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About: Mullite is a(n) research topic. Over the lifetime, 8396 publication(s) have been published within this topic receiving 106006 citation(s). more

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Journal ArticleDOI
Abstract: Mullite has achieved outstanding importance as a material for both traditional and advanced ceramics because of its favourable thermal and mechanical properties. Mullite displays various Al to Si ratios referring to the solid solution Al 4+2 x Si 2−2 x O 10− x , with x ranging between about 0.2 and 0.9 (about 55 to 90 mol% Al 2 O 3 ). Depending on the synthesis temperature and atmosphere mullite is able to incorporate a number of transition metal cations and other foreign atoms. The crystal structure of mullite is closely related to that of sillimanite, which is characterized by chains of edge-connected AlO 6 octahedra running parallel to the crystallographic c -axis. These very stiff chains are cross-linked by tetrahedral chains consisting of (Al,Si)O 4 tetrahedra. In more detail: Parallel to a the tetrahedra are linked to the relatively short more stiff Al–O(A, B) bonds, whereas parallel b they are linked parallel to the relatively long more compliant Al–O(D) bonds. In mullite some of the oxygen atoms bridging the tetrahedra are removed for charge compensation. This gives rise to the formation of oxygen vacancies and of T 3 O groups (so-called tetrahedral triclusters). The anisotropy of the bonding system of mullite has a major influence on the anisotropy of its physical properties. For example: • the highest longitudinal elastic stiffness is observed parallel c , but lower ones parallel a and especially parallel b , • the maximum of the thermal conductivity occurs parallel c , but maller ones parallel a and especially parallel b , • large thermal expansion especially parallel b , • fastest crystal growth and highest corrosion parallel c . Heat capacity and thermal expansion measurements of mullite display reversible anomalies in the temperature range between about 1000 and 1200 °C. It is believed that tetrahedral cations, bridging O atoms, and O vacancies undergo dynamical site exchange processes at high temperatures. At lower temperatures the dynamic disorder may transform to a static one. Diffraction experiments revealed that also partially ordered states may exist. more

770 citations

Journal ArticleDOI
Abstract: Mullite (3AI2O3. 2Si02) is becoming increasingly important in electronic, optical, and high-temperature structural applications. This paper reviews the current state of mullite-related research at a fundamental level, within the framework of phase equilibria, crystal structure, synthesis, processing, and properties. Phase equilibria are discussed in terms of the problems associated with the nucleation kinetics of mullite and the large variations observed in the solid-solution range. The incongruent melting behavior of mullite is now widely accepted. Large variations in the solid solubility from 58 to 76 mot% alumina are related to the ordering/disordering of oxygen vacancies and are strongly coupled with the method of synthesis used to form mullite. Similarly, reaction sequences which lead to the formation of mullite upon heating depend on the spatial scale at which the components are mixed. Mixing at the atomic level is useful for lowtemperature ( more

587 citations

01 Jan 1970-
Abstract: INTRODUCTION. NOMENCLATURE. PREPARATION OF ALUMINA PHASES. Bauxite. Preparation of Bayer Alumina. Wet Alkaline Processes. Wet Acid Processes. Furnace Processes. Carbothermic Processes. Electrolytic Processes. Amorphous and Gel Aluminas. Preparation of the Alumina Trihydroxides. Gibbsite. Bayerite. Nordstrandite, Bayerite II, Randomite. Preparation of the Alumina Monohydroxides. Boehmite. Disapore. Transition Aluminas. Dehydration Mechanism. Sequence of Transition. Phases Formed on Aluminum. Rehydration. Alpha Alumina. Preparation. Factors Affecting Alumina Transitions. Special Ceramic Aluminas. Beta and Zeta Aluminas. Suboxides and Gaseous Phases. STRUCTURE AND MINERALOGICAL PROPERTIES. Structure of the Alumina Phases. Pseudomorphosis. Surface Area of Alumina. Porosity. Sorptive Capacity. MECHANICAL PROPERTIES OF ALUMINA. General Considerations. Bending, Compressive, Tensile, and Torsional Strength. Impact Strength. Moduli of Elasticity (E), and Rigidity (G). Poisson's Ration (i). Creep Characteristics. Thermal Shock. Internal Friction. Fatigue. Hardness and Abrasiveness of Alumina THERMAL PROPERTIES. Thermophysical and Thermochemical Constants. Specific Heat. Thermal Expansion. Thermal Conductivity. Thermal Diffusivity SONIC EFFECTS IN ALUMINA. Velocity of Sound in Alumina. Ultrasonic Absorption. ELECTRICAL PROPERTIES OF ALUMINA. Introduction. Electrical Conductivity of Alumina. Dielectric Constant and Loss Factor of Alumina. Dielectric Strength MAGNETIC PROPERTIES OF ALUMINA. Magnet Susceptibility. Magnetic Resonance of Alumina. OPTICAL PROPERTIES OF ALUMINA. Refractive Index of Alumina. Transmission, Emissivity, and Absorption of Alumina. Phosphorescence, Fluorescence, and Thermoluminescence. Optical Spectra of Alumina. Color in Alumina. Chromia-Alumina System, Laser Applications RADIATION AND ALUMINA. CHEMICAL PROPERTIES OF ALUMINA. Wet Chemical Reactions of Sintered Alumina. Reaction of the Chemical Elements with Alumina. Slagging Effects. Ash Slags. Slags Containing Sulfates. Steel Furnace Slags. Glass Furnace Reactions. Calcium Aluminate Slags. Aluminum Slag Reactions. Miscellaneous Reactions COLLOIDAL PROPERTIES OF ALUMINA. Plasticity. Surface Charge and Zeta Potential of Alumina. Flocculation and Deflocculation Effects. Additives GRINDING CERAMIC ALUMINA. FORMING ALUMINA CERAMICS. Cold Forming of Alumina. Hot-Pressing. Miscellaneous Forming Methods SINTERING. Introduction. Sintering Atmospheres. Sintering Additives ALUMINA IN REFRACTORIES. General. High-Alumina Refractories. Fused Cast Alumina Refractories. Clay-Bonded Alumina Refractories, Mullite Refractories. Spinel, Cordierite, Alumina-Chromite. Refractory Equipment. Refractories for Aluminum and Other Nonferrous Uses. Lightweight Alumina Refractories. Binders for Alumina Refractories ALUMINA AS AN ABRASIVE MATERIAL. Introduction. Loose Grain Abrasive. Grinding Wheels. Ceramic Tools ELECTRICAL APPLICATIONS. Spark Plug Insulators. Electron Tube Elements, High-Frequency Insulation. Alumina Porcelain Insulation. Resistors and Semiconductors CEMENT. Calcium Aluminate Cement. Barium Aluminates ALUMINA IN GLASS. Introduction. Bottle Glass. Devitrified Glasses Containing Alumina. Boron Glasses. Lithium Glasses, Phosphate Glasses. Optical Glasses ALUMINA IN COATINGS. Introduction. Anodic Coatings on Aluminum. Glazes and Enamels. Flame-Sprayed Coatings. Painted, Cast, or Troweled Coatings. Electrolytic Coatings. Evaporated Coatings. Dip Coatings, Cementation Coatings. Coatings on Alumina and Other Ceramic Bases. Alumina Coatings for Electrical Insulation. Alumina Coatings by Sputtering ALUMINA IN CERMETS AND POWDER METALLURGY. Introduction. Chromium-Alumina Cermets. (Iron, Nickel, Cobalt)-Alumina Cermets. Aluminum-Alumina Alloys. Miscellaneous Cermets ALUMINA IN AIRBORNE CERAMICS. Introduction. Gas-Turbine Accessories. Radomes and Rocket Equipment. SEALS, METALLIZING, WELDING. FIBERS, WHISKERS, FILAMENTS. Introduction. Alumina Fibers. Glass Fibers MISCELLANEOUS CERAMIC APPLICATIONS OF ALUMINA. References. more

483 citations

Journal ArticleDOI
Abstract: Mineralogical, textural and chemical analyses of clay-rich materials following firing, evidence that initial mineralogical differences between two raw materials (one with carbonates and the other without) influence the tex- tural and mineralogical evolution of the ceramics as T increases from 700 to 1100° C. Mineralogical and textural changes are interpreted considering local marked disequilibria in a system that resembles a small-scale high- T meta- morphic process ( e.g., contact aureoles in pyrometamorphism). In such conditions, rapid heating induces significant overstepping in mineral reaction, preventing stable phase formation and favoring metastable ones. High- T transfor- mations in non-carbonate materials include microcline structure collapse and/or partial transformation into sanidine; and mullite plus sanidine formation at the expenses of muscovite and/or illite at T ‡ 800° C. Mullite forms by mus- covite-out topotactic replacement, following the orientation of mica crystals: i.e., former (001) muscovite are ^ to (001)mullite. This reaction is favored by minimization of free energy during phase transition. Partial melting followed by fingered structure development at the carbonate-silicate reaction interface enhanced high- T Ca (and Mg) silicates formation in carbonate-rich materials. Gehlenite, wollastonite, diopside, and anorthite form at carbonate-silicate interfaces by combined mass transport (viscous flow) and reaction-diffusion processes. These results may add to a better understanding of the complex high- T transformations of silicate phases in both natural ( e.g., pyrometamor- phism) and artificial ( e.g., ceramic processing) systems. This information is important to elucidate technological achievements and raw material sources of ancient civilizations and, it can also be used to select appropriate clay com- position and firing temperatures for new bricks used in cultural heritage conservation interventions. more

433 citations

Journal ArticleDOI
I. A. Aksaf1, Joseph A. Pask1Institutions (1)
Abstract: Concentration profiles of Al/sub 2/O/sub 3/ in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO/sub 2/--Al/sub 2/O/sub 3/. The intermediate compound mullite, 3Al/sub 2/O/sub 3/.2SiO/sub 2/, melts incongruently at 1828 +- 10/sup 0/C; its stable solid-solution region ranges from 70.5 to 74.0 wt percent Al/sub 2/O/sub 3/ below 1753/sup 0/C and from 71.6 to 74.0 wt percent at 1813/sup 0/C. The microstructures of diffusion zones and heat-treated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: SiO/sub 2/-Al/sub 2/O/sub 3/ in the absence of mullite, SiO/sub 2/-ordered-mullite in the absence of alumina, and SiO/sub 2/-disordered mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at approximately 1880/sup 0/C and its solid-solution range extends up to approximately 77 wt percent Al/sub 2/O/sub 3/. The solid-solution range of disordered mullite extends to approximately equal to 83 wt percent Al/sub 2/O/sub 3/ with an estimated congruent melting temperature of approximately 1900/sup 0/C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum-silicate liquids. (auth) more

395 citations

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Topic's top 5 most impactful authors

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