Mullite Phase Research Articles

The Effect of Alumina (Al<sub>2</sub>O<sub>3</sub>) on the Characteristics of Sintered Mullite-Cordierite Ceramics Synthesized with Kaolin from Naratiwas of Thailand

Mullite-cordierite ceramics was prepared by the three composition of talc, kaolin (kaolin from Naratiwas in Thailand) (NT) and alumina. The different amounts of alumina (5, 10, 15 and 20 wt%) were added to produce various mullite-cordierite alumina mixtures which are denoted as NA, NB, NC and ND, respectively. The mixtures were pressed into rectangular shapes by hydraulic press with the pressure of 150 kg/cm2 then sintered at temperature of 1300 °C and 1350 °C for 2 hours. The morphology of the synthesized mullite-cordierite samples were characterized by x-ray diffraction spectroscopy (XRD), and physical-mechanical properties were investigated. Mullite-cordierite was successfully synthesized. The XRD result was represented phases of mullite and cordierite. The NA samples sintered at temperature of 1300 °C indicated the best physical-mechanical properties including bulk density (2.23 g/cm3), flexural strength (44.4 MPa) and thermal expansion coefficients (5.00x10-6). The other NA samples sintered at temperature of 1350 °C exhibited the bulk density, flexural strength and thermal expansion coefficients are 2.21 g/cm3, 47.8 MPa and 4.99 x10-6, respectively. The thermal expansion coefficient of the NT sintered samples have been plotted the length change relative to measuring temperature ranging of 30-1200 °C. The suitable condition of the synthesized mullite-cordierite ceramics is finally obtained the NA samples with 5 percent weight of alumina composition sintered at the temperature of 1350 °C.

A simple and efficient way to prepare porous mullite matrix ceramics via directly sintering SiO2-Al2O3 microspheres

A novel route was proposed to fabricate porous mullite matrix ceramics by directly sintering SiO2-Al2O3 microspheres (SAMs) in air, which was simple and efficient compared with other methods. The effects of sintering temperature on the phase composition, microstructure, porosity and compressive strength of porous ceramics were studied. It is found that the mullite phase generates when sintered at above 1450°C. SEM results shows that the sintering temperature has a significant influence on the formation of pores morphology. As the sintering temperature increases from 1200°C to 1650°C, the original small holes inside and between the SAMs develop into macropores, whose pore size increases to eight times more than that of SAMs at a maximum, and the distribution of pores is relatively uniform. Porous mullite matrix ceramics with the porosity of 81.37% and compressive strength of 6.25±0.91MPa have been obtained through this method at the sintering temperature of 1550°C.

Effects of SiC on the Properties and Microstructures of Closed-Pore Al2O3-MgAl2O4 Refractory Aggregates

Closed-pore Al2O3-MgAl2O4 refractory aggregates have been prepared successfully based on the superplasticity of Al2O3 ceramic by using a mixed ball-milling method and the gelcasting method. The effects of SiC content on phase composition, porosity and microstructures of the refractory aggregates have been investigated. The properties of Al2O3-MgAl2O4 refractory aggregates fabricated by different methods have been compared to our results. We discuss the influence mechanism of SiC on the refractory aggregate. The results showed that the main phase compositions were corundum and MgAl2O4 spinel with accompanying small amounts of mullite phase. The closed porosity increased with increased addition of SiC. SiC facilitated the sinterability of materials resulting in a low level of apparent porosity. When compared to the mixed ball-milling method, the closed pore size distribution in the refractory aggregates fabricated by the gelcasting method was more uniform.

<i>In Situ</i> Growth of Mullite Whisker Using Different Aluminium Source and Properties of Mullite-Corundum Lightweight Refractory

In-situ growth of mullite whisker from waste coal gangue and different aluminium source mixtures by dry pressing were investigated aiming at the preparation of mullite - corundum lightweight refractory by controlling mullite whisker precursor pseudo particle size and quantity in corundum matrix. The phase composition and microstructure were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Apparent porosity and bending strength were also measured. The samples exhibited characteristic alumina and mullite phases. The formation mechanism and influence factors for mullite whiskers materials were discussed. The experimental results showed that the whisker of the obtained sample changed from the accumulated short column to mesh cross acicular whisker shape, and the length to diameter ratio increases. When the aluminium source was aluminium hydroxide, the length to diameter ratio and apparent porosity reached the maximum of 17.6 and the minimum of 2.15%. The bending strength of all the samples was similar, that of the sample prepared by calcined bauxite was the largest (84 MPa), that of the sample prepared by calcined alumina was inferior, and the sample prepared by aluminum hydroxide was the lowest.

Thermal Exposure Effects on the Strength and Microstructure of a Novel Mullite Fiber

The mechanical behavior of the novel fiber CeraFib75 after various thermal exposures is examined. This fully crystalline mullite fiber was developed to exceed the thermal stability of commercially available oxide fibers. Therefore, heat treatments at temperatures ranging from 1000°C to 1400°C for 25 h were performed and results compared to the well‐established Nextel™ 720 fibers. Mechanical characterization was realized with bundle tensile tests using acoustic emission sensors to determinate the fiber failure distributions. Investigations showed that the initial fiber microstructure of mullite grains with traces of alumina transforms starting at 1200°C. Changes include dissociation of the alumina‐rich mullite phase and grain growth. Thus, strength reduction is measured as a result of these microstructure transformations. Remarkably, at 1400°C, fibers become more fragile and Weibull statistics can no longer describe the failure distribution. A relation between the distribution shape and the load redistribution capability of fibers is suggested. This is more pronounced for Nextel™720 fibers, which present much bigger grains and retain only 10% of their original strength. However, CeraFib75 fibers are more stable and exhibit a strength retention of 50% at the same conditions, which is attributed to the higher amount of mullite phase.

Preliminary Identification of Silantek Clay as Potential Refractory

Silantek clay deposits were characterized and evaluated for their potential as structutral ceramic products raw materials through chemical, mineralogical and refractory (1250 to 1600o C) properties determinations. Results show that the main oxides in the samples were SiO2 and Al2O3, whereas the other oxides present only in small quantity. Kaolinite(Al2Si2O5(OH)4) and quartz(SiO2) present as dominant mineral phases and the minor phases of mullite(Al6Si2O13) and zeolite(Na2Al2Si3O10·2H2O) also appeared in the samples. From the results of the refractoriness tests up to 1600°C, samples ST11, ST23A, ST31A, ST31B, ST33 have potential for refractory products as they demonstrated high thermal resistance properties(over 1600°C). Silantek clay potential to be used as raw materials mainly for refractory, structural ceramics and other common ceramic products

Fabrication and Characterization of Porous Alumina-Based Ceramics Using Silicone Resin as Binder

ABSTRACTPorous alumina based ceramics were fabricated by injection method, where alumina powders were used as raw materials and solvent silicone resin was used as binder. During the heat treatment process, an organic-inorganic transformation occurred in silicone resin. The increase of heating temperature promoted the formation of mullite phase when the temperature was higher than 1500°C. The weight loss of porous alumina based ceramics was maintained at 5.7∼5.9%. When the heating temsperature was 1500°C, porous alumina based ceramics showed the biggest shrinkage rate, lowest apparent porosity and highest bending strength.

Influence of corn flour as pore forming agent on porous ceramic material based mullite: Morphology and mechanical properties

Porous material was processed by the mixing, molding and pressing the ceramic material, afterward burnout and sintering; through the forming porous, using corn flour at different concentration (10, 15 and 20 wt.%) as a pore forming agent; in order to determinate the influence of porous on the mechanical, morphological and structural properties. The effect of the volume fraction of corn flour in the mullite matrix, at various sintering temperature from 1100, 1200, 1300 and 1500?C were tested by Diffraction X ray, showing changes in crystalline phases of mullite (3Al2O3-2SiO2), as result of sintered temperatures. Presence of talcum powder in formula, also cause the formation of the cordierite and cristobalite crystalline phases, giving stability and adhesion to the structure of ceramic material. When sintering at temperatures between 1300 to 1500?C, and it was used the concentration of corn flour 15-20 wt.% as forming agent porous, it was found the better mechanical properties. The scanning electron microscopy analysis shows the presence of open porosity and anisotropy.

Effect of Mullite Formation on Properties of Aluminosilicate Ceramic Balls

In order to study the effect of mullite phase on the physico-mechanical of the aluminosilicate ceramic balls, 3 ceramic balls made of clay, feldspar and quartz were prepared. However, in this work, the clays were selected from i.e., Trong, Perak (TC), Simpang Pulai, Perak (SP) and Selendang, Pahang (SC) and were identified as CBT, CBP and CBS, respectively. The formulation of each clay was fixed to 30wt.% clay; 40wt.% potash feldspar; 40wt.% silica sand. The mixtures later,were dried for 6hours under 25 oC of room temperatures. Prior to shape balls, 30wt.% of water was added to the mixtures in order to form dough. The dough was aged for 24hours to homogenous the plasticity before shaped to 19mm of spherical balls. The balls were dried in an oven at 110°C for 24hours before fired at 1250°C for 2hours. The phase formation of each mixture was investigated via X-ray diffraction analysis (XRD). Meanwhile, the physico-mechanical of ceramic balls was determined according Universal Oil Products (UOP). It is found that CBT exhibited outstanding physico-mechanical which has crushing strength 1270kg compared to CBP and CBS due to the presence of mullite phase. This enables their use for ceramic balls as catalyst bed support in refining applications.

Investigation of the mineral components of porcelain raw material and their phase evolution during a firing process by using a Rietveld quantitative analysis

A ceramic raw material for white porcelain and its phase evolution during a firing process were investigated by using Rietveld method based on powder X-ray diffraction data. The raw material was mainly composed of five mineral phases: quartz (SiO2), microcline (KAlSi3O8), albite (NaAlSi3O8), muscovite (KAl2(AlSi3O10)(OH)2), and kaolinite (Al2Si2O5(OH)4). The amount of each mineral phase could be determined by using Rietveld quantitative phase analyses. During the firing process, the microcline, albite, muscovite and kaolinite phases started to react with each other to produce the mullite phase embedded in an amorphous matrix. The amount of quartz remained nearly unchanged until a temperature above 1200 °C; then, it converted to an amorphous phase at higher temperatures.

Recycling of polished tile waste as a main raw material in porcelain tiles

The manufacture of traditional porcelain tiles is often associated with a large amount of ceramic wastes during the polishing stage. These wastes need to be properly handled due to the corresponding environment problems. This paper aims to evaluate the possibility of reusing the polished tile waste as a main raw material to produce porcelain tiles. The prepared specimens with various amounts (10–70 wt.%) of the polished tile waste were fired at 1100–1180 °C. The sintering behaviors of fired specimens were examined by linear shrinkage, water absorption and bulk density. The flexural strength of fired specimens was measured by the three-point bending method. The test results show that the sample with 50 wt.% polished tile waste fired at 1120 °C has the superior performance (i.e., water absorption of 0.12%, bulk density of 2.49 g/cm3 and flexural strength of 47 MPa), which meets the requirement of porcelain tiles according to the ISO 13006 standard. Thus, the polished tile waste can be effectively reused in porcelain tiles as a main and low-cost raw material by fast firing at low temperature. The X-ray diffraction result indicates that introduction of the polished tile waste into porcelain tiles facilitates the formation of mullite phase. In addition, the relationship between the microstructure and strength of the optimal sample was also discussed.

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.

Effect of Fe2O3 content on the electrical resistivity of aluminous porcelain applied to electrical insulators

In general, the presence of Fe2O3 is claimed to reduce the insulating behavior of electrical porcelains, even if a rigorous proof has not yet been reported. The purpose of this study was to evaluate the real influence of Fe2O3 content on the electrical resistivity of a standard aluminous porcelain widely used in insulators. The electrical resistivity for the composition with 3wt% Fe2O3 was higher than those found for standard aluminous porcelain, which was discussed in terms of the concentration of glassy and mullite phases. A reduction in the electrical resistivity was only observed in porcelain samples containing over 3wt% Fe2O3. The presence of hematite phase was considered responsible for this reduction. These results suggest that low-cost raw materials with greater Fe2O3 content should not extensively affect the insulating properties and could therefore be used in manufacturing aluminous electrical porcelain.

On the sintering mechanisms and microstructure of aluminium–ceramic cenospheres syntactic foams produced by powder metallurgy route

Metal matrix syntactic foams of ceramic (mullite) cenospheres (10–40% v/v) embedded in pure aluminium matrix, were fabricated by powder metallurgy technique at different sintering temperatures (610–710°C). Density, vis-a-vis porosity and microstructural characteristics were examined by using scanning electron microcopy and energy dispersive X-ray spectroscopy. A first systematic description of the sintering mechanism of the composites is presented. The in situ reaction between silica (phase of mullite) and aluminium particles, apart from any degradation of the mullite cell wall and the precipitation of silicon in the matrix, changes the sintering mechanism of syntactic foams due to the formation of eutectic aluminium–silica liquid quantities. Transient liquid phase sintering, with surface material diffusion, and liquid phase sintering, with bulk diffusion, selected to be the main sintering mechanisms, are described in detail for sintering temperatures below and above the melting point of aluminium matrix, respectively.

Cost-effective utilization of mineral-based raw materials for preparation of porous mullite ceramic membranes via in-situ reaction method

Porous mullite ceramic membranes were prepared via in-situ synthesis method using abundant natural minerals including coarse bauxite and fine kaolin powders. Thermal sintering behavior of the green sample shows that a unique dimension expansion appears during the sintering process due to the anisotropic growth of mullite crystals. The XRD analysis suggests the ceramic membrane is mainly composed of mullite phase when sintering above 1450°C. The SEM images confirm that with an increase in sintering temperature, mullitization reaction firstly occurs in kaolin agglomerations, and then bauxite-derived alumina reacts with silica-containing phase by a solution-precipitation mechanism to form an interlocked framework of elongated secondary mullite. Both maximum and average pore sizes increase with increasing sintering temperature. The growth of mullite whiskers has a positive impact on the improvement of porosity and mechanical strength of ceramic membranes. Even after sintering at 1500°C, the sample shows the highest open porosity of 31.6% and good biaxial flexural strength of 100.2MPa.

Utilization of Some Egyptian Waste Kaolinitic Sand as Grog for Bricks and Concrete

The aim of the present study is to assess the suitability of some waste kaolinitic sand as grog for bricks and concretes. Three samples were selected and their chemical and mineralogical compositions as well as their ceramic and mechanical properties were investigated. The solid phase composition as well as the microstructure of the vitrified samples was also carried out using XRD and SEM methods. Refractoriness under load and thermal shock resistance are also investigated. Chemical and phase composition confirm that two samples (1 and 2) are fireclay with total impurities oxide contents (TIOC) less than 3.0 % while the third is mainly silica with TIOC less than 0.5 %. The microstructure of samples 1 and 2 shows predominant mullite crystals, bonded by silicate phases as confirmed by XRD, whereas silica phases are the main components of the third sample with minor intercalation of mullite phase. It is concluded that samples 1 and 2 fired at 1500 ◦C can be used as grog for brick and concrete manufacture in industrial furnaces up to 1400 °C while sample 3 needs some additives to form denser grains.

Mechanical and thermal properties of highly porous Al2TiO5–Mullite ceramics

Al2TiO5–mullite porous ceramics were synthesized by direct foaming method using α-Al2O3, TiO2 and SiO2 as starting materials. The initial suspension for Al2TiO5 was prepared by adding TiO2 suspension to the equimolar amount of partially hydrophobized colloidal Al2O3 suspension. A secondary suspension was prepared using following molar composition 3:2 Al2O3/SiO2 and it was blended to the initial suspension in (0, 10, 20, 30 and 50)vol% to obtain the mullite phase in the sintered sample. Thermal expansion coefficient and thermal hysteresis curves of the porous ceramic samples with pure Al2TiO5 and of the Al2TiO5 ceramic with mullite addition were measured. Hertzian indentations are used to evaluate the damage behavior under constrained loading conditions. Mechanical behavior from indentation load–displacement curves is investigated.

Effect of zinc substitution for calcium on the crystallisation of calcium fluoro-alumino-silicate glasses

A series of 25, 60, 75 and 100mol% zinc substituted for calcium ionomer glasses of the composition 4.5SiO2–3Al2O3–1.5P2O5–3CaO–2CaF2 were produced following a well-established melt quench route. Similar glass compositions have been widely used in dentistry as the glass component in glass ionomer cements. The glasses crystallise to an apatite and mullite phase after appropriate heat treatments and are interesting materials for bone repair. Zinc is a bactericidal agent and has shown ability to stimulate osteogenesis. The new zinc containing glass compositions were characterised by Helium Pycnometer, DSC, TGA, SEM and EDX. The density of the zinc substituted glasses and glass–ceramics increased with increasing the zinc content as expected. The glass transition temperature decreased with zinc substitution, whereas the crystallisation temperature increased with zinc substitution. Zinc substituted glasses lead to the formation of gahnite in expense of mullite. Fluorapatite formation was not favoured in high zinc content glass ceramics, whereas loss of fluorine in the form of SiF4 was suggested. SEM and EDX analysis showed significant changes in the morphology of glass ceramics with zinc substitution. The formation of gahnite during glass crystallisation may open new application avenues for these materials in the field of semiconducting materials.

The sequel of modified fly ashes using high energy ball milling on mechanical performance of substituted past cement

ABSTRACT The consideration of planetary ball mill processing is attempted to obtain modified fly ashes powders that are used as substitutes in a cement paste. Ball milling conditions are related to structural modifications of processed powders. These modifications are retrieved from X-ray diffraction, infrared spectroscopy, powder granulometry, and scanning electron microscopy. As an industrial application, the milled fly ashes are substituted to two different cements to show possible effect on compressive strength of cement pastes. Structural analysis of milled fly ashes show loss of crystallinity of the magnetite phase, nanosizing trend of quartz and mullite phases with some straining that takes place after only 1 h of milling. The drop of particle size of the milled fly ashes is also evidenced but with tendency to agglomeration and particle shape modification from spherical to irregular forms. Mechanical performance results related to modified cement pastes indicate successful substitution of cements by milled fly ashes up to 50%. Substitution results in levels of compressive strength as large as 70 MPa for particular combination of milling time, fly ash content and curing time.

Extraction of alumina from coal fly ash using an acid leach-sinter-acid leach technique

In this paper, a two-step acid leach process for the extraction of aluminium from coal fly ash has been conducted using an acid leach-sinter-acid leach technique. The technique was formulated based on the dissimilar response characteristics of the amorphous and mullite alumina phases present in coal fly ash. The amorphous phase is reactive and soluble in inorganic acids while the mullite phase is refractory, not amenable to inorganic acids, and requires phase transformation before leaching. Sintering of the coal fly ash residue from the first leaching stage was carried out using a mixture of CFA with calcium carbonate. Results presented in this paper show that, sintering of coal fly ash residue from the first leaching stage successfully transformed most of the mullite phase alumina into a leachable plagioclase phase which completely dissolved in sulphuric acid during the second leaching stage. The results further show that the two dissimilar alumina phases can be processed separately by a novel acid leach-sinter-acid leach technique for alumina extraction from coal fly ash as investigated in this work. This study has also revealed that by employing this technique, coal fly ash can be optimally leached. The implications of the findings are that this could provide an alternative economic process for recovering alumina from readily available secondary resources such as coal fly ash.