Sol-Gel Processed Strontium Titanate Ceramics

Ferroelectric strontium titanate (SrTiO₃) ceramics have been prepared by sol-gel processing technique. Strontium acetate [Sr(CH₃COO)₂] and titanium (IV) isopropoxide [Ti(OC₃H₇]₄ were used as the precursors for the fabrication of the samples. The stock solution for the preparation of strontium titanate powder was prepared by dissolving strontium acetate in ethanol and acetic acid solvents and mixing the resultant solution with titanium (IV) isopropoxide in equimolecular amounts. The as fired powder was found to be amorphous. Annealing the amorphous powder at 700°C for one hour in air yields cubic phase crystalline SrTiO₃ powder. Ceramic samples were prepared by pressing and sintering the crystalline powder. The effect of sintering temperature on the structural and dielectric properties were investigated and analyzed.     

Yttrium Iron Garnet/Barium Titanate Multiferroic Composites

Dense multiferroic 0‐3 type composites encompassing BaTiO₃ and Y₃Fe₅O₁₂ were fabricated by the solid‐state reaction method. X‐ray diffraction data combined with scanning electron microscopy imaging show virtual immiscibility between the two phases, with the Y₃Fe₅O₁₂ ferrimagnetic phase well dispersed in the tetragonal BaTiO₃ ferroelectric matrix. Raman spectroscopy analyses corroborate the polar nature of the BaTiO₃ matrix in composites with a Y₃Fe₅O₁₂ content as great as 40 wt%. Ferrimagnetism is detected in all composites and no additional magnetic phases are distinguished. Although these dense ceramics can be electrically poled, they exhibit a very weak magnetoelectric response, which slightly increases with Y₃Fe₅O₁₂ content.     

Modification of Surface Texture by Grinding and Polishing Lead Zirconate Titanate Ceramics

Grinding and polishing affected the orientation of 90° domains at the surface of lead zirconate titanate (PZT) ceramics. This was quantified by using changes in the intensity ratio of the {002} and {200} X-ray reflections. Grinding unpoled PZT with 600-grit SiC paper gave X-ray intensity ratios similar to those of poled material. This implies that 90° domain realignments had occurred in the near surface region probed by the X-rays. Grinding poled samples with 600-grit SiC further increased the X-ray intensity ratio beyond that caused by poling, indicating that additional surface reorientation of 90° domains had occurred. The effects of diamond polishing depended on the size of the diamond particles. The use of 6-μm diamond had no effect on the {002}/{200} intensity ratio of either poled or unpoled samples, while polishing with 15- or 45-μm diamond significantly enhanced the 90° domain rotation. In unpoled samples, the increase in the X-ray intensity ratio then approached that induced by poling or grinding with 600-grit SiC paper. While the observed increase in X-ray intensity ratio upon grinding is attributed to the rotation of 90° domains, the simultaneous formation of 180° domains appears to minimize or reduce the increase in electrical polarization.     

Lead Zirconate-Lead Titanate Piezoelectric Ceramics with Iron Oxide Additions

An investigation of the effects of small additions of Fe₂O₃ to lead zirconate-lead titanate ceramics with compositions between 45 and 60 mol% lead zirconate has shown that, with 2 moles of PbO to balance each mole of Fe₂O₃, the solubility of Fe₂O₃ was about 0.8 wt% in compositions near the tetragonal-rhombohedra1 boundary. The dielectric constant and dissipation factor for both rhombohedra1 and tetragonal materials were decreased by addition of iron oxide, whereas the mechanical quality factor and frequency constant were increased. The dependence of the electromechanical properties on grain size was qualitatively similar for both undoped and iron oxide-doped materials; the presence of iron oxide inhibited grain growth and lowered the limiting grain size below which the electromechanical properties change rapidly with grain size.     

Spectroscopic Assessments of Domain Texture in Barium Titanate: II. Cathodoluminescence Analysis

An analytical procedure has been developed according to basic principles of electro-stimulated luminescence spectroscopy aimed at quickly visualizing with high spatial resolution the domain texture developed in tetragonal BaTiO₃ (BT) materials. In the first part of the paper, the relative intensity of the cathodoluminescence (CL) emission has been systematically collected from different crystallographic planes of BT single-crystal and modeled as a function of anisotropic crystal properties. In this context, an analytical expression has been put forward for quantitatively describing the response function of the CL probe, which links the intensity emission to refractive indexes and absorption coefficients pertaining to different planes of the perovskitic crystal. In the second part of the paper, the CL method is applied to visualize with nanometer-scale resolution the domain texture developed in a polycrystalline BT sample. This study demonstrates that CL spectroscopy is a valuable and efficient tool for the assessment of domain orientation in ferroelectric materials. The CL method possesses both wide-range screening capacity and the scanning flexibility of conventional scanning electron microscopy, coupled with a spatial resolution that is comparable with that obtainable by scanning probe microscopy techniques.     

Crystallographic Texture Development in Bismuth Sodium Titanate Prepared by Reactive-Templated Grain Growth Method

Bi_0.5Na_0.5TiO₃ (BNT) and 0.94Bi_0.5Na_0.5TiO₃·0.06BaTiO₃ (BNT–BT) bulk ceramics with extensive 〈100〉 texture were prepared by the reactive-templated grain growth method, using platelike Bi₄Ti₃O₁₂ (BIT) particles as templates for BNT. Calcined compacts were composed of matrix grains with random orientation and 〈100〉-oriented grains transformed from aligned BIT particles, and the texture developed by the growth of oriented grains during sintering. Ceramics with extensive texture were obtained by using the starting mixture containing the maximum concentration of platelike BIT to form the maximum volume fraction of oriented grains.     

纳米Fe2O3对钛酸铝陶瓷热稳定性能的影响

以溶胶–凝胶法制备的钛酸铝前驱体和固相合成法制备的纳米Fe2O3为原料,在不同温度煅烧保温2h制备出钛酸铝固溶体[Al2(1–x)Fe2xTiO5]。通过X射线衍射、电子探针分析并计算晶格常数和热分解率,研究了纳米Fe2O3含量以及烧成温度对烧后试样热分解性能的影响。结果表明:纳米Fe2O3很容易与Al2TiO5反应,形成固溶体,抑制钛酸铝陶瓷的热分解;随着纳米Fe2O3加入量的增加,Al2(1–x)Fe2xTiO5的晶格常数变大,热分解率降低,但当加入量超过10%时,Al2(1–x)Fe2xTiO5的晶格常数不变甚至减小,热分解率反而会增大;纳米Fe2O3作为添加剂可改善钛酸铝陶瓷的热分解性能。煅烧温度对钛酸铝的热分解率有很大影响,随着温度升高,热分解率降低,当温度大于1350℃时,钛酸铝陶瓷晶格常数保持不变,钛酸铝陶瓷的热分解率变化不大。     

Texture Development in Barium Titanate and PMN–PT Using Hexabarium 17-Titanate Heterotemplates

Bulk BaTiO₃ ceramics with 〈111〉-texture have been prepared by the modified templated grain growth method, using platelike Ba₆Ti₁₇O₄₀ particles as templates, and the mechanism of texture development is examined. The Ba₆Ti₁₇O₄₀ particles induce the abnormal growth of BaTiO₃ grains, and a structure similarity between {001} of Ba₆Ti₁₇O₄₀ and {111} of BaTiO₃ gives 〈111〉-texture to abnormally grown BaTiO₃ grains. Thus, the 〈111〉-texture develops in the BaTiO₃ matrix. The use of platelike Ba₆Ti₁₇O₄₀ particles has been extended to a 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ matrix, but the matrix phase is decomposed by extensive chemical reactions between the matrix and template phases.     

Spectroscopic Assessments of Domain Texture in Barium Titanate: I. Confocal Raman Polarization Analysis

A three-dimensional quantitative Raman analysis of ferroelectric domain orientation is presented for single-crystalline barium titanate (BaTiO₃; BT). The analysis has been developed according to basic principles of polarized/confocal Raman spectroscopy and is aimed at visualizing in three dimensions the geometrical (local) distribution of domain c -axis in the solid angle. In the first part of the paper, the relative intensity modulation of the BT Raman phonon modes has been theoretically modeled for a BT single crystal as a function of three Euler angles; then, the complete set of Raman tensor elements has been experimentally retrieved for the A _g, the B ₁, and the E _g vibrational modes of BT. The proposed polarized Raman algorithm is finally applied to the visualization of domain patterns in BT single crystal and the possibility of nondestructively visualizing domain textures in depth is shown; thus, we demonstrate that polarized Raman spectroscopy is a valuable and efficient tool for nondestructive in space assessments of domain orientation in ferroelectric materials.     

Origin of Texture Development in Barium Bismuth Titanate Prepared by the Templated Grain Growth Method

Microstructure development was examined for BaBi₄Ti₄O₁₅ prepared by the templated grain growth method, and the origin of texture development was discussed. The microstructure development in a compact composed of a platelike template and equiaxed matrix grains was characterized as follows: (1) the template grains thickened at an early stage; (2) the matrix grains changed their shape from equiaxed to platelike, and simultaneously, the plate faces aligned parallel to those of template grains; and (3) a group of large grains with mutually parallel alignment was formed by prolonged heating at high temperature. Texture developed during these microstructural changes, and process (2) made the greatest contribution toward texture development.     

Texture Development in Modified Lead Titanate Thin Films Obtained by Chemical Solution Deposition on Silicon-Based Substrates

An advanced method of X-ray diffractometry analysis, the quantitative texture analysis, is used in this work to study the preferential orientations of ferroelectric lanthanum-modified lead titanate thin films, to establish the factors affecting their development. The new, more reliable, texture data obtained allows us to discuss previous models of texture development in chemical-solution-deposited films. The results show that a fiber type, mixed <100>,<001> preferential orientation, is obtained when high heating rates are used during crystallization. The degree of texture of these films decreases when successive layers are deposited before a simultaneous crystallization of the ensemble is conducted. This undesirable effect is avoided through layer-by-layer crystallization, because of the preferential nucleation of <100>,<001> crystals at the interface between layers. The use of a titanium layer on the platinized silicon-based substrate leads to the development of an additional fiber <111> texture component. In this case, the layer-by-layer crystallization process cannot avoid the loss of the degree of orientation (texture index) with the increase of film thickness. This is mainly caused by the appearance of interlayer porosity, which disrupts the growth of <111>-oriented grains into upper layers.     

Solution-Precipitation of Fine Powders of Barium Titanate and Strontium Titanate

Hydrolytic reactions of metal alkoxides offer a broad range of possibilities for their use in the preparation of ceramic powders. This paper reports a unique and novel process to prepare fine powders of BaTiO₃ and SrTiO₃ from stable precursor solutions by carefully controlling the pH and temperature. This simple route offers good control of stoichiometry and the powders are agglomerate-free with fine particles of size 0.06–0.1 µm and were well sintered at 1200° and 1350°C, respectively. The dielectric properties of the dense ceramics are also reported. The formation aspects of these perovskite phases are also discussed.     

Topotatic-Like Phase Transformation of Amorphous Lead Titanate to Cubic Lead Titanate

The crystallization process of lead titanate (PT) prepared using the polymeric precursor method was investigated using X-ray diffractometry, Raman spectroscopy, electron microscopy, and X-ray absorption spectroscopy techniques. The results showed that amorphous PT was formed by an O–Ti–O structure composed of fivefold and sixfold oxygen-coordinated titanium. The local structure of the amorphous PT phase was similar to that of the cubic PT phase, i.e., similar coordination number and similar bond lengths, leading to a topotactic-like transformation during the phase transformation from amorphous to cubic perovskite PT. Because of the low crystallization temperature, every transformation observed during the crystallization process was associated with a short-range rearrangement process.     

The Effect of Texture and Microstructure on the Macroscopic Properties of Polycrystalline Piezoelectrics: Application to Barium Titanate and PZN–PT

The effects of crystallographic texture and microstructure are analyzed for polycrystalline tetragonal BaTiO₃, pseudotetragonal PZN–PT, and cubic BaTiO₃. For tetragonal BaTiO₃ and pseudotetragonal PZN–PT, we demonstrate that a high anisotropy of the single-crystal properties induces an apparent enhancement in the macroscopic piezoelectric response. For tetragonal BaTiO₃, the predicted macroscopic piezoelectric constants d ₃₁ and d ₃₃ are enhanced with respect to its single-crystal value at the expense of the spatial contributions from d ₁₅. For samples possessing fiber texture, an optimal response is predicted for samples that are not perfectly textured. Similarly, an apparent enhancement of the macroscopic value of d ₁₅ is predicted for PZN–PT. For cubic BaTiO₃, the low anisotropy of the underlying crystal properties induces a uniform decrease of the macroscopic electrostrictive constant, Q ₁₁, with decreasing texture. A completely random polycrystal provides 0.85±0.05 times its single-crystal response.     

镁的钛酸盐制备

本文研究了制备镁的钛酸盐的方法及工艺条件。并用Ｘ－射线衍射仪分析了在各种温度下灼烧样品的相组成，探讨了生成Ｍｇ_２ＴｉＯ_４的条件及历程。     

Dielectric Aging in Tetragonal Solid Solutions of Calcium Titanate in Barium Titanate

The dielectric aging phenomenon was observed in tetragonal polycrystalline solid solutions of calcium titanate in barium titanate between 30° and 90°C, and the 90° ferroelectric domain microstructures were analyzed using replica electron microscopy and statistical procedures. The results show a correlation between aging rate and 90° domains that may be satisfactorily interpreted in terms of the internal residual stress relaxation theory of aging.     

Raman Spectra of Strontium Titanate

At room-temperature the Raman spectrum was second-order, in agreement with, the selection rules for the cubic perovskite structure. The second-order spectrum originates largely from pairs of phonons with wave vectors near the Brillouin zone boundary. First-order Raman-active modes predicted in the tetragonal phase of SrTiO₃ are observed on cooling to 120 K, where a new impurity-related band is observed at 793 cm⁻¹. The presence of impurities raised the cubic to tetragonal phase transition temperature to 120 K.     

Low‐Temperature Selective Catalytic Reduction of NO on an Iron Ore Catalyst in a Magnetically Fluidized Bed

Low‐temperature selective catalytic reduction of NO from simulated flue gas by NH₃ on admixtures of iron ore and iron catalyst in a magnetically fluidized bed was experimentally studied. It was found that iron ore exerts a prominent catalytic effect on NO removal resulting in high removal efficiency without magnetic fields. Since iron ore is insensitive to magnetic fields due to its antiferromagnetic property, NO removal efficiency with iron ore as catalyst is only slightly influenced by magnetic fields. The NO removal efficiency, however, can be greatly improved using admixtures of iron ore and iron as catalyst in a magnetically fluidized bed. At defined magnetic field intensity and the same temperature, the removal efficiency reaches higher values compared with that without magnetic fields.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

Permittivity of Porous Titanate Dielectrics

Relative permittivities ranging from 12 to 100 were obtained from porous titania. Relative permittivities from 8 to 30 were obtained from porous calcium magnesium titanates. Porosity was introduced as microporosity by partial sintering, as 15 μm diameter spherical pores with porogens, and by fabrication of a microcellular structure with cylindrical pores. The relative permittivity as a function of porosity was compared with effective media models. Permittivity could be fitted to the Bruggeman 1/3 power law over a porosity range from 5% to 78%.