Fast and Superfast Motion of Ferroelectric Domain Boundaries

We present the review of our systematic investigation of the propagation of domain walls/boundaries in wide velocity range in uniaxial ferroelectrics with optically distinguished domains, such as lead germanate Pb₅Ge₃O₁₁, congruent and stoichiometric lithium tantalate LiTaO₃ and lithium niobate LiNbO₃. In situ optical observation of domain kinetics was widely used. Three different regimes of boundary propagation have been revealed and investigated in details. It was shown that except of conventional “slow” sideways domain wall motion “fast” and “superfast” domain growth regimes through formation of complicated self-maintained domain boundaries could be obtained. The crucial role of the retardation of screening/compensation of depolarization field has been pointed out. It was demonstrated that realization of proper regime is determined by the ratio of switching rate to bulk screening one. Computer simulation has been applied for verification of proposed approach.     

Evolution of ferroelectric and piezoelectric response by heat treatment in pseudocubic BiFeO<Subscript>3</Subscript>–BaTiO<Subscript>3</Subscript> ceramics

Heat treatment of ceramics is an important process to tailor the fine electromechanical properties. To explore the criteria for optimized heat treatment in a perovskite structure of (1–x)Bi_1.05FeO₃–xBaTiO₃ (BF–BT100x) system, the structural phase relation, ferroelectric and piezoelectric response of BF–BT36 and BF–BT40 ceramics prepared by furnace cooling (FC) and quenching process were investigated. The X-ray diffraction examination showed single pseudocubic perovskite structure for all the ceramics. The homogenous microstructure was obtained for all ceramics with relatively large grain size in the furnace cooled samples. Well saturated ferroelectric hysteresis loops and enhanced piezoelectric constant (d₃₃?=?97 pC/N) were achieved by quenching process. Dielectric curve of BF–BT36 showed large dielectric constant at its Curie temperature, however, BF–BT40 showed diffused relaxor-like dielectric anomalies. Quenched BF–BT36 samples showed typical butterfly like field induced strain curves, however negative strain decreased in BF–BT40 ceramics. From these investigated study, it is observed that BF–BT ceramics are very sensitive to the heat treatment process (furnace cooling and quenching) on the dielectric, electromechanical properties.     

Modeling of transport properties of interfacially controlled electroceramics: Application to n-conducting barium titanate

Grain boundary regions in n-conducting barium titanate (BaTiO₃) are re-oxidized during the cooling process after sintering the ceramics in air. The kinetics of this re-oxidation process is determined by rapid transport of oxygen along the grain boundaries and slow (rate-determining) diffusion of cation vacancies from the grain boundaries into the grains until the diffusion process is frozen-in. Based on numerical calculations of frozen-in diffusion profiles of cation vacancies at grain boundary regions for various cooling rates, a modified Schottky-barrier model is introduced in order to calculate the grain boundary resistivity as a function of temperature from the Curie-point up to 900°C. A change of the activation energy at approximately 500°C is predicted owing to an enrichment of holes in the space charge layers at elevated temperatures. The modeling results are compared with experimental data for BaTiO₃-based positive temperature coefficient resistors (PTCRs).     

Formation Mechanism of Plate-like Bi4Ti3O12 Particles in Molten Salt Fluxes

The plate-like Bi₄Ti₃O₁₂ particles were prepared by molten salt synthesis method. The influence of sintering temperature and cooling process on the microstructure of Bi₄Ti₃O₁₂ powders was studied. Much larger particles were formed at higher temperatures. The particles could grow larger in slow cooling process. The formation mechanism of plate-like Bi₄Ti₃O₁₂ particles in Na₂SO₄-K₂SO₄ system could be viewed as four processes: (1) solid reaction and nucleation, (2) plate-like structure formation, (3) diffusion and edge nucleation, (4) diffusion and epitaxial growth.     

Preparation and properties of nano-crystalline Pb0·72La0·28TiO3 thin films

Abstract

Nano-crystalline Pb_0·72La_0·28TiO₃ (PLT28) thin films were prepared on glass substrate by RF magnetron sputtering using the sequence of post-deposition annealing process. 150 to 200nm thick films were found to be comprised of 5nm grains in the perovskite structure. The effective birefringence shift in the nano-PLT films as a function of applied electric field exhibits a quadratic electro-optical behavior. The hysteresis measurement indicated that the films display slim-loop character. Experimental results showed that the nano PLT film is an electric fieldinduced ferroelectric.     

Dairy cream as a phantom material for biexponential diffusion decay

Commercially available aliquots of dairy cream are shown to have diffusion decay curves characterized by biexponential functions when studied over a wide range of b-factors. The fast and slow diffusion components responsible for the biexponential decay are attributed to water and lipid protons, respectively. The fast diffusion coefficient and relative fast and slow diffusion component fractions obtained from biexponential fits of cream phantoms over a wide range of b-factors up to 3,000 s/mm² are similar to those obtained previously for brain. The slow diffusion coefficient from lipid protons is smaller than that found in the brain. Overall, however, the results suggest that dairy cream can serve as a widely available phantom material for testing software and hardware components designed to perform quantitative, biexponential diffusion decay studies.     

Stability of perovskite-type clusters in melts for relaxor ferroelectric crystal growth

Relaxor ferroelectric single crystals with perovskite structure were novel materials which might revolutionize some applications in medical ultrasonic imaging, telecommunication and ultrasonic devices. But during crystal growth and melt solidification, pyrochlore-type phase presented now and then, which reduced the integrality and properties of ferroelectric single crystals. This work dealt with the stability of perovskite-type clusters in melts for relaxor ferroelectric crystal growth by quenching and slow cooling. Differential thermal analysis (DTA) and X-ray diffraction (XRD) were employed to ascertain the transformation temperature between perovskite-type and pyrochlore-type clusters. Investigated results indicated that clusters in Pb(Mg_1/3Nb_2/3)O₃–32PbTiO₃ (PMN–32PT) melt which was under suitable temperature and prepared by proper pre-synthetic method were all and singular of perovskite-type structure, which established the fundament of PMN–32PT relaxor ferroelectric crystal growth from melts by Bridgman method. Whereas, clusters in Pb(Zn_1/3Nb_2/3)O₃–9PbTiO₃ (PZN–9PT) melt were of perovskite-type structure, pyrochlore-type structure, homologous PbO structure or homologous ZnO structure. During crystal growth from melts by Bridgman method, the pyrochlore phase always appeared.     

Modification on PTCR behavior of (Sr0.2Ba0.8)TiO3 materials by post-heat treatment after microwave sintering

Abstract

Effect of post-sintering treatment on PTCR behavior of (Sr_0.2Ba_0.8)TiO₃ materials prepared by microwave-sintering (ms) process was compared to that prepared by rapid thermal sintering (RTS) process. The microwave-sintering process needed only 1130°C-40 min to effectively densify (Sr_0.2Ba_0.8)TiO₃ materials. The grain size was around 6 μm and PTCR characteristics was around ρ_max/ρ_min≈ 10^1.75, with T_c = 50°C. Lowering the cooling rate after sintering substantially increases the resistivity jump (ρ_max/ρ_min) from 10² to 10^3.4, without altering the microstructure. The annealing at 1250°C for 2 h markedly increased the resistivity jump to (ρ_max/ρ_min)≈10⁶. On the other hand, the rapid thermal sintering (RTS) process required 1320°C-30 min to fully develop the good microstructure (～15 μm) and PTCR property (ρ_max/ρ_min ～ 10^3.0). Post-sintering process, including cooling rate control and annealing, did not improve the electrical properties of these samples, that is ascribed to the slow-cooling rate characteristics of RTS-process for a temperature lower than 800°C.     

TEM observations of magnetic domains and grain boundaries on spin‐sprayed ferrite films exhibiting high permeability usable for gigahertz noise suppressors

Lorentz TEM observations of magnetic domain wall motion, as well as TEM observations of grain boundaries, were performed on spin‐sprayed ferrite films #1 (Ni_0.17Zn_0.22Fe_2.61O₄) and #2 (Ni_0.19Zn_0.20Co_0.03Fe_2.58O₄), both 0.5 µm in thickness. They exhibit much higher natural resonance frequencies than the bulk ferrite and thus have been applied to gigahertz noise suppressors. Films #1 and #2 exhibit prominent and weak in‐plane uniaxial magnetic anisotropy, respectively, which is induced along the liquid flow direction during spin‐spraying. Both films have columnar crystallites with 100‐200 nm widths aligned perpendicular to the film plane, and the boundaries of the crystallites have no pores or impurity phases. Therefore, the crystallites are magnetically exchange‐coupled, which is responsible for the unusually high permeability and high natural resonance frequencies of the films. Under zero bias magnetic field, film #1 exhibits mosaic‐shaped magnetic domains, whereas film #2 exhibits magnetic domains elongated along the easy magnetization axis, both several hundred nanometers in width. For both films the domain structure remains unchanged when an in‐plane bias DC magnetic field,H_dc, of up to 10 Oe is applied along the hard axis. Under a stronger H_dc, the domain structure prominently changes, and the domain walls disappear when H_dc exceeds ∼100 Oe. This confirms our previous finding that the initial permeability is ascribed only to magnetization rotation, with no contribution from domain wall motion [J. Magn. Magn. Mater., 278 , 256 (2004)]. © 2007 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Modal based stability analysis of power systems using energy functions

In this paper a physically based decomposition technique is exploited to perform direct stability analysis of power systems using the energy function method. The slow-coherency analysis decomposes the system into r areas associated with (r- 1) slow modes of the linearized system. The centres of inertia (COIs) of these areas from the slow subsystem and the rest of the fast modes are associated with different areas. The system transient energy function E is decomposed into E_slow associated with the slow variables of the slow subsystem, E_fast associated with the fast variables of the r-areas and E_fast-slow which is a function of both fast and slow variables. Depending on the fault location and strength of connection between areas, the sum of E_slow, E_fast-slow and E_fast of the disturbed arease constitutes a good approximation to the system energy function. Using this partial E-function and the potential energy boundary surface (PEBS) method, both E_cr and t_cr are computed. Numerical results on a 10-machine 39-bus system are presented in support of the technique.     

Refractive index properties of SiN thin films and fabrication of SiN optical waveguide

SiN thin films having excellent surface morphology for the optical device application were synthesized using a plasma enhanced chemical vapor deposition (PECVD) method at low temperature (350^°C) using silane (SiH₄) and nitrogen (N₂). The effects of the SiH₄/N₂ flow ratio, rf power and annealing on the SiN films were investigated. The optical and structural properties of SiN films were characterized using an ellipsometry, a fourier-transform infrared spectroscopy (FT-IR), and an atomic force microscope (AFM). The refractive index increased from 1.6 to 2.3 as the SiH₄/N₂ ratio was increased from 0.17 to 1.67. The rms surface roughness decreased from 14.1 to 3.6Å after post-deposition annealing process performed at 800^°C for 1hr in an air ambient. We could fabricate straight waveguides based on a three layer structure and have no problems with step coverage.     

Microstructures and Piezoelectric Properties of Microwave-Sintered Pb(Mn1/3Nb2/3)0.1Zr0.52Ti0.38O3 Ceramics

In this work, we study the microstructural modifications and the electrical properties of Pb(Mn_1/3Nb_2/3)_0.1Zr_0.52Ti_0.38O₃ + 1.5 wt% PbO (PMNZT) ceramics processed using microwave sintering (MS) and conventional sintering (CS) techniques. Dielectric and piezoelectric properties were evaluated via an impedance/gain phase analyzer and the corresponding microstructures were examined using transmission electron microscopy (TEM). The experimental results imply that smaller grain size and less loss of PbO obtained for specimens using MS process. The MS specimens (1200°C-15 min) show the remanent polarization (P_r) of 24.1 μC/cm², coercive field (E_c) of 15.2 kV/cm, dielectric constants (?_r) of 432 and electromechanical coupling coefficient (k_p) of 0.47, which are superior to CS specimens (1200°C-2 h) with P_r = 19.5 μC/cm², E_c = 16.1 kV/cm, ?_r = 395 and k_p = 0.45. The results revealed that the MS process shows capability of fabrication of good quality PMNZT ceramics with advantages of lowing process temperature and shorten process time. TEM investigations of MS specimens show that smaller grain size and simpler domain arrangements exist inside the specimens. From the energy aspect, the domain structure in piezoelectric ceramics below the temperature of a ferroelectric structural phase transformation is a result of energy minimization. Homogeneous elastic strain energy is reduced at the expense of domain boundary energy during the phase transformation. Density of domain boundaries, 71° (109°) domains in particular, depends on and increases with the grain size. Base on the above argument, therefore, CS specimens exhibit more complicated domain arrangements because they possess larger grain size than MS specimens do. The microstructural investigations suggest that the electrical property difference is intimately related to different sintering methods since the complicated domain arrangements could hinder domain moving and deteriorate electrical performance. In addition, the microstructural characteristics in both MS and CS specimens will be discussed further in the context.     

The effect of Bi4Ti3O12 particles addition in lead-free Bi0.5(Na0.75K0.25)0.5TiO3 ceramics

We studied the effect of Bi₄Ti₃O₁₂ (BiT) platelet addition in Bi_0.5(Na_0.75K_0.25)_0.5TiO₃ (BNKT) ceramics by preparing two kinds of BNKT ceramics. One type of BNKT ceramic was fabricated by a conventional solid state reaction method (normal sample), while the other by addition of 15 wt% BiT platelets to BNKT powders (BiT-added sample). In the case of BiT-added BNKT ceramics, plate like grains were formed by the reaction of BiT platelets with Na₂CO₃, K₂CO₃, and TiO₂ during the sintering process. The grain size of BiT-added BNKT ceramics was 10 times larger than that of normal BNKT ceramic. The piezoelectric strain and d₃₃ values of BiT-added BNKT ceramics were 0.135% and 225 pm/V, respectively. These values were 35% higher than those of normal BNKT ceramics. The piezoelectric properties of BiT-added BNKT ceramics were enhanced by the higher domain activity due to a decrease in domain density at larger grain sizes.     

Structural Investigation of Thin SrTiO3 Films Grown on MgO and LaAlO3 Substrates

Structural investigations of SrTiO₃ thin films deposited by pulsed laser ablation onto MgO and LaAlO₃ substrates are presented. The residual strain along the c-axis (the growth direction) was evaluated with conventional X-ray diffraction. Evaluation of the in-plane strain was accomplished with grazing-incidence X-ray diffraction. The unit cell of the STO film on LAO substrate had an orthorhombic structure with a coexistence of compressive strain along a-axis and tensile strain along b-axis. The film on an MgO substrate was tetragonal, stretched along the c-axis and compressed in-plane. The elemental composition of the samples was investigated using an INCA system for X-ray elemental analysis attached to a Hitachi S-4300 SEM. A comparison of the residual strain and stoichiometric ratio between as-deposited films and films annealed for four hours at 1100°C in flowing oxygen is presented.     

Three Dimensional Domain Structure in Epitaxial Barium Titanate Thin Films

The three dimensional domain structure of barium titanate thin films was determined using a serial sectioning technique. The domain structure varied sharply through the film thickness, being primarily a-oriented near the substrate and increasingly c-oriented away from this interface. The variation in domain structure is explained in terms of a strain gradient due to partial relaxation of epitaxial coherency strains. The refractive index also varied through the film thickness. A simple relationship based on areal fraction of each domain type aptly described the changes in refractive index with domain structure. These results indicate the importance of understanding three-dimensional domain structure and its impact on film properties.     

Novel Common Cell Via and Etch Stopper Technology for 0.25 μM 1T1C 32 MBIT FRAM

In the 0.25 μm FRAM technology generation, it is extremely difficult to define the hole-type cell via on very small top electrode area, because there is no process margin for the hole type cell via. Therefore, a runner via technology based on line-type cell via with Ir etch stopper is developed for 0.25 μm FRAM technology generation. However, it was found that the severe charge degradation occurred during the runner cell via process. It was found that the stress of Ir film plays a dominant role in degrading the capacitor value. Since the Ir film shows severe severe stress variation from compressive to tensile during heating and cooling, the ferroelectric capacitors using the Ir etch stopper show the charge degradation during integration. Therefore, we developed a common cell via scheme and stable PE-SiN etch stopper which possess compressive stress and high etching selectivity against PSG film for replacing Ir etch stopper. The polarization value of ferrolectric capacitor was not degraded after etch-stopper process. The 0.25 μm ferroelectric capacitors exhibited excellent P _r value of 15 μC/cm² after completing whole process integration, which guarantees a reliable high yield.     

Quantitative assessment of regional myocardial function in a rat model of myocardial infarction using tagged MRI

We characterized global and regional left ventricular (LV) function during post myocardium infarction (MI) remodeling in rats, which has been incompletely described by previous MRI studies. To assess regional wall motion, four groups of infarcted animals corresponding to 1–2, 3–4, 6–8 and 9–12 weeks post-MI respectively were imaged using a fast gradient echo sequence with a 2D spatial modulation of magnetization (SPAMM) tagging preparation. An additional group was serially imaged (1–2 and 6–7 weeks post-MI) to assess the global function. Regional and global functional parameters of infarcted rats were compared to non-infarcted normal rats. Compared to normal rats, a decrease in ejection fraction (70 ±7 vs. 40 ± 8%, p<0.05) was observed in rats with MI. Maximal and minimal principal stretches (₁, ₂) and strains (E₁, E₂), principal angle () and displacement varied regionally in normal rats but deviated significantly from the normal values in rats with MI particularly in the infarcted and adjacent zones. Not only was strain magnitude reduced segmentally post-MI, but strain direction became more circumferentially oriented, particularly in rats with larger infarctions. We report the first regional myocardial strain values in normal and infarcted rats. These results parallel findings in humans, and provide a unique tool to examine regional mechanical influences on the remodeling process.     

Empirical Pseudopotential Method for the Band Structure Calculation of Strained-Silicon Germanium Materials

The band structure of strained-silicon germanium (Si_{1 – x} Ge _x ) is calculated as a preliminary step in developing a full band Monte Carlo (FBMC) simulator. The band structure for the alloy is calculated using the empirical pseudopotential method (EPM) within the virtual crystal approximation (VCA). Spin-orbit interaction is included into the calculation via the Löwdin quasi-degenerate perturbation theory, which significantly reduces the computation time. Furthermore, strain is included by utilizing basic elastic theory. Ultimately, the band structure for strained Si_{1 – x} Ge _x is calculated at various germanium concentrations.     

Investigation of ferroelectric domains and structural twins in barium titanate crystal

Abstract

The Ferroelectric domain patterns and the {111} twin boundaries are frequently observed in the BaTiO₃ ceramics. The ferroelectric domain structures in BaTiO₃ single crystal were investigated by a conventional and a synchrotron white beam X-ray topographies. The domain structure of {111} twinned crystal sample was also observed under a polarizing microscope and under a TEM. The relation between the {111} twin and the ferroelectric domain structure was verified by insitu observation of the behaviors of 90° domains during the phase transitions. It is observed that the 90° ferroelectric domain boundaries around the {111} twin boundary which was produced during the crystal growth obey the symmetry relation of the {111} twinning and that they meet each other coherently at the same position of the {111} twin boundary.     

Application of fluorinated SiO2 interlayer dielectrics for ferroelectric memory

Abstract

Fluorine-doped silicon oxide (SiOF) as interlayer dielectric (ILD) was deposited over PZT capacitors by electron cyclotron resonance (ECR) chemical vapor deposition using SiF₄ and N₂O gases. In the conventional deposition of SiO₂ ILD layer using hydrogen-contained source gases, the properties of ferroelectric capacitors are known to be degraded during the formation of SiO₂ layer. In this study, we examined the degradation of electrical properties of SiOF-deposited PZT capacitors. The remnant polarization and leakage currents were not degraded after the deposition of SiOF. We observed that the fluorine atoms were not diffused into the metal electrode in both cases of the SiOF deposited PZT capacitors and post-deposition annealed capacitors. The SiOF films deposited in the high CF₄ flow rate exhibited rough columnar structure on the metal electrodes. We can successfully deposit SiOF in a smooth morphology by introducing TiO₂buffer layer or using the novel deposition method of changing the SiF₄ flow rate, namely two-layer-deposition method.