Equilibrium Polarization Research Articles

Ferroelectric Texture of Individual Barium Titanate Nanocrystals.

Ferroelectric materials display exotic polarization textures at the nanoscale that could be used to improve the energetic efficiency of electronic components. The vast majority of studies were conducted in two dimensions on thin films that can be further nanostructured, but very few studies address the situation of individual isolated nanocrystals (NCs) synthesized in solution, while such structures could have other fields of applications. In this work, we experimentally and theoretically studied the polarization texture of ferroelectric barium titanate (BaTiO3, BTO) NCs attached to a conductive substrate and surrounded by air. We synthesized NCs of well-defined quasicubic shape and 160 nm average size that conserve the tetragonal structure of BTO at room temperature. We then investigated the inverse piezoelectric properties of such pristine individual NCs by vector piezoresponse force microscopy (PFM), taking particular care to suppress electrostatic artifacts. In all of the NCs studied, we could not detect any vertical PFM signal, and the maps of the lateral response all displayed larger displacement amplitude on the edges with deformations converging toward the center. Using field phase simulations dedicated to ferroelectric nanostructures, we were able to predict the equilibrium polarization texture. These simulations revealed that the NC core is composed of 180° up and down domains defining the polar axis that rotate by 90° in the two facets orthogonal to this axis, eventually lying within these planes forming a layer of about 10 nm thickness mainly composed of 180° domains along an edge. From this polarization distribution, we predicted the lateral PFM response, which was revealed to be in very good qualitative agreement with the experimental observations. This work positions PFM as a relevant tool to evaluate the potential of complex ferroelectric nanostructures to be used as sensors.

Local equilibrium and Lambda polarization in high energy heavy ion collisions

The polarization of the Λ hyperon has become an important probe of the Quark-Gluon Plasma produced in relativistic heavy-ion collisions. Recently, it has been found that polarization receives a substantial contribution from a local equilibrium term proportional to the symmetric derivative of the four-temperature vector, the thermal shear tensor. We show that, at very high energies, this term can restore the agreement between the experimental measurements and the predictions of the hydrodynamic model, provided that the hadronization hypersurface is isothermal. We review the theoretical derivation of this new term, discuss numerical computations at RHIC and LHC energies, and compare them with the experimental data. We also present the effect of feed-down corrections.

Checking of the condition of transformers and the efficiency of oil regeneration with RVM (return voltage measurement)

Up to now the insulation in HV power transformers has been made from oil/paper therefore the diagnosis of this kind of insulation will be also important in the next forty-fifty years. The lifetime of this equipment strongly depends on the condition of the insulation system. The ageing process of oil/paper insulating systems is a very complex and complicated phenomenon. In order to get a well-supported decision on the further operation of aged transformers, relevant information would be necessary on the condition of the oil-paper insulation. We also know that the oil-paper insulation has almost always inhomogeneous condition considering the temperature, moistening and ageing processes. The classical methods (insulation resistance, loss tangent, etc.) characterise the insulation by single measured value. This single value is not sufficient for relevant characterisation and diagnosis of such a complex system with lots of tons of insulation and with almost always inhomogeneous distribution of temperature, moisture and ageing product. If we measure the polarization spectra in three states (new insulation in equilibrium and uniform polarization spectrum, new insulation with not uniform distribution of polarization spectrum, later reaching again the uniform distributions) the shape of three polarization spectra will be different. Comparing the polarization and classical methods of this three case we can realize that sometimes the “classical single values” are almost the same but the polarization spectra are different. The three response methods provide very practical information about the insulation system (e.g. moisture, ageing). The moistening and accumulation of ageing by-products change the distribution of interfacial polarisation in the range of long time-constants. These promising response methods measure the polarisation distribution in the range of long time-constant (with other words, in the low frequency range). The three test methods were: Return Voltage measurement (RVM), the C and tgδ measurement in range of some tens of mHz to 50 Hz (FDS=Frequency Domain Spectroscopy), and the measurement of DC charging and discharging currents (PDC=polarisation and depolarization currents) up to some thousands of sec. These equivalent methods (RVM, FDS and PDC) are able to follow the changing of condition of insulation contrary to classical methods. Therefore a little bit surprising that until now the convenient standards is missing considering the polarization methods. In an earlier Hungarian research work (Budapest University of Technology) almost all the necessary measurements have been realized, consequently we are in possession of fundamental data considering the polarization methods. This paper would like to show a short review about the RVM technique, the correct interpretation of RVM data and a case study for the checking of the efficiency of the oil reclamation with RVM technique.

Polarization through the elastic scattering in magnetic field

There exists conceptual possibility to obtain the polarized beams of particles through their elastic scattering in magnetic field. Here such possibility is discussed with the use of classical spin models for, both, the cases of charged and neutral magneton. In the charged particle case the fundamental possibility to observe the effect stems from results of Novosibirsk experiment carried out in 1984. Within the proposed schematic design the polarization process lies in more rapid getting out from the trajectory of those particles which have “wrong” spin directions. For the charged particle the BMT and Frenkel models predict opposite Langevin-type polarizations and for the neutral particles the results obtained from the both are identical being consistent with the previous studies. Parameter [Formula: see text] (Sokolov–Ternov polarization time for electrons) is shown to emerge within the classical theory. The effect of noncomplete equilibrium polarization degree cannot be obtained in such model experiment since the kinetic process of the spin-flip transitions is left beyond the scheme considered. At that time the noninertial nature of motion entails in asymmetry between dependencies of spin–spin radiation rate on two horizontal spin components. This purely classical property is analyzed in the context of so-called “circular Unruh effect.”

Geometric Control of Domain Structure Stability in Ferroelectric Nanotubes

AbstractFerroelectric nanotubes offer intriguing opportunities for stabilizing exotic polarization domains and achieving new or enhanced functionalities by tailoring the complex interplay among the geometry, surface effects, crystal symmetry, and more. Here, phase‐field simulations to predict the room‐temperature equilibrium polarization domain structure in (001)pc PbZr0.52Ti0.48O3 (PZT) nanotubes are used (pseudocubic (pc)). The simulations incorporate the influence of surface‐tension‐induced strains, which have been ignored in existing computational studies. It is found that (001)pc PZT nanotubes can host a unique class of topological polarization domain structures comprising non‐planar flux‐closures and anti‐flux‐closures that are inaccessible with ferroelectrics of planar geometry (e.g., thin‐films, nanodots). It is shown that surface‐tension‐induced strain is significantly enhanced in thin‐walled nanotubes and thereby can lead to noticeable modulation of the flux closures. Domain stability map as a function of the nanotube wall thickness and height is established. The results provide a basis for geometrical engineering of domain structures and associated functional (e.g., piezoelectric, electrocaloric) responses in ferroelectric nanotubes.

Size, Shape, and Material Effects in Ferroelectric Octahedral Nanoparticles

Composite materials composed of multiferroelectric nanoparticles in dielectric matrixes have attracted enormous attention for their potential applications in developing future functional devices. However, the functionalities of ferroelectric nanoparticles depend on shapes, sizes, and materials. In this paper, a time-dependent Landau-Ginzburg method has been used and combined with a method as the coupled-physics finite-element-method-based simulations are used to illustrate the polarization behavior in isolated BaTiO3 or PbTiO3 octahedral nanoparticles embedded in a dielectric medium, like SrTiO3 (ST, high dielectric permittivity) and amorphous silica (a-SiO2, low dielectric permittivity). The equilibrium polarization topology of the octahedral nanoparticle is strongly affected by the choice of inclusion and the size of matrix materials. Also, there are three equilibrium polarization patterns, i.e., monodomain, vortex-like, and multidomain, because of the various sizes and material parameters combination. There is a critical particle size below which ferroelectricity vanishes in our calculations. This size of the PbTiO3 octahedral nanoparticle is 2.5 and 3.6 nm for high- and low-permittivity matrix materials, respectively. However, this size of the BaTiO3 octahedral nanoparticle is 3.6 nm regardless of the matrix materials.

Achieving Maximal Enhancement of Electron Spin Polarization in Stable Nitroxyl Radicals at Room Temperature.

Enhancing the polarization of spin levels at room temperature is one of the active research areas in magnetic resonance. Generation of electron spin hyperpolarization involves a complex interplay of electronic and spin processes. In this work, the optimization of crucial electron spin polarization (ESP) generating parameters and synthesis of a radical-chromophore adduct are described. The ESP of the synthesized adduct is about 550 times the equilibrium polarization at room temperature, which is possibly the maximal value for a chromophore-nitroxyl system. The present work highlights the crucial role of the photophysical quenching process toward the generation of a large ESP. Additionally, a chromophore-diradical adduct is synthesized, and the effects of the additional radical in the ESP generation process are discussed. Enhanced photochemical stability is demonstrated for the diradical adducts, thereby suggesting a potential route toward the generation of photostable radical-chromophore adducts for future studies. The large ESP in these molecules should enable a wide range of applications, such as in DNP, spintronics, and magnetometers.

Continuous energy measurement of the electron beam in the storage ring of Diamond Light Source with resonant spin depolarization

Electron spin polarization occurs naturally in an electron storage ring due to spin-flip synchrotron radiation. The equilibrium polarization level that is achieved in Diamond Light Source was studied theoretically and experimentally. A novel approach that enables measurements during user operation is presented, compatible with beams of small vertical emittance in the range of pm rad. An array of four custom-made scintillation detectors has been installed around the beam pipe downstream from the collimators to capture a significant fraction of the lost particles and witness the depolarization process in the presence of a background loss rate. A new concept for detecting the depolarization resonance even when it coincides with higher-order betatron resonances was introduced. The momentum compaction factor was measured to be in very good agreement with the storage ring numerical model. In the end, we measured a good long-term stability of the beam energy during user operation time.

Nash equilibrium and party polarization in an electoral model with mixed motivations

We study the existence problem of Nash equilibrium as well as the patterns of equilibrium policy outcomes in an electoral competition model with mixed motivations. Each party maximizes a sum of party members’ expected utility and office rent. The inclusion of office rent renders the payoff of each party discontinuous. This makes it difficult to apply usually fixed point arguments to prove the existence of Nash equilibria. By using a recently developed concept, multiple restrictional security (MR‐security) we provide conditions under which a pure‐strategy Nash equilibrium exists within fairly general settings, and further the analysis by presenting conditions under which various patterns of policy choices, including polarization, arise in equilibrium.

Equilibrium and Nonequilibrium Spin Polarization near Filling Factor 3/2

The spin polarization features of an electron system and the relaxation of nonequilibrium spin excitations near an even-denominator fractional state of 3/2 in a two-dimensional electron system based on the GaAs/AlGaAs heterostructure are experimentally investigated. It is shown that the 3/2 state is a singular point in the filling factor dependence of the spin ordering of the two-dimensional electron system, at which the spin subsystem is rearranged. A giant slowing down of the relaxation of spin excitations to the ground state is revealed in a certain range of filling factors near filling factor 3/2.

Chloride removal and corrosion inhibitions of nitrate, nitrite-intercalated Mg Al layered double hydroxides on steel in saturated calcium hydroxide solution

Abstract The aim of this study is to evaluate the chloride removal and corrosion inhibitions of NO3−, NO2− intercalated Mg Al layered double hydroxides (LDH) on steel in saturated calcium hydroxide solution by a comparative experiment. Mg-Al-LDH intercalated with NO2− was synthesized by anion exchange in the host material, Mg-Al-NO3 LDH, which was prepared by a coprecipitation method. The chloride equilibrium isotherm, corrosion potential and polarization resistance of steel (decided by electrochemical impedance spectroscopy (EIS)) in the solution were measured. Besides, scanning electron microscopy (SEM), X-ray diffraction instrument (XRD) and infrared spectroscopy were applied to observe the morphologies and microstructures of LDH. The results indicate that the LDH intercalated with NO3−, NO2− have excellent chloride removal capacities. Compared to LDH-NO3, the maximum chloride uptake amount for LDH-NO2 is relatively lower. However, the corrosion of steel for LDH-NO2 has a higher chloride threshold level. The dual effects, i.e., the uptake of chloride ion as an absorbent and the release of NO2− ions as an inhibitor, contribute to the better inhibition effect for LDH-NO2.

A Simple Method to Establish the Relationship Between the Equilibrium Polarization Resistance and the Elementary Kinetic Parameters of an Electrocatalysed Reaction

A simple and rigorous methodology to establish the relationship between the faradaic equilibrium polarization resistance of an electrocatalytic reaction with the elementary kinetic parameters involved in the reaction mechanism is proposed. It was derived through an alternative method, which avoided the differentiation of the corresponding current-overpotential dependence. This formalism includes the cases where both, reactants and products, exhibit diffusion contributions. It is demonstrated that the equilibrium polarization resistance is the sum of both, faradaic and diffusion, contributions. Each diffusion term has a linear variation with the inverse of the limiting diffusion current density of the species involved. This behavior was exemplified with two different experimental data sets for the hydrogen electrode reaction, obtained on a rotating disc and microelectrodes, respectively.

Phase diagrams of ferroelectric nanocrystals strained by an elastic matrix

Ferroelectric crystallites embedded into a dielectric matrix experience temperature-dependent elastic strains caused by differences in the thermal expansion of the crystallites and the matrix. Owing to the electrostriction, these lattice strains may affect polarization states of ferroelectric inclusions significantly, making them different from those of a stress-free bulk crystal. Here, using a nonlinear thermodynamic theory, we study the mechanical effect of elastic matrix on the phase states of embedded single-domain ferroelectric nanocrystals. Their equilibrium polarization states are determined by minimizing a special thermodynamic potential that describes the energetics of an ellipsoidal ferroelectric inclusion surrounded by a linear elastic medium. To demonstrate the stability ranges of such states for a given material combination, we construct a phase diagram, where the inclusion’s shape anisotropy and temperature are used as two parameters. The ‘shape-temperature’ phase diagrams are calculated numerically for PbTiO3 and BaTiO3 nanocrystals embedded into representative dielectric matrices generating tensile (silica glass) or compressive (potassium silicate glass) thermal stresses inside ferroelectric inclusions. The developed phase maps demonstrate that the joint effect of thermal stresses and matrix-induced elastic clamping of ferroelectric inclusions gives rise to several important features in the polarization behavior of PbTiO3 and BaTiO3 nanocrystals. In particular, the Curie temperature displays a nonmonotonic variation with the ellipsoid’s aspect ratio, being minimal for spherical inclusions. Furthermore, the diagrams show that the polarization orientation with respect to the ellipsoid’s symmetry axis is controlled by the shape anisotropy and the sign of thermal stresses. Under certain conditions, the mechanical inclusion-matrix interaction qualitatively alters the evolution of ferroelectric states on cooling, inducing a structural transition in PbTiO3 nanocrystals and suppressing in BaTiO3 inclusions some transformations occurring in their bulk counterpart. The constructed phase maps open the possibility to calculate dielectric properties of strained PbTiO3 and BaTiO3 nanocrystals and ferroelectric nanocomposites comprising such crystallites.

Topological phase transformations and intrinsic size effects in ferroelectric nanoparticles.

Composite materials comprised of ferroelectric nanoparticles in a dielectric matrix are being actively investigated for a variety of functional properties attractive for a wide range of novel electronic and energy harvesting devices. However, the dependence of these functionalities on shapes, sizes, orientation and mutual arrangement of ferroelectric particles is currently not fully understood. In this study, we utilize a time-dependent Ginzburg-Landau approach combined with coupled-physics finite-element-method based simulations to elucidate the behavior of polarization in isolated spherical PbTiO3 or BaTiO3 nanoparticles embedded in a dielectric medium, including air. The equilibrium polarization topology is strongly affected by particle diameter, as well as the choice of inclusion and matrix materials, with monodomain, vortex-like and multidomain patterns emerging for various combinations of size and materials parameters. This leads to radically different polarization vs. electric field responses, resulting in highly tunable size-dependent dielectric properties that should be possible to observe experimentally. Our calculations show that there is a critical particle size below which ferroelectricity vanishes. For the PbTiO3 particle, this size is 2 and 3.4 nm, respectively, for high- and low-permittivity media. For the BaTiO3 particle, it is ∼3.6 nm regardless of the medium dielectric strength.

Multilevel hysteresis loop engineered with ferroelectric nano-metamaterials

Polarization switching in ferroelectric and multiferroic materials provides a means of designing a broad range of multifunctional devices. To date, such applications have typically exploited 180° ferroelectric polarization switching, where the polarization vector can be alternated between parallel or anti-parallel to the external electric field, i.e., bi-states. However, the coupling between the direction of switching and the long-range order parameter puts a limit on the well-defined polarization state, hindering the use of polarization switching and the advancement of novel applications. Using the ferroelectric nano-metamaterial concept, we demonstrate here that successive polarization switching through multiple well-defined stable states with a long-range order parameter decoupled from the direction of switching can be achieved, and consequently, controllable and multilevel hysteresis loops are obtained under a unidirectional electric field. The equilibrium polarization configuration of stable states are tailored by designing a nanoporous ferroelectric structure based on the ferroelectric nano-metamaterial concept. The orientation of the homogeneous electric field is then controlled to achieve the desired polarization switching behavior and hence the hysteresis loop level, which can pass through all stable polarization states. In addition, a general rule for the nanostructure-state-level of switching is established, from which a high level of hysteresis loop can be designed and achieved. The present study provides a foundation for novel devices based on multilevel ferroelectric switching, such as multilevel data storage memory, multi-logic gates, and electromechanical devices with multilevel control.

Non-woven carbon paper as current collector for Li-ion/Li2S system: Understanding of the first charge mechanism

The use of porous 3D current collectors, especially carbon-based ones, has already been demonstrated as a concept of choice as alternative to standard 2D aluminium foil in sulfur positive electrodes. In this study, we investigated non-woven carbon (NwC) as a 3D current collector for Li2S positive electrodes. Improved charge and discharge capacities, and capacity retention were obtained, as compared with electrodes containing Al-foil current collector. However, irrespective of the current collector used, the initial charge presents a particular and weakly reproducible voltage shape. By applying GITT and XRD measurements an electrochemical mechanism was proposed to explain the initial charge profile, in terms of equilibrium potential and polarization phenomenon. During the 1st charge, competitive oxidation reactions can be evoked involving both Li2S and long polysulfide intermediates (Li2Sn), in the main part of the charge process. While the oxidation of Li2S is thermodynamically favorable over the oxidation of Li2Sn, the kinetics of the latter is much more rapid.

Characterization of deep level defects and thermally stimulated depolarization phenomena in La-doped TlInS2 layered semiconductor

Lanthanum-doped high quality TlInS2 (TlInS2:La) ferroelectric-semiconductor was characterized by photo-induced current transient spectroscopy (PICTS). Different impurity centers are resolved and identified. Analyses of the experimental data were performed in order to determine the characteristic parameters of the extrinsic and intrinsic defects. The energies and capturing cross section of deep traps were obtained by using the heating rate method. The observed changes in the Thermally Stimulated Depolarization Currents (TSDC) near the phase transition points in TlInS2:La ferroelectric-semiconductor are interpreted as a result of self-polarization of the crystal due to the internal electric field caused by charged defects. The TSDC spectra show the depolarization peaks, which are attributed to defects of dipolar origin. These peaks provide important information on the defect structure and localized energy states in TlInS2:La. Thermal treatments of TlInS2:La under an external electric field, which was applied at different temperatures, allowed us to identify a peak in TSDC which was originated from La-dopant. It was established that deep energy level trap BTE43, which are active at low temperature (T ≤ 156 K) and have activation energy 0.29 eV and the capture cross section 2.2 × 10−14 cm2, corresponds to the La dopant. According to the PICTS results, the deep level trap center B5 is activated in the temperature region of incommensurate (IC) phases of TlInS2:La, having the giant static dielectric constant due to the structural disorders. From the PICTS simulation results for B5, native deep level trap having an activation energy of 0.3 eV and the capture cross section of 1.8 × 10−16 cm2 were established. A substantial amount of residual space charges is trapped by the deep level localized energy states of B5 in IC-phase. While the external electric field is applied, permanent dipoles, which are originated from the charged B5 deep level defects, are aligned in the direction of the applied electric field and the equilibrium polarization can be reached in a relatively short time. When the polarization field is maintained, while cooling the temperature of sample to a sufficiently low degrees, the relaxation times of the aligned dipoles drastically increases. Practically, frozen internal electric field or electrets states remain inside the TlInS2:La when the applied bias field is switched off. The influence of deep level defects on TSDC spectra of TlInS2:La has been revealed for the first time.

Scale effects and the formation of polarization vortices in tetragonal ferroelectrics

Vortices consisting of 90° quadrant domains are rarely observed in ferroelectrics. Although experiments show polarization flux closures with stripe domains, it is as yet unclear why pure single vortices are not commonly observed. Here, we model and explore the energy of polarization patterns with vortex and stripe domains, formed on the square cross-section of a barium titanate nanowire. Using phase-field simulations, we calculate the associated energy of polarization patterns as a function of nanowire width. Further, we demonstrate the effects of surface energy and electrical boundary conditions on equilibrium polarization patterns. The minimum energy equilibrium polarization pattern for each combination of surface energy and nanowire width is mapped for both open- and short-circuit boundary conditions. The results indicate a narrow range of conditions where single vortices are energetically favorable: nanowire widths less than about 30 nm, open-circuit boundary condition, and surface energy of less than 4 N/m. Short-circuit boundary conditions tend to favor the formation of a monodomain, while surface energy greater than 4 N/m can lead to the formation of complex domain patterns or loss of ferroelectricity. The length scale at which a polarization vortex is energetically favorable is smaller than the typical size of nanoparticle in recent experimental studies. The present work provides insight into the effects of scaling, surface energy, and electrical boundary conditions on the formation of polarization patterns.

Impact of Proton Concentration on Equilibrium Potential and Polarization of Vanadium Flow Batteries

AbstractThe electrolyte in the vanadium flow battery (VFB) is one of the key materials in determining the energy density, which further limits its commercialization. A novel strategy of varying the proton concentration to improve electrolyte utilization is proposed to enhance the energy density of the VFB. The effects of proton concentration on the equilibrium potential and polarization have been investigated systematically. Superior electrolyte utilization at a proton concentration of 5 mol L−1 is achieved; this corresponds to a high volumetric energy density (22.5 Wh L−1) and volumetric capacity density (14.8 Ah L−1), which is favorable toward the commercial application of VFBs. This general strategy also inspires research on improving electrolyte utilization in other flow battery systems.

Analytic theory of hydrostatic stress effect on switching dynamics in ferroelectrics

Polarization switching of ferroelectric materials is influenced by extensive variables such as electric field, temperature and stress. Currently a variety of theoretical approaches are used to gain a deeper understanding of the influence of extensive variables on the dynamic properties of ferroelectrics. In this paper a Landau-type model for second order bulk materials is used to study the extent to which hydrostatic stress affects the switching dynamic behaviours in ferroelectric material. Exact expressions for equilibrium polarization and switching time are derived analytically and then used to examine the dynamic behaviour in regimes of high electric field and hydrostatic stress. The second aspect of this work is to determine the relationship between empirical rules used to describe dynamic behaviour with the exact expressions derived from this model.