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.     

XPS Analysis of Submicrometer Barium Titanate Powder

A commercial submicrometer BaTiO₃ powder was analyzed using X-ray photoelectron spectroscopy. The analysis revealed the powder surfaces to be covered with a layer of physisorbed H₂O and chemisorbed –OH ions. A BaCO₃ residual not detected with XPS was shown to be present in the powder using X-ray diffraction, suggesting that the carbonate takes the form of discrete particles rather than of a continuous surface layer. A relaxed surface phase detected in previous XPS analyses of bulk BaTiO₃ was also shown to be present. Depth profiling revealed the powder surfaces to be Ti-rich, confirming the presence of a phase, or phases, to stoichiometrically balance the barium carbonate.     

Anomalous Grain Growth in Donor-Doped Barium Titanate with Excess Barium Oxide

Grain growth and semiconductivity of donor-doped BaTiO₃ceramics with an excess of BaO and additions of SiO₂or B₂O₃were studied. The microstructures and electrical measurements on sintered samples revealed that their electrical properties are related to the microstructure development of the sintered samples. Samples heated with an excess of BaO developed a normal microstructure during sintering, as a consequence of normal grain growth (NGG), and were yellow and insulating. In contrast, samples with an excess of BaO and an addition of SiO₂or B₂O₃exhibited anomalous grain growth (AGG) and were dark blue and semiconducting after sintering. When some BaTiO₃seed grains were embedded in a sample of donor-doped BaTiO₃with an excess of BaO (without SiO₂or B₂O₃), AGG was observed, i.e., some seed grains grew into large grains and were blue and semiconducting. An explanation is given for why AGG is responsible for the oxygen release and the formation of semiconducting grains in donor-doped BaTiO₃and not NGG.     

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.     

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.     

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.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Preparation of Highly Dispersed Ultrafine Barium Titanate Powder by Using Microbial-Derived Surfactant

To uniformly disperse ultrafine BaTiO₃ particles with a stoichiometric composition and several tens of nanometers in diameter to primary particles during the sol–gel synthesis process, a new aqueous surfactant with a high hydrophilic group density and special cis-structure was prepared from a microbial product and added to solution before the sol–gel synthesis reaction. Because of the rapid formation of large and porous aggregates which were 30–50 μm in diameter in suspension without addition of this unique structural surfactant, the prepared ultrafine BaTiO₃ particles caused rapid sedimentation in suspension. The addition of the surfactant in the range of 7.1 wt% for the synthesized BaTiO₃ particles made it possible to decrease the size of the aggregates in suspension as well as the sedimentation velocity while maintaining the stoichiometric composition. The optimum additive content to obtain the minimum aggregate size of about 100–200 nm in diameter and the highest dispersion stability in suspension while maintaining the stoichiometric composition of prepared ultrafine BaTiO₃ particles without other phases was determined at about 7.1 wt%. Because the excess addition of this surfactant at more than 8.5 wt% inhibited the uniform synthesis of BaTiO₃ particles, an amorphous phase with a highly specific surface area and a BaCO₃ phase formed in the synthesized particles.     

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.     

Fluorine as a Donor Dopant in Barium Titanate Ceramics

We have studied the electrical properties and microstructure of fluorine-doped BaTiO₃ ceramics. The samples were prepared using a classical ceramic technology that involved the calcination of intimately mixed powders of BaCO₃, TiO₂, and BaF₂. When the samples were sintered in untreated ambient air, the fluorine from the sample reacted with water vapor and formed gaseous HF. To prevent this hydrolysis of the fluorine, we sintered the samples in dried air. The fluorine-doped BaTiO₃ ceramics sintered in a dry atmosphere showed microstructures and electrical properties typical of donor-doped BaTiO₃. The samples containing up to 0.3 mol% of fluorine were coarse-grained, semiconducting, and displayed a remarkable PTCR effect. In contrast, the samples with a higher fluorine concentration were fine grained and insulating. A SIMS elemental mapping of the samples showed that the fluorine was distributed throughout the microstructure of the semiconducting samples; however, the fluorine concentration was enriched at grain boundaries and in the BaTi₂O₅ intergranular phase.     

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.     

Effects of Temperature and Atmosphere on the Formation Mechanism of Barium Titanate Using the Citrate Process

In this investigation, several experiments have been conducted to study the effects of temperature and atmosphere on the formation of BaTiO₃ using the citrate process. It was found that BaTiO₃ could be completely formed when the precursor powder was heated at 300°C in O₂ for 24 h and then heated in situ at 475°C for 1 h, during which no intermediate phase was observed throughout the process. Moreover, BaTiO₃ could be formed at 400°C in O₂ via the combustion of organic materials, in which the amount of the residual BaCO₃ depended on the partial pressure of O₂. The development of BaCO₃ and oxycarbonate intermediate was found to be dependent on the temperature, the atmosphere, and the organic materials, and their formation mechanisms are discussed.     

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO₃ was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO₂) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO₂ partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba_1.054Ti_0.946O_2.946→Ba₂TiO₄+BaTiO₃ and second 1250°C for BaTi₂O₅→Ba₆Ti₁₇O₄₀+BaTiO₃, respectively. All other regimes of the BaO–TiO₂ were found to be consistent with the reported diagrams in the literature.     

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.     

Effects of Silver on Barium Titanate as a Function of Stoichiometry

This work examines the effects of Ag on stoichiometric and nonstoichiometric BaTiO₃ in terms of the unit cell dimensions, the polycrystalline microstructure, and the dielectric properties. Stoichiometric BaTiO₃ and compositions with 0.5 mol% TiO₂ excess and 0.5 mol% BaO excess were prepared via solid-state synthesis with varying amounts of Ag up to 0.3 mol%. Experimental results indicate that stoichiometry plays a significant role in the solubility of Ag and its effects on the physical properties. Overall, the solubility of Ag was negligible in stoichiometric BaTiO₃. However, compositions with excess TiO₂ stabilized the solubility of Ag as evidenced from changes in the unit cell dimensions and dielectric properties. Based on these data, it is proposed that Ag occupies the A-site in the perovskite structure with an upper limit of Ag solubility of 0.06 mol% Ag in BaTiO₃ with 0.5 mol% excess TiO₂. Dielectric measurements showed that Ag concentrations approaching 0.3 mol% gave rise to an increase in the space charge effect, especially at temperatures above T _C. In both nonstoichiometric compositions, the presence of a liquid Ag phase during thermal processing was found to affect microstructural development and sintering.     

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.     

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.     

Preparation of Barium Titanates from Oxalates

The decomposition of mixed barium titanium oxalates was studied by means of various thermochemical as well as spectroscopic methods. Infrared (IR) spectra of the mixed oxalates indicate the existence of an octahedral complex with Ti chelated by oxalate groups. In general, the results of both IR and the thermochemical analyses suggest that the oxalates are first converted to unidentate carbonate, then to ionic carbonate, and finally to mixed oxides of perovskite structure. The decomposition processes were found to depend upon the atmosphere. In the presence of oxygen the stoichiometrically mixed oxalates decompose by forming TiO₂ and BaCO₃ as intermediates. Under vacuum, two routes of decomposition occur in parallel. In one route, TiO₂ and BaCO₃ are formed as intermediates; in the other, partially reduced TiO_x(x < 2) is formed, which further reacts with BaCO₃ to produce also BaTiO₃, CO₂ and CO.     

Solubility of BaO in BaTiO3

The solubility and mode of incorporation for BaO in BaTiO₃ were studied by X-ray powder diffraction, scanning and transmission electron microscopy, electron probe microanalysis, and equilibrium electrical conductivity measurements. The presence of barium orthotitanate, Ba₂TiO₄, as a second phase for samples containing >0.1 mol% excess BaO was confirmed by direct microscopic examination. There was no evidence to support the incorporation of excess BaO into BaTiO₃ by a Ruddlesden-Popper type of superlattice ordering mechanism. Measurement of the equilibrium electrical conductivity showed no detectable shift in the conductivity profile resulting from excess BaO, thus setting an upper limit of 100 ppm for the solubility of BaO in BaTiO₃.     

Formation of BaTiO3 from BaCO3 and TiO2 in Air and in CO2

The formation of BaTiO₃ from equimolar BaCO₃ and TiO₂ (rutile) mixtures was studied in air and in CO₂. A small amount of BaTiO₃ is formed first directly from BaCO₃ and TiO₂ at the surface of contact. From then on it is a diffusion-controlled reaction, and both BaTiO₃ and Ba₂TiO₄ are produced, with Ba₂TiO₄ being formed in much larger amounts. In 1 atmosphere of CO₂, the intermediate Ba₂TiO₄ was suppressed up to a temperature of about 1100°C. in agreement with thermodynamic calculations. Ba₂TiO₄ reacts fast with 1 atmosphere of CO₂ below about 1100°C. to produce BaTiO₃and BaCO₃