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.     

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.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.     

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.     

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.     

Kinetics of Barium Titanate Synthesis

Reaction curves were obtained at various temperatures and concentrations for the formation of BaTiO₃ from particulate titania in Ba(OH)₂ solution. Kinetic analyses were performed by constructing mathematical models which took into account the particle size distribution of the reactant titania for both the topochemically-rate-controlled and the diffusion-rate-controlled reactions. At [Ba(OH)₂] > ca. 0.1 M the rate-controlling step is the Ba reaction with TiO₂ at the interface. The measured activation energy is 105.5 kJ/mol. The rates are independent of Ba(OH)₂ concentration, indicating that the TiO₂ interface is saturated. At [Ba(OH)₂] < ca. 0.1 M the rate-determining step shifts to diffusion through the product BaTiO₃ layer, the rates are concentration dependent, and the BaTiO₃ particle sizes are inversely proportional to the Ba(OH)₂ concentrations used.     

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.     

Novel Fabrication of Nickel Hydrosilicate Hollow Spheres

Hollow Ni₃Si₂O₅(OH)₄ nanospheres were synthesized via a facile deposition process at room temperature. The diameters of the products are in the range of 300–320 nm, and the average wall thickness is about 10 nm. Furthermore, the synthesis process of Ni₃Si₂O₅(OH)₄ hollow spheres was briefly described and this solution-based approach could be extendable to the preparation of other spherical materials with hollow interiors.     

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.     

Structure Evolution in Hydrothermally Processed (<100°C) BaTiO3 Films

Thin films of cubic BaTiO₃ were processed hydrothermally at 40°–80°C by reacting thin layers of titanium organo metallic liquid precursors in aqueous solutions of either Ba(OH)₂ or a mixture of NaOH and BaCl₂. All films (thickness ∼1 μm) were polycrystalline with grain sizes ranging from nano- to micrometer dimensions. BaTiO₃ formation was facilitated by increasing [OH^-], [Ba²⁺], and the temperature. The film structure was related to the nucleation and growth behavior of the BaTiO₃ particles. Films processed at relatively low [OH^-], [Ba²⁺], and temperatures were coarse grain and opaque, but increasing [OH^-], [Ba²⁺], and temperature caused the grain size to decrease, resulting in transparent films.     

Spark Plasma Sintering Behavior of Nano-Sized (Ba, Sr)TiO3 Powders: Determination of Sintering Parameters Yielding Nanostructured Ceramics

Nano-powders of BaTiO₃, SrTiO₃, Ba_0.6Sr_0.4TiO₃ (BST64), and a mixture of the composition (BaTiO₃)_0.6(SrTiO₃)₀₄ with particle sizes in the range of 60–80 nm were consolidated by spark plasma sintering (SPS). An experimental procedure is outlined that allows the determination of a "kinetic window," defined as the temperature interval within which the densification process can be kinetically separated from the grain growth one, enabling preparation of dense nanostructured ceramics. The width of this window varied from almost zero for BST64 to 125°C for the (BaTiO₃)_0.6(SrTiO₃)_0.4 mixture. During the densification (sintering) of the (BaTiO₃)_0.6(SrTiO₃)₀₄ mixture, BST64 is formed. The main part of this reaction occurs in a fully densified body through which suggesting that the constitutional phase(s) have a self-pinning effect on the grain growth.     

Solid-State Synthesis of Ultrafine BaTiO3 Powders from Nanocrystalline BaCO3 and TiO2

Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO₂ (70 nm) with BaCO₃ of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO₃ in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO₃ already leads to formation of the single-phase BaTiO₃ after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d ₅₀=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO₃ and TiO₂ represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.     

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.     

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.     

Rapid Hardening of Cement by the Addition of a Mechanically Activated Al(OH)3–Ca(OH)2 Mixture

Rapid hardening of cement was achieved in the present study by adding a mechanically activated Al(OH)₃–Ca(OH)₂ mixture to the starting cement paste. Among the dominant parameters for hardening were the mechanical treatment time for the Al(OH)₃ powder and the Al(OH)₃/Ca(OH)₂ ratio. The hardening mechanisms are discussed here in terms of the ionic concentration of the solution and the hydration products created when the Al(OH)₃–Ca(OH)₂ mixture was added to water. Mechanical activation of the Al(OH)₃ powder accelerated dissolution into an aqueous alkaline solution and induced the formation of calcium aluminate hydration products. Those hydration products increased the compressive strength of the cement paste at a very early stage of hardening.     

Single-Calcination Synthesis of Pyrochlore-Free 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 and Pb(Mg1/3Nb2/3)O3 Ceramics Using a Coating Method

A coating approach for synthesizing 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃ (0.9PMN–0.1PT) and PMN using a single calcination step was demonstrated. The pyrochlore phase was prevented by coating Mg(OH)₂ on Nb₂O₅ particles. Coating of Mg(OH)₂ on Nb₂O₅ was done by precipitating Mg(OH)₂ in an aqueous Nb₂O₅ suspension at pH 10. The coating was confirmed using optical micrographs and zeta-potential measurements. A single calcination treatment of the Mg(OH)₂-coated Nb₂O₅ particles mixed with appropriate amounts of PbO and PbTiO₃ powders at 900°C for 2 h produced pyrochlore-free perovskite 0.9PMN–0.1PT and PMN powders. The elimination of the pyrochlore phase was attributed to the separation of PbO and Nb₂O₅ by the Mg(OH)₂ coating. The Mg(OH)₂ coating on the Nb₂O₅ improved the mixing of Mg(OH)₂ and Nb₂O₅ and decreased the temperature for complete columbite conversion to ∼850°C. The pyrochlore-free perovskite 0.9PMN–0.1PT powders were sintered to 97% density at 1150°C. The sintered 0.9PMN–0.1PT ceramics exhibited a dielectric constant maximum of ∼24 660 at 45°C at a frequency of 1 kHz.     

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.     

Interfacial Reaction of BaTiO3 Ceramics with PbO–B2O3 Glasses

Interfacial reactions of pure, lead-, and zirconium-substituted BaTiO₃ ceramics with PbOB₂O₃ glasses were studied, with an emphasis on the effect of glass composition. Microstructures were analyzed by scanning electron microscopy and electron-probe microanalysis aided with X-ray diffractometry of powder mixtures in the system BaTiO₃PbOB₂O₃ heated at 850°C. The interfacial microstructures were divided into two types, depending on the glass composition. The first type was characterized by precipitates of TiO₂ dispersed in the glass matrix. Extended heating or limited glass volume resulted in the formation of a continuous layer of BaTi(BO₃)₂. The second type of microstructure was characterized by a lead-rich perovskite phase, which developed at the glass/ceramic interfacial region. Growth kinetics for this phase denied the diffusion-controlled mechanism. The substitution of lead in BaTiO₃ enhanced the penetration of glass into the ceramics along the grain boundaries and developed a coreshell structure.