Showing papers on "Mullite published in 1997"

Mullite Based Oxidation Protection for SiC-C/C Composites in Air at Temperatures up to 1900 K

Abstract: For an industrial Si-SiC coated C/C material (reference material) the temperature dependence of the linear rate of mass loss is interpreted in the temperature range 773 < T < 1973 K. The Arrhenius plot of the thermogravimetrically determined oxidation rate shows four typical regimes. Only in the temperature range 1323 < T < 1823 K is the oxidation rate close to or lower than the limit for long-term application. Pulsed Laser Deposition (PLD) allows the ablation of nonconductive and high melting targets and the preparation of films with complex composition. High energy impact CO 2 laser pulses (j= 3.10 7 W cm -2 ) lead to melting and evaporation of the target material in a single step. Therefore the flux of the metal components is stoichiometric. Deposited green layers did not show IR peaks typical for mullite. After a short oxidation treatment (15 min at 1673K) the formation of mullite in the coating was completed as was confirmed by IR spectroscopy and XRD investigations. Thin PLD-mullite layers (900 nm) did not markedly improve the oxidation resistance of the reference material in the high temperature range 1473 < T < 1973 K. However, a preoxidation treatment of the substrate material and mullite coatings with a thickness of 2.5 μm improved the oxidation behaviour significantly. Because of SiO 2 formation at the mullite-SiC interface all samples exhibited a mass increase on oxidation. The inward diffusion of oxygen across the outer mullite-containing layer controlled the kinetics of the reaction as was deduced from 18 O diffusivity measurements in PLD mullite layers. The calculated oxidation rates resulting from the diffusion parameters in SiO 2 and mullite are close to the thermogravimetric data. For oxidation durations of three days only amorphous SiO 2 is formed at the mullite-SiC interface.

Effect of Composition on Mullitization Behavior of α-Alumina/Silica Microcomposite Powders

Abstract: The effect of composition on mullite formation was investigated using submicrometer composite particles that consisted of α-alumina cores and amorphous silica coatings. Powders with varying alumina/silica ratios were prepared by varying the thickness of the silica coating. The mullitization behavior was monitored using differential thermal analysis, X-ray diffractometry, and scanning electron microscopy. The results were consistent with previous studies that indicated that mullite forms initially by nucleation and growth within the siliceous phase, but also that chemical interdiffusion within the mullite grains is required to complete the reaction. The reaction rate in both stages was affected by the alumina/silica ratio. Available evidence has indicated that the first stage of the reaction is controlled by the dissolution of alumina in the siliceous phase. Results for the second stage suggested that alumina diffuses more rapidly through mullite than silica.

Fabrication, properties, and modelling of engineering ceramics reinforced with nanoparticles of silicon carbide

Abstract: The present paper prepared by Professor R. W Davidge, briefly summarises the results obtained from a multinational project aimed at assessing nanocomposite ceramics reinforced with fine (∼ 200 nm) particles of SiC. Various fabrication procedures were developed, e.g. by attritor milling in a water medium, followed by freeze drying and hot pressing. Matrixes were alumina, silicon nitride, mullite, and cordierite. Although modest improvements in properties were obtained for all systems, the most interesting effects were observed for alumina. For a given grain size, strength was increased by ∼ 70% and toughness by ∼ 10% in material with 5 vol.-%SiC, and for the range studied was relatively independent of nanophase content and particle size (10-400 nm). Based on microscopy observations and estimates of the Al 2 O 3 /SiC boundary fracture energy, it is postulated that a key feature of the nanocomposite effect is associated with a strongly increasing R curve behaviour over a small number of grain dimensions. The consequence is that fracture is predominantly transcrystalline, as opposed to intercrystalline for pure material. This gives the nanocomposite a significantly improved wear resistance. The material also showed good strength retention at high temperatures in corrosive atmospheres, and a considerably reduced creep rate.

Sol-Gel transition and structural evolution on multicomponent gels derived from the alumina-silica system

Abstract: The capacity of the sol-gel process of producing highly pure, homogeneous alumina-silica based materials had been demonstrated in the last few years. However, a full understanding on the mechanisms associated to sol formation and sol to gel transition has not yet been achieved and is required for the development of a new generation of nano-structurally tailored materials that will significantly enhance the technological importance of the sol-gel process. In this work, tetraethyl orthosilicate (TEOS) and aluminum isopropoxide were used to prepare materials within the entire silica-alumina system. Process parameters, such as gelation time, were correlated to variables of the initial stage of the process, such as pH, temperature of hydrolysis and water/alkoxide ratio. The obtained gels were dried at 105°C and subsequently heat treated at 500 and 1100°C for 3 hours. X-ray diffraction and infrared spectroscopy were used to characterize the materials and phase transformations. Structural information obtained from phase characterization and phase transformations was correlated to the effects of the process variables on sol formation and gelation, providing insights related to the mechanisms involved. The influence of temperature of aluminum isopropoxide hydrolysis on peptization and gelation of the mixtures was noted. The different behavior of mixtures hydrolyzed at low and high temperatures was suggested to be caused by different mechanisms of surface charge formation on the structurally different aluminum hydroxides. Monophasic and diphasic mullite xerogels were produced by changing temperature of aluminum isopropoxide hydrolysis, and led to formation of mullite and Al-Si spinel phases respectively, when treated at 1100°C.

Sintering of industrial mullites

Abstract: Two mullite powders, different in chemical and morphological characteristics, were compared with respect to their sinterability. For the development of a low-cost sintering step, MgO was added as sintering aid and its content was optimized for both mullites: the enhancement in sintering was imputed to a liquid phase formation. The two mullites presented a strongly different sinterability; the one gave rise to high density sintered bodies while the other, even after a grinding step which made its particle size distribution similar to that of the first, did not reach high final density. In both mullites, there was a preexisting glassy phase: the strong influence of its content, composition and spatial distribution on the sintering behaviour was demonstrated. For better clarifying the action of the sintering aid, the glassy phase was eliminated by HF washing and the sintering behaviour of washed and unwashed samples with added MgO was compared. The contribution of MgO during sintering as well as the new phase formation in the sintered bodies seem to be linked to the alumina and impurity content of the mullites.

Mullite evolution in ceramics derived from kaolinite, kaolinite with added α-alumina, and sol-gel precursors

Abstract: The microstructural evolution of mullite prepared from sol-gel (boehmite and colloidal silica), kaolinite, and kaolinite-α-alumina has been investigated. For the sol-gel route, calcination at 600°C for 1 h produced a powder consisting of γ-alumina and amorphous silica. The density of sintered compacts increased with temperature to a maximum of 3g cm -3 after 2h at 1700°C. Calcination of sol-gel powder at 1400°C for 2h resulted in mullite powder which exhihited inferior sinterability, achieving a maximum density of 2.6 g cm -3 after 2 h at 1700°C. Both sintered samples had an equiaxed grain structure with some glassy phase located mainly at triple junctions. The maximum density achieved in kaolinite derived ceramics was 2.68 g cm -3 on sintering at 1450°C for 2 h, and the resultant microstructure consisted of high aspect ratio mullite grains in a glassy matrix. Addition of α-alumina to the kaolinite to give a total alumina content of ∼72 (wt-)%, led to a density of 2.92 g cm -3 on sintering at 1700°C for 2 h. The microstructure consisted of primary mullite derived from the kaolinite, and secondary mullite produced by precipitation from a liquid phase.

Microstructural evolution and growth of crystallite size of mullite during thermal transformation of kyanite

Abstract: The microstructural evolution of mullite during the thermal transformation of kyanite has been studied in the temperature range 1200–1600 °C. The shape and size of the grains were analysed by means of SEM-EDS while crystallite size evolution was studied by X-ray line profile analyses. The results obtained showed that total transformation of kyanite to mullite takes place between 1350 and 1400 °C. At temperatures below 1350 °C needle-like mullite grains are always produced. At higher temperatures the mullite grains reveal rounded end platelet morphology. Evolution from needle-like to platelet shape was correlated with the X-ray data.

Microwave-assisted Acid Dissolution of Sintered Advanced Ceramics for Inductively Coupled Plasma Atomic Emission Spectrometry

Abstract: The microwave-assisted acid dissolution of sintered bodies of 28 structural and electronic advanced ceramic materials was systematically evaluated. These materials included zirconia-based ceramics, such as m-ZrO2 (a non-stabilized monoclinic zirconia), Ca-PSZ and Mg-PSZ (two partially stabilized zirconias), Y-FSZ (a fully stabilized zirconia) and Ce-TZP, Yb-TZP and Y-TZP/Ce (three tetragonal polycrystalline zirconias); alumina-based ceramics, such as Al2O3, mullite and spinel; ceria-based ceramics, such as CeO2–Gd2O3 (cubic ceria gadolinia); titania-based ceramics, such as TiO2; titanate-based ceramics, such as Al2TiO5, BaTiO3 and BIT (bismuth titanate); lead titanate-based ceramics, such as Ca-PT, La-PT, Nd-PT, Sm-PT and Gd-PT; lead zirconate titanate-based ceramics, such as PZT and PLZT; niobate-based ceramics, such as PMN (lead magnesium niobate); non-oxide-based ceramics, such as AlN, BN, Si3N4 and SiC; and oxide and non-oxide-based ceramics, such as β′-sialon (silicon aluminium oxynitride). Fifteen acids or mixtures of acids were tried, including HCl, HNO3, H2SO4, aqua regia, H2SO4–(NH4)2SO4 and mixtures of these acids with HF and with H2O2. A commercially available laboratory medium pressure microwave oven was used. Eleven optimized microwave methods were developed. These methods are simple (three stages maximum), fast (15–35 min digestion time) and mild (20–60% of the microwave oven power). By applying these microwave methods, it was possible to dissolve completely all the sintered advanced ceramics, except SiC and β′-sialon. These two non-oxide ceramics were the only samples that could not be dissolved by any of the acids or mixtures of acids tested. The microwave-assisted acid dissolution was compared for ICP-AES with conventional dissolution procedures, i.e., alkali fusion in a platinum crucible and in a graphite crucible and acid decomposition by conductive heating at elevated pressure (in a PTFE bomb). It was demonstrated that microwave-assisted dissolution presents many advantages over the other procedures. When compared with acid decomposition by conductive heating in a PTFE bomb, one of the most important advantages is the drastic shortening of the digestion time from hours to minutes. When compared with alkali fusions, one of the most important advantages is the use of smaller amounts of high-purity acids, which contain less impurities than the fluxes; because of this, matrix effects and contamination from the attack reagents are lower, and consequently there is an improvement in the analytical figures of merit of ICP-AES.

The development of CVD mullite coatings for high temperature corrosive applications

Abstract: Adherent crystalline mullite coatings have been chemically vapor deposited on SiC and Si 3 N 4 -based substrates to enhance its corrosion/ oxidation resistance. Thermodynamic and kinetic considerations have been utilized to produce mullite coatings with a variety of growth rates, compositions, and morphologies. The flexibility of processing can be exploited to produce coated ceramics with properties tailored to specific applications and varied corrosive environments. The coating and been tested in several corrosive environments including oxidation, jet burner exhaust, and Na 2 SO 4 .

Influence of Grinding in Pyrophyllite−Mullite Thermal Transformation Assessed by 29Si and 27Al MAS NMR Spectroscopies

Abstract: The influence of grinding on the thermal transformation of pyrophyllite to mullite is analyzed by 27Al and 29Si MAS NMR spectroscopies, using X-ray powder diffraction and thermal analysis (DTA-TG) as complementary techniques. Mechanical treatments by dry grinding produce partial destruction of pyrophyllite and rearrangement of Al ions in tetra- and pentahedral environments. Tetrahedral sheet breakdown favors Al atomic diffusion and formation of Si-poor nuclei of mullite at 1000 °C. In particular, the exothermic peak detected at 1000 °C in DTA curves is ascribed to transformation of pentahedral coordination of Al in ground samples into tetra- and octahedral aluminum of mullite. During thermal decomposition of pyrophyllite, segregation of amorphous silica is produced. These results are compared with those obtained in kaolinite−mullite transformation, which were previously analyzed.

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Abstract: Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples. The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of γ-Al2O3 spinel and mullite, or simple γ-Al2O3 spinel, which subsequently transformed to mullite at 1260°C. Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of γ-Al2O3 spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.

Deposition of mullite and mullite-like coatings on silicon carbide by dual-source metal plasma immersion

Abstract: Mullite and mullite-like coatings on silicon carbide have been produced by a metal plasma immersion ion implantation and deposition (Mepiiid) technique based on two cathodic vacuum arc sources and concurrent pulse biasing of the substrate in an oxygen atmosphere. The deposition was carried out at oxygen partial pressures of between 0.66 and 3.33 Pa. The Al:Si ratio in the films varied from 1:1 to 8:1 and was controlled by varying the pulse duration of the separate plasma guns. High bias voltage was used early in the deposition process in order to produce atomic mixing at the film-substrate interface, while lower bias voltage was used later in the deposition; low ion energy allows control of the physical properties of the film as well as faster deposition rates. The as-deposited films were amorphous, and crystalline mullite was formed by subsequent annealing at 1100{degree}C for 2 h in air. Strong adhesion between the mullite and the SiC was achieved, in some cases exceeding the 70 MPa instrumental limit of our pull-tester. {copyright} {ital 1997 Materials Research Society.}

Thermodynamic study of inclusion formation in low alloy steel weld metals

Abstract: The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO2.3Al2O3) is found to be formed in addition to Ti3O5 in the steel melt at 1800 K when the melt has a wt-%Al/wt-%O ratio of ∼0·6 optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al2O3), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with pre-existing mullite. Titanium additions are found to be beneficial to...

Thermal oxidation of carbothermal β′-sialon powder:reaction sequence and kinetics

Abstract: The oxidation of carbothermally synthesised β′-sialon powder (z=2.45) was found by X-ray powder diffraction and solid-state 29 Si and 27 Al MAS NMR spectroscopies to result in the initial formation of amorphous SiO 2 and a non-crystalline mullite-like aluminosilicate which becomes crystalline at higher temperatures. The evolution of crystalline mullite is accompanied by a decrease in both the sialon z value and in the proportion of tetrahedral Al. The ratio of 29 Si in the SiO 2 to 29 Si in the mullite oxidation products is about 4, consistent with the formation of Al 2 O 3 as an additional oxidation product. The oxidation kinetics at 1100–1300 °C are described by a parabolic rate law with an activation enthalpy of 161 kJ mol -1 , suggesting the rate-determining step to be the permeation of oxygen through the oxidised layer coating the sialon grains.

Preparation of Al2TiO5 from alkoxides and the effects of additives on its properties

Abstract: Al2TiO5 was prepared by the sol-gel method from alkoxides and its mechanical and thermal properties measured. The prepared Al2TiO5 powder was very fine and had a narrow particle-size distribution. The addition of mullite and Al2O3 to the prepared Al2TiO5 inhibited the grain growth during sintering, resulting in a decrease of microcracking and an increase of fracture strength. Al2TiO5/mullite composite exhibited a higher fracture strength than Al2TiO5/alumina composite. The thermal expansion coefficient of Al2TiO5 increased with the addition of mullite and alumina, and also increased with temperature up to 1000°C; however, it decreased in the temperature range between 1000 and 1200°C during heating, due to decomposition of Al2TiO5. The addition of mullite inhibited the decomposition of Al2TiO5, but the addition of Al2O3 accelerated it. Al2TiO5 prepared from metal alkoxides was also more stable than that prepared from the commercial alumina and titania powders.

Effect of BN Coating on the Strength of a Mullite Type Fiber.

Abstract: Nextel 480 is a polycrystalline essentially mullite fiber (70 wt.-% Al2O3+28 wt.-% SiO2+2 wt.-% B2O3). Different thicknesses of BN were applied as coatings on this fiber. Optical, scanning electron, and transmission electron microscopy were used to characterize the microstructure of the coatings and fibers. The effects of coating and high temperature exposure on the fiber strength were investigated using two-parameter Weibull distribution. TEM examination showed that the BN coating has a turbostratic structure, with the basal planes lying predominantly parallel to the fiber surface. Such an orientation of coating is desirable for easy crack deflection and subsequent fiber pullout in a composite. The BN coated Nextel 480 fiber showed that Weibull mean strength increased first and then decreased with increasing coating thickness. This was due to the surface flaw healing effect of the coating (up to 0.3 μm) while in the case of thick BN coating (1 μm), the soft nature of the coating material had a more dominant effect and resulted in a decrease of the fiber strength. High temperature exposure of Nextel 480 resulted in grain growth, which led to a strength loss.

Electrophoretic deposition infiltration of 2-D woven SiC fibre mats with mixed sols of mullite composition

Abstract: Electrophoretic deposition (EPD) has been used to infiltrate a dual-component mixed sol of mullite composition into woven, electrically conducting SiC (Nicalon) fibre preforms. Both silica and alumina precursors had a near-equiaxed particle shape and their controlled colloidal mixing at an optimised pH (3.1) has lead to their extensive heterocoagulation on a nanometre scale. The mechanisms of particle migration in the EPD cell are not fully clear at this stage. It is suggested that both sol species migrate simultaneously as a ‘composite’ sol particle to the fibre mat serving as one electrode in the cell. Owing to the small particle size of the silica and alumina used, both components were able to infiltrate the spaces within the fibre tows. Good particle packing and a high solids-loading were achieved, producing a firm matrix deposit which adhered to the fibres. The maintenance of the stoichiometric mullite composition in the deposited material was confirmed by XRD analysis of the deposited material heat-treated at 1350 °C for 5h. The high-quality infiltrated fibre mats serve as preforms for the fabrication of mullite matrix composites.

Cobalt aluminate spinel-mullite composites synthesized by sol-gel method

Abstract: CoAl 2 O 4 spinel-mullite composites were prepared by double substitution of Al by Ti and Ni in stoichiometric 3:2 mullite. Gels with compositions 3(Al 2 − 2 x Co x Ti x O 3 )·2SiO 2 (x = 0.025, 0.05 and 0.2) and 3(Al 2 − 2 x M x O 3 )·2SiO 2 (x = 0.05; M = Co 2+ or Ti 4+ ) were synthesized by sol-gel techniques. The structure of the gels was investigated by infra-red spectroscopy. Heating gels at 750 °C produced an amorphous silicoaluminate network. The reaction sequence was investigated by differential thermal analysis, X-ray diffraction and ultraviolet-visible spectroscopy. All samples crystallized at temperatures lower than 1000 °C from the amorphous state. Al-Si spinel and/or CoAl 2 O 4 were the first crystalline phases detected, which appeared almost simultaneously. This finding indicated the important role played by Co 2+ and Ti 4+ in phase development and crystallization kinetics, even for compositions very close to the stoichiometric 3:2. The reaction sequence can understood by assuming that CoAl 2 O 4 spinel was developed by epitaxial growth on Al-Si spinel. This fact permitted direct control of the final microstructure of the CoAl 2 O 4 spinel-mullite composite as revealed by scanning and transmission electron microscopies.

Production and properties of low density engineering ceramic foams

Abstract: A new manufacturing route for foam ceramics has been developed and is now being commercialised. The primary advantage of the process is its inherent flexibility. The foams can be near net shape manufactured in a variety of shapes and sizes and after production are simply dried and fired. Machining is easily per formed in the green and/or fired state. In addition, the porosity and pore size distribution can be varied to suit the application and a wide range of ceramic materials can be foamed. The primary criterion is the ability to produce a stable ceramic suspension. Foams have been produced in alumina, partially and fully stabilised zirconia, mullite, cordierite, and hydroxyapatite with densities ranging from 5 to 40% of theoretical. Applications are diverse and range from furnace linings, high temperature kiln furniture and crucibles, to catalyst supports, hot gas filtration, and artificial bone.

Mullitization of Andalusite in Refractory Bricks

Abstract: Mullite formation was studied in andalusite bricks fired in the range 1300-1600°C Mullitization of andalusite grains retains the grain shape and bulk chemistry. An andalusite monocrystal is transformed into a composite made of a mullite monocrystal (≃ 80 %)with a capillary network filled with a silica rich glass. The capillaries form interconnected tubes parallel to the c axis, common to the mullite crystal and the parent andalusite grain. Heating andalusite crystals reveals that only ≃10-15% of the glass is expelled out of the host grain. This observation is supported by volume balance calculations In spite of a high amount of glass, andalusite based refractories exhibit an excellent behaviour in industrial use resulting from the trapping of most of the glass in the mullite.

The quantification of different forms of cristobalite in devitrified alumino-silicate ceramic fibres

Abstract: When heat treated, amorphous alumino-silicate fibres will devitrify into crystalline phases which on cooling become stable at room temperature. X-ray diffraction has been used to identify and quantify the phases present in such products; mullite and two distinct forms of cristobalite have been observed. One form of cristobalite has been identified as α-cristobalite, the second form has been designated as α′-cristobalite. An internal standard approach was used to enable calibration curves to be obtained for the three crystallite phases, including the α′- cristobalite for which no commercially available standard exists. The accuracy of the methods was estimated as ± 5 wt% for all three crystalline phases present. This was achieved by quantifying the proportion of α′-cristobalite in a standard alumino-silicate fibre after heat treatment for 48 h at 1773 K and then using this material as the `standard' for α′-cristobalite. This `standard' contained 32.9 wt% α′-cristobalite, 1.6 wt% α-cristobalite and 65.5 wt% mullite. The exact nature of the α′- cristobalite phase has also been examined using high-temperature X-ray diffraction and differential scanning calorimetry. It has been shown to be more similar in nature to α-cristobalite than β-cristobalite since it undergoes an α′ → β phase transition on heating. Compared with α-cristobalite it has a different lattice size, and both the temperature and enthalpy of the α′ → β phase transition are lower than for the α → β phase transition. It has been proposed that the α′-cristobalite observed is a defect form of α-cristobalite but with a constant amount of defects/substituted cations which gives it a well defined and consistent structure.

A new silicothermal route to the formation of X-phase sialon: The reaction sequence in the presence and absence of Y2O3

Abstract: A new method is described for the preparation of essentially monophase X-phase sialon, Si 12 Al 18 O 39 N 8 , by silicothermal reduction of mixtures of kaolinite, γ-Al 2 O 3 and elemental Si in purified nitrogen at 1450–1500 °C. Powder X-ray diffraction and solid state 29 Si and 27 Al MAS-NMR of reaction mixtures heated to 1200–1570 °C suggest that the reaction proceeds in several parallel steps, including the nitridation of the elemental Si to Si 3 N 4 , thermal decomposition of the kaolinite to mullite and amorphous silica, reaction between the amorphous silica and γ-alumina, and reaction between silicon nitride and the aluminosilicate phase. Two forms of O′-sialon also appear as minor phases, possibly formed in the early stages by nitridation of the silica or aluminosilicate. The addition of up to 3 wt% of Y 2 O 3 to the reaction mixture increases the rate of all the reactions occurring above 1100 °C without changing the reaction sequence, but the X-phase product formed in the presence of Y 2 O 3 retains a high proportion of SiO 2 N 2 groups, by comparison with X-phase formed without Y 2 O 3 , in which SiO 3 N groups predominate. XRD and MAS-NMR show the fully reacted silicothermal product to be comparable with reputable X-phase sialon prepared by a two-stage carbothermal method.

Continuous fibre reinforced mullite matrix composites by sol–gel processing: Part I Fabrication and microstructures

Abstract: The fabrication of ceramic composites based on a mullite matrix uniaxially reinforced with either high strength carbon or Nicalon-SiC continuous fibres is described. Sols of high solids contents of up to 40 vol% were prepared using colloidal silica sol with either α-alumina powder or colloidal boehmite as reactants for the preparation of the mullite matrix, 3Al2O3·2SiO2. Single-stage infiltration was used for the preparation of prepregs for laminating into composites. The composites were consolidated by uniaxially hot-pressing at 1300 or 1550°C, depending on the matrix precursors and the final phases in the matrix. Highly densified composites with fibre volume fractions of 32–60% were obtained and the microstructures characterised by mercury porosimetry, X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Ion irradiation-induced amorphization in the Al2O3-SiO2 system: A comparison with glass formation

Abstract: The ion beam-induced crystalline-to-amorphous transition has been investigated for crystalline phases in the Al2O3–SiO2 system: Al2O3, SiO2 (quartz), Al2SiO5 (kyanite, andalusite, sillimanite), and 3Al2O3⋅2SiO2 (mullite). Xe+ 1.5 MeV was used to irradiate samples at temperatures from 15 to 1023 K in situ in a transmission electron microscope to determine the critical amorphization doses. The susceptibility to amorphization is (highest to lowest): quartz, sillimanite, kyanite, andalusite, mullite, and alumina. These data are compared to viscosities and activation energies for viscous flow of melts in this system. The doses required for amorphization by ion irradiation are related to the viscosities of the melts. The activation energies for irradiation-enhanced annealing are qualitatively correlated with the activation energies of viscous flow. These results suggest a parallel between ion beam irradiation-induced amorphization and glass formation. Glass-forming “ability’’ correlates with susceptibility to r...

Electrochemical Measurement of Critical Supersaturation in Fe- O - M (M=Al, Si, and Zr) and Fe- O - Al - M (M=C, Mn, Cr, Si, and Ti) Melts by Solid Electrolyte Galvanic Cell

Abstract: The supersaturation ratio with respect to alumina precipitation, S° Al2O3 (=(a Al .A O 3 ] ss /(a 2 Al .a O 3 ) eq ), in Fe-O-0.008∼0.090mass%Al melt was electrochemically measured at 1 873K in an Al 2 O 3 crucible by blowing CO 2 gas or adding an Fe-2mass%Al alloy intermittently, using plug-type ZrO 2 -9mol%MgO and tube-type mullite probes. The S° ZrO2 (=(a Zr .a O 2 ) ss /(a Zr .a O 2 ) eq ) value in Fe-O-0.04mass%Zr melt was also measured at 1 873 K in a ZrO 2 -11mol%CaO crucible by adding an Fe-4mass%Zr alloy. As a result, the critical values for log S° Al2O3 and log S° ZrO2 were 3.5 and 1.3, respectively. No supersaturation for the precipitation of SiO 2 was observed. The presence of solute element M (M=C, Mn, Cr, Si, and Ti) in an Fe-O-0.148∼0.158mass%Al melt was found to significantly influence the supersaturation phenomenon.

Damage tolerant ceramic matrix composites by a precursor infiltration

Abstract: Damage-tolerant, continuous fiber ceramic matrix composites are fabricated to fill the space between the fibers with a powder. The powder particles are heat treated to form a porous framework without shrinkage, which is then strengthened with an inorganic synthesized from a precursor in solution. High particle packing densities is achieved within the fiber preform using a small particle-to-fiber diameter ratio. Filling the interstices with a powder increases the composite density and also limits the size of the crack-like voids within the matrix. The ceramic matrix composite (CMC) has mechanical characteristics similar to those found in wood. It is also affordable and inherently oxidation resistant. The composite is characterized by a heterogeneous distribution of fibers within a porous matrix having a homogeneous, fine porosity. A residual stress from thermal expansion mismatch of the matrix and fibers is created in the composite. The illustrated embodiment uses Al2 O3 fibers with either mullite or Si3 N4 matrices.

Mullite/SiAlON/Alumina Composites by Infiltration Processing

Abstract: The formation of mullite/SiAlON/alumina composites was studied by infiltrating a SiAlON/alumina-base composite with two different solutions, followed by thermal treatment. The base composite was prepared from a mixture of tabular Al{sub 2}O{sub 3} grains, fume SiO{sub 2}, and aluminum powders. The mixture was pressed into test bars and nitrided in a nitrogen-gas (N{sub 2}) atmosphere at 1,480 C. The infiltrants were prehydrolyzed ethyl polysilicate solution and ethyl polysilicate-aluminum nitrate solution. The composites were infiltrated under vacuum, cured at 100 C, and precalcined in air at 700 C. This infiltration process was repeated several times to produce bars that had been subjected to multiple infiltrations, then the bars were calcined in a N{sub 2} atmosphere at 1,480 C to obtain mullite/SiAlON/alumina composites. The infiltration process increased the percentage of nitrogenous crystalline and mullite phases in the matrix; therefore, a decrease of the composite microporosity was observed. The infiltration increased the mechanical strength of the composites. Of the two composites, the one produced using prehydrolyzed ethyl polysilicate as the infiltrant had a higher mechanical strength, before and after being subjected to a severe thermal shock.