Revised Phase Diagram for the System Al2O3—SiO2

On the basis of 190 runs made up to 1860°C in sealed noble-metal containers the following revisions have been made in the equilibrium diagram for the system A1₂O₃–SiO₂. Mullite melts congruently at 1850°C. The extent of equilibrium solid solution in mullite at solidus temperature is from approximately 60 mole % Al₂O₃ (3/2 ratio) to 63 mole % A1₂O₃. Metastable solid solutions can be prepared up to about 67 mole % Al₂O₃. There is no evidence for stable solubility of excess SiO₂ beyond the 3/2 composition at pressures below 3 kbars. Refractive indices are presented for glasses containing up to 60 mole % Al₂O₃ and from them the composition of the eutectic is confirmed at 5 mole % SiO₂. The variation in lattice constants of the mullite solid solution is not an unequivocal guide to composition since mullites at one composition produced at different temperatures show differences in spacing, no doubt reflecting Al-Si ordering phenomena. The possibility of quartz and corundum being the stable assemblage at some low temperatures and pressures cannot be ruled out. A new anhydrous phase in the system is described, which was previously thought to be synthetic andalusite; it is probably a new polymorph of the Al₂SiO₅ composition with ortho-rhombic unit-cell dimensions a =7.55 A, b =8.27 A, and c = 5.66 A.     

The System AI2SiO5 at High Temperatures and Pressures

Experiments on the system Al₂SiO₅ at high temperatures and pressures with the belt apparatus indicate that kyanite melts incongruently above about 1500°C at 25,000 bars to Al₂O₃ (corundum) plus liquid. The pressure-temperature curves obtained by starting with either a 1/1 Al₂O₃/SiO₂ gel or with kyanite are essentially identical but differ considerably from the results with andalu-site and sillimanite. The structure of the starting material has considerable influence on the kinetics of the reaction and the metastable formation of corundum in this system. An "equilibrium" curve based on the andalusite-sillimanite data is described by P = 33.8 × 10-³T - 26.4 (Pin kbars, Tin°C).     

Reinvestigation of Al–Si Spinel Phase in Diphasic Al2O3–SiO2 Gel

Mechanical mixture of γ-Al₂O₃ and amorphous SiO₂, and diphasic Al₂O₃/SiO₂ gels of three different compositions were synthesized. They were subjected to heat treatment to various temperatures in the range 900°–1600°C. Qualitative X-ray diffraction data show that these diphasic gels do not crystallize to a combined mixture of θ-Al₂O₃ and α-Al₂O₃ polymorphs at the intermediate stage, prior to mullite formation. Estimated mullite formation data show that the course of its formation from mixed oxides was different from that of diphasic gels. Results are compared with previous findings and the concept of Al–Si spinel formation in the phase transformation of stoichiometric diphasic gel system is substantiated.     

The System KAlSiO4–Mg2SiO4–KAlSi2O6

Schairer's study (1954) on phase relations in the system KalSi₂O₆–Mg₂SiO₄–SiO₂ was extended to include the system KalSiO₄–Mg₂SiO₄–KalSi₂O₆. It is shown that this join is ternary; however, the relatively high vapor pressure of the condensed phases prohibits study by the usual quenching techniques. The apparent intersection of the (KalSiO₄–Mg₂SiO₄–SiO₃) join with the primary phase volume of spinel is attributed to loss of the alkali-silicate constituents by vapor transport. This results in the effective bulk composition being moved away from this join toward the primary phase volume of spinel in the system K₂O–MgO–Al₂O₃–SiO₂.     

Structural Analysis of SiO2-Al2O3 Glasses

The structure of SiO₂-Al₂O₃ glasses with up to 60 wt% Al₂O₃ was investigated using the radial distribution function together with the correlation method based on X-ray scattering intensity data. Radial distribution curves are interpreted on the basis of glass-in-glass separation with the constituents of SiO₂-rich and Al₂O₃-rich glasses. The structure of the Al₂O₃-rich glass has a short-range ordering similar to the crystal structure of mullite. The calculated S- i (S) curve of this model gives good agreement with the observed one.     

The System Al2O3-Cr2O3-Sio2

Liquidus phase equilibrium data are presented for the system Al₂O₃-Cr₂O₃-SiO₂. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al₂O₃-ICr₂O₃-93SiO₂) coexists with a mullite solid solution (61Al₂O₃-10Cr₂O₃-29SiO₂), a corundum solid solution (19Al₂O₃-81Cr₂O₃), and cristobalite (SO₂). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.     

Application of Compositionally Diphasic Xerogels for Enhanced Densification: The System Al2O3-SiO2

Stoichiometric mullite (71.38 wt% Al₂O₃-28.17 wt% SiO₂) and 80 wt% Al₂O₃-20 wt% SiO₂ gels were prepared by the single-phase and/or diphasic routes. Dense sintered bodies were prepared from both sets of gels in the Al₂O₃-SiO₂ system. Apparent densities of 96% and 97% of theoretical density were measured for the diphasic (using two sols) mullite samples sintered at 1200° and 1300°C for 100 min, respectively; this compared with 85% and 94% for the single-phase xerogels under the same conditions, and to much lower values for mullite prepared from conventional mixed powders. The microstructure of the mullite pellets from diphasic xerogel precursors is also considerably finer.     

Change in Chemical Composition of Mullite Formed from 2SiO2°3Al2O3 Xerogel During the Formation Process

The chemical composition of mullite which was termed from 2SiO₂3Al₂O₃ xerogel by firing was examined by analytical TEM. Mullite formed at 950°C firing showed around 66 mol% Al₂O₃, which was fairly Al₂O₃ rich compared with the bulk composition. The chemical composition of mullite gradually approached the bulk composition as the firing temperature increased to 1400°C and slightly departed again above that firing temperature.     

Phase Equilibrium Relationships in a Portion of the System MgO–Al2O3–2CaO·SiO2

The system MgO–Al₂O₃–2CaO·SiO₂ comprises a plane through the tetrahedron CaO–MgO–Al₂O₃–SiO₂. A total of 108 compositions were prepared having an alumina content below the line joining 2CaO·Al₂O₃SiO₂ (gehlenite) and MgO·Al₂O₃ (spinel). Quenching experiments were carried out on 96 of these compositions at temperatures up to 1590°C. Three binary eutectic systems and two ternary eutectic systems are described. Compositions on this plane are of significance in an investigation of the constitution of basic refractory clinkers made from Canadian dolomitic magnesites. They also concern the compositions of certain blast furnace slags.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Cell Dimensions, Solid Solution, Polymorphism, and Identification of Mullite and Sillimanite

X-ray, optical, and chemical data are presented for sillimanites and for a wide range of natural and synthetic mullites. Single-crystal X-ray studies have revealed a new type of mullite (called S mullite) characterized by subsidiary sharp reflections in distinction to the extra diffuse reflections of common mullite (called D mullite). Atomic substitution of aluminum in D mullite leads to expansion of a and c but not of b, whereas substitution of iron and titanium in natural mullites gives a slight initial decrease in a and b followed by a subsequent increase; c increases uniformly for all concentrations. The data are consistent with solid solution ranging between the extremes 3Al₂O₃.2SiO₂, 2Al₂O₃.-SiO₂, and 3(Al_0.9Fe_0.1)₂O₃.2SiO₂. Cell dimensions of sillimanite vary only slightly. Consideration of the various methods of distinguishing mullite and sillimanite shows that the measurement of cell dimensions is the most reliable and for mullite has the added advantage that the S and D polymorphs may be distinguished and an estimate of the composition obtained for D mullite.     

Effect of Sol-Gel Mixing on Mullite Microstructure and Phase Equilibria in the α-Al2O3-SiO2 System

Starting powders containing 72 wt% Al₂O³ and 28 wt% SiO₂, were prepared by sol-gel methods classified as colloidal and polymeric. Compacts fired at 1700°C showed significant differences in microstructure. The specimens formed with the colloidal powder had mullite grains of prismatic shape and a liquid phase; with polymeric powder, mullite grains were granular with no liquid phase present. It is shown that the mullite grains in the first case are higher in AI₂O₃ content, resulting in an excess of SiO₂ which is the base for the liquid phase. In the second case, the mullite grains have the same Al₂O₃ content as the starting powders. The presence of a liquid phase in the first case is considered to be metastable, resulting from the nature of the starting materials and processing conditions employed.     

Rapid Crystallization of SiO2-Al2O3 Glasses

The crystallization of Al₂O₃-rich glasses in the system SiO₂-Al₂O₃ which were prepared by flame-spraying and/or splat-cooling was studied by DTA, electron microscopy, and X-ray diffraction. Over a wide range of compositions, the crystallization temperature ( T_x ) remained near 1000°C, changing smoothly with composition. In all cases crystallization of mullite was detected by X-ray diffraction. In the low-Al₂O₃ region, coarsening of the microstructure during crystallization was observed by electron microscopy. In the high-Al₂O₃ region mullite and γ-Al₂O₃ cocrystallized; this behavior may be interpreted as evidence of a cooperative process of crystallization at the respective T_x 's. The crystallite size of the mullite immediately after rapid crystallization increased continuously with increasing Al₂O₃ content. In light of the T_x data, the adequacy of the evidence for the proposed metastable miscibility gap in the SiO₂-Al₂O₃ system is questioned.     

Dissolution Rate of Silicon Nitride Ceramics into Molten CaO–Al2O3–SiO2 Slags

The dissolution rates of silicon nitride (Si₃N₄) ceramics into CaOAl₂O₃SiO₂ slags were investigated by using a rotating specimen method in the temperature range of 1773–1873 K. Dissolution rates in the present study increased as the revolution speed and temperature increased and decreased as the SiO₂ content of the slags increased. The nitrogen content of the slags increased after the Si₃N₄ ceramics had been immersed into them. The slags contained two types of nitrogen ions—N³⁻ and CN^-—because a graphite crucible was used for the experiment. N³⁻ ions were confirmed in all the slags that were used in the present work; the CN^- content was much lower than that of the N³⁻ ions, except in the slag without SiO₂. Based on those results, Ficks law of diffusion was used to analyze the dissolution rates. The dissolution mechanism of the Si₃N₄ ceramics into CaO–Al₂O₃SiO₂ slags has been discussed in this paper.     

Phase Diagram for the SiF4 Join in the System SiO2-Al2O3-SiF4

Alumina reacts with 1 atm of SiF₄ below 660°± 7°C to form A1F₃ and SiO₂. At higher temperatures the product is a mixture of fluorotopaz and AIF₃. Mixtures of fluorotopaz and AIF3 decompose in 1 atm of SiF₄ at 973°± 8°C and form tabular α-alumina. The equilibrium vapor pressure of SiF₄ above mixtures of fluorotopaz and AlF₃ is log p (atm) = 9.198 – 11460/ T (K). Fluorotopaz itself decomposes at 1056°± 5°C in 1 atm of SiF₄ to give acicular mullite, 2Al₂O₃^.1.07SiO₂. Alumina and mullite are stable in the presence of 1 atm of SiF₄ above 973° and 1056°C, respectively. The phase diagram of the system SiO₂-Al₂O₃-SiF₄ shows only gas-solid equilibria.     

Novel Low-Temperature Processing Route of Dense Mullite Ceramics by Reaction Sintering of Amorphous SiO2-Coated γ-Al2O3 Particle Nanocomposites

Dense mullite ceramics were successfully produced at temperatures below 1300°C from amorphous SiO₂-coated gamma-Al₂O₃ particle nanocomposites (AS-gammaA). This method reduces processing temperatures by similar/congruent300°C or more with respect to amorphous SiO₂-coated alpha-Al₂O₃ particle microcomposites (AS-alphaA) and to other Al₂O₃-SiO₂ reaction couples. The good densification behavior and the relatively low mullite formation temperature make AS-gammaA nanocomposites an excellent matrix raw material for polycrystalline aluminosilicate fiber-reinforced mullite composites.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.     

Phase Equilibria in High-Alumina Part of the System CaO–MgO–Al2O3–SiO2

Results are presented of a study of phase equilibria among crystalline and liquid phases in the quaternary system CaO–MgO-Al₂O₃–SiO₂ at Al₂O₃ contents greater than 35%. Equilibrium diagrams shown are for the five triangular joins CaAl₂Si₂O₃-Ca₂Al₂SiO₇-MgAl₂O₄, Ca₂Al₂SiO₇-MgAl₂O₄-Al₂O₃, CaAl₂Si₂O₈-MgO-Al₂O₃, CaAl₂Si₂O₈-Mg₂SiO₄-MgAl₂O₄, and CaAl₂Si₂O₈-MgO-Mg₂SiO₄. The composition and nature of the four quaternary peritectic points and the relationships of univariant lines and primary phase volumes are discussed.     

Quaternary System Al2O3–CaO–MgO–SiO2: I, Study of the Crystallization Volume of Al2O3

Compatibility relations of Al₂O₃ in the quaternary system Al₂O₃–CaO–MgO–SiO₂ were studied by firing and quenching followed by microstructural and energy-dispersive X-ray examination. A projection of the liquidus surface of the primary phase volume of Al₂O₃ was constructed in terms of the CaO, SiO₂, and MgO contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, were defined, and the positions of the isotherms were tentatively established. The effect of SiO₂, MgO, and CaO impurities on Al₂O₃ growth also was studied.     

Mullite Crystallization from SiO2-Al2O3 Melts

SiO₂-Al₂O₃ melts containing 42 and 60 wt% A1₂O₃ were homogenized at 2090°C (∼10°) and crystallized by various heat treatment schedules in sealed molybdenum crucibles. Mullite containing ∼78 wt% A1₂O₃ precipitated from the 60 wt% A1₂O₃ melts at ∼1325°± 20°C, which is the boundary of a previously calculated liquid miscibility gap. When the homogenized melts were heat-treated within this gap, the A1₂O₃ in the mullite decreased with a corresponding increase in the Al₂O₃ content of the glass. A similar decrease of Al₂O₃ in mullite was observed when crystallized melts were reheated at 1725°± 10°C; the lowest A1₂O₃ content (∼73.5 wt%) was in melts that were reheated for 110 h. All melts indicated that the composition of the precipitating mullite was sensitive to the heat treatment of the melts.