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.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

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.     

Thermal Behavior of Alumina—Silica Xerogels During Calcination

Xerogels of 3Al₂O₃·2SiO₂ mullite were prepared by hydrolyzing Al(NO₃)₃·9H₂O and Si(OC₂H₅)₄ solutions with pH values of 8.3, 9.4, 10.1, and 10.4; the xerogels were composed of a combination of singlephase and diphasic materials. A strong alkaline solution enhanced bayerite formation in the gels. Mullite from the diphasic xerogels was produced by reacting θ-Al₂O₃ with amorphous SiO₂, whereas mullite from the single-phase xerogels was transformed from Al-Si spinel. For the single-phase xerogel, the DTA curve closely resembled the kaolinite-to- mullite reaction. For the diphasic xerogels, the Al³⁺ -containing solution gelled to pseudoboehmite, which transformed to bayerite in solution. The bayerite then decomposed to η-Al₂O₃ and to θ-Al₂O₃ sequentially on heating.     

Reexamination of the Kaolinite-to-Mullite Reaction Series

The kaolinite-to-mullite reaction series was reexamined with special attention to the nature of the metaphase, the eontroversial spinel phase, and the cause of the exothermic peak at 980°C. Amorphous SiO₂ forms during the exothermic reaction; it can be leached by alkali extraction. When the residual cubic phase is heated further, it forms mullite only. This result indicates that the cubic phase is an Al-Si spinel and that metakaolinite is an AI₂O₃-SiO₂ compound. It was established that the exotherm exhibited by kaolinite at 980°C represents the sudden transformation of metakaolinite to Al-Si spinel, the crystallization of mullite, and the liberation of amorphous SiO₂. The AI-Si spinel has the same composition as mullite, containing both AI(IV) and AI(VI). This spinel transforms into mullite at the second exothermic peak; no amorphous SiO₂ is liberated.     

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.     

Characterization and Sintering Behavior of Alkoxide-Derived Aluminosilicate Powders

Submicrometer SiO₂-Al₂O₃ powders with compositions of 46.5 to 76.6 wt% Al₂O₃ were prepared by hydrolysis of mixed alkoxides. Phase change, mullite composition, and particle size of powders with heating were analyzed by DTA, XRD, IR, BET, and TEM. As-produced amorphous powders partially transformed to mullite and Al-Si spinel at around 980°C. The compositions of mullite produced at 1400° and 1550°C were richer in Al₂O₃ than the compositions of stable mullite solid solutions predicted from the phase diagram of the SiO₂-Al₂O₃ system. Particle size decreased with increasing Al₂O₃ content. The sintered densities depended upon the amount of SiO₂-rich glassy phase formed during sintering and the green density expressed as a function of particle size.     

Outstanding Problems in the Kaolinite-Mullite Reaction Sequence Investigated by 29Si and 27Al Solid-state Nuclear Magnetic Resonance: 11, High-Temperature Transformations of Metakaolinite

Solid-state ²⁹Si and ²⁷Al NMR spectra of kaolinite fired at 800° to 1450°C, interpreted in light of a newly proposed metakaolinite structure and complementary X-ray diffraction results, lead to the following conclusions about the hightemperature reactions: (1) Removal of the final residual hydroxyl radicals of metakaolinite at ∼9707deg;C triggers the separation of a considerable amount of amorphous free silica and the formation of poorly crystalline mullite and a spinel phase. (2) Mullite and spinel form in tandem, the former originating in the vicinity of AI-0 units of regular octahedral and tetrahedral symmetry randomly distributed throughout the metakaolinite structure. (3) The initially formed mullite is alumina-rich but at higher temperatures progressively gains silica, approaching the conventional 3Al₂O₃· 2SiO₂ composition. (4) The spinel phase contains insufficient Si to be detected by ²⁹Si NMR but has a ²⁷Al NMR spectrum consistent with γ-Al₂O₃. On further heating, the spinel is converted to mullite by reaction with some of the amorpholls silica, the balance of which eventually becomes cristobalite.     

Structural Changes Induced on Mullite Precursors by Thermal Treatment: A 27Al MAS-NMR Investigation

NMR study of mullite precursors has shown that local arrangement of Al in samples synthesized by spray pyrolysis differs considerably from the one adopted by samples obtained by polymeric or colloidal routes. Aluminum is tetra- and pentahedrally coordinated in the first type of samples but is tetra- and octahedrally coordinated in the second ones. Segregation of SiO₂ and Al₂O₃ is directly produced in colloidal preparation; however, this phenomenon occurs only in polymeric gels when they are heated between 980° and 1100°C. In polymeric samples, thermal treatment at ∼980°C produces the formation of γ-Al₂O₃. A similar treatment in spray-pyrolized powders gives directly 3:2 mullite. From these results, exothermic and expansive effects detected at ∼980°C were ascribed to changes in coordination of Al produced during the atomic rearrangement that accompanies formation of these two phases (γ-Al₂O₃ or mullite). Above 1200°C, incorporation of Si in the Al-rich phase induces the formation of 3:2 mullite in polymeric and colloidal samples.     

Silica-Free Phases with Mullite-Type Structures

Silica-free phases which have a structure similar to that of mullite can be crystallized from gels in the Na₂O-Al₂O₃ and (Na,K)₂O-BaO-Al₂O₃ systems. A gelation step appears to be necessary, since a solid-state reaction between Na₂CO₃ and Al(OH)₃ does not give the mullite-type phase. Crystallization of this phase requires a high alkali content during formation of the gel. A well-crystallized phase is formed at 950°C and is stable to at least 1000°C; at higher temperatures (i.e. 1200°C), β-Al₂O₃ and corundum are formed. The mullite-type phase appears to crystallize with an increase in temperature at the expense of a γ-Al₂O₃ phase, indicating adsorption of Na on the defect spinel structure, which is then rearranged to give the mullite-type phases.     

Anomalous Temperature Dependence of the Growth Rate of the Reaction Layer between Silica and Molten Aluminum

An aluminum/Al₂O₃ composite body is produced by a displacement reaction between SiO₂ and molten aluminum. The growth rate of the reaction layer possesses negative (anomalous) temperature dependence at 1000–1300 K. This study compared reported reaction-kinetic data and investigated causes for this temperature dependence. The reaction product, Al₂O₃, changed from the γ-/θ-Al₂O₃ phase to the α-Al₂O₃ phase in this temperature range and α-Al₂O₃ became the dominant phase at >1273 K. Isothermal transformation of the γ-/θ-Al₂O₃ product phases to the α-Al₂O₃ phase was also observed. Morphologies and scales of the Al₂O₃ phases change drastically at 1173 K; this transition occurred in a spatially discontinuous manner. Reaction-rate retardation was interpreted in terms of occurrence of the competitive and simultaneous reactions to produce different Al₂O₃ phases in this temperature range. It was also found that the hydrogen release from the raw SiO₂ and the SiO₂ phase transformation were not related to the negative temperature dependence.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

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.     

Spinel Phase Formation During the 980°C Exothermic Reaction in the Kaolinite-to-Mullite Reaction Series

With the use of differential thermal analysis, X-ray diffraction, and transmission electron microscopic techniques, we showed that γ-Al₂O₃ type spinel phase is solely responsible for the 980°C exotherm in the kaolinite-to-mullite reaction series. Transmission electron microscopic characterization indicated that the spinel formation is preceded by a phase separation in the amorphous dehydroxylated kaolinite matrix. Chemical analysis of the spinel phase by energy dispersive X-ray spectroscopy revealed a nearly pure Al₃O₂ composition.     

Lowering Crystallization Temperatures by Seeding in Structurally Diphasic Al2O3-MgO Xerogels

Thermal reactions in 93% Al₂O₃-7% MgO and 95.8% Al₂O₃-4.2% MgO gels seeded with α-Al₂O₃, MgAl₂O₄, α-Fe₂O₃, and SiO₂, sols were investigated by differential thermal analysis to determine the extent of nucleation catalysis of solid-state reactions. Seeding with α-Al₂O₃ lowered the α-Al₂O₃ crystallization temperature in these xerogels by 100° to 150°C. Spinel seeds have much less effect on the γ-α transition, and α-Fe₂O₃ and SiO₂ seeds do not affect it significantly. Isostructural seeding of gels may therefore permit lower ceramic processing temperatures.     

Contribution to the Study of Moisture Expansion in Ceramic Materials

Measurements were made of the expansions produced by autoclaving coprecipitated and mixed gels of silica and alumina fired over the temperature range 850° to 1200°C. The effect of adding soda to gels of kaolinite composition was investigated and the compositions of the phases were determined. The results show that amorphous silica has a limited influence and must be modified by alumina or by soda and alumina to produce expansions comparable with those of ceramic bodies. The active material is an amorphous alkali aluminosilicate, to be distinguished from permutites and glass. Formation of glass and crystalline compounds reduces moisture expansion. At low firing temperatures (below 950° C.) the hydration of γyAl₂O₃ to boehmite produces high moisture expansions, but γ-Al₂O₃ modified by silica (silicon spinel) has only a limited influence. Some observations are made on the nature of cristobalite developed during the firing of pure amorphous silica and amorphous silica into which additives were introduced.     

Preparation of Submicrometer A12O3 Powder by Gas-Phase Oxidation of Tris (acetylacetonato) aluminum (111)

Fine A1₂O₃ powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al₂O₃ at 1000° and 1200°C, and δ-Al₂O₃ at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas.     

Characterization of Spinel Phase Formed in the Kaolin-Mullite Thermal Sequence

The spinel phase formed on the thermal reaction of kaolin group minerals was characterized by X-ray quantitative analysis, lattice constant determination, and chemical analysis by analytical TEM. The spinel phase was determined to be nearly γ-Al₂O₃ with ∼8 wt% of SiO₂, significantly less than previously reported.     

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.     

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.