Influence of interface intermixing and periodicity on internal electric field and polarization in ferroelectric superlattices

A thermodynamic model is developed to study the effect of interface intermixing and modulation period on the internal electric field of a superlattice comprising alternate layers of ferroelectric and dielectric. The intermixed layer plays an important role in determining both the polarization and internal electric field modulation profiles of the superlattice. The presence of interface intermixed layer gives rise to inhomogeneous internal field and polarization, which enhances the depolarization field of the superlattices. Interface intermixing; however, does not have much influence on the polarization behavior of the superlattice. Ferroelectric volume fraction or thickness ratio of ferroelectric to dielectric is one of the key parameter for controlling the properties of a superlattice consisting alternate layers of ferroelectric and dielectric.     

Calculations on switching characteristics of ferroelectric–paraelectric superlattices

We study the switching characteristics of a superlattice consisting of a periodic ferroelectric and paraelectric layer with interlayer coupling using the Landau–Khalatnikov equation. The inhomogeneity of polarization at the ferroelectric or paraelectric surface is described by the extrapolation length δ. The effects of various parameters such as the viscosity coefficient, layer thickness, interlayer coupling and temperature on the switching characteristics are investigated. Our study shows that the hysteresis loop and switching current versus applied field depend strongly on those parameters. Interlayer coupling increases the ferroelectricity of the superlattice, leading to a strong enhancement in polarization and coercive field.     

Static experimentations of the piezoceramic d15-shear actuation mechanism for sandwich structures with opposite or same poled patches-assembled core and composite faces

Static d₁₅-shear actuated smart composites consisting of glass fiber/epoxy layers sandwiching piezoceramic shear patches-assembled cores were investigated experimentally and numerically. The piezoceramic cores were formed by connecting two or three patches with the same or opposite polarization directions. For each cantilevered benchmark the shear-induced transverse tip deflection, under increasing actuation voltage, ranging from 61.5 V to 198 V, was measured by an electronic speckle pattern interferometer system. The performance of the shear actuated smart composites was characterized by their shear-induced transverse deflection per length per voltage. It was found that this performance is much better at high voltages for which the response becomes nonlinear. For verification of the experimental results the proposed benchmarks were simulated within ABAQUS^® commercial code using three-dimensional piezoelectric finite elements. The comparison of the obtained experimental and simulation results show a nonlinear dependence of the transverse deflection for voltages higher than around 92 V.     

Modulated internal electric field,dielectric susceptibility and polarization in ferroelectric superlattices

A thermodynamic model for describing interface intermixing in a superlattice combining a ferroelectric and a paraelectric is developed. Formation of intermixed layer at interfaces leads to a periodic modulation of ferroelectric properties in these superlattices. Spatially-varying internal electric field, dielectric susceptibility and polarization of these superlattices are calculated. Effects of modulation period and temperature on the internal electric field, dielectric susceptibility and polarization of these superlattices with inhomogeneous properties are examined. Correlation between these ferroelectric properties is established and discussed.     

Computation of the static polarizabilities of multi-wall carbon nanotubes and fullerites using a Gaussian regularized point dipole interaction model

A Gaussian regularized dipolar model has been developed for carbon sp² structures. It is based on three parameters: the two components of the local polarizability tensor of the carbon atoms and a regularization parameter related to the size of the electronic clouds. The parameters of the model have been adjusted to reproduce the polarizability of isolated and FCC aggregates of C₆₀ and C₇₀ molecules, while avoiding any polarization catastrophes. The model has been applied to semiconducting single-wall nanotubes of finite and infinite length. Asymptotically, the axial and transverse polarizability per unit length vary linearly and quadratically with the tube radius, respectively, as already predicted by a tight-binding model and ab initio methods. As for multi-wall nanotubes, we show that the transverse polarizability is close to the transverse polarizability of the external layer taken alone, while the axial polarizability is close to the sum of the axial polarizabilities of the individual layers.     

Concentration polarization in a narrow reverse osmosis membrane channel

Concentration polarization in a narrow reverse osmosis channel is bounded by the channel height and under the influence of variable transverse velocity. An attempt was made in this article to quantify concentration polarization in such a narrow membrane channel. The transverse velocity in the membrane channel was first determined and its impact on concentration polarization was investigated. Based on the concept of retained salt, analytical equations were developed for the wall salt concentration at an arbitrary point in the narrow membrane channel. Finally, development of concentration polarization in typical reverse osmosis channels under various conditions was numerically simulated and discussed. Interesting results on the details of concentration polarization in the narrow reverse osmosis channel that had never been reported before were revealed with this mechanistic model. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Fabrication of a new type of organic-inorganic hybrid superlattice films combined with titanium oxide and polydiacetylene

We fabricated a new organic-inorganic hybrid superlattice film using molecular layer deposition [MLD] combined with atomic layer deposition [ALD]. In the molecular layer deposition process, polydiacetylene [PDA] layers were grown by repeated sequential adsorption of titanium tetrachloride and 2,4-hexadiyne-1,6-diol with ultraviolet polymerization under a substrate temperature of 100°C. Titanium oxide [TiO₂] inorganic layers were deposited at the same temperatures with alternating surface-saturating reactions of titanium tetrachloride and water. Ellipsometry analysis showed a self-limiting surface reaction process and linear growth of the nanohybrid films. The transmission electron microscopy analysis of the titanium oxide cross-linked polydiacetylene [TiOPDA]-TiO₂ thin films confirmed the MLD growth rate and showed that the films are amorphous superlattices. Composition and polymerization of the films were confirmed by infrared spectroscopy. The TiOPDA-TiO₂ nanohybrid superlattice films exhibited good thermal and mechanical stabilities.     

Observation of linear and quadratic magnetic field-dependence of magneto-photocurrents in InAs/GaSb superlattice

We experimentally studied the magneto-photocurrents generated by direct interband transition in InAs/GaSb type II superlattice. By varying the magnetic field direction, we observed that an in-plane magnetic field induces a photocurrent linearly proportional to the magnetic field; however, a magnetic field tilted to the sample plane induces a photocurrent presenting quadratic magnetic field dependence. The magneto-photocurrents in both conditions are insensitive to the polarization state of the incident light. Theoretical models involving excitation, relaxation and Hall effect are utilized to explain the experimental results.     

Temperature-dependent compressive creep of ferroelectric ceramics and evolution of remnant state variables

A pre-poled lead titanate zirconate cube specimen is subject to constant compressive stress of six different magnitudes at four different room and high temperatures. Electric displacement in poling direction and strains in longitudinal and transverse directions are measured and plotted versus time. The effects of stress and temperature on creep behavior of the material are discussed. Then compressive stress of impulse type with gradually increasing magnitude is applied to the specimen and it is found that linear material properties depend linearly on so-called relative remnant polarization at all four temperatures. Using the linear relations, the evolution of remnant state variables during compressive creep is calculated and discussed. Longitudinal and transverse remnant strains are shown to depend linearly on relative remnant polarization, and the relations between relative remnant polarization and applied compressive stress are discussed.     

BaTiO3铁电薄膜在硅基片上的取向生长

采用脉冲激光沉积方法,在硅基片上先沉积MgO或CeO2缓冲层后再制备BaTiO3(BTO)铁电薄膜.通过原位反射高能电子衍射来监测MgO,CeO2缓冲层在硅基片上的生长行为.用X射线衍射测定BTO薄膜的结晶取向.并利用压电响应力显微镜观察了铁电薄膜的自发极化形成的铁电畴.结果表明:BTO薄膜在不同的缓冲层硅基片上以不同的取向生长,在织构的MgO/Si(001)基片上为(001)择优,择优程度与MgO织构品质有关,其中在双轴织构MgO缓冲层上为(001)单一取向;在CeO2(111)缓冲层上为(011)单一取向.(001)取向的BTO薄膜具有更大的面外极化,而(011)取向的BTO薄膜具有更大的面内极化.     

Domain morphology of newly designed lead-free antiferroelectric NaNbO3-SrSnO3 ceramics

Reversible antiferroelectric-ferroelectric phase transitions were recently observed in a series of SrSnO₃-modified NaNbO₃ lead-free antiferroelectric materials, exhibiting well-defined double polarization hysteresis loops at ambient conditions. Here, transmission electron microscopy was employed to investigate the crystallography and domain configuration of this newly designed system via electron diffraction and centered dark-field imaging. It was confirmed that antiferroelectricity is maintained in all compositions, manifested by the characteristic ¼ superlattice reflections in the electron-diffraction patterns. By investigating the antiferroelectric domains and domain boundaries in NaNbO₃, we demonstrate that antiphase boundaries are present and their irregular periodicity is responsible for the streaking features along the ¼ superlattice reflections in the electron-diffraction patterns. The signature domain blocks observed in pure NaNbO₃ are maintained in the SrSnO₃-modified ceramics, but disappear when the amount of SrSnO₃ reaches 7 mol.%. In particular, a well-defined and distinct domain configuration is observed in the NaNbO₃ sample modified with 5 mol.% SrSnO₃, which presents a parallelogram domain morphology.     

Enhanced hydrogen entry into iron from 0.1 M NaOH at definite potentials

This work aimed at explaining the enhancement of hydrogen entry into iron from alkaline solution occurring at definite potentials. Hydrogen permeation rate (HPR) through a 35-μm thick iron membrane was measured with the electrochemical technique in 0.1 M NaOH at 25 °C during cathodic and anodic polarizations. Enhanced HPR was observed at potentials of oxide reduction or iron oxidation, and potentials more cathodic than about −1.65 V_NHE during prolonged galvanostatic polarization. XPS analysis showed that after the polarization, surface layers contained hydrated iron oxides and that amount of these products increased with the polarization time. It is suggested that the enhanced hydrogen entry can be explained by acidification of the near-metal solution due to iron oxidation and/or oxide reduction, and probably by a promoting effect of some Fe-O species. It is proposed that these effects are associated with surface layers. They can affect hydrogen entry as a source of protons in the oxide reduction, as a diffusion barrier making the near-metal acidification possible, and as a resistance causing an IR drop. Strong enhancement of HPR after prolonged galvanostatic polarizations can be associated with the formation of thick surface layers with IR drop enabling anodic oxidation of iron under these layers.     

Incommensurately Modulated Polar Structures in Antiferroelectric Sn-Modified Lead Zirconate Titanate: The Modulated Structure and Its Influences on Electrically Induced Polarizations and Strains

Tin-modified lead zirconate titanate ceramics, Pb_(0.99)Nb_0.02-[(Zr_0.55Sn_0.45)_0.98Ti_y]O₃ (PZST 45/ y /2), have been studied by hot-stage transmission electron microscopy (TEM), dielectric spectroscopy, Sawyer-Tower polarization ( P—E ) measurements, and electrically induced strain (ε- E ) techniques. The compositional range chosen for investigation was 0 < y < 0.09. Electron diffraction confirmed the existence of 1/x〈110〉 superlattice reflections in the previously designated antiferroelectric tetragonal state. For PZST 45/0/2, the room-temperature modulated structure was found to be commensurate with the lattice and isomorphous with the antiferroelectric orthorhombic phase of PbZrO₃. With increasing Ti content, the modulation wavelength (λ) increased and became incommensurate. At high temperatures, λ was found to be incommensurate for all compositions. A model is discussed which suggests that the origin of incommensuration is due to a competition between anti-ferroelectric and ferroelectric tendencies. P—E studies revealed antiferroelectric-ferroelectric (AFE—FE) polarization curves which possessed no hysteresis in the incommensurate region, indicating a thermally reversible AFE—FE transformation. On cooling, strong hysteresis developed as the incommensurate structure became metastably locked. Investigations then revealed that the electrically induced strain associated with the AFE—FE transformation was not realized until field strengths were attained significantly above that required for polarization saturation. It is believed that the electrically induced strain is decoupled from the polarization due to AFE—FE switching by the field modulation of the incommensurately modulated polar structure.     

Effect of periodic number of [Si/Sb80Te20]x multilayer film on its laser-induced crystallization studied by coherent phonon spectroscopy

The periodic number dependence of the femtosecond laser-induced crystallization threshold of [Si(5nm)/Sb₈₀Te₂₀(5nm)]_x nanocomposite multilayer films has been investigated by coherent phonon spectroscopy. Coherent optical phonon spectra show that femtosecond laser-irradiated crystallization threshold of the multilayer films relies obviously on the periodic number of the multilayer films and decreases with the increasing periodic number. The mechanism of the periodic number dependence is also studied. Possible mechanisms of reflectivity and thermal conductivity losses as well as the effect of the glass substrate are ruled out, while the remaining superlattice structure effect is ascribed to be responsible for the periodic number dependence. The sheet resistance of multilayer films versus a lattice temperature is measured and shows a similar periodic number dependence with one of the laser irradiation crystallization power threshold. In addition, the periodic number dependence of the crystallization temperature can be fitted well with an experiential formula obtained by considering coupling exchange interactions between adjacent layers in a superlattice. Those results provide us with the evidence to support our viewpoint. Our results show that the periodic number of multilayer films may become another controllable parameter in the design and parameter optimization of multilayer phase change films.     

Fatigue improvement in modified lead zirconate titanate ceramics through employment of La0.8Sr0.2MnO3 buffer layers

The dielectric, ferroelectric and fatigue properties of modified lead zirconate titanate (PZT) ceramics were investigated in terms of the effect of La_0.8Sr_0.2MnO₃ (LSM) buffer layers. The double sided LSM buffer layers resulted in a lower dielectric loss, a weaker frequency dependence of dielectric constant, a lower leakage current density, and an increase in the saturation polarization. Moreover, it was found that up to 1.4×10⁷ cycle numbers, the Ag||LSM/PZT/LSM||Ag capacitor, with remanent polarization decreased by 55%, was superior to the Ag||PZT||Ag capacitor by 85%. The results indicate that the LSM buffer layers can improve the fatigue endurance of the PZT ceramics with Ag electrodes, mainly because the accumulated charges were compensated at the interface junctions between the LSM buffer layers and the Ag electrodes. We fit the polarization fatigue data using a modified model and calculated the characteristic decay time of oxygen vacancy migration in the Ag||LSM/PZT/LSM||Ag and the Ag||PZT||Ag capacitors, respectively.     

Electromechanical Determination of the High-Field Phase Transition of Pb(Mg1/3Nb2/3)O3–PbTiO3–(Ba,Sr)TiO3 Relaxor Ferroelectrics

Ceramics in the (1 – x )[(1 – y )Pb(Mg_1/3Nb_2/3)O₃· y PbTiO₃] · x MeTiO₃ system, where Me is Sr or Ba, exhibit very large electrostrictive strains at reasonable drive fields. However, the optimum use temperature and frequency vary with the particular composition used. As relaxor ceramics, each composition has a broad transition from electrostrictive to partially piezoelectric behavior. The transition temperature ( T _t) can be roughly determined from strain or polarization properties; however, it can be more quantitatively determined from the effective electro-mechanical Q . A plot of induced transverse strain/induced polarization squared (effective Q ₁₂) as a function of temperature shows a sharp and unmistakable change in slope—this defines T _t. The slope of induced transverse strain/polarization (effective g₃₁) also shows a change in slope at T _t, although this is more gradual than that of effective Q . The indicated T _t correlates with those found from measurement of strain and polarization.     

Dark-field microscopy studies of polarization-dependent plasmonic resonance of single gold nanorods: rainbow nanoparticles

Orientation sensors require the monitoring of polarization-dependent surface plasmons of single nanoparticles. Herein, we present both the longitudinal and transverse surface plasmonic resonance from a single gold nanorod (AuNR) using conventional dark-field microscopy. The relative peak intensities of the transverse and longitudinal surface plasmons of a single AuNR can be successfully tuned by polarized excitation, which is an important step towards the use of transverse plasmon resonance of single AuNRs without photo-induced reshaping of nanoparticles. More interestingly, compared with AuNRs with small diameters, unique optical properties from AuNRs with diameters greater than 30 nm are revealed. As a result, optical images with different colors, rainbow nanoparticles (sea green, brown, red, yellow and green), depending on the polarization angle, can be revealed by a single AuNR. This result holds great promise for polarization-controlled colorimetric nanomaterials and single particle tracers in living cells and microfluidic flows.     

Formation of Catalytic Layers from Tetraethoxysilane-Based Sols for Use in Polymer Fuel Cells

The catalytic layers are designed and synthesized using sol–gel technology with the aim of improving catalytic layers and membranes of polymer fuel cells. The layers synthesized consist of a silicate matrix with uniformly distributed platinum nanoparticles. The current density is estimated from the steady-state polarization curves of anodic oxidation of hydrogen. An analysis of these estimates shows that the amount of platinum required to attain a certain current density with the use of the catalysts prepared according to the proposed technique is one order of magnitude smaller than that with the catalytic layers produced by a traditional activation.     

Effect of anodic oxidation on the corrosion behavior of Ti-based materials in simulated physiological solution

Anodic oxidation in acetic acid was investigated as a means of improving the corrosion resistance, in simulated physiologic solution, of titanium and two titanium-based alloys, Ti–6Al–7Nb and Nitinol. The composition and the thickness of the surface layers formed by anodization were analyzed using X-ray photoelectron spectroscopy. The electrochemical characteristics were investigated by linear polarization, cyclic polarization, and electrochemical impedance spectroscopy at the open circuit potential. Anodization of all these three metals resulted in the formation of TiO₂ as the main oxide. These layers improved the corrosion behavior in simulated physiological solution, as evidenced by significant increase in polarization resistance and decrease in corrosion current density. Electrochemical impedance results were interpreted based on the two-layer structure of the passive film. Anodization has thus been shown to provide a simple and effective means of improving the corrosion behavior of titanium-based alloys in simulated physiological solution.     

共振介质中极化强度的经典振子描述

文章用经典振子模型,讨论共振跃迁介质中宏观极化强度的驰豫过程;用图形说明了单个振子由于辐射而产生的驰豫过程和振子间由于弹性碰撞而产生的驰豫过程;介绍了影响驰豫时间的因素和几种典型介质的驰豫时间;由振子模型推导了共振介质的线性极化率公式.