Crystallographic Facetting in Sintered Barium Titanate

A commercial TiO₂-excess BaTiO₃ powder has been sintered and its microstructure analyzed for crystallographic facetting via both scanning and transmission electron microscopy (SEM and TEM). Facetted grain surfaces are developed initially from {111} at a low temperature of 1215°C, which are then altered to {111} and {100} at 1290°C in the presence of a grain-boundary liquid phase. The grain shape is also modified correspondingly from platelike to polygonal. Facetting of the intragranularly located residual pores in BaTiO₃ along the {141} planes further develops on the (quasi-)equilibrium shape after annealing at 1400°C for 100 h from the initially well-characterized {111}, {110}, and {100} in as-sintered samples sintered at the same temperature for 10 h. The Wulff plots derived from the residual pores in as-sintered and annealed samples are constructed for the 〈011〉 zone. Microstructural analysis also suggests that the shape of grains and intragranular residual pores is modified progressively upon annealing. The initial solid–vapor surface energy has become less anisotropic crystallographically. Abnormal grain growth in relation to the surface energy anisotropy is discussed.     

Preparation of Semiconducting Barium Titanate by Excimer Laser Irradiation

A drastic drop of the electrical resistivity was observed on a ferroelectric insulator BaTiO₃ surface, after KrFlaser irradiation in air at atmospheric pressure. The temperature dependence of the resistivity was employed to demonstrate semiconducting behavior. Such behavior was seen when the resistance was measured in nitrogen. When measured in air, the semiconducting layer was oxidized and the insulating behavior restored.     

Effects of Excess Barium Ions on Aqueous Barium Titanate Tape Properties

The effects of excess free barium ions in aqueous barium titanate slip on the resulting BaTiO₃ tape properties were investigated in terms of the slip behavior, green/sintered tape density and morphology, and dielectric properties. The excess free barium ions expressed by means of the Ba/Ti ratio adversely affected most tape properties. Increase in the slip viscosity, green porosity, and agglomeration along with a decrease in mechanical properties and green/sintered density were found with the increase in the Ba/Ti ratio. However, dielectric permittivity was increased with increase in the Ba/Ti ratio. An effort was made to correlate these phenomena with Ba²⁺ leaching in water for realistic multilayer ceramic capacitor applications.     

Dielectric Properties of Fluxed Barium Titanate Ceramics with Zirconia Additions

BaTiO₃ compacts, when fluxed with <2 vol% of a complex borate glass phase, were sintered to near theoretical density at temperatures <1175°C in 2 h. Microstructural analysis showed a uniform grain size <1.0 μm with 0.75 wt% ZrO₂ added to the flux phase as a grain growth inhibitor. TEM analysis revealed a microcrystalline grain-boundary phase with the ZrO₂ resident along the grain boundaries. These samples displayed an essentially flat dielectric profile, low dissipation factors (<2%) over the range 25° to 125°C, a near linear dependence (≅±15%) between 25° and −55°C, and significantly increased voltage stability. X-ray diffraction analyss of these small-grained materials indicated a suppression of the tetragonal structure toward a more cubic modification.     

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.     

Initial Specific Surface Area and Graln Growth in Donor-Doped Barium Titanate

The grain growth of donor-doped BaTiO₃ prepared from BaTiO₃ powders with different initial specific surface areas was studied. Results show that a higher initial surface area and, consequently, a smaller critical grain size at the phase boundary drastically increase the critical amount of donor dopant, causing the grain size anomaly during sintering.     

Point Defect Concentrations in Barium Titanate Revisited

An analytic link between the oxygen partial pressure and the concentrations of the point defects for given temperatures and A/B conditions is presented for perovskites. A clear distinction between three different conditions is made. These are the sintering conditions, an intermediate metastable state, and a low-temperature metastable state. The analytical solution for a metastable state resulting from nonequilibrated metal vacancies permits a more accurate and self-consistent approach to calculating the equilibrium constants from conductivity– P (O₂) data. One of the reasons for the higher accuracy is that there is no need to divide the existence regime into subregimes with different approximations to the electroneutrality equation (Brouwer approximation). An excellent fit of the experimental conductivity data to a single function with only two adjustable parameters over all conductivity– P (O₂) space is obtained. The relative importance of frozen-in metal vacancies and foreign acceptors is discussed for BaTiO₃.     

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.     

Effect of Green States on Sintering Behavior and Microstructural Evolution of High-Purity Barium Titanate

Compacts prepared from three differently agglomerated powders were studied. Hg-penetration results and SEM observations were employed to compare the uniformity of powder compacts and to investigate the pore-size evolution and the microstructural development during sintering. It was found that the more nonuniform the powder compact, the higher the degree of pore growth in the initial and at the beginning of intermediate stages of sintering. Moreover, a higher sintering temperature and a nonuniform microstructure with larger grains could not be avoided. Microstresses might develop because of the differential shrinkage, but they would be released thereafter via the change of grain morphology. It was observed that the aggregate and pore-boundary separation might not be the primary reason for the initiation of discontinuous grain growth.     

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.     

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.     

Preparation and Characterization of Barium Titanate Suspensions for Stereolithography

Ferroelectric photoactive suspensions for stereolithography have been developed by dispersing a high volume fraction of barium titanate powder in hexanediol diacrylate (HDDA) with the aid of effective photoinitiators and dispersants. Rheological properties showed a shear thinning behavior and a low viscosity at a shear rate adequate for the recoating process. The barium titanate–HDDA suspension showed poor curing behavior due to the large refractive index difference between the ceramic and the resin. The coarse barium titanate–HDDA suspension showed a smaller surface reflectance and a larger cure depth than the fine barium titanate–HDDA suspension.     

Grain-Boundary Chemistry of Barium Titanate and Strontium Titanate: I, High-Temperature Equilibrium Space Charge

Direct observations using scanning transmission electron microscopy (STEM) of the grain-boundary chemistry of selectively doped SrTiO₃ and BaTiO₃ show the predominant solute segregation in both systems to be that of acceptors (negative effective charge). Appreciable donor segregation is not observed even at lattice concentrations as high as 10 mol%. Donor and acceptor codoped materials show segregation of the acceptor only. The results are consistent with a grain-boundary space-charge distribution consisting of a positive boundary and negative space charge. All grain boundaries examined also show an excess of Ti relative to the A-site cations, suggesting that the positive boundary charge is at least partially accommodated by an excess of Ti ions. The sign and magnitude of the electrostatic potential appear to be remarkably insensitive to changes in lattice defect structure with solute doping. Grain-boundary chemistry appears dominated by space-charge segregation, in contrast with the predictions of recent atomistic simulations which neglect the space-charge potential.     

Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Chloride and Titanium Tetrachloride under Moderate Conditions

Tetragonal BaTiO₃powders were prepared hydrothermally at 240°C, in only 12 h, using BaCl₂·2H₂O and TiCl₄, which are rather easy to manipulate. Characterization via X-ray diffractometry, scanning electron microscopy, Brunauer–Emmett–Teller analysis, and differential scanning calorimetry confirmed that increasing the NaOH excess concentration (from 0.5 M to 2.0 M ) and decreasing the initial TiCl₄concentration (from 0.625 M to 0.15 M ) promotes the formation of tetragonal BaTiO₃powders. After reaction, the powders were proved to be phase-pure BaTiO₃, with no impurities, such as Cl⁻ and CO₃²⁻.     

Defect Chemistry and Microstructure of Hydrothermal Barium Titanate

Hydrothermal powders of BaTiO₃ and (Ba,Ca)(Ti,Zr)O₃ contain large amounts of protons in the oxygen sublattice. The proton defects are compensated by vacancies on metal sites. When the powder is annealed, water is released and the point defects disappear in the temperature range of 100°–600°C. Metal and oxygen vacancies combine to small nanometer-sized intragranular pores. At temperatures of >800°C, the intragranular pores migrate to the grain boundaries and disappear. In multilayer ceramic capacitors that have been prepared from hydrothermal powders, the intragranular pores are preferentially collected at the inner electrodes, which results in bloating, cracks, and delamination.     

Effects of Zirconia on Microstructure and Dielectric Properties of Barium Titanate Ceramics

Dielectric properties and structural characteristics of BaTiO₃ ceramics are significantly influenced by small addition (2 wt%) of ZrO₂. SEM and TEM observations revealed enhanced microstructural uniformity and retarded grain growth depending on sintering temperature. Above 1320°C, Zr diffusion into the BaTiO₃ lattice resulted in a chemical modification of the tetragonal structure and the development of core–shell grains. Below 1320°C, TEM analysis showed ZrO₂ at the grain boundaries as discrete particles (∼0.03μm). X-ray diffraction analysis revealed a decrease in the axial (c/a) ratio with decreasing grain size. A corresponding decrease in the spontaneous polarization, and twinned domain structures, were also observed in the fine-grained ceramics. These samples also showed a flattened permittivity response with temperature and significantly lower losses.     

Influence of Stoichiometry on the Microstructure and Positive Temperature Coefficient of Resistivity of Semiconducting Barium Titanate Ceramics

The influence of stoichiometry, i.e., Ba/Ti ratio, on the microstructure and positive temperature coefficient of resistivity (PTCR) characteristics of BaTiO₃ was investigated. Fine-grain microstructures are obtained for Ba-rich, stoichiometric, low-temperature-sintered, Ti-rich materials. The room-temperature resistivities ( ρRT ) of the fine-grain Ti-rich samples are large (>10⁸Ω·cm). Excess Ba²⁺ ions can decrease the ρRT , by more than 2 orders of magnitude, because of the compensation of barium vacancies near the grain-boundary regions. Rapid cooling after sintering can also decrease ρRT (⋍100×) and is ascribed to the suppression of reoxidation. Large-grain microstructures and low ρRT , on the other hand, are generally observed for Ti-rich and Al₂O₃-SiO₂-TiO₂-added samples after sintering at a temperature higher than the corresponding eutectic point.