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.     

Thermal Evolution and Sintering Behavior of a 2:1 Mullite Gel

The thermal evolution of a mullite gel of composition 2Al₂O₃·SiO₂ has been investigated. The gel crystallized at 1300°C into an alumina-rich mullite and corundum, instead of single-phase 2Al₂O₃·SiO₂ mullite. The amount of Al₂O₃ that dissolved in the mullite structure has been determined in the 1300–1780°C temperature range by measuring the mullite lattice parameters. A maximum limit for the amount of Al₂O₃ in solid solution has been observed. Densification of the gel powders has been analyzed up to temperatures of 1780°C. The microstructure of dense materials always showed the presence of residual Al₂O₃ particles.     

Formation of Silicon-Aluminum Spinel

Characterization of the intermediate cubic phase formed during the transformation of coprecipitated SiO₂-Al₂O₃ gel on heating was studied and X-ray diffraction methods are reviewed and criticized. Coprecipitated gels of different SiO₂/Al₂O₃ ratios were prepared; all showed a 980°C exotherm followed by crystallization of the cubic phase and liberation of SiO₂. Alkali extraction of SiO₂ showed two types present in the 980°-heated product. One variety is free amorphous SiO₂ and the other, chemically bonded to alumina in the crystalline cubic phase, was isolated and characterized as Si-Al spinel with the same composition as mullite. Thus, its formation from the gel of mullite composition shows the highest exotherm and the measured density agrees approximately with the theoretically calculated value.     

Reaction-Forming of Mullite Ceramics Using an Aqueous Milling Medium

Mullite and mullite/ZrO₂ ceramics were fabricated starting from Si/Al₂O₃ and Si/Al₂O₃/ZrO₂ powder mixtures, which were mixed and attrition milled with TZP balls in water. Isopressed powder compacts were subjected to a heat treatment in air, during which the Si was oxidized to SiO₂. At } 1410°C, reaction between Al₂O₃ and SiO₂ occurred, resulting in mullite (3Al₂O₃·2SiO₂). Depending on the composition of the starting powders, the end product was either single-phase mullite or a mullite composite. The reaction process was monitored by thermogravimetry and dilatometry. It was found that the microstructure and mechanical properties of the reaction-formed mullite ceramics were significantly improved by ZrO₂ additions.     

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.     

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.     

Infrared-Transparent Mullite Ceramic

Mullite ceramic, transparent in the infrared, was prepared by hot-pressing and hot-isostatically pressing starting materials derived from alkyloxides. A composition with 72.3 wt% Al₂O₃ yielded transparent, submicrometer grain size bodies at 1630°C, whereas higher temperatures produced glass-containing microstructures. A composition with 76 wt% A1₂O₃ formed precipitates of α-Al₂O₃ at the consolidation temperature, which could be removed by subsequent annealing between 1800° and 1850°C. Spectral transmittance and absorption coefficients of the bodies are reported. The formation of the second phases was linked to phase equilibria and grain growth that promoted compositional equilibration of the mullite phase. The results suggest adjustments to phase boundaries in the high-temperature segment of the SiO₂-Al₂O₃ phase diagram.     

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.     

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.     

Phase Transformation of Diphasic Aluminosilicate Gels

Aluminosilicate gels with compositions Al₂O₂/SiO₂ and 2 were prepared by gelling a mixture of colloidal pseudo-boehmite and a silica sol prepared from acid-hydrolyzed Si(OC₂H₅)₄. Upon heating the pseudo-boehmite transforms to γ-Al₂O₃ around 400°C, then to δ-Al₂O₃ at 1050°C, and at 1200°C reacts with amorphous SiO₂ to form mullite. Some twinned θ-Al₂O₃ forms before mullite. Nonstoichiometric specimens have a similar transformation sequence, but form mullite grains with inclusions of either Al₂O₃ or cristobalite, often associated with dislocation networks or micropores. Mullite grains are formed by nucleation and growth and have equiaxed shape.     

Novel Synthesis of Mullite Powder with High Surface Area

Boehmite (AlO(OH)) solid-solution gel, which yields stoichiometric mullite (3Al₂O₃2SiO₂) at high temperatures, has been prepared by the hydrazine method. The formation process leading to 3Al₂O₃2SiO₂ is discussed. The as-prepared powder and powders heated below 1200°C consist of very fine particles showing needlelike morphology, whereas the particles of mullite powder show thin prismatic morphology. The mullite powder after heating at 1300°C has a high surface area (87 m²/g).     

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.     

Crystallization Behavior of Al2O3 in the Presence or SiO2

The independent crystallization sequence of an Al₂O₃ component is modified in the presence of SiO₂ and vice versa. Mixed SiO₂-Al₂O₃, gel (28 wt% SiO₂ and 72 wt% Al₂O₃) forms neither cristobalite nor γ-Al₂O₃ and corundum at 1000°C but forms Si-Al spinel; an amorphous aluminosilicate phase invariably also forms after the gel is heated. However, the composition of this amorphous aluminosilicate phase is not as yet known.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

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.     

Effect of Oxygen Vacancies on the Hot Hardness of Mullite

The structure of mullite, which has a composition ranging from 3Al₂O₃·2SiO₂ to Al₂O₃·2SiO₂, contains ordered oxygen vacancies. Sillimanite, Al₂O₃·SiO₂, has a similar structure but with no vacancies. The indentation hardness of polycrystalline mullite (3Al₂O₃·2SiO₂) was measured from room temperature up to 1400°C and compared with that of single-crystal sillimanite (Al₂O₃·SiO₂) up to 1300°C. It was found that both materials show the same variation in hardness with temperature, suggesting that the structures have a similar resistance to plastic deformation, and therefore that the oxygen vacancies in the mullite structure are not the primary cause of mullite's resistance to high-temperature deformation.     

Calculated Thermodynamic Data and Metastable Immiscibility in the System SiO2-Al2O3

Thermodynamic data on activities, activity coefficients, and free energies of mixing in SiO₂-Al₂O₃ solutions were calculated from the phase diagram. Positive deviations from ideal mixing in the thermodynamic data suggest a tendency for liquid immiscibility in both SiO₂- and Al₂O₃-rich compositions. The calculated data were used to estimate regions of liquid-liquid immiscibility. A calculated metastable liquid miscibility gap with a consolute temperature of ∼1540^°C at a critical composition of ∼36 mol% Al₂O₃ was considered to be thermodynamically most probable; the gap extended from ∼11 to °49 mol% Al₂O₃ at 1100^°C. SiO₂-rich glass compositions showed evidence of glass-in-glass phase separation when examined by direct transmission electron microscopy.     

Temperature-Dependent Iron Solubility in Mullite

Sample disks prepared from Al₂O₃ (61 wt%), SiO₂ (28 wt%), and Fe₂O₃(II wt%) powders were sintered at 1270° and 1440°C and then annealed between 1300° and 1670°C. The annealed samples consisted of mullite as the main compound with minor amounts of glass and sometimes magnetite. The iron content of the mullites decreases strongly from ∼ 10.5 wt% Fe₂O₃ at 1300°C to ∼ 2.5 wt% Fe₂O₃ at 1670°C. A complex temperature-controlled exsolution mechanism of iron from mullite is considered.     

Influence of Thermal Aging on Microstructural Development of Mullite Containing Alkalis

The influence of annealing treatments at temperatures of 900°C up to 1630°C on the microstructure of a 3Al₂O₃2SiO₂ mullite that contains a small amount of alkali (<3 wt%) has been studied. Annealing treatments of a base mullite material at the sintering temperature (1630°C) and at two temperatures lower (900°C) and higher (1200°C) than the lowest invariant points of the SiO₂-Al₂O₃-Na₂O system have been performed. Microstructures have been characterized by using scanning and transmission electron microscopy. Special attention has been given to grain-boundary characteristics-particularly the amount, composition, and distribution of the remaining glasses. Aging of this material at high temperature leads to a redistribution of the microstructure toward an equilibrium that involves the dissolution of the mullite grains, formation of a liquid phase, and liquid-phase grain growth. As the aging temperature increases, liquid-phase grain growth progressively overcomes the effect of the dissolution of mullite and a bimodal microstructure with an increasing number of large, tabular grains develops.     

Mechanical Activation of the Decomposition and Sintering of Kyanite

The influence of attrition milling on the thermal decomposition of kyanite (Al₂O₃·SiO₂) to mullite (3Al₂O₃·2SiO₂) and SiO₂, and its subsequent sintering, was studied. A commercial kyanite was attrition-milled for times up to 12 h. Dilatometry confirmed that as-received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one-half of the decomposition occurs in the last 35°C. For the attrition-milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition-milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering.