The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Hartmut Schneider - One of the best experts on this subject based on the ideXlab platform.
-
Mullite-2 c – a natural polytype of Mullite
European Journal of Mineralogy, 2020Co-Authors: Stephan Lenz, Hartmut Schneider, Johannes Birkenstock, Lennart A. Fischer, Reinhard X. FischerAbstract:Abstract. A single crystal ( ∼ 20 µ m × 20 µ m × 330 µ m) of Mullite-2 c , a natural polytype of Mullite, was separated from a radially grown cluster of acicular crystals from Ettringer Bellerberg (Quarternary Eifel volcanic fields, Germany). The chemical composition determined from electron microprobe analysis (EMPA) is Na 0.01 Mg 0.05 Al 8.52 Fe 0.29 3 + Si 3.13 Ti 0.02 O 19.55 , corresponding to x=0.22(8) in the generalised mineral formula M y + Mg z 2 + M 8 + 4 x + y - 2 z 3 + M 4 - 4 x - y + z 4 + O 20 - 2 x . Only Fe3+ as foreign cation was considered in the refined structure model, partially replacing Al3+ in the octahedral chains. A crystal of a similar type, though exhibiting a significantly different composition with x=0.02 , was first described in 2015, tentatively named “silliMullite” by Fischer et al. (2015). This crystal and our new sample have similar structural properties, now classified as a polytype of Mullite, designated Mullite-2 c . Single-crystal X-ray diffraction showed that the Mullite-2 c crystal investigated here exhibits partial Si ∕ Al ordering in the double chains of (Si,Al)O4 tetrahedra in contrast to the sample described in 2015 as being completely ordered. The ordering in Mullite-2 c results in a doubled c lattice parameter with respect to Mullite. It crystallises in space group Pnam, with cell parameters for the new sample of a=7.5432(5) A, b=7.7048(5) A, c=5.7965(3) A, V=336.89(6) A 3 and Z=1 . X-ray powder diffraction data are presented with a detailed discussion of the differences between the diffraction patterns of sillimanite, Mullite and Mullite-2 c . Crystals of Mullite-2 c are translucent to lightly violet, they possess a vitreous lustre and the calculated density is 3.199 g cm −3 . The optical character is biaxial ( + ), with refractive indices determined by spindle-stage microscopy of nx=1.6673 , ny=1.6687 and nz=1.680(4) (adjusted to conform to 2VZ=39(4) ∘ ). Applying the Gladstone–Dale approach, the compatibility index is 0.007, representing superior compatibility. In terms of chemical composition and structural features Mullite-2 c is an outstanding example of Mullite-type compounds falling into the postulated miscibility gap between sillimanite and Mullite. Its crystal structure combines characteristics from both Mullite (oxygen vacancies, triclusters of tetrahedral building units) and sillimanite (high degree of Si ∕ Al ordering in the tetrahedral building units, causing the doubled c parameter). The lattice parameters (normalised to 1 c ) of the new sample lie between those of sillimanite and 3 / 2 Mullite; the chemical composition is close to 3 / 2 Mullite and thus differs significantly from the silica-rich composition of the species previously determined by Fischer et al. (2015), indicating a relatively large compositional variation.
-
Mullite crystal structure and related properties
Journal of the American Ceramic Society, 2015Co-Authors: Reinhard X. Fischer, Hartmut Schneider, Jurgen SchreuerAbstract:Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2-2x)O10-x. Main members are sillimanite (x = 0), stoichiometric 3/2-Mullite (x = 0.25), 2/1-Mullite (x = 0.40), and the SiO2-free phase ι-alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal Mullite. Following a short introduction, the second part of the review focuses on the crystal structure of Mullite. In particular, the characteristic Mullite-type structural backbone of parallel chains consisting of edge-sharing MO6 octahedra and their specific cross-linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of Mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new Mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.
-
High Pressure Behavior of 7:4 Mullite and Boron-Substituted Mullite: Compressibility and Mechanisms of Amorphization
Journal of the American Ceramic Society, 2014Co-Authors: Patricia E. Kalita, Hanna Lührs, Reinhard X. Fischer, Andrew Cornelius, Kristina Lipinska, Stanislav V. Sinogeikin, Hartmut SchneiderAbstract:The high-pressure elastic properties behavior, phase stability, and mechanisms of amorphization of the alumino-silicate 7:4 Mullite and a corresponding Mullite doped with boron were investigated in situ by powder synchrotron X-ray diffraction with a diamond anvil cell in quasi-hydrostatic conditions. The samples of 7Al2O3:4SiO2 (Al4.66Si1.33O9.67), referred to as 7:4 Mullite and an alumino-silicate Mullite with 3.5(4) mol% B2O3, referred to as B-Mullite, were compressed, in small pressure steps, up to 27.8 and 28.9 GPa, respectively, and then decompressed back to ambient pressure. All along the compression path both samples' patterns are indexable with a Mullite structure. Compression data are smooth up to a threshold pressure, from which point the diffraction peaks appeared to broaden, and the refined unit cell parameters deviate significantly down from the compressional trend. Above ~23 GPa the diffraction patterns are not indexable anymore, suggesting amorphization. Rietveld structural refinements allow for a description of the pressure-induced main deformation mechanisms and structural trends. Pressure-induced mechanisms of amorphization are also discussed. A third-order Birch Murnaghan equation of state is fitted to the pressure-volume data to obtain experimental bulk moduli, as well as axial compressibilities for 7:4 Mullite and B-Mullite. Finally the volume compression in response to the applied pressure, combined with thermal expansion coefficients, allows a P–T–V equation-of-state for B-Mullite to be proposed.
-
Boron Mullite: Formation and basic characterization
Materials Research Bulletin, 2012Co-Authors: Hanna Lührs, Reinhard X. Fischer, Hartmut SchneiderAbstract:Graphical abstract: Display Omitted Highlights: ► Decrease of B-Mullite formation temperature with increasing boron content. ► Decrease of lattice parameters b and c with increasing boron content. ► Significant reduction of thermal expansion (−15%) due to incorporation of boron. ► Decomposition of B-Mullite at 1400 °C, long-term stability at 800 °C. -- Abstract: A series of boron doped Mullites (B-Mullite) was prepared from single-phase gels with initial compositions based on a 1:1 isomorphous substitution of Si by B, starting from a 3:2 Mullite composition (Al{sub 4.5}Si{sub 1.5}O{sub 9.75}). A high amount of boron (>10 mol.%) can be incorporated into the crystal structure of Mullite where it most likely replaces Si. In situ phase formation of B-Mullites was studied with high temperature X-ray diffraction and thermal analysis. A decrease of the formation temperature for B-Mullite with increasing boron content was observed. With increasing boron content lattice parameters b and c significantly decrease, while no systematic evolution of a is observed. Long annealing at 1400 °C results in decomposition of B-Mullite to boron free Mullite and α-alumina. At 800 °C B-Mullite appears to be stable over a period of at least 12 days. The mean thermal expansion coefficient was reduced by 15%more » upon incorporation of boron which makes the material technologically interesting.« less
-
Mullite precursor phases
Journal of The European Ceramic Society, 2003Co-Authors: Hartmut Schneider, B. Saruhan, D. Voll, L. Merwin, A. SebaldAbstract:Abstract Admixtures of tetraethoxysilane (TEOS) and aluminium sec.-butylate (AlOBu) with stoichiometric 3Al2O3.2SiO2 Mullite composition were used for the syntheses. Depending on the amount of H2O, the velocity of the hydrolysis process and the pH of the solvent, three different types of temperature-induced Mullite formation processes are observed. Type I Mullite precursors: produced by slow hydrolysis with very little H2O. From ≈350°C and up to ≈900°C these precursors are non-crystalline and show a homogeneous mixture on an atomic level. Above this temperature limit the precursors transform to Al2O3-rich Mullite and non-crystalline SiO2. Type II Mullite precursors: produced by rapid hydrolysis with excess H2O in a very basic environment (pH > 10). Above ≈350°C these precursors are more phasic, consisting of relatively large crystalline γ-Al2O3, and of non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C. Type III Mullite precursors: produced by rapid hydrolysis with excess H2O in a moderately basic environment (pH ≤ 10). From ≈350°C and up to ≈900°C these precursors are non-crystalline. Above this temperature limit the precursors gradually transform to γ-Al2O3 and non-crystalline SiO2-rich areas. Mullite formation is observed at ≳1200°C.
J M F Ferreira - One of the best experts on this subject based on the ideXlab platform.
-
influence of raw material type and of the overall chemical composition on phase formation and sintered microstructure of Mullite aggregates
Ceramics International, 2009Co-Authors: Ibram Ganesh, J M F FerreiraAbstract:Abstract Dense Mullite aggregates with varied (47–70%) alumina contents have been prepared by a conventional dry-powder pressing technique followed by heat treatments at temperatures in the range of 1450–1725 °C. Different types of clays, beach sand sillimanite (BSS) and a high purity aluminium hydroxide were used as starting materials. Mullites derived from BSS consisted of equi-axed grains whereas those obtained from clay containing precursor mixtures exhibited elongated grains. The bulk density (BD), apparent porosity (AP) and water absorption (WA) capacity of sintered Mullites were found to be strongly influenced by the pre-mullitization step of the precursors and in a less extent by the type of raw material, its hydration degree and the impurity contents of Fe 2 O 3 , CaO and Na 2 O. Mullite aggregates obtained from the three different types of aluminosilicate raw materials (i.e., ball clay, china clay and beach sand sillimanite) through a double-stage heat treatment process exhibited better sintered properties in terms of bulk density, apparent porosity, water absorption capacity and higher Mullite contents in comparison to those obtained following a single-stage firing process.
Mehmet Sarikaya - One of the best experts on this subject based on the ideXlab platform.
-
Mullite for structural electronic and optical applications
Journal of the American Ceramic Society, 1991Co-Authors: Ilhan A. Aksay, Daniel M. Dabbs, Mehmet SarikayaAbstract: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 (
-
Mullite for Structural, Electronic, and Optical Applications
Journal of the American Ceramic Society, 1991Co-Authors: Ilhan A. Aksay, Daniel M. Dabbs, Mehmet SarikayaAbstract: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 (
Krishan K. Chawla - One of the best experts on this subject based on the ideXlab platform.
-
interface engineering in Mullite fiber Mullite matrix composites
Journal of The European Ceramic Society, 2008Co-Authors: Krishan K. ChawlaAbstract:Mullite fiber/Mullite matrix composites are attractive because of their inherent oxidation resistance at high temperatures. Mullite has better creep resistance than alumina. However, chemical interactions between oxides are often very severe; with the result no gain is made over monolithic Mullite in terms of toughness. Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Thus, the microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. This body of work on Mullite/Mullite composites shows the feasibility of producing fully dense, tough oxide/oxide composites by interface engineering. Coatings such as BN alone or SiC/BN double coating function effectively for Mullite fiber/Mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. It would appear that for use at high temperatures in air, one needs to identify structural analogs of BN among oxides.
-
Interface engineering in Mullite fiber/Mullite matrix composites
Journal of The European Ceramic Society, 2007Co-Authors: Krishan K. ChawlaAbstract:Mullite fiber/Mullite matrix composites are attractive because of their inherent oxidation resistance at high temperatures. Mullite has better creep resistance than alumina. However, chemical interactions between oxides are often very severe; with the result no gain is made over monolithic Mullite in terms of toughness. Even in the absence of chemical bonding, a strong mechanical bond component may be present. This originates from radial compressive stress due to thermal expansion mismatch and/or the surface roughness of interface. Thus, the microstructure and behavior of the interface region are the key factors in obtaining an effective control of damage in composites and enhancement of toughness. This body of work on Mullite/Mullite composites shows the feasibility of producing fully dense, tough oxide/oxide composites by interface engineering. Coatings such as BN alone or SiC/BN double coating function effectively for Mullite fiber/Mullite matrix composites in that they provide a nonbrittle fracture and increased work of fracture at room temperature. It would appear that for use at high temperatures in air, one needs to identify structural analogs of BN among oxides.
-
Processing, structure, and properties of Mullite fiber/Mullite matrix composites
Journal of The European Ceramic Society, 1999Co-Authors: Krishan K. ChawlaAbstract:Abstract Oxide fiber/oxide matrix composites form an important and very attractive subpart of ceramic matrix composites because of their inherent stability in oxidizing atmospheres at high temperature. In particular, Mullite fiber/Mullite matrix composites have the potential of high temperature usage in oxidizing atmospheres. The interface in Mullite fiber/Mullite matrix was engineered by using thick BN (1 μm) or BN/SiC double coating on Mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the room temperature mechanical properties of these Mullite fiber/Mullite matrix composites could be achieved by incorporation of these interfacial coatings and by using a colloidal processing route to make dense Mullite matrix.
-
Interfacial characteristics of Mullite fiber/BN coating/Mullite matrix composites
1994Co-Authors: Krishan K. ChawlaAbstract:The interface in Mullite fiber Nextel 480/Mullite matrix was engineered by using a thick BN (1 {mu}m) coating on Mullite fibers, such that deformation mechanisms conducive to toughness enhancement could be brought to play. Significant improvements in the mechanical properties of these Mullite fiber/Mullite matrix composites could be achieved by incorporation of BN interfacial coating and by using a colloidal processing route to make dense Mullite matrix. An interfacial testing system with a flat-bottomed, diamond indenter was used to obtain the interface characteristics. Using a progressive debonding of the interface model, it was determined that the average interfacial frictional sliding shear stress in this composite was about 50 MPa. Fracture surfaces of these BN coated composites obtained in flexure test showed fiber pullout.
Ramón Torrecillas - One of the best experts on this subject based on the ideXlab platform.
-
Mullite refractory metal mo nb composites
Journal of The European Ceramic Society, 2008Co-Authors: J S Moya, Marcos Gonzalez Diaz, C F Gutierrezgonzalez, Ramón Torrecillas, Luis A Diaz, Jose F BartolomeAbstract:Abstract Two refractory metals were chosen as a second phase to fabricate Mullite-metal composites: (a) molybdenum which is thermodynamically compatible with Mullite up to 1650 °C and (b) niobium, a metal which is solid state incompatible with Mullite giving rise to compounds such as NbO and Nb5Si3 which are both electrical conductors and more oxidation resistant than the Nb metal. The Mullite-refractory metal electrically conductive composites with a content of metal (30–50 vol%) were obtained by a wet-processing route and subsequent hot pressing at 1650 °C. The effect of Mo particle size and volume fraction on the mechanical properties (σf, damage resistance, crack growth resistance i.e., R-curve behaviour, etc.) and wear resistance of the Mullite–Mo composites have been assessed. Additionally these composites could be machined by electro discharge machining (EDM) technique. In the case of Mullite–Nb system Mullite–Al2O3–Nb5Si3–NbO composites showed electrically conducting properties. Its microstructural feature, mechanical properties (σf and R-curve behaviour) and EDM were studied and compared with the results obtained for the Mullite–Mo system.
-
Thermomechanical behavior of a zircon–Mullite composite
Ceramics International, 2007Co-Authors: M. Hamidouche, Ramón Torrecillas, N. Bouaouadja, Gilbert FantozziAbstract:Abstract A zircon–Mullite composite was made by reaction sintering from a mixture of Mullite and zircon powders. The thermomechanical characterization (strength and toughness) revealed an intermediate behavior between Mullite and zircon. The composite presents a high cooling thermal shock resistance for temperatures between the ambient and 1000 °C. Bending creep tests were made at temperatures between 1100 and 1300 °C using stresses from 10 up to 90 MPa. The stress exponent value is between 2 and 3 while the activation energy varies from 280 up to 900 kJ mol −1 between 1000 and 1300 °C. Microscopic observations suggest an intergranular creep mechanism. The grain interface forces between Mullite and zircon are more important than those of zircon–zircon grains. The incorporation of Mullite particles in a zircon matrix produces a composite that has both the good Mullite behavior toward creep and the high thermal shock resistance of zircon.