Synthesis of Single-Crystal Barium Titanium Isopropoxide Complex to Form Barium Titanate

Single-crystal barium titanate double alkoxide complex with a composition presumed to be BaTi(OCH(CH₃)₂)₆·xC₆H₆ was successfully synthesized in this work. The crystal was converted to BaTiO₃ at 100°C. The preparation of the Oxides via single crystals of metal alkoxides has some advantages over the well-known sol-gel method in that it results in oxides with well-defined and homogeneous compositions at the atomic level and in fine particle sizes, smaller than 50 nm.     

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.     

Preparation of Dense BaTiO3 Ceramics from Sol-Gel-Derived Monolithic Gels

Dense, small-grained BaTiO₃ ceramics, with a grain size around 1 μm and a relative sintered density >98%, were obtained at 1100°C from sol-gel-derived gel monoliths without using any sintering additives. The monolithic gels asprepared had a relative density of about 50% and consisted of ultrafine pseudo-cubic BaTiO₃ particles (<50 nm). These gels, with a significantly high density compared with that of previous ones (∼30%), have been synthesized at room temperature from a sol solution with a concentration of equimolar mixture of titanium isopropoxide and barium ethoxide (0.8 mol/L), using the methanol/2-methoxyethanol mixed-solvent system. Microstructural development of the gel monoliths with increasing sintering temperature and the dielectric properties of the obtained dense BaTiO₃ ceramic have been investigated.     

Hydrothermal Synthesis of Nanometer-Sized Barium Titanate Powders: Control of Barium/Titanium Ratio, Sintering, and Dielectric Properties

Nanometer-sized BaTiO₃ powders have been synthesized hydrothermally from Ba(OH)₂ and titanium alkoxide at 150°C for 2 h, and the Ba/Ti ratio has been measured with an accuracy of ±0.003. Stoichiometric powders can be obtained by adjusting the Ba/Ti ratio of the reactants to a value of 1.018. At a lower Ba/Ti ratio, the solubility of Ba(OH)₂ prevents full incorporation of barium, and barium-deficient powders result. A higher Ba/Ti ratio leads to the incorporation of excess barium in the powder. K _s(BaTiO₃,-25°C) = 7 × 10^-8 has been calculated for the equilibrium reaction. From this result, two reproducible processes for the synthesis of stoichiometric BaTiO₃ are proposed. The processes rely only on very accurate control of the chemical composition (Ba/Ti ratio) of the precursor suspension. The sintering behavior of powders having Ba/Ti ratio values between 0.965 and 1.011 is described from results of dilatometric measurements and isothermal sintering. Room-temperature dielectric constants as high as 5600 and losses as low as 0.009 have been obtained for a stoichiometry slightly less than 1.000. It is expected that optimum sintering behavior and electrical properties are obtained in the stoichiometry range 0.995-1.000.     

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 Highly Dispersed Barium Titanate Nanoparticles by a Novel Solvothermal Method

A novel solvothermal route has been developed to synthesize highly dispersed nanocrystalline barium titanate (BaTiO₃), using a mixture of ethylenediamine and ethanolamine as a solvent. The as-synthesized BaTiO₃ nanoparticles were characterized by X-ray powder diffraction, transmission electron microscopy (TEM), high-resolution TEM, Fourier transform infrared spectroscopy, and thermal analysis. Based on the results of characterizations, the organic solvent was found to influence strongly the crystal growth and dispersibility of BaTiO₃. The BaTiO₃ nanoparticles obtained were highly dispersed and crystalline with a cubic perovskite structure. The particle size derived from the TEM ranged from 5 to 20 nm.     

Low-Temperature Synthesis of Fully Crystallized Spherical BaTiO3 Particles by the Gel–Sol Method

The synthesis of spherical BaTiO₃ particles was attempted by a new technique, the "gel–sol method," at 45°C. The (Ba–Ti) gel used as a starting material was prepared by aging mixtures of titanyl acylate with a barium acetate aqueous solution ([glacial acetic acid (AcOH)]/[titanium isopropoxide (TIP)] = 4, [barium acetate]/[TIP] = 1) at 45°C for 48 h. Potassium hydroxide (KOH) was used as a catalyst for the formation of BaTiO₃. Powder X-ray diffractometry (XRD) results and Fourier-transform infrared (FT-IR) measurements for the (Ba–Ti) gel showed that the gel was amorphous, but the spatial arrangement of barium and titanium in the (Ba–Ti) gel is similar to that in crystalline BaTiO₃ particles. Fully crystallized spherical BaTiO₃ powder with a particle size of 40–250 nm formed at the very low reaction temperature of 45°C. Scanning electron microscopy images showed that the final particles formed via aggregation of the fine particles that seem to be the primary particles of bulk (Ba–Ti) gel. From the XRD, FT-IR, and Raman spectroscopy analysis, it was found that the crystal structure of the as-prepared particles continuously transformed from cubic to tetragonal as the calcination temperature increased, and high crystalline tetragonal BaTiO₃ phase was obtained at 1000°C after 1 h of heat treatment.     

Phase Equilibria in the System BaO–TiO2

The system BaO-TiO₂ was investigated using quenching, strip-furnace, and thermal techniques. Five compounds were found to exist in the system: Ba₂TiO₄, BaTiO₃, BaTi₂O₅, BaTi₃O₇, and BaTi₄O₉. Of these, only barium metatitanate (BaTiO₃) melts congruently (at 1618°C.). The dititanate melts incongruently at 1322° C. to yield BaTiO₃ and liquid; the trititanate melts at 1357°C. to yield BaTi₄O₉ and liquid; the tetra-titanate melts to TiO₂ and liquid at 1428° C. The nature of melting of the orthotitanate could not be determined accurately because of the high temperature involved and the rapid reaction with platinum. The two eutectics in the system occur between Ba₂TiO₄ and BaTiO₃ at 1563°C. and between BaTi₂O₅ and BaTi₃O₇ at 1317°C. The temperature of the cubic-hexagonal transition in barium metatitanate was determined as 1460°C. and the transition has been shown to be reversible. The transition temperature is raised sharply by the addition of a small percentage of TiO₂ although the extent of solid solution is quite limited. Some applications to the manufacture of titanate bodies and to the growth of single crystals of barium metatitanate are discussed.     

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.     

A Novel Reaction Path to BaTiO3 by the Oxidation of a Solid Metallic Precursor

Multilayer capacitors with thin, dielectric BaTiO₃ layers can possess a relatively high capacitance per unit volume. A solid metallic precursor method has recently been developed for preparing thin BaTiO₃/noble metal laminates. In the present paper, the phase and microstructural evolution of Ba-Ti metallic precursors were examined after oxidation at 300° to 900°C in pure oxygen at 1 atm pressure. Barium peroxide, BaO₂, formed rapidly during oxidation at 300°C, with titanium largely remaining as unoxidized particles in the peroxide matrix. Between 375° and 500°C, the solidstate reaction of barium peroxide with metallic titanium yielded barium orthotitanate, Ba₂TiO₄. Further exposure to temperatures between 500° and 900°C resulted in the oxidation of residual metallic titanium. The oxidized titanium then reacted with the orthotitanate matrix to form barium metatitanate, BaTiO₃. The rate of formation of BaTiO₃ was found to be strongly dependent on the degree of milling of the Ba-Ti precursors and on the heating rate between 300° and 500°C.     

Influence of Rare-Earth Dopants on Barium Titanate Ceramics Microstructure and Corresponding Electrical Properties

In this article, ytterbium and erbium oxides are used as doping materials for barium titanate (BaTiO₃) materials. The amphoteric behavior of these rare-earth ions leads to the increase of dielectric permittivity and decrease of dielectric losses. BaTiO₃ ceramics doped with 0.01–0.5 wt% of Yb₂O₃ and Er₂O₃ were prepared by conventional solid-state procedure and sintered at 1320°C for 4 h. In BaTiO₃ doped with a low content of rare-earth ions (0.01 wt%) the grain size ranged between 10 and 25 μm. With the higher dopant concentration of 0.5 wt%, the abnormal grain growth is inhibited and the grain size ranged between 2 and 10 μm. The measurements of capacitance and dielectric losses as a function of frequency and temperature have been carried out in order to correlate the microstructure and dielectric properties of doped BaTiO₃ ceramics. The temperature dependence of the dielectric constant as a function of dopant amount has been investigated.     

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.     

Ruddlesden–Popper Planar Faults and Nanotwins in Heteroepitaxial Nonstoichiometric Barium Titanate Thin Films

Nonstoichiometric 10-mol%-excess-BaO–BaTiO₃ (Ba_1.1TiO_3.1) thin film grown on a (100) SrTiO₃ substrate consisted of heteroepitaxial c -axis-oriented BaTiO₃ perovskite crystals including Ruddlesden–Popper planar faults and nanometer-scale multiple (111) twin lamellae. High-resolution electron microscopy and electron energy-loss spectroscopy revealed that nanotwins with coherent (111) Σ3 coincident site lattice boundaries were terminated in the BaTiO₃ matrix to form incoherent (211) Σ3 boundaries accommodating excess barium ions. Both Ruddlesden–Popper planar fault and incoherent (211) boundary formation were proposed as possible accommodation mechanisms of excess barium ions in the perovskite film.     

Formation Mechanisms of Tetragonal Barium Titanate Nanoparticles in Alkoxide–Hydroxide Sol-Precipitation Synthesis

The early stage of barium titanate (BaTiO₃) nanoparticle formation is investigated by in situ X-ray diffraction (XRD) and X-ray absorption near-edge structure (XANES) using synchrotron radiation. BaTiO₃ nanoparticles are synthesized via dissolution of barium hydroxide octahydrate and hydrolysis of titanium (IV) isopropoxide in isopropanol. In the course of raising the temperature of the alkoxide–hydroxide mixture solution to 80°C, in situ synchrotron XRD reveals that BaTiO₃ nanocrystals smaller than 6 nm begin to nucleate at 50°C without intermediate TiO₂ anatase formation, and Ti K edge absorption spectra also confirm the formation of corner-sharing TiO₆ octahedra at 60°C. The average size of BaTiO₃ precipitates increases to about 7.5 nm at 80°C. The synthesized nanopowders show an anomalously high tetragonality according to the Rietveld refinement of synchrotron XRD data.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Crystal Chemistry and Dielectric Properties in the Systems BaTiO3-UO2 and BaTiO3-BaUO3

Polycrystalline barium titanate fired in nitrogen at 1300° to 1400°C accommodates up to 3 mole % UO₂ in solid solution; its structure is then cubic at room temperature. With BaUO₃ additions the structure becomes disordered and quasi-cubic. In air, about 1 mole % UO₂ goes into solid solution in BaTiO₃ but the structure remains tetragonal. Diffraction peaks of a new phase, possibly a ternary oxide of barium, uranium, and titanium, appear in patterns of specimens containing more than 2 mole % UO₂. The dielectric constant of BaTiO₃ ceramics fired in air, steam, or oxygen increases with up to about 0.5 mole % UO₂ but declines rapidly above this level. The dielectric constant of BaUO₃ is about two orders of magnitude lower than that of BaTiO₃, and additions of BaUO₃ invariably lower the dielectric constant of BaTiO₃.     

Synthesis of Tetragonal Barium Titanate Nanoparticles Via Alkoxide–Hydroxide Sol-Precipitation: Effect of Water Addition

Barium titanate (BaTiO₃) nanoparticles are synthesized under N₂ atmosphere by the hydrolysis and condensation of barium hydroxide octahydrate and titanium (IV) isopropoxide with controlled water addition. With increasing the amount of water addition, the primary nanoparticles tend to grow from 7.5 to 68.5 nm in average diameter and to agglomerate to form spherical secondary particles of 0.5–3 μm in diameter. The influence of water content on the crystal size and structure was studied by X-ray diffraction via synchrotron radiation. As the amounts of added water increased, Rietveld analysis revealed that the lattice volume shrinks along the c -axis changing the crystal structure of BaTiO₃ nanoparticles gradually from tetragonal to cubic. FT-IR spectra also indicated that the concentration of hydroxyl ions in BaTiO₃ nanoparticles decreases with increasing water contents. It is thus concluded that adding sufficient amount of water is effective in reducing the concentration of protons in the oxygen sublattice during synthesis of BaTiO₃ nanoparticles via alkoxide–hydroxide sol-precipitation process.     

Crystallinity and Stoichiometry of Nano-Structured Sol-Gel-Derived BaTiO3 Monolithic Gels

The crystallization behavior and stoichiometric changes of barium titanium alkoxide-derived monolithic gels prepared by the sol-gel process using a high-concentration Ba₂Ti precursor solution (0.8 mol/L) were investigated during aging at room temperature. Crystallization of the gels (which were amorphous, per X-ray diffraction analysis immediately after gelation) into the BaTiO₃ perovskite phase increased during aging and was associated with significant shrinkage of the gels. Crystallization reached a value of ∼82% by the final stage of shrinkage, assuming the degree of crystallization of a gel treated at 600°C to be 100%. The stoichiometry of the gels (Ba/Ti molar ratio) also changed considerably during aging, as estimated by the concentrations of Ba and Ti that remained in the expelled liquid resulting from syneresis at any time during the aging process. Deviation in the Ba/Ti ratio of the precursor solution ranged from 0.015 at the initial stage of shrinkage to 0.003 at the final stage, a value determined by inductively coupled plasma atomic emission spectroscopy. The present study demonstrates the great advantage of using high-concentration precursor solutions of barium titanium alkoxides, rather than low-concentration solutions, to obtain BaTiO₃ gel monoliths with high density and crystallinity and little stoichiometric deviation, by sol-gel processing at room temperature.     

Direct Writing of Dielectric Ceramics and Base Metal Electrodes

Three-dimensional structures were assembled by direct writing of colloidal gel-based inks of electro-ceramic and metal electrode compositions. The ternary mixtures of BaTiO₃, BaZrO₃, and SrTiO₃ were explored via an active mixing nozzle print strategy that allowed for rapid screening of complex oxide ceramics. In a second experiment, Ni and BaTiO₃ inks were developed from commercially available powders in the form of concentrated, aqueous colloidal gels having shear thinning with yield stress rheology. The different surface chemistry of Ni and BaTiO₃ was accommodated by adsorbing a bi-layer of oppositely charged polyelectrolytes on the BaTiO₃ particles. Alternating layers of metal and ceramic were printed to build a multilayer structure. The multilayer Ni–BaTiO₃ structures were sintered in a reducing atmosphere and the capacitance was measured. These two examples illustrate the use of discrete or graded deposition capabilities of direct writing.     

Dielectric Bodies in the Quaternary System BaTiO3–BaSnO3–SrSnO3–CaSnO3

Dense bodies were prepared of compositions in the quaternary system BaTiO₃–BaSnO₃–SrSnO₃–CaSnO₃ containing from 3 to 60 mole % stannate. The general effect of the stannate addition to barium titanate was to decrease the Curie temperature and broaden the peak. On a molar basis the three stannates were approximately equivalent in their effect on the dielectric properties of barium titanate, although the rate of shift of the Curie temperature was slightly greater when SrSnO₃ was used. Bodies containing calcium or strontium stannate had lower power factors than those containing barium stannate. Bodies compounded with calcium stannate matured most readily and at lower temperatures. Bodies having dielectric constants ( K ) of 2300 to 2800 at 1 kc. with low positive temperature coefficients up to about 55°C. were obtained with a 3 mole % addition of stannate. Bodies with minimum K 's of 3000 to 4000 at 1 kc. over the range 25° to 85°C. were obtained from BaTiO₃ with an addition of about 6 mole % BaSnO₃, SrSnO₃, or CaSnO₃. Bodies with negative temperature coefficients of K ranging from about 13,000 to about 1000 p.p.m. per °C. were obtained with stannate additions of from 10 to 60 mole %.