Modeling of enhanced electrocaloric effect above the Curie temperature in relaxor ferroelectrics

The electrocaloric (EC) effect offers promise as a means to realize solid-state refrigeration, which requires EC materials possessing a pronounced pyroelectric effect over a broad temperature range. Pauli's master equation is adopted to investigate the recently observed phenomenon of enhanced EC effect above the Curie temperature in relaxor ferroelectrics. The proposed approach allows the EC coefficient to be determined within the framework of classic Landau-Ginzburg-Devonshire thermodynamics and the Maxwell relation, taking into account both the depolarization effect and dielectric permittivity dispersion based on the concept of superparaelectricity and the nanopolar region. We analyze three contributions of the EC effect: temperature-dependent dielectric dispersion, intrinsic pyroelectric effect and enhanced dielectric stiffness. The maximum EC coefficient is determined through the derivatives of the three components with respect to temperature. The proposed approach, in which the evolution of polarization correlation length is accounted for, cannot only provide a microscopic explanation for the thermally driven enhancement of EC responses, but also improves upon the existing models for estimating the EC effect in paraelectric phase of relaxors. Finally, some potential approaches for engineering the enhancement of EC coefficient are also suggested. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.postprin

Effects of volume evolution of static and dynamic polar nanoregions on the dielectric behavior of relaxors

Recent experiments revealed unusual dielectric behaviors of Pb(Mg 1/3Nb 2/3)O 3 originated from polar nanoregions (PNRs). Thus, Pauli's master equation is adopted to investigate the distinct dielectric responses, correlation strength, and volume evolutions of static and dynamic PNRs. Our findings have not only validated the percolation theory but also ascertained Lorentzian distribution of the rate of thermal change of PNR volume. Finally, based on Maxwell's equation the observed dielectric deviations of bulk permittivity from Curie-Weiss law are attributed to the thermal effects on static volume fraction and polarization rotation. © 2011 American Institute of Physics.published_or_final_versio

Effects of frequency and temperature evolution of polar nanoregions on susceptibility dispersion and polarization mechanism in relaxors

A generic theory for the frequency and temperature effects on the characteristic evolution of polar nanoregions (PNRs) is essential for improving and optimizing the design of relaxor-based piezoelectric and electrocaloric cooling devices. Pauli's master equation was used to deduce analytical expressions for both the polarization dynamics and frequency-tunable susceptibility dispersions of relaxors. This was done by coupling the intrinsic equilibrium and dynamic factors of bulk relaxors and thin-films. It has been found that for relaxors to comply with Vogel-Fulcher relation, the evolution of PNR mean volume and coercive field of localized electric hysteresis for PNRs has to obey the classic Merz's switching law. The evolution of PNR mean volume in Pb(Mg 1/3Nb 2/3)O 3 crystal was calculated in the temperature range of 200-300 K and at different frequencies up to 10 12 Hz. Our results were in good agreement with the mean-field percolation theory and experimental correlation lengths. Hence, the proposed theory may serve as a new basis for studying the relationship between macroscopic dielectric, electrocaloric as well as other important properties of relaxors and evolutions of their typical microstructures. © 2011 American Institute of Physics.published_or_final_versio

Unique vortex and stripe domain structures in PbTiO 3 epitaxial nanodots

The domain structures of PbTiO 3 epitaxial nanodots under the influences of depolarization fields and mismatch strains have been studied using three dimensional phase field simulations. The single-vortex structure and mixed domain configuration, which consisted of zigzag stripe domain and closure dipole flux near the interfaces, were found to be effective in annihilating the depolarization fields in the isotropically tensile and compressive ferroelectric nanodots, respectively. These domain structures were produced by the combined effect of electrostatic and mismatch elastic energies. The width of stripe domain was found to be related to the volume percentage of polarization dipoles along the z-axis, which varied remarkably with the change of compressive mismatch strain. In the case of nanodots under anisotropic mismatch strains, double-vortex domain patterns and stripe domains with nearly straight domain walls were formed. Moreover, the domain structures with electrostatic energy neglected were also studied. © 2011 Elsevier Ltd. All rights reserved.postprin

Simulation of surface effects in energy dissipation of ultrahighfrequency (UHF) nanocantilevers

Devices composed of nanoelectromechanical systems (NEMS) possess distinguished properties which make them quite suitable for a variety of applications including ultra-high-frequency (UHF) resonators. However, most GHz resonators have low quality factor even though it has been well above 10 3∼ 10 5 for very-high-frequency (VHF) microresonators. The motivation for our investigation of single crystal silicon nanoresonator arises from both its technological importance and its extraordinary surface effects. Our simulation results show that the quality factor decreased in a nearly linear manner as the surface area to volume ratio (SVR) was increased, which suggests that surface losses play a significant role in determining the quality factor of nanoresonators.published_or_final_versio

An optimization-based "phase field" model for polycrystalline ferroelectrics

An optimization-based computational model is proposed to study domain evolution in polycrystalline ferroelectrics composed of numerous grains, each of which consists of multiple domains. Domain switching is realized by an optimization process to minimize the free energy of each grain. Similar to phase field modeling, no priori domain-switching criterion is imposed in the proposed model. Moreover, by focusing on the volume fractions of domains only, the computational complexity of this model becomes much smaller and the domain textures evolution can be captured. Simulation results on both tetragonal and rhombohedral lead titanate zirconate ceramics illustrate the efficiency of this model. © 2010 American Institute of Physics.published_or_final_versio

Ductility enhancement of layered stainless steel with nanograined interface layers

Combination of surface mechanical attrition treatment (SMAT) and co-rolling is a promising experimental methodology to design metals with high strength and high ductility. Recent results have revealed that brittle nanograined interface layer (NGIL) can enhance the ductility of the co-rolled SMATed stainless steel (SS). In the present study, the cohesive finite element method is used to show that the SS ductility is significantly enhanced with the increase of fracture toughness of coarse-grained layers and failure strain of NGIL. However the ductility will not increase if the NGIL thickness goes beyond 60 μm. © 2011 Elsevier B.V. All rights reserved.postprin

Effects of film thickness and mismatch strains on magnetoelectric coupling in vertical heteroepitaxial nanocomposite thin films

The phase field model is adopted to study the magnetoelectric coupling effects in vertical heteroepitaxial nanocomposite thin films. Both the lateral epitaxial strains between the film and the substrate and the vertical epitaxial strains between the ferroelectric and ferromagnetic phases are accounted for in the model devised. The effects of the film thickness on the magnetic-field- induced electric polarization (MIEP) are investigated. The results obtained show that the MIEP is strongly dependent on the film thickness, as well as on the vertical and lateral epitaxial strains. © 2011 American Institute of Physics.published_or_final_versio

Anisotropic mechanism on distinct transition modes of tip-activated multipolorizaion switching in epitaxial BiFeO 3 films

Based on the extended Kittel's law, an anisotropic mechanism has been developed to investigate the complex multipolarization switching in (001) and (110) epitaxial BiFeO 3 films, under a biased-tip field. Switching inhomogeneity and domain wall width evolution have been specifically accounted for. It has been found that distinct switching modes, i.e., the breakdown mode of 71°-switched domain and the activation mode of 180°/ 109°switching, exist and dominate the switching orders within switching process. Our predicted switching orders show excellent agreements with the existing experimental data and phase-field results. A two-step procedure is also proposed to fabricate single-phase 71°ferroelastic domain array of controllable density using (001) BiFeO 3 films, which is favored in practice to significantly enhance the magnetoelectric coupling and photovoltage. © 2011 American Institute of Physics.published_or_final_versio

Acoustic leakage in electromagnetic waveguides made from piezoelectric materials

We study the propagation of coupled acoustic and electromagnetic waves in a piezoelectric plate waveguide embedded in two half-spaces of another dielectric material. It is shown that certain waves are guided electromagnetically but not so acoustically and that these waves are effectively damped waves with acoustic leakage of energy. An estimate of the acoustic radiation damping is given. © 2007 American Institute of Physics.published_or_final_versio