Mullite Precursor Sol Research Articles

Dispersion of SiC powder suspension in mullite sol and influence on properties of sintered ceramics

The dispersion behavior of SiC powders in various concentration of mullite sol was studied by zeta potential, rheological, and sedimentation measurements. Effect of pH, sol concentration, solid loading on the rheological properties are discussed in detail. The lowest viscosity and sedimentation was obtained for 45 wt% SiC powders dispersed in 15 wt% mullite sol, at pH similar to 2. The adsorption isotherms belong to Langmuir type and optimum concentration of mullite for maximum dispersion was found to be similar to 0.036 g/m(2). The stabilization mechanism of SiC powder into mullite precursor sol was explained by Derjaguin-Landau-Vervey-Overbeek (DLVO) theory. Coating was confirmed by transmittance electron microscopy and energy-dispersive X-ray spectroscopy analysis. Further sintering of the mullite-coated SiC powder compact at 1300 degrees C produced porous SiC ceramic with porosity similar to 40 vol% and pore diameter similar to 3 mu m. The corrosion behavior of the ceramics was studied in strong acid medium at 90 degrees C and compared with the ceramics obtained from uncoated SiC powder.

Properties of mullite-bonded SiC ceramics from mullite precursor sol coated SiC powder and crystallization kinetics of mullite formation

Mullite bonded -bonded porous silicon carbide (SiC) ceramics were fabricated by sintering sol–gel mullite-coated SiC particles. The coated powder was characterized by measuring zeta potential at different pH, X-ray photoelectron spectroscopy (XPS), XRD analysis and thermal analysis. The thermal oxidation behaviour of coated SiC powder was examined by thermogravimetry (TG)-Differential thermal analysis (DTA) technique in dry air. Mullite formation exhibited two exothermic peaks; the first 1195–1213°C and the second at 1319–1358°C. The activation energy for mullite crystallization in the first step ranged from 884 to 1250 KJ mole− 1 and in second step it ranged from 759 to 1018 KJ mole− 1. Characterization of resulting SiC ceramics were done by XRD, SEM, TEM, EDS, porosity, pore size distribution analysis and measurement of mechanical strength. The ceramics with porosity ∼42 vol.-% exhibited flexural strength of ∼23 MPa. Microstructure observations revealed that neighbouring SiC particles were bonded by well-developed neck consisting of needle-shaped mullite grown in a siliceous matrix.

Microstructure and oxidation behavior of sol–gel mullite coating on SiC-coated carbon/carbon composites

A dense, uniform, crack-free and well-bonded sol–gel mullite coating with thickness of about 178μm was successfully fabricated on SiC-coated carbon/carbon (C/C) composites. Mullite coating is produced by airbrush spraying a 3/2-mullite coating precursor solution consisting of fine mullite powder dispersed in a mullite precursor sol and air sintering the as-deposited coating. This method has several benefits such as simplicity of the process, high efficiency, ability to coat complex geometries and cost-effectiveness. The mullite/SiC coating was found to protect C/C composites from oxidation at 1773K for 100h with a weight loss rate of only 6.85×10−5gcm−2h−1 and retained compression strength of 97.44%. The oxidation rate was found to be significantly decreased with oxidation time, possibly due to the densification of mullite coating during the oxidation process. The corresponding high temperature oxidation activation energy of mullite/SiC coated C/C in the temperature range of 1573–1773K is calculated to be 110.63kJ/mol.

Comparative study of conventional and microwave sintered mullite fibres: structural study

Mullite fibres were synthesised from a mixture of aluminium, aluminium chloride and acidic silica solutions as monophasic salts using a sol–gel technique. The viscosity and rheological behaviour of the mullite precursor sol were examined. Mullite fibres were synthesised using both conventional and microwave sintering techniques. The samples were characterised by X-ray diffraction and scanning electron microscopy. Pure mullite fibres were synthesised by microwave sintering at a relatively low temperature of 1200°C. The grain size of the fibre samples sintered using the microwave technique was finer than that produced by conventional sintering. These results show that microwave sintering is a promising technique for processing mullite fibres.

Preparation of uniaxially aligned mullite ceramic fibers by electrospinning

Uniaxially aligned mullite ceramic fibers were prepared by electrospinning a polyacrylonitrile/mullite sol mixture with a rotating drum simultaneously acting as a collection device followed by sintering. A novel mullite precursor sol was synthesized using N,N-dimethylformamide as a solvent. The mean diameter of the mullite sol particles was 15.6±5.7nm. The crystal development and micromorphological evolution of both the as-electrospun composite fibers and the sintered mullite fibers were investigated by scanning electron microscopy, X-ray diffractometry, thermogravimetry–differential scanning calorimetry, and Fourier transform infrared spectroscopy. The sintering temperature played an important role in controlling the surface morphology, diameters, and crystal structure of the mullite fibers. Upon sintering the composite fibers at 1000 and 1300°C, metastable tetragonal phase mullite and stable orthorhombic phase mullite fibers were obtained.

Mullite fibers prepared from an inorganic sol–gel precursor

Mullite fibers were prepared by sol–gel process using alumina sol and silica sol. Alumina sol was synthesised from dissolution of aluminum powder in aluminum chloride hexahydrate solution. The optimal spinning alumina sol could be obtain in the composition range of Al/AlCl3·6H2O molar ratio 3.4–3.8. The Al and Si components were mixed at the molecular level and linear molecules were formed in the mullite precursor sol. The dried mullite precursor gel fibers completely transformed to mullite fibers at 1,000 °C with a smooth surface and uniform diameter.

Effects of precursor pH and composition on the grain morphology and size of mullite ceramics in aqueous system

The effects of the precursor pH and composition on the synthesizing behavior, grain morphology, and grain size of mullite ceramics have been investigated using silica-rich (Al 2O 3·2SiO 2), stoichiometric (3Al 2O 3·2SiO 2), and alumina-rich (7.5Al 2O 3·2SiO 2) mullite precursor sols. Mullite precursor sol was prepared by the dissolution of aluminum nitrate enneahydrate (Al(NO 3) 3·9H 2O) into the mixture of colloidal silica sol. Precursor pH of the sols was controlled to the acidic (pH≈1.5–2) and basic (pH≈8.5–9) conditions. The synthesizing temperature of mullite phase was found to be above 1200 °C for pH≈1.5–2, and above 1300 °C for pH≈8.5–9 through X-ray diffraction (XRD) and Fourier transfer infrared spectroscopy (FT-IR) results, independent of composition. The grain size of the synthesized mullite for pH≈8.5–9 was larger than that for pH≈1.5–2 in the overall heat-treated temperature. Aspect ratio of the mullite grains was increased with an increase of the silica concentration. It was found that the grain morphology and grain size were predominantly governed by the initial composition and precursor pH, respectively.

Synthesis behavior and grain morphology in mullite ceramics with precursor pH and sintering temperature

The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite were studied using a stoichiometric mullite (3Al2O3 · 2SiO2) precursor sol. The mullite precursor sol was prepared by the dissolution of aluminum nitrate enneahydrate [Al(NO3)3 · 9H2O] into the mixture of silica sol. The precursor pH of the sols was controlled to the acidic (pH ≈ 1.5 to 2), intermediate (pH ≈ 4.5 to 5) and basic (pH ≈ 8.5 to 9) conditions. The gels dried from the synthesized aluminosilicate sols were formed into a disk shape under 20 MPa pressure; then the green bodies were sintered for 3 h in the temperature range of 1100–1600 °C. The synthesizing temperature of mullite phase was found to be above 1200 °C for pH ≈ 1.5 to 2, and above 1300 °C for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9. The grain morphology of the synthesized mullite was changed to a rod shape for pH ≈ 1.5 to 2, and granulate shape for pH ≈ 4.5 to 5 and pH ≈ 8.5 to 9 with increasing sintering temperature. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature. However, at higher pH, the precursor pH and sintering temperature did not affect the synthesis behavior and grain morphology.

Effects of Precursor pH on Synthesizing Behavior and Morphology of Mullite in Stoichiometric Composition

Stoichiometric mullite (<TEX>$3Al_2$</TEX><TEX>$O_3$</TEX>. <TEX>$2SiO_2$</TEX>) precursor sol has been prepared by sol-gel method. The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite have been studied. Mullite precursor sol was prepared by dissolution of aluminum nitrate enneahydrate (Al(<TEX>$NO_3$</TEX>)<TEX>$_3$</TEX>.9H<TEX>$_2</TEX>O) into the mixture of silica sol. Precursor pH of the sols was controlled to acidic condition (<TEX>$PH\leq$</TEX> 1~1.5) and to basic condition (<TEX>$pH\geq$</TEX>8.5~9). The synthesized aluminosilicate sols were formed under 20 MPa pressure after drying at <TEX>$150^{\circ}C$</TEX> for 24 hours, and then sintered for 3hours in the temperature range of <TEX>$1100~1600^{\circ}C$</TEX>. From TGA/DTA analysis, total weight loss in the aluminosilicate gel of the acidic sample was (equation omitted) 56% and that of the basic sample was (equation omitted) 85%, indicating that the synthesizing temperature of mullite phase for acidic and basic samples was above <TEX>$1200^{\circ}C$</TEX> and <TEX>$1300^{\circ}C$</TEX>, respectively. The morphologies of the synthesized mullite were fine and needle-like (or rod-like) for acidic sample, and granular for basic sample that has been sintered above <TEX>$1300^{\circ}C$</TEX>. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature.

Microstructure and Mechanical Properties of Sol-Gel Derived Mullite Containing Needle-Like Grains

Synthesis of mullite whiskers in F− atmosphere was carried out by means of vapour-phase reaction. A mullite precursor sol was prepared by mixing aluminium nitrate and ethyl silicate in propanol medium. The addition of HF triggers the gelation of mullite. The powders was uniaxialiy pressed into bars and was heat-treated at 1100°C in an airtight container. During this process, mullite whiskers were formed and they were used as growth centres for needle like grains. The sapmles when sintered at 1600°C for 4 h in air, showed needle morphology and the fracture toughness was 2.3 MPa.m½.

Densification of mullite–SiC nanocomposite sol–gel precursors by pressureless sintering

Mullite–SiC nanocomposite has been synthesised based on a nanoprecursor route and sintered to high density through pressureless sintering technique. The approach has been first to evolve a method to obtain high density, fine-grained mullite matrix phase through a sol–gel seeded route. Nanosize SiC particles (∼200 nm) were dispersed in a seeded mullite precursor sol to obtain mullite-coated SiC particles, which were further compacted and sintered to hybrid composites, resulting in distribution of SiC particles at the inter- and intra-mullite grain positions.

Mullite Whiskers from Precursor Gel Powders

A mullite precursor sol was prepared by mixing boehmite and silica sols. On addition of 3.5 wt% F− ion as a 47% solution of HF, the mullite sol was gelled. The dried gel was ball‐milled and calcined at 1400°C for 1 h in an airtight container. Mullite whiskers grew on the (111) plane along the 〈001〉 direction.