Characterization and sintering of gels in the system MgO–Al2O3–SiO2

Cordierite aerogels, made by supercritical drying, and xerogels, formed by ambient pressure drying, have been prepared by combining two different recipes. The chemical composition of the gels varied from stoichiometric cordierite 2MgO·Al₂O₃·5SiO₂ to 0·5MgO·1·4Al₂O₃·5SiO₂ due to different procedures for washing of the gels. The crystallization of nearly stoichiometric cordierite gels was shown to be relatively complex involving the formation of several metastable phases such as μ-cordierite (Mg₂Al₄Si₅O₁₈), spinel (Al₆Si₂O₁₃) and sapphirine (Mg₄Al₈Si₂O₂₀) before the equilibrium phase composition was obtained at around 1350°C. On the other hand, during crystallization of gels with stoichiometry close to 0·5MgO·1·4Al₂O₃·5SiO₂ the equilibrium phases mullite, cristobalite and α-cordierite were the major phases formed during heat treatment. A lower densification rate was observed for aerogels compared to xerogels due to a larger pore size. A lower crystallization temperature in aerogels probably due to heterogeneous nucleation reduced the densification. For gels with a composition near 0·5MgO·1·4Al₂O₃·5SiO₂ nucleation and densification occur simultaneously and large differences in the densification behavior was observed. ©     

Rh–Sr/Al2O3 Catalyst for N2O Decomposition in the Presence of O2

The improving effect of Sr in the catalytic activity of Rh for N₂O decomposition has been studied under 1,000 ppm N₂O/He and 1,000 ppm N₂O/5% O₂/He (GHSV = 10,000 h^?1). Different techniques have been used for catalysts characterization: TEM, SEM-EDX, XRD, N₂ adsorption at ?196 °C and in situ XPS. Sr favours the Rh dispersion and reduction under reaction conditions, and allows the low temperature removal of N₂O in the presence of O₂ (100% decomposition at 350 °C).     

Characterization of chemosynthetic H3PO4–Al2O3–2SiO2 geopolymers

Pure chemosynthetic Al₂O₃–2SiO₂ powders fabricated by a sol–gel method exhibit high phosphoric acid-activated properties and high compressive strengths. The phosphoric acid-activated properties could be characterized by compressive strength. The phase structure evolution of synthetic powders and the resulting geopolymers were investigated by DTA-TG, XRD, FTIR and MAS NMR analysis. These results show that the phosphoric acid-activation region of the synthetic powders was in the range of 200–800 °C, which was much lower than the temperature at which kaolinite was converted into metakaolinite. ³¹P MAS NMR analysis revealed that [PO₄] tetrahedra were part of the geopolymer structure.     

Synthesis and Characterization of Silica–Polymer Nanocomposites Functionalized with Piperazine for the Synthesis of β-Nitro Alcohols

Abstract  

In order to compare the activity and selectivity for the synthesis of β-nitro alcohols, piperazine was functionalized directly and after surface modification into the ordered mesoporous SBA-15 framework. The materials were characterized by powder X-ray diffraction, N₂-adsorption–desorption isotherm, FT-IR, SS-NMR and scanning electron microscopy. The catalyst synthesized via surface modification under solvent free conditions showed very high activity and selectivity of β-nitro alcohols compared to the one synthesized by direct functionalization of SBA-15. Finally the possible reaction pathways were explained mechanistically.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

Heat-Insulating Properties of High-Alumina Materials Filament-Reinforced with Glass Fiber of the Al2O3–SiO2 and Al2O3–SiO2–ZrO2 Systems

Refractories and Industrial Ceramics - Refractory heat-insulating materials produced in Russia are characterized. The thermal stability of high-alumina heat-insulating materials filament-reinforced...     

Synthesis of menthols from citral on Ni/SiO2–Al2O3 catalysts

The one-pot synthesis of menthols from citral was studied on Ni/SiO₂–AlO₃ catalysts containing 3.6%, 8.8% and 11.4% Ni. The yield of menthols increased with the amount of Ni up to 94% on Ni(11.4%)/SiO₂–AlO₃, reflecting the diminution of byproducts formation via acid-catalyzed reactions. The sample deactivation was studied by performing two consecutive catalytic tests. Results showed that Ni(11.4%)/SiO₂–AlO₃ was a stable, active, and highly selective catalyst because it contained the appropriate density and strength of bifunctional acid/Ni⁰ active sites to efficiently promote the hydrogenation/isomerization pathway involved in the reaction network while avoiding coke formation.     

Y2O3–Al2O3–SiO2-based glass-ceramic fillers for the laser-supported joining of SiC

Four glass-ceramic filler compositions within the Y₂O₃–Al₂O₃–SiO₂ system were tested for their suitability in laser-supported joining of SiC materials. The compositions differed in terms of their primary crystallization behavior. Joint zone microstructures were investigated and joint tightness was determined using helium leak rate measurements after joining and subsequent annealing at 900 °C and 1050 °C.Yttria- and silica-rich compositions showed a partial crystallization of yttrium silicates during the short laser processing. Subsequent heat treatment effected further crystallization toward equilibrium conditions. Despite the strong change in the degree of crystallization no reduction of the tightness was observed for the best compositions; after 500 h annealing at 1050 °C tightness values of less than 10⁻⁸ mbar l s⁻¹ were measured. These results demonstrate the potential of the investigated filler system for high temperature stable hermetic sealing. At the same time the creation of homogeneously structured joints from glass-ceramic fillers using a laser-supported technology needs the understanding of the crystallization kinetics.     

Fabrication and properties of macro-porous SiC using Al2O3–Y2O3–SiO2 as bonding additives

Macro-porous SiC was fabricated without using pore-forming agents by an in situ reaction bonding process. A bonding additive, Al₂O₃–Y₂O₃–SiO₂, with a low melting temperature was mixed with SiC particles and sintered at 1500 °C and 1600 °C for 1 h in Ar. Macro-porous SiC with a porosity of 32.7–45.9%, a pore size of 3.4–4.2 μm, and a relatively narrow and uniform pore size distribution was fabricated by varying the amount of bonding additive. The flexural strength of macro-porous SiC prepared at 1500 °C increased from 47.2 MPa to 71.2 MPa while the porosity decreased from 45.9% to 42.8%, respectively. When the sintering temperature of the macro-porous SiC was increased to 1600 °C, the flexural strengths were significantly reduced to 32.6–35.6 MPa, along with a reduction in porosity and pore size. The permeability of macro-porous SiC prepared at 1500 °C varied from 1.59 × 10^?12 m² to 1.26 × 10^?12 m², depending on the porosity. As the sintering temperature increased from 1500 °C to 1600 °C, the permeability decreased to less than 1.00 × 10^?12 m² because of the reduced porosity and average pore size. The electrical resistivity of macro-porous SiC prepared at 1500 °C and 1600 °C varied from 2.7 × 10⁸ Ω-cm to 1.4 × 10⁹ Ω-cm and from 1.3 × 10⁸ Ω-cm to 1.7 × 10⁹ Ω-cm, respectively, with increasing volume percent of bonding additives. The relatively high electrical resistivity was apparently due to neck bonding phase between SiC particles formed by phases consisting of Y₂Si₂O₇, YAG, and residual Al₂O₃.