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.     

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.     

Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al₂O₃–SiO₂ system. The amorphous precursors have been obtained by hydrolysing TEOS directly in a diluted aqueous solution of aluminium nitrate, spray drying the clear solution and heating the resulting powder. Up to 70 mol % Al₂O₃, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al₂O₃ mullite and spinel crystallise together; and for more than 80 mol % Al₂O₃ only spinel is formed. In the 70–80 mol % Al₂O₃ range of composition, when both mullite and spinel crystallise, low heating favours the crystallisation of mullite and it is nearly possible to crystallise only mullite from a 75 mol % Al₂O₃ sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al₂O₃ precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al₂O₃. Heating the samples up to 1700 °C for 1 h, the phase equilibrium is not reached, particularly when both mullite and spinel crystallise together, and θ-Al₂O₃ is still present.     

Mullite and mullitelike compounds in the Al2O3-SiO2 system

Conclusions The congruence or incongruence of mullite's melting is not finally explained, and requires further study; apparently, it depends on the conditions in which the experiments are carried out, and also on the presence of stabilizing impurities in the lattice of the mullite and the partial pressure of the oxygen.The phase diagram for A1₂O₃-SiO₂ system contains only one compound — mullite, of the composition 3Al₂O₃·2SiO₂, with a completely ordered lattice. Solid solutions having a mullitelike structure can be obtained in the compositional range from Al₂O₃·SiO₂ to 2Al₂O₃· SiO₂ with the use of special additives.Translated from Ogneupory, No. 1, pp. 23–27, January, 1984.     

Phase diagram of the system Al2O3-SiO2

Phase diagrams of the system Al₂O₃ - SiO₂ as presented in the domestic and foreign literature are analyzed. The congruent and incongruent states of mullite, liquation in the solid phase, and the structures of mullite and sillimanite are discussed.Translated from Ogneupory, No. 8, pp. 11 – 14, August, 1995.A lecture delivered to metallurgists and silicate producers at the Ural State Engineering University.     

The physicochemical processes in the reactions of the components of system Mg(Al1−XCrx)2O4-SiO2-iron oxide

Conclusions The phase transition in the firing of compositions based on the system Mg(Al_1–XCr_X)₂O₄-(Fe₂O₃, SiO₂) consist essentially of the formation of solid solutions of spinels and the ferrous component of the reagent, and the solution of the spinel in the iron-silica melt.In compositions with magnesia-alumina spinel the solution of the spinel in the melt is the dominant process in every case while in compositions with a magnesite-chromite spinel the dominance of a given process depends on the composition of the reagent.The solubility of spinels in an iron-silica melt depends on the composition of the spinel and the percent iron oxides in the primary melt while the total percent melt depends mainly on the percent silica in the original mixture. The solubility of high-alumina spinels in melts of Fe₂O₃ and SiO₂ is greater than that of high-chromite spinels in every case.Deceased.Translated from Ogneupory, No. 7, pp. 45–49, July, 1974.     

Cast refractories of the Al2O3-Cr2O3-CaO system

Conclusions In order to develop stable materials for glassmaking, we studied the physical and technological properties of the refractories belonging to the Al₂O₃-Cr₂O₃-CaO system that contain 5–20% CaO, 15–35% Cr₂O₃ and 45–80% Al₂O₃.The glass resistance of the refractories of the experimental systems (compositions) exceeds that of the BKCh-33 baddeleyite-corundum products by 3–5 times and their thermal shock resistance is superior to that of the well known chromium-containing refractories at comparable levels of mechanical properties.The developed refractories are recommended for the top or the bottom structures of the glassmaking furnaces depending on their glass resistance and thermal shock resistance and for making the refractory components of ferrous metallurgical units that are in contact with highly basic slags.Translated from Ogneupory, No. 3, pp. 23–26, March, 1989.     

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.     

Integrated experimental phase equilibria study and thermodynamic modelling of the binary ZnO–Al2O3, ZnO–SiO2, Al2O3–SiO2 and ternary ZnO–Al2O3–SiO2 systems

Phase equilibria of the ZnO–SiO₂, Al₂O₃–SiO₂ and ZnO–Al₂O₃–SiO₂ systems at liquidus were characterized at 1340–1740 °C in air. The ZnO–Al₂O₃ subsolidus phase equilibria were derived from the experiments with the SiO₂- and CaO + SiO₂-containing slags. High-temperature equilibration on silica or platinum substrates, followed by quenching and direct measurement of Zn, Al, Si and Ca concentrations in the phases with the electron probe X-ray microanalysis (EPMA) was used to accurately characterize the system. Special attention was given to zincite phase that was shown to consist of two separate ranges of compositions: round-shaped low-Al zincite (<2 mol.% AlO_1.5) and platy high-Al zincite (4–11 mol.% AlO_1.5). A technique was developed for more accurate measurement of the ZnO solubility in the low-ZnO phases (corundum, mullite, tridymite and cristobalite) surrounded by the ZnO-containing slag, using l-line for Zn instead of K-line, avoiding the interference of secondary X-ray fluorescence. Solubility of ZnO was found to be below 0.03 mol.% in corundum and cristobalite, and below 0.3 mol.% in mullite. Present experimental data were used to obtain a self-consistent set of parameters of the thermodynamic models for all phases in this system using FactSage computer package. The modified quasichemical model with two sublattices (Zn²⁺, Al³⁺, Si⁴⁺) (O^2?) was used for the liquid slag phase; the compound energy formalism was used for the spinel (Zn²⁺,Al³⁺)[Zn²⁺,Al³⁺,Va]₂O^2-₄ and mullite Al³⁺₂(Al³⁺,Si⁴⁺) (O^2?,Va)₅ phases; the Bragg-Williams formalism was used for the zincite (ZnO, Al₂O₃); other solid phases (tridymite and cristobalite SiO₂, corundum Al₂O₃, and willemite Zn₂SiO₄) were described as stoichiometric. Present study is a part of the research program on the characterization of the multicomponent Pb–Zn–Cu–Fe–Ca–Si–O–S–Al–Mg–Cr–As–Sn–Sb–Bi–Ag–Au–Ni system.     

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.     

Visible Light Transparency for Polycrystalline Ceramics of MgO·2Al2O3 and MgO·2.5Al2O3 Spinel Solid Solutions

Magnesium aluminate spinel solid solutions with the alumina‐rich compositions MgO·2Al₂O₃ and MgO·2.5Al₂O₃ have been prepared as polycrystalline ceramics with average in‐line transmissions at 550 nm of 85.5 ± 0.3% and 80.9 ± 0.4%, respectively. Starting powders are prepared from combinations of high purity Mg(OH)₂ and γ‐Al₂O₃ thoroughly mixed in an aqueous slurry, and the solids are collected, dried, calcined, mixed with LiF sintering aid, and sieved. The optimum amount of LiF added varies with the alumina composition of the spinel solid solution. The powders are sintered into dense ceramics by hot pressing at 1600°C under vacuum and 20 MPa uniaxial load followed by hot isostatic pressing at 1850°C under 200 MPa in Ar. Both compositions exhibit exaggerated grain growth with average sizes well over 500 μm. Knoop hardness measurements are 11.2 ± 0.3 GPa for MgO·2Al₂O₃ and 11.0 ± 0.4 GPa for MgO·2.5Al₂O₃.     

Subsolidus Structure of the System Al2O3 – SiO2 – MgO – P2O5 (A Review)

The structure of the Al₂O₃ – SiO₂ – MgO – P₂O₅ system in the subsolidus range is analyzed. Splitting of the system into elementary tetrahedra is shown, their volumes and degrees of asymmetry are calculated, a topological graph of relationships between the tetrahedra is constructed, and a geometrical-topological characteristic of phases in the considered system is given. Eutectic temperatures, as well as compositions and conode lengths, are calculated for some pseudobinary and ternary sections. Ranges of the system compositions suitable for fire-resistant ceramic technology and for heat-insulating materials are specified.     

Effect of SiO2, Fe2O3, and TiO2 on stabilization of cubic ZrO2 in the presence of Al2O3

Several compositions are investigated in order to determine the essence of the phase transformations occurring at temperatures up to 1600°C in the ZrO₂ — Ln₂O₃ (Ln is Nd, Y, Yb) — Al₂O₃ — SiO₂ (Fe₂O₃, TiO₂) systems. The efficiency of using Y₂O₃ and Yb₂O₃ to stabilize cubic ZrO₂ in the presence of a mixture of Al₂O₃ and SiO₂ (Fe₂O₃, TiO₂) is shown. Data show the possibility of fabricating high-quality zirconium-corundum articles with any proportion of Al₂O₃ and ZrO₂.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 17 – 20, June 1996.     

Molten cast materials of the Al2O3-MgO system

Molten cast alumomagnesia refractory materials containing up to 28.3% MgO (MgAl₂O₄) are investigated. It is shown that materials with 10–15% MgO have a complex structure and phase composition. The latter can be interpreted as a solid solution of corundum and spinel or as a combination of MgO · 2.5Al₂O₃. The behavior of these materials in molten TK16 and K8 glass is investigated.     

Research on high-temperature creep properties of Al2O3-MgAl2O4 refractory

In this study, the Al₂O₃-MgAl₂O₄ refractory samples were prepared with tabular alumina and fused magnesia-alumina spinel as the raw materials. A creep resistance test was performed at 1400°C and studied. The tested samples after the creep resistance test were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy to investigate the effect of adding the amount and particle size of MgAl₂O₄ on high-temperature creep resistance of Al₂O₃-MgAl₂O₄ refractory, as well as its mechanism. As indicated by the results, under the same amount of spinel added, the sample with 3-1 mm spinel had higher creep resistance than the sample with 1-0 mm spinel. In the creep test, spinel aggregates would react with the alumina matrix to form a secondary spinel layer around the spinel particles, thereby connecting the matrix and the aggregates and improving the creep resistance of the specimens. As clearly indicated by the observed result of the microstructure, in the formation of the secondary spinel layer, Mg²⁺ had a higher migration rate and showed the secondary spinel layer's higher thickness ratio on both sides of the reaction interface, which would induce the Kirkendall effect. Accordingly, considerable pores generated and accumulating at the interface tended to reduce the creep resistance of the material, and this effect was highly dependent on the size of the spinel and the activation solid solution reaction intensity.     

Rearrangement of conodes of the phase diagram of the MgO-Al2O3-SiO2 system and its technological prospects

The subsolidus structure of the phase diagram of the MgO - Al₂O₃ -SiO₂ system is studied and the boundaries of the domains of existence and the probability of the existence of all stable phases in the entire temperature region are determined. The temperature of the pseudobinary spinel — mullite eutectic is refined. The results are taken into consideration in constructing the corresponding nonisothermal section of the phase diagram, which opens new technological possibilities due to the rearrangement of the conodes of the MgO -Al₂O₃ - SiO₂ system.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 4 – 8, November, 1996.     

Fusion diagram for the pseudosystem CaO·Al2O3-MgO·Al2O3-MgO

Conclusions The use of statistical methods will considerably reduce the number of experiments needed to construct fusion diagrams, and yield good results.We constructed a calculation fusion diagram for the pseudosystem CaO·Al₂O₃-MgO·Al₂O₃-MgO in which we determined the region of optimum compositions for the cements of the highest refractoriness, which is interesting in relation to producing highly refractory cements with prescribed properties.Translated from Ogneupory, No. 4, pp. 27–28, April, 1989.     

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.     

Phase Relationships in the NaPO3–Al2O3 Glass-Forming System

The phase formation is studied in the NaPO₃–Al₂O₃ system (0–25 mol % Al₂O₃). It is shown that two individual compounds, namely, Na₆Al₂(P₂O₇)₃ and Na₃Al₂(PO₄)₃, exist along the studied join. The phase diagram of the NaPO₃–Al₂O₃ system (0–25 mol % Al₂O₃) is constructed. The phase separation phenomena are revealed in the melt.     

Nanoindentation of Al2O3/Al2TiO5 composites: Small-scale mechanical properties of Al2TiO5 as reinforcement phase

The mechanical properties of alumina/aluminum titanate composites (Al₂O₃/Al₂TiO₅) were evaluated and analyzed by nanoindentation. Indentations with different penetration depths were performed, and residual imprints on specimens were located and observed by combining complementary characterization techniques. The mechanical response of composites was found to be determined by grain size of alumina and aluminum titanate, as evaluated from indentations performed at 1500 nm of penetration depth. On the other hand, small indents in individual grains permitted to assess the hardness as well as the elastic modulus of non-cracked particles of Al₂TiO₅ through implementation of different analytical indentation models. The attained values for the local mechanical properties were validated through critical comparison of them with those predicted by the rule of mixtures. Results showed no evidence of microcracking on grains of the reinforcing phase for all the tested composites, before and after low penetration depth indentations. Elastic modulus of Al₂TiO₅ was found to be higher than the values reported on bulk aluminum titanate, presumably because of the absence of microcracking for small grain sizes. The bulk composite mechanical response is finally discussed on the basis of contributions from those of the individual phases.