Alumina-rich Mullite Research Articles

Evolution of Ti2O3 and mullite in Si-Al2O3-fused mullite containing Ti2O3 composite at 1600 °C

Si-Al2O3-fused mullite containing Ti2O3 composite was sintered at 1600 °C in N2. Changes in the structure and activity of fused mullite containing Ti2O3 raw material during the coupled reaction of metallic silicon and Ti2O3 are explored. The mechanism of regulating the phase composition and microstructure of the composite with the changes of Ti2O3 and fused mullite is also revealed. At high temperatures and low oxygen partial pressures, the stability of fused mullite decreases and decomposes to form SiO(g) and alumina-rich mullite. SiO(g) reacts with N2 to form Si3N4, which solidifies with mullite to form X-Sialon, further optimizing the phase composition. With the transformation of Ti2O3 to Ti(C,N)ss, the inhibitory effect of Ti3+ on the growth and development of fused mullite disappears, and the morphology of the fused mullite is gradually transformed from a dense structure to a classical elongated columnar morphology. A model for the mechanism of non-oxide formation and the regulation of micro-morphology is developed.

Preparation of mullite ceramics with equiaxial grains from powders synthesized by the sol-gel method

Four different alumina content of mullite ceramics were fabricated by powders synthesized using the sol-gel method. The synthesis process of powders, microstructure evolution, mechanical and optical properties of the mullite ceramics were studied. The XRD results showed that the precursors transformed into aluminosilicate spinel phase at 1000 °C and mullite phase at 1200 °C. Equiaxial grains were easy to form in the alumina-rich mullite ceramics while elongated grains were easy to form in the alumina-poor mullite ceramics. With the increase of alumina content, the grain size of the samples firstly increased and then decreased, the number of elongated grains decreased while equiaxed grains increased. The flexural strength, compression strength, fracture toughness, and Vickers hardness all decreased firstly and then increased. While the infrared transmittance increased firstly and then decreased. The transmittance at 4 μm (thickness of 0.75 mm) of the ceramics containing 66mol% Al2O3 reached the highest (72%) when sintered at 1780 °C because of the equiaxial grains.

Solid-state synthesis of mullite from spent catalysts for manufacturing refractory brick coatings

This paper shows the results of the solid-state synthesis of mullite from spent catalysts discarded from fluid catalytic cracking (FCC); the catalysts are mainly composed of silica and alumina but are polluted with SOX, forming a non-crystalline network. The synthesized mullite was used as a feedstock to thermally spray a coating onto a silica-alumina refractory brick, and its chemical resistance at high temperature was subsequently evaluated by contact with K2CO3 at 950°C. Initially, the spent catalyst was thermally treated for 2h at 600, 900, and 1200°C to eliminate the SOX pollutant. The heat treatment at 1200°C completely removed the SOX in the sample. Additionally, four thermal processes were performed by heating the spent FCC catalyst in an electrical furnace to 1500 and 1600°C and by using an oxyacetylene flame to synthesize mullite. Thermal treatments at 1500°C were performed with and without alumina added to the spent FCC catalyst, whereas those conducted at 1600°C and using a flame were performed using only added alumina. In the powders thermally treated at 1500°C, silica-rich mullite (3Al2O3.2SiO2) accompanied by an excess of alumina or silica was obtained with or without alumina added, respectively. In contrast, the materials treated at 1600°C formed alumina-rich mullite (2Al2O3.SiO2), which was accompanied by an excess of alumina. Mullite was not synthesized in the flame-heated powder. The silica-rich mullite accompanied by an excess of alumina was used as feedstock powder to modify the surface of a refractory brick, improving its resistance to chemical attack by K2CO3 at high temperature.

Microwave sintering of mullite-Al2O3 from kaolin precursor

In the present work the effect of the microwave hybrid fast sintering of mullite-Al2O3 with non-stoichiometric mullite composition was investigated. Samples were fast sintered using a range of microwave power from 1.2 to 2.1 kW. The mullitization process of the silica-rich composition was faster than that of the alumina-rich composition. Alumina-rich mullite showed higher densities and smaller grain size than alumina-poor mullite. It was concluded that the microwave hybrid sintering of mullite was very sensitive to composition.

Stability and phase evolution of mullite in reducing atmosphere

Abstract The aim of this study is to investigate the stability of mullite and its phase evolution when heated at 1600–1650 °C in reducing atmosphere created by a carbon bed. The stoichiometric mullite (3Al 2 O 3 ·2SiO 2 ) was synthesized by reacting the proper precursors at 1400 °C in air atmosphere. Samples containing the primary mullite and graphite or carbon were prepared by pressing the mix and heated at 1600–1650 °C. Products were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the stoichiometric mullite could decompose to corundum and alumina-rich mullite. During firing under reducing atmosphere, the stoichiometric mullite became richer in Al 3+ cations. The gaseous SiO compound was found to be released from the sample and reacting with C to form SiC phase. Comparison of general mullite solid solution (Al 4 + 2 x Si 2 − 2 x O 10 − x ) with alumina-rich mullite obtained from decomposition revealed that x would take different values depending on temperature and atmosphere. The ultimate decomposition product was found to be corundum.

High-Energy X-ray Diffraction from Aluminosilicate Liquids

Containerless high energy X-ray diffraction measurements have been performed on molten Al(2)O(3)-SiO(2) as a function of composition. The data show a strong distortion of the SiO(4) tetrahedral units and a breakdown of intermediate range order as alumina is added. For silica-rich compositions, the X-ray pair distribution functions are consistent with the presence of 4-fold oxygen triclusters predicted by molecular dynamics simulations, where these liquids exhibit relatively high viscosities compared to other binary silicates. For higher alumina content liquids, the average cation-oxygen coordination number gradually increases with increasing Al(2)O(3) content, but the pair distribution functions change very little with composition, suggesting the increased presence of disordered AlO(n) (n = 4,5,6) polyhedra are associated with low viscosity melts. A comparison of the liquid and glassy X-ray pair distributions functions at the alumina-rich mullite composition indicate a significant distortion of the polyhedra in the melt, suggesting structural changes are temperature dependent.

Functionally graded chemical vapor deposited mullite environmental barrier coatings for Si-based ceramics

Dense, uniform, crystalline mullite (3Al 2O 3·2SiO 2) environmental barrier coatings have been deposited by chemical vapor deposition on SiC substrates, using the AlCl 3–SiCl 4–CO 2–H 2 system. These coatings have exhibited excellent high temperature oxidation and hot-corrosion resistance. For very long-term exposures, it is desirable to reduce the silica activity at the coating surface. A graded coating composition has been achieved in the coatings, with the Al/Si ratio being stoichiometric (∼3) at the coating/substrate interface for coefficient of thermal expansion (CTE) match, and increasing monotonically towards the outer coating surface to some of highest alumina-rich mullite reported to date. The structure of the high alumina mullite has been examined by high-resolution transmission electron microscopy. The effects of the CVD deposition parameters on the coating microstructure and growth kinetics have been investigated.

Crystallographical analysis of surface layers of refractory ceramics formed using combined flame spray and simultaneous laser treatment

The crystalline characteristics of the surface layers of an alumina-based refractory ceramic, prepared using a novel combined flame spraying and laser melting method, have been investigated using X-ray diffraction and the Rietveld method. The results have indicated that 20% Cr2O3 by weight added to Al2O3 powder retards the metastable γ-Al2O3 to α-Al2O3 phase transition during flame spraying. The mullite crystals grew preferentially along the [111] direction for surface layers prepared by laser melting only or by the combined process using pure Al2O3 powder as the feedstock for flame spraying. In the latter case, the surface layer contained more alumina-rich mullite and more crystalline cristobalite than the surface layer prepared by laser melting. However, using a composite powder of 20% Cr2O3 and 80% Al2O3 by weight as feedstock in the combined process, not only did the preferred orientation of the mullite grains change to the [220] direction, but, also, the amount of SiO2 incorporated in the mullite crystals was closer to that for stoichiometric mullite.

Factors influencing the preparation of mullite coatings from metal chloride mixtures in CO 2 and H 2

The preparation of mullite coatings through chemical vapor deposition (CVD) from mixtures of silicon tetrachloride, aluminum trichloride, carbon dioxide, and hydrogen is addressed in this study. The results of past experimental studies on the kinetics of this deposition system and the predictions of a mathematical model that involves detailed homogeneous and heterogeneous chemistry mechanisms for the codeposition process are used to identify operating conditions where deposition of coatings consisting of mullite or alumina-rich mullite is possible. Deposition experiments are carried out in a hot-wall reactor using as substrates both small thin plates and refractory wires traversing the centerline of the reactor. Results are presented on the dependence of the film composition in silica and alumina on temperature, pressure, and residence time of the reactive mixture in the reactor. The results showed that the alumina content of the deposits increased towards the entrance of the reactor, exceeding in many cases the value that corresponds to stoichiometric mullite. The microstructure and surface morphology of the deposit depended strongly on the operating conditions. In general, the deposition of material having the composition of mullite (2SiO 2·3Al 2O 3) or alumina-rich mullite was found to be favored by low operating pressures, high Si : Al feed ratios, and low residence times in the CVD reactor.

Structure and high-temperature stability of compositionally graded CVD mullite coatings

Dense, uniform and crack-free mullite (3Al 2O 3·2SiO 2) coatings were deposited on SiC by chemical vapor deposition. The coatings were compositionally graded, with the Al/Si ratio increasing towards the outer surface of the coatings for improved corrosion resistance. The coatings were found to start out as a nanocrystalline layer, which is an intimate mixture of γ-Al 2O 3 nanocrystallites imbedded in a vitreous silica-rich matrix at the substrate/coating interface. Mullite grains nucleated when the surface composition of the growing coating was in a narrow range close to that of stoichiometric mullite. The phase transformations occurring in these coatings during high-temperature anneals in the range 1100–1400 °C were studied. These phase transformations, which include a tetragonal-to-orthorhombic transformation, mullitization and devitrification of silica in the nanocrystalline layer, and α-alumina precipitation and twinning of the alumina-rich mullite, are discussed in light of the adhesion and corrosion resistance of the coatings.

Microstructural stability and room temperature mechanical properties of the Nextel 720 fibre

The microstructure and tensile properties of the as-received and heat-treated Nextel 720 fibre have been studied. During its fabrication the Nextel 720 fibre is pyrolysed at a temperature lower than 1400°C for a very short time which does not allow the microstructure to be stabilised. A pseudo-tetragonal metastable alumina rich mullite is formed which crystallises in the form of mosaic grains containing low angle boundaries. These mosaic grains enclose some rounded and elongated α-alumina grains. The evolution of the mullite to the stable orthorhombic symmetry is seen from 1200°C for post heat treatments lasting several hours. For longer heat treatments at 1200°C or at 1300°C and higher temperatures, the mosaic grains begin to recrystallise into single grains and the alumina rejected from the mullite contributes to the growth of elongated α-alumina grains which suggests diffusion through an intergranular liquid silicate phase. At 1400°C the mullite has the 3:2 composition and after 24 h the growth of the elongated α-alumina grains leads to a reduction of the room temperature tensile strengths. The α-alumina creation coupled to the phase transformation and dissolution of mullite leads to an increase of the Young's modulus after heat treatments from 1200°C.

New aqueous mullite precursor synthesis. Structural study by 27Al and 29Si NMR spectroscopy

A mullite precursor gel is prepared by the slow and homogeneous generation of ammonia inside an aqueous solution of aluminium nitrate and silicic acid, urea being used as the base generator. At 80–100 °C a silica gel is rapidly formed, by a catalytic reaction, and then this gel is slowly digested by the partially hydrolysed aluminium atoms which link to silica by SiOAl bonds. The structural evolution has been studied by 27Al and 29Si NMR spectroscopy. The first aluminium atoms are incorporated in tetrahedral symmetry and the following ones remain hexacoordinated. The system evolves to soluble aluminosilicate colloids with a local structure close to that of imogolite or allophane: each silicon atom being linked to three AlO 6 hexahedra and a hydroxyl group. By the completion of the hydrolysis the colloids are cross linked to a gel precursor of mullite. With thermal treatment the imogolite-like orthosilicate units arrangement is rapidly lost, but the xerogel remains chemically homogeneous with a random distribution of Al and Si atoms in the lattice, the Al atoms being four-, five- and six-fold coordinated. The xerogel crystallises at 980 °C into an alumina-rich mullite and amorphous silica, and the 3Al 2O 3· 2SiO 2 stoichiometry is reached at 1300 °C by reincorporating the silica in the lattice.

Solid-state NMR examination of the formation of β-sialon by carbothermal reduction and nitridation of halloysite clay

β-Sialon (Si6 –zAlzOzN8 –z; 0 < z < 4.2] can be readily formed from halloysite clay [Al2Si2O5(OH)4·2H2O] at 1400 °C by carbothermal reduction using brown coal and nitridation. A combination of X-ray diffraction, transmission electron microscopy and 29Si and 27Al solid-state NMR are used to establish the details of the intermediate stages of the reaction. NMR clearly shows a distinct sequence of reactions of silica and mullite formed from the thermal decomposition of halloysite with carbon. The carbon initially reacts with free silica and then removes silicon from the mullite, to form a β-SiC-like phase, leaving an alumina-rich mullite framework. SiC subsequently reacts with the remaining silicon from the mullite phase to form some Si3N4 which then combines with silicon from SiC, the alumina-rich mullite and nitrogen to form β-sialon. The 29Si and 27Al NMR spectra from sialon X-phase are reported.