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.     

Role of Length Scale on Pressure Increase and Yield of Poly(vinyl butyral)–Barium Titanate–Platinum Multilayer Ceramic Capacitors during Binder Burnout

The binder-burnout kinetics of poly(vinyl) butyral from BaTiO₃ multilayer ceramic capacitors with platinum metal electrodes were analyzed by combining thermogravimetric analysis with infrared spectroscopy. The rate of weight loss was accelerated when both BaTiO₃ and platinum metal were present, and the presence of both metal and ceramic enhanced the production of CO₂. The activation energy and pre-exponential factor were determined by analysis of the weight-loss data with a first-order kinetics model. Then, the decomposition kinetics were incorporated into a coupled heat- and mass-transport model to predict pressure increases as a function of the heating cycle. The heating cycles determined in this manner then were used to evaluate the yield of capacitors 1.3–3.8 cm long and 0.3–1.3 cm high. The optimum yield was realized at an aspect ratio (height:length) of 1:3.     

Reversible Weight Change of Acceptor-Doped BaTiO3

The oxygen vacancy concentration of BaTiO₃ doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O₂ partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO₃. The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO₃ band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres.     

Effect of Heating Rates during Sintering on the Electrical Properties of Ultra-Thin Ni–BaTiO3 Multilayer Ceramic Capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCCs) is one of the present day challenges for increasing capacitive volumetric efficiency and high voltage dielectric properties. The present paper continues a series of investigations aimed at engineering the stability of ultra-thin Ni layers in base-metal electrode MLCCs. A kinetic approach based on the control of sintering profiles is found to not only prevent Ni electrode discontinuities, but also to significantly improve the interfacial electrical properties. Increasing sintering heating rates from 200 to 3000°C/h leads to a decrease in its temperature dependence of capacitance. Faster heating rates also reduce the BaTiO₃ grain size, which is beneficial to the reliability of multilayer capacitors. A direct correlation between heating rates, the thickness of an interfacial (Ni, Ba, and Ti) alloy reaction layer and the interfacial contact resistance has been observed. The decrease in the alloy layer thickness at high heating rates leads to an increased effective Schottky barrier height between the dielectric and electrode toward its theoretical value of 1.25 eV for pure Ni–BaTiO₃ interfaces.     

Dispersion of Aqueous Barium Titanate Suspensions with Ammonium Salt of Poly(methacrylic acid)

The colloidal stability of aqueous barium titanate (BaTiO₃) suspensions with the ammonium salt of poly(methacrylic acid) (PMAA-NH₄), as a function of pH, has been investigated. The concentration of PMAA-NH₄ required to stabilize aqueous BaTiO₃ suspensions decreases as the pH increases. A stability map that delineates the critical amount of PMAA-NH₄ necessary to achieve colloidal stability, as a function of pH, has been constructed.     

Hydrothermal Synthesis for Large Barium Titanate Powders at a Low Temperature: Effect of Titania Aging in an Alkaline Solution

Monodisperse and spherical barium titanate (BaTiO₃) powders with diameters of 200–470 nm were directly prepared by a low-temperature hydrothermal method at 90°C. Spherical titania (TiO₂) powders, ranging in size from 150 to 420 nm, were initially prepared by a controlled hydrolysis and condensation reaction, aged in a highly alkaline solution for 12 h, and then hydrothermally reacted with barium hydroxide to be converted to BaTiO₃ without a morphological change. The aging step of the TiO₂, where the surface of TiO₂ was highly densified through elimination of the pores, was indispensable to retain the sizes and shapes of TiO₂ in the resulting BaTiO₃. This was due to the fact that the formation of BaTiO₃ proceeded by an in situ reaction mechanism. The resulting BaTiO₃ powders exhibited dense and nonporous structures even after calcination at 1000°C.     

Synthesis of Ultrafine and Spherical Barium Titanate Powders Using a Titania Nano-Sol

A novel synthetic method for the preparation of spherical, homogeneous, and ultrafine barium titanate (BaTiO₃) powders is described. An aqueous titania nano-sol was prepared by peptizing coarse aggregate of hydrous titania with nitric acid. BaTiO₃ powders could be synthesized through a simple reflux method using the titania nano-sol and barium hydroxide. As decreasing the titanium concentration, the particle size of the resulting spherical BaTiO₃ powder was increased from 40 to 130 nm and the porosity also increased. It was revealed that the smaller as-prepared BaTiO₃ powder was less porous and became more tetragonal with less intragranular pores after annealing. With this method, a highly tetragonal BaTiO₃ powder ( c / a ∼1.008) with a particle size of 120.0 nm was successfully prepared and would be very suitable for the thinner dielectrics in higher capacitance multilayer ceramic capacitors.     

Preparation of Hollow BaTiO3 and Anatase Spheres by the Layer-by-Layer Colloidal Templating Method

Hollow BaTiO₃ and anatase spheres were prepared from multilayered colloidal titanate particles. An inorganic precursor, titanium (IV) bis(ammonium lactate) dihydroxide (TALH) (chemical formula: [CH₃CH(O–)CO₂–NH₄]₂Ti(OH)₂) was used. First, a layer-by-layer (LBL) colloid-templating method was employed using TALH to generate monodispersed hollow titanate spheres. These spheres were then treated in a Ba(OH)₂ solution or distilled water under hydrothermal conditions to transform them into hollow BaTiO₃ or anatase spheres, respectively.     

Defects of Base Metal Electrode Layers in Multi-Layer Ceramic Capacitor

An ultra-thin Ni-based metal used as the electrode layer in multilayer ceramic capacitor determines the dielectric performance of the capacitor. The warpage and the continuity of the inner electrode layers, and a dihedral angle between BaTiO₃ layers and metal electrodes of two ceramic capacitors (X7R and X5R) were characterized by optical microscopy and scanning/transmission electron microscopes. The results show that the warpage of the chips is closely related to the discontinuity of the inner electrode. The discontinuity takes place mainly because of Rayleigh instability of the Ni layer, but is less induced by the tensile stress from sintering.     

Effects of Copper Coating on the Crystalline Structure of Fine Barium Titanate Particles

We have investigated the effect of a metal coating—copper—on the tetragonal structure of fine barium titanate (BaTiO₃) particles. The BaTiO₃ particles were synthesized by a sol-gel method and heat treated at temperatures >900°C for various amounts of time before coating. The copper coating was achieved by an electroless coating technique. The transmission electron microscopy micrographs revealed that the coated powder contained fine BaTiO₃ particles that were embedded in copper patches. The X-ray diffractometry patterns showed that the copper coating increased the c/a ratio of the fine BaTiO₃ particles. For powders that were heat treated at 900°C for 10 h and were initially cubic, the copper coating changed the c/a ratio, from 1 to 1.0034. For powders that were calcined at 900°C for 20 h and were initially tetragonal, the copper coating further enhanced the c/a ratio, from 1.0028 to 1.0043. When the copper-coated BaTiO₃ particles were oxidized, the c/a ratio was reduced to a value that was approximately equal to or below that of the uncoated powders. A conductive coating can eliminate the depolarization energy of an insulating polar particle. The fact that the copper coating promoted the polar tetragonal phase but the nonconductive copper-oxide coating did not was consistent with the interpretation that the presence of the cubic phase (nonpolar) in small BaTiO₃ particles was caused by the depolarization effect.     

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.     

Process Window of BaTiO3–Ni Ferroelectric–Ferromagnetic Composites

Experimental study and thermodynamic analysis are conducted to identify a process window for the preparation of BaTiO₃–Ni ferroelectric–ferromagnetic composites. A tiny amount of Ni solutes (<0.5 mol%) can play a key role in the preparation of the composites. The Ni solute acts as an acceptor that enhances the reduction resistance of BaTiO₃. Owing to the presence of Ni acceptors, dense BaTiO₃–Ni composites can be prepared by sintering above 1330°C in an atmosphere of 1 Pa oxygen partial pressure.     

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.     

Effect of Densification Mismatch on Camber Development during Cofiring of Nickel-Based Multilayer Ceramic Capacitors

The development of camber during the cofiring of a two-layered structure of Ni-electrode/BaTiO₃-dielectric as a function of temperature has been investigated. At a given thickness of Ni electrode, less camber and camber rate with increasing thickness of BaTiO₃ dielectric have been observed. This phenomenon is attributed to the densification mismatch between the Ni electrode and the BaTiO₃ dielectric during cofiring. The mathematical analysis of camber development, based on a viscous model, shows significant agreement with experimental observations.     

High Tc in Electrospun BaTiO3 Nanofibers

BaTiO₃ nanofibers were prepared by electrospinning. The morphology of synthesized BaTiO₃ nanofibers was investigated under different heat treatment conditions. The phase transformations in BaTiO₃ nanofibers were monitored using Raman spectroscopy. It has been found that the Curie temperature of BaTiO₃ nanofibers increased to 220°C, which is notably higher than the bulk BaTiO₃ ceramics.     

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.     

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.     

Phase Transitions in the System BaTiO3—KnbO3

Solid solution formation in the system BaTiO₃—KnbO₃ was established by X-ray diffraction and dielectric measurements. Solid solutions with cubic symmetry were observed in the composition range from 4 to 90 mole % KnbO₃ at room temperature. The lattice parameter for the BaTiO₃ solid solutions increased with increasing KNbO₃; that for the KnbO₃ solid solutions decreased with the addition of BaTiO₃. A distinct discontinuity in lattice parameter was observed at the composition containing about 65 mole % BaTiO₃. Dielectric measurements were made from-195° to 400°C. The cubic-tetragonal transition temperature of BaTiO₃ was rapidly lowered with increasing addition of KNbO₃, whereas the two lower phase transition temperatures were raised. All three phase transitions of KnbO₃ were rapidly lowered with increasing addition of BaTiO₃. The observed phase transitions, lattice parameters, and electron probe data suggest a complex region in the subsolidus which extends from 35 to about 75 mole % KNbO₃.     

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.     

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.