Showing papers on "Mullite published in 1999"

Development of Textured Mullite by Templated Grain Growth

Abstract: Highly textured mullite was obtained by enhancing anisotropic grain growth by TiO 2 doping and by templating grain growth on oriented acicular mullite seed particles in a mullite precursor. Upon heating, the mullite precursor crystallized and densified to an equiaxed microstructure of 1-2 μm mullite grains at which time the mullite seed particles grew rapidly in the length direction (c-axis) to produce a highly textured microstructure. By changing the seed (template) particle concentration, a range of oriented microstructures and anisotropic grains could be produced.

Suitability of Mullite for High Temperature Applications

Abstract: High temperature mechanical behaviour of mullite has been studied. Our study include tensile, flexural and compressive creep behaviour and fracture up to 1400 °C. The results obtained in creep are analysed and compared with previous work in the literature. Two regions with different behaviour can be distinguished. The creep rates in bending, tension and compression are very similar in the first region at low stresses and temperatures. It is shown that in this region creep takes place by accommodated grain boundary sliding assisted by diffusion. At higher stresses slow crack growth from defects present in the sample occurs. The stress at which this transition in the deformation mechanism happens is dependent on several factors, the loading system during testing, the grain size, the amount and distribution of glassy phase and the environment. It is claimed the existence of a network of mullite–mullite grain boundaries free of glassy phase associated to the low surface energy of [001] planes. The diffusion rate through these boundaries controls the creep rate, and explains the high creep resistance of mullite. The results presented in this work lead to the conclusion that the mechanism controlling high temperature deformation resistance of mullite materials in a wide range of stress–temperature working conditions is independent of the glassy phase content. Slow crack growth limit the use of mullite at high stresses and temperatures.

Fabrication of low thermal expansion, high strength cordierite structures

Abstract: The present invention is directed at sintered ceramic articles exhibiting a crystalline phase assemblage comprising, by weight, of 65-95 % cordierite and 5-35 % of a secondary phase selected from the group consisting of mullite, magnesium-aluminate spinel, and sapphirine and having a bulk analytical composition consisting esentially of about, by weight, 32-51 % SiO2, 35-49 % Al2O3, 7-16% MgO. Furthermore, the ceramic articles exhibit an effective strength of greater than about 3000, a CTE of less than about 15x10-7/°C, over the temperature range of 25 °C to 1000 °C, and a total intrusion porosity, as measured by a Hg intrusion method, of at least 20 %. This invention also relates to a method for producing a sintered ceramic article having the aforementioned cordierite and secondary minor phase mixture.

Influence of time and temperature on reactions and transformations of muscovite mica

Abstract: Members of the mica group of clay minerals are important constituents of clays used as raw materials for the production of bricks, tiles, and clay pipes. Using muscovite, K2 Al4 (Si6 Al2 )O20 (OH)4 ), as the pure end member representative of the mica group, their thermal behaviour has been determined and is expressed as a time–temperature–transformation (TTT) diagram. Water loss occurs over a prolonged temperature range with no discrete event resulting in significant weight loss. The thermal decomposition of muscovite is dominated by the presence initially of the three sheet types which make up the layered mica structure. Initially γ-Al2O3 and then corundum form from the octahedrally co-ordinated aluminous sheet which is sandwiched by two tetrahedral sheets. With potassium from the interlayer site, the tetrahedral sheets break down to form a melt, from which leucite crystallises. Topotactic influences of this type account for the failure of muscovite to produce potassium feldspar and mullite as pr...

High temperature single crystal properties of mullite

Abstract: Extensive neutron diffraction and Rietveld studies of dense, hot pressed mullite (3Al 2 O 3 ·2SiO 2 ) have been conducted up to 1650 °C in air, yielding a complete set of lattice parameters and axial thermal expansion coefficients. Unconstrained powders of the same stoichiometric composition were also analyzed by X-ray diffraction and Rietveld techniques up to 900 °C in air, from which lattice parameters and thermal expansion coefficients were obtained. An earlier reported structural discontinuity was confirmed by XRD to lie in the temperature range 425 to 450 °C. Single-crystalline mullite fibers of composition 2·5Al 2 O 3 ·SiO 2 were grown from the melt by a laser-heated, float zone method. A partial set of the single-crystal elastic moduli were determined from various sections of fiber, by Brillouin spectroscopy, from room temperature up to 1400 °C. They indicated a roughly 10% reduction in stiffness over that temperature range.

Semiclosed-Cell Mullite Foams: Preparation and Macro- and Micromechanical Characterization

Abstract: The preparation and characterization of the properties of mullite ceramic foams suitable for diesel exhaust filtration systems are described. Two foams of different cell sizes, 32 and 61 ppi (pores per inch), were prepared from slurries by the replication process with polyurethane sponges as the templates. The microstructure and the dependence of the porosity and the mechanical properties on the preparation and resulting foam cell size and normalized density are discussed. In addition the micro- and macromechanical properties have been characterized using instrumented indentation techniques; with sharp (Berkovich) and small spherical-tipped indenters the hardness and modulus of the dense strut materials have been measured. The macroproperties have been measured with a large sapphire indenter, ∼5 mm radius using a load partial-unloading technique. The latter enables the modulus and the contact pressure versus penetration to be measured. These values are compared with traditional bulk measurements for crushing strength and elastic modulus. The results are discussed in terms of available theoretical treatments of the indentation of brittle porous materials.

Photoacoustic thermal characterization of kaolinite clays

Abstract: The usefulness of photoacoustic techniques in characterizing the thermal properties of kaolinite clays, which are a very abundant mineral in some regions of Brazil, was investigated. Different behavioural patterns are observed below and above a heating temperature of 950 °C and can be attributed to the change from metakaolinite to an amorphous three-dimensional mineral which crystallises to mullite and crystobalite above 1200 °C. The implications of the results in the manufacture of clay bricks are briefly considered. This is important in tropical regions where a knowledge of the thermal properties of materials used in the building construction industry is highly desirable.

Production of porous mullite bodies

Abstract: Porous sintered ceramic materials having mullite as its primary phase and a method for producing them. The method includes preparing a plasticizable raw material mixture comprised, by weight, of 75 to 99% pre-reacted mullite powder, and 1.0 to 25% of a water-swelling clay, adding an organic binder system to the mixture and mixing the mixture to form an extrudable mixture, and extruding the mixture to form a substrate of the desired configuration. The green body is dried and fired for a time and at temperature sufficient to form a sintered mullite structure having a narrow through-pore size distribution comprised of pores exhibiting an average intrusion-pore size of between about 2-15 μm, and a total intrusion porosity, as measured by Hg intrusion method, of at least 30%.

Low temperature pressureless sintering of sol-gel derived mullite

Abstract: Mullite of stoichiometric composition has been synthesized by sol-gel process. Aluminium nitrate and ethyl silicate are used as the starting materials to prepare the single phase mullite powder. DTA shows that the mullitization starts at 971°C. It is observed from XRD analysis that the amorphous mullite completely transforms to orthorhombic crystalline mullite at 1300°C. Wet milling operation has enhanced the sintering density of pure mullite by 12% more than that of dry milling. The naturally available raw material, clay has been identified as one of the suitable sintering aids for mullite. The addition of clay to the mullite powder enhances the density up to 95% of TD even at a very low temperature, 1450°C by pressureless sintering. A viscous flow sintering mechanism during densification is one of the main criteria to achieve good densification.

The effect of water vapour atmospheres on the thermal transformation of kaolinite investigated by XRD, FTIR and solid state MAS NMR

Abstract: The dehydroxylation of kaolinite and its subsequent transformation to cubic Al-spinel and mullite has been studied under water vapour atmospheres up to 0·8 atm pressure. Dehydroxylation, as determined by XRD, FTIR and solid state MAS NMR, can be accomplished at low temperatures (ca 500°C) under water vapour atmospheres, the efficiency increasing with increasing water vapour pressure. The subsequent thermal transformations to crystalline products are also facilitated by water vapour atmospheres, which also enhance the mechanical properties of the fired kaolinite (crushing strength and softening coefficient) by 3–4 times. These effects are discussed in terms of proton attack on the Si–O bonds followed by polycondensation of the resulting silanol groups to siloxane, facilitating the nucleation of mullite from amorphous aluminosilicate and strengthening the chemical bonds between the grains.

Electrical resistivity of transition metal ion doped mullite

Abstract: The electrical resistivity of pure mullite (3Al 2 O 3 .2SiO 2 ) varies from 10 13 ohm-cm at room temperature (r.t.) to 10 4 ohm-cm at 1400 °C. It was observed that by doping mullite with the 3d-type transition metal ions, e.g. Mn, Fe, Cr and Ti, the resistivity of mullite could be reduced to 10 11 ohm-cm, i.e. 1/100 that at r.t. and 1/5 that at 1400 °C. The resistivity of doped and undoped mullite decreased by 6–5 orders at about 500–600 °C but 4–3 orders between this temperature and 1400 °C. The 3d orbital electrons, the oxidation states and the concentration of the transition metal ions as well as the sites of mullite lattice occupied by the ions were found responsible for lowering of resistivity of mullite. Evidence of the presence of Mn 2+ , Mn 3+ , F e 3+ , Cr 3+ and Ti 4+ ions in mullite had been obtained which entered the octahedral site. The Ti 4+ ion which substituted Al 3+ ion in the octahedral site of mullite structure appeared to be the most efficient one to reduce the resistivity. This has been confirmed by the results of activation energy of resistivity/band gap energy, Eg which was the lowest for mullite doped with 1·0 wt% Ti 4+ ion. At 1·0 wt% concentration level, these ions lowered the resistivity of mullite to minimum.

Low sintering temperature cordierite batch and cordierite ceramic produced therefrom

Abstract: The invention is directed at sintered ceramic articles exhibiting a primary crystalline phase of cordierite and an analytical oxide composition, in weight percent, of 44-53 % SiO2, 30-38% A12O3, 11-16% MgO and 0.05 to 10% of a metal oxide. The ceramic article exhibits a coefficient of thermal expansion in at least one direction no greater than about 15.0x10-7/°C over the temperature range of about 25°C to about 800°C. The sum of the weight percentages of residual mullite, corundum, and spinel, as measured by X-ray diffractometry of the crushed and powdered body, is not greater than 15%. Furthermore, the invention discloses a method of producing the aforementioned ceramic body, comprising (a) selecting a raw material batch mixture for forming the cordierite ceramic body, comprising two or more compounds which serve as an alumina source, a silica source and a magnesia source, and at least one metal oxide source in an amount to result in the cordierite body comprising, on an analytical oxide basis, in weight percent, between about 0.05-10% of the metal oxide; (b) adding an organic binder system to the inorganic mixture and forming the mixture into a green body; (c) drying the green body and thereafter firing the green body at a time and at a temperature no greater than about 1300°C to result in sintered ceramic body. Preferred metal oxide sources include the oxides or oxide-forming compounds of the metals selected from the group consisting of molybdenum, tungsten, bismuth, copper, yttrium, lanthanide metals and boron.

Fabrication and mechanical properties of SiC platelet reinforced mullite matrix composites

Abstract: Hot-pressing of mullite and SiC–mullite matrix composites was performed at temperatures and pressures between 1500 and 1650°C and 5 and 15 MPa, respectively. Composites were produced using different precursors; sol–gel derived mullite and kaolinite/α-alumina. The precursor did not strongly affect the optimum density achieved, reaching 97·5% of theoretical for a 20 vol% SiC addition in both cases. The SiC platelet addition impaired densification kinetics in all composites compared to mullite monoliths. Fracture toughness, measured by the indentation strength in bending technique, was marginally higher for the sol–gel precursor material in both monolith and composite. Fracture toughness increased with SiC content for both materials. For example, for the sol-gel precursor material it increased from 2.9±0.1 MPa m1/2 for the monolith to 3.9±0.1 MPa m1/2 for the 20 vol% SiC composite. Similarly, hardness increased with SiC addition for both materials, but the hardness of the sol–gel material was greater than that of the kaolinite+α-alumina material for all compositions. The relationship between microstructure and mechanical properties is discussed.

Crack Healing Behavior and High Temperature Strength of Mullite/SiC Composite Ceramics.

Abstract: A mullite/SiC composite ceramic was sintered. Four point bend tests were conducted according to JIS standard. Semi-elliptical surface crack of 100 or 200μm in diameter was made on the specimen. For four kinds of specimens (asreceived, heal treated, pre-cracked and heal treated cracked), crack healing behavior and high temperature strength were tested systematically. The main conclusions were obtained as follows: (a) Mullite/SiC composite ceramic has ability to heal crack. (b) Best healing condition was found to be 1300°C, in air for 1h. (c) Maximum crack size to be healed is semi-elliptical crack of 100μm in diameter. (d) Crack healed part has enough strength up to 1100°C and most specimen failed outside the pre-cracked zone and healed crack is not sensitive to static fatigue.

Structural development of single phase (type I) mullite gels

Abstract: Single phase (type I) mullite gels were prepared by sol-gel techniques starting from alkoxides (Al-butylate, tetraethylorthosilicate) and alkoxides plus nitrates (tetraethylorthosilicate, Al(NO3)3·9H2O). After drying at 150°C the aluminosilicate gels are non-crystalline and remain so up to ≈ 900°C. Above 900°C the gels transform into Al2O3-rich mullite plus a coexisting SiO2 phase. Structural studies on temperature-dependent dehydroxylation and condensation of the gels were carried out by large angle X-ray scattering, by infrared spectroscopy and by29 Si NMR spectroscopy. Heat-treatment (<150°C) of dried gels first causes removal of the H2O and organic residuals weakly bound at the open pore surfaces of the gels while the stronger, structurally bound OH groups are not affected. At temperatures <600°C OH groups are released and recombine to molecular H2O. If the temperature does not exceed 800°C the newly formed H2O is trapped in closed nanopores of the gel-network. Corresponding electron microscopical investigations reveal agglomerates of ≈10 nm sized primary particles virtually unaffected by the heat-treatment below 900°C. NMR investigation provided a new structural model on type and distribution of coordination polyhedra in aluminium silicate gel networks. Unlike Si, which according to 29Si NMR is always 4-fold coordinated with O, 27Al NMR spectroscopy revealed that Al cordination is more complex and is influenced by thermal treatment. Al occurs six-fold (octahedrally) and four-fold (tetrahedrally) coordinated. A third 27Al NMR signal which has been attributed to five-fold-coordinated Al in the literature increased in intensity with the heat-treatment. A comparison of NMR data of the gels with those of mullite suggests that tetrahedra triclusters (3 tetrahedra having one oxygen atom in common) occur as major structural units in aluminium silicate gels rather than five-fold-coordinated Al. Triclusters of tetrahedra may compensate the excess negative charge in the network caused by Si4+ → Al3+ substitution. The charge compensation model is supported by aluminosilicate gels doped with network modifiers (e.g., Na+). Since equimolar addition of Na+ compensates Si4+ → Al3+ substitution the formation of triclusters is no longer required which actually can be deduced from27 Al NMR studies.

A process for sintering of MgO and CaO based ceramics

Abstract: MgO, CaO and Doloma (phase mixture of MgO and CaO, obtained from the mineral dolomite) based materials are attractive steel-making refractories because of their potential cost effectiveness and worldwide abundance. The study of the sintering and the important properties of these materials is well documented [1–6]. In order to improve the corrosion and hydration resistance of these refractories, high density bodies are recommended [2]. To increase the density of sintered bodies, additions of SiO2, Fe2O3, Al2O3 and their fluxing agents are usually used. Nevertheless, these additions degrade the refractoriness, slag resistance and the hot strength [3, 4]. An alternative process is proposed, as a result of a substantial study, to increase the density of MgO and/or CaO based ceramics without the addition of lower melting constituents. This process consists of activating and minimizing the average particle size of doloma powders, before sintering. In this work, the sintering of CaO, MgO and doloma activated powders according to the proposed process is presented. Moreover, the addition effect of these activated powders on the sintering of aluminum silicate based ceramics such as mullite and kaolin is also studied. The domestic limestone and dolomite raw materials, derived from local regions (Constantine and Batna regions), were used as starting materials. Their quantitative analysis, using atomic absorption spectroscopy and X-ray fluorescence showed that the purity of these raw materials is about 99.5 wt %. Doloma contains, mainly, 0.27 wt % Fe2O3, 0.07 wt % Al2O3 and 0.02 wt % Na2O, whereas limestone contains, mainly, SiO2 (0.11 wt %), MgO (0.09 wt %), Al2O3 (0.04 wt %) and (K, Na)2O (0.07 wt %). The starting materials were subjected to calcination at 900 ◦C followed by crushing. Then, three sintering processes were applied, as shown in Fig. 1 (routes 1, 2 and 3). The first route is usually used to produce MgO, CaO and doloma based refractories, even though it requires a high sintering temperature (>1600 ◦C); it is called process 1. The second and the third ones are the proposed processes used in this work. Individual specimens of each of the studied types were die-pressed at 75 MPa to form sample disks (13 mm in diameter and approximately 7 mm in thickness). Afterwards, they were fired on a platinum crucible foil, in air at relatively lower sintering temperatures, with a heating rate of 10 ◦C/min. The preselected heating times at these temperatures were from 10 to 720 min. The bulk densities of the sintered samples were simply determined by geometrical and weighing measurements. The relative density, which is the ratio between the measured and the theoretical density was then calculated. More detailed preparation conditions are well described elsewhere [7, 8]. Fig. 2 illustrates the results of doloma densification, using the 3 processes shown in Fig. 1. In the case of process 1, only 43% of the theoretical density can be obtained at 1200 ◦C. However, a density level in excess of 90% of theoretical is achieved for the highly hydrated doloma (process 3) while the partially hydrated doloma (process 2) achieved only a low sintered density, i.e. lower than 58% of theoretical. Even when increasing the sintering time to 6 h, such a density could not be exceeded. This can be explained by the influence of the non hydrated MgO particles’ morphology on particle packing and particle-particle interaction. Changing the particles’ morphology by total hydration of MgO (process 3) leads to a considerable increase in densification. A previous study carried out on doloma [9] showed that the particles’ morphology change as well as their agglomeration are closely related to the process by which doloma refractory is prepared (processes 1 and 3). The last process gives much finer particles than the dead-burned aggregates and it is expected to fill the interstitial voids (porosity) between the packed aggregate particles and thus increase the green density prior to firing. Nevertheless, the green densities for the three processes are very close and approach 36% of the theoretical. This indicates that, rather than the voids ratio, the role of their morphology and distribution is the determinant factor. As far as lime sintering is concerned, density levels as high as 90% of theoretical were achieved for powder, activated according to process 2 (Fig. 3) [8, 10]. At firing temperatures up to 1400 ◦C, such a density cannot be exceeded even when the sintering time is increased. For the non-treated lime (process 1), however, only 75% of theoretical density was achieved. Furthermore, the density of non-treated lime increased with holding time, while the density of the treated lime tended to saturate with sintering time. The density level of 90% of theoretical can also be achieved for the treated lime at a firing temperature as low as 1250 ◦C but for a relatively longer holding time (Fig. 3). Following process 1, this level (90%) is not achieved even when lime samples were sintered at higher temperatures (1600 ◦C) [11]. Moreover, the sintering of MgO powder, prepared from doloma and activated according to process 3 is

Oxygen Self‐diffusion in Cylindrical Single‐Crystal Mullite

Abstract: A mixture of SiO 2 and Al 2 O 3 was prepared from metal alkoxides and fired at 1650°C to form mullite and an SiO 2 -rich glass phase. After leaching out the glass phase, crystalline mullite powder was obtained. Transmission electron microscopy examination revealed the powder to be cylindrical particles of single-crystal mullite. A diffusion experiment was performed on the mullite to determine the oxygen diffusion coefficient. This could be expressed as D ox = 1.32 × 10 -6 exp[-397 kJ/RT] m 2 s -1 in the range of 1100 to 1300°C. This falls along the line extended through the oxygen diffusion coefficient of forsterite, which is similar in crystal structure to mullite.

Crystallization kinetics of the aluminum silicate glass fiber

Abstract: The crystallization behavior and kinetics of aluminum silicate glass fiber (Al2O3 79 wt%) have been investigated using a non-isothermal DTA method The precipitation phase of the aluminum silicate glass fiber is mullite, the glass transformation temperature is 1451∼1472 K, the crystallization reaction begins to occur from 1516∼1536 K, and the temperature at maximum precipitation is 1538∼1568 K under the experimental conditions The crystallization kinetic parameters were investigated using several kinetic models and the optimal kinetic parameters for crystallization process of the aluminum silicate glass fiber are Ee=652 KJ/mol, n=161, k0=498×1019 (s−1)

Nucleation mechanisms in chemically vapor-deposited mullite coatings on sic

Abstract: Dense, uniform, and adherent chemically vapor-deposited mullite coatings were deposited on SiC substrates using the AlCl{sub 3}{endash}SiCl{sub 4}{endash}H{sub 2}{endash}CO{sub 2} system. Typical coating morphology consisted of a thin interfacial layer of {gamma}{endash}Al{sub 2}O{sub 3} nanocrystallites embedded within a vitreous SiO{sub 2}-based matrix. When a critical Al/Si ratio of 3.2{plus_minus}0.29 was reached within this nanocrystalline layer, mullite crystals nucleated and grew as columnar grains. The thickness of the nanocrystalline layer decreased as the input AlCl{sub 3}/SiCl{sub 4} ratio was increased. In all cases, the Al/Si composition in the coating increased from the coating/substrate interface to the coating surface. Critical factors leading to the nucleation and growth of mullite crystals are discussed in this article. {copyright} {ital 1999 Materials Research Society.}

Study of ceramic coatings formed by microarc oxidation on Al matrix composite surface

Abstract: Research was conducted on the microstructure, composition, and properties of a ceramic coating deposited on the surface of an aluminium matrix composite (10Al2O3f –ZL109) using microarc oxidation. The influence of different matrixes and reinforcements on the microstructure, properties, and deposition parameters of the ceramic coatings was studied. Both factors showed considerable effects, though the matrix composition had more influence. The results also showed that mullite ceramic coatings can be successfully deposited on the surfaces of aluminium matrix compos ites. The coating was composed of two layers, a porous layer and a dense layer.

Growth and crystallization of YAG- and mullite-composition glass fibers

Abstract: This paper describes a new process to synthesize crystalline oxide fibers for high temperature structural applications. Strong and chemically homogeneous precursor fibers of 5–40 μ m diameter were made at rates of up to 1·6 m s −1 by glass fiber pulling techniques from undercooled molten oxides. The precursor fibers were heat treated at temperatures up to 1873 K to make crystalline fibers with controlled grain size and properties. Tensile strengths of the precursor fibers were up to 5–6 GPa (900 ksi) for YAG- (Y 3 Al 5 O 12 ) and mullite- (Al 6 Si 2 O 13 ) compositions. Research to optimize fiber compositions and crystallization processes, and to scale up precursor fiber production is discussed.