Effects of Ethanol Addition and Ba/Ti Ratios on Preparation of Barium Titanate Nanocrystals Via a Spray Pyrolysis Method

BaTiO₃ nanocrystals were synthesized by using a low-pressure spray pyrolysis (SP) method. A new strategy was proposed to improve nanocrystal formation and phase evolution by ethanol addition and Ba/Ti molar ratio variation, respectively. A mixture of submicron and nanoparticles was found with SP of precursors without ethanol addition, while only nanoparticles could be obtained by ethanol addition. BaTiO₃ crystal growth was improved by increasing the Ba/Ti ratio. In comparison, SP of the same precursor under atmospheric conditions resulted in only submicron particles with smooth surfaces and irregular morphology.     

Effect of the Ba/Ti Ratio on the Microstructures and Dielectric Properties of Barium Titanate-Based Glass–Ceramics

A series of BaTiO₃ (BT)-based ferroelectric glass–ceramics have been prepared via controlled crystallization by varying the Ba/Ti ratio in an aluminum silicate glass composition, and the subsequent microstructure, phase evolution, and dielectric properties have been investigated. X-ray diffraction indicated that an increasing Ba/Ti ratio promoted the crystallization of BaTiO₃ and BaAl₂Si₂O₈ from the glass matrix, and the cracking of glass–ceramics appears to be correlated to mismatch in the thermal expansion coefficient among BaTiO₃, BaAl₂Si₂O₈, and the glass matrix. In addition, it was found that increasing the Ba/Ti ratio facilitated the formation of a dendrite structure with obvious porosity. The change in the Ba/Ti ratio in the glass notably modified the dielectric properties: a high Ba/Ti ratio in the glass resulted in an increased dielectric constant and decreased breakdown strength.     

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.     

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.     

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.     

Effect of Ba: Ti Ratio on Densification of LiF-Fluxed BaTiO3

The literature on LiF-fluxed BaTiO₃ has shown substantially lower sintering temperatures compared to that of unfluxed BaTiO₃. In an effort to understand densification behavior in this system, shrinkage was studied for various Ba: Ti ratios and 2 wt% LiF additions. Sintering of BaTiO₃ with LiF is sensitive to the Ba:Ti ratio. Excess Ba reduced the sintering temperature and increased the fired bulk density. The starting powder stoichiometry, net Ba:Ti ratio, and addition of LiF appear to be independently important.     

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.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

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.     

Preparation of Fine Tetragonal Barium Titanate Powder by a Microwave–Hydrothermal Process

A microwave–hydrothermal (MH) process was performed at 240°C to prepare tetragonal BaTiO₃ from TiCl₄ and Ba(OH)₂. No alkali hydroxide was used to avoid contaminations. MH BaTiO₃ powder with a c / a ratio of 1.010 and a mean size of 180 nm was synthesized within only 9 h. The MH BaTiO₃ contains a very low concentration of lattice hydroxyl group, associated with a very small lattice strain. The measured density of the MH BaTiO₃ is favorably consistent with the theoretical value, and the Ba/Ti stoichiometry determined is 0.996. The formation of a tetragonal structure in BaTiO₃ and the particle growth were strongly promoted by the MH process. The effects of lattice defects on the stoichiometry and the determination of transition enthalpy were discussed.     

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.     

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.     

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.     

Electroceramics from Source Materials via Molecular Intermediates: BaTiO3 from TiO2 via (Ti(catecholate)3)2-

Rutile or anatase may be depolymerized and complexed by sequential treatment with (i) H₂SO₄/(NH₄)₂SO₄, (ii) H₂O, and (iii) catechol/NH₄OH to produce the intermediate (NH₄)₂(Ti(catecholate)₃) · 2H₂O. Treatment with Ba(OH)₂· 8H₂O leads to an acid-base reaction generating Ba(Ti(catecholate)₃) · 3H₂O, in which the Ba:Ti ratio is held at 1:1 at the molecular level. Calcination produces BaTiO₃ powder.     

Synthesis of Nano-sized BaTiO3 Powders by the Rotary-Hydrothermal Process

Nano-sized BaTiO₃ powders with narrow size distribution and high tetragonality were attempted to be synthesized by the rotary-hydrothermal process in a water system as a novel technique, using a mixture of anatase-type TiO₂ and Ba(OH)₂ as starting material. The rotary-hydrothermal syntheses were performed under conditions with a rotary-speed of 20 revolutions per minute at 423–523 K for 3–96 h. Highly- and mono-dispersed BaTiO₃ powders mainly composed of coarse-faceted particles with the tetragonal phase were successfully synthesized by controlling the conditions for rotary-hydrothermal treatments. TEM and TG results revealed that these coarse-faceted BaTiO₃ particles contained very few structural defects such as hydroxyl content. Thus, the rotary-hydrothermal process was a useful method to synthesize very high-quality BaTiO₃ particles, and the further control of various conditions of the rotary-hydrothermal treatment is expected to control the crystalline phase and microstructures of final BaTiO₃ powders.     

Electroceramics from Source Materials via Molecular Intermediates: PbTiO3 from TiO2 via [Ti(catecholate)3]2-

The precursor [NH₄]₂[Ti(catecholate)₃] · 2H₂O is known to react with Ba(OH)₂· 8H₂O in an acid/base process that generates Ba[Ti(catecholate)₃] · 3H₂O, a compound which undergoes low-temperatue calcination to produce BaTiO₃ powder. Attempts to develop similar routes to PbTiO₃ have been frustrated, since lead(II) hydroxide does not exist. The amphoteric yellow PbO and the basic oxide, Pb₆O(OH)₆⁴⁺, are both insufficiently basic to react with [NH₄]₂[Ti(catecholate)₃] · 2H₂O. Based on the large sizes of both the [Ti(catecholate)₃]^2- anion and the Pb²⁺ cation, a precipitation method has been developed in which lead nitrate and [NH₄]₂[Ti(catecholate)₃] · 2H₂O are added together in an aqueous medium causing precipitation and leaving only NH₄NO₃ in solution. The lead-titanium-catecholate complex that forms in this manner undergoes low-temperature pyrolysis to produce PbTiO₃. SEM indicates a submicrometer ultimate crystallite size.     

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.     

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.     

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.     

Hydrothermal Synthesis of Tetragonal Barium Titanate from Barium Hydroxide and Titanium Dioxide under Moderate Conditions

Tetragonal BaTiO₃ powders were prepared hydrothermally, using Ba(OH)₂·8H₂O and TiO₂ (anatase), in the absence of anions such as chloride ions, at a temperature of 220°C for several days. Characterization via X-ray diffractometry, scanning electron microscopy, and differential scanning calorimetry confirmed that increasing the Ba:Ti molar ratios (from 1:1 to 4:1) and alkaline concentrations (from 1.0 M to 3.0 M ) promotes the formation of tetragonal BaTiO₃.