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.     

Study of the service of high-alumina and kaolin refractories in the checkers and structure of blast-furnace stoves

Conclusions The structure of kaolin brick in the high-temperature zone of the stove after 6.7–7.2 years service at subcupola temperatures of 1300°C underwent substantive external and structural changes.In the structure of the stove made from high-alumina brick VGO-62 after six years service we detected substantially less change.In the kaolin brick of the upper rows of the checker three clearly defined zones are formed: working (slag), impregnated with alkalis, the transition zone (porcelain-type), and the least-changed zone. During the service of high-alumina refractories mullite crystallizes, which confirms the results of work carried out previously.Owing to the creep of kaolin and high-alumina refractories 4–5% shrinkage of the brick occurs in the upper rows of the checker, and at a depth of 2.5–6.0 m — 1.5–2.0%. The height of the checker diminishes under these conditions by 0.8–1.0 m.The densification of the structure of the upper rows of the checker corresponds to a reduction in porosity and an increase in the density of the brick.The temperature of initial deformation under load of 2 kg/cm² of the slagged kaolin refractories taken from the upper rows of the checker diminishes by 150–200°C, and in the lower layers of the checker it does not alter. This factor for high-alumina refractories in service increases by 160–190°C. In the high-temperature zone of the stoves it is desirable to test dense, high-alumina refractories containing 72–75% Al₂O₃.Translated from Ogneupory, No. 5, pp.14–19, May, 1972.     

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.     

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.     

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.     

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.     

The long-term high-temperature behavior of refractories produced from ZrO2 stabilized with Nd2O3

Conclusions A technology was developed for the production of zirconia refractories from ZrO₂ stabilized with Nd₂O₃. The thermal strength of the product is adequate for long-term service at a large (200–300°C/mm) temperature gradient. Products based on a zirconia — neodymium solid solution can be used several times.It was established that no appreciable Nd₂O₃ vaporization and, consequently, no appreciable destabilization of the ZrO₂ develops in a neutral medium at 2100–2500°C.The solid-phase processes developing at 2100–2500°C in products from a mixture of 70% cubic solid solution (88 mole % ZrO₂+12 mole % Nd₂O₃) and 30% unstabilized ZrO₂ fired at 1750°C consist of the redistribution of the Nd₂O₃ between the cubic solid solution Nd₂Zr₂O₇-ZrO₂ and the unstabilized ZrO₂, and the diffusion of some of the Nd₂O₃ from the cooler to the working zone.Translated from Ogneupory, No. 3, pp. 52–55, March, 1976.     

Aluminium dissolution in the NaF-AlF3-Al2O3 system: Effects of alumina,temperature, gas bubbling and dissolved metal

Current reversal chronopotentiometry, with and without a delay time between the forward and reverse current pulses, was employed to evaluate the effects of temperature, alumina content, gas bubbling (argon and carbon dioxide) and dissolved metal on the rate of aluminium dissolution in NaF–AlF₃–Al₂O₃ molten bath. The working electrode was a tungsten wire electrode and the temperature range studied was 824–1040°C. The effect of the alumina content was determined in melts with CR=1.45 and CR=4.3 at 1029±3°C (CR = mol NaF/mol AlF₃). The experiments involving gas bubbling and dissolved metal were carried out in melts similar to industrial compositions, i.e. CR=2.4, 4.8 wt. % Al₂O₃ at 980°C. In general, the dissolution rate of aluninium increased with increasing temperature, decreased slightly with increasing alumina content in acidic melts (CR<3) but changed little in basic melts (CR>3), increased with bubbling and decreased in the presence of dissolved metal. The rate of Al dissolution is thus mass transport controlled.     

Thermal transformations during the interaction of chamotte and orthophosphoric acid

Conclusions Up to 300°C chamotte does not react with orthophosphoric acid. In the range 300–800°C crystallization of the type SiO₂·P₂O₅ occurs. With further increase in temperature the silicophosphate dissolves, and at 1030°C is converted from the low- to the high-temperature form, and at 1200°C it changes completely into the melt.Starting from 500°C the mullite decomposes. The intensity of the mullite line at 1400°C is slight.In the range 700–1000°C a large quantity of liquid phase, formed during decomposition of the silicophosphates and mullite, sharply depresses the refractoriness-under-load and increases their shrinkage.At 1200°C AlPO₄ crystallizes in the cristobalite form. An increase in the quantity of AlPO₄ increases the refractoriness-under-load of the specimens.Mixtures of chamotte and orthophosphoric acid after firing at 1400°C contain (in reducing order) AlPO₄ (cristobalite form), cristobalite, mullite, and quartz; they may be recommended as low sintering mortars.Translated from Ogneupory, No.2, pp. 39–43, February, 1970.     

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.     

More efficient utilization of alumina in refractories production

Conclusions An investigation was carried out of ways of utilizing commercial alumina more efficiently in refractories production by improving the composition of the products and the technology of their manufacture. Commercial alumina should be used mainly for the production of mullite refractories from synthetic mullite and of corundum refractories containing 90–99% Al₂O₃. Commercial alumina must be used for the production of spinel and spinel-containing refractories.More extensive use should be made of the alumina-containing waste from chemical plants for the production of refractories with a low mullite content, of ramming compounds, fusioncast refractories, and electrofused spinel as a substitute for chromite in roof bricks. To produce refractories with a higher Al₂O₃ content than can be achieved with kaolin more extensive use should be made of natural alumina-containing starting materials.Some mullite and corundum refractories should be produced with low porosity by using high molding pressures, actively sintering starting materials, and hydrostatic molding.Translated from Ogneupory, No. 8, pp. 33–39, August, 1978.     

Sintering and some properties of compositions in the Al2O3-Al2TiO5 system with additions of amorphous silica

Conclusions We studied the effect of addtions of amorphous silica in amounts of 6–11% on the properties of compositions in the Al₂O₃-Al₂TiO₅ system.The presence of SiO₂ enables us to obtain, at reduced firing temperatures (1580°C), adequately dense (P_open=3–4%) compositions while preserving the high thermal-shock resistance and compressive strength at 1050°C (360–420 MPa).Translated from Ogneupory, No. 8, pp. 19–20, August, 1986.     

Investigation of mullite obtained by fusion in Solar furnaces

Conclusions Melts in the Al₂O₃-SiO₂ system that are fused and cooled at different rates form a wide region of solid solutions on the basis of mullite and glass phase. The concentration of alumina in the solid solution of the specimens chilled at a rate of 10² and 10³ K/sec changes from 71.8 to 80%.An increase in the chilling rate to 10⁵ K/sec contributes to the formation of metastable, saturated solid solutions with a maximum content of up to 87% Al₂O₃ and also completely amorphous states with a content of 71.8–76% Al₂O₃.The amount of glass phase in the stoichiometric compositions increases from 20–25 to 100% with a chilling rate of 10²-10³ and 10⁵ K/sec respectively.It was established experimentally that the use of microadditive in the form of TiO₂ can affect the nature of the crystallization of the melt and yield a monophase crystalline mullite from the batch of any composition.Translated from Ogneupory, No. 6, pp. 22–26, June, 1989.     

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.     

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.     

Sintering and some properties of mullite-corundum ceramic obtained from pure oxides

Conclusions The effect of the total concentration of SiO₂ and the method of preparation of the batch on the sintering and some of the properties of a mullite-corundum ceramic made from pure oxides has been studied.It is established that the sintering of the ceramic made from batches of various compositions containing mixtures of SiO₂ and Al₂O₃ is significantly more difficult than when analogous batches containing previously sintered mullite are used. With an increase in the concentration of Al₂O₃ in the batch with the previously synthesized mullite, the apparent density and strength increase while the open porosity decreases.The sintering of the mullite-corundum ceramic of the composition 10% SiO₂+90% Al₂O₃ containing previously sintered mullite differs from corundum and mullite by its significantly higher thermal-shock resistance.Translated from Ogneupory, No. 6, pp. 51–54, June, 1980.     

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.     

Calculation of the liquid phase in the Al2O3-B2O3-SiO2 system for the composition of commercial quartzite powders

Conclusions A calculation of the percent liquid phase from the phase diagram Al₂O₃-B₂O₃-SiO₂ showed that quartzite powders containing 97.5% or less SiO₂ are unsuitable as lining for crucible, especially at the reheating temperature of cast iron (1550°C), owing to the possible melting of about 30% of the working zone of the lining, the migration of the melt into the colder zones, and the consequent increase in the rate of sintering.The erosion of a crucible in service depends on the percent and viscosity of the liquid phase being formed. The viscosity of the boroaluminosilicate melts forming in the Al₂O₃-B₂O₃-Si₂ system at 1550°C is high so that their activity relative to the substance in the solid state is insignificant and the sintering effect is inhibited.However, owing to the nonequilibrium course of the process mainly low-temperature liquid phases are formed and then overheated. The B₂O₃ content of these phases is high (up to 18.3%) so that their fluidity is high; Since the erosion rate of the lining depends on the action of these melts the quartzite powders used for the lining of induction furnaces should contain not more than 1.1% Al₂O₃ and not more than 1% B₂O₃.Translated from Ogneupory, No. 6, pp. 68–70, June, 1978.     

Preparation of self-reinforcement of porous mullite ceramics through in situ synthesis of mullite whisker in flyash body

Self-reinforced porous mullite ceramics were fabricated by a starch consolidation method with flyash, different aluminium sources (Al(OH)₃ and Al₂O₃) and the additive AlF₃ as raw materials. The reinforcement mechanism of needle-like mullite whiskers through in situ synthesis in ceramic body was investigated. The bulk density, apparent porosity and bending strength of the samples were tested. Phase compositions and microstructures of the sintered samples were measured by XRD and SEM, respectively. It showed that AlF₃ as additive was helpful to the formation of mullite whiskers at a low temperature. As the aluminium sources, Al(OH)₃ was more suitable for the preparation of mullite whiskers than Al₂O₃. The in situ synthesized mullite whiskers formed an interlocking structure, which enhanced the mechanical strength of the porous mullite ceramics. Porous mullite ceramics with bending strength of about 100 MPa and apparent porosity of about 55% were made at 1550 °C.     

Triangulation and Specific Features of the Phase Formation in Strontium Aluminosilicate Glass-Forming Systems

The SrAl₂Si₂O₈–Sr₂Al₂SiO₇–SrSiO₃ system obtained by the triangulation of the SrO–Al₂O₃–SiO₂ ternary system is studied. The ternary eutectic is determined at a temperature of 1310°C. In this region of the system, homogeneous glasses that correspond to the eutectic composition and garnet stoichiometry are synthesized at a temperature of 1350°C. It is established that the main phases formed upon crystallization of the glasses are strontium anorthite, strontium helenite, and strontium metasilicate.