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.     

Kaolinite–Mullite Reaction Series: The Development and Significance of a Binary Aluminosilicate Phase

The semiquantitative estimations of 980°C exothermic reaction products of kaolinite by quantitative X-ray diffraction (QXRD) and chemical leaching techniques show the formation of a significant amount of amorphous aluminosilicate phase (∼ 30 to 40 wt%). The theoretically expected AlO₄/AlO₆ ratio in the 980°C reaction is in close agreement with the value measured by the X-ray fluorescence (XRF) technique and the experimental radial electron distribution (RED) profile agrees with the suggested 980°C formation of Si-Al spinel with mullite-like composition. Mullitization of kaolinite has been compared with a synthetic Al₂O₃—SiO₂ mixture. In synthetic mixtures development of an intermediate amorphous aluminosilicate phase is an essential step prior to mullitization. Kaolinite forms mullite in two ways: (i) by polymorphic transformation of cubic mullite at 1150° to 1250°C and (ii) by nucleation of mullite in the amorphous aluminosilicate phase and its subsequent growth above 1250°C. Thus chemical continuity is maintained throughout the reaction series and the intermediate spinel phase is silicon bearing and its subsequent transformation to mullite confirms the topotactic concept in the kaolinite transformation.     

Reactions in Silica-Alumina Mixtures

Examination of mixtures of extremely pure silica and alumina shows that the greatest reactivity is not encountered with stoichiometric ratios to form mullite with the formula 3Al₂O₃. 2SiO₂ but rather with the formula 2A1₂O₃-SiO₂, and that reactivity also depends on the crystalline modification of alumina. A sharp exothermic differential thermal peak at 980°C. is attributed to three simultaneous reactions dependent on the silica-alumina ratio of the mixture: (1) the crystallization of gamma alumina, (2) the crystallization of a hydrogen aluminum spinel (HAl₅O₅), and (3) the reaction of silica with the hydrogen aluminum spinel to form mullite.     

Interpretation of the Kaolinite-Mullite Reaction Sequence from Infrared Absorption Spectra

The phases in the kaolinite-mullite reaction sequence were reexamined by ir absorption spectrophotometry. Particular attention was paid to the controversial intermediate Al-containing phases. Amorphous materials were leached from fired kaolinite samples with NaOH to help identify crystalline phases. Metakaolinite partially decomposes, releasing amorphous γ-Al₂O₃ and SiO₂, before the "950°C" exothermic reaction in which metakaolinite is completely decomposed. The resulting spinel-type phase, which is associated with amorphous SiO₂ and some poorly crystalline "primary" mullite, is γ-Al₂0₃ (crystalline) rather than an Al-Si spinel. There is some evidence, however, that a fraction of the γ-Al₂O₃, may be an Al-Si spinel. At ≥1100°C secondary mullite therefore forms primarily from the γ-Al₂O₃/amorphous SiO₂ reaction and the recrystallization of primary mullite, whereas excess amorphous SiO₂ eventually crystallizes as cristobalite.     

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.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Resolution of Thermal Peaks of Kaolinite in Thermomechanical Analysis and Differential Thermal Analysis Studies

The recent findings for the kaolinite metakaolinite, cubic-mullite, and orthorhombic-mullite reaction series have been thoroughly examined by differential thermomechanical analysis (DTMA) and differential thermal analysis (DTA). Metakaolinite shows two differential contraction peaks in the vicinity of 980°C caused by final dehydroxylation at the endothermic dip just before 980°C in DTA with expulsion of 35–37 wt% SiO₂, formation of a defect aluminosilicate phase and simultaneous contraction of the latter phase, and crystallization of cubic mullite at the 980°C exotherm in DTA. Mullitization takes place in two simultaneous reaction steps: (i) polymorphic transformation of cubic mullite to orthorhombic mullite during the ∼1250°C exotherm shown by DTA which coincides with the differential expansion peak in DTMA and (ii) nucleation followed by crystallization of orthorhombic mullite from the residual aluminosilicate compact phase during the ∼1330°C exotherm shown by DTA. The aluminosilicate formed during the large differential contraction at 1100°–1400°C as shown by DTMA. These results, obtained by the two physical techniques, corroborate earlier findings of the kaolinite transformation series.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Synthesis and Sintering of Cordierite from Ultrafine Particles of Magnesium Hydroxide and Kaolinite

Thermal reactions of magnesium compounds and kaolinite were investigated to obtain dense cordierite ceramics without additives. Magnesium hydroxide was precipitated from aqueous solution in the form of ultrafine hexagonal tabular particles of about 0.1 µm, and heating this mixture with kaolinite in a mole ratio of MgOAl₂O₃2SiO₂ resulted in formation of an amorphous state at about 900°C following thermal decomposition. µ-Cordierite was then crystallized from the amorphous phase at about 950°C, and gradually transformed into alpha-cordierite. Firing the pressed specimens yielded a dense alpha-cordierite ceramic with a relative density of 97.7% at 1350°C.     

Carbon Interfacial Layers Formed by Oxidation of SiC in SiC/Ba-Stuffed Cordierite Glass-Ceramic Reaction Couples

Reaction couples between α-SiC and cordierite (2MgO·2Al₂O₃·5SiO₂═ Mg₂Al₄Si₅O₁₈) were prepared by sandwiching (and enclosing) SiC single crystals between plates of Ba-stuffed magnesium aluminosilicate (Ba-MAS) glass and hot-pressing; the Ba-MAS was subsequently crystallized at 1000° to 1200°C in argon or air. No reaction occurred at the SiC/Ba-MAS interfaces during hot-pressing, but crystallization heat treatments caused formation of amorphous carbon reaction layers at the SiC/cordierite interfaces, due to concurrent oxidation via the reaction SiC + O₂→ SiO₂+ C. The thickness of the carbon of the carbon layer was variable. These results suggest that formation of C layers at SiC/silicate interfaces in other composites (containing Nicalon fibers, for example) depends more on thermochemistry and less on the details of SiC nonstoichiometry than has heretofore been supposed.     

Structural Characterization of the Spinel Phase in the Kaolin-Mullite Reaction Series Through Lattice Energy

Kaolinite undergoes structural transformation on heating. X-ray photographs reveal the existence of a spinel-type phase when kaolin is heated at 980°C. The kaolinite decomposes into a spinel phase with the expulsion of silica. A controversy arises as to whether the spinel phase is γ-Al₂O₃ or Si-Al spinel. Calculating the lattice energies of the structures confirms that the spinel phase is γ-Al₂O₃ and not Si-Al spinel, as proposed earlier. The heat involvement in phase transformation, as obtained from experimental observation at 980°C, is also explained in the light of lattice energies.     

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.     

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.     

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.     

Controlled Spherulitic Crystallization in 3BaO-5SiO2 Glass

Morphological stages in the internal crystallization of a 3BaO- 5SiO₂ glass were investigated. Spherulitic crystallization occurred between 800° and 950°C. After long times at the crystallization temperatures, the spherulites changed, apparently by secondary crystallization. Above 950°C, lath-like crystals are formed by recrystallization of the spherulites. The effects of nucleation-and-growth heat treatments on the rate and morphology of crystallization were studied. The peak nucleation temperature was between 700° and 725°C, and all the crystalline forms were 3BaO.5SiO₂.     

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.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Formation of TiO2(B) Nanocrystallites in Sol-Gel-Derived SiO2-TiO2 Film

TiO₂(B), one of the polymorphs of TiO₂, has been formed by annealing a sol-gel-derived SiO₂-TiO₂ amorphous film on a silicon wafer at 900°C in air. Transmission electron microscopy (TEM) revealed that nanocrystallites with a size of 5-10 nm were dispersed in the amorphous SiO₂ matrix in the film. The X-ray diffraction pattern and lattice fringe spacing in high-resolution TEM images corresponded to those of TiO₂(B). These TiO₂(B) nanocrystallites are probably stable with the presence of surrounding SiO₂ in the film at 900°C, because previous works reported that this phase should be converted to anatase at temperatures higher than 550-700°C.     

Preparation of Porous Anatase Coating from Sol-Gel-Derived Titanium Dioxide and Titanium Dioxide-Silica by Water-Vapor Exposure

The effects of water vapor on the crystallization behavior of sol-gel-derived titanium dioxide (TiO₂) thin films that contained 0-50 mol% silica (SiO₂) were investigated. Anatase formed on exposure to water vapor at 60°-180°C, with a simultaneous decrease in the concentration of OH groups. An increase in the SiO₂ content of the exposed films led to an increase in the average crystalline size. Because crystallization of the exposed films of the films was not accompanied by shrinkage, porous anatase coatings were obtained via exposure at a relatively low temperature. Phase separation of the immiscible TiO₂-SiO₂ system was induced with water vapor, which resulted in acceleration of the crystallization of the sol-gel films.