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.     

Formation of (111) Twins in BaTiO3 Ceramics

(111) twins are found frequently in the microstructure of BaTiO₃ ceramics prepared by the mixed-oxide technique. The processing steps by which the (111) twins are formed are in question. A BaCO₃-TiO₂ powder mixture is milled and then Calcined at 1100°C for 2 h, where BaTiO₃ is formed by the release of CO₂. After the powder is calcined, it is analyzed by TEM. Image and diffraction pattern data are combined to prove the presence of (111) twins in powder particles; the formation of (111) twins occurred frequently during calcination. The formation mechanism of such twins and their role during anomalous grain growth are discussed.     

Preparation of 〈110〉-Textured BaTiO3 Ceramics by the Reactive-Templated Grain Growth Method Using Needlelike TiO2 Particles

Dense, highly 〈110〉-textured BaTiO₃ ceramics were prepared by the reactive-templated grain growth method. Needlelike TiO₂ (rutile) particles with their needle axis parallel to 〈001〉 were used as reactive template particles. Slurry containing an equimolar mixture of TiO₂ and BaCO₃ was tape cast to form a green compact, in which TiO₂ particles were aligned with their needle axis parallel to the casting direction. Calcination of the green compact changed TiO₂ particles into BaTiO₃ grains with their 〈110〉 direction parallel to the casting direction, for which the topotaxial relation of was responsible. Sintering yielded a dense, highly textured BaTiO₃ compact.     

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.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

Effect of Grain Boundary Structure on Diffusion-Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO₃. SrTiO₃ particles were scattered on the polished surfaces of two kinds of BaTiO₃ samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H₂ for those with rough boundaries. In the BaTiO₃ samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO₃ samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

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.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

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.     

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     

Interaction between Barium Titanate and Binary Glasses

Interactions between BaTiO₃, and three binary glasses were studied through the reaction of BaTiO₃, powder with glass powder. For PbO–B₂O₃ and PbO–SiO₂ glasses, the reaction led to stable compound formation, the substitution of Pb in the BaTiO₃ structure, and noticeable grain growth of BaTiO₃. The interaction phenomena for these two glass systems were very similar. The substitution of Pb into BaTiO₃ is assisted by chemical reactions in which BaB₂O₄ or Ba₂SiO₄ is formed. The substitution into BaTiO3 also seems to be closely related to the grain growth of BaTiO₃. On the other hand, only compound formation was observed during the processing of BaTiO₃ with Bi₂O₃–B₂O₃ glass. Neither BaTiO₃ grain growth nor Bi substitution took place with the Bi₂O₃–B₂O₃ glass system. Based on the observed reactions and the glass viscosity, several sintering aids for BaTiO₃ ceramic products are suggested in this paper.     

Influence of Grain Growth on Dielectric Properties of Nb-Doped BaTiO3

Effects of grain size and grain growth in Nb-doped BaTiO₃ on temperature and frequency dependencies of the dielectric constant were investigated. When 0.65 μm powder is sintered to an average grain size of 1 μm, two dielectric constant peaks indicate the presence of Nb-free BaTiO₃ and of Nb-containing material. Single peaks are observed above room temperature after additional grain growth or when 0.07 μm powder is sintered to an average grain size of 1 μm. The Curie point of pure BaTiO₃ with 1 μm grains is 4 to 6°C lower than that of material with grains >10 μm. Thermodynamically, this behavior is accounted for by a phase inversion stress ∼ the room-temperature stress.     

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.     

Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Columnar and highly oriented (100) BaTiO₃ and SrTiO₃ thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO₃ and SrTiO₃ thin films were fabricated on LaAlO₃ by a seeded growth process that appeared to follow a standard "two-step" growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO₃ throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO₃ and PbTiO₃ seed layers were effective in promoting the growth of highly oriented (100) BaTiO₃ films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO₃ and TiO₂ appeared to dictate thin film microstructural evolution.     

Effect of Sintering Atmosphere on Grain Boundary Segregation and Grain Growth in Niobium-Doped SrTiO3

To investigate the donor segregation in grain boundary regions and its effect on grain growth in SrTiO₃, SrTiO₃ powder compacts were doped with Nb₂O₅ and sintered in air or in hydrogen. The Nb-doped SrTiO₃ sintered in air did not show any detectable Nb segregation at the grain boundary region while an appreciable segregation was observed in the space-charge region of the sample sintered in H₂. The observed donor segregation in H₂ suggests a negative grain boundary charge and compensating positive space charge, which are the opposite to those in air. The negative grain boundary core was attributed to the segregation of inherently present acceptor impurities and the trapping of electrons at grain boundaries. In the H₂-sintered sample, where the added Nb ions were segregated in the space-charge region, the grain growth was suppressed. This result may indicate that the grain growth suppression in H₂ is due to the Nb solute drag of the boundary motion and the reduction in Ti vacancies.     

Grain-Growth Behavior during Stepwise Sintering of Barium Titanate in Hydrogen Gas and Air

To study the effect of oxygen partial pressure on grain growth in BaTiO₃, TiO₂-excess samples have been sintered in air with and without a prior H₂ heat treatment. Without prior H₂ treatment, abnormal grain growth occurs below and above the eutectic temperature ( T _e). An introduction of H₂ treatment before air sintering, however, increases the average grain size and suppresses the formation of abnormal grains during subsequent air sintering below and above T _e. This H₂ treatment effect has been explained in terms of a decrease of the driving force for the growth of faceted grains below a critical value for formation of abnormal grains. The observed grain-growth behavior under various atmospheres demonstrates the possibility of having various microstructures via control of oxygen partial pressure and initial grain size.     

Microcrystalline BaTiO3 by Crystallization from Glass

The properties and composition of glasses suitable for crystallization of BaTiO₃ are described. The crystallization of certain glasses results in a nearly complete recovery of BaTiO₃, besides the feldspar BaAl₂SiO₃ as a minor phase. The mechanism of crystallization was investigated by thermal analysis, viscosity, and grainsize measurements as a function of the temperature whereas density data were used for evaluation of the BaTiO₃ content. Within the range 30 to 60% by volume of BaTiO₃ at about 1μ grain size, the measured dielectric constant increased from 100 to 1200. The calculated partial dielectric constant of the Titanate phase at this grain size was about 3500. As the grain size approached 0.1μ, the dielectric constant decreased and became nearly independent of the temperature because of the predominance of surface states. Other effects were attributed to special structural characteristics, such as absence of porosity and clamping of the titanate particles within the microcrystalline matrix. Data are also presented on dielectric constant and loss tangent at different frequencies, dc breakdown strength, dc resistivity, and ferroelectric properties as a function of the grain size of the crystallized material.     

A New Glycothermal Process for Barium Titanate Nanoparticle Synthesis

Barium titanate (BaTiO₃) nanoparticles were synthesized at the low temperature of 80°C through a glycothermal reaction using Ba(OH)₂·8H₂O and amorphous titanium hydrous gel as precursors and a solution of 1,4-butanediol and water as solvent. This processing method provides a simple low-temperature route for producing BaTiO₃ nanoparticles, which could also be extended to other systems. It is demonstrated that the size of BaTiO₃ particles can be controlled by reaction conditions, such as reaction temperature and various volume ratios of 1,4-butanediol/water.     

Formation of Core-Shell Structure in the BaTiO3-SrTiO3 System

When a 1-mol%-Fe₂O₃-added 0.67BaTiO₃-0.33SrTiO₃ (mole ratio) powder compact was sintered at 1380°C, a core-shell structure was developed. The core phase formed via solid-state interdiffusion of barium and strontium ions between BaTiO₃ and SrTiO₃ particles. In contrast, the shell phase formed via a solution-precipitation process in the presence of an Fe₂O₃-containing liquid phase. Energy-dispersive X-ray analysis showed that the core was a strontium-rich paraelectric phase and the shell was a barium-rich ferroelectric phase at room temperature. The core-shell structure developed in the BaTiO₃-SrTiO₃ system suggests the possibility of obtaining a variety of phase distributions with different Curie temperatures.