Structural, microstructural and vibrational characterization of apatite-type lanthanum silicates prepared by mechanical milling

Apatite-type lanthanum silicates have been successfully prepared at room temperature by dry milling hexagonal A-La₂O₃ and either amorphous or low cristobalite SiO₂. Milling a stochiometric mixture of these chemicals in a planetary ball mill with a moderate rotating disc speed (350 rpm), allows the formation of the target phase after only 3 h although longer milling times are needed to eliminate all SiO₂ and La₂O₃ traces. Thus, the mechanically activated chemical reaction proceeds faster when using amorphous silica instead of low cristobalite as silicon source and pure phases are obtained after only 9 and 18 h, respectively. As obtained powder phases are not amorphous and show an XRD pattern as well as IR and Raman bands characteristic of the lanthanum silicate. The domain size of the as-prepared phases varies gradually with the temperature of post-milling thermal treatment with activation energies of about 26(8) and 52(10) kJ mol⁻¹ K⁻¹ for the apatites obtained from amorphous silica and low-cristobalite, respectively. These values suggest crystallite growth to be favored when using amorphous silica as reactant.     

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.     

Mullite-Alumina Composites Prepared by Sol-Gel

Ceramic composite gels in the mullite-alumina system (with mullite phase contents ranging from 20 to 100 wt%) were prepared by sol-gel in acid media and using a water/alkoxide molar ratio of 4. Powders were uniaxially pressed to obtain discs.The effect of heat treatment on the materials was followed by thermal analysis and X-ray diffraction (XRD). The crystalline phases formed from the gels were dependent upon the sample composition and the heat treatment temperature. For samples with composition close to the stoichiometric mullite composition the crystallization of mullite was observed after heat treatment at temperatures 900°C, and corundum appeared at higher temperatures. For samples with higher alumina contents mullite formation together with the spinel phase was observed at the lower heat treatment temperatures, while mullite, silimanite, corundum, and other transient phases of alumina were also detected at higher temperatures. The densities and microstructures of materials sintered at 1400°C were studied. Densification of the samples was low due to the agglomeration of the powders, as confirmed by scanning electron microscopy.     

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.     

Thermal Evolution of Hybrid Organic-Inorganic Gels Derived from Reaction of 1,4 Butanediol with Tetraethoxysilane

Transparent monolithic discs of organic-inorganic hybrid gels have been prepared by hydrolysis-condensation reactions of tetraethyl orthosilicate with 1,4 butanediol. The gels and glasses have been characterized by infrared spectroscopy and ²⁹Si MAS NMR. The characterization of the gels by infrared spectroscopy showed the incorporation of carbonaceous groups in the polymeric structure and Si–C bonding in the glasses. Pyrolysis of the gels has been studied using thermal analysis. It showed that the pyrolysis of the gels occurs in two temperature domains. The first is below 400°C due to condensation reactions and second is in the temperature range 450–550°C due to decomposition of carbonaceous groups and crosslinking. Pyrolysis of the gels at 1000°C resulted in X-ray amorphous, hard black glasses similar to oxycarbide glasses obtained by pyrolysis of siloxanes. On further heat treatment to 1400–1600°C, development of cristobalite structure and crystalline silicon carbide is observed in the otherwise amorphous black mass. The pyrolysed materials have been found to exhibit good resistance towards oxidation at 1000°C.     

Surface oxidation of high-surface-area silicon carbide: FT-IR studies

The surface of ultrafine silicon carbide powders, prepared by a laser-driven gas-phase reaction was studied as a self-supporting disk by FT infrared spectrometry. After evacuation silicon and carbon atoms located at the surface give rise tovSiH andvCH bands. When heating in oxygen, subtraction spectra showed features which could be strictly correlated with a progressive growth of a silica layer: SiH and CH bands were replaced by new bands characteristic of amorphous silica and the typical band of surface silanol groups on silica (3745 cm^–1) simultaneously increased.     

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 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.     

Photoluminescence Properties of Silica Aerogel During Sintering Process

We report photoluminescence spectra measured for two series of silica aerogel sintered at 1000°C in different time intervals. In the photoluminescence spectra of nonsintered sample, bands at 1.8, 2.0 and 2.2 eV are identified. During sintering process, the bands at 1.8 and 2.0 eV decrease and vanish, while the band at 2.2 eV shows more complicated behavior. According to infrared spectra of the same samples we find that the photoluminescence band at 1.8 eV originates from nonbridged oxygen hole center defect, and that at 2.0 eV originates from silane in the gel network. Nonstoichiometric SiOx causes photoluminescence band at 2.2 eV.     

The microstructural study of thermal treatment montmorillonite from Heping, China

The montmorillonite samples from Heping, China had been studied by chemical analysis, DAT, TG, XRD and MAS NMR. The results showed that the hydroxyl in octahedra sheets begins dehydrating, when the thermal treatment temperature reaches 650 degrees C, but the layer structure remains, the corresponding Al(VI) was turned into Al(IV) in octahedra sheets. When the temperature reaches 900 degrees C, the layer structure of montmorillonite is destroyed, and the new mineral phase mu-cordierite is found. When the temperature reaches 1200 degrees C, the mu-cordierite phase loses stability, and decomposes into cristobalite phase and mullite phase, meanwhile, the recrystallization phenomena in thermal treatment products is obvious. There is a small quantity of Al(VI) signal in MAS NMR spectrum, this Al(VI) corresponds to the Al of mullite. When the temperature reaches 1350 degrees C, the cristobalite and mullite phases reduce slightly, and more Fe-cordierite phase appear. There is corresponding Fe-cordierite spectrum in XRD and MAS NMR.     

Thermal transformations up to 1200 °C of Al-pillared montmorillonite precursors prepared by different OH–Al polymers

Aluminum-pillared montmorillonites are useful materials for their application as catalysts, adsorbents and ceramic composites. The precursor is a pillared montmorillonite that is not thermally stabilized. The precursor preparation methods, textural properties and catalytic activity have been extensively investigated, but comparatively, studies concerning their thermal transformations at high temperature are limited. In this work, precursors were prepared using two types of montmorillonites, Cheto (Ch) and Wyoming (W), and using two different OH–Al polymer sources: hydrolyzed (H) and commercial (C) solutions. Structural and thermal transformations of the precursors with heating up to 1200 °C were determined by X-ray diffraction and thermogravimetric analysis. Thermal analysis of these precursors below 600 °C revealed the influence of OH–Al polymers from the two solutions. The major phases developed at 1200 °C from the original montmorillonites were mullite for W and cordierite for Ch. The content of these phases depended on the aluminum in the octahedral sheet of the pristine montmorillonites. Amorphous phase, cristobalite, spinel, sapphirine and others phases were also found. The intercalation of OH–Al polymers in montmorillonites caused an increase in amorphous content after treatment at 1030 °C; however, it favored mullite development above 1100 °C. Although total aluminum content of both W and Ch precursors was similar, the transformation to mullite was directly related to the octahedral aluminum/magnesium ratio. The phase composition of the products at 1200 °C was not dependent on the type of intercalated OH–Al polymers. The increase in mullite content of the thermally treated precursors contributes to its possible application as advanced ceramic products.     

Effect of degradation of montmorillonite by vibration grinding on the dTA curves in the range 20–1500°C

The effect of vibration grinding on the DTA curves of montmorillonite isolated from the most important locality of bentonite in the Slovak Republic (Jel?ovy potok, Middle Slovakia) was studied in the temperature range 20–1500°C. Interpretation is offered for 6 endothermic and 3 exothermic peaks. vibration grinding modified the course of dehydration of the mineral, suppresses the “dehydroxylation’ peak at 700°C, enables the crystallization of high-temperature quartz, cristobalite and cordierite at lower temperatures in comparison with unground natural montmorillonite. Vibration griding slows down the crystallization of mullite which—unlike quartz, cristobalite and cordierite—does not belong to high-temperature phases of Cheto-montmorillonite. The order in which these high-temperature phases occur is not influenced by vibration grinding.     

Synthesis of Cristobalite Containing Ordered Interstitial Mesopores using Crystallization of Silica Colloidal Crystals

Cristobalite with ordered interstitial dual-sized mesopores was synthesized through the crystallization of silica colloidal crystals composed of monodispersed amorphous silica nanoparticles. An aqueous solution containing both a flux (Na₂O) and a carbon precursor (an aqueous low-molecular weight phenolic resin) was infiltrated into the interstices of silica colloidal crystals. The organic fraction in the nanocomposite was further polymerized and subsequently carbonized in an Ar flow at 750 °C to reinforce the colloidal crystal structure. The thermal treatment resulted in the crystallization of the colloidal crystals into cristobalite while retaining the porous structure. The cristobalite-carbon nanocomposite was calcined in air to remove the carbon and create interstitial ordered mesopores in the cristobalite. The surfaces of crystalline mesoporous silica are quite different from those of various ordered mesoporous silica with amorphous frameworks; thus, the present findings will be useful for a precise understanding and control of the interfaces between the mesopores and silica networks.     

Low-temperature infrared and raman studies: XI. The vibrational behaviour of ammonium perchlorate- its phase changes and the rotational freedom of its ions

An infrared, Raman and X-ray diffraction study on NH₄CIO₄. was conducted in the temperature ranges 300 to 17 °K, 300 to 130 °K and 300 to 120 °K respectively. The infrared studies also included the deuterated salt. The spectra are discussed and vibrational assignments made. Changes in the infrared spectra, as well as intensity measurements of the vibrational bands indicate anomalous behaviour in the region 100–110 °K as well as 17–50 °K. Possible reasons for these changes are proposed. The rotational behaviour of the ammonium ion upon cooling and hydrogen bonding are discussed.     

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%.     

Structural Study of Silica Xerogel Composites Containing Pd Aggregates

Composites containing Pd aggregates dispersed in amorphous silica are of interest from both a fundamental and applied point of view because of their attractive catalytic properties. The silica powders added with palladium, prepared by the sol-gel method, were studied using X-ray diffraction, and IR- and UV-Vis-spectrophotometry. Silica xerogel samples were prepared using a ethanol/H₂O/TEOS molar ratio of 4:11.6:1 and loaded with sodium tetrachloropalladate. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the palladium aggregates in the SiO₂ matrix. We found in our samples partial crystallization of the glass matrix in form of quartz and cristobalite phases with palladium oxide and metallic palladium phase at 1000^∘C. The Rietveld refinement method was used in order to determine the percentage of the phase contents.     

FT-Raman and photoacoustic infrared spectroscopy of Syncrude heavy gas oil distillation fractions

FT-Raman and photoacoustic (PA) infrared spectra of six distillation fractions derived from Syncrude heavy gas oil (HGO), which has a boiling range from 343 to 524 degrees C, were analyzed in detail in this study. Most of the information on the fingerprint region (200-1,800 cm(-1)) is provided by the FT-Raman spectra, which display approximately 30 bands that are assignable to functional groups in alkanes or aromatics. Monocyclic, bicyclic and tricyclic aromatics in the six fractions were also monitored using bands in this region. The C-H stretching region in both the FT-Raman and PA infrared spectra of the HGO distillation fractions was analyzed according to a curve-fitting algorithm used in previous investigations of samples with lower boiling points. The PA spectra of the HGO fractions were also analyzed by integration. The curve-fitting results show that the frequencies of the 11 Raman and 8 infrared bands used to model the aliphatic (approximately 2,775-3,000 cm(-1)) parts of the respective spectra are approximately constant across the entire HGO boiling range. These band positions are consistent with the results obtained in earlier studies of other distillation fractions obtained from Syncrude sweet blend. Both curve-fitting and integration show that the respective proportions of CH(2) and CH(3) groups do not vary significantly within the HGO region.     

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.     

Microstructure and luminescence of transparent glass ceramic containing Er:BaF2 nano-crystals

Transparent BaF₂-SiO₂ glass ceramics doped with different content of Er³⁺ were prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscope (TEM), absorption and infrared spectra (IR). BaF₂ nano-crystals with 2-15 nm in size, depending on the crystallization temperature, distributed homogeneously among the amorphous silica matrix. The BaF₂ lattice parameters decreased with the increasing of Er³⁺ doping, indicating the incorporation of Er³⁺ into nano-crystals, which was further confirmed by energy dispersive X-ray spectroscopy (EDS) and absorption spectra analysis. The upconversion emissions of Er³⁺ emerged under the excitation at 980 nm for glass ceramic heat-treated at 800 °C.     

Pectin based cerium (IV) and thorium (IV) phosphates as novel hybrid fibrous ion exchangers synthesis,characterization and thermal behaviour

Summary Pectin based cerium (IV) and thorium (IV) phosphates have been synthesized as new phases of hybrid fibrous ion exchangers. Both materials were characterized using X-ray diffraction, infrared (IR) spectra, thermogravimetric analysis (TG), differntial thermogravimetry (DTG), differntial thermal analysis (DTA) and scanning electron microscopy (SEM), as well as the determination of their ion exchange capacity, elution and pH titration. The X-ray study reveals the amorphous nature of the materials, while SEM studies confirm the fibrous nature of the materials. The thermal studies of these materials indicate that both of them are highly stable on heating as they retain about 97% of their ion-exchange capacity (i.e.c.) on heating up to 100°C and about 81% on heating up to 200°C.