Showing papers on "Mullite published in 2010"

Review of mullite synthesis routes by sol-gel method

Abstract: The sol–gel method for the mullite synthesis is reviewed, with particular emphasis on the characterization of monophasic and diphasic gels at low, intermediate and high temperatures and the factors that influence the hydrolysis and condensation rate of the sol–gel process, which in turn determine the properties of the final material. A wide range of studies about mullite precursors synthesized via sol–gel is discussed here.

The Effects of Temperature on the Local Structure of Metakaolin-Based Geopolymer Binder: A Neutron Pair Distribution Function Investigation

Abstract: Neutron pair distribution function (PDF) analysis is utilized to advance the understanding of the local atomic structural characteristics of geopolymer binders derived from metakaolin, specifically the nature and amount of the water associated with these materials. Samples were heated in air to temperatures up to 1200°C, then analyzed ex situ by high momentum transfer neutron total scattering and PDF analysis. Water contained in large pores, along with water associated with hydration of potassium cations in the geopolymer framework structure, comprise the majority of water in this material. The remaining water is situated in small pores and as terminal hydroxyl groups attached to the Si–Al framework. The Si–Al framework structure undergoes only subtle rearrangement upon heating, but maintains a tetrahedral aluminosilicate framework environment. After crystallization with heating beyond 1000°C, the geopolymer gel is predominantly converted to leucite, with small amounts of amorphous mullite and glassy silica, which have never before been observed in heated geopolymers. This demonstrates the value of neutron PDF analysis to probe the local structure of these important geopolymeric materials.

Mullite development on firing in porcelain stoneware bodies

Abstract: Microstructural evolution on heating was investigated in a reference industrial composition (50% kaolinitic clay, 40% feldspar and 10% quartz) of porcelain stoneware, fast fired at different temperatures (500–1400 °C). The evolution of mullite crystals, regarding shape and size progress, was examined by scanning electron microscopy (SEM). The proportion of Type I mullite crystals decreases with firing temperature and simultaneously, the size of crystals increases, reaching the maximum value of aspect ratio (3:1) at 1400 °C. Type II and Type III secondary mullite needles increase with temperature in both number and length, which leads to an increase in the aspect ratio from 5:1 to ∼20:1 in Type II crystals and from ∼33:1 to 50:1 in Type III mullite needles. Finally, clusters of Type III mullite fibres are observed in porcelain stoneware samples fast fired in the 1250–1280 °C interval.

Effect of microstructure on mechanical properties of porcelain stoneware

Abstract: This work examines the effect of microstructure (aspect ratio of mullite crystals and proportion of crystalline and amorphous phases) as well as different physical features (bulk density, closed and open porosity and absolute density) on the mechanical properties of a standard porcelain stoneware composition (50% kaolinitic clay, 40% feldspar and 10% quartz) fired in the 1200–1300 °C temperature interval using a fast firing schedule. The mechanical behaviour was evaluated in terms of bending strength, Vickers microhardness, fracture toughness and Young's modulus. After viewing the results, it can be concluded that increased σ f , H v and E values were mainly due to open porosity, percentage of mullite phase and morphology of secondary mullite needles, whereas closed porosity and quartz particles have no influence on these properties.

Wetting and interactions of Ni- and Co-based superalloys with different ceramic materials

Abstract: In this work, a systematic study of the wetting behavior of two Ni-based and one Co-based superalloys, used, in particular, for the fabrication of turbine blades, is presented with reference to different ceramic substrates: sapphire, polycrystalline alumina, zirconia, and mullite. Wettability tests have been performed by means of the “sessile drop” method at 1500 °C; the characterization of the interfaces between the molten drop and the substrates has been performed by SEM/EDS analysis in order to check the final characteristics of the solidified interfaces. The results are discussed in terms of chemical interactions in relation to the processing parameters and as a function of the surface and interfacial, morphological and energetic properties of the systems.

Elaboration and characterization of tubular macroporous ceramic support for membranes from kaolin and dolomite

Abstract: A low cost macroporous support for ceramic membranes was prepared by in situ reaction sintering from local natural mineral kaolin with dolomite as sintering inhibitor. The characterization focused on the phase evolution, microstructure, pore structure, mechanical strength and water permeability at various compositions and sintering temperatures. The sintering of kaolin was improved with 5 wt% dolomite, but clearly inhibited with ≥10 wt% dolomite. For the 20 wt% dolomite samples, the crystalline phases were mainly composed of mullite, cordierite and anorthite after sintering between 1,150 and 1,300 °C. Moreover, both mean pore size and mechanical strength increased with increasing sintering temperature from 1,100 to 1,300 °C, but the water permeability and porosity decreased. The 1,250 °C sintered macroporous support with 20 wt% dolomite exhibited good performances such as porosity 44.6%, mean pore size 4.7 μm, bending strength 47.6 MPa, water permeability 10.76 m3 m−2 h−1 bar−1, as well as good chemical resistance. This work provides opportunities to develop cost-effective ceramic supports with controllable pore size, porosity, and high strength for high performance membranes.

A review of the possible applications of nanotechnology in refractory concrete

Abstract: This article reviews the manufacturing nanotechnologies of modern refractory concretes and some other cementitious materials. The main part of the article focuses on the results obtained by the authors who analyzed the application of nanotechnology for manufacturing refractory concretes and examined the influence of nanostructure formation in the binding material on the properties of refractory concretes. In one case, investigations were carried out using two‐component (sodium silicate solution mixed with dicalcium silicate) and three‐component (sodium silicate solution mixed with dicalcium silicate plus calcium aluminate cement) binding materials, whereas in other case, multi‐component material, middle cement refractory concrete with mullite aggregates, microsilica and additives of single and hybrid deflocculant (polycarboxylate ether Castament FS20 and sodium tripolyphosphate) were researched. Preliminary investigations showed that the three‐component binding material under development hardens ...

Sintering, Phase Stability, and Properties of Calcium Phosphate-Mullite Composites

Abstract: In this contribution, we report the results of a study to probe into the combined effect of the sintering conditions and mullite (3Al2O3·2SiO2) addition (upto 30 wt%), on the densification mechanism, phase assemblage, and microstructure development in calcium phosphate (CaP)-mullite composites. The experimental results reveal that close to 95% theoretical density can be achieved by sintering the compositions in the temperature range of 1300°–1350°C. Finer scale microstructural analysis using transmission electron microscopy reveals the presence of both β- and α-TCP (Ca3(PO4)2–tricalcium phosphate) and a crystalline residue of gehlenite at triple junctions. The shrinkage kinetics of the composites have been analyzed to qualitatively understand the sintering mechanisms. For the composites, solid-state sintering in the initial stage, followed by liquid-phase sintering at or near the sintering temperature are postulated to explain the effect of temperature and volume fraction of the second phase. Some important features of the liquid-phase sintering have been discussed using the CaO–Al2O3–SiO2 ternary phase diagram. A comparison with the earlier published results reveals that a better combination of long-crack fracture toughness, compressive strength, and flexural strength can be obtained with the newly developed composites.

Effect of TiO2 on the mullite formation and mechanical properties of alumina porcelain

Abstract: The effect of adding TiO 2 to standard alumina porcelain on its microstructure and flexural strength was investigated. A series of alumina porcelain bodies containing increasing amounts of TiO 2 were prepared by extruding mixtures of raw materials and TiO 2 . Porcelain rods were fired under industrial scheduling in a manufacturing kiln. The overall degree of crystalline and amorphous phase content within the porcelain bodies was quantitatively determined using a Rietveld analysis. Results indicated a higher amount of mullite formation in porcelain bodies containing TiO 2 . Examination of the product materials using field emission scanning electron microscopy showed a high density of secondary mullite crystals present in the earlier feldspar grain areas of specimens with TiO 2 . Energy dispersive X-ray analysis of secondary mullite crystals revealed that Ti 4+ enters into the secondary mullite structure forming a Ti 4+ -mullite solid solution. Assessment of the mechanical properties of the TiO 2 containing bodies indicated that small addition raises the flexural strength of the standard porcelain. The improvement in mechanical properties could be associated with an increase of both specimen density and relative content of types II and III secondary mullites. Both observations may be attributed to a decrease in the viscosity of melted feldspar grains, which in turn favours the nucleation and growth of secondary mullite crystals and thus increases the final density of porcelain bodies.

Aluminosilicates with varying alumina–silica ratios: synthesis via a hybrid sol–gel route and structural characterisation

Abstract: Aluminosilicates with varying Al2O3:SiO2 molar ratios (3 : 1, 3 : 2, 3 : 3 and 3 : 4) have been synthesized using a hybrid sol–gel route using boehmite sol as the precursor for alumina and tetraethyl orthosilicate (TEOS) as the precursor for silica. The synthesis of boehmite sol from aluminium nitrate, and its use as the alumina precursor, is cost effective compared to alkoxide precursors. Structural aspects, including bonding and coordination, are studied in detail for samples calcined in the temperature range 400–1400 °C using both NMR and FTIR spectroscopy: the results are correlated with phase formation data (spinel and high temperature phases) obtained from XRD and thermal analysis. FTIR results show a broadening of peaks at 800 °C indicating a disordered distribution of octahedral sites caused by crosslinking between AlO6 octahedral and SiO4 tetrahedral units prior to the formation of mullite. 27Al MAS NMR spectra are consistent with a progressive decrease in the number of AlO6 polyhedra with increasing temperature corresponding to Al in these units being forced to adopt a tetrahedral coordination due to the increasing presence of similarly coordinated Si species. XRD results confirm the formation of pure mullite at 1250 °C for a 3Al2O3:2SiO2 system. At 1400 °C, phase pure mullite is observed for all compositions except 3Al2O3:SiO2 where α-Al2O3 is the major phase with traces of mullite. The synthesis of aluminosilicates through a hybrid sol–gel route and the detailed insight into structural features gained from spectroscopic and diffraction techniques contributes further to the development of these materials in applications ranging from nanocatalysts to high-temperature ceramics.

Porous mullite composite with controlled pore structure processed using a freeze casting of TBA-based coal fly ash slurries

Abstract: Porous ceramic composite with controlled “designer” pore structure has been prepared by a freeze casting route using a tertiary-butyl alcohol (TBA)/coal fly slurry system. Unidirectional aligned macropore channels were developed by controlling the solidification direction. Simultaneously, small sized dendrite-like pores formed in the outer walls of the pore channels. The compressive strength of the sintered porous composite increased as the porosity decreased, i.e. low porosity gave a high compressive strength. After sintering at 1500 °C, the mullite composite with a porosity of 67% exhibited a compressive strength of ∼23 MPa.

Solvent based environmental barrier coatings for high temperature ceramic components

Abstract: Environmental barrier coatings for high temperature ceramic components including a bond coat layer; an optional silica layer; and at least one transition layer including: from about 85% to about 100% by volume of the transition layer of a primary transition material including a rare earth disilicate, or a doped rare earth disilicate; and from 0% to about 15% by volume of the transition layer of a secondary material selected from Fe2O3, iron silicates, rare earth iron oxides, Al2O3, mullite, rare earth aluminates, rare earth aluminosilicates, TiO2, rare earth titanates, Ga2O3, rare earth gallates, NiO, nickel silicates, rare earth nickel oxides, Lnb metals, Lnb2O3, Lnb2Si2O7, Lnb2SiO5, borosilicate glass, alkaline earth silicates, alkaline earth rare earth oxides, alkaline earth rare earth silicates, and mixtures thereof; where the transition layer is applied to the component as a slurry including at least an organic solvent, the primary transition material and at least one slurry sintering aid, and where a reaction between the slurry sintering aid and the primary transition material results in the transition layer having a porosity of from 0% to about 15% by volume of the transition layer.

Phase composition of alumina–mullite–zirconia refractory materials

Abstract: Refractories in the Al 2 O 3 –SiO 2 –ZrO 2 system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al 2 O 3 /SiO 2 ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al 2 O 3 /SiO 2 ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al 2 O 3 /SiO 2 ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO 2 is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.

Preparation of Mullite- and Zircon-Based Ceramics Using Kaolinite and Zirconium Oxide: A Sintering Study

Abstract: The interactions between zirconia (ZrO2) powder and three Algerian hydrated kaolinitic clays were studied at high temperatures. The analysis by X-ray diffraction of the prepared products allowed to follow the different phase developments during heat treatment and to identify the parameters controlling the zirconia conversion into zircon (ZrSiO4). It was found that ZrSiO4 formation, occurring at temperatures above 1150°C, is enhanced by the presence, in the clays, of fusing impurities such as K, Fe, Ca, and Mn, and by a decrease in zirconia particle size. A reactional mechanism, involving zirconia, a flux, and cristobalite is proposed. Moreover, the effect of zirconia additions on sintering was studied. It was also found that the increase in the porosity ratio of the final products for zirconia levels above 20 wt% was governed by a decrease in the flux amount, due to its lower clay content. Finally, it was shown that ceramics obtained by sintering at 1400°C for 2 h of a mixture of 38 wt% of fine zirconia powder and 62 wt% of the more reactive clay were mainly constituted of zircon and mullite.

In vitro dissolution of calcium phosphate-mullite composite in simulated body fluid.

Abstract: In our recent research, we have developed novel CaP-mullite composites for bone implant applications. In order to realize such applications, the in vitro dissolution behaviour of these materials needs to be evaluated. In this perspective, the present paper reports the dissolution behavior of pure hydroxyapatite (HAp) and hydroxyapatite composites with 20–30 wt% mullite in simulated body fluid (SBF). The in vitro dissolution experiments were carried out for different time duration starting from 7 days up to 28 days. XRD and SEM results show almost no dissolution for pure HAp and HAp composite with 30 wt% mullite. However, HAp-20 wt% mullite composite exhibits considerable dissolution after 7 days. The α-TCP phase mainly contributes to the dissolution process. Based on the dynamic changes in pH, ionic conductivity, Ca and P ion concentration in SBF as well as microstructural observations of the bioceramic surfaces after various time frames of immersion in SBF, the differences in dissolution behaviour are discussed.