Showing papers on "Tridymite published in 2007"
TL;DR: In this paper, a novel type of mechanically strong zeolite extrudates is introduced, in which the Zeolite crystals, here ZSM-5, are embedded in a sintermatrix of aluminium phosphate with the crystal structure of tridymite.
Abstract: A novel type of mechanically strong zeolite extrudates is introduced, in which the zeolite crystals, here ZSM-5, are embedded in a sintermatrix of aluminium phosphate with the crystal structure of tridymite. The pore size distribution in the matrix can be tailored through both, the choice of the size of the embedded particles and their amount in the extrudates. At zeolite concentrations of 50% (w/w) or higher, the mean pore diameter of the matrix ( D pore ) depends on the zeolite crystal size only, and amounts to D pore ≈ 0.25 D particle . Other than conventional matrices on alumina basis, the AlPO 4 matrix does neither provide catalytic self-activity, nor does cause any alumination of the zeolite framework.
65 citations
TL;DR: It was found that the number of covalent bonds to the surface formed by the functional reagents is affected by the surface morphology, and for high-quality MCM-41 silicas the distance between neighboring surface silanol groups is greater than 0.5 nm.
Abstract: MCM-41 and SBA-15 silicas were studied by 29 Si solid-state NMR and 15 N NMR in the presence of 15 Npyridine with the aim to formulate generic structural parameters that may be used as a checklist for atomicscale structural models of this class of ordered mesoporous materials. High-quality MCM-41 silica constitutes quasi-ideal arrays of uniform-size pores with thin pore walls, while SBA-15 silica has thicker pore walls with framework and surface defects. The numbers of silanol (Q 3 ) and silicate (Q 4 ) groups were found to be in the ratio of about 1:3 for MCM-41 and about 1:4 for our SBA-15 materials. Combined with the earlier finding that the density of surface silanol groups is about three per nm 2 in MCM-41 (Shenderovich, et al. J. Phys. Chem. B 2003, 107, 11924) this allows us to discriminate between different atomic-scale models of these materials. Neither tridymite nor edingtonite meet both of these requirements. On the basis of the hexagonal pore shape model, the experimental Q 3 :Q 4 ratio yields a wall thickness of about 0.95 nm for MCM-41 silica, corresponding to the width of ca. four silica tetrahedra. The arrangement of Q 3 groups at the silica surfaces was analyzed using postsynthesis surface functionalization. It was found that the number of covalent bonds to the surface formed by the functional reagents is affected by the surface morphology. It is concluded that for high-quality MCM-41 silicas the distance between neighboring surface silanol groups is greater than 0.5 nm. As a result, di- and tripodical reagents like (CH3)2Si(OH)2 and CH3Si(OH)3 can form only one covalent bond to the surface. The residual hydroxyl groups of surface-bonded functional reagents either remain free or interact with other reagent molecules. Accordingly, the number of surface silanol groups at a given MCM41 or SBA-15 silica may not decrease but increase after treatment with CH3Si(OH)3 reagent. On the other hand, nearly all surface silanol groups could be functionalized when HN(Si(CH3)3)2 was used.
64 citations
TL;DR: In this paper, a series of micro-crystalline opals from siliceous rocks, geodes, and bentonite clays from East Rhodopes, Bulgaria, were analyzed by Raman spectroscopy, X-ray powder diffraction, electron microscopy, and thermogravimetric and differential thermal analysis.
Abstract: The structure of natural hydrous silica is complex and its study requires the complementary application of several methods. To elucidate the structural state of opaline silica of different geneses, microcrystalline opals from siliceous rocks, geodes, and bentonite clays from East Rhodopes, Bulgaria, were analyzed by Raman spectroscopy, X-ray powder diffraction, electron microscopy, and thermogravimetric and differential thermal analysis. Comparison of X-ray diffraction and spectroscopic data for a series of microcrystalline opals showed that the fraction of tridymite-like structural units can be estimated using the relative intensity of the Raman scattering near 350 cm –1 . Opals displaying an intense, poorly resolved Raman band centered near 330–360 cm –1 contain a larger proportion of nanosized spatial regions with tridymite-type atomic arrangements as opposed to cristobalite-type arrangements. The results demonstrate the ability of Raman spectroscopy to characterize the Þ nescale structure of opal and to better distinguish opals showing similar XRD patterns. The application of Raman micro-spectroscopy showed that on intermediate-range scale the atomic structure of opal lepispheres is closer to the framework topology of tridymite than to that of cristobalite.
47 citations
TL;DR: In this paper, the Schreinemakers theory of PT-diagrams is applied to the high-temperature and medium-pressure part of the SiO2-H2O system.
Abstract: The high-temperature and medium-pressure part of the PT-diagram of the system SiO2-H2O has been investigated experimentally. The equilibrium diagram is discussed in the light of Schreinemakers general theory of PT-diagrams.
The triple invariant point cristobalite + tridymite + quartz lies at 1190°C and 1430 atm. Neither cristobalite nor tridymite are stable at high pressure. Quartz may precipitate from the melt at a very high temperature (1360°C and higher), if the pressure is great enough, and if the water content is low.
Using new experimental and published data a PT-diagram of the system SiO2-H2O in the large PT-region is given.
40 citations
TL;DR: In this paper, traces of volatile lithium species transported through the gas phase catalyze the crystallization of silica integrated into common silicon microstructures, and the selected crystallization temperature determines which polymorph forms.
Abstract: Amorphous silica formed by thermally oxidizing silicon is commonly eligible for lithographic patterning or can be grown on patterned substrates. However, it is still a challenge to combine controlled microstructuring with controlled crystallization of SiO 2 . Here, it is shown that traces of volatile lithium species transported through the gas phase catalyze the crystallization of silica integrated into common silicon microstructures. The selected crystallization temperature determines which polymorph forms. As an example, the formation of quartz, tridymite, and cristobalite microtubes by thermally oxidizing macroporous silicon is investigated. Lithium-induced crystallization may extend state-of-the-art silicon technology and yield nano- and microstructures consisting of different silica polymorphs, which are, in contrast to many functional oxides, nontoxic.
26 citations
TL;DR: In this paper, a spin-coated LiCoO 2 thin-film cathode with amorphous nanoparticles and annealed at 400°C showed the best cycle life performance, and effectively suppressed the degradation of Li + -diffusion kinetics during cycling.
Abstract: To control the nanostructure of an AlPO 4 -coating layer, nanoparticles with three AlPO 4 phases were synthesized: amorphous, tridymite, and cristobalite phases. These colloids were layered on LiCoO 2 thin films by spin coating, and subsequently annealed at 400°C. The interdiffusion variations at the interface were eliminated by the spin-coating method, while the cycle-life performance of the coated cathode depended on the nanostructure of the AlP04-nanoparticle-coating layer. The LiCoO 2 thin-film cathode coated with amorphous nanoparticles and annealed at 400°C showed the best cycle-life performance, and effectively suppressed the degradation of Li + -diffusion kinetics during cycling.
20 citations
TL;DR: In this article, the rates of dissolution in dilute sodium hydroxide solution of a series of size-graded powders of the four common polymorphic forms of silica (quartz, tridymite, cristobalite and vitreous silica) have been studied and found to be in keeping with their physical properties.
Abstract: The rates of dissolution in dilute sodium hydroxide solution of a series of size-graded powders of the four common polymorphic forms of silica (quartz, tridymite, cristobalite and vitreous silica) have been studied and found to be in keeping with their physical properties. The effect of variation of particle size on the dissolution of quratz has been related to the ‘amorphous layer’. The results provide no support for the ‘solubility theory’ of silicosis.
18 citations
31 Aug 2007-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this article, the authors provided relevant data at temperatures between 1573 and 1673 K and partial pressures of oxygen between 10−9 and 10−4 K at intermediate oxygen partial pressures and temperatures relevant to sulfide smelting and nonferrous metallurgy.
Abstract: At present, there is a scarcity of data on the activities of iron oxides in the FeO-Fe2O3-CaO-SiO2 slag system at intermediate oxygen partial pressures and temperatures relevant to sulfide smelting and nonferrous metallurgy. The present study provides relevant data at temperatures between 1573 and 1673 K and partial pressures of oxygen between 10−9 and 10−4 atm. The experiments were carried out by equilibrating the slag in a CO-CO2 gas mixture in a platinum crucible, after which the phases of all the experimental samples, including the platinum foil, were analyzed by electron probe microanalysis (EPMA). Where only liquid phase or liquid phase and tridymite (SiO2) were observed, wet chemical analysis was used to determine the ratio of (mass pct Fe2+)/(mass pct Fe3+). Activity and activity coefficients for FeO (liquid) and FeO1.33 (solid) were calculated. Tendencies of the effect of the (CaO/SiO2) ratio, temperature, and oxygen partial pressure on these thermochemical quantities are discussed in this article.
18 citations
TL;DR: Investigation of natural volcanic opal samples with the destabilization phenomena from Mexican deposits found systematic changes in frequencies of both the Raman and the infrared bands, caused by increasing regularities of bond-lengths and bond-angles Si-O-Si groups under the effect of stability.
17 citations
25 Sep 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the authors used X-ray diffraction, GISRD, and secondary ions mass spectroscopy (SIMS) to characterize the oxidation of Ti 3 SiC 2 in air from 25 to 1450°C.
Abstract: The oxidation of Ti 3 SiC 2 in air from 25 to 1450 °C is characterised by differential thermal and gravimetric analysis (DTA/TGA), X-ray diffraction (XRD), grazing-incidence synchrotron radiation diffraction (GISRD), neutron diffraction (ND), transmission electron microscopy (TEM), secondary ions mass spectroscopy (SIMS) and Vickers indentation. The diffraction results show that rutile formed at a temperature of ∼750 °C. A glassy phase – formed at >1000 °C – devitrified upon cooling to room temperature to form tridymite but crystallised to cristobalite at temperatures ≥ 1300 °C. Composition depth-profiling of the surface layer oxides by XRD, GISRD and SIMS revealed a graded distribution of phases (TiO 2 , SiO 2 and Ti 3 SiC 2 ) both at the nanoscale (≤1100 °C) and microscale level (≥1200 °C), which is particularly distinct at the interfaces. The oxide layers also exhibit a graded variation in microhardness.
17 citations
TL;DR: In this article, the electrochemical reaction mechanisms of tridymite FePO4 with Li ions were studied in the voltage range of 2.4-0 V. The authors found that the lithiation/delithiation of FePO 4 was similar to that of binary 3d-metal oxides, which involve the formation and decomposition of Li2O along with the reduction and oxidation of nanoscale 3d metal nanoparticles.
TL;DR: In this article, the effects of varying the crystallinity of the alumina source (boehmite) and the use of different silica sources such as freshly prepared silica either from sodium silicate or paddy husk extract, silica gel from commercial water-glass, and tetraethyl orthosilicate have been studied.
Abstract: Silicoaluminophosphate (SAPO-5) molecular sieves have been synthesised from reaction mixtures having a molar composition of: 0.7–1.0 Al2O3:0.7–1.0 P2O5:0.01–2.0 SiO2:xR:40 H2O (where R = (C2H3)3N or (C2H3)4NOH and x = 1.5–2.5 for (C2H5)5N and 0.5 for (C2H5)4NOH, at 473 K using various sources of alumina and silica. The effects of (i) varying the crystallinity of the alumina source (boehmite) and (ii) the use of different silica sources such as freshly prepared silica either from sodium silicate or paddy husk extract, silica gel from commercial water-glass, and tetraethyl orthosilicate have been studied. The crystallinity of boehmite has been found to have a strong effect on its reactivity towards the formation of SAPO-5. The activity of boehmite for SAPO-5 formation increased with a decrease in its crystallinity (or with increase in its moisture content). Any silica source devoid of sodium ions could be employed for the synthesis of SAPO-5. The process of crystallisation started as early as within 1.5 h of reaction and incorporation of silicon into the AlPO4 framework has been noted at this stage. Formation of some tridymite phase as impurity has been observed under conditions such as (i) SiO2 concentration > 1.7 mole and (ii) x > 2.0 when R = (C2H5)3N.
TL;DR: In this paper, the elastic coefficients of the mechanoluminescent material SrAl2O4 stuffed tridymite determined by the first-principles plane-wave pseudopotential total energy method were reported.
Abstract: We report for the first time the complete set of elastic coefficients of the mechanoluminescent material SrAl2O4 stuffed tridymite determined by the first-principles plane-wave pseudopotential total energy method We calculate other mechanical properties (ie, bulk modulus, Young’s modulus, and Poisson’s ratio) of this material from the elastic coefficients determined, and compare them with the corresponding experimental and calculated results of typical oxides, indicating that SrAl2O4 stuffed tridymite is a relatively soft ceramic oxide In addition, we investigate the crystal structure of SrAl2O4 stuffed tridymite at elevated hydrostatic pressures up to 10GPa and confirm anisotropic deformation in the SrAl2O4 lattice
TL;DR: In this paper, the dissolution rates of size-graded powders of quartz, tridymite, cristobalite and vitreous silica, having particle diameters in the range 0.2-5.0 μ, have been measured by a conductimetric micromethod.
Abstract: As part of fundamental studies in silicosis research, the rates of dissolution in 0.1 M-hydrofluoric acid of size-graded powders of quartz, tridymite, cristobalite and vitreous silica, having particle diameters in the range 0.2–5.0 μ, have been measured by a conductimetric micromethod. The rates of dissolution per unit area of the vitreous silica powders were an order of magnitude larger than those of the tridymite or cristobalite powders, and two orders of magnitude larger than those of the quartz powders. Quartz, unlike the other polymorphic forms of silica, showed an increase in rate (per unit area) with decreasing particle size as well as an increase in the relative amount of ‘easily soluble layer’.
These results are adduced as evidence against the ‘solubility theory of silicosis’ and in favour of a biological activity dependent on the surface structure.
TL;DR: In this article, the structural Fe(III) reduction of low-ferruginous phyllosilicates has been studied and shown to be irreversible for the most part.
Abstract: Microorganisms are capable of reducing the structural Fe(III) of some phyllosilicates, i.e., montmorillonite, nontronite, and illite. Two reduction results are possible. For the low-ferruginous minerals, the solid-phase partial reduction of the structural Fe(III) does not cause any decay of particles. In this case, reduction leads to a partial dehydroxylation of the lattice and magnetic disordering of the mineral, as well as to changes in the layer packaging, the crystallization degree, the texture, the color, the surface charge, and the cation-exchange capacity. Except for dehydroxylation and the changing Fe oxidation status, the transformations of minerals’ properties are irreversible for the most part. In highly ferruginous phyllosilicates, the structural Fe(III) reduction leads to the particle dissolution. As a result of microbial reductive dissolution of crystallized nontronite, new amorphous minerals are formed, i.e., aluminosilicates with an elevated Al/Si ratio, Si globules, and Fe hydroxides. Another biogenically crystallized Si mineral (tridymite βSiO2) is formed in light-textured soils and sediments after smectite decomposition. The bacterial reduction of highly ferruginous illite leads to transformation of the shape of some particles: initially acicular illite crystals were transformed into plates. The amorphous structure is formed in illite particles contacting with bacterial cells. The solid-phase reduction of structural Fe(III) does not lead to particle dissolution in the case of low-ferruginous illite.
TL;DR: In this paper, an empirical relationship has been established between the volume/surface diameter obtained from microscope measurements, and the specific surface by gas adsorption, and has been shown to apply to all samples except unetched quartz.
Abstract: The methods of gas adsorption, air permeability and microscopy have been used to determine values of the specific surface of samples of different modifications of silica and of glass spheres that lie in the size range 1–12 μ. Quartz, cristobalite, tridymite and fused silica were studied both before and after etching with hydrofluoric acid.
An empirical relationship has been established between the volume/surface diameter (Dvs) obtained from microscope measurements, and the specific surface by gas adsorption, and has been shown to apply to all samples except unetched quartz, for a given size of which the specific surface by gas adsorption is about twice that of the other samples.
Below 6 μ. the air-permeability method gives lower results than does microscopy or gas adsorption.
TL;DR: In this paper, phase relationships in the system MgO-Al 2 O 3 -SiO 2 -Cr 2 O3 (MASCr) were investigated experimentally from 1250 to 1560 °C using a 1-atm vertical quench furnace on 12 oxide mixtures covering a wide range of chemical compositions.
Abstract: Phase relationships in the system MgO-Al 2 O 3 -SiO 2 -Cr 2 O 3 (MASCr) were investigated experimentally from 1250 to 1560 °C using a 1-atm vertical quench furnace on 12 oxide mixtures covering a wide range of chemical compositions. Spinel, corundum, mullite, and sapphirine were found to display a wide range of solid solutions toward CR-rich end-members, whereas enstatite, cordierite, forsterite, and glasses were always Cr-poor. Glasses formed in relatively MgO-rich bulk compositions from 1250 °C, in assemblage with cordierite, enstatite, forsterite, and spinel, suggesting a lowering of the eutectic minimum in MASCr compared to available melting relationships in the model system MAS. Although unexpected, the relatively low-temperature melting of Cr-bearing mixtures is supported by topological constraints in the subsystems SACr and MSCr. The extent of Cr solubility in mullite has been found to reach ~14 wt% at 1560 °C, and 33 wt% in sapphirine at 1340 °C, which are the highest Cr values reported so far for these minerals. Liquidus surfaces are constrained by glass compositions that are displaced systematically toward silica compared to corresponding liquids in the ternary system MAS. As a consequence, the primary crystallization fields of Cr-spinel, Cr-sapphirine, and Cr-mullite are found to face the cristobalite/tridymite primary fields. A tentative liquidus diagram for the system MgO-Al 2 O 3 -SiO 2 -Cr 2 O 3 is proposed, which could serve as a basis for the interpretation of the high-temperature evolution of planetary materials as well as for the development of new ceramic materials.
Patent•
31 Jul 2007
TL;DR: In this paper, a piece is made by sintering a nanoparticle/ceramic mixture in air or under inert gas at temperatures of between 900° C and 1250° C.
Abstract: The invention relates to a piece comprised in part or in full of a pigmented ceramic in which the pigment is comprised of nanoparticles based on a metal from column IB of the periodic table of the elements or of an alkaline metal, or an alloy of both, coated with a layer of silica, the silica being crystalline silica, particularly cristobalite or tridymite. The ceramic is preferably zirconia or alumina. The nanoparticle silica is crystallized, for example, by a thermal treatment in air or under inert gas at temperatures of between 900° C. and 1400° C. The piece is made by sintering a nanoparticle/ceramic mixture in air or under inert gas at temperatures of between 900° C. and 1250° C.
TL;DR: The specific surface of tridymite samples in the size range 0.5-40 μ has been determined by gas-adsorption (with nitrogen and krypton as adsorbates), and by air permeability and microscopy as mentioned in this paper.
Abstract: The specific surface of tridymite samples in the size range 0.5-40 μ has been determined by gas-adsorption (with nitrogen and krypton as adsorbates), and by air permeability and microscopy. The results, together with those from X-ray diffraction studies, suggest that tridymite in contrast to quartz has neither a fine external surface nor an ‘amorphous’ layer. Specific surface values determined by nitrogen adsorption and by krypton adsorption are in better agreement when the krypton atom is assigned a value of 19.5A2 rather than the calculated value.
It is concluded that the dissolution of silica and of its easily soluble surface layer play a very insignificant role in silicosis. Unetched tridymite has greater fibrogenic activity than unetched quartz.
TL;DR: In this paper, the structure of the tridymite form of AlPO 4 was refined in first harmonic approximation from X-ray powder diffraction data in the 3 + 1 dimensional superspace group P2 1 (αβ0).
Abstract: Similar to silica tridymite, the corresponding AlPO 4 analogue shows a sequence of displacive phase transitions at elevated temperatures. The transformation temperatures are lower than for SiO 2 tridymite. The incommensurate structure of the tridymite form of AlPO 4 between ca. 94 and 167 °C was refined in first harmonic approximation from X-ray powder diffraction data in the 3 + 1 dimensional superspace group P2 1 (αβ0). The major components of the displacive modulation are wavy transversal shifts of the centers of the PO 4/2 and AlO 4/2 tetrahedra associated with additional tilting of the rigid tetrahedra. The wave vector varies continuously from q 1 = 0.068 a* + 0.006 b* at 165 °C to q 2 =0.051 a*+ 0.003 b* at 96 °C. The amplitudes of the wave-like displacements increase with decreasing temperature to 0.2 A at 96 °C for the Al and P atoms and up to ca. 0.6 A for some of the oxygen atoms. The modulation is assumed to arise from the condensation of a rigid unit lattice vibration mode like in isostructural intermediate silica tridymite.
TL;DR: The solubility of all fractions between 0.5 and 70μ of a Belgian glass sand (Snowit) was found to be proportional to the surface area available for solution as mentioned in this paper.
Abstract: Measurements have been made of the silica solubility of quartz samples in the size range 0.5–500μ, prepared from different sources. The solubility of all fractions between 0.5 and 70μ of quartz from a Belgian glass sand (‘Snowit’) was found to be proportional to the surface area available for solution; with quartz from Madagascar the same proportionality was found for samples with sizes between 4 and 50μ, but particles below 2μ in diameter had a much higher solubility per unit area. No colloidal polymerised silicic acid was found in the extracts of the quartz samples of sizes from 0.5 to 500μ, and its alleged occurrence could be explained by quartz and kaolin of small particle size.
The solubility characteristics of quartz, tridymite, cristobalite and fused silica were found to be very similar.
The heat of wetting by water of unetched and etched samples of quartz and tridymite was found to be of the order of 260 ergs./cm.2, which was attributed to the presence of silanol groups on the surface.
TL;DR: In this article, the authors used X-ray diffraction, IR spectroscopy and differential temperature analysis to study the silica powders added with chlorophyll species, and found partial crystallization of the glass matrix in form of tridymite and cristobalite phases and quenching centers.
Abstract: Composites containing chlorophyll aggregates dispersed in amorphous silica are of interest because of their optical attractive properties. The silica powders added with chlorophyll species, prepared by the sol-gel method, were studied using X-ray diffraction, IR spectroscopy and differential temperature analysis. Silica xerogel samples were prepared using an ethanol/H2O/TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of the annealing temperature. We found in our samples partial crystallization of the glass matrix in form of tridymite and cristobalite phases and quenching centers or nonfluorescing aggregates due to denaturation of photosystem promoted by chlorophyll decomposition after 400∘C.
TL;DR: In this paper, samples of the composition of 10Fe2O3·10CaO·80SiO2 were prepared by the sol-gel method and heat-treated in different atmospheres.
Abstract: Samples of the composition of 10Fe2O3·10CaO·80SiO2 were prepared by the sol-gel method and heat-treated in different atmospheres. They were investigated by X-ray diffraction, scanning electron microscopy and Mossbauer spectroscopy. In the heat-treated samples in air iron is present up to 1000 °C in form of hematite and as Fe3+ in the tetrahedral sites. A wide range of hematite particle sizes was observed, the average size increased with heating temperature. At 1000 °C wollastonite was observed, at 1200 °C tridymite was formed and all the iron was incorporated in hematite. A heat-treatment at 500 °C under reducing conditions led to poorly crystallized maghemite and at 700 °C to metallic iron and fayalite formation.