Microwave-Hydrothermal Synthesis of Monodispersed Nanophase α-Fe2O3

Synthesis of monodispersed nanophase α-Fe₂O₃ (hematite) powder to be used as a red pigment in porcelains was investigated using microwave-hydrothermal and conventional-hydrothermal reactions using 0.018 M FeCl₃·6H₂O and 0.01 M HCl solutions at 100°–160°C. Acicular and yellow β-FeOOH (akaganite) particles 300 nm in length and 40 nm in thickness were dominantly formed at 100°C after 2–3 h, while spherical α-Fe₂O₃ particles 100–180 nm in diameter were preferentially formed after 13 h using a conventional-hydrothermal reaction. However, a microwave-hydrothermal reaction at 100°C led to monodispersed and red α-Fe₂O₃ particles 30–66 nm in diameter after 2 h without the formation of β-FeOOH particles. In this paper, the effect of microwave radiation during hydrothermal treatment at 100°–160°C on the formation yield, kinetics, morphology, phase type, and color of α-Fe₂O₃ was investigated.     

Glycothermal Reaction of Rare-Earth Acetate and Iron Acetylacetonate: Formation of Hexagonal ReFeO3

The reaction of a mixture of iron acetylacetonate and rare-earth (Tm-Lu) acetate in 1,4-butanediol at 300°C yielded a novel phase of ReFeO₃ having a hexagonal crystal system (a0 = 6.06, c₀= 11.74 A) together with small amounts of Fe₃O₄and/or the garnet phase. The particle size of the product distributed in a narrow range and selected area electron diffraction from a particle having apparent polycrystalline outlines suggested that each particle was actually a single crystal grown from one nucleus. On calcination, the hexagonal phase irreversibly transformed into the perovskite phase at around 980°C. The use of ethylene glycol in place of 1,4-butanediol of the present procedure afforded Fe₃O₄, while hydrothermal reaction of the same starting materials yielded a mixture of Fe₂O₃and an amorphous rare-earth phase.     

Synthesis of Celsian (BaAl2Si2O8) from Solid Ba-Al-Al2O3-SiO2 Precursors: I, XRD and SEM/EDX Analyses of Phase Evolution

An intimate Ba-Al-Al₂O₃-SiO₂ powder mixture, produced by high-energy milling, was pressed to 3 mm thick cylinders (10 mm diameter) and hexagonal plates (6 mm edge-to-edge width). Heat treatments conducted from 300° to 1650°C in pure oxygen or air were used to transform these solid-metal/oxide precursors into BaAl₂Si₂O₈. Barium oxidation was completed, and a binary silicate compound, Ba₂SiO₄, had formed within 24 h at 300°C. After 72 h at 650°C, aluminum oxidation was completed, and an appreciable amount of BaAl₂O₄ had formed. Diffraction peaks consistent with hexagonal BaAl₂Si₂O₈, BaAl₂O₄, β-BaSiO₃, and possibly β-BaSi₂O₅ were detected after 24 h at 900°C. Diffraction peaks for BaAl₂O₄ and BaAl₂Si₂O₈ were observed after 35 h at 1200°C, although SEM analyses also revealed fine silicate particles. Further reaction of this silicate with BaAl₂O₄ at 1350° to 1650°C yielded a mixture of hexagonal and monoclinic BaAl₂Si₂O₈. The observed reaction path was compared to prior work with other inorganic precursors to BaAl₂Si₂O₈.     

Synthesis of Mesoporous Needle-Shaped Ytterbium Oxide Crystals by Solvothermal Treatment of Ytterbium Chloride

The reaction of rare-earth (RE; Y, Er, and Yb) chloride hydrates in 1,4-butanediol at 300°C for 2 h gave mixtures of RE(OH)₂Cl and RE₂O₃· x H₂O, and the products were composed of irregularly shaped particles. A prolonged reaction (10 h) yielded a mixture of RE(OH)₂Cl and RE₂O₃· x H₂O for Er or Y, but phase-pure RE₂O₃· x H₂O was obtained for Yb. The product for Yb comprised needle-shaped single crystals of Yb₂O₃· x H₂O with a width of 0.2–0.6 μm and a length of 5–15 μm. The Yb₂O₃· x H₂O phase decomposed to Yb₂O₃ at 350°–500°C, preserving the needle-shaped morphology; this was maintained even after calcination at 1100°C. Single crystals of Yb₂O₃ obtained by the calcination of Yb₂O₃· x H₂O at 500°C had very small voids and the voids were enlarged to 35 Å in diameter by calcination at 800°C.     

Growth of Highly Oriented TlBa2Ca2Cu3O9—y Superconducting Films on Ag Substrates Using a Dip-Coated Barium Calcium Copper Oxide Sol—Gel Precursor

Thin films of Ba_zCa₂Cu₃O₇ a precursor of TIBa₂, Ca₂-Cu₃O_9-y, were prepared by sol-gel synthesis from an all alkoxide solution. The barium and calcium precursors were the respective metals reacted with 2-methoxyethanol, and the copper precursor was copper methoxide complexed by triethanolamine. Silver substrates were coated using the sol-gel solution by dip-coating. Subsequent processing included a low-temperature drying step (600°C), repeated coating to provide the desired thickness, heat treatment at 850°C in oxygen to remove carbon, and reaction at 860°C in a two-zone thallination furnace to produce a TIBa₂Ca₂-Cu₃O_9-y, superconducting film. These films were strongly c-axis-aligned, had a zero-resistance temperature (T₂) of 110 K, and a critical-current density (J_c) of 1.9 × 10₄ A/cmZ at 77 K and zero magnetic field.     

Formation and Stability Regions of the High-Temperature Fluorite-Related Phase in the R2O3-Ta2O5 System (R = La, Nd, Sm, Ho, Er, and Yb)

Solid-state reaction and annealing of melts of samples in the R₂O₃-Ta₂O₅system (R is a rare-earth element, i.e., La, Nd, Sm, Ho, Er, and Yb) revealed a defect-fluorite phase (F-phase) at high temperatures. The formation region of this F-phase was in the region of ˜70-80 mol% R₂O₃for small rare-earth ions, such as erbium and ytterbium, but only in the region of ˜80 mol% R₂O₃at temperatures of >1800°C for large rare-earth ions, such as the lanthanum-through-samarium series. This F-phase exhibited disordered cation and anion sublattices, such as (R0.8Ta0.2)(O1.7box_0.3). The F-phase decomposed to R₂O₃ and an ordered phase–R₃TaO₇ (orthorhombic weberite)–through a eutectic reaction at temperatures in the range of 1500°-1700°C for gadolinium or larger rare-earth ions, whereas the F-phase was stable at 1500°C for the small rare-earth ions (the erbium-through-ytterbium series).     

Preparation of Metal Ruthenates by Spray Pyrolysis

Submicrometer crystalline metal ruthenate powders with perovskite structure, MRuO₃ (M = Sr, La), and pyrochlore structure, M₂Ru₂O_{7- x} (0.5 < x < 1; M = Bi, Pb, Y, Eu, Gd, Tb, Dy, Ho, Er, Tm), were prepared by spray pyrolysis using metal nitrates and glycolates under an oxygen-gas atmosphere at temperatures up to 1100°C. Submicrometer-sized solid single crystals (SrRuO₃), submicrometer-sized hollow spheres consisting of nanocrystallites (pyrochlore rare-earth ruthenates, Bi₂Ru₂O₇, and Pb₂Ru₂O_6.5 below 1000°C), and nanometer-sized particles (Pb_2.31Ru_1.69O_6.5 and Bi-Pb-O above 1000°C) were observed. Particle formation proceeded by intraparticle reaction and intraparticle reaction followed by evaporation of volatile metal oxides to form metal oxide vapors followed by condensation and reaction to form particles. The former was observed for systems where no volatile metal oxides were formed, whereas the latter occurred for the Pb-Ru-O and Bi-Ru-O systems, where volatile metal oxides, such as Bi₂O, PbO, and RuO _x could occur. Particle morphology depended strongly on precursor properties. Submicrometer-sized single-crystal SrRuO₃ particles could be formed from the metal nitrates but not from Sr(NO₃)₂ and ruthenium glycolate, which gave hollow polycrystalline particles. In general, crystallite size could be controlled by varying precursor properties and reactor temperature, with higher temperatures giving larger crystallite sizes.     

Synthesis of Rare-Earth Gallium Garnets by the Glycothermal Method

Single-phase rare-earth gallium garnets were obtained by the reaction of stoichiometric mixtures of gallium acetylacetonate and rare-earth (Nd-Lu) acetates in 1,4-butanediol at 300°C (glycothermal reaction). Particles of gadolinium gallium garnet (GGG) and other gallium garnets with rare-earth elements larger than Gd were spherical with diameters of 0.5-2 μm, while particles of garnets with smaller rare-earth ions (Tb-Lu and Y) were much smaller (100-300 nm) with particle size distributed in a quite narrow range. TEM observation revealed that each particle was essentially a single crystal grown from one nucleus, but that defects frequently occurred during the crystal growth. Cerium and praseodymium gallium garnets were also formed when the reaction was carried out in the presence of GGG seed crystals. Hydrothermal reactions of the same starting materials under identical conditions yielded mixtures of gamma-Ga₂O₃ and the garnet phase.     

Synthesis and Characterization of Nanocrystalline Lanthanide Oxysulfide via a La(OH)3 Gel Solvothermal Route

A La(OH)₃ gel solvothermal process has been developed for the preparation of nanocrystalline lanthanide oxysulfide (La₂O₂S) in polar solvents at 300°C through a reaction between a La(OH)₃ gel and K₂S. X-ray powder diffraction (XRD) indicated that the product was hexagonal La₂O₂S with cell parameters a = 4.046 Å and c = 6.951 Å. Transmission electronic microscopy (TEM) showed that different morphology nanocrystallites were formed, including particles with diameters of about 10 nm and nanorods about 10 nm in diameter and 300 nm in length, depending on the solvent.     

Synthesis of V2O3 Powder by Evaporative Decomposition of Solutions and H2 Reduction

The evaporative decomposition of solutions method was used to form V₂O₅. Spraying above the congruent melting temperature of V₂O₅ (690°C) resulted in dense spherical particles with a smooth surface. Spraying below the V₂O₅ melting temperature yielded porous V₂O₅ powder with a rough surface. Reduction of the V₂O₅ to V₂O₃ was done in a H₂ atmosphere. Spherical V₂O₃ powder was attained when the reduction temperature was low enough to reduce the V₂O₅ surface before partial sintering (necking) between V₂O₅ particles occurred. The resulting V₂O₃ particle size was smaller than the precursor V₂O₅ powder as expected by the differences in densities between V₂O₅ ( p = 3.36 g/cm³) and V₂O₃ ( p = 4.87 g/cm³).     

Subsolidus Relations in the System ZrO2-ThO2-P2O5

Subsolidus phase equilibrium studies and linear thermal expansion data in the binary system ZrP₂O₇-ThP₂O₇ show that a series of metastable, low-expansion cubic solid solutions can be obtained at room temperature by a process of mutual stabilization. These solid solutions are ordinarily stable only above the inversions of ZrP₂O₇ and ThP₂O₇ at 300° and 1294°, respectively. Compatibility relations in other areas of the ternary system are shown in a diagram for equilibrium at 1400° and in another showing the influence of the metastable but very persistent form of 2ZrO₂·p₂O₅.     

Para-to-Ortho H2 Conversion on Gd, Y, Gd2O3, and Y2O3

The mechanism of parahydrogen conversion was studied on Gd₂O₃ and Y₂O₃ powders and on Gd and Y evaporated metal films at low and high temperatures (77° to 90°K and 298° to 418°K). Absolute rates of conversion are compared to theoretical values for 3 possible reaction mechanisms, and it is concluded that a paramagnetic vibrational mechanism is operative on Gd₂O₃, Gd, and Y. On Y₂O₃ the reaction rate is enhanced by additional surface paramagnetic sites. The portion of the surface which is active is ∼1 for the metals and ∼0.01 for the oxides.     

Effect of Hydrothermal Conditions on the Morphology of Colloidal Boehmite Particles: Implications for Fibril Formation and Monodispersity

The synthesis of colloidal boehmite (AlOOH) is studied by heating basic aluminum chloride solutions under constant stirring. The temperature and Al₂O₃: Cl molar ratio influence the product morphology. Synthesis at 140°C generates highly fibrous polycrystalline particles that are on average 360 nm long, 30 nm broad, and 8 nm thick. They contain 0.11 mol of excess H₂O per 1 mol of AlOOH. Synthesis at temperatures between 140° and 190°C produces broader fibrils and less excess H₂O. Preparation at 220°C eventually produces fully crystalline platelike boehmite particles about 260 nm long, 95 nm broad, and 14 nm thick, without excess H₂O. Fibril synthesis requires an Al₂O₃:Cl molar ratio exceeding 1.0 to yield noncoagulated particles. The fibrils are fairly monodisperse with 20% standard deviation in their length for an Al₂O₃: Cl molar ratio about 1.0.     

Synthesis and Growth Kinetics of Monodispersive Indium Hydrate Particles

Nano- or submicron In(OH)₃ and In₂O₃ particles of different morphologies were synthesized from a nitrate solution by a homogeneous precipitation process. By using X-ray diffractometer, thermogravimetric analysis, transmission and scanning electron microscopes, and inductively coupled plasma-optical emission spectrometry, the properties of particle growth were analyzed. The results indicated that the kinetics of the hydrolysis reaction of In³⁺ was a zero-order reaction with an activation energy of 128 kJ/mol, which implied that the reaction was controlled by the decomposition kinetic of urea additive. The growth anisotropic of particles, pH value of reaction solutions, residual In³⁺ concentration relative to aging time with different temperatures and starting concentrations were reported in this study. Calcination of the hydrate to form In₂O₃ particles between 300° and 900°C did not greatly change the morphologies of the particles.     

THIS ARTICLE HAS BEEN RETRACTEDEffect of Silica Sol on the Properties of Alumina-Based Duplex Ceramic Cores

A series of alumina-based ceramic cores sintered at 1300°C, 1400°C, and 1500°C for 5 h were prepared, and the phases and microstructures were characterized by X-ray diffraction and scanning electron microscopy. The effect of colloidal silica sols on the properties of ceramic core was discussed. The properties of these materials were determined. The results indicated that the microstructure of the core is characterized by the presence of substantially unreacted Al₂O₃ particles having a polycrystalline composition consisting essentially of in situ synthesized 3Al₂O₃·2SiO₂ on the surface of the Al₂O₃ particles. The colloidal silica sol contents do not have an appreciable effect on the densification and shrinkage of the alumina ceramic core. The ceramic cores of 5 wt% colloidal silica sol contents sintered at 1500°C for 5 h showed the smallest creep deformation in the present research.     

Microstructural Characterization of ZrO2 Particles Prepared by Reaction of Gaseous ZrCl4 with Fe2O3

The microstructure of ZrO₂ fine particles produced by a novel synthesis method at 450° and 950°C has been studied. The fundamentals of the synthesis method, which involves both chemical and diffusion phenomena, are presented. The method is based on mass transport through the gaseous phase between metallic zirconium and Fe₂O₃ powder. The mass-transporting chemical species are zirconium and iron chlorides. This article focuses on the microstructure and structure of ZrO₂ particles formed by the reaction between gaseous ZrCl₄ and solid Fe₂O₃, which is a relevant reaction step that occurs during the synthesis process. The resulting ZrO₂ crystals grown on Fe₂O₃ particles have been analyzed using transmission electron microscopy. Microstructural characterization has been complemented by X-ray diffractometry analysis. Tetragonal-ZrO₂ is produced at 450°C and monoclinic-ZrO₂ single crystals are produced at 950°C.     

Low-Temperature Synthesis of BaAl2O4/Aluminum-Bearing Composites by the Oxidation of Solid Metal-Bearing Precursors

BaAl₂O₄/aluminum-bearing composites have been synthesized via the low-temperature oxidation of Ba-Al precursors. Ba-Al powder mixtures that were prepared via high-energy vibratory milling were uniaxially pressed into bar-shaped specimens that were then exposed to a series of heat treatments in pure, flowing oxygen at temperatures up to 640°C. Oxidation at a temperature of 300°C resulted in the formation of barium peroxide (BaO₂). Additional heat treatment at a temperature of 550°C resulted in the consumption of BaO₂ and some aluminum to yield BaAl₂O₄ and Al₄Ba. The oxidation of Al₄Ba at a temperature of 640°C yielded additional BaAl₂O₄. Microstructural analyses revealed that a well-dispersed, co-continuous mixture of Al₂O₃-excess BaAl₂O₄ and 99.5% pure aluminum was produced.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

Electrical Conductivity in the ZrO2-Rich Region of Several M2O3—ZrO2 Systems

The electrical conductivity of M₂O₃-ZrO₂ compositions containing 6 to 24 mole % M₂O₃, where M represents La, Sm, Y, Yb, or Sc, was examined. Only Sm₂O₃, Y₂O₃, and Yb₂O₃ formed cubic solid solutions with ZrO₂ over most of this substitutional range. Scandia forms a wide cubic solid solution region with ZrO₂ at temperatures above 130°C whereas the cubic solid solution region at room temperature is narrow (6 to 8 mole % Sc₂O₃). Lanthana additions to ZrO₂produced no fluorite-type cubic solid solutions within the compositional range investigated. Generally, the electrical conductivity of these cubic solid solutions increased as the size of the substituted cation decreased and the electrical conductivity for each binary system attained a maximum at about 10 to 12 mole % M₂O₃.     

Preparation of Submicrometer A12O3 Powder by Gas-Phase Oxidation of Tris (acetylacetonato) aluminum (111)

Fine A1₂O₃ powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al₂O₃ at 1000° and 1200°C, and δ-Al₂O₃ at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas.