Fusion-Cast High-Chrome Refractories Synthesized in the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> - MgO - SiO<Subscript>2</Subscript> System

The synthesis and comprehensive study of fusion-cast refractories based on the Cr₂O₃ - MgO - SiO₂ system containing more than 60% Cr₂O₃ are reported. Analyzed by x-ray diffractometry and petrography, the synthesized materials display a phase composition represented by a complex spinel and escolaite. Tested for stability by molten alkali-free borosilicate E-glass, the synthesized refractories are shown to be not inferior in corrosion resistance to a chromium oxide-based ceramic refractory. __________ Translated from Novye Ogneupory, No. 10, pp. 68 – 74, October, 2005.     

Volumetric nucleation of crystals catalyzed by Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in glass based on furnace slags

The features of the volumetric nucleation of crystals in glass obtained by melting furnace slags with the additive of SiO₂, chromium sesquioxide Cr₂O₃, are studied by the methods of differential thermal and Xray phase analysis and optical microscopy. Upon the introduction of Cr₂O₃ as the catalytic additive, two phases are sequentially formed in the glass: magnesiochromite (MgO · Cr₂O₃) and diopside (CaO · MgO · 2SiO₂). The characteristics of homogeneous and heterogeneous crystallization are determined: the stationary nucleation rate, nonstationary nucleation time, crystal growth rate, and their temperature dependences are obtained. Practical recommendations on the use of the obtained glass are given.     

Synthesis and properties of flux-cast refractories in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Details are given of the synthesis and testing of flux-cast refractory materials in the alumina-rich region of the Al₂O₃-MgO-B₂O₃ system; XRD and petrography indicate that the main structure-forming phases are corundum and magnesian spinel. In subordinate amounts there are the boroaluminate 9Al₂O₃·2B₂O₃ and the previously unknown compound 4Al₂O₃·MgO·2B₂O₃, whose composition has been established by microprobe analysis. Corrosion tests showed that three-component systems containing magnesium and boron oxides at levels of 5–10% do not increase the corrosion resistance of refractories in molten sodium-calcium-silicate glass and electrovacuum borosilicate glass. __________ Translated from Novye Ogneupory, No. 3, pp. 161–163, March, 2008.     

Characterization of Fischer–Tropsch Cobalt-Based Catalytic Systems (Co/SiO<Subscript>2</Subscript> and Co/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) by X-ray Diffraction and Magnetic Measurements

Results of the characterization of six Co-based Fischer–Tropsch (FT) catalysts, with 15% Co loading and supported on SiO₂ and Al₂O₃, are presented. Room temperature X-ray diffraction (XRD), temperature and magnetic field (H) variation of the magnetization (M), and low-temperature (5 K) electron magnetic resonance (EMR) are used for determining the electronic states (Co⁰, CoO, Co₃O₄, Co²⁺) of cobalt. Performance of these catalysts for FT synthesis is tested at reaction temperature of 240 °C and pressure of 20 bars. Under these conditions, 15% Co/SiO₂ catalysts yield higher CO and syngas conversions with higher methane selectivity than 15% Co/Al₂O₃ catalysts. Conversely the Al₂O₃ supported catalysts gave much higher selectivity towards olefins than Co/SiO₂. These results yield the correlation that the presence of Co₃O₄ yield higher methane selectivity whereas the presence of Co²⁺ species yields lower methane selectivity but higher olefin selectivity. The activities and selectivities are found to be stable for 55 h on-stream.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and CeO<Subscript>2</Subscript>-promoted MgO sorbents for CO<Subscript>2</Subscript> capture at moderate temperatures

A series of Al₂O₃ and CeO₂ modified MgO sorbents was prepared and studied for CO₂ sorption at moderate temperatures. The CO₂ sorption capacity of MgO was enhanced with the addition of either Al₂O₃ or CeO₂. Over Al₂O₃-MgO sorbents, the best capacity of 24.6 mg- CO₂/g-sorbent was attained at 100 °C, which was 61% higher than that of MgO (15.3 mg-CO₂/g-sorbent). The highest capacity of 35.3 mg-CO₂/g-sorbent was obtained over the CeO₂-MgO sorbents at the optimal temperature of 200 °C. Combining with the characterization results, we conclude that the promotion effect on CO₂ sorption with the addition of Al₂O₃ and CeO₂ can be attributed to the increased surface area with reduced MgO crystallite size. Moreover, the addition of CeO₂ increased the basicity of MgO phase, resulting in more increase in the CO₂ capacity than Al₂O₃ promoter. Both the Al₂O₃-MgO and CeO₂-MgO sorbents exhibited better cyclic stability than MgO over the course of fifteen CO₂ sorption-desorption cycles. Compared to Al₂O₃, CeO₂ is more effective for promoting the CO₂ capacity of MgO. To enhance the CO₂ capacity of MgO sorbent, increasing the basicity is more effective than the increase in the surface area.

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Combustion of a Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript>-Al-C Powder Mixture in the SHS Regime and the Structure of the Combustion Products

The combustion of pressed specimens of a stoichiometric Fe₂O₃-TiO₂-Al-C powder mixture burning in the SHS regime is studied. It is found that the final product has the same shape and size as the initial specimen. The composition and structure of the final product, which is highly porous high-melting Al₂O₃-TiC-Fe(Ti) composite ceramics, are studied. The mechanism of phase and structure formation in the examined system is proposed.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 60–66, July–August, 2005.     

Understanding the solidification and leaching behavior of synthesized Cr-containing stainless steel slags with varying Al2O3/SiO2 mass ratios

This study investigated the effect of Al₂O₃/SiO₂ mass ratios on the equilibrium crystallization behavior of synthesized CaO–SiO₂–MgO–Al₂O₃–Cr₂O₃ stainless steel slags to understand the selective concentration behavior of Cr into a primary Mg(Cr,Al)₂O₄ spinel phase during slag solidification and to determine the leaching stability of Cr-containing slags. The spinel solid solution was precipitated within the temperature range of 1600-1400 °C, where the Cr/(Cr+Al) mole ratio in the Mg(Cr,Al)₂O₄ spinel phase gradually decreased for slags with higher Al₂O₃/SiO₂ mass ratios. When the Al₂O₃/SiO₂ mass ratio increased from 0.125 to 0.5, the Cr content in the amorphous glass phase gradually decreased, with a subsequent increase in the Cr content in the crystalline phase. For slags with a unit Al₂O₃/SiO₂ mass ratio and MgO mole percent comprising less than the combined sum of the Cr₂O₃ and Al₂O₃ mole percents, the Cr content in the amorphous glass phase increased, which was correlated with the enhanced substitution of Cr³⁺ with Al³⁺ in the spinel. The trend of the amount of Cr-related ions in the leachate was consistent with the trend of Cr in the amorphous glass phase: the amount decreased for slags with Al₂O₃/SiO₂ mass ratios from 0.125 to 5 and then increased for slags with an Al₂O₃/SiO₂ mass ratio of 1. The results suggest that the addition of appropriate amounts of Al₂O₃ to stainless steel slags could be conducive to stabilizing Cr into the primary spinel phase to minimize Cr leaching into the environment.     

Synthesis and properties of binary spinels in a NiO–CuO–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The conditions for the formation of a spinel structure from a NiO–CuO–Fe₂O₃–Cr₂O₃ oxide mixture using several technological approaches have been examined. Addition of KCl is accompanied with the formation of two spinel-like phases, whereas in the absence of KCl just one solid solution of nickel–copper ferrite–chromite with the structure of a cubic spinel is formed. At the temperature of thermal treatment of 900°C, the presence of an admixture phase of the delafossite (CuCrO₂) type was established. The conditions for the fabrication of samples containing two spinel phases (cubic and tetragonal) characterized with the most developed surface and manifesting = increased catalytic activity in the reaction of the decomposition of an organic substance by hydrogen peroxide have been formulated. The studied features of spinel synthesis can be of interest for developing materials with an active surface promising for application as adsorbents of catalysts and sensors.     

Refractive index and compressibility of the KAlSi<Subscript>3</Subscript>O<Subscript>8</Subscript> glass at pressures up to 6.0 GPa

The refractive index of potassium aluminosilicate glass of the KAlSi₃O₈ composition in the pressure range up to 6.0 GPa has been measured using a polarizing interference microscope and an apparatus with diamond anvils. The changes in the relative density, which characterize the compressibility of the K₂O · Al₂O₃ · 6SiO₂ glass, have been estimated in the pressure range under investigation from the measured refractive indices within the framework of the theory of photoelasticity. The results have been compared with the data previously obtained for the Na₂O · Al₂O₃ · 6SiO₂ glass. Although the molar contents of Al₂O₃ and M ₂O (where M = K or Na) are identical in these glasses, the KAlSi₃O₈ glass exhibits a higher compressibility, which agrees with the lower degree of depolymerization of this glass as compared to that observed in the NaAlSi₃O₈ glass. The pressure derivative of the bulk modulus K′ _t , which is calculated from the Birch-Murnaghan equation for the KAlSi₃O₈ glass (K′ _t = 7–9), is higher than that for the NaAlSi₃O₈ glass (K′ _t = 5.5–6.0). An increase in the pressure derivative of the bulk modulus K′ _t upon replacement of the Na⁺ cations by the K⁺ cations is explained by the inhibition of compression of the large K⁺ cations, which are located in cavities and have a considerably larger orbital radius than the Na+ cations. This manifests itself in the fact that the curves describing the dependences of the change in the relative density (d − d0)/d (compressibility) on the pressure P for the KAlSi₃O₈ and NaAlSi₃O₈ glasses converge at pressures above 4.0 GPa.     

Properties of compositions based on the system Al2O3-Mg(Al1−x,Crx)2O4

Conclusions Magnesia-alumina spinel and highly chromic spinel MgAl_0.4Cr_1.6O₄ and MgCr₂O₄ retard the grain growth of corundum during firing of the product. Low-chromic spinel MgAl_1.6Cr_0.4O₄ and MgAlCrO₄ added in small quantities intensify the corundum recrystallization.Small (5–10% by weight) additions of spinel Mg(Al_1–x, Cr_x)₂O₄ increase the strength of the corundum specimens, but only the high-alumina spinel improves their sintering.The sintering of mixtures of Al₂O₃ and Mg(Al_1–x Cr_x)₂O₄ is impaired during the substitution of magnesia-alumina spinel by magnesia chromite, and with an increase in the quantity of spinels from 5 to 30%.A small addition (5–10%) of high-alumina spinel of the composition Mg(Al_1–x, Cr_x)₂O₄ where x 0.5 to the alumina precalcined at 1450°C enables us to obtain dense, strong, and thermally shock-resistant corundum products.Translated from Ogneupory, No. 3, pp. 53–56, March, 1972.     

The phase conversions and properties of refractories based on the MgO-Al2O3-Cr2O3system

Conclusions An investigation of the phase conversions in the MgO-Al₂O₃-Cr₂O₃ system at a below-unity ratio MgO/R₂O₃ and varied content of sesquioxides showed that the magnesiochromite initially being formed in all mixtures interacts with the Al₂O₃ at temperatures above 1000°C with the result that two kinds of solid solutions are formed, viz., a spinel one Mg(Cr_xAl)_1–x)₂O₄ and (Al_xCr_1–x)₂O₃. The predominance of a given component in the original composition remains preserved in both phases.The presence was established of a solid solution of Al₂O₃ in the spinel phase which contained a signficant amount of MgAl₂O₄. No solid solutions of R₂O₃ are formed in the spinel when the chromite component predominates in both phases.The changes in the properties of the specimens correspond to the phase conversions. After high-temperature firing materials of the type Mg(Al, Cr)₂O₄-(Al, Cr)₂O₃ possess adequate property indices.The analysis is concerned with that part of the system for which MgO/(Al₂O₃ + Cr₂O₃) is less than unity.Translated from Ogneupory, No. 8, pp. 48–53, August, 1976.     

Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO₂/Al₂O₃ (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV–Vis, NH₃-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO₂/Al₂O₃ molar ratio. When the ratio of SiO₂/Al₂O₃ is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO₂/Al₂O₃ gave similar and high isobutane conversions. However, when the ratio of SiO₂/Al₂O₃ was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO₂/Al₂O₃ (≥80). The ratio of SiO₂/Al₂O₃ has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO₂/Al₂O₃ ratio effect on the catalytic performances.     

Europium(II) in glasses of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>-MnO-Eu<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Triply and doubly charged states of europium are revealed by ¹⁵¹Eu Mössbauer spectroscopy in the structure of glasses of the composition (mol %) 19.5Al₂O₃, 31.5SiO₂, 26.5MnO, and 22.5Eu₂O₃. The isomer shifts in the Mössbauer spectra of Eu³⁺ and Eu²⁺ ions in the structure of glasses differ from the isomer shifts in the spectra of the Eu₂O₃ and EuO compounds. This difference is explained by the fact that the electron density at ¹⁵¹Eu nuclei is affected by the manganese and aluminum atoms, which are not bound directly to the europium atoms. The broadening of the spectra of the Eu²⁺ ions in glasses is caused by the nonuniform isomer shift.     

Thermogravimetric Studies of Solid-Phase Reactions in the System MgO – Al2O3 – SiO2 and Their Analysis in Terms of Graph Theory

Changes in the phase composition of the MgO – Al₂O₃ – SiO₂ system in the subsolidus region are studied by the thermogravimetric method. Specific features of the DTA curves are correlated with periodic changes in the concentration of phases involved in chemical reversible solid-phase reactions. Critical kinetic effects associated with conjugated solid-phase reactions in the MgO – Al₂O₃ – SiO₂ system are discussed in terms of graph theory. The phase composition of materials with a dissipative structure is shown to form by a self-organization mechanism.     

The Oxidative Polycondensation of Benzylidene-4′-hydroxyanilene using Air O<Subscript>2</Subscript>, NaOCl and H<Subscript>2</Subscript>O<Subscript>2</Subscript>: Synthesis and Characterization

In this work, the oxidative polycondensation reaction conditions of benzylidene-4′-hydroxyanilene (B-4′-HA) were studied using oxidants such as air O₂, H₂O₂ and NaOCl in an aqueous alkaline medium between 40 and 95 ^○C. Oligo-benzylidene-4′-hydroxyanilene was characterized by ¹H-NMR, FT-IR, UV-Vis, size exclusion chromatography (SEC) and elemental analysis techniques. The solubility of oligomer using organic solvents such as DMF, THF, DMSO, methanol, ethanol, CHCl₃, CCl₄, toluene, acetonitrile, ethyl acetate was investigated. According to air O₂ oxidant (flow rate 8.5 L/h), the conversion of B-4′-HA was 82.0% in optimum conditions such as [B-4′-HA]₀=[KOH]₀=0.1015 mol/L at 50 ^○C for 25 h. According to the SEC analysis, the number-average molecular weight (M_n), weight-average molecular weight (M_w) and polydispersity index (PDI) values of O-B-4′-HA were found to be 1852 g mol⁻¹, 3101 g mol⁻¹ and 1.675; 2123 g mol⁻¹, 4073 g mol⁻¹ and 1.919; 2155 g mol⁻¹, 4164 g mol⁻¹ and 1.932, using air oxygen, NaOCl and H₂O₂ oxidants, respectively. Also, Thermo gravimetric analysis (TGA) showed oligo-benzylidene-4′-hydroxyanilene to be unstable against thermo-oxidative decomposition. The weight loss of O-B-4′-HA was found to be 95.87% at 1000 ^○C.     

Analysis of the formation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> + Fe nanocomposites

The formation of Al₂O₃ + Fe nanocomposites (in the range 0–20 wt % Fe) in the course of three sequential processes, such as dispersion, compaction, and sintering at a temperature of 1573 K, is investigated. It is revealed that the sintering is accompanied by the formation of the spinel phase at interfaces. It is demonstrated that the composition of the sintered samples corresponds to an equilibrium composition at a temperature of approximately 1073 K and that the spinel phase serves as a barrier layer preventing oxidation of iron     

A method for determining the charge state of chromium in Cr-doped Bi<Subscript>12</Subscript>SiO<Subscript>20</Subscript> and Bi<Subscript>12</Subscript>TiO<Subscript>20</Subscript> materials

A chemical method for determination of the charge state of chromium in B₂O₃-based materials (Bi₁₂SiO₂₀ and Bi₁₂TiO₂₀ single crystals) in a wide Cr concentration range (1 × 10^–5 – 2 × 10^–2 wt.%)is proposed. The method is based on a color reaction between Cr⁶⁺ and diphenylcarbazide in an acid medium.Translated from Novye Ogneupory, No. 8, pp. 61 – 63, August, 2004.     

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF₂–SiO₂–Al₂O₃–MgO melts generally increased by decreasing the CaO/SiO₂ ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO₂ ratio was 0.5, whereas CaAl₁₂O₁₉ (CA6) and Mg(Mn)Al₂O₄ (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO₂ ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO₂ ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO₂ ratio increased. From thermodynamic analysis considering the equilibrium constant (K_SP) and activity quotient (Q_SP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was proposed.     

Synthesis and electrochemical properties of layered LiNi<Subscript>1/2</Subscript>Mn<Subscript>1/2</Subscript>O<Subscript>2</Subscript>prepared by coprecipitation

Spherical LiNi_1/2Mn_1/2O ₂ powders were synthesized from LiOH . H₂O and coprecipitated metal hydroxide, (Ni_1/2Mn_1/2)(OH)₂. The average particle size of the powders was about 10 m and the size distribution was quite narrow due to the homogeneity of the metal hydroxide, (Ni_1/2Mn_1/2)(OH)₂. The tap-density of the LiNi_1/2Mn_1/2O₂ powders was approximately 2.2 g cm_–3, which is comparable to the tap-density of commercial LiCoO₂. The LiNi_1/2Mn_1/2 O₂electrode delivered a discharge capacity of 152, 163, 183, and 189 mA h g^–1 in the voltage ranges of 2.8–4.3, 2.8–4.4, 2.8–4.5, and 2.8–4.6 V, respectively, with good cyclability. Furthermore, Al(OH)₃-coated LiNi_1/2Mn_1/2O₂exhibited excellent cycling behavior and rate capability compared to the pristine electrode.     

Oxidative reforming of methane on structured Ni-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/cordierite catalysts

The catalytic properties of Ni/Al₂O₃ composites supported on ceramic cordierite honeycomb monoliths in oxidative methane reforming are reported. The prereduced catalyst has been tested in a flow reactor using reaction mixtures of the following compositions: in methane oxidation, 2–6% CH₄, 2–9% O₂, Ar; in carbon dioxide and oxidative carbon dioxide reforming of methane, 2–6% CH₄, 6–12% CO₂, and 0–4% O₂, and Ar. Physicochemical studies include the monitoring of the formation and oxidation of carbon, the strength of the Ni-O bond, and the phase composition of the catalyst. The structured Ni-Al₂O₃ catalysts are much more productive in the carbon dioxide reforming of methane than conventional granular catalysts. The catalysts performance is made more stable by regulating the acid-base properties of their surface via the introduction of alkali metal (Na, K) oxides to retard the coking of the surface. Rare-earth metal oxides with a low redox potential (La₂O₃, CeO₂) enhance the activity and stability of Ni-Al₂O₃/cordierite catalysts in the deep and partial oxidation and carbon dioxide reforming of methane. The carbon dioxide reforming of methane on the (NiO + La₂O₃ + Al₂O₃)/cordierite catalyst can be intensified by adding oxygen to the gas feed. This reduces the temperature necessary to reach a high methane conversion and does not exert any significant effect on the selectivity with respect to H₂.