Crystallization of Gels in the Apatite-Mullite System

Crystallization processes in gels of the apatite-mullite system were studied to obtain information for the synthesis of bioglass-ceramics and composite materials. SiO2-sol, Al(NO3)3·9H2O, Ca(NO3)2·4H2O, (NH4)3PO4·3H2O and CaF2 were used as precursors. CaF2 was added before and after gelation. Mixtures of mullite gel-glass and fluorapatite in the range 10 to 90 mol% were investigated for synthesis of composites. All the samples were heat treated at different temperatures in the range 950–1250°C and the structural changes were established using X-ray diffraction and IR-spectroscopy. When the gels were treated at 1050°C and at 1150°C, the main crystalline phases found were fluorapatite and mullite independent of the CaF2 content and the manner of its addition. At 1250°C the relative amounts of fluorapatite and mullite decrease and gehlenite appears. Composite materials containing fluorapatite and mullite as main crystalline phases can be obtained only when the content of mullite gel-glass in the initial mixture is more than 60 mol%.     

The Effect of Precursor Chemistry on the Crystallisation and Densification of Sol-Gel Derived Mullite Gels and Powders

Stoichiometric and silica-rich mullite gels and powders were prepared using four different sol-gel methods. Thermal analysis, X-ray powder diffraction and dilatometry techniques were used to investigate the thermal decomposition, crystallisation and sintering of these mullite precursor gels. The method of preparation, by controlled hydrolysis of various mixtures of tetraethylorthosilicate, aluminium sec-butoxide and aluminium nitrate, affected the texture of the gels, producing single-phase or diphasic samples.The crystallisation sequence of the gels depended on the composition and method of preparation. Single phase mullite crystallised from homogeneous gels at 980°C, while diphasic gels initially formed of a mixture of -Al₂O₃ spinel and mullite, or simple -Al₂O₃ spinel, which subsequently transformed to mullite at 1260°C.Dilatometry and density measurement were used to investigate the sintering of compacts formed by pressing powders prepared from gels precalcined at 500°C. Varying the heating rates from 2 to 10°C min^-1 had little effect on the densification to 1500°C. However, the densification rate was sensitive to the degree of crystallinity and the amount and type of phases present at the sintering temperature. The presence of -Al₂O₃ spinel in the structure initially promoted densification, but the sintering rate was reduced considerably after mullite crystallised. Diphasic materials, especially those with an excess amount of silica in the original gel, sintered to higher densities due to the presence of excess silica promoting densification by viscous phase sintering.     

Mullite Based Compositions Prepared by Sol-Gel Techniques

Ceramic powders and gels in the multi-alumina system with different compositions were prepared by sol-gel method. The preparation of gels was done in strong acidic conditions and also with the addition of ammonia. The powders were prepared by dropping the sols on a hot plate. The effect of water/alkoxide and ethanol/alkoxide ratios was studied together with the type of catalyst used (hydrochloric acid or ammonia). The thermal evolution of materials was followed by thermal analysis, X-ray diffraction, specific surface area measurements and electron microscopy. The morphology of powders was also studied. The characteristic exothermic peak attributed to a good mixing degree of the Al and Si species was observed at ∼980°C. The intensity of this exothermic peak was more pronounced in gels than in sol-gel derived powders. The crystalline phases formed are related with the preparation conditions and with the thermal treatment. The crystallisation of mullite and transient phases of alumina were observed in powder samples heat treated at 1000°C while in gel samples mullite was the only crystalline phase detected.     

Preparation and characterization of diphasic sol-gel derived unsupported mullite membranes

Diphasic gels prepared by mixing freshly prepared polymeric silica and polymeric boehmite sols through a modified Al-alkoxide route in mullite compositions led to the crystallization of mullite upon heat treatment at 775 °C. Mullite formation was observed at a 100 °C higher temperature when diphasic gels were formed by mixing aged polymeric sols containing about 2 nm in diameter boehmite species. These relatively low mullite formation temperatures were attributed to the nanoscale sizes of the polymeric species of the two amorphous phases present in the diphasic gels.     

Study of thermally treated dickite by infrared and micro-Raman spectroscopy using curve-fitting technique

The products of dickite heated in air at 1000 to 1300°C were studied using curve-fitting of transmission and photoacoustic infrared and micro-Raman spectra. The spectra were compared with those of mullite, Al-spinel, corundum, cristobalite, amorphous silica and meta-dickite. Bands that characterize crystalline phases appeared at 1100°C and became stronger with increasing temperature. Mullite, Al-spinel, corundum and amorphous silica were identified by their characteristic bands. The characteristic IR bands of cristobalite overlap those of mullite and amorphous silica, and its presence was therefore established from intensity ratios of the appropriate bands. The research clearly demonstrated the advantage of using curve-fitting for the identification of high temperature phases in the study of the thermal treatment of kaolin-like minerals by infrared and Raman spectroscopy. This technique seems to be a useful method for materials analysis in the ceramic industry.This revised version was published online in November 2005 with corrections to the Cover Date.     

Cordierite-Like Catalyst Supports Based on Clay Materials

The possibility of lowering the temperatures of crystallization of spinel-like cordierite, mullite, MgAl₂O₄, and corundum was studied. The following naturally occurring clay materials were used for the synthesis of supports: bentonite, clay, and kieselguhr. The effect of the preparation procedure on the structure and texture characteristics of the resulting supports was found. The composition of the resulting samples varied over the following ranges: clay material, from 45 to 60%; Al₂O₃, from 30 to 42%; and MgO, from 10 to 20%. Conditions responsible for spinel formation in cordierite and mullite phases over the temperature range 1050–1150°C were found. At the same parent composition, the thermal stability of supports prepared by coprecipitation was higher than that of composites prepared by mechanical mixing, as found using X-ray diffraction analysis and measuring specific surface areas and mechanical strength. The supports synthesized are promising for the preparation of supported catalysts for high-temperature processes.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 648–651.Original Russian Text Copyright © 2005 by Stoyanova, Vladov, Kasabova, Mekhandzhiev.     

Hydroxyapatite gels and nanocrystals prepared through a sol-gel process

We have investigated the effect of the Ca/P molar ratio on the structural and morphological properties of hydroxyapatite (HA) gels and nanocrystals. The sol-gel process was carried out in aqueous, and alternatively in alcoholic medium (50% water-50% ethanol), at 37°C. Gel samples were obtained by drying the sols at 37°C or at 80°C, whereas powder samples were obtained by filtering the sols. Heat treatment at temperatures as low as 300°C is enough to obtain pure HA from the gels with a Ca/P molar ratio of 1.00 and 1.67. At variance, heat treatment of the gels with a Ca/P of 2.55 always produces secondary phases. The degree of crystallinity of HA increases with the Ca/P molar ratio of the sols, and it is slightly affected by the presence of ethanol in the precipitation medium. Filtering of the sols provides powders constituted of nanocrystalline HA that exhibit degree of crystallinity, crystal morphology and thermal stability closely related to the sols composition.     

Relationships among Preparative Technique,Phase Composition and Bending Strength of Bauxite Porcelain

1 INTRODUCTION Bauxite porcelain is a novel “K2O–Al2O3–SiO2” system ceramic by using sintered bauxite, clay and potash feldspar and albite as the main raw materials with the Al2O3’s content of 50%～60(massfraction). Compared to the traditional“K2O–Al2O3–SiO2” feldspar porcelain by using quartz, bolus alb and potash feldspar and albite as the main raw materials, the bauxite porcelain possesses such advantages as high mechanical strength, excellent electrical insula- tion prop…     

Mullite crystallization using fully hydrolyzed silica sol: the gelation temperature influence

Mullite is an aluminosilicate widely used as a structural material for high temperature applications. This paper studies the effect of the gelation temperature on the synthesis of two mullite precursors: polymeric and colloidal silica, using both in fully-hydrolyzed silica sol, derived from sodium silicate. The gels were synthesized using aqueous silicic acid and aluminum nitrate. Ethylene glycol was added into polymeric gels. Two gelation temperatures were used: 80 and 100 °C. In the polymeric precursor, the increasing of the gelation temperature caused an increase in the silica incorporation inside the mullite crystalline lattice at 1,000 °C, and it also generated an increase in the reaction extent at all calcination temperatures. In the colloidal precursors, these effects were more intense than in the polymeric precursors in terms of yield. Colloidal samples calcined at 1,250 °C crystallized cristobalite and alpha alumina in addition to mullite when they were previously gelled at 80 °C. On the other hand, the same sample gelled at 100 °C led to only crystallized mullite. The reaction extent increased by more than 20 % for colloidal samples gelled at 100 °C compared to colloidal samples gelled at 80 °C (calcined at 1,250 °C). This increase was due to the almost total incorporation of alumina and silica in the crystalline lattice of mullite.     

Sol-Gel Synthesis and Pyrolysis Study of Oxyfluoride Silica Gels

Silicon oxyfluoride materials are synthesized by the sol-gel method using triethoxyfluorosilane as precursor, bearing the Si—F bond. SiO_(2–0.5x) F _x gel preparation requires peculiar experimental control of hydrolysis and condensation reactions. Maintenance of the Si—F bond during gelling, heating and aging was studied in the case of processes carried out under an argon atmosphere or in air. Fluorine contents in resulting samples were quantified by FT-IR and X-ray photoelectron spectroscopy (XPS); specific surface area and porosity of powdered samples were determined by N₂ adsorption. The thermal stability of oxyfluoride gels was studied by thermogravimetric-mass spectrometric (TG-MS) coupled analyses during heat treatment, under He flow. Mass spectra recorded during principal weight losses indicate the release of variously fluorinated silicon species resulting from Si—F/Si—O exchange reactions. The evolution of these species was observed at different temperatures, depending on gelling conditions. In particular, degradation of Si—F moieties was prominent for gels aged in air, whereas samples processed under an argon atmosphere preserve the Si—F bond up to 300°C.     

Reaction sintering and microstructural evolution in metakaolin-metastable alumina composites

Fine needles of mullite grains were obtained successfully in a compact and low porous matrix using solid state sintering. We treated high-grade kaolin and sand-rich kaolin at 750 °C to amorphous metakaolins, and bauxite at 1,000 °C to metastable alumina. By designing a stochiometric composition of mullite, each amorphous metakaolin was added to metastable alumina. Fine grains of mullite with almost complete crystallization were obtained from 1,350 °C in a case of amorphous metakaolin from high-grade kaolin and at 1,550 °C in the other case where amorphous metakaolin is from sand-rich kaolin. The difference in the temperatures of mullitization was linked to the late dissolution of silica from the cristobalite and quartz phases which were still present in the sand-rich metakaolin sample at 1,350 °C. The use of metastable alumina and metakaolin instead of kaolin to design the mullite matrix allows the increase in number of mullite nucleation sites. This results to high densification and crystallization, fine grain size, and high mechanical properties of the final matrix.     

Structural Development of Single Phase (Type I) Mullite Gels

Single phase (type I) mullite gels were prepared by sol-gel techniques starting from alkoxides (Al-butylate, tetraethylorthosilicate) and alkoxides plus nitrates (tetraethylorthosilicate, Al(NO₃)₃·9H₂O). After drying at 150°C the aluminosilicate gels are non-crystalline and remain so up to 900°C. Above 900°C the gels transform into Al₂O₃-rich mullite plus a coexisting SiO₂ phase. Structural studies on temperature-dependent dehydroxylation and condensation of the gels were carried out by large angle X-ray scattering, by infrared spectroscopy and by²⁹ Si NMR spectroscopy. Heat-treatment (<150°C) of dried gels first causes removal of the H₂O and organic residuals weakly bound at the open pore surfaces of the gels while the stronger, structurally bound OH groups are not affected. At temperatures <600°C OH groups are released and recombine to molecular H₂O. If the temperature does not exceed 800°C the newly formed H₂O is trapped in closed nanopores of the gel-network. Corresponding electron microscopical investigations reveal agglomerates of 10 nm sized primary particles virtually unaffected by the heat-treatment below 900°C. NMR investigation provided a new structural model on type and distribution of coordination polyhedra in aluminium silicate gel networks. Unlike Si, which according to ²⁹Si NMR is always 4-fold coordinated with O, ²⁷Al NMR spectroscopy revealed that Al cordination is more complex and is influenced by thermal treatment. Al occurs six-fold (octahedrally) and four-fold (tetrahedrally) coordinated. A third ²⁷Al NMR signal which has been attributed to five-fold-coordinated Al in the literature increased in intensity with the heat-treatment. A comparison of NMR data of the gels with those of mullite suggests that tetrahedra triclusters (3 tetrahedra having one oxygen atom in common) occur as major structural units in aluminium silicate gels rather than five-fold-coordinated Al. Triclusters of tetrahedra may compensate the excess negative charge in the network caused by Si⁴⁺ Al³⁺ substitution. The charge compensation model is supported by aluminosilicate gels doped with network modifiers (e.g., Na⁺). Since equimolar addition of Na⁺ compensates Si⁴⁺ Al³⁺ substitution the formation of triclusters is no longer required which actually can be deduced from²⁷ Al NMR studies.     

Effect of alumina concentration on thermal and structural properties of mas glass and glass-ceramics

Magnesium aluminum silicate (MAS) glass samples with different concentrations of alumina (7.58 to 14.71 mol%) were prepared by melt and quench-technique. Total Mg content in the form of MgF₂+MgO was kept constant at 25 mol%. MAS glass was converted into glass-ceramics by controlled heat treatment at around 950°C. Crystalline phases present in different samples were identified by powder X-ray diffraction technique. Dilatometry technique was used to measure the thermal expansion coefficient and glass transition temperature. Scanning electron microscopy (SEM) was employed to study the microstructure of the glass-ceramic sample. It is seen from X-ray diffraction studies that at low Al₂O₃ concentrations (up to 10.5 mol%) both MgSiO₃ and fluorophlogopite phases are present and at higher Al₂O₃ concentrations of 12.3 and 14.7 mol%, fluorophlogopite and magnesium silicate (Mg₂SiO₄), respectively are found as major crystalline phases. The average thermal expansion co-efficient (_avg) of the glass samples decreases systematically from 9.8 to 5.5·10^–6 °C^–1 and the glass transition temperature (T g) increases from 610.1 to 675°C with increase in alumina content. However, in glass-ceramic samples the _avg varies in somewhat complex manner from 6.8 to 7.9·10^–6 °C^–1 with variation of Al₂O₃ content. This was thought to be due to the presence of different crystalline phases, their relative concentration and microstructure.Authors wish to thank Dr V. C. Sahni, Director Physics Group and Dr J. V. Yakhmi, Head TPPED, BARC for encouragement and support to the work. They would like to thank Dr S. K. Kulshreshtha for many useful discussions. Technical assistance from Shri B. B. Sawant, Mrs Shobha Manikandan, Mr Rakesh Kumar and Shri P. A. Wagh is gratefully acknowledged. One of authors (BIS) would like to thank BRNS-DAE for awarding him KSKRA fellowship.     

Facetting of Fe-15% Cr-N, Fe-3% V-C and Fe-3% V-C,N (111) surfaces

Summary Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning electron microscopy (SEM) have been used to study the surface chemistry of crystallographically open bcc (111) surfaces of Fe-15% Cr-N, Fe-3% V-C and Fe-3% V-C,N alloys. The binary and ternary surface phases CrN, VC and V(C,N) were formed at temperatures ranging from 450 to 750°C depending on the alloy. On Fe-15% Cr-30ppmN (111) two-dimensional surface compounds CrN were formed at temperatures above 600°C according to the bulk phase diagram of the Fe-Cr-N system, whereas on Fe-15% Cr-N samples with nitrogen contents of more than about 100 ppm the precipitation of three-dimensional CrN took place at temperatures below 600°C. Optical and SEM micrographs as well as oxidation experiments at room temperature indicated that the substrate surfaces are inhomogeneously covered by the surface phases. Facetting of the bcc (111) surfaces induced both by cosegregation of the solutes and by surface precipitation was observed in real space (SEM) as well as in reciprocal space (LEED). It is shown that the surface phases are epitaxially arranged on (100) facets of the substrate.     

The effect of heat and IR radiation on the fluorescence of cellulose

The emission spectra of pure cellulose samples of various origins were monitored during several heating/cooling cycles. During heating the emission intensity decreased due to the greater probability for internal conversion at higher temperatures. Cooling resulted in an emission recovery that was nearly reversible over several heating/cooling cycles, provided that the final 0temperature was sufficiently low. The change in the relative emission yield with temperature showed two regimes, both with linear decreases but different slopes, suggesting different mechanisms for the internal conversion in these regions. Heating to temperatures higher than 160°C for filter papers and higher than 145°C for microcrystalline cellulose initiated reactions that caused changes in the emission spectra typical of thermal degradation. If the samples were heated beyond these threshold temperatures the emission recovery on cooling after the first heat treatment occurred to a much higher intensity level than that observed initially, indicating the formation of a multitude of new chromophores due to thermal reactions. Exposure of the samples to IR radiation caused a slow increase in the emission intensity for almost 600h.     

Novel non-hydrolytic sol-gel route to metal oxides

Monolithic alumina and aluminosilicate gels have been prepared using a novel sol-gel process based on the non-hydrolytic condensation reaction between a metal halide and a metal alkoxide. XRD indicated that the alumina gel remained amorphous at 750°C; solid state ²⁷Al NMR indicated the presence in the dried gel and in the amorphous calcined sample of a large amount of pentacoordinated aluminum atoms. A study of the sol formation using liquid state ²⁷Al NMR suggested that the gel structure was reminiscent of the oligomeric structure of the chloroisopropoxide precursors. Differential thermal analysis and XRD indicated that the aluminosilicate gels were converted to mullite below 1000°C, suggesting a high degree of homogeneity in these precursors.     

Some significant advances in sol-gel processing of dense structural ceramics

This review deals with the solution-sol-gel processing of some structural ceramics such as alumina, zirconia, mullite and cordierite and brings out the most significant advances in the preparation of dense ceramics. In the Al₂O₃ system, seeding of gels with -Al₂O₃ and other isostructural seeds led not only to lower crystallization temperature but also enhanced densification with refined microstructure through solid-state epitaxy. This breakthrough of seeding has led to improved abrasive grains with large commercial market. Although highly dense, partially stabilized zirconia ceramics were prepared using monodisperse and spherical or nanophase zirconia sol-gel powders, no commercial applications seem to have been realized thus far. In the Al₂O₃–SiO₂ system, compositionally different sol-gel nanocomposites (diphasic gels) led to enhanced densification of mullite at lower temperatures because of the occurrence of densification and crystallization processes almost simultaneously. Mullite powders derived from the diphasic gel route are a breakthrough and are now commercially marketed by Chichibu Cement Company. Highly dense cordierite was prepared by using three sols, i.e., the compositionally different sol-gel nanocomposites in an analogous manner to that of mullite. Although this process appears to be highly cost-effective, especially for the fabrication of substrates, it has not yet been utilized for commercial applications. The use of sol-gel nanocomposites in the processing of various ceramics is expected to be fully exploited in the future.     

The Effect of Interactions between γ-AlOOH and SiO2 Hydrosols on the Phase Formation in Their Mixed Products during Thermal Treatment

The effect of the sign and the value of a relative change in the -potentials of -AlOOH and SiO₂ sols on the composition homogeneity of the products of their mixing, as well as on the phase formation during thermal treatment of the latter is studied. A sample prepared by mixing -AlOOH and SiO₂ sols at pH 2.5 demonstrates the highest homogeneity of mixed gels and forms the only phase of mullite 3Al₂O₃ · 2SiO₂ during its thermal treatment (1350°C). At this pH value, a slow heterocoagulation of the sol particles with different signs of the -potential takes place. The coagulation of the sols at pH 1.5 and 5.0 results in the preparation of more or less heterogeneous mixed products and the formation of several phases during thermal treatment.     

Synthesis of Bi2O3 and Bi4(SiO4)3 Thin Films by the Sol-Gel Method

Bi2O3 thin films were prepared by dipping silica slides in ethanolic solutions of tris(2,2-6,6-tetramethylheptane-3, 5-dionato)bismuth(III) [Bi(dpm)3] [1] and heating in air at temperatures 500°C. Bi4(SiO4)3 homogeneous thin films were obtained from the reaction of the bismuth oxide coating with the silica glass substrate at temperatures higher than 700°C. For heat treatments at temperatures between 600°C and 700°C, Bi2SiO5 coatings were obtained. The composition and microstructure evolution of the films were determined by Secondary Ion-Mass Spectrometry (SIMS), X-Ray Photoelectron Spectroscopy (XPS) and Glancing Angle X-Ray Diffraction (GA-XRD). The synthesis procedure was reproducible and allowed the control of the Bi2O3 phase composition. Moreover, the thin film annealing parameters were correlated with the formation of bismuth silicates, among which Bi4(SiO4)3 (BSO) is very appealing for the production of fast light-output scintillators [2].     

Structural characterisation of heat-treated anodic alumina membranes prepared using a simplified fabrication process

A facile method for forming porous anodic alumina membranes based on one-step anodising in sulphuric acid is reported. A flat and well-ordered basal surface incorporating uniformly sized pores was obtained without the need for electrolytic polishing. Excess metallic aluminium was removed from the film using a saturated solution of iodine in methanol. The high-temperature properties of the oxide ceramic membranes were investigated using thermal analysis, mass spectrometry, X-ray diffraction and solid-state nuclear magnetic resonance. At 970 °C the amorphous alumina crystallises to γ-Al₂O₃ with the release of SO₂ and O₂. Finally at 1228 °C the alumina converts into the thermodynamically preferred phase, corundum. The pore structure of the oxide membrane was found to be very stable at elevated temperatures, suggesting applications in materials synthesis, catalysis and gas separation.