Showing papers on "Tridymite published in 2011"

Hi‐NicalonTM‐SSiC Fiber Oxidation and Scale Crystallization Kinetics

Abstract: The oxidation and scale crystallization kinetics of Hi-NicalonTM-S SiC fibers were measured after oxidation in dry air between 700° and 1400°C. Scale thickness, composition, and crystallization were characterized by TEM with EDS, supplemented by SEM and optical microscopy. TEM was used to distinguish oxidation kinetics of amorphous and crystalline scales. Oxidation initially produces an amorphous silica scale that incorporates some carbon. Growth kinetics of the amorphous scale was analyzed using the flat-plate Deal-Grove model. The activation energy for parabolic oxidation was 248 kJ/mol. The scales crystallized to tridymite and cristobalite, starting at 1000°C in under 100 h and 1300°C in under 1 h. Crystallization kinetics had activation energy of 514 kJ/mol with a time growth exponent of 1.5. Crystalline silica nucleated at the scale surface, with more rapid growth parallel to the surface. Crystalline scales cracked from thermal residual stress and phase transformations during cool-down, and during oxidation from tensile hoop growth stress. High growth shear stress was inferred to cause intense dislocation plasticity near the crystalline SiO2–SiC interphase. Crystalline scales were thinner than amorphous scales, except where growth cracks allowed much more rapid oxidation.

Oxidative coupling of methane: Influence of the phase composition of silica-based catalysts

Abstract: A simple solid-phase synthesis technique was applied to prepare a series of silica-based composite materials. Each material contains tungsten, manganese and an alkali oxide. To determine the influence of the alkali nature on the catalytic properties of SiO 2 -based composites, a W–Mn/SiO 2 material without any alkali was prepared. In addition, material containing tungsten, manganese and lithium without SiO 2 was synthesized. The composites were studied by XRD, TPR, BET, XPS, SDB/SEM and EDS methods. A thorough analysis of the phase parameters, which were crystallized in alkali-doped W–Mn/SiO 2 catalysts and determined by XRD, was performed for the first time. Most synthesized materials demonstrated higher catalytic activity in the oxidative coupling of methane (OCM) in comparison with the analogous materials prepared according the convenient wet impregnation method. The OCM reaction was carried out without any dilution of the methane–oxygen mixture. The results of catalytic tests were interpreted in relation to the catalyst silica-matrix phase composition. It was found that two polymorphic phases of SiO 2 coexist in each composite material. These phases are cristobalite and quartz in Li- and Na-containing material or cristobalite and tridymite in K-, Rb- and Cs-containing materials. The hypothetical T – x phase diagrams of Me 2 O–SiO 2 (where Me is an alkali metal) systems were constructed to explain the coexistence of two phases of SiO 2 in the studied catalysts.

Krotite, CaAl2O4, a new refractory mineral from the NWA 1934 meteorite

Abstract: Krotite, CaAl_2O_4, occurs as the dominant phase in an unusual Ca-,Al-rich refractory inclusion from the NWA 1934 CV3 carbonaceous chondrite. Krotite occupies the central and mantle portions of the inclusion along with minor perovskite, gehlenite, hercynite, and Cl-bearing mayenite, and trace hexamolybdenum. A layered rim surrounds the krotite-bearing regions, consisting from inside to outside of grossite, mixed hibonite, and spinel, then gehlenite with an outermost layer composed of Al-rich diopside. Krotite was identified by XRD, SEM-EBSD, micro-Raman, and electron microprobe. The mean chemical composition determined by electron microprobe analysis of krotite is (wt%) Al_2O_3 63.50, CaO 35.73, sum 99.23, with an empirical formula calculated on the basis of 4 O atoms of Ca_(1.02)Al_(1.99)O_4. Single-crystal XRD reveals that krotite is monoclinic, P2_1/n; a = 8.6996(3), b = 8.0994(3), c = 15.217(1) A, β = 90.188(6), and Z = 12. It has a stuffed tridymite structure, which was refined from single-crystal data to R_1 = 0.0161 for 1014 F_o > 4σF reflections. Krotite is colorless and transparent with a vitreous luster and white streak. Mohs hardness is ~6½. The mineral is brittle, with a conchoidal fracture. The calculated density is 2.94 g/cm3. Krotite is biaxial (–), α = 1.608(2), β = 1.629(2), γ = 1.635(2) (white light), 2V_(meas) = 54.4(5)°, and 2V_(calc) = 55.6°. No dispersion was observed. The optical orientation is X = b; Y ≈ a; Z ≈ c. Pleochroism is colorless to very pale gray, X > Y = Z. Krotite is a low-pressure CaAl_2O_4 mineral, likely formed by condensation or crystallization from a melt in the solar nebula. This is the first reported occurrence of krotite in nature and it is one of the earliest minerals formed in the solar system.

Do H-bond features of silica surfaces affect the H2O and NH3 adsorption? Insights from periodic B3LYP calculations.

Abstract: The adsorption of a single H2O and NH3 molecule on different fully hydroxylated α-quartz, cristobalite, and tridymite surfaces has been studied at the B3LYP level of theory, within a periodic approach using basis sets of polarized triple-ζ quality and accounting for basis set superposition error (BSSE). Fully hydroxylated crystalline silica exhibits SiOH as terminal groups whose distribution and H-bond features depend on both the considered silica polymorph and the crystallographic plane, which gives rise to isolated, H-bond interacting SiOH pairs or infinitely connected H-bond chains. A key point of the present study is to understand how the H-bond features of a dry crystalline silica surface influence its adsorption properties. Results reveal that the silica-adsorbate (H2O and NH3) interaction energy anticorrelates with the density of SiOH groups at the surface. This counterintuitive observation arises from the fact that pre-existing H-bonds of the dry surface need to be broken to establish new H-bonds ...

Monoclinic tridymite in clast-rich impact melt rock from the Chesapeake Bay impact structure

Abstract: X-ray diffraction and Raman spectroscopy confirm a rare terrestrial occurrence of monoclinic tridymite in clast-rich impact melt rock from the Eyreville B drill core in the Chesapeake Bay impact structure. The monoclinic tridymite occurs with quartz paramorphs after tridymite and K-feldspar in a microcrystalline groundmass of devitrified glass and Fe-rich smectite. Electron-microprobe analyses revealed that the tridymite and quartz paramorphs after tridymite contain different amounts of chemical impurities. Inspection by SEM showed that the tridymite crystal surfaces are smooth, whereas the quartz paramorphs contain irregular tabular voids. These voids may represent microporosity formed by volume decrease in the presence of fluid during transformation from tridymite to quartz, or skeletal growth in the original tridymite. Cristobalite locally rims spherulites within the same drill core interval. The occurrences of tridymite and cristobalite appear to be restricted to the thickest clast-rich impact melt body in the core at 1402.02–1407.49 m depth. Their formation and preservation in an alkali-rich, high-silica melt rock suggest initially high temperatures followed by rapid cooling.

Silica brick and preparation method thereof

Abstract: The invention specifically relates to a silica brick and a preparation method thereof. The technical scheme comprises the following steps: mixing 55-75wt% of silica aggregate, 17-32wt% of fine silica powder, 2-8wt% of micro silicon powder and 2-6wt% of mineralizer; adding lignosulfite which accounts for 0.5-4wt% of the mixture; calendering; carrying out pressure molding; drying; and heating to 1000-1400 DEG C at a heating rate of 25-35 DEG C/h in a high-temperature furnace, and preserving the temperature for 8-10 hours to obtain the silica brick. The preparation process is simple, environment friendly and energy-saving, the production cost is low, the firing temperature is low, and a large amount of tridymite can be generated in the firing process of the prepared silica brick. Besides, the silica brick has the characteristics of high softening point under load, low residual quartz content, less volume change, high density and high strength.

Phase formation in Li2MoO4-K2MoO4-MMoO4 (M = Ca, Pb, Ba) systems and the crystal structure of α-KLiMoO4

Abstract: Solid-phase interactions in Li2MoO4-K2MoO4-MMoO4 (M = Ca, Pb, Ba) systems were studied, and the subsolidus regions of these systems were triangulated. The lead and barium systems were studied in a more detailed way to discover that, along KLiMoO4-K2M(MoO4)2 (M = Pb, Ba), KLiMoO4-PbMoO4, and Li2MoO4-K2Ba(MoO4)2 quasi-binary sections, there are homogeneity regions reaching 6–11 mol % based on K2M(MoO4)2 and lead molybdate. Triple molybdates are formed in none of the systems, which is verified by experiments on spontaneous crystallization from solution in melt. Crystallization experiments yielded crystals of potassium dimolybdate and simple and double molybdates from the boundary systems. The crystal structure was solved for a hexagonal KLiMoO4 phase: (Na,K){ZnPO4}, a = 18.8838(7) A, c = 8.9911(6)A, Z = 24, space group P63, R = 0.065. The structure comprises a three-dimensional tridymite framework built by an alternation of corner-sharing LiO4- and MoO4 tetrahedra wherein voids are occupied by potassium cations.

Experimental study of phase equilibria in the “SnO2” – CaO – SiO2 system in air

Abstract: Experimental studies have been conducted to determine the primary phases and liquidus temperatures in the “SnO2” – CaO – SiO2 system in air between 1693 and 1898 K, using high temperature equilibration and quenching followed by electron probe X-ray microanalysis The following primary phase fields were identified: CaSiSnO5 (malayaite), Ca3Si2SnO9 (tricalcium tin silicate), CaSiO3 (pseudo-wollastonite), Ca2SiO4 (dicalcium silicate), SiO2 (tridymite and cristobalite), SnO2 (cassiterite) and CaSnO3 (calcium stannate)

Vibrational States in Opals Revisited

Abstract: Quantitative assessment of the vibrational features of nanocrystalline hydrated silicas is still not yet ascertained, although this is a relevant issue in silica physics. In this work, we examine the vibrational states in two generic opaline silicas. PM3 semiempirical calculations on finite SiO(2) hydrated clusters prove that the low-frequency Raman lines experimentally observed at similar to 307, 340, and 370 cm(-1) and the high-frequency ones at 790 cm(-1) and 1080 cm(-1) stem from O-Si-O bending vibrations in six-membered rings of corner-shared SiO(4) tetrahedra and O-Si-O symmetric/asymmetric stretch vibrations, respectively. Lattice dynamical calculations on crystalline silica polymorphs show that the predominance of tridymite over cristobalite in opaline silicas cannot be established due to the intrinsic nanoscale crystalline order of the materials. We conclude that the vibrational variations observed between opaline silicas are due to gradual formations of six-membered rings in a disordered SiO(2) matrix.

Computational studies of silica

Abstract: There are three areas of research in this thesis. The �first is concerned with the silica polymorph, tridymite, with simulations carried out using three computational methods: free energy minimisation, molecular dynamics and Density Functional Theory. A number of tridymite structures with di�fferent atomic configurations have been found in nature. The simulations explore various properties of these di�fferent forms of tridymite and investigate whether it is possible to distinguish between them using the three computational techniques. It was found that the interatomic potential and simulation technique used, rather than the simulation temperature, were the main factors a�ffecting the resulting structure. There are a number of possible explanations for this result: The techniques may not be sensitive enough to deal with an energy landscape as at as in the case of tridymite. Another reason is that the potentials have been parameterised to distinguish between structures which have reconstructive transitions (where bonds are broken and formed) and may not be able to deal with displacive transitions (where only angles between atoms change) as with tridymite. The �final possible explanation is that a number of the known structures may be meta-stable and/or poorly characterised. For the second research area molecular dynamics simulations using a rigid ion two body potential were carried out in order to investigate the properties of silica melts and glasses. A number of different silica crystals were melted to see whether the melts are all similar or whether their properties can be di�fferentiated according to the original crystal structure. At sufficiently high temperatures the starting structure did not a�ffect the properties of the melt. Several properties of silica melts and glasses were investigated: mean square displacement, autocorrelation functions, pair distribution functions, the extent to which silicon and oxygen atoms move together, Arrhenius plots, coordination number, bond lengths and angles. Investigations were also carried out as to whether it is possible to use a shell model to simulate a silica melt. Various properties were calculated and it was found that agreement with experiment was not as accurate as when using the rigid ion model. The third research area is an exploration of the properties of amorphous silica at elevated pressures and a range of temperatures, using molecular dynamics with a rigid ion two body potential. Calculations show that, at low temperatures, the distortion of the tetrahedra is not recovered upon decompression whereas experimental results �find complete recovery of the tetrahedra. There is little available experimental data on the behaviour of silica at both high pressures and temperatures. Calculations show that at high temperatures all properties of the initial structure before compression are recovered.

Deformations of Crystal Frameworks

Abstract: We apply our deformation theory of periodic bar-and-joint frameworks to tetrahedral crystal structures. The deformation space is investigated in detail for frameworks modelled on quartz, cristobalite and tridymite.

다양한 산화물들이 리튬규산염 유리의 결정화에 미치는 영향

Abstract: Glass-ceramics based on lithium disilicate(Li₂Si₂O?) are prepared by heat-treatment of glasses in a system of SiO₂-Li₂O-K₂O-Al₂O₃ with different compositions. The crystallization heat-treatment was conducted at the temperature range of 700~900℃ and samples were analyzed by XRD and SEM. Mechanical properties were determined by a Vicker’s hardness and 4 point bending strength. When SiO₂/Li₂O ratio increased, cristobalite and tridymite crystals showed more predominate than lithium disilicate crystals. Increase in Al₂O₃ contents in the glass suppressed crystallzation of lithium disilicate crystals. Increase in ZnO, B₂O₃ contents in the glass decreased crystallization temperature of lithium disilicate crystals, and increased mechanical properties because of the reduction of the lithium disilicate crystal size.

Silica crown refractory corrosion in glass melting furnaces

Abstract: The critical parameters of silica refractories, such as compressive strength, bulk, density, quantity of silica, microstructure and porosity were evaluated of unused and used bricks to line the crowns of glass furnaces, when the rate of corrosion of crowns were about 2 times greater. The change of these parameters, the chemical composition and formation of the microcracks in the used silica refractories material were studied. It was established that the short time at service of container glass furnace crown can be related to low quality of silica brick: high quantity of CaO and impurities, low quantity of silica, low quantity of silica, transferred to tridymite and cristobalite and formation of 5-10 μm and more than 100 μm cracks in the crown material. The main reason of corrosion high quality silica bricks used to line the crown of electrovacuum glass furnace is the multiple cyclic change of crown temperature at 1405 - 1430°C range in the initial zone of crown and at 1575 - 1605°C range in the zone of highest temperatures.

Implications of the Presence of Tridymite in the Fukang Pallasite

Abstract: Introduction: We report the results of a study of the main-group pallasite Fukang [1]. Fukang was recovered in 2001 in the Gobi Desert, China, with a main mass weight of 1003 kg. Like many main-group pallasites, Fukang consists of rounded to semi-angular olivine grains imbedded in an Fe-Ni matrix; accessory schreibersite is also present. Minor phases include troilite, euhedral chromites, rounded whitlockite, and low-Ca pyroxene. Observations: Unusual incompatible-element rich silicate inclusions, several tens to hundreds of microns in length, are observed in some Fukang olivine grains (Fig. 1). All inclusions contain silica, K-rich orthoclase-normative glass. Accessory phases differ in each inclusion, but include Fe-metal, phosphates, sulfides, and chromite. All inclusions are rimmed by an unusual Cr-rich silicate with a stoichiometry closely approximating (Ca,Mg,Fe)7(Cr,Al)3(SiO4)6. Ca zoning (up to 10.5 wt. %) is observed in portions of the Cr-silicate rims, sometimes enriched near the inclusion rim-olivine phase boundary. EPMA of this material show that it is homogenous with Si/O of ~0.25. However, its composition is not consistent with any known mineral. Structural analysis is needed to definitively identify this phase. Raman spectra of the silica were collected from an un-oriented crystal at 100% power on a Thermo Almega microRaman system, using a solid-state laser with a frequency of 780 nm, and a thermoelectric cooled CCD detector (Fig. 2). The laser is partially polarized with 4 cm resolution and a spot size of 1 μm. Raman spectroscopy identified the silica phase as monoclinic tridymite, which was confirmed by a comparison to the results of [2]. A nearly equidimensional crystal (0.05 x 0.05 x 0.04 mm) was dug out from one of the inclusions and mounted on a Bruker X8 APEX2 CCD X-ray diffractometer equipped with graphite-monochromatized MoKα radiation. All collected reflections were indexed using the Bruker program SAINT. XRD analysis determined cell dimensions to be a = 25.938(5), b = 5.0150(9), c = 18.547(3) A, β = 117.680(9)°, and V = 2136(1) A. These parameters agree well with those for monoclinic tridymite reported by [2]. Implications: Tridymite appears to have crystallized directly out of a liquid phase leaving behind the orthoclase-normative glass. The size of the tridymite crystals varies with each inclusion (up to 200 μm) and the crystal shape becomes rounded in larger inclusions. Tridymite is a relatively high-temperature, lowpressure phase. It forms from a melt at pressures less than 0.15 GPa and at temperatures of approximately 870 oC to 1470 oC [3]. These constraints help determine the conditions under which the Fukang pallasite formed. The presence of such a low-pressure phase in Fukang allows us to make an upper estimate of the size of the Main-group pallasite parent-body. It also implies that the Fukang parent-body did not undergo significant high pressure shock processing. We estimate the upper limit on the size of the Main-group parent body using the pressure constraints provided by the presence of tridymite. To model the pressure experienced within a spherical planetismal, as a function radius, we assume hydrostatic equilibrium and integrate the following differential equation:

Pigmented ceramic element and manufacturing method

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 DEG C. and 1400 DEG C. The piece is made by sintering a nanoparticle/ceramic mixture in air or under inert gas at temperatures of between 900 DEG C. and 1250 DEG C.

Synthesis, Crystal and Band Structures, and Optical Properties of a New Framework Mercury Pnictides: [Hg4As2](InBr3.5As0.5) with Tridymite Topology.

Abstract: Single crystals of the title compound are prepared by solid state reaction of Hg2Br2, elemental In, and As (400 °C, 120 h).