Low-Temperature Hydrothermal Synthesis of Phase-Pure (Ba,Sr)TiO3 Perovskite using EDTA

A thermodynamic model was developed to describe the stability of (Ba,Sr)TiO₃ (BST) solid solutions in the Ba–Sr–Ti–K–(EDTA)–H₂O (EDTA = ethylenediaminetetraacetic acid) system. Phase diagrams were computed to identify the range of conditions suitable for making phase-pure BST. Hydrothermal experiments were performed to validate the thermodynamic model. The model was found to be more useful when an ideal solid solution was used to estimate the energetics for the BST phase instead of experimental thermodynamic data. In addition, EDTA was found to promote stable conditions for BST formation. When attempting to prepare Ba_0.50Sr_0.50TiO₃ without EDTA, BaTiO₃-rich and SrTiO₃-rich phases precipitated separately, at 70°–160°C. However, in the presence of EDTA, a phase-pure Ba _0.55Sr_0.45TiO₃ solid solution was obtained at 90°–120°C. EDTA is effective because it prevents phase heterogeneities from forming and equalizes the adsorption affinity of strontium and barium species.     

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO₃ or SrTiO₃ powders of high surface activity. As-prepared helium-dried SrTiO₃ is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO₃ crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.     

Reaction Products in the System MgCl2-NaOH-H2O

The precipitation process of solid phases Mg₃(OH)₅CI-4H₂O (phase 5), Mg₂(OH)₃CI-4H₂O (phase 3), and Mg(OH)₂ was followed by the addition of NaOH water solution in MgCl₂ water solutions of different concentrations (0.001 to 4.8 mol dm⁻³) and characterized by chemical, potentiometric, coulometric, and X-ray diffraction analyses. The concentration range in which the precipitation of solid phases occurs was determined. The phase distributions relative to the pH of solution and concentrations of magnesium and chloride were defined by the equilibrium diagram. The approximate solubility products of stable solid phases formed at different ionic strengths and at 293 K were determined.     

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.     

Effect of Polyethylene Glycol Content on the Optical Properties of Ba0.9Sr0.1TiO3 Multilayers

Quasiperiodic Ba_0.9Sr_0.1TiO₃ (BST) multilayers have been fabricated on SrTiO₃ substrates by using chemical solutions containing polyethylene glycol (PEG), and effect of PEG content on optical properties of the BST multilayers have been investigated. Each multilayer with a single perovskite phase displays a layered structure consisting of dense and porous BST layers. It is found that the central wavelength of the reflection-band for BST multilayers shifts to longer wavelength with increasing the PEG concentration within a given polymer amount range. With the same numbers of period and processing condition, the BST multilayers derived from chemical solutions containing PEG additives with a relative molar amount of 0.5, exhibit the highest optical reflectivities.     

Preparation of Silicon Nitride-Titanium Nitride and Titanium-Titanium Nitride Composites from (CH3)3SiNHTiCl3-Coated Si3N4 and Ti Particles

[(Trimethylsilyl)amino]titanium trichloride, (CH₃)₃-SiNHTiClj, was isolated as a red-orange crystalline solid in 58% yield from the reaction of TiCl₄ with [(CH₃)₃Si]₂NH in 1:1 molar ratio in dichloromethane at —78°C. Pyrolysis of (CH₃)₃SiNHTiCl₃ at 600°C furnished titanium nitride. This precursor is suitable for the preparation of composites and was employed to prepare Si₃N₄-TiN and Ti-TiN powders by adding Si₃N₄ particles or titanium powders to a solution of (CH₃), SiNHTiCl₃ in dichloromethane, drying and pyrolyzing the resulting solid. This precursor also has been used as a binder to prepare Si₃N₄-TiN and Ti-TiN bodies. High-resolution transmission electron microscopic studies of the Si₃N₄-TiN composite showed that titanium nitride is concentrated on the surface of the Si₃N₄ particles.     

Preparation and Characterization of BaxSr1-x, TiO3 Thin Films by a Sol-Gel Technique

Barium strontium titanate, (Ba_x,Sr_1-x TiO₃, thin films of various compositions were prepared by a sol-gel method. Solutions consisting of acetate powders and titanium IV isopropoxide in a mixture of acetic acid and ethylene glycol were spin-coated onto silicon and platinum-coated silicon substrates. Processing parameters were optimized to develop stable solutions which yielded films with relatively low crystallization temperatures. It was determined that ethylene glycol was a necessary component of the solution to increase stability to precipitation and to decrease the crystallization temperature of the films. The grain size of the films varied with annealing temperature and atmosphere and directly affected the dielectric properties. A dielectric constant of 400 and a dissipation factor of 0.04 were measured at 1 kHz for (Ba_0.8,Sr_0.2) TiO₃ films heated to 700°C for 1 h with a thickness of approximately 400 nm. Films of this composition maintained low leakage current densities for extended time periods when measured at an applied field of 75 kV/cm.     

Alkoxide-Hydroxide Route to Syntheltize BaTiO3-Based Powders

When preparing homogeneous, fine barium titanate powders, the major difficulty is to avoid the spontaneous self-condensation between the Ti-OH groups. In the usual way of preparing fine barium titanate powders, chelating agents (citrate, oxalate) or simply unidentate ligands (alkoxy or carboxyl groups) are used to complex titanium atoms. Another way is to mix barium and titanium precursors in a strongly basic medium. The condensation between the Ti(OH)^2-₆Ba²⁺ species directly gives the perovskite compound. Using an alkoxide-hydroxide route, a homogeneous Ba-Ti solution was prepared that completely advanced by condensation between the Ti(OH)^2-₆Ba²⁺ species and led to a controlled-stoichiometry powder. Concerning pure barium titanate, dried powders exhibited the cubic perovskite structure, and a direct sintering at 1150°C, without calcination, led to highly dense BaTiO₃ bodies with fine-grained uniform microstructure (1 μm) that exhibited a high permittivity value at room temperature ( K = 5400). The alkoxide-hydroxide method was also used to prepare dense alkaline-earth perovskite ceramics with complex compositions.     

Mechanochemical Synthesis and Electrochemical Properties of LiMn2O4

Spinel LiMn₂O₄ as a cathode material for lithium secondary batteries has been synthesized by a mechanochemical process, and its electrochemical properties have been characterized. Highly disordered nanocrystalline LiMn₂O₄ powders have been prepared by the mechanochemical processing of Li₂O and MnO₂ powder mixtures for 24 h. Electrochemical characterization of mechanochemically processed powder has shown that the intercalation of Li⁺ takes place with an initial capacity of 167 mA·h/g in the 2.5–4.3 V range and has better capacity retention as compared to the well-ordered crystalline LiMn₂O₄ powders. The better capacity retention of the mechanochemically processed LiMn₂O₄ powder may be attributed to the highly disordered structure that could accommodate the Jahn–Teller distortion of the spinel structure during Li⁺ intercalation around the 3 V region.     

Phase Transformation Kinetics in the Doped System LiAl5O8—LiFe5O8

Solid solutions of various compositions in the system LiAl₅O₈–LiFe₅O₈ were prepared by calcining mixtures of Li₂CO₃, Al₂O₃, and Fe₂O₃. Lattice parameters of the solid solutions were determined by X-ray diffraction (XRD). Samples of equimolar composition were also prepared with titanium and magnesium as dopants. The doped and undoped samples were annealed at 1050°C for up to 192 h in air. Annealing at this temperature, which is inside the miscibility gap for an equimolar composition, led to the decomposition of the originally single-phase solid solution. The extent of phase transformation was followed by XRD. The fraction transformed was estimated using integrated peak intensities. The kinetic data were analyzed in light of the conventional kinetic equation and the previously derived equation which takes into account the diffusive and the interface transfer processes. It was observed that titanium enhances and magnesium suppresses the kinetics of phase separation. These results are rationalized on the premise that the predominant point defects are Schottky defects in this spinel system.     

Crystallization of Fluorite-Type Solid Solution from Alkoxides in the System Y2O3-TiO2

Fluorite-type solid solutions crystallize directly at lower temperatures from amorphous materials prepared by the simultaneous hydrolysis of yttrium and titanium alkoxides. The lattice constant of the solid solution changes continuously as a function of composition. The kinetics of crystallization of the fluorite phase have been studied by X-ray measurements. The initial stage at each temperature proceeds rapidly in a short time. The final stage can be expressed in terms of the contracting cube equation 1—(1 —f)¹¹³= kt, the activation energy being 184 mol⁻¹. A pyrochlore-type solid solution corresponding to the composition Y₂Ti₂O₇ crystallizes at 8l5° to 900°C.     

Effect of Nd2O3 Concentration on the Defect Structure of CeO2–Nd2O3 Solid Solution

An extensive X-ray study of CeO₂–Nd₂O₃ solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO_1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO_1.5 were synthesized by coprecipitation from Ce(NO₃)₃ and Nd(NO₃)₃ aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO₂–NdO_1.5 solid solutions with 0–40 mol% NdO_1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO_1.5. The change in the lattice parameters of CeO₂–NdO_1.5 solid solutions, when plotted with respect to the NdO_1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO_1.5 in CeO₂ solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO_1.5) of solid solution.     

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.     

Self-Propagating High-Temperature Synthesis of Ti3SiC2: Study of the Reaction Mechanisms by Time-Resolved X-Ray Diffraction and Infrared Thermography

Ti₃SiC₂ is synthesized by self-propagating high-temperature synthesis (SHS) of elemental titanium, silicon, and graphite powders. The reaction paths and structure evolution are studied in situ during the SHS of the 3Ti+Si+2C mixture by time-resolved X-ray diffraction coupled with infrared thermography. The proposed reaction mechanism suggests that Ti₃SiC₂ might be formed from Ti–Si liquid phase and solid TiC _x . Finally, the effect of the powders starting composition on the Ti₃SiC₂ synthesis is studied. For the investigated initial mixtures, TiC _x is always formed as a major impurity together with the Ti₃SiC₂ phase.     

Fabrication and Electrical Properties of Textured Sr0.53Ba0.47Nb2O6 Ceramics by Templated Grain Growth

Textured Sr_0.53Ba_0.47Nb₂O₆ ceramics with a relative density of >95% were fabricated using templated grain growth (TGG). Acicular KSr₂Nb₅O₁₅ template particles synthesized via a molten salt process were aligned by tape casting in a mixture of solid-state-synthesized SrNb₂O₆ and BaNb₂O₆ powders. The resulting ceramics possessed strong fiber texture along the polar axis ([001]) of the strontium barium niobate. Samples with 15.4 wt% templates attained a textured fraction of 0.82 after sintering at a temperature of 1450°C for 4 h. These materials showed peak dielectric constants of 7550 at 1 kHz, remanent polarizations of 13.2 μC/cm², saturation polarizations of 21 μC/cm² (60%–85% of the single-crystal value), piezoelectric strain coefficients of 78 pC/N (70%–85% of the single-crystal value), and room-temperature pyroelectric coefficients of 2.9 × 10⁻²μC·(cm²·°C)⁻¹ (52% of the single-crystal value). These results show that TGG is a viable option for accessing single-crystal properties in polycrystalline ceramics.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

Synthesis of Vanadate and Arsenate Spodiosites and Interpretation of Optical Spectra of Manganese-Doped Sr2VO4C1

New vanadate and arsenate analogs of spodiosite (Sr₂-(VO₄)C1 and Sr₂(AsO₄)C1) were prepared. Isomorphism between chromium and vanadium spodiosites was established by formation of complete solid solutions. The vanadate analogs of calcium and barium, the arsenate analog of calcium, phosphate analogs of strontium and barium, and the fluorine analogs, Ca₃(VO₄)₂·CaF₂ and Sr₃(VO₄)₂·SrF₂, could not be synthesized. The optical absorption spectra of manganese-doped Sr₂(VO₄)Cl showed the presence of Mn⁵⁺ in the vanadium position, the site symmetry being approximately _Td. Three transitions, ³T₂( F ), ³T₁( F ), and ¹A_l( G ) at (12,000 and 13,000 cm^-1), (16,700 cm^-1), and (14,300 cm^-1), respectively, were observed from the ground state, ³A₂( F ). The lower symmetry components were strong enough to split the energy level, ³T₂( F ). The crystal field parameter Dq and the Racah parameters B and C were calculated to be 1200, 470, and 1927 cm^-1, respectively.     

Molten-Salt Synthesis of Ba1–xPbxTiO3 in the System BaTiO3–PbCl2

Ba_{1– x} Pb _x TiO₃ powder with a fixed composition was prepared by the reaction of BaTiO₃ powders with molten PbCl₂at various PbCl₂/BaTiO₃ molar ratios at 600° and 800°C in a nitrogen atmosphere. When 0.1 μm powder was used, the reaction was finished when x = 0.9. Two phases of BaTiO₃and a solid solution of Ba_{1– x} Pb _x TiO₃ coexisted, but the final phase gave a solid solution of Ba_{1– x} Pb _x TiO₃ at 800°C. When 0.5 μm powder was used, the two phases coexisted in the products at 600°C at PbCl₂/BaTiO₃= 1.0. A sintered compact of Ba_{1– x} Pb _x TiO₃ powders solid solution was prepared by hot isostatic pressing, and its dielectric constant was measured in the temperature range 20°–550°C.     

Effects of Composition on the Reaction Kinetics of Hydrothermally Derived Barium Strontium Titanate

Barium strontium titanate (BST, Ba _x Sr_{1− x} TiO₃) powders were fabricated by reacting nanocrystalline TiO₂ with aqueous alkaline solutions containing Ba and Sr at 80°C. Measurements of reaction kinetics showed that Ba-rich BST compositions exhibited more rapid reaction rates compared with Sr-rich BST compositions, and the reaction rate increased monotonically with increasing Ba content. The average particle size also increased with increasing Ba content, with the particle growth rate of BaTiO₃ being approximately a factor of 10 greater than SrTiO₃. The increase in growth rate from Sr-rich to Ba-rich BST corresponded to a morphological transition from 20 to 30 nm cuboidal particles to 80 nm raspberry-like particles, respectively.     

Optical Spectra of Synthetic Spinels in the System MgAl2O4–MgCr2O4

Unpolarized optical spectra were measured in the wavelength range 322–1666 nm by the diffuse reflection technique from spinel powders synthesized in the system MgAl₂O₄–MgCr₂O₄. The spectra were interpreted by the crystal-field theory on the basis of trigonally distorted spinel octahedra with D_3d symmetry. For chromium-rich solid solutions, including the MgCr₂O₄ end-member, results after peak fittings showed octahedral D_3d local symmetry around Cr³⁺ ions, identical to the crystallographic site symmetry. For chromium-poor solid solutions, however, octahedral C_3v local symmetry was suggested around Cr³⁺ ions, different from the D_3d crystallographically expected.