Showing papers on "Mullite published in 1995"

New generation of plasma-sprayed mullite coatings on silicon carbide

Abstract: Mullite is promising as a protective coating for silicon-based ceramics in aggressive high-temperature environments. Conventionally plasma-sprayed mullite on SiC tends to crack and debond on thermal cycling. It is shown that this behavior is due to the presence of amorphous mullite in the conventionally sprayed mullite. Heating the SiC substrate during the plasma spraying eliminated the amorphous phase and produced coatings with dramatically improved properties. The new coating exhibits excellent adherence and crack resistance under thermal cycling between room temperature and 1000 to 1400 C. Preliminary tests showed good resistance to Na2CO3-induced hot corrosion.

Kinetic study of the kaolinite-mullite reaction sequence. Part II: Mullite formation

Abstract: The present work is a follow-up of the investigation on the decomposition reaction of kaolinite as a function of the defectivity of the starting material and the temperature of reaction. In the present work we study the high temperature reaction of mullite synthesis from kaolinite, from the starting point of the results obtained in the first part. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation in isothermal conditions. The apparent activation energy for mullite nucleation and growth is found to be related to the defective structure of the starting kaolinite, which thus must have an influence on the chemical homogeneity of the amorphous intermediate phase. The analysis of the kinetic data indicate that the initial reaction mechanism is controlled by mullite nucleation, while as the reaction proceeds it shifts towards a grain growth-limited process which is intermediate between phase boundary and diffusion controlled. The order of the reaction obtained from standard analysis of the isothermal kinetic data is lower in the case of the ordered kaolinite KGa-1, in agreement with a rate limiting process more strongly limited by diffusion. For each sample there is a small but significant decrease in the order of the reaction at higher temperature: we interpret the change as related to the variation of the diffusion process in the amorphous phase due to the growing grains of mullite and cristobalite. The values of the activation energies and induction times are comparable neither to a model of mullite formation from a monophasic gel, nor mullite formation from a diphasic gel, being intermediate between the two. We can infer that the amorphous precursors from natural kaolinites can be considered pseudo-monophasic gel-like phases, approaching the monophasic gel-like behaviour as the defectivity of the initial kaolinite increases.

Apatite-mullite glass-ceramics

Abstract: Two series of fluoro-alumino-silicate glasses, one with varying phosphate content (P2O5) and one with fixed phosphate content and varying fluorite (CaF2) content have been investigated as potential bioglass-ceramics. Compositions with intermediate phosphate contents that contained fluorite crystallized to fluoroapatite and mullite. Compositions with high fluorite contents exhibited a low liquidus temperature, were readily castable and crystallized by a bulk nucleation mechanism. The fluoroapatite phase with appropriate heat treatment formed as thin needle-like crystals with a high aspect ratio.

Processing of Silicon Carbide‐Mullite‐Alumina Nanocomposites

Abstract: Nanocomposite materials in the form of nanometer-sized second-phase particles dispersed in a ceramic matrix have been shown to display enhanced mechanical properties. In spite of this potential, processing methodologies to produce these nanocomposites are not well established. In this paper, we describe a new method for processing SiC-mullite-Al2O3 nanocomposites by the reaction sintering of green compacts prepared by colloidal consolidation of a mixture of SiC and Al2O3 powders. In this method, the surface of the SiC particles was first oxidized to produce silicon oxide and to reduce the core of the SiC particles to nanometer size. Next, the surface silicon oxide was reacted with alumina to produce mullite. This process results in particles with two kinds of morphologies: nanometer-sized SiC particles that are distributed in the mullite phase and mullite whiskers in the SiC phase. Both particle types are immersed in an Al2O3 matrix.

Synthesis of a Mullite Precursor from Aluminum Nitrate and Tetraethoxysilane via Aqueous Homogeneous Precipitation: An 27Al and 29Si Liquid‐ and Solid‐State NMR Spectroscopic Study

Abstract: A simple aqueous process is described for the preparation of aluminosilicate colloids and chemically homogeneous mullite precursor gel. Starting from a solution of aluminum nitrate and silicic acid, aluminum is slowly hydrolyzed at 80–100°C by in situ generation of ammonia. A silica gel is rapidly made, probably by a catalytic effect of urea, the base generator. This gel is then slowly digested by partially hydrolyzed aluminum species which break the Si-O-Si bonds and link to the gel by Si-O-Al bonds. Progressively a clear colloidal sol is obtained and the colloidal particle size continues to decrease toward aluminosilicate species where the silicon atoms are in a single environment and may be linked to three hexacoordinated aluminum atoms and a hydroxyl group, by reference to natural imogolite. When the hydrolysis of aluminum is nearly complete, these particles are cross-linked and a final gel precursor of mullite is obtained. This gel is chemically very homogeneous and crystallizes to mullite at 980°C. The structural evolution, from the first gel to the ceramic, has been followed by 27Al and 29Si liquid- and solid-state MAS NMR spectroscopy.

Constitution of Mullite Glasses Produced by Ultra-rapid Quenching of Plasma-sprayed Melts

Abstract: Spherical shaped spray-dried admixtures of chemical pure and very fine (Y-AI,‘O~ and quartz powders with mullite composition (72 wt% Al,O,, 28 wt% SiO,) were used as starting materials. The spray-dried powders (IO-100 km) were melted in a nitrogen plasma flame and subsequently quenched in water thus producing spherical, hollow, and porous particles (I 100 pm). The as-quenched spherules consist of mullite glass, some residual ~Alz0, and quartz, and a very low amount of newly formed mullite. Double quenching of the material increases the glass content to >90 wt. %. 27Al and 29Si MAS NA4R studies show that the rapidly quenched mullite glass is composed of a network of (SiO)-tetrahedra and (AlO)-octahedra, -pentahedra, and -tetrahedra. The frequency distribution oj‘ (AlO)-structural units is similar to those in metakaolinite, type I (polymer) mullite precursors, and ‘in other melt-quenched aluminium-silicate glasses suggesting strong structural similarities of these phases. This has been supported by the exothermic mullite crystallization process taking place at e 980°C in all cases.

Effect of Seeding on Phase Development, Densification Behavior, and Microstmcture Evolution in Mullite Fabricated from Microcomposite Particles

Abstract: An investigation was carried out concerning the effect of mullite seed particles on phase development, densification behavior, and microstructure evolution in powder compacts prepared with silica/alumina microcomposite particles. The incorporation of ∼2 wt% seed particles in the microcomposite powder compacts had relatively little effect on densification, but resulted in significant decreases in the temperature for mullite formation and the grain sizes in mullitized samples. Samples could be sintered to almost full density and subsequently converted to mullite with average grain sizes ≤0.4 μm at temperatures in the range of 1300°-1400°C. The available evidence indicated that mullite formation occurred primarily by nucleation and growth in the siliceous matrix phase.

Transmission electron microscopy study of Al/Al2O3 composites fabricated by reactive metal infiltration

Abstract: The microstructure of Al/α-Al2O3 composites made by infiltrating molten Al into dense mullite preforms has been characterized using transmission electron microscopy. The growth of the Al/Al2O3 composites was found to proceed through three stages. Initially, Al infiltrates into a dense mullite preform through grain boundary diffusion, and reacts with mullite at grain boundaries to form a partial reaction zone. Then, a complete reaction takes place in the reaction region between the partial reaction zone and the full reaction zone to convert the dense mullite preform to a composite of α-Al2O3 (matrix) and an Al-Si phase (thin channels). Finally, the reduced Si from the reaction diffuses out of the Al/Al2O3 composite through the metal channels, whereas Al from the molten Al pool is continuously drawn to the reaction region until the mullite preform is consumed or the sample is removed from the molten Al pool. Based on the observed microstructure, infiltration mechanisms have been discussed, and a growth model of the composites is proposed in which the process involves repeated nucleation of Al2O3 grains and grain growth.

Cristobalite Phase Formation in Glass/Ceramic Composites

Abstract: The effect of several different aluminum-containing ceramic additions to borosilicate glass on suppressing cris-tobalite precipitation has been examined. The results showed that mullite or aluminum nitride suppresses cristo-balite formation more effectively than alumina or spinel. Although both follow a simple rule of mixtures, glass/mullite composites can be fabricated with lower dielectric constants than glass/alumina composites, while maintaining a thermal expansion coefficient close to Si. Electron micro-analysis using X-ray energy dispersive spectroscope showed that the measured interdiffusion coefficient between alumina and glass is in good agreement with the data which have already been published.

Mechanical Behavior of Mullite Fiber Reinforced Aluminum Alloy Composites

Abstract: Discontinuously reinforced aluminum alloys are viewed as candidate materials for elevated temperature applications because of their attractive high temperature strength properties and wear resistance. The elevated temperature elastic properties and the failure characteristics in relation to the preform flaws, however, have not received much attention in spite of their potential significance. These issues are studied for an aluminum-silicon alloy reinforced with mullite discontinuous fibers, fabricated using the squeeze infiltration technique. The effect of preform flaws (shot) on room temperature strength and ductility is investigated for composites seeded with different amounts of shot. The Young's modulus of the composite exceeds that of the unreinforced alloy over a wide range of temperatures, and the beneficial influence of the fibers is especially significant at elevated temperatures. The primary contribution to the reduction in the modulus of the composite at higher temperatures is shown to be the d...

Preparation of alumina and alumina-silica powders by wire explosion resulting from electric discharge

Abstract: Wires of aluminium and aluminium-silicon alloys containing 5.2 and 12.1 wt% Si were exploded in air by electric discharge, and the properties of the powders obtained were examined. The powders consisted of large particles of several micrometres and very fine spherical ones, which were formed from the metallic droplets and vapours, respectively, and were mostly oxidized. The large particles decreased with an increase in the charged energy. Although the particle-size distribution was broad, the average particle sizes were small and were increased from 0.7 μm to 1.3 μm with an increase in the charged energy. These powders were identified as γ-alumina, amorphous alumina-silica compound and a small amount of the metal. The amorphous phase crystallized to mullite in the temperature range 1153–1243 K, and the crystallization temperature fell with increase in the charged energy because the amount of fine particles formed from the vapours increased.

Transformation of Ba-Al-Si precursors to celsian by high-temperature oxidation and annealing

Abstract: Celsian (monoclinic BaO · A12O3 · 2SiO2) is being considered as a matrix material for ceramic composites used in high-temperature structural applications. The present article describes the synthesis of celsian by the oxidation and annealing of solid, malleable, metallic Ba-Al-Si precursors. The phase and microstructural evolution after various stages of oxidation at 300 °C to 1260 °C in pure oxygen at 1 atm pressure have been examined by X-ray diffraction (XRD) and electron microprobe analyses (EPMA). Barium peroxide, BaO2, formed rapidly during oxidation at 300 °C, with aluminum and silicon remaining largely as unoxidized particles in a BaO2 matrix. Between 300 °C and 500 °C, barium orthosilicate, Ba2Si04, formed by a solid-state reaction between barium peroxide and unoxidized silicon. Further exposure to temperatures between 500 °C and 1200 °C resulted in the oxidation of aluminum and of residual silicon. The oxidized silicon reacted with the barium orthosilicate matrix to yield higher silica-containing barium silicates that, in turn, reacted with alumina or mullite to form metastable hexacelsian (hexagonal BaO-A12O3 · 2SiO2). Celsian was then obtained by further exposure to peak temperatures ≤1260°C.

Effect of Al4SiC4 Addition to Carbon-Containing Refractories

Abstract: The behavior and the effect of Al4SiC4, an excellent hydration-resistant compound, as an antioxidant for carbon-containing refractories have been investigated, and the corresponding mechanisms have been discussed. Al4SiC4 added to the refractories initially reacts with CO to form Al2O3, SiC and C. After the reaction, if the temperatures are below -1560°C, the formed SiC and Al2O3 will further react with CO to form mullite (Al6Si2O13) and C. The above reaction processes can form protective layers on the surfaces of the refractories, which inhibit the oxidation of the refractories. From the results obtained, Al4SiC4 was found to markedly inhibit the oxidation of the carboncontaining refractories.

Phase Diagram for Mullite-SiF4

Abstract: At 1 atm of SiF4, mullite and SiF4 react below 660° 7°C to form A1F3 and SiO2. From 660° to 1056°± 5°C, the product is fluorotopaz. Mullite is stable in the presence of 1 atm of SiF4 above 1056°C. The transition temperatures at other pressures of SiF4 can be calculated from log p(atm) = 11.587 – 10811/T(K) and log p(atm) = 9.9609 – 13238/T(K). The phase diagram shows only gas–solid equilibria, but there is evidence for a metastable melt from which acicular mullite and fluorotopaz grow.

Combustion synthesis, characterization, sintering and microstructure of mullite-cordierite composites

Abstract: Mullite and cordierite are promising materials for electronic packaging due to their dielectric constant and coefficient of thermal expansion being lower than that alumina [1-5]. Recent advances in very large scale integrated circuits (VLSI) have led to materials with low dielectric constant and thermal expansion tailored over a wide range. Cordierite- dispersed-mullite composites allow a selection of materials with a wide range of values of dielectric constant and thermal expansion [6]. Mullite/ cordierite composites have been prepared by the sol-gel method using metal alkoxides [7] or by the use of composite sols [8, 9]. Such composites have aslo been prepared by solid state mixing of mullite and cordierite in the desired ratio [10].

Design and processing of all-oxide composites

Abstract: All-oxide ceramic composites as a material with potential for long life-time applications at temperatures in the 1400-1600{degrees}C range in combustion environments were studied. The properties of available polycrystalline and single crystal oxide fibres were summarised. The literature on stable weak interfaces in all-oxide composites was reviewed. Composites with single crystal fibres, a polycrystalline matrix of the same material as the fibres, and a compatible high temperature stable weak oxide interphase was suggested to be the most promising approach. Processing of all-oxide composites was performed. ZrO{sub 2}-coated sapphire fibres in reaction bonded alumina and in hot pressed alumina showed crack deflection and fibre pull-out. In reaction bonded mullite crack deflection and pull-out was observed even for un-coated sapphire fibres. This was attributed to thermal expansion mismatch. A recently started European project aiming at development, scale-up and property evaluation of all-oxide composites is briefly outlined.

Oxidation-resistant interfacial coatings for continuous fiber ceramic composites

Abstract: Developing an oxidation-resistant interfacial coating for continuous fiber ceramic composites (CFCCs) continues to be a major challenge. CFCCs` mechanical behavior are influenced by the interfacial bonding characteristics between the fiber and the matrix. Finite element modeling studies suggest that a low-modulus interfacial coating material will be effective in reducing the residual thermal stresses that are generated upon cooling from processing temperatures. Nicalon/SiC composites with carbon, alumina and mullite interfacial coatings were fabricated with the SiC matrix deposited using a forced-flow chemical vapor infiltration process. Composites with mullite interfacial coatings exhibited considerable fiber pull-out even after oxidation and have potential as a composite system.

Constitution of porcelain before and after heat-treatment. I: Mineralogical composition

Abstract: Porcelain compositions were prepared with and without the addition of mineralizers/nucleating agents and their phase assemblages were studied before and after subjecting them to heat-treatment. Mullite, quartz and glass were detected in the compositions before heat-treatment, but cristobalite appeared as an additional phase after heat-treatment. Heat-treated compositions were comparatively rich in mullite following crystallization from the glassy phase. Nb 2 O 5 was the most efficient nucleating agent among those tried.

Single‐Phase Mullite Gels Doped with Chromium

Abstract: The influence of chromium on homogeneity of gels and on mullite formation at 980°C was examined by DTA coupled with quadrupole mass spectrometry (QMS), X-ray diffraction (XRD), and SEM, EDX, and TEM analyses. Mullite precursors were prepared according to the method which provides completelyhomogeneous gels in the Al2O3-SiO2 system, with a constant molar ratio of (Al + Cr)/Si = 3/1 and various Al/Cr ratios (0-4 mol% Cr2O3). DTA and QMS analyses revealed the same processes of dehydraton, combustion of organic, as well as the decomposition of nitrates independently of chromium content. The differences were observed in the crystallization path at 980°C. Only for the undoped mullite precursors was the crystallization of mullite as a sole phase determined by XRD, because chromium promotes spinel formation. The incorporation of chromium into mullite structure under nonisothermal and isothermal conditions was followed for the sample with 2 mol% Cr2O3, by measuring the unit cell parameters as a function of temperature. Under nonisothermal conditions, two processes of mullite crystallization were observed, which were attributed to the formation of areas with different scales of homogeneity and distribution of Al + Cr and Si species. From the areas with a higher scale of homogeneity aluminum-rich chromium-doped mullite crystallized at as low as 910°C. On the other hand, a lower degree of homogeneity led to the crystallization of Al-rich chromium mullite as well as spinel. Above 1050°C, spinel and the rest of the amorphous Cr2O3, not yet incorporated into mullite, reacted with amorphous SiO2-rich phase, forming Al+Cr-richer mullite. This brought about an apparent discontinuity in the plot of α-axis vs temperature. Under isothermal conditions, gel segregation is suppressed; therefore, no discontinuities in α-axis length were observed. SEM, TEM, and EDX analysis of gel foil at 900°C proved the same phase separation as for the undoped sample, but also additional formation of rather large inhomogeneous areas enriched with alumina.

Sintering and crystallization of mullite in diphasic gels

Abstract: Diphasic monolithic mullite gel of stoichiometric composition shows a multiple aggregation of colloidal SiO2 (∼ 12 nm) and AlOOH (∼10 nm) particles. The identity of the colloidal particles is retained up to the crystallization of mullite at a temperature of 1240°C in unpressed or 1220°C in gels cold-isostatically pressed at 1.5 GPa. Intensive sintering is closely followed by crystallization of mullite. Both sintering and crystallization are apparently related with the formation of a silica continuum. Nucleation of mullite appears to occur at points of contact (interface) of SiO2/δ-Al2O3 particles.

Chemical vapor deposition of mullite coatings

Abstract: This invention is directed to the creation of crystalline mullite coatings having uniform microstructure by chemical vapor deposition (CVD). The process comprises the steps of establishing a flow of reactants which will yield mullite in a CVD reactor, and depositing a crystalline coating from the reactant flow. The process will yield crystalline coatings which are dense and of uniform thickness.

The topaz to mullite transformation on heating

Abstract: The decomposition of topaz to mullite and other siliceous phases on heating above about 1100 °C was found to depend on sample size and the presence of water vapor in the heating atmosphere. The principal experimental technique employed was scanning electron microscopy, but the data were supported by x-ray diffraction, thermal analysis, mass spectroscopy of volatile emissions, and solid-state nuclear magnetic resonance. In relatively large samples, the transformation to mullite evidently takes place by a vapor phase mechanism within the bulk. The surface reaction that took place for samples heated in a wet atmosphere allowed the formation of high-silica glass, as well as mullite. The use of a hydrogenous heating atmosphere resulted in the sublimation and reformation of mullite whiskers, well outside the boundary of the original topaz.

Homogeneous precipitation of mullite precursors

Abstract: The synthesis of mullite powders or gels from an aqueous precursor solution has been studied. The starting solution is made by hydrolyzing TEOS into an aqueous solution of aluminium nitrate. When this solution is sprayed into a solution of ammonia in isopropanol, a nearly monophasic mullite precursor is obtained, whereas if this precipitation is made into an aqueous solution of ammonia, or ammonium carbonate, a diphasic colloidal precipitate is formed. By a slow and homogeneous precipitation in aqueous solution, induced by in-situ generation of ammonia by thermal hydrolysis of urea, a monophasic gel is also obtained. The samples have been studied by DTA, DSC, TGA, XRD and dilatometry.

Mechanical behaviour of mullite composites reinforced with mullite fibres

Abstract: Mullite matrix composites uniaxially reinforced with uncoated and BN (0.3 or 1 μm thick) coated mullite fibres were consolidated to 82 to 98% of TD (theoretical density) at 1300 °C by using a colloidal processing route to make mullite matrix. The mechanical properties were characterized by flexural testing and chevron-notch tests for fracture toughness measurement. The samples with uncoated and 0.3 μm BN-coated fibres failed in a brittle manner without any toughening effect of the fibres. In contrast, the sample with 1 μm BN-coated fibres showed toughened composite behaviour (i.e. load-carrying capacity following maximum load) with a significant enhancement in toughness (e.g. about 12 MPa m 1 2 in the as-fabricated state), despite a relatively low density of about 90% TD. The room temperature ultimate strength (320 MPa) and fracture toughness of this composite decreased a little by heat treatment at 1320 °C for inducing complete transformation to mullite of the matrix, probably due to the degradation of fibre strength.