Microstructure Of Mullite Research Articles

Effects of partial substitution of calcium alumino-titanate on the properties and microstructure of mullite–cordierite composites

In order to realize the recycling of calcium alumino-titanate (CAT, a solid waste), the effect of small amounts (3–9% weight ratio) of CAT on the sintering properties of the mullite–cordierite system was investigated. The thermal-shock stability, volume stability, and mechanical properties of samples with different amounts of CAT were characterized and tested. The influence of CAT on the phase composition and microstructure of the mullite–cordierite system was also deduced from thermodynamic calculations. Experimental results, which agreed with the calculations, demonstrate that CAT could promote the close combination and aggregation of matrix materials, thus strengthening the consolidation process and enhancing desirable properties.

A novel monolithic mullite microfiltration membrane for oil-in-water emulsion separation

Monolithic ceramic microfiltration membranes made up of an interlocking network of mullite crystals have been developed from low-cost raw materials, consisting of China clay, AlF3·3H2O and Al2O3. The characteristics of the developed membranes have been studied. The membrane made from clay+40 wt% AlF3·3H2O powder mixture with 10 wt% Al2O3 displayed excellent properties, with apparent porosity of 64%, median pore diameter of 0.3 μm, flexural strength of 43 MPa, and pure water permeance of 1031 L/m2 h bar. The membrane also displayed excellent pore connectivity and chemical stability owing to the crosslinked needle-like mullite microstructure. The performance of the mullite microfiltration (MF) membrane in oil-in-water emulsion separation was studied with three test solutions having oil concentrations of 200 mg/L, 500 mg/L and 1000 mg/L. The membrane showed excellent permeate flux of more than 200 L/m2. h at a transmembrane pressure of 2 bar and high oil rejection of more than 96% with all the feed concentrations. The fabrication of the monolithic mullite MF membrane in comparison to the conventional asymmetric ceramic MF membranes is highly economical and can be used as a cost-effective alternative for oil-in-water emulsion separation.

Microstructural analysis (Voigt function method) of mullite in whiteware triaxial porcelains

Triaxial porcelains were produced of two whiteware blends containing kaolin B (of “high-crystallinity” kaolinite) or halloysitic kaolin M (also with “low-crystallinity” kaolinite) in two different industrial firing cycles (fast or slow) with final temperatures 1270 °C, 1300 °C, 1320 °C and 1340 °C. The crystalline microstructure of mullite in all porcelain samples was studied by X-ray diffraction through analysing 110 and 001 reflections using the Voigt function method and by optical and electron microscopy. Mean crystallite sizes were determined independently for both 110 and 001 diffraction directions as principal semi-quantitative characteristics for all sixteen specimens. They illustrated well the influence of kaolin kind, firing cycle and processing temperature on crystalline microstructure. Significant differences were observed as functions of each of these three parameters for most pairs of specimens. The differences of mean crystallite sizes were significant for 110 diffraction direction and smaller for 001 (especially for samples fired in higher temperatures). Additionally, the influence of fluxes of a triaxial composition on growth of mullite crystallites was shown by comparing crystallite sizes of mullite in the studied industrial porcelains to those produced by firing a reference kaolin. In most samples positive correlation of mean crystallite size estimated for [001] direction with length of crystalline prism estimated from electron microscopy images was noted, and this can be considered relevant for the investigation of formulations and processing conditions, since microstructural observations by electron microscopy require much longer sample preparation times, and are less representative of the studied whole sample.These results are significant in formulation of industrial porcelains in order to select both raw materials and firing cycles for obtaining required mullite development. That will improve the technical properties of final industrial porcelain.

Structural, microstructural and mechanical properties of sintered iron-doped mullite

The study of an effect of iron doping on the structural, microstructural and mechanical properties of sintered iron-doped mullite is presented. The results of phase composition, performed in detail by Mössbauer spectroscopy and XRD analysis, revealed that all added iron was inside the mullite lattice forming the single phase up to 12% by weight of Fe2O3 and 1300 °C. Samples, which were processed at 1550 °C, contained secondary phases, hematite or magnetite, regardless of the amount of added iron. Furthermore, the addition of iron decreases the values of relative linear shrinkage comparing to the values of undoped one (~18%) while the densities of the sintered samples rise as well as their values of microhardness. Even though the density values were not too high (90 TD%), the obtained values of microhardness were excellent, 1634 HV0.1 for maximum iron content due to the characteristic mullite microstructure.

Effect of silver nanoparticless in the structure and mechanical properties of mullite/Ag cermets

The objective of this work was to study the influence of the addition of silver nanoparticles in the microstructure of mullite at two different temperatures of sintering (1500 and 1600?C), in order to decrease the porosity and increase the density as well better the hardness and fracture toughness. The microstructural characteristics were studied by scanning electron microscopy, confocal scanning microscopy and X-ray diffraction. Mullite/Ag cermets with homogenous microstructure were and a fracture toughness of 2.42 MP??m1/2.

Impact of inorganic waste fines on structure of mullite microspheres by reaction sintering

The impact of waste composition and morphology on the properties of the mullite microstructure were studied by reaction sintering two types of siliceous waste powders with calcined alum sludge. Stoichiometric mixtures of fly ash or rice husk ash with the alum sludge were shaped into mullite microspheres by droplet coagulation. The mullitization reaction was followed by HT-XRD with a distinct difference in onset temperature. The specific powder composition of fly ash (presence of primary mullite) shifted the onset temperature to higher values as compared to the rice husk ash mixture. After sintering at 1600 °C, the mullitization reaction resulted in the same yield for both samples. Although the mullite content in the microspheres was equal, distinct differences on the microstructure could be observed. Reaction sintering of the fly ash mixture resulted in lower total porosity with smaller pore sizes.

Low-temperature sintering and microstructure control of mullite–Mo composites

Dense mullite–Mo (45vol%) composites with homogeneous microstructure have been obtained by plasma activated sintering of a mixture of Mo and mullite precursors at a relatively low temperature (1350°C) and a pressure of 30MPa. The mullite precursor was synthesized by a sol–gel process followed by a heat-treatment at 1000°C. The influence of different mullite precursors on the densification behavior and the microstructure of mullite–Mo composites has been studied. The precursor powder heat-treated at 1000°C with only Si–Al spinel but no mullite phase shows an excellent sintering activity. Following the sintering shrinkage curves, a two-stage sintering process is designed to enhance the composite densification for further reducing the sintering temperature. The study reveals that viscous flow sintering of amorphous SiO2 at low temperatures effectively enhances the densification. Moreover, microstructure of these composites can be controlled by selecting different precursors and sintering temperatures.

Microstructure and properties of mullite–ZrO2 ceramics with silicon nitride additive prepared by spark plasma sintering

The process of densification and development of the microstructure of mullite–ZrO2/Y2O3 ceramics from mixture of Al2O3, SiO2, ZrO2 and Y2O3 by gradually adding of α–β Si3N4 nanopowder from 1 to 5wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400°C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si3N4 additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33g/cm3, while for traditionally sintered samples, the value is 2.55g/cm3, and the compressive strength for SPS grows with Si3N4 additives, reaching 600N/mm2. In the case of traditional sintering, it decreases to approximately 100N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO2 grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si3N4 additive, are filled with mullite crystalline formations.

Cost Effective And Minimal Time Synthesis of Mullite From A Mine Waste By Thermal Plasma Process

The synthesis of mullite from an aluminous-rich mine waste (shale) closely associated with iron/manganese mines from Bonai-Keonjhar belt, Odisha is reported. The shale constitutes major kaolinite with minor halloysite, quartz, orthoclase and plagioclase minerals and compositionally contains 36.40 % Al2O3 and 52.10% SiO2. In order to convert this mine waste to a refractory product, ‘Mullite’, alumina powder was added in 1:0.9; 1:1 and 1:1.1 weight ratios with the shale and thoroughly homogenized. Individual mixtures were fed in to the thermal plasma reactor and processed for only 5 minutes to form mullite. The phase and microstructure developed in the processed samples were investigated from their XRD patterns, Raman spectra and SEM images. Diagnostic peaks of the mullite phase are distinctly marked in the XRD pattern and Raman spectra of plasma treated products. Microstructure of mullite observed under SEM clearly exhibits a fused layered structure. The integrated results confirm the formation of high quality mullite from a mixture of 1:1 weight ratio. This low cost process can be implemented in industrial scale for processing of such mine waste to a value added refractory product.

Synthesis of mullite from sillimanite dissociation through transferred arc plasma torch

The transferred arc plasma process is characterized by extremely high temperatures, low environmental impact and short processing time. By virtue of high temperature and power density it paves the way for a high production rate. Hence an experiment was undertaken to synthesize mullite from the dissociation of sillimanite through thermal plasma processing. For this purpose, a transferred arc plasma (TAP) torch was specially designed on laboratory scale level and the operating parameters were optimized in order to achieve a maximum percentage of mullite with time and cost effectiveness. The phase and microstructure formation of the processed samples were analyzed by X-ray diffraction (XRD) and scanning electron microscope (SEM) images. The obtained results clearly show that the torch input power and processing time strongly influence the mullite microstructure and phase formation. The present TAP torch processing gives maximum percentage of mullite (95%) through sillimanite dissociation at 15 kW power level with 4 min of processing time.

Preparation of Mullite by Coating Al(OH)3 on the Surface of Fly Ash

Porous mullite were prepared by coating Al(OH)3 on the surface of the particles of waste fly ash. The effects of content of urea on formation of mullite and sintering temperature on microstructure of mullite were investigated by XRD and SEM analysis. It was showed that two kind mullite were obtained, one was growing from original Needle-like mullite existing in original ashfly, and the other was formed through by the reaction between glassy phase and Al2O3 coated on spherical waste fly ash.

Formation mechanism and microstructure development in acicular mullite ceramics fabricated by controlled decomposition of fluorotopaz

The Dow Chemical Company has developed a ceramic wall flow filter for use as a diesel particulate filter (DPF). It is composed of mullite grown from the low temperature (1000–1200 °C) decomposition of fluorotopaz in the presence of silicon tetrafluoride. Mullite grains are acicular (aspect ratio ∼20), and their size can be controlled by raw material and processing variables (diameters chosen from 3 to 50 μm). The acicular mullite grains interlock forming an efficient filter with low back pressure. Heating precursors in air or introducing SiF 4 at temperature does not lead to a similar mullite microstructure. Characterization of samples quenched during the transformation from fluorotopaz to mullite indicates that growth of mullite was facilitated by a melt. The mullite formed is alumina-poor and chemically zoned from a molecular ratio Al 2O 3/SiO 2 = 1.25 in the core to Al 2O 3/SiO 2 = 1.7 in the rim. This mullite composition (5:4–7:4) appears to be stable across a broad range of temperatures.

Microstructural variations in mullite ceramics derived from diphasic sols using different forming techniques

Mullite ceramics with controlled microstructure in terms of residual glassy phase and density were produced from diphasic silica/boehmite sols using pressure filtration, die-pressing, and extrusion techniques. The effect of the compaction process on the green and sintered densities and on the final mullite microstructure was investigated. It is shown that glassy-phase-free mullite with high green and sintered densities can be produced from engineered diphasic colloidal suspensions using pressure filtration. The results show that the green body formation technique strongly determines the final sintered density, but it has no effect on the final phase composition of diphasic sol-derived sintered mullites.

Sintering and microstructure of mullite–Mo composites

Abstract Mullite–Mo composites of different compositions (0–100 vol.% Mo) were sintered to near theoretical density by pulse electric current sintering (PECS). The densification behaviour and the microstructure of mullite–Mo composites as a function of Mo content were studied. The addition of 10 vol.% Mo significantly enhanced the strength and toughness of monolithic mullite to 556 MPa and 2.9 MPa m 1/2 , respectively. SEM observations revealed the modification of discrete isolated Mo particles to continuosly interconnected network with the increase in the Mo content. Mo grains were located at the grain boundaries as well as inside the mullite grains. The addition of Mo to monolithic mullite led to a change in the fracture mode.

Colloidal Processing of Glassy-Phase-Free Mullite from Heterocoagulated Boehmite/Silica Nano-composite Sol Particles

The effects of nano scale designed ultrafine mullite precursor microstructures on the formation and location of silica-rich glassy phase in the final sintered mullite matrix have been examined. By engineering and designing the stoichiometric mullite precursors comprising ultrafine boehmite and silica particles, the location and amount of glassy phase in sintered mullite can be controlled. Particle-particle interactions of the two dissimilar species in the nanoscale were controlled in terms of the particle size ratio and particle shape. An obtained glassy-phase-free mullite microstructure is expected to provide very good mechanical properties at high temperature in terms of creep and thermal shock resistance.

Densification behaviour and microstructure of gel-derived phase-pure mullite in the presence of sinter additives

Gels with high pure mullite composition have been synthesized using aluminum nitrate nanohydrate and ethyl silicate. Pure mullite has been sintered to 88.5% of the T.D., when sintered at 1600°C for 3 h. Suitable sintering aids such as SrO (0.5 wt.%) and clay (2 wt.%) have tremendous effect in promoting the densification behavior of mullite to attain density to 98 and 95% of T.D. respectively at relatively low temperatures by pressureless sintering. SrO promotes the densification of pure mullite via liquid phase sintering and also does clay assist the viscous flow sintering. Clear differences found in the microstructures of mullite could be related to the different sintering aids, which have apparent effect on the densification behavior. SrO doped samples show a duplex microstructure of fine equiaxed and elongated grain morphology, while the clay doped samples show equiaxed and acicular morphology.

Mulita a partir de hidróxido de alumínio e de sílica da casca de arroz

Mulita foi sintetizada a partir do hidróxido de alumínio, Al(OH)3, purificado de rejeito industrial, e de sílica obtida a partir da queima, a 500 oC, da casca de arroz purificada. Ainda que o processo de formação da mulita se inicie a 1300 oC as experiências foram conduzidas a 1600 oC durante 8,0 h a fim de se medir a densidade final das peças e se observar o efeito da adição de ítria no processo. As densidades aparentes encontradas foram de 2,70 g/cm3 e 2,84 g/cm3, respectivamente, para a mulita pura e para a formulação mulita com 5,0% em peso de Y2O3. A microestrutura da mulita dopada com ítria mostra alguns grãos longos, até 30 mm. A análise de EDS mostrou que o ítrio foi incorporado na mulita

Effects of Y<sub>2</sub>O<sub>3</sub> Addition on the Sinterability and Microstructure of Mullite (Part 2)

The phenomena in the sintering process of Y2O3-added mullite ceramics and the crystallization behavior of liquid phase in sintered bodies were studied using differential thermal analysis, X-ray powder diffractometry, scanning and transmission electron microscopies. The mullite powder compacts containing 2-10wt% Y2O3 were sintered at 1500°C for 2h, and were then cooled in furnace or quenched in order to study the crystallization phenomena. During the firing process, a liquid phase containing Y, Al, Si and O components was formed in the sintered body. When the specimen was cooled in furnace, the liquid phase solidified into glassy state or crystallized depending on the amount of Y2O3. The glass phase in the furnace-cooled specimen could also crystallize on annealing. Some crystallization phenomena of the liquid phase were observed. A small α-Al2O3 grain, which was formed by the solid state reaction between mullite and Y2O3 during the firing process, was observed inside a mullite grain. Larger α-Al2O3 grains, which were formed from the solution-reprecipitation process of liquid phase, were observed in the sintered body.

Effects of Y<sub>2</sub>O<sub>3</sub> Addition on the Sinterability and Microstructure of Mullite (Part 1)

Effects of Y<sub>2</sub>O<sub>3</sub> Addition on the Sinterability and Microstructure of Mullite (Part 1)

Production of Ti powders by calcio-thermic reduction of TiO2

The process of densification and development of the microstructure of mullite–ZrO2/Y2O3 ceramics from mixture of Al2O3, SiO2, ZrO2 and Y2O3 by gradually adding of α–β Si3N4 nanopowder from 1 to 5 wt% by traditional and spark plasma sintering were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and some ceramic and mechanical properties. The processes of DTA for all samples are characterised by a low-pitched endo-effect, when gradual mullite formation and noticeable densification at temperatures of 1200–1400 °C is started. It is testified by shrinkage and density both for traditionally and by SPS-sintered samples. The influence of the Si3N4 additive on the density characteristics is insignificant for both sintering cases. For SPS samples, the density reaches up to 3.33 g/cm3, while for traditionally sintered samples, the value is 2.55 g/cm3, and the compressive strength for SPS grows with Si3N4 additives, reaching 600 N/mm2. In the case of traditional sintering, it decreases to approximately 100 N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS is created from mullite (or pseudo-mullite) crystalline formations with the incorporation of ZrO2 grains. The microstructure of ceramic samples sintered by SPS shows that mullite crystals are very densely arranged and they do not have the characteristic prismatic shape. The traditional sintering process causes the creation of voids in the microstructure, which, with an increasing amount of Si3N4 additive, are filled with mullite crystalline formations.