Sintering of Corundum Mixtures Modified by a Vitreous Phase of a Eutectoid Composition

The effect of a vitreous phase of a eutectoid composition on sintering of corundum mixtures is considered. It is established that the additive corresponding to a eutectic composition of the CaO – Al₂O₃ system melting at a temperature of 1395°C improves the service properties of corundum concrete through activating its sintering.     

Gunited concrete linings of conveyer sintering machines

Conclusions The construction of unique heating equipment requires the development of composite programs with a single center of control, coordination, and financing of all work: scientific research, planning and design, construction and assembly, and start-up and adjustment. Further introduction of gunited concrete linings requires standard production by the refractory industry of guniting mixtures, for which successful use may be made of VTs-75 cement of ferroalloy production clinker, using additional working with the addition of special inspection on the basis of petrographic or x-ray diffraction analyses or on the basis of the criterion M (limits of variation 3.2–4.1). In mineral composition the clinker must meet the following requirements: 73–78.2% CaO·2Al₂O₃; 5% max·CaO·6Al₂O₃; 3% max· MgO·Al₂O₃; 6–9% CaO·Al₂O₃; up to 4% chromium regulus.As the result of the work conducted, a method was developed for guniting linings of various degrees of complexity with a refractory layer thickness up to 200 mm operating at a temperature up to 1450°C in an oxidizing, iron, gas, and dust atmosphere.The method may find wide use for lining of thermal equipment both in the steel industry and in other branches of industry.Translated from Ogneupory, No. 4, pp. 51–57, April, 1988.     

Thermal stability of palladium on ceria-doped alumina

Several palladium on alumina and ceria/alumina catalysts were prepared and oxidized in air between 400 and 1000°C. The metal dispersion was determined by hydrogen titration of adsorbed oxygen. Dispersions above 50% were maintained on 0.2% Pd/Al₂O₃ up to 900°C. Adding 5.0% ceria, or increasing the metal loading to 2.5%, greatly reduces the thermal stability of the palladium, such that the dispersion falls rapidly at 600°C. The rates of methane oxidation (moles of CO₂/g Pd h) at 250°C and 5% excess oxygen are nearly equal on 0.22–2.50% Pd/3.5–5.2% CeO₂/Al₂O₃, dispersion 14–42%, and 0.20–0.46% Pd/Al₂O₃, dispersion 59–86%, but are 10 to 20 times lower than the rate on 2.3% Pd/Al₂O₃, dispersion 11%. The lower rate of methane oxidation on ceria-promoted and highly dispersed palladium on alumina might be due to the conversion of the palladium into less active palladium oxide during reaction.     

Ceramic cements and ceramic concretes of quartz-chamotte composition

Conclusions On the basis of quartz sand and scrap chamotte refractories (28.0% Al₂O₃, 67.0% SiO₂) we obtained highly concentrated bonding suspensions (cement slips) of mixed composition enabling us to form castings with a porosity of 12.2–18.3% characterized by bending strengths of up to 4.5 MPa and compressive strengths of up to 32.4 MPa.During the use of quartz-chamotte suspensions as bond and of chamotte scrap (grog) as the filler we obtained ceramic concretes with an original compressive strength of 18.4–25.6 MPa, and a porosity of 14.6–19.2%.We studied the change in the physicomechanical properties of the bonds (cements) and the ceramic concretes during heat treatment in the range 100–1450°C.The chamotte ceramic concretes based on quartz-chamotte bonds of the optimal compositions possess an increased spalling resistance (16 water-heat cycles from 1300°C during testing of specimen cubes with edges of 50 mm).Translated from Ogneupory, No. 5, pp. 5–9, May, 1986     

Properties of hot aluminosilicate concretes with various binders

Conclusions On the basis of results of a study of aluminosilicate concretes in the heated state, the following recommendations can be made for the use of these concretes, depending on the form of the binder and on the concentration of Al₂O₃ in the filler.Semiacid concrete in APB has fairly good strength and deformation properties, a high thermal-shock resistance, and can be used up to temperatures of 1350°C.Chamotte concrete in HAC is clearly not economical to manufacture. For performance, it can be totally replaced by chamotte concrete in AC or APB. The chamotte concrete in APB has better characteristics in the heated state than the concretes in other binders. It can be successfully used under conditions where there are sudden variations in the temperature and mechanical action.Aluminosilicate concretes in WG can reasonably be used in conditions of fairly intense abrasive action up to temperatures of 800°C. In places where the concrete is not subject to the action of molten metal or slag, it would not be reasonable to use concretes in WG containing >60% Al₂O₃.The high-alumina concretes in HAC have greater strength at high temperatures and therefore they must be used in those conditions; analogous concretes in APB can reasonably be used where there are sudden variations in temperature.The properties considered here of the concretes in HAC and APB are improved by increasing the concentration of Al₂O₃ and therefore the form of the filler for the concretes must be chosen in relation to the actual conditions of use.Translated from Ogneupory, No. 7, pp. 52–60, July, 1980.     

The structure and composition of high-frequency fused mullite

Conclusions The high-frequency smelting of a charge containing 69–75% Al₂O₃ and 31–25% SiO₂ gives a product with a stable phase composition of mullite and glass. Corundum occurs only in the form of discrete crystals.Mullite from a charge with alumina modulus 3.0 contains a minimum of glass and a maximum of Al₂O₃, so that it is the most interesting one from the standpoint of refractories production.Firing the fused materials promotes the formation of mullite from the vitreous phase. A similar phenomenon occurs during the firing of products manufactured from fused mullite by the ceramics technology. In this case the pre-firing stage can be dispensed with.Deceased.Translated from Ogneupory, No. 4, pp. 40–46, April, 1977.     

Properties of RO-Y2O3-Al2O3-SiO2 glasses

The coefficient of linear expansion, glass-transition temperature, temperature at the orset of deformation (strain point), density. Young modulus, microhardness, crystallizability, and contact angle are studied as a function of the composition in RO–Y₂O₃–Al₂O₃–SiO₂ (R=Ca and/or Mg) glass systems. The composition ranges for glasses (with strain point >900°C and coefficient of linear expansion of (32–45)×10^–7°C^–1) that can be used for soldering silicon-nitride ceramics were established.Translated from Steklo i Keramika, No. 12, pp. 5–7, December, 1996     

On the Phase Separation and Crystallization of Glasses in the MgO–Al2O3–SiO2–TiO2 System

The changes in the structure and phase composition of glasses in the MgO–Al₂O₃–SiO₂–TiO₂ system (at different TiO₂ contents and ratios MgO : Al₂O₃) upon their heat treatment in the temperature range 700–960°C are investigated by small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. The influence of gallium oxide additives on the phase separation and crystallization is analyzed. It is demonstrated that the heat treatment results in the phase separation, which occurs through the spinodal decomposition mechanism. A regular structure formed upon phase separation is retained after the completion of crystallization in inhomogeneity regions. The interference effects due to the regularity in the distribution of nanocrystals in the vitreous matrix bring about a decrease in the light scattering intensity and provide transparency of glass-ceramic materials.     

Synthesis of Glass-Ceramic Materials in the BaO–PbO–B2O3–Al2O3–TiO2 System

The influence of PbO, B₂O₃, and Al₂O₃ additives on the glass formation and crystallization of glasses with a high total content of BaO and TiO₂ (65–75 wt % or 76–86 mol %) is investigated. It is shown that glasses of the compositions (wt %) 31–35 BaO, 12–17 PbO, 34–42 TiO₂, 10–13 Al₂O₃, and 2–3 B₂O₃ are promising materials for use in preparing glass-ceramic ferroelectrics based on the melting–molding–crystallization technology. These compounds are characterized by a relatively low melting temperature (1450°C), the absence of spontaneous crystallization during molding, and the possibility of controlling the phase composition of the material through the appropriate choice of the crystallization temperature.     

Wettability of aluminosilicate refractories with molten steel

Conclusions An increase in the Al₂O₃ content of the base causes the wetting angle of steel 10kp to increase with an increase in the temperature from 1490 to 1620°C.Chromium-nickel steel wets refractories to a greater degree than rimmed steel. In this case the wetting angle is largest on mullite-corundum refractories containing 75–78% Al₂O₃.The introduction of small amounts of modifying additives in the form of MgO and Cr₂O₃+ ZrSiO₄ into mullite-corundum and corundum refractories causes the wetting angle to increase by 7–10° and helps to increase the resistance of the refractory materials to the action of rimmed steel.Translated from Ogneupory, No. 4, pp. 53–56, April, 1978.     

Thermomechanical properties of ceramics in the systems Al2O3-TiO2 and Al2O3-TiO2-mullite

Conclusions We studied certain properties of ceramics in the systems Al₂O₃-TiO₂ and Al₂O₃-TiO₂-mullite, obtained by the use of the double-stage synthesis of aluminum titanate.We established the nature of the change in the high-temperature strength in relation to the ratio of Al₂O₃ and aluminum titanate. The maximum high-temperature strength (bending) at 1200°C is possessed by ceramic with a corundum matrix and a volume proportion of aluminum titanate equal to 40–45%.It is established that the addition of CaO + SiO₂ made in amounts of up to 1.0–1.5% contributes to the partial breakdown of the aluminum titanate in the compositions Al₂O₃-TiO₂ and the production of a ceramic with a bending strength of 160–190 N/mm² at 20–200°C, thermal-shock resistance 650–800°C, and thermal conductivity of 1.9–2.1 W/(m·K).We studied the effect of the mullite concentration on the properties of the ceramic in the system Al₂O₃-TiO₂-mullite. The introduction of mullite in amounts of not more than 50%, containing up to 3% of impurities, contributes to an increase in the ceramic's strength in the range 20–1300°C and in the thermal shock resistance.Translated from Ogneupory No. 2, pp. 22–26, February, 1988.     

Origin of rate bistability in Mn–O/Al2O3 catalysts for carbon monoxide oxidation: role of the Jahn–Teller effect

Peculiarities in catalytic activity in carbon monoxide oxidation as well as some structure, electronic and magnetic properties of the three oxide catalysts, Mn³⁺–O/Al₂O₃ (1), Mn³⁺–O–Fe/Al₂O₃ (Mn-substituted spinel, 2) and -Fe₂O₃/Al₂O₃ (3), were studied by kinetic measurements and by Mössbauer spectroscopy. The catalysts 1 and 2 showed a kinetic bistability with a sharp transition towards more reactive state at 200°C (ignition point). In contrast, for catalyst 3, at 200–250°C, the behavior of reaction rate against temperature did not display noticeable hysteresis. On cooling the catalysts 1 and 2, extinction was observed at about 170 and 120°C, respectively, i.e., at 30–80°C lower than the corresponding ignition points. Proximity of activation energy for the high and low activity (15–19 kJ/mol) for both Mn-containing catalysts suggests an increase in the number of active sites at high temperature with no changes in the reaction mechanism. The considerable difference between Mn-containing catalysts 1, 2 and Fe-containing catalyst 3 may be caused by Jahn–Teller (JT) type distortions of the oxygen polyhedron around Mn³⁺. A significant spontaneous axial bond stretching within the local polyhedron seems to diminish Mn–O binding energy, facilitate the participation of surface oxygen species, O_S, in the oxidation of CO by a redox mechanism and promote oxygen vacancies at the surface that would cause considerable effect on the activity. An increase in the width of the counterclockwise hysteresis loop for the catalyst 2 compared to the catalyst 1 indicates that clusters of mixed spinel provide more active sites and more labile O_S species than clusters of the binary Mn oxide.     

Effect of nickel nitrate on phase changes in mullite and mullite — Corundum refractories when heated in oxidizing and reducing conditions

Conclusions During heating in oxidizing atmospheres of mullite-corundum specimens with an addition of nickel nitrate at 500–900°C, NiO is formed, and an increase in temperature from 900 to 1500°C is characterized by the development of nickel spinel, NiAl₂O₄, the quantity of which increases with an increase in the content of the Ni(NO₃)₂ · 6H₂O, and the firing temperature.The high temperature diffractometric method showed that in the system 3Al₂O₃ · 2SiO₂-NiO heating in oxidizing conditions, and commencing at 1000°C, NiO decomposes the mullite with the formation of aluminonickel spinel, whose quantity rapidly increases with further temperature rise. At 1500°C the mullite is preserved in small quantities.The nickel spinel, formed as a result of heating to 1300°C the mullite-corundum refractory containing 25% Ni(NO₃)₂ · 6H₂O during firing in the silicon carbide mixture and in the hydrogen atmosphere at 1400°C for 30 min, in the main decomposes with the separation of metallic nickel and corundum. However, x-ray patterns confirm that in these conditions the nickel spinel is not completely decomposed.Translated from Ogneupory, No.5, pp.32–36, May, 1972.     

Mechanical properties and recrystallization of alumina ceramics reinforced by silicon carbide whisker

The mechanical properties and recrystallization of a material consisting of Al₂O₃ and SiC whisker are considered for different hot pressing regimes. The best properties are obtained at an SiC whisker content of 20% in hot pressing in an argon medium at 1600°C (the ultimate bending strength is 600 MPa, the critical coefficient of stress intensityK _Ic=5.5 MPa · m^1/2). The bending strength measured at 1000°C is at least 400 MPa for the material pressed at 1700°C in air. It is established that the presence of SiC whisker on grain boundaries of Al₂O₃ noticeably impedes the growth of alumina grains in recrystallization for all chosen regimes of hot pressing (1650 – 1900°C, 15 – 45 min). The dominant grain size is 1 –5 µm. A mathematical model is suggested for evaluating the maximum grain size in uniform grain growth, which isD*=0.571 [1++(1+16.151/f)^1/2]d, whered, f are the diameter and the volume fraction of SiC whisker. The material can be recommended for use under conditions of thermomechanical stress and abrasive wear.Translated from Ogneupory, No. 5, pp. 8 – 12, May, 1995.     

Dynamics of Phase Transitions in the SHS of a 3Cu–Al Powder Mixture in the Regime of Thermal Explosion

The paper reports results of dynamic xray photography of selfpropagating hightemperature synthesis of a 3Cu–Al powder mixture in the regime of thermal explosion. The stages of formation of the final product are considered, beginning with the heating of the starting mixture and interaction of its components and ending with phase transitions during cooling of the product — intermetallides Cu₉Al₄ and Cu₃Al, which are the main components of Cu based tribotechnical materials. It is shown that upon heating in the temperature range of 550 – 590°C, the intensity of diffraction lines of Al drops to the background level. The exothermicreaction of synthesis of the intermetallide is initiated at a temperature of 610 – 630°C. In the period of sharp increase in temperature to 1040°C, the presence of the starting copper and the newly formed hightemperature phase with the Cu₉Al₄ tructure are detected simultaneously in the combustion wave. When cooled to 300$°C, the material contains two cubic phases — Cu₉Al₄ and an solid solution of Al in Cu. Further cooling is accompanied by formation and increase of peaks of the Cu₃Al phase with an orthorhombic lattice. The phase composition of aluminum bronze produced by SHS with combustion in the regime of thermal explosion is not in equilibrium.     

Changes in high alumina brick used in hot air stoves

Conclusions Aluminous brick made by the Semiluks Refractories Plant containing 60% Al₂O₃ sinters intensely, with sharp changes in properties and reductions in refractoriness from 1810 to 1750°C when used at temperatures of 1500–1600°C.The least-change zone and working zone of the refractory consist mainly of mullite and glassy substance. The amount of mullite increases in the direction from the least-change to the working zone.It is necessary to use high alumina refractories with a higher content of Al₂O₃ (70–72%), preferably with increased density, in order to increase the life of the linings in hot air stoves.Translated from Ogneupory, No. 6, pp.29–32, June, 1968.     

Effects of alkali addition on the mechanical properties and durability of concrete

Increasing the concrete alkali content from 0.6% to 1.25% of Na₂O_e of the cement mass by adding NaOH to the mixture water has harmful effects on most mechanical properties (compressive, splitting, direct tensile, and flexure strengths) of concrete made with a water-to-cement (w/c) ratio of 0.41 and limestone aggregates not susceptible to alkali-silica reaction (ASR), however not on the elasticity modulus measured under compression or direct tension. Shrinkage tests at 50% RH and 23 °C started after 7 days at 100% RH and 23 °C show that the low-alkali concrete shrinks more than the high-alkali one, despite similar water losses. Freeze-thaw tests performed on air-entrained concretes show that the two concretes resist well to freezing and thawing while showing similar air-void systems. When examined under the scanning electron microscope (SEM), the hydrates in the two concretes present similar microstructure; however, the high-alkali concrete shows a more reticular and porous microtexture, which could explain the reduction in strength.     

Thermal aging of ceramics based on Al2O3-TiO2, Al2O3-SiO2 compositions

Conclusions Following an investigation of the thermal aging at 800°C in air of ceramics based on Al₂O₃ TiO₂ (Nos. 1–3) and Al₂O₃-TiO₂-SiO₂ (No. 4), it was established that the ceramics based on Al₂O₃-TiO₂-SiO₂ are the most heat-stable at 800°C over prolonged periods.Translated from Ogneupory, No. 1, pp. 21–23, January, 1990.     

Compositions Based on MgAl2O4, Al6Si2O13, AND Al2TiO5

Binary and ternary fused compositions based on alumomagnesian spinel, aluminum titanate, and mullite are studies using various techniques of physicochemical analysis. The minimum incipient melting temperature in the MgAl₂O₄ – Al₆Si₂O₁₃ – Al₂TiO₅ system is [1645 ± 15] °C. Fused materials with tailored phase compositions are obtained under industrial conditions. Technological properties of a fused composite corundum – alumomagnesian spinel – mullite – aluminum titanate are studied.     

Deterioration Mode of Barium-Containing NOx Storage Catalyst

Barium-containing NO _x storage catalyst showed serious deactivation under thermal exposure at high temperatures. To elucidate the thermal deterioration of the NO _x storage catalyst, four types of model catalyst, Pt/Al₂O₃, Ba/Al₂O₃, Pt–Ba/Al₂O₃, and a physical mixture of Pt/Al₂O₃ + Ba/Al₂O₃ were prepared and their physicochemical properties such as BET, NO TPD, TGA/DSC, XRD, and XPS were evaluated while the thermal aging temperature was increased from 550 to 1050°C. The fresh Pt–Ba/Al₂O₃ showed a sorption capacity of 3.35 wt%/g-cat. but the aged one revealed a reduced capacity of 2.28 wt%/g-cat. corresponding to 68% of the fresh one. It was found that this reduced sorption capacity was directly related to the deterioration of the NO _x storage catalyst by thermal aging. The Ba on Ba/Al₂O₃ and Pt–Ba/Al₂O₃ catalysts began to interact with alumina to form Ba–Al solid alloy above 600°C and then transformed into stable BaAl₂O₄ having a spinel structure. However, no phase transition was observed in the Pt/Al₂O₃ catalyst having no barium component, even after aging at 1050°C.