic field effectively enhances the magnetoelectric effect. [Copyright &y& Elsevier]