Sc2O3 Nanopowders via Hydroxyl Precipitation: Effects of Sulfate Ions on Powder Properties

Hydroxyl-type Sc₂O₃ precursors have been synthesized via precipitation at 80°C with hexamethylenetetramine as the precipitant. The effects of starting salts (scandium nitrate and sulfate) on powder properties are investigated. Characterizations of the powders are achieved by elemental analysis, X-ray diffractometry (XRD), differential thermal analysis/thermogravimetry (DTA/TG), high-resolution scanning electron microscopy (HRSEM), and Brunauer-Emmett-Teller (BET) analysis. Hard-aggregated precursors (γ-ScOOH·0.6H₂O) are formed with scandium nitrate, which convert to Sc₂O₃ at temperatures ≥400°C, yielding nanocrystalline oxides of low surface area. The use of sulfate leads to a loosely agglomerated basic sulfate powder having an approximate composition of Sc(OH)_2.6(SO₄)_0.2·H₂O. The powder transforms to Sc₂O₃ via dehydroxylization and desulfurization at temperatures up to 1000°C. Well-dispersed Sc₂O₃ nanopowders (∼64.3 nm) of high purity have been obtained by calcining the basic sulfate at 1000°C for 4 h. The effects of SO₄²⁻ on powder properties are discussed.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

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.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Stability of SrFeO3-Based Materials in H2O/CO2-Containing Atmospheres at High Temperatures and Pressures

The chemical stability of SrFeO₃-based perovskites in H₂O- and CO₂-containing atmospheres at high temperatures and pressures has been examined. The extent of reaction as a function of p CO₂, p H₂O, temperature, and time has been determined. Either strontium carbonate or Sr(OH)₂·H₂O was observed on sample surfaces after exposure. Observation of two different reaction-rate behaviors could be explained by the formation of different products. The stability of the perovskite has been found to increase when the activity of Sr is decreased. Chemical stability in H₂O/CO₂ is important to understand in order to use these membrane materials for syngas production.     

Preparation of Dicalcium Siliciate at 950°C

β-Ca₂Si0₄ can be obtained from a mixture ofCaC₂O₄-H₂O and amorphous silica by firing at 950°C as opposed to a normal sintering temperature around 1450°C. If CaCO₃ is used instead of CaC₂O₄·H₂O, four repeated firings under CO₂ atmosphere are needed to obtain β-Ca₂SiO₄. The role of CO₂ atmosphere during firing and the influence of specific surface of reactanm on the rate of reaction are discussed.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Mechanochemical Changes of Weinschenkite-Type RPO4·2H2O (R = Dy, Y, Er, or Yb) by Grinding and Thermal Reactions of the Ground Specimens

It is reported that, on mechanochemical treatment, weinschenkite-type RPO₄·2H₂O (R = Dy, Y, or Er) gradually transforms into rhabdophane-type RPO₄· nH₂O (n = 0.5 to 1) and weinschenkite-type YbPO₄·2H₂O into xenotime-type YbPO₄, at room temperature in air. Rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O obtained by grinding weinschenkite-type RPO₄·2H₂O (R=Y or Er) are new. The new rhabdophane-type YPO₄·0.8H₂O and ErPO₄·0.9H₂O gradually transform to xenotime-type YPO₄ and ErPO₄ when heated above 900°C (R = Y) and 700°C (R = Er) in air.     

Optimization of Chemical Vapor Deposition Parameters for Fabrication of Oxidation-Resistant Mullite Coatings on Silicon Nitride

Fabrication of mullite (3Al₂O₃·2SiO₂) coatings by chemical vapor deposition (CVD) using AlCl₃–SiCl₄–H₂–CO₂ gas mixtures was studied. The resultant CVD mullite coating microstructures were sensitive to gas-phase composition and deposition temperature. Chemical thermodynamic calculations performed on the AlCl₃–SiCl₄–H₂–CO₂ system were used to predict an equilibrium CVD phase diagram. Results from the thermodynamic analysis, process optimization, and effects of various process parameters on coating morphology are discussed. Dense, adherent crystalline CVD mullite coatings ∼2 μm thick were successfully grown on Si₃N₄ substrates at 1000°C and 10.7 kPa total pressure. The resultant coatings were 001 textured and contained well-faceted grains ∼0.3–0.5 μm in size.     

Reactive Ceria Nanopowders via Carbonate Precipitation

Nanocrystalline CeO₂ powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce³⁺ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO₃·2.5H₂O, decomposes to CeO₂ at temperatures ≥300°C. The CeO₂ powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.     

A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO₃)₃·6H₂O, and Al(NO₃)₃·9H₂O or AlCl₃·6H₂O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO₃⁻, etc. X-ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry-spinning fibers with the aid of some organic polymer.     

Fabrication and Photoluminescence Characteristics of Eu3+-Doped Lu2O3 Transparent Ceramics

A novel co-precipitation process was adopted for the preparation of highly sinterable europium-doped lutetia powders using ammonium hydroxide (NH₃·H₂O) and ammonium hydrogen carbonate (NH₄HCO₃) as the mixed precipitant. The resultant powders calcined at 1000°C for 2 h showed good dispersity and excellent sinterability. Highly transparent polycrystalline lutetia ceramics with a relative density of ∼99.9% were fabricated by pressureless sintering in flowing H₂ atmosphere at 1850°C for 6 h without any additives. The average grain sizes of the transparent material were estimated to be 50–60 μm. Optical in-line transmittance in the visible wavelength region for Lu₂O₃ ceramics (1 mm in thickness) reached 80%. The luminescence and decay behavior of the obtained transparent plate and the corresponding nanophosphors were also investigated.     

Nano-Blast Synthesis of Nano-size CeO2–Gd2O3 Powders

CeCl₃·7H₂O and GdCl₃·6H₂O that were dissolved in water were precipitated with urea (NH₂CONH₂) to produce matrix agglomerates for three-component nano-reactors. Mixing hexamethylenetetramine with dilute nitric acid resulted in the formation of well-dispersed nano-particles of cyclotrimetilene trinitramine (C₃H₆N₆O₆) (RDX) in the solvent. Nano-reactors were produced by impregnating the nano-C₃H₆N₆O₆ into the matrix agglomerates of an intermediate complex of cerium and gadolinium compounds. Blast initiation of the C₃H₆N₆O₆ resulted in extremely rapid detonation and gaseous products formation at temperatures of 2000°–5000°C, which were compressed into a volume nearly equal to the initial volume of each RDX nano-particle. Multiple "nano-blasts" occurred in the volume of each nano-reactor. The impact of the blast waves led to fragmentation of the surrounding matter. The evolution of a large volume of gaseous products dissipated the heat of the process and limited temperature increase, thus reducing the possibility of local sintering among the primary particles. The short-term high temperature generated during the blasts enhanced the solid solubility of the metal oxides. Uniform aggregates of 22∼74 nm consisting of 6∼14 nm crystallites of gadolinia in ceria solid solution were synthesized.     

Effect of Some Additives on the Specific Surface of Alumina Obtained from Aluminum-Ammonium Alum

The effect of Al(NO₃)₃·9H₂O, AlCl₃·6H₂O, Al(CH₃COO)₃, and NH₄F on the specific surface of Al₂O₃ obtained from aluminum-ammonium alum by calcining was studied. It was found that the use of these additives makes it possible to obtain Al₂O₃ with specific surface varying from 1 to 135 m²/g after thermal treatment in the interval from 1273 to 1423 K. The changes in the morphology and structure of powederd Al₂O₃ obtained from alum containing these additives were studied by electron microscope observations.     

Fabrication of NiO Nanoparticle-Coated Lead Zirconate Titanate Powders by the Heterogeneous Precipitation Method

NiO nanoparticle-coated lead zirconate titanate (PZT) powders are successfully fabricated by the heterogeneous precipitation method using PZT, Ni(NO₃)₂·6H₂O, and NH₄HCO₃ as the starting materials. The amorphous NiCO₃·2Ni(OH)₂·2H₂O are uniformly coated on the surface of PZT particles. XRD analysis and the selected-area diffraction (SAD) pattern indicate that the amorphous coating layer is crystallized to NiO after being calcined at 400°C for 2 h. TEM images show that the NiO particles of ∼8 nm are spherical and weakly agglomerated. The thickness of the nanocrystalline NiO coating layer on the surface of PZT particle is ∼30 nm.     

Synthesis and Properties of Dielectric Ceramics from Metallo-Organic Precursors through the Solid-State Reaction of Mixed Gels

A perovskite structure of 0.4Pb(Mg_1/3Nb_2/3)O₃·0.3Pb(Mg_1/2W_1/2)O₃·0.3PbTiO₃ was prepared from metallo-organic precursors through the solid-state reaction of the mixed gels. Three types of mixed gels were crystallized to obtain PbTiO₃, MgNb₂O₆, and MgWO₄ powders. These powders were calcined at 900°C after mixing with a stoichiometric amount of Pb(CH₃COO)₂·3H₂O. The dielectric constant of the ceramic fired at 900°C was improved by adding an excess of 10 mol% Mg(OC₂H₅)₂, and the ceramic achieved X7T specification of the Electric Industries Association standard. The dielectric loss was reduced by adding an excess of 5 mol% Pb(CH₃COO)₂·3H₂O.