In Situ Surface Modification of Silicon Carbide Particles Using Al3+ Complexes and Polyelectrolytes in Aqueous Suspensions

Silicon carbide (SiC) particles were modified in situ , using Al³⁺ complexes in aqueous media, via control of Al(NO₃)₃concentration and pH. The Al³⁺ formed hydrolyzed complexes that adsorbed onto the charged sites on the SiC particle surfaces. As a result, the surface-modified SiC particles behaved in an alumina-like manner in the approximate pH range 5–8. The modified SiC particles were further treated with two types of polyelectrolytes that were sequentially adsorbed onto the particle surface, to give a maximum surface charge. As a result of this surface-modification process, the SiC could be dispersed with Al₂O₃in aqueous media without heteroaggregation.     

Chemisorption of Yttrium Nitrate on AlN Particles

Homogenization of Y₂O₃ as a sintering additive in AlN green compact has been achieved by chemisorption of Y(NO₃)₃ on AlN particles in isopropanol suspension. Substantial adsorption of Y(NO₃)₃ equivalent to a Y₂O₃ content up to a maximum of 1.4 wt% of AlN has been observed. Electrokinetic mobility measurements of the coated particles and the adsorption studies show that the adsorption takes place in isopropanol suspension. SEM shows a good homogeneous distribution of Y₂O₃ in the green ceramic compacts, and no trace of secondary crystallization has been observed.     

Influence of a Ceramic Substrate on Aqueous Precipitation and Structural Evolution of Alumina Nano-Crystalline Coatings

Either boehmite (γ-AlOOH) or gibbsite (γ-Al(OH)₃) nanocrystalline thin films (h≈100 nm) can be precipitated from AlCl₃ solution at fixed pH and temperature onto different substrates. It depends on the nature of the substrate (mica flakes, SiO₂ flakes, or α-Al₂O₃ flakes), on their crystallographic properties (crystalline or amorphous), and on some experimental parameters (agitation rate, addition rate). According to the surface charge of the substrates, different alumina species are involved in the precipitation process. When negative charges are present on the substrate, the [Al₃O(OH)₃(OH₂)₉]⁴⁺ polycation is promoted, leading to the formation of the (Al₄) tetramer ([Al₄O(OH)₁₀(OH₂)₅]^o) and then to the precipitation of bohemite. When positive charges are present, a ligand bridge containing complex ([Al₃O(OH)₃(O₂H₃)₃(OH₂)₉]⁺) is likely favored, giving rise to hexagonal ring structures or amorphous solids that lead to the formation of gibbsite. Besides the surface effects, crystalline substrates can act as a template during precipitation of aluminum species as shown for the formation of gibbsite on muscovite. Finally, calcination at 850°C of boehmite samples leads to porous γ-Al₂O₃ layers, while calcination of gibbsite leads to δ-Al₂O₃ layers.     

Sinterability of β-Calcium Orthophosphate Powder Prepared by Spray-Pyrolysis

Mixed solutions of Ca(NO₃)₂ and (NH₄)₂HPO₄ with Ca/P = 1.50 were spray-pyrolyzed at 600°C to produce β-calcium orthophosphate (β-Ca₃(PO₄)₂) powder; the spray-pyrolyzed powder was ground and then calcined at 600°C for 1 h. The best crystalline β-Ca₃(PO₄)₂ powder was obtained from the solution with 1.80 mol.L^–1 Ca(NO₃)₂, 1.20 mol.L^–1 (NH₄)₂HPO₄. The resulting powder was composed of primary particles with sizes of <0.5 μm. Dense β-Ca₃(PO₄)₂ ceramics with a relative density of 96.1% could be fabricated by firing this compressed powder at 1070°C for 5 h.     

High-Yield Synthesis of B4C/BN Ceramic Materials by Pyrolysis of Polymeric Lewis Base Adducts of Decaborane(14)

Polymers of type [B₁₀H₁₂˙diamine]x (diamine=H₂NCH₂CH₂NH₂, (CH₃)₂NCH₂CH₂NH₂, (CH₃)₂NCH₂CH₂N(CH₃)₂, etc.) have been found to be useful ceramic precursors. In a stream of argon, their pyrolysis gives B4C/BN; in a stream of ammonia, BN.     

Growth of ZnO Nanorods by the Chemical Solution Method with Assisted Electrical Field

Zinc oxide (ZnO) nanorod arrays on the ZnO-coated seed substrates were prepared by the solution chemical method from Zn(NO₃)₂/NaOH under an assisted electrical field. The influence of the electrical field on ZnO nanorod growth was primarily explored, and the positive effects of the electrical field were demonstrated by adding polyethylene glycol in growth solution. It has been proved that the electrical field enhances ion adsorption to the substrate and lowers the nucleation energy barrier by increasing charge intensity; meanwhile, it produces H⁺ through oxidation of OH⁻ and increases properly the degree of solution supersaturation near the substrate surface. XRD results show that the nanorods grown under the electrical field primarily have a zincite structure. With increasing precursor concentration, the average diameter and length of ZnO nanorods increase. The maximum rod growth rate at a given concentration of Zn²⁺ ion occurs at a specific temperature.     

Synthesis of Anatase-Type TiO2 Nanocrystallites Via a Redox Route

Anatase nanocrystallites showing high surface area (∼62 m²/g) and good photocatalytic property have been obtained by pyrolyzing at 600°C for 4 h an ammonium titanyl double sulfate precursor (α-(NH₄)₂TiO(SO₄)₂) synthesized via a redox approach, that is, by oxidizing an aqueous solution of titanium trichloride (TiCl₃) with ammonium peroxodisulfate ((NH₄)₂S₂O₈), followed by reacting with ammonium sulfate ((NH₄)₂SO₄).     

Investigation of the Phase Diagram for the Pseudobinary System Li2SO4–La2(SO4)3 and Its Ionic Conductivity

The phase diagram of the pseudobinary system Li₂SO₄–La₂(SO₄)₃ has been investigated by means of X-ray diffraction and differential thermal analysis. LiLa(SO₄)₂ is formed by a peritectic reaction in this system; the peritectic temperature is 653±3°C. The eutectic reaction of Li₂SO₄ and LiLa(SO₄)₂ occurs at 553±3°C; the composition at the eutectic point is 17 mol% La₂(SO₄)₃. LiLa(SO₄)₂ is monoclinic with a=1.375 nm, b=0.6744 nm, c=0.7068 nm, and β=105.4°. The ionic conductivity of LiLa(SO₄)₂ has been studied from room temperature to 350°C and is found to be relatively low at room temperature or at lower temperatures. Its activation energy is 0.66 eV. Thus it is not suitable as a fast ion conductor.     

Reduction and Sintering of a Nickel–Dispersed-Alumina Composite and Its Properties

High-density nickel–dispersed-alumina (Al₂O₃/nickel) composites with superior mechanical properties were obtained by the hydrogen reduction and the hot pressing of alumina–nickel oxide (Al₂O₃/NiO) mixed powders. The mixtures were prepared by using NiO or nickel nitrate (Ni(NO₃)₂· n H₂O) as a dispersion source of nickel metal. Microstructural investigations of the composite fabricated using nitrate powder revealed that fine nickel particles, } 100 nm in diameter, dispersed homogeneously at the matrix grain boundaries, forming the intergranular nanocomposite. High strength (.1 GPa) and high-temperature hardness were registered for the composite that contained a small amount of nickel dispersion. The ferromagnetic properties of nickel, such as high coercive force, were observed, because of the fine magnetic dispersions, which indicates a functional value of structural composites.     

Transition Metal Ions as Spectroscopic Probes for Detection of Network Structure in Novel Inorganic Glasses

Spectra of divalent transition metal ions in ZnCl₂, KOAc-Ca(OAc)₂, KNO₃-Ca(NO₃)₂, and K₂SO₄-ZnSO₄ glasses are presented. Ions with low octahedral site preference energies (e.g. CO²⁺) can be used as "probes" for divalent cations in glass and can indicate the presence or absence of network structures. Thus ZnCl₂ glass has a tetrahedral network structure, analogous to vitreous BeF₂, but KOAc-Ca(OAc)₂ and KNO₃-Ca(NO₃)₂ glasses contain potassium ions and discrete acetatocalcate(II) and nitratocalcate(II) "complex ions." The structure of acetate and nitrate glasses is discussed in terms of the ideal glass concept.     

Dehydration Behavior of Aluminum Sulfate Hydrates

An exothermic transition is observed near 400°CC on thermal dehydration of highly crystalline AI₂(SO₄)3.16H₂O, Al₂(S0₄)₃ 14H₂O, and Al₂(S0₄)₃ 9H₂O when the early stages of heating are carried out in vacuum. Amorphous or partially crystalline hydrates do not show the exotherm. No systematic relation is apparent between the decomposition behavior and the pore volume distribution of the various anhydrous A1₂(SO₄)₃ products.     

Preparation of Nickel Powders by Spray Pyrolysis of Nickel Formate

The preparation of nickel powders by the ultrasonic spray pyrolysis of Ni(HCOO)₂ was studied. Phase-pure nickel powder was obtained at as low as 350°C. HCOOH was a reducing source for nickel formation. Moreover, metallic nickel was obtained at a residence time as short as 0.1 s at 600°C. A broad range of particle morphologies, which included agglomerated nanoparticles, nonagglomerated submicrometer particles, hollow particles, and spherical dense particles, were obtained from Ni(HCOO)₂ pyrolysis and were shown to depend on the precursor solution and the operating condition.     

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.     

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.     

Effects of Transition-Metal Substitution on the Catalytic Properties of Barium Hexaaluminogallate

The effects of the substitution of transition-metal ions and/or reductant gases on the catalytic properties of barium hexaaluminogallate were investigated. Transition-metal-substituted hexaaluminogallates (BaM(Al,Ga)₁₁O₁₉, M = transition metal, Al/Ga = 9/3) were synthesized from aqueous metal nitrates and ammonium carbonate by the coprecipitation followed by crystallization at 1100°C. The direct NO _x reduction was observed over BaM(Al,Ga)₁₁O₁₉ to be around 10%. The NO _x removal activity of BaM(Al,Ga)₁₁O₁₉ powders was improved by addition of C₃H₆ as a reductant gas. Co-, Ni- and Cu-substituted BaM(Al,Ga)₁₁O₁₉ catalysts exhibited about 40% NO _x reduction with C₃H₆ in excess oxygen at a high space velocity of 10 000 h⁻¹. The NO _x reduction on Mn- and Fe-substituted BaM(Al,Ga)₁₁O₁₉ catalysts was less than 10% even in the presence of C₃H₆. The temperature of the effective NO _x reduction on BaM(Al,Ga)₁₁O₁₉ catalysts could be adjusted from 350° to 500°C by the selection of the transition-metal substitution in the catalysts. The catalysts hold high activities for NO _x reduction even at 500°C in water vapor produced in the combustion system of reductant gases.     

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.     

Preparation and Properties of Alumina/Nickel-Cobalt Alloy Nanocomposites

High-density nickel-cobalt alloy-dispersed Al₂O₃ (Al₂O₃/Ni-Co alloy) composites were obtained via the hydrogen reduction and hot pressing of Al₂O₃, Ni(NO₃)₂6H₂O, and Co(NO₃)₂6H₂O powder mixtures. Microstructural investigations revealed that nanometer-sized alloy particles were dispersed homogeneously at the matrix grain boundaries, forming the intergranular-type nanocomposite. High strength (>1 GPa) was registered for the Al₂O₃/10 wt% Ni-Co alloy composite. An inverse magnetostrictive response to applied stress was observed, because of the Ni-Co alloy dispersions, which indicates promise for incorporating new functions such as stress and fracture sensing into the structural ceramics without any loss of mechanical properties.     

Water-Based Gelcasting of Surface-Coated Silicon Nitride Powder

A layer of Y₂O₃–Al₂O₃, used as a sintering aid, was coated onto the surface of Si₃N₄ particles by the precipitation of inorganic salts from a water-based solution containing Al(NO₃)₃, Y(NO₃)₃, and urea. The electrokinetic and colloidal characteristics of the Si₃N₄ powder were changed significantly by the coating layer. As a result, dispersion of the Y₂O₃–Al₂O₃-coated Si₃N₄ powder was significantly greater than that of the original powder. Furthermore, the Y₂O₃–Al₂O₃ coating layer prevented the hydrogen-gas-discharging problem that occurred during gelcasting of the original Si₃N₄ powder because of reaction between the uncoated powder and the basic aqueous solution in suspension. Surface coating, as well as the gelcasting process, significantly improved the microstructure, room-temperature bending strength, and Weibull modulus of the resulting ceramic bodies.     

Preparation by a Sol-Gel Process of Borosilicate Glasses Containing Microcrystals of Tellurium and Zinc Telluride

Borosilicate glasses, 5B₂O₃· 95SiO₂ (mol%), containing TeO₂ and ZnO nominally equivalent to 10 wt% Te and ZnTe were prepared by a solgel method from Si(OC₂H₅)₄, B(OCH₃)₃, H₆TeO₆, and Zn(NO₃)₂. A study by electron spectroscopy for chemical analysis (ESCA) showed that glasses heated at high temperature (450°C) in air contained both Te⁶⁺ and Te⁴⁺ ions on the surface layer, but that mainly Te⁴⁺ ions occurred inside the bulk glass. When solgel-derived borosilicate glasses containing the TeO₂ compound were reduced at elevated temperature in a hydrogen atmosphere, Te crystallites ranging in size from 4 to 15 nm were produced at a lower temperature, between 200° and 250°C. The absorption edge moved from the infrared to the visible wavelength region as the particle size decreased to about 4 nm. For glasses containing both TeO₂ and ZnO, ZnTe crystallites formed at high temperature—over 300°C—and existed along with the Te phase.     

Phase Equilibrium in a Portion of the Ternary System Ba OA-I2,O3,-SiO2,

Phase-equilibrium relations on the liquidus surface in the system Ba0-A1₂O₃-SiO₂ have been investigated by the quenching method. The compositions investigated within the ternary area were those containing less than 30%, A1₂O₃ and more than 20% SiO₂ by weight. Petrographic and X-ray techniques were employed in the determination of the crystalline phases.
The crystal phases that separate from melts within the area investigated are barium orthosilicate (2BaO. SiO₂,), barium metasilicate (BaO 2SO₂,), solid solutions, sanbornite (BaO 2SiO₂), tridymite and cristobalite (SO₂), mullite (3A1₂O₃ 2SiO₂), and celsian (BaO A1₂O3_.2SiO₂). Diagrams show the isotherms and indices of refraction of the glasses.
Five quintuple points and eleven boundary curves have been determined within = .5yo compositional variations. The liquidus-surface temperatures have been obtained within limits of ± 125°C.