Experimental investigation of the LiAlSi2O6‐MgSiO3 and LiAlSi2O6‐CaMgSi2O6 isopleths at 1 atm

The phase diagrams of the LiAlSi₂O₆‐MgSiO₃ and LiAlSi₂O₆‐CaMgSi₂O₆ isopleths were experimentally investigated at 1 atm using the quenching method and differential scanning calorimetry and the phases produced were characterized with the help of X‐ray diffraction and electron probe microanalysis. With the help of thermodynamic optimization, the phase diagrams of both systems were more accurately reported. No detectable solubility of Li₂O in diopside and enstatite was found. However, both systems are not simple binary eutectic systems because their phase equilibria are somewhat complex due to the presence of some β‐spodumene solid solution. In the β‐spodumene solid solution, no notable solubility of MgO and CaO was detected; evidence of significant solubility of SiO₂ was confirmed.     

Synthesis In Situ, Characterization, and Photostability of Europium β-Diketone Chelates in Organically Modified Silicates (ORMOSILs)

Europium β-diketonates have desirable optical properties, and transparent inorganic materials that have been doped with these chelates seem to be promising for potential applications in luminescence-display devices, integrated optical device sources, and new visible lasers. However, these chelates may be decomposed or not dissolved in sol—gel precursor solutions and, thus, cannot be doped in gel glasses that have been prepared via traditional sol—gel processing. An in situ synthesis technique has been used for the first time to synthesize five binary and ternary coordination compounds of europium β-diketonates in transparent organically modified silicates (ORMOSILs) during heat treatment or during the process of sol-to-monolithic ORMOSIL conversion. ORMOSILs that have been doped with in situ europium β-diketonates exhibit rather-high fluorescence intensity and good monochromati-city. These materials result in enhanced red-light emission under ultraviolet excitation. The chelates that have been synthesized in situ in the ORMOSILs reveal better photostability, in comparison to that of the pure chelates that have been dissolved in ethanol solutions.     

Heterogeneous enantioselective carbon–carbon bond formation: role of the inorganic support in the synthesis and activity of supported chiral auxiliaries

Heterogeneous enantioselective alkylation of benzaldehyde with diethylzinc was performed using (-)-ephedrine grafted on mesoporous micelle templated aluminosilicates or silicates, as chiral auxiliary. Supports were characterized by a regular mesoporosity and a same initial pore diameter. Immobilization of the chiral aminoalcohol was performed through covalent anchoring of 3-halogenopropyltrimethoxysilane (XPTMS) and substitution of the halogen by (-)-ephedrine. Comparison of the efficiency of the catalysts was carried out. Results were analyzed taking into account the accessibility to the catalytic sites by changing their density (decrease of XPTMS concentration, spacing of the sites by alkyl goups) and the effect of the uncovered mineral surface on activities and enantioselectivities. This revised version was published online in June 2006 with corrections to the Cover Date.     

Surface Relief Induced by Martensitic Transformation in Phosphorus-Bearing Dicalcium Silicate

Crystals of (Ca_1.955□_0.045)(Si_0.91P_0.09)O₄, where □ denotes a vacancy, have been prepared and examined by XRD, optical microscopy, SEM, and AFM. At 20°C, the crystals are composed of 38%α' _L and 62%β phases. Upon cooling to −185°C, the α' _L - to β-phase transformation occurs, which increases the β-phase composition to 72%. The transformation is also accompanied by the formation of platelike surface reliefs. The surface relief angles have been determined from observations (7.9°± 0.2°) and calculations based on a phenomenological analysis (7.84°). The fair agreement of these values indicates that the transformation is martensitic and mainly governed by a shear mechanism.     

Quantitative prediction of the structure and properties of Li2O–Ta2O5–SiO2 glasses via phase diagram approach

Tantalum silicate glasses serve as laser host materials to take advantage of their high refractive index and the ability to tailor their physical properties in the design of high-performance photonic and photoelectric components. However, successful attainment of feature control in tantalum-doped materials remains a longstanding problem due to the limited understanding of local structure around the tantalum ions, a problem that lies at the heart of predicting the micro- and macroscopic properties of these glasses. Herein, we present a novel approach for predicting the local structural environments in tantalum silicate glass based on a phase diagram approach. The phase relations and glass formation region of Li₂O–Ta₂O₅–SiO₂ ternary systems are explored to calculate the structure and additive physical properties of lithium tantalum silicate glasses. These measured and calculated results are in good quantitative agreement, indicating that the phase diagram approach can be applied broadly to Li₂O–Ta₂O₅–SiO₂ ternary glass systems. Using the phase diagram approach, the local structure of tantalum can be directly obtained. Each Ta atom is surrounded by six atoms, and its polyhedron, the TaO₆ octahedron, bonds through oxygen to Li and Ta. As a network modifier, Ta⁵⁺ depolymerizes the silicate glass structure by modulating the local structure of lithium atoms in Li₂O–Ta₂O₅–SiO₂ ternary glass system. The compositional dependence of structure in lithium tantalum silicate glasses is quantitatively determined based on the structure of the nearest neighbor congruent compound through the lever rule. These findings offer a precise prediction of tantalum silicate glass properties with quantitative control over local structural environment of the disordered materials.     

Phase evolution of reactive sputtering synthesized holmium silicate coatings

Holmium silicates are promising environmental barrier coatings (EBCs) materials for Si-based non-oxide ceramics due to their excellent thermal and mechanical properties. In the present work, the phase evolution of holmium silicate coatings synthesized by reactive sputtering was systematically studied. The monosilicate crystallized in an X1-Ho₂SiO₅ phase at 1000°C and transformed to an X2-Ho₂SiO₅ phase at 1200°C. The phase transition is not problematic for the coating process because the X1- and X2-Ho₂SiO₅ phases have similar coefficients of thermal expansion (CTEs). For the holmium disilicates, the stoichiometry obviously influenced polymorphic phase evolutions. Relatively Ho₂O₃-rich compounds promoted the formation of γ-Ho₂Si₂O₇ phase at lower temperatures. This study on phase evolution vs temperature and stoichiometry can provide a guideline to optimize the processing and the properties of advanced holmium silicate EBCs.     

Hafnium silicate formation during the reaction of β-cristobalite SiO2 and monoclinic HfO2 particles

Hafnium silicate (HfSiO₄; hafnon) is under consideration as an environmental barrier coating material for high-temperature applications. However, its rate of formation from mixtures of monoclinic HfO₂ and crystalline (β-cristobalite) SiO₂ powders is unknown. Here it has been synthesized and its formation rate measured during their solid-state reaction at temperatures from 1250°C to 1400°C. Rietveld refinement of X-ray diffraction patterns indicates that at 1250°C the hafnon phase fraction increases linearly with time, while at the highest reaction temperature, the hafnon phase fraction exhibited a parabolic dependence upon time. Between these two limiting temperatures, a region of linear behavior preceded a transition to parabolic kinetics, with the transition occurring at an earlier time as the reaction temperature increased. Arrhenius relations fitted the kinetics of hafnium silicate formation in both the linear and parabolic regimes. Scanning electron microscopy indicated that the reaction proceeded by diffusion of SiO₂ into HfO₂, similar to the mechanism by which zirconium silicate has been formed from vitreous SiO₂ and tetragonal ZrO₂. The initial linear rate of reaction is consistent with the growth of the contact area between the SiO₂ and HfO₂ particles combined with rapid permeation of the Si⁴⁺ and O²⁻ through the initial, incompletely formed hafnon. After a thin hafnon layer had formed between the reactants, the rate of hafnium silicate growth slowed and further growth was governed by the rate of diffusion of Si⁴⁺ and O²⁻ through the reaction product consistent with the observed parabolic dependence of the phase fraction upon time.     

Alkali-ash reactions and deposit formation in pulverized-coal-fired boilers: the thermodynamic aspects involving silica,sodium, sulphur and chlorine

Thermodynamic calculations predict the behaviour of alkalis in the furnace gases of pulverized-coal-fired boilers. Results imply that a large proportion of the volatile sodium in the coal reacts with silica or silicate fly ash and forms a low viscosity surface layer of sodium silicate on the ash which, depending on its thickness, may enhance retention of the ash particles impinging on boiler tubes. The thickness of the sodium silicate layer depends mainly on the ratio of alkalis (non-chloride) to ash surface area in the furnace gases and the kinetics of the silicate-forming reaction in the temperature range 1300–1850K. The calculations show that sodium sulphate is stable only below 1300–1400 K in pulverized-coal-fired furnace atmosphere, and, therefore, sodium-ash reactions at higher temperatures will greatly reduce sulphur retention in the boiler as sulphate deposits.     

Investigation of silicates as a catalyst in biodiesel production: A review

Hiking of crude oil prices and diesel fuel shortage is incentive for the researchers to develop bioenergy sources. Biodiesel has environmental beneficial attributes, and its production processes are worthy of continued studies. Many biodiesel production processes are available but, most of them are not on a commercial scale. Biodiesel production using solid catalysts involved fewer unit operations compared with homogeneous catalyzed processes. Many heterogeneous catalysts have been extensively investigated in the recent years and well established. Researchers' focus is how to obtain active and more stable silicates catalyst that can be recycled for several times in the process. Silicates catalyst activity and stability are critically discussed in this work to assess their industrial application, as excessive purification steps could be avoided. This review provides a brief overview on semi‐novel heterogeneous catalyst types ‘silicates’ used in the transesterification of vegetable oils for biodiesel production. Process conditions and leaching out of catalyst active sites are also highlighted. Product quality analysis is presented, in addition to concluded remarks regarding silicates as a selected catalyst. A preliminary economic assessment of biodiesel production catalyzed by the suggested catalyst ‘silicates’ compared with potassium hydroxide (KOH) and lime (CaO) is performed. Copyright © 2016 John Wiley & Sons, Ltd.