Intermediate Phase Development in Phosphorus-Doped Barium Titanate

In the present work, the phase formation and thermal evolution in phosphorus-doped BaTiO₃ have been studied using differential thermal analysis, X-ray diffractometry, scanning electron microscopy coupled with energy-dispersive spectroscopy, transmission electron microscopy, and high-temperature nuclear magnetic resonance. Phosphorus cations that are incorporated from ester phosphate form a surface layer that covers the BaTiO₃ particles. This layer acts as a reactive coating during sintering. Phosphorus-doped BaTiO₃ samples that have been treated at temperatures of 650°–900°C show the presence of crystalline Ba₂TiP₂O₉ and/or Ba₃(PO₄)₂ phases. The appearance of secondary phases is dependent on the cooling rate. Higher temperatures (900°–1200°C) result in the presence of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles. As a consequence, the remaining titanium-rich BaTiO₃ drives the formation of a liquid phase at temperatures >1200°C. In regard to the reported sintering behavior of P⁵⁺-doped BaTiO₃, the formation of a phosphorus–BaO-rich phase that covers the BaTiO₃ particles could be the origin of the improved porosity coalescence and removal that is observed at the earlier stages of sintering.     

Nanosized Barium Titanate Powder by Mechanical Activation

Mechanical activation, without any additional heat treatment, is used to trigger the formation of a perovskite BaTiO₃ phase in an oxide matrix that consists of BaO and TiO₂ in a nitrogen atmosphere. The resulting BaTiO₃ powder exhibits a well-established nanocrystalline structure, as indicated by phase analysis using X-ray diffractometry. A crystallite size of ∼14 nm is calculated, based on the half-width of the BaTiO₃ (110) peak, using the Scherrer equation, and an average particle size of 20–30 nm is observed using transmission electron microscopy for the activation-derived BaTiO₃ powder.     

Barium Carbonate Phase on the Surfaces of Barium Titanate Particles and in Situ Transmission Electron Microscopy Observation of Its Decomposition

A BaCO₃ phase is found on the surfaces of hydrothermally synthesized BaTiO₃ particles; it occurs as aggregates or small protuberances. A small proportion of the phase decomposes to BaO crystallites when heated by a convergent electron beam in a transmission electron microscope. The BaO and BaCO₃ crystallites disappear when they are irradiated successively by the convergent electron beam. The BaO crystallites and the BaCO₃ phase sublimate and/or react with BaTiO₃ crystals whose surface layers are deficient in Ba²⁺ ions.     

Preparation of a Monodispersed Suspension of Barium Titanate Nanoparticles and Electrophoretic Deposition of Thin Films

A transparent and stable monodispersed suspension of nanocrystalline barium titanate was prepared by dispersing a piece of BaTiO₃ gel into a mixed solvent of 2-methoxyethanol and acethylacetone. The results of high-resolution transmission electron microscopy (HR-TEM) and size analyzer confirmed that the BaTiO₃ nanoparticles in the suspension had an average size of ∼10 nm with a narrow size distribution. Crystal structure characterization via TEM and X-ray diffraction indicated BaTiO₃ nanocrystallites to be a perovskite cubic phase. BaTiO₃ thin films of controlled thickness from 100 nm to several micrometers were electrophoretic deposited compactly on Pt/Ti/SiO₂/Si substrates. The deposited thin film had uniform nanostructure with a very smooth surface.     

Influence of Barium Dissolution on the Electrokinetic Properties of Colloidal BaTiO3 in an Aqueous Medium

Hysteresis in the electrokinetic behavior of colloidal hydrothermal BaTiO₃ occurs during sequential acid and base titrations. Ba dissolution during acid titration results in an oxide-rich surface. When the acid-treated BaTiO₃ is titrated back to pH 10, dissolved Ba is specifically adsorbed and/or precipitated onto the particle surface. The combined effects of dissolution and subsequent adsorption–precipitation results in titration hysteresis. Most of the labile Ba can be removed by multiple acid treatments, which result in a TiO₂-like surface layer composition. Barium dissolution increases with decreasing pH but levels off below pH 4 due to diffusion through the surface oxide layer as predicted previously. A phenomenological model is offered to explain the electrokinetic behavior as a function of pH. It is suggested that inherent BaCO₃ contamination is not the primary source of dissolved Ba from hydrothermal BaTiO₃ in acidic solution.     

Necessary Conditions for the Formation of {111} Twins in Barium Titanate

The experimental conditions for {111} twin formation in BaTiO₃ were investigated. When BaTiO₃ compacts without excess TiO₂ were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N₂–5H₂), no {111} twins formed within the BaTiO₃ grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess-TiO₂-containing BaTiO₃ samples sintered in air, while no twins were observed in the excess-TiO₂-containing samples sintered in 95N₂–5H₂. X-ray diffraction analysis showed that excess TiO₂ forms a Ba₆Ti₁₇O₄₀ phase during sintering with the space group A 2/ a in air and a Ba₆Ti₁₇O_{40− x} phase with the space group C in 95N₂–5H₂. It appears therefore that excess TiO₂ and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO₃. These results may also indicate that the interface structure between BaTiO₃ and Ba₆Ti₁₇O₄₀ influences the twin formation.     

Room-Temperature Solubility Behavior of Barium Titanate in Aqueous Media

In the present study, the incongruent dissolution of barium from barium titanate (BaTiO₃) has been studied as a function of dispersion pH and powder volume fraction for two different BaTiO₃ powders. In alkaline dispersions, the barium solubility strongly increases as the pH increases, as suggested by thermodynamic considerations. At pH <7, the barium solubility reaches a plateau, the height of which is dependent on the surface area of the powder and the solids loading of the slip. The BaTiO₃ surface is completely depleted of barium in this region.     

Solvothermal Synthesis and Characterization of Barium Titanate Powders

Cubic BaTiO₃ powders have been synthesized from gel powders after thermal treatment in an alcohol solution and characterized using X-ray diffractometry, transmission electron microscopy, and other techniques in detail. The borderline reaction conditions, such as the reaction temperature and time, for the synthesis of crystalline BaTiO₃ powder in different solvents are established. The single-phase BaTiO₃ powder has low agglomeration and seems to have a regular morphology. The formation of BaTiO₃ powders via solvothermal reaction is more difficult, in comparison to hydrothermal processing. The crystalline powders, which have a small particle size (∼20–60 nm) and narrow particle-size distribution, can be synthesized in methanol, ethanol, or n -propanol systems. Unlike the hydrothermal reaction, tetragonal-phase BaTiO₃ powders cannot be prepared via the solvothermal reaction, even with alkali catalysts.     

Effect of Silver on the Sintering and Grain-Growth Behavior of Barium Titanate

Silver and its alloys frequently are used as electrode material for BaTiO₃-based dielectrics. In the present study, a small amount of fine silver particles have been intimately mixed with BaTiO₃ powder. The sintering and grain-growth behavior of the silver-doped BaTiO₃ in air are investigated. The solubility of silver in BaTiO₃, as revealed by lattice-parameter measurement, electrical measurement, and electron probe microanalysis, is <300 ppm. The densification of BaTiO₃ is slowed slightly by the addition of silver inclusions. However, the presence of a small amount (<0.3 wt%) of silver increases the amount and size of abnormal grains. When the silver content is >0.3 wt%, the grain growth of BaTiO₃ then is prohibited by the silver inclusions.     

Phase Transition and Physical Characteristics of Nanograined BaTiO3 Ceramics Synthesized from Surface-Coated Nanopowders

Nanograined BaTiO₃ ceramics prepared from 40-nm-size BaTiO₃ nanopowders exhibited the cubic as well as the tetragonal phase, while nanograined BaTiO₃ ceramics prepared from BaTiO₃ nanopowders coated with Mn had only the tetragonal phase. The dielectric constant of the latter was 10 times larger than that of the former; the latter exhibited PTCR behavior with a resistivity jump ratio of about 5.0 × 10⁴. These physical properties of the BaTiO₃ ceramics appeared to be significantly affected by the strain near grain boundaries; such strain resulted in a phase transition from the cubic to the tetragonal phase in the nanograined BaTiO₃ ceramics, even though the grain size was about 40 nm.     

Effect of MgO Doping on the Phase Transformations of BaTiO3

The successive phase transformations in MgO-doped BaTiO₃ were studied. Upon MgO doping, dielectric anomalies corresponding to lower phase transformations were broadened and depressed, while an anomaly for a cubic–tetragonal transformation remained and shifted to a lower temperature. XRD peak splitting upon tetragonality of BaTiO₃ was decreased, and the peaks exhibited abnormally broadened profiles which are different from the one for cubic BaTiO₃ above T _c. Raman spectroscopy revealed the existence of orthorhombic phase at room temperature for the solid solution with 0.5 mol% or more MgO. The temperature dependence of the Raman spectrum showed that orthorhombic and rhombohedral phases in MgO-doped BaTiO₃ were stabilized at higher temperatures than pure BaTiO₃.     

Wetting Reaction between Lithium Fluoride and Barium Titanate

The wetting reaction between LiF and BaTiO₃ was studied by examining the reaction products on the LiF-wetted surface of BaTiO₃. A reduction reaction occurred between the melt of LiF and BaTiO₃, causing the formation of LiTiO₂ and volatilization of fluorine. The wetted surface consisted of spherical particles of LiTiO₂ and eutectically decomposed phases of BaLiF₃ and LiTiO₂. The formation of LiTiO₂ was repressed by the addition of excess BaO in the melt of LiF, and a well-spread film was formed on the surface of BaTiO₃. The wetting between BaLiF₃ and BaTiO₃ was also examined. The reaction of forming LiTiO₂ did not occur, and a mosaic film was formed on the surface of BaTiO₃.     

Low-Temperature Synthesis of Fully Crystallized Spherical BaTiO3 Particles by the Gel–Sol Method

The synthesis of spherical BaTiO₃ particles was attempted by a new technique, the "gel–sol method," at 45°C. The (Ba–Ti) gel used as a starting material was prepared by aging mixtures of titanyl acylate with a barium acetate aqueous solution ([glacial acetic acid (AcOH)]/[titanium isopropoxide (TIP)] = 4, [barium acetate]/[TIP] = 1) at 45°C for 48 h. Potassium hydroxide (KOH) was used as a catalyst for the formation of BaTiO₃. Powder X-ray diffractometry (XRD) results and Fourier-transform infrared (FT-IR) measurements for the (Ba–Ti) gel showed that the gel was amorphous, but the spatial arrangement of barium and titanium in the (Ba–Ti) gel is similar to that in crystalline BaTiO₃ particles. Fully crystallized spherical BaTiO₃ powder with a particle size of 40–250 nm formed at the very low reaction temperature of 45°C. Scanning electron microscopy images showed that the final particles formed via aggregation of the fine particles that seem to be the primary particles of bulk (Ba–Ti) gel. From the XRD, FT-IR, and Raman spectroscopy analysis, it was found that the crystal structure of the as-prepared particles continuously transformed from cubic to tetragonal as the calcination temperature increased, and high crystalline tetragonal BaTiO₃ phase was obtained at 1000°C after 1 h of heat treatment.     

Uniform Coating of Nanometer-Scale BaTiO3 Layer on Spherical Ni Particles via Hydrothermal Conversion of Ti-Hydroxide

A uniform BaTiO₃ nano layer was coated on spherical Ni particles for multilayer ceramic capacitor applications via a Ti-hydroxide coating using the controlled hydrolysis of a TiCl₄ butanol solution containing (C₂H₅)₂NH (diethylamine, DEA) and its subsequent hydrothermal reaction at various [Ba(OH)₂], residual [DEA], and hydrothermal temperatures. The hydrothermal conversion was successful at [Ba(OH)₂]≥0.065 M (Ba/Ti≥1.3) and T ≥150°C, and the residual DEA in the Ti-hydroxide coating layer not only affected the formation of the BaTiO₃ phase but also resulted in a rough surface morphology. When a minimal amount of DEA was involved in the formation of Ti-hydroxide, a uniform BaTiO₃ coating with a clean surface morphology could be attained, which was confirmed by elemental mapping of the coated powder and the observation of hollow spheres after removing the Ni core. The BaTiO₃ coating was very effective not only in preventing Ni oxidation but also in shifting the starting point of Ni densification to a higher temperature.     

In–Situ Transmission Electron Microscopy Crystallization Studies of Sol–Gel-Derived Barium Titanate Thin Films

Barium titanate (BaTiO₃) thin films that were derived from methoxypropoxide precursors were deposited onto (100) Si, Pt/Ti/SiO₂/(100) Si, and molecular-beam-epitaxy-grown (MBE-grown) (100) BaTiO₃ on (100) Si substrates by spin coating. The crystallization behavior of the amorphous-gel films was characterized using in-situ transmission electron microscopy heating experiments, glancing-angle X-ray diffraction, and differential thermal analysis/thermogravimetric analysis. Amorphous-gel films crystallized at a temperature of ∼600°C to an intermediate nanoscale (5–10 nm) barium titanium carbonate phase, presumably BaTiO₂CO₃, that subsequently transformed to nanocrystalline (20–60 nm) BaTiO₃. Random nucleation in the bulk of the gel film was observed on all substrates. In addition, oriented growth of BaTiO₃ was concurrently observed on MBE-grown BaTiO₃ on (100) Si. High-temperature decomposition of the intermediate carbonate phase contributed to nanometer-scale residual porosity in the films. High concentrations of water of hydrolysis inhibited the formation of the intermediate carbonate phase; however, these sols precipitated and were not suitable for spin coating.     

Thermal Behavior of BaTiO3 Particles Synthesized by Plasma Chemical Vapor Deposition

The thermal behavior of nanoparticles BaTiO₃, prepared by a radio-frequency plasma chemical vapor deposition (RF-plasma CVD) method, was characterized by various analysis methods. The BaCO₃ phase was included in the powder as byproducts, which is also observed in hydrothermal BaTiO₃ powder. The BaCO₃ phase decomposed and disappeared by annealing at 873 K for 30 min. H₂O, N₂, CO₂ and H₂, were detected by a thermal desorption spectra measurement from BaTiO₃ powder. The annealed powder became well-crystallized particles without grain growth, although as-prepared powder included polycrystalline particles. We successfully observed in-situ grain growth for BaTiO₃ nanoparticles by thermal transmission electron microscope. At the initial step of normal grain growth, very fine particles with 40–60 nm diameters started to merge into the larger grains around 1083 K. The migration rate was measured by video images and a grain boundary diffusion coefficient D_gb was calculated.     

Electrophoretic Deposition and Characterization of Micrometer-Scale BaTiO3 Based X7R Dielectric Thick Films

Uniform and relatively dense BaTiO₃ thick films of 1–5 μm were prepared by an electrophoretic deposition process using submicrometer BaTiO₃ powders (mean particle size: ∼0.2 μm). Two different BaTiO₃ powders and solvent media were used to investigate the film quality and thickness control. The surface charge mechanism of BaTiO₃ particles was explained according to the observed results. The microstructures were examined by means of SEM. The experimental results show that the thickness could be controlled independently of suspension concentration by keeping a constant applied voltage and a constant current drop in a given suspension. BaTiO₃ thick films have good insulation resistance and dielectric properties such as a dielectric constant and a dissipation factor that are compatible with the data from conventional tape-cast BaTiO₃ thin layers.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Thickness of Cubic Surface Phase on Barium Titanate Single-Crystalline Grains

The crystallographic structure of BaTiO₃ particles prepared by conventional solid-state reaction has been investigated by X-ray diffractometry and dielectric measurements. The systematic analysis of the diffraction pattern has clearly shown the coexistence of tetragonal and cubic phases at room temperature. The relative amount of cubic phase increases and the dielectric constant decreases with decreasing particle size. These experimental results are consistent with the model that the cubic phase is located at the surface of BaTiO₃ grains. The thickness of the surface layer is estimated to be approximately 5 nm, irrespective of the grain size.     

Preparation of Orthorhombic Ba2YCu3O7 Powder by Single-Step Calcining

A single calcination step, solid-state process that provides orthohombic Ba₂YCu₃O₇ powder is described. BaCO₃, Y₂O₃, and CuO are used as precursor materials. The only phase identifiable by X-ray diffraction is the orthorhombic Ba₂YCu₃O₇. The use of a vacuum during the inital stages of the calcining process promotes complete decomposition of the carbonate, and no residual carbonate is observed. An oxygen atmosphere during the later stages of calcining ensures proper oxidation to Ba₂YCu₃O₇. The use of a similar combination vacuum-oxygen calcining schedule should also be beneficial in the preparation of chemically derived powders.