Phase equilibria in the CaO·SiO2Na2O·SiO2Na2O·Al2O3·6SiO2 system

Equilibrium phase relations in the system CaO·SiO₂Na₂O·SiO₂Na₂O·Al₂O₃·6SiO₂ at 40–80 wt% Na₂O·Al₂O₃·6SiO₂ composition range have been experimentally studied at temperatures between 800 °C and 1200 °C. The liquidus temperature was determined with differential scanning calorimetry. The equilibrated samples were quenched with pressurized nitrogen, and examined with electron probe X-ray microanalysis and X-ray diffraction for identification of microstructure and phase relations. Five primary phase fields, CaO·SiO₂, Na₂O·SiO₂, Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ were established. The ternary eutectic point of CaO·SiO₂, Na₂O·2CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ was determined to be at 1030 °C with the composition of 29.0 wt% CaO·SiO₂, 12.0 wt% Na₂O·SiO₂ and 59.0 wt% Na₂O·Al₂O₃·6SiO₂. Peritectic reaction of Na₂O·2CaO·3SiO₂, 2Na₂O·CaO·3SiO₂ and Na₂O·Al₂O₃·6SiO₂ occurred at 930 °C with the composition of 13.0 wt% CaO·SiO₂, 29.0 wt% Na₂O·SiO₂ and 58.0 wt% Na₂O·Al₂O₃·6SiO₂. The liquidus surface projection of the ternary system has been constructed in the composition region important for the bottom ash application.     

Crystallization of Glasses in the K2O–Nb2O5–SiO2System

The changes in the structure and phase composition of glasses in the K₂O–Nb₂O₅–SiO₂system upon their heat treatment in the temperature range 700–800°C are studied by the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction. It is demonstrated that the crystallization is the primary process giving rise to microinhomogeneities in glasses due to heat treatment. Nanocrystals of an unidentified niobium-containing phase precipitate in glasses with the formation of regions with a decreased content of potassium and niobium oxides. An increase in the duration of heat treatment at the studied temperatures results in an increase in the size of nanocrystals without change in their phase composition. This is accompanied by the disappearance of diffusion zones, which leads to a decrease in the SAXS intensity in the range of small scattering angles and, correspondingly, to a decrease in the light scattering intensity.     

On the Possibility of Forming Complexes with the Participation of O2– Ions in Melts and Glasses of the Na2O–SiO2–Cu2O System

The structural role of copper ions in melts (glasses) of the Na₂O–SiO₂–Cu₂O–CuO system is analyzed in the framework of the acid–base concept with due regard for the geometric (the radius ratio for Cu²⁽¹⁾⁺ and O^2– ions) and energy (the mean enthalpies of the Cu²⁽¹⁾⁺–O bonds) factors. It is demonstrated that copper ions in the structure fulfill the function of modifier cations. In these melts, the Cu¹⁺–Cu²⁺ redox equilibrium can be described without regard for the formation of [Cu²⁽¹⁾⁺O_4/2]^2(3)– ionic complexes (which could be incorporated into the structure of silicon–oxygen anions) and [Cu²⁺O _b/k ]^{2 – b/k} polyhedra providing the interaction between Cu²⁺ ions and anions. The influence of the formation of these polyhedra on the redox equilibrium is considered within the formalism of chemical thermodynamics. The composition dependence of the oxygen ion exponent pO is measured by an electromotive force (emf) technique. The ratio between the numbers of copper atoms with different valences is determined by chemical analysis. The experimental data obtained are in agreement with the theoretical inferences.     

Calculation of the Viscosity of Glass-Forming Melts: VI. The R2O–B2O3–SiO2and RO–B2O3–SiO2Ternary Borosilicate Systems

A method for calculating the viscosity from composition and temperature for melts in the R _m O _n –B₂O₃–SiO₂systems is proposed. The change in the concentrations of structural groups depending on the melt composition is taken into account in calculations. The results of calculations are compared with the experimental data available in the literature on the viscosity of 1200 melts with the use of the SciGlass information system. The root-mean-square deviation between the experimental and calculated characteristic temperatures varies from 30 K (for the glass transition temperature and the Littleton point) to 50 K (for a viscosity of 10⁴P).     

Influence of the support composition on the hydrogenation of acrolein over Ag/SiO2–Al2O3 catalysts

The gas phase hydrogenation of acrolein over supported silver catalysts has been investigated with a focus on the influence of the support acidity. Acidity has been varied by preparing silver catalysts supported on silica/alumina supports with varying SiO₂/Al₂O₃ ratio. After the catalytic experiments the Ag catalysts exhibit similar particle sizes, as revealed with TEM (transmission electron microscopy). The acidity of the samples was estimated using TPD of adsorbed ammonia which gives the total acidity of the samples, furthermore by IR of adsorbed pyridine to identify the Brønsted and Lewis acidity. No Brønsted acidity was found, and the Lewis acidity showed a clear dependence on the support composition. It is shown that a high total acidity and a high amount of strong Lewis acid sites on the catalysts cause a low conversion of acrolein and low selectivity to allyl alcohol. The interaction of silver with the support or effects of the metal–support perimeter are discussed as possible reasons for this behaviour.     

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.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Improvement in the non-linear electrical characteristics of the SnO2·Co2O3·Ta2O5 varistor material with Pr6O11 additive

The electrical and microstructural properties of SnO2-based varistor ceramic sample were improved by addition of Pr6O11. It was found that the introduction of Pr6O11 can lead to a great improvement in the threshold voltage and the non-linear electrical properties of SnO2-based varistors. As the amount of Pr6O11 increased from 0.00 to 0.5 mol%, the mean grain size decreases from 16.64 to 7.58 μm, the relative dielectric constant (at 1 kHz) increases from 1243.2 to 4534.6, the non-linear coefficient increases from 15.44 to 18.25 and the break down electrical field increases from 275.3 to 880.5 V/mm. The structure-property relationship is discussed systematically.     

Crystallization behavior of iron- and boron-containing nepheline (Na2O·Al2O3·2SiO2) based model high-level nuclear waste glasses

This study focuses on understanding the relationship between iron redox, composition, and heat-treatment atmosphere in nepheline-based model high-level nuclear waste glasses. Glasses in the Na₂O–Al₂O₃–B₂O₃–Fe₂O₃–SiO₂ system with varying Al₂O₃/Fe₂O₃ and Na₂O/Fe₂O₃ ratios have been synthesized by melt-quench technique and studied for their crystallization behavior in different heating atmospheres—air, inert (N₂), and reducing (96%N₂–4%H₂). The compositional dependence of iron redox chemistry in glasses and the impact of heating environment and crystallization on iron coordination in glass-ceramics have been investigated by Mössbauer spectroscopy and vibrating sample magnetometry. While iron coordination in glasses and glass-ceramics changed as a function of glass chemistry, the heating atmosphere during crystallization exhibited minimal effect on iron redox. The change in heating atmosphere did not affect the phase assemblage but did affect the microstructural evolution. While glass-ceramics produced as a result of heat treatment in air and N₂ atmospheres developed a golden/brown colored iron-rich layer on their surface, those produced in a reducing atmosphere did not exhibit any such phenomenon. Furthermore, while this iron-rich layer was observed in glass-ceramics with varying Al₂O₃/Fe₂O₃ ratio, it was absent from glass-ceramics with varying Na₂O/Fe₂O₃ ratio. An explanation of these results has been provided on the basis of kinetics of diffusion of oxygen and network modifiers in the glasses under different thermodynamic conditions. The plausible implications of the formation of iron-rich layer on the surface of glass-ceramics on the chemical durability of high-level nuclear waste glasses have been discussed.     

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. ©     

Enhanced electromagnetic wave absorption properties of a novel SiC nanowires reinforced SiO2/3Al2O3·2SiO2 porous ceramic

To realize the broad-bandwidth and high-efficiency absorption characteristics, a novel SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ porous ceramic was successfully fabricated by method of precursor infiltration pyrolysis (PIP). Polycarbosilane (PCS) and ferrocene (Fe(C₅H₅)₂) were used as the precursor and catalyst to incorporate SiC nanowires into the SiO₂/3Al₂O₃·2SiO₂ porous ceramic. The curvy SiC nanowires formed three-dimensional (3D) networks with a proper nanometer heterostructure, thereby consuming the microwave energies. The influence of SiC nanowires contents on the microwave absorption properties was investigated. The results indicate that the SiC nanowires contents can be tuned by controlling the PIP cycles, thereby modifying the dielectric properties of as-prepared composite ceramics. The dielectric and electromagnetic wave absorption performances are gradually enhanced with an increasing of SiC nanowires contents. The SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ composite ceramic exhibits excellent electromagnetic wave absorption abilities when the SiC nanowires content is 23.9% (PIP5). The minimum reflection coefficient (RC_min) of the composite ceramic is −30 dB at 10.0 GHz, corresponding to more than 99.9% of the electromagnetic wave consumption. The effective absorption bandwidth (EAB) can cover the frequency ranges of 8.2–12.4 GHz (the entire X-band) at the thickness of 5.0 mm. In general, the novel SiC nanowires reinforced SiO₂/3Al₂O₃·2SiO₂ composite ceramic can be considered as a promising electromagnetic wave absorbing material.     

Preparation and characterization of crednerite glass–ceramics in the MnO·CuO·SiO2 system

Nanoscale crednerite (CuMnO₂) was prepared in the system MnO·CuO·SiO₂, using glass–ceramics technique for the first time. Based on obtained data from differential thermal analysis (DTA), the prepared samples were heat-treated at 700 and 800 °C for 2 h. The presence of crystalline phases after and before heat treatment was investigated by X-ray diffraction analysis. Crystallization of crednerite (CuMnO₂), manganese silicate (Mn₂SiO₄) and traces of cuprite (Cu₂O) and cristobalite (SiO₂) phases were recognized. Transmission electron microscopy showed nanoscale crystals in the range 5–10 nm. The prepared glass–ceramics showed ferrimagnetic properties with wide range coercivity from 53 to 2217 Hci and magnetization saturation from 0.21708 to 1.2 emu/g. From IR reflection data; the reflection intensity of the light is high in the range of orange–red color and violet–blue colors and low in the range of green color.     

Production of Middle Distillate Through Hydrocracking of Paraffin Wax Over NiMo/SiO2-Al2O3 Catalysts: Effect of Solvent in the Preparation of SiO2-Al2O3 by a Sol–Gel Method

Mesoporous Ni-Al₂O₃ (XNiAl) catalysts with different Ni/Al atomic ratio (X) were prepared by a co-precipitation method for use in hydrogen production by steam reforming of liquefied natural gas (LNG). The effect of Ni/Al atomic ratio of mesoporous XNiAl catalysts on their physicochemical properties and catalytic activity for steam reforming of LNG was investigated. Physical properties of XNiAl catalysts did not show a consistent trend with respect to Ni/Al atomic ratio, while chemical properties of XNiAl catalysts strongly influenced by Ni/Al atomic ratio. Nickel species were highly dispersed on the surface of XNiAl catalysts through the formation of nickel aluminate phase or solid solution of nickel oxide and nickel aluminate phase. In the steam reforming of LNG, both LNG conversion and hydrogen composition in dry gas showed volcano-shaped curves with respect to Ni/Al atomic ratio. Nickel surface area of XNiAl catalysts was well correlated with LNG conversion and hydrogen composition over the catalysts. Among the catalysts tested, 0.8NiAl (Ni/Al = 0.8) catalyst with the highest nickel surface area showed the best catalytic performance.     

Ru-Pt catalysts supported on Al2O3 and SiO2─Al2O3 for the selective ring opening of naphthenes

Alumina and silica-alumina supported Ru-Pt catalysts were evaluated for the ring opening of naphthenes. Pt, Ru, and Ru-Pt catalysts were prepared by the impregnation of inorganic precursors over γ-alumina and silica-alumina supports. The catalysts were evaluated by temperature programmed reduction (TPR), pyridine temperature programmed desorption (TPD), CO-FTIR, and by test reactions of 33DM1B isomerization, cyclohexane dehydrogenation, cyclopentane hydrogenolysis, and the ring opening of decalin. The strong interaction between the metals (Pt-Ru) was attributed to their reductions occurring simultaneously. The acidity and strength of the acid sites of the monometallic Ru catalyst were higher than those of the monometallic Pt catalyst. The total acidity (Lewis and Brønsted) and the strength of the acid sites were higher for the silica-alumina supported catalysts. The silica-alumina catalysts had 10 times more Brønsted acidity than the γ-alumina ones and an increased activity and selectivity to decalin ring opening products. Supported monometallic Ru had the best performance for the ring opening reaction.     

Investigation of the formation of fractal structures in SiO2 · SnO x · CuO y thin films prepared by the sol-gel method

The processes of fractal structure formation have been considered in SiO₂ · SnO _x · CuO _y nanofilms prepared by the sol-gel method from a tetraethoxysilane alchohol solution modified by metal salts. The atomic force microscopy images of these films have been obtained. The surface morphology has been analyzed using the Takens embedding method and Grassberger-Procaccia algorithm. The correlation and fractal dimensions of the film samples and the coefficient of surface area increase have been calculated. The effects of the annealing temperature and concentration of the doping component on the formation of fractal structures in the materials under study have been estimated.     

Effect of P2O5 on evolution of microstructure of MgO–Al2O3–SiO2 glass ceramics

Abstract

MgO–Al₂O₃–SiO₂ (MAS) cordierite based glass ceramics were prepared by volume crystallisation. X-ray diffraction, Scanning electron microscopy and Energy diffraction scanning were used to investigate crystallisation behaviour and the influence of P₂O₅ on microstructure MAS based glass ceramics. The results showed that P⁵⁺ could promote the phase separation of MAS glass and that the glass was divided into two areas, such as Mg₄Al₂Ti₉O₂₅ and the containing P⁵⁺ area at <900°C. Mg₄Al₂Ti₉O₂₅ and Mg₃(PO₄)₂ in the area were both advantageous to the precipitation of μ cordierite, which further transformed to α cordierite due to P⁵⁺ in the residual glassy phase. However, P⁵⁺ inhibited the presence of cordierite when the heat treatment temperature was >900°C.     

Effect of steam on the acid strength of H3PW12O40/SiO2 · nH2O and its application in skeletal isomerization of n-butane

The effect of steam on the acid strength of H₃PW₁₂O₄₀ (HPW)/SiO₂ · nH₂O was determined by the Hammett indicators. When the steam content in N₂ was 1.6%, the acidity (H₀ > −13.7) of HPW/SiO₂ · nH₂O could be kept for over 10 h at 300 °C, but for only 2 h when steam was lacking. When it was used as a catalyst for skeletal isomerization of n-butane to isobutane at 300 °C, the HPW/SiO₂ · nH₂O showed stability in the presence of steam (1.6% in feed) better than that without steam, due to a suppression of the loss of crystalline water.     

Phase equilibria studies in the CaO–SiO2–Al2O3–MgO system with CaO/SiO2 ratio of 0.9

Phase equilibria and liquidus temperatures in the CaO–SiO₂–Al₂O₃–MgO system at a CaO/SiO₂ weight ratio of 0.9 in the liquid phase have been experimentally determined employing high-temperature equilibration and quenching technique followed by electron probe X-ray microanalysis. Isotherms at 1573, 1623, 1673, and 1773 K were determined and the primary phase fields of wollastonite, melilite, olivine, periclase, spinel, and corundum have been located. Compositions of the olivine and melilite solid solutions were analyzed and discussed. Comparisons between the newly constructed diagram, existing data, and FactSage predicted phase diagrams were performed and differences were discussed. The present study will be useful for guidance of industrial practices and further development of thermodynamic modeling.     

A Study of Ionic Conductivity of Glasses in the PbCl2–PbO · B2O3 and PbCl2–2PbO · B2O3 Systems

The steady-state electrical conductivity of oxychloride glasses in the PbCl₂–PbO · B₂O₃ and PbCl₂–2PbO · B₂O₃ systems is investigated. In the temperature range from 190 to 380°C, the dependence of log on the reciprocal of the temperature exhibits a linear behavior. The nature of charge carriers is studied using the Hittorf technique. It is demonstrated that protons and chlorine ions are charge carriers in solid glasses. The concentration dependence of the transport numbers of chlorine ions is examined by the Tubandt method. The contribution of the electronic component to the total electrical conductivity is estimated with the use of the Liang–Wagner technique. The concentration dependences of the electrical conductivity and the transport numbers of chlorine ions are interpreted in terms of the microinhomogeneous glass structure associated with the selective interaction of components during synthesis of glasses.