Diphasic Mullite Gel Research Articles

Si-incorporated alumina phases formed out of diphasic mullite gels

The diphasic mullite gel forms o-mullite on heating via intermediate spinel phase. Characterization of the latter phase with various physico-chemical techniques is concisely reviewed. It is noticeable that XRD intensity of both the amorphous scattering band and the diffraction peak of Al–Si spinel phase changes during each step of transformation processes of diphasic gel. Accordingly, the integrated area of the intensity peak of amorphous band and that of Al–Si spinel phase generated during heating diphasic gels were measured by XRD technique with the help of X’Pert Graphics and Profit softwares. The amount of free SiO2 (A) content present at various stages of heating diphasic gels was estimated by classical alkali leaching study standardized earlier. The results show that diphasic gel which forms an aluminosilicate (A) phase initially by dehydration and dehydroxylation, subsequently crystallizes to Al–Si spinel phase. In consequence, the ratio of XRD peak of spinal phase to that of amorphous band increases in the temperature range of 600–1000 °C. This study confirms the earlier view of incorporation of silica into the alumina structure with formation of Al–Si spinal phase. Complementary alkali leaching study indicates the existence of non-crystalline silica-rich aluminous phase other than free non-crystalline silica during heating diphasic gel at ~1000 °C.

Structural Parameters of Mullite Formed During Heating of Diphasic Mullite Gels

Diphasic gels of four different compositions were synthesized. Lattice parameters, crystallite size, and strain values of mullite formed at different stages of annealing of diphasic gels have been calculated by quantitative standard less analysis using X‐ray diffraction. Results show that all three parameters change on progressive heating. Changes in the microstrain of mullite show a reciprocal relationship with crystallite size. Silica‐rich gel shows that the relative changes in the calculated values of three structural parameters of mullite formed during its formation process are because of an increase in its crystallinity rather than changes in composition. However, gels of composition in the solid solution range show that the relationship between lattice constants and chemical composition of mullite formed at high temperature agrees with Cameron data.

The structural change of diphasic mullite gel studied by XRD and IR spectrum analysis

Abstract A systematic study on the structural change of diphasic mullite gel with heat treatment was carried out by combination of XRD and IR analysis. Aluminium ions are prevalently hexacoordinated in the as-received gel. After heated at high temperatures, part of them transforms to tetrahedral coordination. Phase segregation occurs at ≈1000 °C, resulting in the formation of γ-Al 2 O 3 (containing 9–11 wt.% SiO 2 ) and almost pure amorphous silica. Mullite forms at ≈1250 °C by reaction between (δ,θ)-Al 2 O 3 and amorphous silica. The incorporation of SiO 2 into γ-Al 2 O 3 raises the transformation temperature of γ-Al 2 O 3 →(δ,θ)-Al 2 O 3 , the formation of pure amorphous silica and mullite render Si-O bands in IR spectrum shift stepwisely towards higher wave numbers.

27Al and 29Si MAS–NMR studies of structural changes in hybrid aluminosilicate gels

Abstract Aluminosilicate gels with stoichiometric and nonstoichiometric compositions were synthesized by means of colloidal sol-gel method and their mullitization behavior was studied by X-ray diffraction (XRD), 27Al and 29Si magic angle spinning nuclear magnetic resonance (MAS–NMR) experiments. Particular attention was given to the structural changes of matrix accompanying the formation of mullite. The various coordinated Al occupancies were clarified by simulating the 27Al MAS–NMR spectra with Gaussian lines. The results demonstrate that the so-synthesized aluminosilicate gel is a hybrid gel containing a mixture of a single-phase gel and a diphasic gel. The mullitization of so-formed hybrid gel exhibits a consecutive one-step conversion process, but not a two-step process, much similar to that of a true diphasic mullite gel. The mullite formation from hybrid aluminosilicate gel mainly depends on the nature of dominant matrix part, but not on the nature of minor matrix part in gel. During the formation process of mullite, amorphous Si-rich phase appears as a transitional phase. The effects of gel composition and heating rate on the phase transformation behavior of hybrid aluminosilicate gel were also discussed here.

Effect of composition on the sintering and microstructure of diphasic mullite gels

Diphasic gels with alumina/silica ratio of 68/32, 71.8/28.2, 74/26, and 76/24 were prepared and used to study the effect of the precursor composition on the mullite formation process, densification behavior, and microstructure development. Alumina contents of mullites in the samples sintered at high temperature (1650°C) for 4 h or at low temperature (1550°C) for longer time increased with the increase in alumina/silica ratio of the gels. A sharp decrease in densification rate was observed on transition from the mullite+silica-rich liquid phase field to the mullite solid solution. This decrease was associated with elimination of the viscous flow of the amorphous silica phase. However, the densification rate was higher for the samples with excess alumina than for the 74/26 sample. Both in the silica-rich sample (68/32) and the stoichiometric mullite sample (71.8/28.2), elongated mullite grains were observed. Such elongated grains grew at the expense of the surrounding smaller mullite grains. In the 74/26 and 76/24 samples, mullite grains were equiaxed. Furthermore, in the latter, elongated α-Al 2O 3 grains were observed.

Characterization of the spinel phase in a diphasic mullite gel using dynamic X-ray diffraction

Dynamic X-ray diffraction (DXRD) has been used in an effort to identify the specific phase changes which are responsible for observed thermal events at ∼980 °C in mullite gel precursors. Specifically, changes in the evolution of the common and strongest diffraction peak (d = 0.139 nm) corresponding to both transient alumina phases and the Al-Si spinel were followed in order to descriminate between these two phases. Results which compare the DXRD results for a diphasic mullite gel and a boehmite gel are presented and suggest that the Al-Si spinel phase forms at ∼980 °C in diphasic gels along with δ- and/or γ-Al2O3. These results are corroborated by separate TEM measurements which indicate the presence of both phases in samples quenched from 1000 °C.

Effect of mechanochemical treatment on the crystallization behaviour of diphasic mullite gel

The effect of mechanochemical treatment on the structure and crystallization behaviour of diphasic mullite gel was investigated. The structural differences caused by mechanochemical treatment in diphasic gel were characterized by 27Al and 29Si MAS-NMR, XRD, DTA-TG and FT-IR. These investigations show that the crystallization process in this system is affected by the structural changes caused by grinding. Grinding for 20 h changed a diphasic system containing discrete octahedral Al–O units (8 ppm) and silica units (−111 ppm) to a more homogeneous system containing tetrahedral, “pentahedral” (30 ppm) and octahedral alumina, and a strong 29Si resonance at −83 ppm (≈70% of the total Si) associated with aluminosilicate units. The ground gel converts to mullite via the spinel-phase at 1150–1200°C. Mechanochemical treatment thus converts the diphasic type II gel precursor into a more homogeneous type III gel precursor, as reflected by its thermal behaviour.

Low-Temperature Mullitization in Boehmite-Tetraethoxysilane Gel Precursor Containing γ-Alumina and Mullite Nucleating Seeds

The formation of mullite from diphasic mullite gels of stoichiometric composition 3Al2O3·2SiO2 through a mixture of seed nuclei has been studied. The precursor gels are prepared from boehmite (AlOOH) sol and tetraethoxysilane (TEOS) seeded with mullite (2 wt %) and γ-Al2O3 (2 wt %) individually and as a mixture consisting of γ-Al2O3 (1 wt %) and mullite (1 wt %). The phase formation in such gels has been compared with sample containing no seeds. The presence of mullite is observed as early as 1150 °C in the mixed seeded sample, while in the unseeded one, this is observed only at 1250 °C, as reported generally. On the otherhand, the mullitization reaction has been initiated at a temperature of 1200 °C in gels seeded separately with 2 wt % γ-Al2O3 and 2 wt % mullite. The results suggest that the mechanism of mullite formation in seeded gel is similar to that of diphasic gels in which the transitional alumina−amorphous silica reaction predominates to form mullite. Mixed seeding appears to be appreciably favo...

Anisotropic Grain Growth in Diphasic‐Gel‐Derived Titania‐Doped Mullite

Densification and anisotropic grain growth in diphasic‐gel‐derived, titania‐doped mullite were studied. Titania enhanced initial and intermediate stage densification in diphasic mullite gels by reducing the glass viscosity. Rodlike anisotropic mullite grains started to grow in titania‐doped diphasic mullite gels once a dense, equiaxed microstructure was achieved. The onset temperature for anisotropic grain growth decreased with increasing titania concentration because the sintering temperature for final‐stage densification decreased. The lowest onset temperature for anisotropic grain growth was ∼1500°C in 5 wt% titania‐doped mullite. The aspect ratio and area fraction of anisotropic mullite grains increased with higher titania concentration and were strongly dependent on the initial titania particle size. Kinetic studies demonstrated that anisotropic grain growth in titania‐doped diphasic mullite gels followed the empirical equation Gn ‐ G0n=Kt, with growth exponents of 3 and 6 for the length [001] and thickness [110] directions, respectively. The activation energies for grain growth were 690 kJ/mol for the length and 790 kJ/mol for the thickness directions.

Interfacial precipitation in titania-doped diphasic mullite gels

Interfacial precipitation in sol-gel derived, titania-doped diphasic mullite gels was investigated using conventional and high resolution transmission electron microscopy. Rutile, anatase, and brookite precipitated on the interface between {110} planes of mullite and glass pockets in the sintered body. The formation of brookite may be attributable to the Si- and Al-rich environment during precipitation. Each polymorph of titania has a unique morphology and orientation relationship with mullite. Brookite exhibits a truncated pill box shape, and anatase displays a vermicular morphology. Quenching experiments suggest that the precipitates grow and undergo phase transformations during cooling.

Mullite Transformation Kinetics in P2O5‐, TiO2‐, and B2O3‐Doped Aluminosilicate Gels

Mullite transformation kinetics of sol‐gel‐derived diphasic mullite gels doped with P2O5, TiO2, and B2O3 were studied using quantitative X‐ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P2O5 doping because of phase separation and formation of α‐alumina and cristobalite. In TiO2‐doped samples, the mullite transformation temperature decreased with TiO2 doping, and the transformation rate increased with decreasing TiO2 particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid‐state mass transport through lattice defects. B2O3 doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.

Further Studies on Thermal Transformation of Diphasic Al2O3-SiO2 Gel

Diphasic mullite gel (Si/Al=1/3) has been synthesized in.situ by hydrolyzing an aqueous solution of silica sol and aluminium salt with the help of NH4OH solution. It has been characterized by DTA. The formation of Si-Al spinel during non-eqilibrium heating of the gel and its subsequent transformation have been recorded by XRD technique. The result shows that Si—Al spinel crystallizes very slowly at 500°-800°C and then very rapidly at ∼ 1200°C which corresponds with the exhibition of a broad exotherm between 1100° and 1300°C in DTA. Accordingly, this exotherm is attributed to the formation of Si—Al spinel. It suddenly transforms to predominant amount of orthorhombic form of mullite with consequent exhibition of an exotherm at 1320°C in DTA. The residual non-crystalline aluminosilicate phase also crystallizes into mullite and accounts for the second mullitization process in diphasic gel.

Diphasic mullite gel pressed at 1.5 GPa: Mechanical properties and microstructure of ceramics

Mullite gels containing 72–85 wt% Al2O3 were prepared by mixing commercial boehmite and silica sols and gelled by heating. Powdered gels were either dried at 60°C or calcined at 550°C, and cold isostatically pressed under 0.5, 1.0 or 1.5 GPa. The effect of pressure is greater for the calcined gels than for the dried gels. The bulk density and room-temperature flexural strength of ceramics increased with increasing compacting pressure, while the fracture toughness is independent of the pressure. Mullite containing 75 wt% Al2O3 and sintered at 1500°C for 3 h has a density which is about 99% of the theoretical density, a flexural strength of 523±40 MPa and a fracture toughness of 3.5 MPa m1/2.

Transparent mullite ceramics from diphasic aerogels by microwave and conventional processings

Transparent mullite ceramics were developed by both microwave and conventional sintering of compacts starting with a diphasic aerogel near 1300 °C. Both sintering processes were carried out in air at ambient pressure. The conventionally sintered sample was essentially non-crystalline, whereas the microwave sintered sample was highly crystalline mullite. Using a xerogel of the same composition, no transparency was achieved under the same conditions. The results indicate that the agglomeration-free microstructure of the starting aerogel was the key for achieving the transparency. The achievement of transparent mullite ceramics by microwave processing is attributed to the rapid-heating, accelerated-mullitization, enhanced densification, and limited grain-growth of the diphasic mullite gel in the microwave field.

New data on the thermal analysis of diphasic mullite gel

A diphasic mullite gel has been synthesized by using Ludox and Al(NO3)3-9H2O in ammoniacal solution. Both DTA and DDTA studies have been performed with a sensitive differential thermal analyzer system. Besides usual exotherm due to mullite formation, a pronounced exotherm between 800–1300°C has been noted. Considering previous XRD data, it is explained as due to formation of Si-Al spinel phase, which is subsequently transformed exothermally at ∼1320°C to mullite.

Single-phase and diphasic aerogels and xerogels of mullite: Preparation and characterization

Single-phase mullite composition gels have been synthesized using tetraethoxysilane [Si(OC 2H 5) 4] and aluminum nitrate nonahydrate as precursors. Diphasic mullite gels have been prepared using colloidal silica and boehmite as precursors. Xerogels and aerogels of both the above gels have been obtained by ordinary drying in air at 60 °C and critical point drying in methanol, respectively. Single-phase xerogels show an intense exothermic peak at about 980 °C while their counterparts, aerogels, do not show any detectable exotherm by differential thermal analysis (DTA). These results suggest that the structure of single phase gels changed during critical point drying and the structural changes were investigated by solid-state 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. No differences between diphasic xerogels and aerogels could be detected by DTA because there was little or no effect of drying on the discrete silica and alumina phases. The tetrahedral coordination of Al in single-phase mullite gel changed to octahedral coordination as detected by MAS NMR during critical point drying, which suggests that the alumina component segregated. Both single-phase and diphasic aerogels of mullite composition showed very high surface areas in the temperature range 1000–1400 °C and these may be useful for high temperature catalytic applications.

Anisotropic grain growth in seeded and B 2O 3-doped diphasic mullite gels

Anisotropic grain growth in mullite was investigated in B 2O 3-doped diphasic gels seeded with either mullite particles or whiskers. Anisotropic grain growth was observed in all systems. The largest anisotropic grains were obtained with a system seeded with 2 wt% mullite whiskers and doped with 2 wt% B 2O 3. The mullite whiskers act as sites for multiple nucleation and subsequently as templates for mullite overgrowth. Boria lowered the mullite formation temperature by 150 °C and it significantly enhanced anisotropic grain growth independent of the presence of seed particles. This enhancement was attributed to increased dissolution of alumina.

Intermediate Si-Al spinel phase formation in phase transformation of diphasic mullite gel

Al2O3-SiO2 diphasic mullite gel (Si/Al=1/3) has been synthesized by using Al(NO3)3·9H2O and Ludox in basic conditions. Its phase transformation behaviour has been studied by qualitative and quantitative X-ray diffraction techniques. The results indicate that a noncrystalline alumino-silicate phase forms together with the slow crystallization of Si-Al spinel phase of a mullite-like composition up to 1250 °C. Thereafter, it transforms suddenly to orthorhombic mullite around 1325 °C. Earlier studies, e.g. alkali leaching, and measurement of co-ordination number of aluminium in 1000 °C heated diphasic gel, confirm Si-Al spinel formation as an intermediary phase.

The effect of precursor characteristics on the crystallization and densification of diphasic mullite gels

The crystallization and densification behavior of diphasic mullite gels was studied with the gels prepared by using various sources of boehmite powder and silica sol to find the effect of precursor characteristics such as the pH of mixed sols, particle size, and the type of the stabilizing alkali ion in silica sol. Both low pH and fine particle size were found to promote mullite crystallization and thus result in low densification. However, even with a high pH and a large particle size, densification was low without the presence of sodium because of the crystallization of cristobalite and mullite before densification. A small amount of sodium (0·5 wt% as Na 2O), present as a stabilizing ion in coilloidal silica or intentionally added, suppressed the crystallization of cristobalite and mullite before densification. As a result, bulk densities of more than 95% of the theoretical density were obtained by sintering at 1320°C for 1 h without a controlled heating.

Preliminary Characterization of Gel Precursors and Their High‐Temperature Products by 27Al Magic‐Angle Spinning NMR

The environment of the Al atom in alumina gels, single‐phase and diphasic mullite gels, and their transformation products was studied by 27Al magic‐angle spinning nuclear magnetic resonance (MASNMR). Alumina gels made from inorganic and organic precursors showed exactly the same 27Al MASNMR spectra. Heat treatment of gels at 500°C caused artial conversion of octahedral Al to tetrahedral Al. The Al in single‐phase mullite gel appears to be highly ordered compared to that in diphasic mullite gel.