Mullite Bodies Research Articles

Properties of Durable Mullite Bodies Manufactured from Waste Clay-Diatomite

Durable mullite bodies have been fabricated using diatom frustules from diatomite powder as the Si source and Al-nitrate as the Al precursor, resulting in fibrous pore morphology. The hard mullite ceramics prepared by mold pressing without additives showed high compressive strength (up to 133 MPa when sintered at 1500 °C). The diatomite-nitrate samples were sintered at three temperatures (1300, 1400, and 1500 °C) for 2 hours. XRPD analysis of the sintered samples showed that the crystalline mineral phases mainly comprise mullite, cristobalite, and corundum. SEM results indicate the presence of rod-like mullite grains measuring 5 µm in length and 500 nm in diameter (aspect ratio 1:10). XRPD analysis of the samples sintered at 1300 °C demonstrated good thermo-mechanical stability and the formation of new hard phases (mullite, corundum, and cristobalite), making the analyzed diatomaceous earth suitable to produce various types of ceramic, construction, and thermal insulating materials.

Physico-Mechanical and Microstructure Characteristics of Porous Mullite Ceramics

Porous mullite ceramics were prepared from mixtures of Egyptian kaolinite-clay and calcined α-alumina powder. The effects of two methods of compaction as well as two types of binder were investigated. The influence of sintering temperature on the physical properties, crystalline phase, compressive strength and microstructure was studied. Mullite bodies processed by uniaxial pressing utilizing 15wt. % water at 1500°C for 2h exhibited the highest compressive strength of 150 MPa and bulk density of 2.1 g/cm3, as well as, an open porosity of 35%. In the present study, the only stoichiometric mullite was achieved by the crystallization of metakaolin. The mullite crystallizing out from the liquid phase was rich in silica, as determined by EDS, while that resulting from alumina grains was rich in alumina. The microstructure showed that primary mullite appeared at 1200°C, beside the unreacted alumina particles. On the other hand, at 1400°C alumina particles started to react with the glassy phase to form parallel laminates of mullite.

Mechanical and thermal behavior of cellular mullite materials

In this paper, the mechanical behavior and thermal properties of cellular mullite bodies obtained by thermal direct-consolidation of foamed aqueous suspensions of mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose (MC) were studied. The mechanical behavior of cellular mullite materials sintered at 1600 °C was evaluated by diametral compression at room temperature, 1000 °C and 1300 °C. The variation in the thermal diffusivity and thermal conductivity at temperatures up to 900 °C was determined using the laser-flash method. The results of the mechanical and thermal evaluation were analyzed based on the porosity features of the sintered materials, which was determined in turn by the starting system used for shaping the bodies.

Fly Ash Utilisation in Mullite Fabrication: Development of Novel Percolated Mullite

Fly ash is an aluminosilicate and the major by-product from coal combustion in power stations; its increasing volumes are major economic and environmental concerns, particularly since it is one of the largest mineral resources based on current estimates. Mullite (3Al2O3·2SiO2) is the only stable phase in the Al2O3-SiO2 system and is used in numerous applications owing to its high-temperature chemical and mechanical stabilities. Hence, fly ash offers a potential economical resource for mullite fabrication, which is confirmed by a review of the current literature. This review details the methodologies to utilise fly ash with different additives to fabricate what are described as porous interconnected mullite skeletons or dense mullite bodies of approximately stoichiometric compositions. However, studies of pure fly ash examined only high-Al2O3 forms and none of these works reported long-term, high-temperature, firing shrinkage data for these mullite bodies. In the present work, high-SiO2 fly ashes were used to fabricate percolated mullite, which is demonstrated by the absence of firing shrinkage upon long-term high-temperature soaking. The major glass component of the fly ash provides viscosities suitably high for shape retention but low enough for ionic diffusion and the minor mullite component provides the nucleating agent to grow mullite needles into a direct-bonded, single-crystal, continuous, needle network that prevents high-temperature deformation and isolates the residual glass in the triple points. These attributes confer outstanding long-term dimensional stability at temperatures exceeding 1500 °C, which is unprecedented for mullite-based compositions.

Cellular mullite materials processed by direct foaming and protein casting

In this paper, cellular mullite bodies were developed by thermal direct-consolidation of foamed aqueous mullite-bovine serum albumin (BSA) and mullite-BSA-methylcellulose suspensions, burning out (650 °C, 2 h) and sintering (1600 °C, 2 h). Some modifications to the shaping route conventionally used in protein casting were incorporated in the proposed processing to obtain bodies with controlled and homogeneous microstructures. The materials were characterized by porosity measurements, analysis of phases by XRD, and microstructural analysis by SEM and Hg-porosimetry. Characteristic parameters of cell size distributions, percentage of open and closed cells, window size and interstitial pore size distributions, and microstructural features of the mullite matrix were determined. Moreover, basic 2D cell size parameters and global 3D stereological parameters were analysed. The obtained results showed that mullite bodies with hierarchical porosity and different microstructural features were developed from the design and control of processing routes, which use BSA as a foaming and binder/consolidator agent.

Shaping of porous mullite green bodies by foaming and thermal gelation of bovine serum albumin

The shaping of cellular microstructures of mullite bodies prepared by thermo-gelation of foamed mullite-bovine serum albumin (BSA) suspensions using both a novel forming route and the conventionally reported route were studied. Mullite-BSA and mullite-BSA-methylcellulose (MC) suspensions specific for each route were prepared and then characterized by measuring viscosity. They were subsequently foamed by stirring, and their foaming capacity and foam stability over time were evaluated. Bubble size distributions vs. stand time were obtained by analyzing images captured by confocal laser-scanning microscopy (CLSM). Green bodies were formed by pouring each system into heated molds, by heating and by drying, and were characterized by porosity measurements and microstructural analysis by SEM. The obtained results showed that the use of certain experimental conditions for each route allowed the shaping of homogeneous mullite porous bodies with different microstructural features, namely, ceramic foams with open cells or cellular ceramics with closed cells and thick walls.

Processing of porous mullite ceramics using novel routes by starch consolidation casting

Abstract In this article, the development and characterization of porous mullite bodies prepared using two novel forming routes with native starches were studied with the aim of developing bodies without deformation and with homogeneous porous microstructures. Mullite–starch suspensions specific for each route were prepared by mixing and characterized by measuring viscosity. Mullite green bodies were fabricated by heating the suspensions in metallic molds and by burning out the starch, while final porous materials were obtained by sintering at different temperatures. Bodies obtained before and after the burning-out process, and sintered disks, were characterized with porosity measurements and microstructural analysis by SEM. The phases generated after the sintering process were determined by XRD, and pore size distributions were studied by Hg-porosimetry. The obtained results showed that the use of both routes allowed the shaping of homogeneous mullite bodies without causing cracks or deformations and the consequent development of controlled porous mullite microstructures.

Porous Mullite Ceramics Formed by Direct Consolidation Using Native and Granular Cold‐Water‐Soluble Starches

In this article, the processing and microstructures of porous mullite bodies prepared by modifying the conventional route of the starch consolidation casting method were studied. The proposed route, called the “soluble route”, involves the use of native starches (i.e., potato, cassava, and corn starches) and a synthesized granular cold‐water‐soluble (GCWS) starch. Stable aqueous mullite‐starch suspensions (0.25 starch volume fraction of 40 vol% total solids) were prepared by mixing. The total starch content was a mixture of ungelatinized native starch and GCWS starch with a 1:10 ratio of GCWS starch to total starch. Steady‐state shear flow properties of the suspensions were analyzed by measuring viscosity. The addition of CGWS starch increased the starting suspension viscosity and thus prevented the particle segregation. Porous mullite bodies were obtained by heating (80°C, 2 h) the suspensions in metallic molds and by drying (40°C, 24 h) and sintering (1650°C, 2 h) the green disks after burning out the starch (650°C, 2 h). Green bodies obtained before and after the burning‐out process, and the sintered disks were characterized with density and porosity measurements (Archimedes method) and microstructural analysis by scanning electron microscopy. The phases generated after the sintering process were determined by X‐ray diffraction analysis, and pore size distributions were studied by Hg‐porosimetry. The obtained results showed that the use of the GCWS starch made the shaping of homogeneous mullite bodies without cracks or deformations possible along with the development of controlled porous microstructures.

Influência do Y2O3 na sinterização rápida de mulita comercial

A mulita tem se tornado um material forte candidato para uso como cerâmica avançada e funcional. Nesse sentido, muito interesse tem sido devotado no estudo de aditivos de sinterização. Assim, este trabalho tem por objetivo analisar a influência do Y2O3 na queima rápida de mulita comercial. Pó de mulita comercial foi dopado com 0,5, 1,0 e 2,0% de Y2O3 prensado isostaticamente e submetida a ciclos de sinterização rápida a 1600 ºC. Foi determinada a densidade dos corpos sinterizados e executada sua análise por difração de raios X e microscopia eletrônica. A sinterização rápida da mulita gera corpos com baixa densidade, enquanto a adição de Y2O3 possibilita atingir elevadas densificações nos corpos mulíticos. O aumento do teor de Y2O3 não provoca alterações significativas na distribuição de tamanho de grão da mulita quando de sua sinterização rápida. Elevadas taxas de aquecimento permitem a obtenção de microestruturas com distribuições de tamanho de grão mais estreitas.

Porous mullite templated from hard mullite beads

Porous mullite bodies were developed by spark plasma sintering (SPS) amorphous mullite beads of about ∼30 μm in diameter at two temperatures, 950 and 1300 °C. Materials showed a close random stacking of solid spheres that retained their original packing but slightly flattened at the contacts in some cases. Depending on the thermal history, the beads were partially or fully crystallized. The thermal conductivity of the different porous mullite materials was analyzed as a function of the microstructure. Owing to the particular porous network, high gas permeability and very low thermal conductivities (1–2 W m −1 K −1) were achieved, among the lowest reported for sintered mullite materials.

Preparation and characterization of porous fibrous mullite bodies doped with TiO 2

The preparation and characterization of porous mullite ceramic bodies doped with TiO 2 are described. The bodies were prepared from sol–gel by the replication process with cellulosic nonwoven fabric as the template. The microstructure and the dependence of the porosity and the mechanical properties on the TiO 2 content are discussed. Clear differences were found in the microstructure of doped and undoped samples. Rodlike anisotropic mullite grains were grown in titania-doped mullite bodies fired at 1650 °C, while fine almost homogeneously distributed mullite grains were formed in the latter. Strength measurements showed that increasing TiO 2 content increased the bending strength of the bodies.

Effect of Y 2O 3 additive on conventional and microwave sintering of mullite

Mullite has become a strong candidate material for advanced structural and functional ceramics. Much interest has recently focused on sintering aids for mullite. The aim of this study was to evaluate the effect of Y 2O 3 as a sintering aid in the conventional and microwave sintering of mullite. To accomplish this study, a highly pure industrial mullite was used. Mullite with and without Y 2O 3 was pressed under a cold isostatic pressure of 200 MPa. Samples were sintered conventionally at 1400, 1450, 1500, 1550 and 1600 °C for 2 h and microwave-sintered for up to 40 min using a large range of power. The microstructure and physical properties of the microwave-sintered samples were compared to those of the conventionally sintered samples. The results showed that Y 2O 3 improved the densification of mullite bodies in the conventional and microwave sintering processes, but high densifications were achieved in just a few minutes when Y 2O 3 was used with microwave processing.

Kaolin processing waste applied in the manufacturing of ceramic tiles and mullite bodies

In the last few years, mineral extraction and processing industries have been identified as sources of environmental contamination and pollution. The kaolin processing industry around the world generates large amounts of waste materials. The present study evaluated the suitability of kaolin processing waste as an alternative source of ceramic raw material for the production of ceramic tiles and dense mullite bodies. Several formulations were prepared and sintered at different temperatures. The sintered samples were characterized to determine their porosity, water absorption, firing shrinkage and mechanical strength. The fired samples were microstructurally analysed by X-ray diffraction. The results indicated that ceramic tile formulations containing up to 60% of waste could be used for the production of tiles with low water absorption (approximately 0.5%) and low sintering temperature (1150 degrees C). Mullite formulations with more than 40% of kaolin waste could be used in the production of bodies with high strength, of about 75 MPa, which can be used as refractory materials.

Use of Kaolin Processing Waste for the Production of Mullite Bodies

Mullite is a versatile material used in traditional and advanced ceramic, due to low thermal expansion, high thermal shock and creep resistance. The production of waste from ore industry is a problem that is increasing nowadays. The kaolin processing industry produces residue rich in Al2O3 and SiO2. These oxides are good precursors to produce mullite. The aim of this work is to use the residue from kaolin industry to produce mullite ceramic bodies. It was studied alumina, clay and residue from kaolin processing as raw materials. The material was dried and pressing in uniaxial press (30MPa). The samples were sintered at temperatures of 1450oC, 1500oC, 1550oC and 1600oC. The ceramic bodies were characterized by X-ray diffraction. The density was measured by Archimedes method and the flexural strength by the three point bending technique. The results showed that is possible to produce mullite with high amount of waste from kaolin processing.

Sintering of commercial mulite powder: Effect of MgO dopant

MgO is one of the sintering aids most commonly used in the processing of mullite bodies. However, few studies have investigated the influence of MgO on the densification and microstructural development of mullite bodies, and the amount of MgO to be used as dopant is still a matter of controversy. Thus, this work investigated the efficiency of small amounts of MgO in the sintering of industrial mullite. MgO was added to obtain dopant concentrations of 0.1 and 0.5 wt.% in the mullite samples. Doped and nondoped samples were produced by cold isostatic pressing (CIP) at 200 MPa and pressureless sintering at 1500, 1550, 1600 and 1650 °C for 2 h. The use of 0.1 and 0.5 wt.% of MgO increased the final density of the sintered samples, with the doped samples reaching densities of 99% and the nondoped samples reaching densities of 95%. Elongated mullite grains were observed in the nondoped samples when their density fell below 95%, while the microstructures of bodies containing 0.1 wt.% of MgO were controlled up to densities of 98%. The 0.5% doped samples required lower sintering temperatures, however elongated mullite grains were observed when densities of 99% were reached.

Influence of Na2O contamination on changing the cristobalite phase into amorphous in mullite ceramics with excess silica

Authors have been trying to fabricate cristobalite-containing mullite bodies with high strength and toughness by reheating mullite-glass sintered bodies. In this process, a phenomenon that once-crystallized cristobalite layer changed back to amorphous again was observed by the microstructure observation of the sample contaminated with Na2O. In this study, the minimum amount of Na2O to change the cristobalite phase into amorphous was quantitatively determined by the electron probe microanalysis (EPMA) method. As a result, the amount of Na2O in the amorphous phase just near the cristobalite-amorphous phase boundary was 1.6-1.9 mass%, which almost corresponded with the minimum amount of Na2O in the liquid phase coexisting with mullite, estimated from a phase diagram.

Utilização do resíduo do beneficiamento do caulim para a produção de corpos mulíticos

Nos últimos anos a indústria da extração e beneficiamento mineral vem sendo mencionada como fonte de agressão ambiental e poluição devido a grande quantidade de resíduos gerados. A indústria do beneficiamento do caulim reflete esse quadro, gerando elevada quantidade de resíduo por ano. Assim, este trabalho tem por objetivo analisar a viabilidade de utilização do resíduo do beneficiamento do caulim para a produção de corpos mulíticos. Formulações contendo resíduo de caulim e de alumina foram desenvolvidas e homogeneizadas por mistura via úmida em moinho de bolas. Em seguida confeccionaram-se corpos de prova por prensagem uniaxial, que foram submetidos a sinterização em diferentes temperaturas. Após queima os materiais foram caracterizados por difração de raios X e determinação da absorção de água, densidade aparente, retração de queima e módulo de ruptura à flexão. Os resultados evidenciaram que a temperatura de queima mais adequada varia de acordo com a composição e o teor de resíduo presente nas formulações, que temperaturas de queima muito elevadas podem conduzir o material a superqueima e que formulações contendo teores de resíduos superiores a 40% podem ser utilizadas para a produção de corpos mulíticos com elevada resistência mecânica, da ordem de 75MPa.

Fabrication and Structure of TiO<sub>2</sub> Coated Open-Cell Mullite Ceramics

TiO2 coated open-cell mullite ceramics were fabricated with coating TiO2 sol on pores of mullite, and its processing parameters were investigated. Open-cell mullite ceramics were fabricated through a gel-casting process. Two kinds of mullite precursor powders were prepared by dissolution of two kinds of aluminum salts (Al(NO3)3·9H2O; type I and Al2(SO4)3·16H2O; type II) into colloidal silica sols. To produce porous mullite ceramics, both mullite precursor powders and PMMA beads (≈ 5μm) were co-dispersed by electrosteric stabilized mechanism in an aqueous system and then gel-casted. The green bodies were sintered above 1300°C for 3hrs in air. The PMMA was offered as pores in sintered mullite bodies. The prepared open-cell mullite ceramics were dipped in TiO2 precursor solution. The sintered bodies coated with TiO2 sols were re-sintered below 1000°C for 3hrs after drying at room temperature. The TiO2 was successfully coated into the open-cell mullite ceramics. The characteristics of each TiO2 coated porous mullite ceramics were investigated by XRD, SEM, porosimetry, as functions of aluminum salt and pH of sol. It was found that the synthesis behavior and the porosity of the mullite are strongly dependent upon aluminum salt species, resulting in different grain size, morphology, and pore size.

Electrical Characterization of Mullite Bodies Containing Al-Rich Anodizing Sludge

Al-rich sludge produced from industrial anodising and surface treatment processes had been tested in the fabrication of mullite-based materials, by using unidirectional dry pressing as shaping technique. Mixture with common natural silica and magnesium-containing materials, like ball clay, kaolin and diatomite were prepared in order to achieve interesting final fired refractory and/or electrical insulating materials. Microstructural changes upon sintering and crystalline phase evolution are detailed and their relationship with the electrical behaviour is also studied, by the use of impedance spectroscopy. Finally, different models were used to fit the experimental electrical resistivity values and to estimate the percolation limit of the bi-phase material.

Preparation of zirconia–mullite composites by an infiltration route

In order to improve the mechanical properties of pure mullite ceramics, a process has been developed that incorporates Zr(Y)O 2 into mullite by infiltrating partially reaction sintered (porous) mullite compacts with mixed ZrOCl 2·8H 2O and YCl 3·6H 2O solutions, corresponding to ZrO 2/Y 2O 3 = 97/3 (mol.%). The infiltration of small amount (∼7.2 wt.%) of Zr(Y)O 2 significantly improved the densification behavior and mechanical properties of mullite bodies sintered at 1620 °C, for 10 h. The mocrostructure contained a few elongated mullite crystals (aspect ratio > 6) which were embedded in a fine-grained matrix. The relatively large Zr(Y)O 2 inclusions were mainly dispersed intergranularly in the matrix, but small (<0.7 μm) inclusions were present in the grains. During sintering, the liquid phase having higher Al 2O 3 content than that of stoichiometric mullite composition formed at triple points of the mullite/mullite grains.