Showing papers on "Mullite published in 2011"

The vitreous phase of porcelain stoneware: Composition, evolution during sintering and physical properties

Abstract: High performance ceramic tiles (ISO 13006 Group BIa, water absorption < 05%) are composed of porcelain stoneware: a compact and light-colored material containing a large amount of vitreous phase, which governs sintering behavior and affects geometrical, mechanical and functional properties of finished products Ninety-three porcelain stoneware tiles were analyzed for bulk chemistry (XRF) and quantitative phase composition (XRD-Rietveld) in order to calculate both chemical composition and physical properties of the vitreous phase; their evolution during the sintering process was followed by lab simulation of industrial firing and quenching in the 1100–1200 °C range Porcelain stoneware tiles contain 40% to 75% wt of a vitreous phase having a quartz-feldspathic composition with an alumina excess coming from clay minerals breakdown Vitreous phase formation by feldspars melting is a fast phenomenon, starting from ~ 1050 °C, that is mostly accomplished before viscous flow begins densification, which goes on involving a slow-rate quartz dissolution Sintering kinetics is expected to be controlled by viscosity and surface tension of the liquid phase, which appear to depend essentially on the alumina content (hence on the mullite stability) along with the Na/K and Na/Ca ratios At any rate, a microstructural control on sintering is claimed as the rheological behavior of the viscous phase (ie the matrix containing both liquid phase and fine-grained crystals of quartz and mullite) is substantially different from that of the liquid phase only

Sintering and characterization of flyash-based mullite with MgO addition

Abstract: Mullite ceramics were fabricated at relatively low sintering temperatures (1500–1550 °C) from recycled flyash and bauxite with MgO addition as raw materials. The densification behavior was investigated as function of magnesia content and sintering temperature. The results of thermal analysis, bulk density and pore structure indicate that MgO addition effectively promoted sintering, especially above 1450 °C. Due to the presence of large interlocked elongated mullite crystals above 1450 °C, associated with enhanced densification, an improvement in mechanical strength was obtained for the samples containing magnesia. The addition of magnesia slightly decreases the LTEC at 1300 °C due to the formation of low-expansion α-cordierite, but slightly increases the LTEC above 1400 °C due to the formation of high expansion corundum and MgAl2O4 spinel.

Copper Stabilization via Spinel Formation during the Sintering of Simulated Copper-Laden Sludge with Aluminum-Rich Ceramic Precursors

Abstract: The feasibility of incorporating copper-laden sludge into low-cost ceramic products, such as construction ceramics, was investigated by sintering simulated copper-laden sludge with four aluminum-rich ceramic precursors. The results indicated that all of these precursors (γ-Al(2)O(3), corundum, kaolinite, mullite) could crystallochemically stabilize the hazardous copper in the more durable copper aluminate spinel (CuAl(2)O(4)) structure. To simulate the process of copper transformation into a spinel structure, CuO was mixed with the four aluminum-rich precursors, and fired at 650-1150 °C for 3 h. The products were examined using powder X-ray diffraction (XRD) and scanning electron microscopic techniques. The efficiency of copper transformation among crystalline phases was quantitatively determined through Rietveld refinement analysis of the XRD data. The sintering experiment revealed that the optimal sintering temperature for CuAl(2)O(4) formation was around 1000 °C and that the efficiency of copper incorporation into the crystalline CuAl(2)O(4) structure after 3 h of sintering ranged from 40 to 95%, depending on the type of aluminum precursor used. Prolonged leaching tests were carried out by using acetic acid with an initial pH value of 2.9 to leach CuO and CuAl(2)O(4) samples for 22 d. The sample leachability analysis revealed that the CuAl(2)O(4) spinel structure was more superior to stabilize copper, and suggested a promising and reliable technique for incorporating copper-laden sludge or its incineration ash into usable ceramic products. Such results also demonstrated the potential of a waste-to-resource strategy by using waste materials as part of the raw materials with the attainable temperature range used in the production of ceramics.

The effect of iron-bearing mineral melting behavior on ash deposition during coal combustion

Abstract: Mineral matter in coal is one of the most important sources of problems in coal combustion, including fouling, slagging and corrosion, etc. Mineral matter transformation and slag formation are specific properties of coal that provide more information on the suitability for coal combustion or gasification. In this research, the sintering behavior of Chinese pulverized coal (PC) ash as well as mineral transition mechanism under various atmospheres, especially iron-bearing minerals, was studied by heating microscopy, X-ray diffraction (XRD), Scanning Electron Microscope (SEM) combined with EDX (SEM-EDX). Furthermore, the molecular chemical characteristics of iron-bearing minerals, fayalite (2FeO·SiO2), were also calculated using quantum chemistry theory to provide an insight into the mineral reaction mechanism at molecular level in detail. The results showed that the different states of iron (Fe2+ or Fe3+) are the dominant reasons for different sintering behaviors of coal ash under different conditions. The iron-bearing minerals in ash, such as wustite (FeO), almandite (3FeO·Al2O3·3SiO2) and fayalite, etc., are the most important factors influencing the initial sintering behavior of coal ash in the temperature range from 1273 to 1373 K under reducing condition during coal combustion. In fayalite molecular cluster, it is because that the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO) are mostly constituted by Fe atoms and the minimum energy difference between HOMO and LUMO in fayalite cluster (1.659 eV) is much lower than that of mullite (3Al2O3·2SiO2, 6.8 eV) and kaolinite (Al2O3·2SiO2·2H2O, 3.49 eV), the thermal stability of fayalite is relatively low and Fe atoms (especially Fe(6), Fe(8) and Fe(3)) have relatively higher reactivity than any other atoms in fayalite molecular cluster. That is the reason why fayalite starts to melt at relatively low temperature during ash sintering behavior.

Synthesis and properties of mullite-type (Bi1−xSrx)2(M11−yM2y)4O9−x (M = Al, Ga, Fe)

Abstract: Mullite-type A 2 M 4 O 9 phases (M = Al, Ga, Fe), representing promising oxygen conducting materials for solid oxide fuel cells (SOFCs), were synthesized using the glycerine- and the EDTA/citric acid synthesis method. For strontium-doped material pure phases could be obtained only by washing the samples after the heating in both synthesis methods. Temperature dependent investigations were carried out to show the influence of the metal atoms on the structural stability and thermal expansion coefficients. Whereas the Sr-free phases show a quasi linear thermal expansion behavior in all three directions up to their incongruent melting points, a discontinuity in the measured range is observed for the investigated strontium doped dibismuth-nonaoxotetrametallate(III) caused by the decomposition into Bi 2 M 4 O 9 , strontium metallates and bismuthoxide. Big single crystals were only observed for the Sr-free compound, of which the structure of Bi 2 (Ga 0.45 Fe 0.55 ) 4 O 9 will be presented here in the Barnighausen tree corresponding mullite-type setting.

Densification Study and Mechanical Properties of Microwave-Sintered Mullite and Mullite–Zirconia Composites

Abstract: The objective of this research is to evaluate microwave sintering as a viable technique to reliably process high-strength mullite and mullite–zirconia composites. In this report, the advantage of microwave processing over conventional heating is realized by comparing the densification behavior and mechanical properties of sintered mullite and mullite–zirconia composites. Commercially procured mullite powders were sintered using a 3 kW, 2.45 GHz microwave furnace in the range of 14001–15001C, for 1 h. For pure mullite, an increase in sintering temperature from 14001 to 15001C decreased the porosity from 30% to 13%, which increased the compressive strength from 128718 to 387721 MPa with 1 wt% MgO as a sintering aid to mullite. Furthermore, yttria-stabilized tetragonal zirconia (3Y-TZP) was incorporated (up to 20 wt%) to improve the microwave absorption efficiency of mullite. A maximum density of 92% can be achieved for mullite with 1 wt% MgO and 10 wt% ZrO2 composites sintered at 15001C in microwave furnace and exhibited a maximum hardness of B10.2470.61 GPa, compressive strength of B740738 MPa and a moderately high indentation fracture toughness of B3.6570.43 MPa . m 1/2 .M icrowave sintering data, when compared with those of conventional sintering, revealed that volumetric heating of microwaves led to considerable improvement in strength of mullite and mullite–zirconia composites while utilizing reduced time and energy.

Phase Equilibria in Synthetic Coal–Petcoke Slags (Al2O3–CaO–FeO–SiO2–V2O3) under Simulated Gasification Conditions

Abstract: Phase equilibria of the Al2O3–CaO–FeO–SiO2–V2O3 system in synthetic slag mixtures simulating coal–petcoke slag chemistry at 1500 °C in an oxygen partial pressure of 10–8 atm were investigated by a series of quench experiments. Quenched samples were analyzed by inductively coupled plasma optical emission spectrometry (ICP), X-ray diffractometry (XRD), transmission electron microscopy (TEM), and wavelength dispersive X-ray (WDX). Two precipitated crystal phases were identified in molten slags: mullite (3Al2O3·2SiO2) in Al2O3-rich slags and karelianite (V2O3) in V2O3-rich slags. Scanning electron microscopy and TEM diffraction patterns confirmed the presence of the mullite and karelianite phases. On the basis of experimental results, an isothermal phase diagram of the Al2O3–CaO–FeO–SiO2–V2O3 system at 1500 °C and PO2 = 10–8 atm is proposed while keeping CaO = 7.0 wt % and FeO = 13.5 wt %.

Creation of microcracks in porcelain during firing

Abstract: Quartz porcelain samples were tested by acoustic emission (AE) and sonic resonant methods (sensitive to nucleation and motion of structural defects) during a cooling stage of the firing process from temperature of 1250 °C. As a consequence of the mismatch of thermal expansion coefficients of the glass and the mullite phases, and the quartz particles, microcracking begun at the temperature of the glass transition (∼800 °C) and continued in several stages until the temperature reached 300 °C. A non-monotonous behaviour of the Young's modulus and temporary vanishing of the AE signals on cooling between 573 and 500 °C confirm the significance of the β → α transition, which lies mainly upon the reversal of thermal stresses acting on the glass matrix and the quartz particles. Consequently, above this temperature, radial cracks nucleating at the quartz particles appear, whereas below this temperature, circumferential cracks around the particles are produced.

Mullite–gadolinium silicate environmental barrier coatings for melt infiltrated SiC/SiC composites

Abstract: Slurry based mullite/gadolinium silicate (Gd 2 SiO 5 ) environmental barrier coatings (EBCs) were developed for melt infiltrated (MI) SiC/SiC composites. The coating chemically adhered well on the substrates. Thermal cycling of uncoated MI-SiC/SiC composites conducted between 1350 °C and 90 °C (one hour hot and 15 min cold) in a 96.5% H 2 O–3.5% O 2 environment caused severe oxidation damage after 100 cycles resulting in the formation of dense silica layer of about 25 μm maximum thickness. Mullite/Gd 2 SiO 5 EBCs provided excellent protection to MI-SiC/SiC against moisture damage with significantly less oxidation of the substrate; only about a 2 μm thick oxide layer formed even after 400 similar thermal cycles. The hair-line cracks formed at the coating/substrate interface after 400 cycles causing partial coating de-lamination.

Enhancement of Oily Wastewater Treatment by Ceramic Microfiltration Membranes using Powder Activated Carbon

Abstract: Results of a hybrid microfiltration-powdered activated carbon (MF-PAC) system for the treatment of synthetic oily wastewaters with mullite and mullite-alumina (50 % alumina content) ceramic membranes are presented. The experiments were conducted to determine the effects of the presence of PAC on MF process performance in terms of flux decline, membrane fouling, and total organic carbon (TOC) rejection. The experimental results demonstrate that PAC addition at low concentration (200–400 ppm) increases permeate flux by 19.6 % for mullite and 61 % for mullite-alumina MF membranes. However, high PAC concentration (1200 ppm) decreases the permeate flux by 22 % for mullite and 25 % for mullite-alumina MF membranes. Also, TOC rejection increases from 93.8 to 97.4 % for mullite and from 89.6 to 92.4 % for mullite-alumina MF membranes.

Strength of aluminium titanate/mullite composites containing thermal stabilizers

Abstract: This work contains a study on the room temperature-fracture strength of three aluminium titanate-based materials containing mullite and different thermal stabilizers (namely Fe 2 O 3 and MgO). The highest inert strength was reached by the material sintered without any stabilizer. The MgO-doped material had a comparable inert strength, but a significantly higher Weibull modulus. Finally, the Fe 2 O 3 -doped material showed the worst mechanical properties. In all cases, a critical load above which strength degraded was apparent. These behaviours have been analyzed in terms of the type of additives and the particular microstructures. Conclusions about the potential use of these materials are briefly stated.

Studies on the formation of mullite from diphasic al2o3-sio2 gel by fourier transform infrared spectroscopy

Abstract: Al2O3-SiO2 diphasic gel was synthesized by sol gel route from aluminium nitrate and silicic acid following aqueous phase colloidal interaction. The precursor gel powder was thoroughly characterized by chemical analysis, measurement of surface area and bulk density measurement. The gel powder was further characterized by thermogravimetry, XRD diffraction study of the heat treated samples and Scanning Electron Microscopic (SEM) study of the fractured surfaces of the heat treated compacts. The gel was observed to be a truly di-phasic in nature and was capable of forming nano-structured well distributed mullite phase in the microstructure of the heat treated sample. The gel powder was heated at different temperatures and the formation of Al-O-Si linkage towards the formation of mullite phase (3Al2O3 , 2SiO2) was studied by Fourier Transform Infra Red (FT-IR) spectroscopy. The characteristic Al-O-Si linkage was found to develop after 600oC indicating the formation of mullite gel and at 16000oC the linkage became very strong indicating complete crystallization of mullite.

Influence of body composition on the technological properties and mineralogy of stoneware: A DOE and mineralogical–microstructural study

Abstract: This paper reports a systematic and comprehensive investigation of the effects of the starting mixture composition on the mineralogy and properties of porcelain stoneware tiles using mixture design and full quantitative phase analyses by the Rietveld method. Functional relationships between properties and the raw material mixture proportions were obtained and related to the mineralogical composition of the fired product. Mullite crystallisation depended on the chemical environment. Dissolved quartz mounted to 10 wt% of the dry body regardless on initial amount, indicating saturation of the surrounding melt. The paramount role of the amorphous content on the stoneware properties was disclosed quantitatively. Open porosity decreased with increasing amount of amorphous content, and consequently both the stain and wear resistance increased. The CIE-Lab colour parameters a * and b * increased with increased amorphous content due to interaction with surface iron in hematite. The mullite content increased wear resistance, thus supporting the mullite strengthening theory.

Effects of mechanical activation on the non-isothermal kinetics of mullite formation from kaolinite

Abstract: The non-isothermal kinetics of mullite formation from both non-activated and mechanically activated kaolinite have been studied by differential thermal analysis (DTA). Kaolinite was mechanically activated in a planetary mill, while amorphization in the structure was studied by X-ray diffraction analysis. It was established that the mechanical activation especially affected the loss of structural water. The activation energies depending on the conversion for mullite formation have been calculated from the DTA curves by using the non-isothermal method of Coats and Redfern at heating rates of 5, 10, 15, and 20 °C min−1. The mechanical activation and amorphization of the kaolinite brings to the formation of mullite at a lower heating temperature.

Effect of Excess MgO Mole Ratio in a Stoichiometric Cordierite (2MgO·2Al2O3·5SiO2) Composition on the Phase Transformation and Crystallization Behavior of Magnesium Aluminum Silicate Phases

Abstract: The effect of excess MgO, 2.0–4.0 mol ratios on the crystallization behavior and purity of μ-cordierite was investigated using X-ray diffraction (XRD), differential thermal analyzer, and TGA. Quantitative XRD, the Rietveld technique, was carried out using the HighScore Plus software. The glass crystallization process route was used with talc and kaolin as the main raw materials and compensated with MgO, Al2O3, and SiO2 accordingly. The crystallization temperature of the glasses decreased as a function of the MgO mole ratio. Less than 2.8 mol MgO increased the formation of α-cordierite up to 94 wt%. However, above 2.8 mol ratio, the forsterite phase started to appear, together with mullite, μ-cordierite, and spinel.

Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Abstract: Si-based ceramics (e.g., SiC and Si3N4) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H2O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al2O3·2SiO2) interlayer and (1 − x)BaO·xSrO·Al2O3·2SiO2, 0 ≤ x ≤ 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H2O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.

Mineral melting behavior of chinese blended coal ash under gasification condition

Abstract: The fusibility and mineral melting behavior of Chinese blended coal ash under gasification condition were studied by standard ash fusion temperature (AFT) test, X-ray diffraction (XRD), scanning electron microscope (SEM) and phase diagram theory. The experimental results show that the trend of blended ash's AFT is not linearly related to the blending ratios, but mirrors by changes in liquidus temperature from ternary phase diagram systems. The AFTs of those blended ashes whose main ash chemical compositions locate near the boundaries between two mineral regions or triple points are lower and change more evident than that of those blended ashes whose main ash chemical compositions are far away from the boundaries or triple points. With increasing mass fraction of low AFT ash in blended coal ash, the location of blended ash in ternary systems is changed from the mullite region to the anorthite region, as well as the dominant crystal mineral at deformation temperature (DT) is also transformed from mullite to anorthite via mullite–mullite and anorthite coexisting–anorthite. The fluxing minerals (anhydrite, wustite, albite, hercynite, augite) in low AFT ash can react with the refractory minerals (mullite, quartz) complicatedly to form new low-melting minerals (anorthite, hercynite, fayalite, gehlenite) at high temperature (HT) (1273–1403 K). These minerals can also interact to form eutectic mixtures in FeO-SiO2-Al2O3 and CaO-SiO2-Al2O3 systems and act as the most key factors for lowing melting mechanism of blended ash under gasification condition. Copyright © 2010 Curtin University of Technology and John Wiley & Sons, Ltd.

A novel low-clay translucent whiteware based on anorthite

Abstract: This study was carried out to characterize the properties of a novel low-clay translucent whiteware suitable for daily use. The low-clay whiteware is produced from coarsely and finely milled prefired materials of the same composition plus a small amount of clay. It consists of anorthite (CaO·Al2O3·2SiO2) and mullite (3Al2O3·2SiO2) crystalline phases and a glassy phase with high crystalline to glassy phase ratio. The development of needle shaped long mullite crystals that were forming three dimensional interlocking network had significant effect on the elimination of pyroplastic deformation during glaze firing. Typical flexural strength and fracture toughness values were ∼110 MPa and ∼1.85 MPam1/2, respectively. The low-clay whiteware had relatively low (4.6 × 10−6/°C) thermal expansion coefficient which made possible to glaze the whiteware with a typical hard porcelain glaze. A continuous interface layer was produced between the whiteware and the glaze and no crack was present through layer because of expansion mismatch.

Effect of aluminum hydroxide content on porosity and strength of porous mullite-bonded silicon carbide ceramics

Abstract: Porous mullite-bonded silicon carbide (SiC) ceramics were prepared from SiC and aluminum hydroxide [Al(OH)3] powders. The Al(OH)3 content was varied from 14.5 to 47.3 wt %, and porous SiC ceramics were fabricated via reaction sintering at 1450–1550°C for 2 h. The microstructure showed large SiC grains embedded in a matrix of small and loosely packed mullite/alumina particles. It was demonstrated that the porosity decreased and flexural strength increased with increase in Al(OH)3 content. The porosity varied from 46 to 54%, and flexural strength varied from 3 to 14 MPa with the variation in Al(OH)3 content and sintering temperature. Typically, porous mullite-bonded SiC ceramics of 14 MPa strength and 47% porosity were obtained when SiC and 47.3 wt % Al(OH)3 powders were sintered at 1500°C.