Influence of [Ba + Ca]/[Ti + Zr] Ratio on the Interfacial Property of (Ba,Ca)(Ti,Zr)O3 (BCTZ) Powders in an Aqueous Medium

We report findings on the electrokinetic and solubility behaviors of (Ba,Ca)(Ti,Zr)O₃ (BCTZ) powders having three different [Ba + Ca]/[Ti + Zr] ratios: 0.995, 1.000, and 1.005. Electrokinetic and solubility properties of BCTZ powders in aqueous media are phenomenologically similar to BaTiO₃. Ba and Ca ions, occupying primarily A-sites on the perovskite lattice, dissolve during acid titration, which results in surface depletion of A-site cations in the surface region of BCTZ particles. The electrokinetics of colloidal BCTZ powders reflects changes in the surface chemistry that occur as a result of dissolution and adsorption/reprecipitation of surface ions. An increase in [Ba + Ca]/[Ti + Zr] ratio results in an increase in the dynamic mobility at all pH values, an increase in the titration hysteresis, and an increase in the isoelectric pH. Each of these effects can be attributed to Ba and Ca in the near-surface region of BCTZ.     

Interaction between Dissolved Ba2+ and PAA-NH4 Dispersant in Aqueous Barium Titanate Suspensions

The interaction between dissolved Ba²⁺ and dissociated ammonium salt of poly(acrylic acid) (PAA-NH₄) in aqueous suspension has been studied. The dissolved Ba²⁺ causes flocculation of dissociated PAA-NH₄, thus degrading its dispersing effectiveness in aqueous BaTiO₃ suspensions. The concentration of PAA-NH₄ required to stabilize aqueous BaTiO₃ suspension increases with increasing Ba²⁺ concentration at a given pH. A stability map, which is determined by a rheological study, is constructed to describe the amount of PAA-NH₄ required to have well-dispersed aqueous BaTiO₃ suspensions as a function of Ba²⁺ concentrations at different pH values.     

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.     

Influence of Solids Concentration on the Isoelectric Point of Aqueous Barium Titanate

We report on findings that the particle surface charge is influenced by solids concentration in aqueous suspensions of BaTiO₃. Three decades in solids concentration were analyzed by combining results from two different electrokinetic methods. Combined results demonstrate a systematic acidic shift in the isoelectric pH with decreasing solids concentration. The shift is attributed to the development of a Ba-depleted, TiO₂-rich surface layer. Using kinetic arguments, it is shown that the thickness of this layer will be proportional to the surface-to-volume ratio.     

Nucleation and Inhibition Control during the Hydrothermal–Electrochemical Growth of Barium Titanate Films

The control of the microstructure of BaTiO₃ films grown on titanium by the hydrothermal–electrochemical method was investigated. Experiments were conducted in a three-electrode high-pressure electrochemical cell in a 0.1 M Ba(OH)₂ electrolyte at 150°C. Results showed that the spontaneous initial nucleation linked to pure hydrothermal BaTiO₃ formation can be inhibited by cathodically protecting the titanium electrode from the moment it is immersed in the electrolyte. The application of initial nucleation pulses of varying cathodic potentials affected the grain size of the deposit. It is suggested that the formation of a titanium oxide layer is a necessary step previous to the nucleation of BaTiO₃.     

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.     

Hydrothermal Synthesis of Thin Films of Barium Titanate Ceramic Nano-Tubes at 200°C

A novel, low-temperature synthesis method for producing BaTiO₃ thin films patterned in the form of nano-tubes ("honeycomb") on Ti substrates is reported. In this two-step method, the Ti substrate is first anodized to produce a surface layer (∼200–300-nm thickness) of amorphous titanium oxide nano-tube (∼100-nm diameter) arrays. In the second step, the anodized substrate is subjected to hydrothermal treatment in aqueous Ba(OH)₂, where the nano-tube arrays serve as templates for their hydrothermal conversion to polycrystalline BaTiO₃ nano-tubes. This opens the possibility of tailoring the nano-tube arrays and of using various precursor solutions and their combinations in the hydrothermal bath, to produce ordered, patterned thin-film structures of various Ti-containing ceramics. These could find use not only in a variety of electronic device applications but also in biomedical applications, where patterned thin films are also desirable.     

Dislocation Loop Formation in Nonstoichiometric (Ba,Ca)TiO3 and BaTiO3 Ceramics

Dislocation loops observed in nonstoichiometric and stoichiometric (Ba,Ca)TiO₃, and in stoichiometric BaTiO₃ sintered in a reducing atmosphere, were characterized by conventional transmission electron microscopy (TEM) under two-beam conditions and high-resolution TEM atomic structure analysis. Dislocation loops mostly lay on {100} planes with Burgers vectors of type 〈100〉. The dynamic behavior of these dislocation loops during the electron beam irradiation (EBI), however, was classified into two different types of dislocation loops: in A-site-excess (Ba,Ca)TiO₃, contrasts of dislocation loops faded completely away; in BaTiO₃ and B-site-excess (Ba,Ca)TiO₃, fine-line contrasts remained. Dislocation loops with Burgers vectors of type 1/2〈100〉 and the resultant crystallographic shear (CS) structure with a displacement vector of type 1/2〈110〉 after EBI were proposed to interpret residual line images. Disappearance of these line images in A-site-excess (Ba,Ca)TiO₃ strongly suggests preferential Ca ion site occupancy at the CS structure.     

Discontinuous Dissolution of a Liquid Phase in Ba(Ni1/3Nb2/3)O3 Ceramics

Discontinuous dissolution of a liquid phase accompanied by diffusion-induced grain-boundary migration (DIGM) in Ba(Ni_1/3Nb_2/3)O₃ (BNN) ceramics has been studied. When the liquid-phase-sintered BNN specimens are heat-treated at low temperatures, the liquid phase formed during sintering is dissolved discontinuously and during the reaction grain boundaries migrate. As the heat-treatment temperature is lowered, the degree of grain boundary migration and dissolution of the liquid phase increase. These results are qualitatively consistent with the coherency strain energy model proposed for the driving force for DIGM.     

Effects of Solids Loading, pH, and Polyelectrolyte Addition on the Stabilization of Concentrated Aqueous BaTiO3 Suspensions

Colloidal stability of concentrated aqueous BaTiO₃ suspensions with ammonium salts of poly(acrylic acid) (PAA-NH₄) and poly(methacrylic acid) (PMAA-NH₄) as a function of pH and solids loading is investigated. For suspensions with solids loading less than 40 vol%, the required polyelectrolyte concentration to stabilize aqueous BaTiO₃ suspensions decreases with increasing pH, but remains relatively unchanged with increasing solids loading. As the solids loading continuously increases (e.g., >50 vol%), the required amount of polyelectrolyte increases, but exhibits a minimum at pH ∼ 9.2. The critical amount of polyelectrolyte needed to achieve colloidal stability of aqueous BaTiO₃ suspensions as a function of pH and solids loading is summarized in a three-dimensional stability map.     

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.     

Hysteresis in the I–V Tunneling Characteristics of n-Type BaTiO3 Ceramics

Scanning tunneling microscopy (STM) and spectroscopy (STS) have been applied to study the surface electronic properties of n -type BaTiO₃ ceramics under ultrahigh vacuum and at various oxygen partial pressures. I – V tunneling characteristics of vacuum-annealed BaTiO₃ do not exhibit rectifying behavior, implying that the Fermi level is pinned at the surface. The surface band gap of BaTiO₃ annealed under vacuum at 540°C is equal to 1 eV. The top edge of the surface valence band is located 0.7 eV below the Fermi level. Hysteresis in the I – V characteristics has been observed at high oxygen partial pressures. Dosing of the BaTiO₃ with oxygen increases the surface band gap and unpins the Fermi level. As a result, the I – V characteristics acquire rectifying features similar to those observed for BaTiO₃ Schottky-type diodes. Hysteresis in the I – V spectra observed at high oxygen partial pressures is attributed to the changes of the surface potential barrier due to adsorption/desorption of oxygen modulated by the tip-sample potential difference.     

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.     

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.     

Determination of Inversion Temperature of Sb2O3-Doped BaTiO3 Positive Temperature Coefficient of Resistivity (PTCR) Ceramics by the Finite Difference Method

The effect of the cooling rate on the PTCR (positive temperature coefficient of resistivity) characteristics of 0.1 mol% Sb₂O₃-doped BaTiO₃ ceramics has been investigated. Resistances both below and above the Curie temperature were increased by slow cooling, which indicated that the resistive layer width at the grain boundary increased as the cooling rate decreased. Concentration profiles of the Ba vacancies as a function of distance from the grain boundary have been simulated by the finite difference method. The inversion temperature of the 0.1 mol% Sb₂O₃-doped BaTiO₃ system was determined to be 1160°C from the measured electrical properties and computed concentration profiles.     

Hydrothermal Soft Chemical Synthesis and Particle Morphology Control of BaTiO3 in Surfactant Solutions

Barium titanate (BaTiO₃) particles with book-like and spherical morphology were prepared by using a hydrothermal soft chemical process in the presence of a cationic surfactant. A layered titanate of H_1.07Ti_1.73O₄ with a lepidocrocite-like structure and plate-like particle morphology was used as the precursor. The layered titanate was hydrothermally treated in a Ba(OH)₂–(HTMA-OH) ( n -hexadecyltrimethylammonium hydroxide) solution or a Ba(OH)₂–(HTMA-Br) ( n -hexadecyltrimethylammonium bromide) solution in a temperature range of 80°–250°C to prepare BaTiO₃. The intercalation reaction of HTMA⁺ with the layered titanate promotes the structural transformation reaction from the layered titanates to BaTiO₃, while it inhibits the structural transformation reaction to anatase under the hydrothermal conditions. The particle morphology of BaTiO₃ prepared by this method dramatically changes with changing reaction conditions. HTMA⁺ plays an important role in changing particle morphology in the hydrothermal soft chemical process.     

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 pH on the Chemistry of the Barium Titanium Citrate Gel and Its Thermal Decomposition Behavior

In this investigation, X-ray diffractometry and ¹³C NMR spectroscopy were used to characterize the effect of pH on the formation of gel and its thermal decomposition by the Pechini process. It was found that the major effect of pH was to destroy the esterification between citric acid (CA) and ethylene glycol, which in turn could influence Ba species and the formation of a mixed-metal CA complex. It was also found that barium carbonate was derived from the Ba species and was not related to the instability of Ba₂Ti₂O₅CO₃, but Ba₂Ti₂O₅CO₃ was formed by the thermal decomposition of a mixed-metal CA complex. Moreover, BaTiO₃ formed via two routes, the reaction of BaCO₃ with Ti species and thermal decomposition of Ba₂Ti₂O₅CO₃.     

Characterization of Internal Pores in Hydrothermally Synthesized BaTiO3 Particle by Transmission Electron Microscopy

Hydrothermally synthesized barium titanate, BaTiO₃ (commercially available), has internal pores in small particles. The three-dimensional structures of the pores were successfully observed using transmission electron microscopy (TEM) and reconstructed by a tomography system. The behavior of the internal pore is observed in situ with increasing temperature on the thermal stage of a TEM device. We succeed in recording the movement of pores and small grains. At >1128 K, some pores move out from the particle's surface during TEM observation. The temperature roughly agrees with the temperature at which the density of BaTiO₃ powder sharply increases. During observation with increasing temperature, a thin layer appeared on the particle's surface at >73 K and then disappeared at 1193 K.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.