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This study, investigates how adaptable silica made from rice husks could be used in energy storage applications. It is show that rice husk silica (SiO2) can be used in a variety of applications with the right processing. Cold plasma is used to prepare the silica samples in a variety of structural phases which improves their electrochemical characteristics. Using (X-ray diffraction analysis) XRD, Raman spectroscopy, (Fourier-transform infrared spectroscopy) FTIR, (Field emission scanning Electron microscopy) FESEM, and (Energy Dispersive spectroscopy) EDS techniques are used to investigate and evaluate the differences in their structural, morphological, and chemical composition. A three-electrode system is used to assess the material's electrochemical performance. Surprisingly, it is found that after plasma treatment, amorphous silica materials have a large increase in specific capacitance (297 F/g) as a result of an increase in functional groups and better adhesion characteristics. These silica samples also have an outstanding electron reversibility and coulombic efficiency. The effect of the various properties of the silica on super capacitance and catalyst performances in metal air battery is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Silica bricks with cemented silica and crystalline silica as raw materials show different properties. In this paper, the phase and structural characteristics of different kinds of silica raw materials and silica bricks after heat treatment at different temperatures are explored by the means of XRD, SEM and CT, and the formation mechanism of tridymite in the silica bricks is clarified. Results show that the liquid phase formed by impurities in both crystalline silica and cemented silica raw material cannot promote the transformation of quartz to tridymite at high temperatures. The phase composition of both cemented silica and crystalline silica after heat treatment at 1460 and 1550 °C is cristobalite. [ABSTRACT FROM AUTHOR]

Opaline mudstone (OM) composed of opal-CT (SiO2·nH2O) has high potential use as a cesium (Cs) adsorbent, due to its high specific surface area (SSA). The objective of this study was to investigate the Cs adsorption capacity of chemically activated OM and the adsorption mechanism based on its physico-chemical properties. We used acid- and base-activation methods for the surface modification of OM. Both acid- and base- activations highly increased the specific surface area (SSA) of OM, however, the base-activation decreased the zeta potential value more (− 16.67 mV), compared to the effects of acid-activation (− 6.60 mV) or non-activation method (− 6.66 mV). Base-activated OM showed higher Cs adsorption capacity (32.14 mg/g) than the others (acid: 12.22 mg/g, non: 15.47 mg/g). These results indicate that base-activation generates pH-dependent negative charge, which facilitates Cs adsorption via electrostatic attraction. In terms of the dynamic atomic behavior, Cs cation adsorbed on the OM mainly exist in the form of inner-sphere complexes (IS) containing minor amounts of water molecules. Consequently, the OM can be used as an effective Cs adsorbent via base-activation as an economical and simple modification method. [ABSTRACT FROM AUTHOR]

This article provides data on cristobalite and tridymite from the Arsenatnaya active fumarole of the Tolbachik volcano, Kamchatka, Russia. The minerals occur in associations with fumarolic sylvite, sanidine, cassiterite, hematite, pseudobrookite, johillerite, tilasite, and badalovite. Fumarolic cristobalite is tetragonal (α-modification); the unit-cell parameters for one sample are: а = 4.975 (7) Å, с = 6.944 (13) Å, and V = 171.89 Å3. There are two types of tridymite, that is, monoclinic (MC) and orthoorthorhombic (PO-10), in the Arsenatnaya fumarole. The unit-cell parameters of these tridymite modifications are: a = 18.553 (5) Å, b = 5.006 (1) Å, с = 25.952 (10) Å, β = 117.68 (2)°, V = 2134.3 (11) Å3 (MC); a = 9.941 (2) Å, b = 17.165 (4) Å, с = 82.362 (18) Å, and V = 14053.4 (29) Å3 (PO-10). Mineral assemblages of cristobalite and tridymite indicate high-temperature formation conditions of these minerals not lower 450–500°С with considerable participation of HCl and HF in process of basalt alteration by fumarolic gas. The surrounding basalt was a source of silicon. This element was probably transported in the form of SiX4, where X = F, Cl. [ABSTRACT FROM AUTHOR]

Textures and whole-rock chemistry, as well as mineral composition, were analyzed in megaspherulites (high-temperature crystallization domains [HTCDs]) that formed in different geographical and geotectonic contexts and during different geological periods (Silver Cliff, CO, USA—Paleogene; El Quevar, Argentina—Miocene; Meissen Volcanic Complex, Germany—Late Carboniferous). All of these megaspherulites have formed exclusively in rhyolitic lava, and their mineral composition is dominated by K-feldspar (sanidine) and SiO2 phases (quartz, cristobalite, tridymite). All megaspherulites represent composite HTCDs, comprising three zones: inner domain (ID), outer domain (OD), and a marginal domain (MD). Early evolution of megaspherulites is characterized by either central cavities and sector- to full-sphere spherulites or dendritic quartz-sanidine domains. The latter consist of bundles of fibrils each radiating from a single point reflecting relatively high growth rates. A common feature of OD and MD of all three megaspherulite occurrences is autocyclic banding. It mainly comprises fibrous (≤ 100 μm length), radially oriented sanidine and quartz, which formed at a temperature close to glass transition temperature (Tg). The termination of megaspherulite growth is marked by centimeter-sized sector-sphere spherulites on the surface. Megaspherulite formation requires limited nucleation, which is probably related to the low phenocryst content of the hosting lava. Latent heat from overlying crystallizing lithoidal rhyolite maintained low undercooling conditions keeping nucleation density low and facilitating high diffusion and growth rates. Late megaspherulite growth and its termination under low diffusion conditions is controlled by cooling close to Tg. Calculations based on literature data suggest that the megaspherulite growth presumably lasted less than 60 years, perhaps 30 to 40 years. [ABSTRACT FROM AUTHOR]

New experimental data on the distributions of silver (Ag), bismuth (Bi), and antimony (Sb) between liquid iron-silicate slag and liquid copper metal in equilibrium with tridymite in the Cu-Fe-O-Si system at T = 1250 °C and 1300 °C (1523 K and 1573 K) have been obtained. The experimental methodology involved high-temperature equilibration, rapid quenching of the equilibrated phases, followed by direct measurement of the equilibrium phases with electron probe X-ray microanalysis (EPMA) for measurement of major element concentrations and laser ablation inductively coupled plasma mass spectrometry (LAICPMS) for the measurement of minor element concentrations. The 4-point test approach was employed to confirm the achievement of chemical equilibrium in the current study. Open-system equilibration on quartz substrates in controlled gas atmospheres (CO/CO2/Ar) and closed-system equilibration in sealed quartz ampoules were used. The new experimental data resolve significant discrepancies found in previous studies and can be used as input for the development of thermodynamic databases for copper-making processes. [ABSTRACT FROM AUTHOR]

The use of low-temperature quartz modifications in pulverized form creates additional risks in the manufacture of molds, since polymorphic transformations decrease their crack resistance. In practice, individual layers of the shell wall or the mold as a whole often crack and even disintegrate. In many casting shops, the lining is preliminarily roasted. That may somewhat improve the consequences of polymorphic transformations in the quartz, but steady heating of the molds through the filler to reduce the odds of cracking prolongs the process and increases the energy consumption. The best-known approach to minimizing the cracking and disintegration of shell molds on roasting is to replace the filler: instead of quartz dust, it is possible to use disperse quartz sand of polyfractional composition, sillimanite, pulverized alumosilicate, globular corundum, or fused quartz. However, those materials are expensive and their use is inconsistent with the requirements of resource conservation in casting. A possible material might be silica-based ceramic cullet from the shells used in steel and aluminum investment casting. At present, such cullet is not recycled. It is dumped or used as filler in the flasks used for shaping shells. Chemical and phase analysis shows that the ceramic cullet formed after removing the steel and aluminum castings contains not only quartz and high-temperature tridymite and cristobalite phases (the base) but also 5-10% iron and iron scale and 3-5% aluminum and its oxides. The use of such ceramic cullet in shell production eliminates repeated polymorphic transformation of the quartz in roasting and sealing the molds, which would change the volume, density, and crystal lattice of the material. That increases the crack resistance and strength of the shells and minimizes the waste of the castings produced. Residual iron and aluminum and their oxides improve the utility of the mold. Tests of this recycling method in production conditions confirm its effectiveness. [ABSTRACT FROM AUTHOR]

To assist in the optimization of copper smelting and converting processes, accurate new measurements of the phase equilibria of the Cu-Fe-O-S-Si system have been undertaken. The experimental investigation was focused on the characterization of gas/slag/matte/tridymite equilibria in the Cu-Fe-O-S-Si system at 1523 K (1250 °C), P(SO2) = 0.25 atm, and a range of P(O2)s. The experimental methodology, developed in PYROSEARCH, includes high-temperature equilibration of samples on substrate made from the silica primary phase in controlled gas atmospheres (CO/CO2/SO2/Ar) followed by rapid quenching of the equilibrium condensed phases and direct measurement of the phase compositions with electron-probe X-ray microanalysis (EPMA). The data provided in the present study at 1523 K (1250 °C) and the previous study by the authors at 1473 K (1200 °C) has enabled the determination of the effects of temperature on the phase equilibria of the multicomponent multiphase system, including such characteristics as the chemically dissolved copper in slag and Fe/SiO2 ratio at silica saturation as a function of copper concentration in matte. The new data will be used in the optimization of the thermodynamic database for the copper-containing systems. [ABSTRACT FROM AUTHOR]

Phase equilibria of the Pb-Cu-Si-O system have been investigated in the temperature range from 1073 K to 1673 K (800 °C to 1400 °C) for oxide liquid (slag) in equilibrium with solid Cu metal and/or liquid Pb-Cu alloy, and solid oxide phases: (a) quartz or tridymite (SiO2) and (b) cuprite (Cu2O). High-temperature equilibration on silica or copper substrates was performed, followed by quenching, and direct measurement of Pb, Cu, and Si concentrations in the liquid and solid phases using the electron probe X-ray microanalysis has been employed to accurately characterize the system in equilibrium with Cu or Pb-Cu metal. All results are projected onto the PbO-“CuO0.5”-SiO2 plane for presentation purposes. The present study is the first-ever systematic investigation of this system to describe the slag liquidus temperatures in the silica and cuprite primary phase fields. [ABSTRACT FROM AUTHOR]

We have synthesized powders of the CsMIIPO4 (MII = Mg, Mn, Co) phosphates with the β-tridymite structure. Phosphate ceramics were produced by conventional cold pressing with inorganic sintering aids, followed by programmed annealing, and by an innovative method: high-speed spark plasma sintering. The thermal diffusivity and thermal conductivity (0.3-0.4 W/(m K)) of the CsMIIPO4 ceramics indicate that they are heat insulators with a high working temperature. [ABSTRACT FROM AUTHOR]

This article studies the influence of raw material, sintering temperature, particle size, and Al2O3 content on densification of mullite ceramics. Thermal cycle treatments were conducted to evaluate thermal shock resistance of samples. Microstructures of samples were also investigated to reveal effects of various parameters and thermal test. Flexural strength retention rises while density decreases, resulting from increasing extra space for stress release. Tridymite precipitation is found to be harmful in improving thermal shock resistance. [ABSTRACT FROM AUTHOR]

The majority of primary pyrometallurgical copper making processes involve the formation of two immiscible liquid phases, i.e., matte product and the slag phase. There are significant gaps and discrepancies in the phase equilibria data of the slag and the matte systems due to issues and difficulties in performing the experiments and phase analysis. The present study aims to develop an improved experimental methodology for accurate characterisation of gas/slag/matte/tridymite equilibria in the Cu-Fe-O-S-Si system under controlled atmospheres. The experiments involve high-temperature equilibration of synthetic mixtures on silica substrates in CO/CO/SO/Ar atmospheres, rapid quenching of samples into water, and direct composition measurement of the equilibrium phases using Electron Probe X-ray Microanalysis (EPMA). A four-point-test procedure was applied to ensure the achievement of equilibrium, which included the following: (i) investigation of equilibration as a function of time, (ii) assessment of phase homogeneity, (iii) confirmation of equilibrium by approaching from different starting conditions, and (iv) systematic analysis of the reactions specific to the system. An iterative improved experimental methodology was developed using this four-point-test approach to characterize the complex multi-component, multi-phase equilibria with high accuracy and precision. The present study is a part of a broader overall research program on the characterisation of the multi-component (Cu-Fe-O-S-Si-Al-Ca-Mg), multi-phase (gas/slag/matte/metal/solids) systems with minor elements (Pb, Zn, As, Bi, Sn, Sb, Ag, and Au). [ABSTRACT FROM AUTHOR]

Experimental studies were undertaken to determine the gas/slag/matte/tridymite equilibria in the Cu-Fe-O-S-Si system at 1473 K (1200 °C), P(SO) = 0.25 atm, and a range of P(O)'s. The experimental methodology involved high-temperature equilibration using a substrate support technique in controlled gas atmospheres (CO/CO/SO/Ar), rapid quenching of equilibrium phases, followed by direct measurement of the chemical compositions of the phases with Electron Probe X-ray Microanalysis (EPMA). The experimental data for slag and matte were presented as a function of copper concentration in matte (matte grade). The data provided are essential for the evaluation of the effect of oxygen potential under controlled atmosphere on the matte grade, liquidus composition of slag and chemically dissolved copper in slag. The new data provide important accurate and reliable quantitative foundation for improvement of the thermodynamic databases for copper-containing systems. [ABSTRACT FROM AUTHOR]

Synthesis and thermomechanical characterization of polyethylene glycol-silica cristobalite composites have been successfully performed. Commercial Polyethylene glycol (PEG) 4000 and natural-silica-sand-derived cristobalite (SiO) powders were used in this research. The cristobalite ceramic powder was produced by a coprecipitation method using silica sand from Tanah Laut, South Kalimantan as the raw material. XRD data analysis for the cristobalite powder showed that it contained 98.8 wt% cristobalite and 1.2 wt% tridymite - both are polymorphs of silica. The PEG-cristobalite composition was varied in the composites at PEG weight fraction of 100, 80, 60 and 40 %. PEG and cristobalite dry powders were simply mixed and then uniaxially pressed before a heat treatment at 50 °C to form dense composite prior to characterization. Further FTIR and XRD data analyses for the densed samples showed the success of the formation of the composite. The thermomechanical properties was characterised using Dynamic mechanical analysis (DMA) instrument with shear mode. In general, at all measurement temperatures, the storage modulus ( G') of the composites was significantly improved with the addition of cristobalite, e.g. its values for 60 % cristobalite sample was 13 times that of the pure PEG sample. The pure samples showed a rubbery plateau behaviour which was not found in the composite samples. It was also found that by increasing the amount of cristobalite, the glassy transition temperature ( T ) increase from 56 °C for the pure and 68 °C for the 60 % cristobalite samples respectively. [ABSTRACT FROM AUTHOR]

The effect of repeated nonisothermal heating to 1500°C on shrinkage and crystallization of quartz ceramic with added alkali (0.5% NaO) and monolithic fuzed quartz is studied. Marked (up to 10%) tridymite formation is detected in specimens with added alkali and with a predominant cristobalite content. Some properties are provide for specimens after single-stage heating to 1500°C. Depending on additive content specimens have open porosity of 6 - 9% and ultimate strength in compression of 130 - 185 MPa. [ABSTRACT FROM AUTHOR]

SrAlO:Eu/Yb powders were synthesized through the hydrothermal process. The phase and morphology of powders were characterized by XRD and SEM. The corresponding luminescence was recorded by the spectrophotometer. The XRD results indicated that the obtained SrAlO:Eu/Yb samples have the monoclinic phase. The SEM images showed that the obtained SrAlO:Eu/Yb samples are microspheres with hierarchical structure. Under the excitation at 360 nm, the SrAlO:Eu/Yb powders showed emission bands in visible and near infrared regions. The Yb concentration has obvious influence on the emission intensities of SrAlO:Eu/Yb powders. The results showed that Eu ion is an efficient sensitizer for Yb ion in SrAlO host lattice. [ABSTRACT FROM AUTHOR]

Cesium-containing phosphates CsMgPO, CsZr(PO), and CsMgZr(PO) of framework structure were synthesized. The chemical stability of the crystalline phosphates in neutral, acidic, and alkaline media was studied. Quantitative data on the chemical stability in a wide pH range and on the ion-exchange capacity of phosphates containing cesium, magnesium, and zirconium ions were obtained by potentiometric titration. The Cs leach rates from the ceramic samples upon long contact with water were from 3 × 10 to 4 × 10 g cm day. The phosphates CsZr(PO) and CsMgZr(PO) are thermally stable in the temperature range from-268 to 1000°C, and CsMgPO undergoes a low-temperature phase transition at-37°C without break of chemical bonds in the structure. The stability of ceramic and powdered samples of CsMgPO, CsZr(PO), and CsZrMg(PO) was studied in different aspects, and these materials proved to be promising for cesium consolidation. High cesium content of CsMgPO allows its use as a material of the active core of Cs/Ba sources. [ABSTRACT FROM AUTHOR]

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si-O-Si network comprising a four-membered ring structure of SiO tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C. [ABSTRACT FROM AUTHOR]

Based on the poudretteite (K Na B Si O) glass composition, six transparent glass frits were prepared. TiO, LiF and CrO were experimented on the base glass composition as nucleation catalysts. Only cristobalite and tridymite were developed through the sintering process of the glasses between 900 and 1100 °C. LiF enhances mainly tridymite with traces of cristobalite and in the opposite side the parent and both TiO and CrO containing glasses catalyzed mainly cristobalite with traces of tridymite. The microstructure of the glass samples heat-treated at 1000 °C showed backbone-like, small rods, quadrant-, hexagonal crystals referred to the cristobalite and tridymite. The EDS microanalysis shows the presence of both alkalis, i.e. Na and K, in the cristobalite crystal structure while the residual glass contains K. The infrared spectroscopy indicates the band patterns of cristobalite and tridymite. [ABSTRACT FROM AUTHOR]

The exceptional occurrence of fluorine-rich mineral phases in the benmoreitic lava dome of Mt. Calvario (south-western flank of Mt. Etna) has given the opportunity to understand the genetic process allowing their crystallization. Both primary and secondary mineral associations were found, namely: plagioclase, clinopyroxene, olivine, fluorapatite and iron oxides as primary assemblage, whereas fluoro-edenite and fluorophlogopite, ferroan-enstatite, hematite, pseudobrookite and tridymite as secondary mineralization. In addition to some major and trace elements (e.g., Fe, Ti, Na, K, P, Ba, Rb, Sm, Zr), particularly fluorine and chlorine concentrations of the whole rock are significantly higher than other Etnean prehistoric benmoreites, and cannot be accounted for common differentiation processes in the feeding system. The selective enrichment in some elements has been here attributed to volatile flushing occurring in the plumbing system, with fluid/melt ratio of ~0.65:1. The resulting high amount of fluorine, coupled with its high solubility even at low pressure for benmoreitic melts, finally led to nucleation and growth of F-rich mineral phases during syn- and post-eruptive conditions. [ABSTRACT FROM AUTHOR]

Copper concentrates and fluxes can contain variable levels of SiO, CaO, and MgO in addition to main components Cu, Fe, and S. Metal recovery, slag tapping, and furnace wall integrity all are dependent on phase equilibria and other properties of the phases and are functions of slag composition and operational temperature. Optimal control of the slag chemistry in the copper smelting, therefore, is essential for high recovery and productivity; this, in turn, requires detailed knowledge of the slag phase equilibria. The present work provides new phase equilibrium experimental data in the FeO-FeO-SiO-CaO-MgO-AlO system at oxygen partial pressure of 10 atm within the range of temperatures and compositions directly relevant to copper smelting. For the range of conditions relevant to the Kennecott Utah Copper (South Magna, UT) smelting furnace, it was confirmed experimentally that increasing concentrations of MgO or CaO resulted in significant decreases of the tridymite liquidus temperature and in changes in the position of the tridymite liquidus in the direction of higher silica concentration; in contrast, the spinel liquidus temperatures increase significantly with the increase of MgO or CaO. Olivine and clinopyroxene precipitates appeared at high MgO concentrations in the liquid slag. The liquidus temperature in the spinel primary phase field was expressed as a linear function of 1/(wt pctFe/wt pctSiO), wt pctCaO, wt pctMgO, and wt pctAlO. The positions of each of the liquidus points (wt pctFe)/(wt pctSiO) at a fixed temperatures in the tridymite primary phase field were expressed as linear functions of wt pctCaO, wt pctMgO, and wt pctAlO. [ABSTRACT FROM AUTHOR]

A new phase transition (III–IV) was found at 311 K in CsCoPO4 by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO4 was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO4 had the same crystal structure as that of corresponding phases in CsZnPO4. An extremely large particle size effect was found on III–IV phase transition in CsCoPO4; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO4. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO4, while such a critical size was not found in CsCoPO4. [ABSTRACT FROM AUTHOR]

A new phase transition (III–IV) was found at 311 K in CsCoPO4 by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO4 was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO4 had the same crystal structure as that of corresponding phases in CsZnPO4. An extremely large particle size effect was found on III–IV phase transition in CsCoPO4; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO4. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO4, while such a critical size was not found in CsCoPO4. [ABSTRACT FROM AUTHOR]

Ferrous calcium silicate slags are used in primary and secondary metallurgical processes (described by the FeO-Fe2O3-CaO-SiO2 system). Despite the industrial and scientific importance of this system, the phase equilibria have not been fully investigated. Characterization of this slag system is necessary to improve the design and optimization of new and existing metallurgical processes, particularly in relation to fluxing practice and operating temperatures. Experimental methods have been developed to investigate the phase equilibria of these slags involving the equilibration of samples at fixed oxygen partial pressures, rapid quenching, and the analysis of the compositions of solid and liquid phases using electron probe X-ray microanalysis (EPMA) with wavelength dispersive detectors. Liquidus and solidus data are reported for the primary phase fields of spinel, pseudo-wollastonite, and tridymite in the temperature range of 1200 °C to 1350 °C at an oxygen partial pressure of 10−6 atm, and at 1250 °C at an oxygen partial pressure of 10−5 atm. The resulting data have been used to construct liquidus and solidus isotherms in the “FeO”-CaO-SiO2 system directly relevant to industrial processes. [ABSTRACT FROM AUTHOR]

Under standard conditions, carbon dioxide (CO2) is a simple molecular gas and an important atmospheric constituent, whereas silicon dioxide (SiO2) is a covalent solid, and one of the fundamental minerals of the planet. The remarkable dissimilarity between these two group IV oxides is diminished at higher pressures and temperatures as CO2 transforms to a series of solid phases, from simple molecular to a fully covalent extended-solid V, structurally analogous to SiO2 tridymite. Here, we present the discovery of an extended-solid phase of CO2: a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II (refs 1,2) above 50 GPa at 530–650 K. Together with the previously reported CO2-V (refs 3–5) and a-carbonia, this extended phase indicates a fundamental similarity between CO2 (a prototypical molecular solid) and SiO2 (one of Earth’s fundamental building blocks). We present a phase diagram with a limited stability domain for molecular CO2-I, and suggest that the conversion to extended-network solids above 40–50 GPa occurs via intermediate phases II (refs 1,2), III (refs 7,8) and IV (refs 9,10). The crystal structure of phase VI suggests strong disorder along the c axis in stishovite-like P42/mnm, with carbon atoms manifesting an average six-fold coordination within the framework of sp3 hybridization. [ABSTRACT FROM AUTHOR]

AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al2O3 : P2O5 : Et3N : H2O = 1:1:1.5:x, where x is between 100 and 750 H2O molar ratio. The structure and morphology of the AlPO-5 molecular sieves depend on the gel mixture's composition, hydrothermal temperature, hydrothermal reaction time, and pH. Without pH control, the AlPO-5 structure changed from a spherical shape at H2O = 100 to a hexagonal pillar shape at H2O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H2O = 100 and an island of hexagonal pillars with radiation form appeared at H2O = 300-450 due to the formation of a tridymite type of dense AlPO4 phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO4 phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H2O concentration and pH value have a dramatic effect on the AlPO-5 structure. [ABSTRACT FROM AUTHOR]

Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase of (Si0.9 Ge0.1)O2, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking faults on (1 1 1)β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases. Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation mediated synchro-shear process on (1 1 1)β planes; the geometry of this process is analyzed. [ABSTRACT FROM AUTHOR]

The cristobalite α-β inversion has been studied using DSC on cristobalites produced by firing high purity quartz with and without addition of a mineraliser. If no mineraliser was used, the inversion temperatures and hysteresis on heating and cooling increased with firing temperature. Firing time had little or no effect on inversion temperature. When a mineraliser was used, the same general trend was observed with increases in firing time at low temperatures leading to splitting of the inversion peak. The amount of mineraliser added had little effect. Tridymite inversions were also observed. The results are explained in terms of the degree of order of the cristobalite structure. [ABSTRACT FROM AUTHOR]

The 7.05 Ma Rattlesnake Tuff covers ca. 9000 km, but the reconstructed original coverage was between 30000 and 40000 km. Thicknesses are remarkably uniform, ranging between 15 and 30 m for the most complete sections. Only 13% of the area is covered with tuff thicker than 30 m, to a maximum of 70 m. The present day estimated tuff volume is 130 km and the reconstructed magma volume of the outflow is 280 km DRE (dense rock equivalent). The source area of the tuff is inferred to be in the western Harney Basin, near the center of the tuff distribution, based mainly on a radial exponential decrease in average pumice size, and is consistent with a general radial decrease in welding and degree of post-emplacement crystallization. Rheomorphic tuff is found to a radius of 40-60 km from the inferred source. Four facies of welding and four of post-emplacement crystallization are distinguishable. They are: non-welded, incipiently welded, partially welded and densely welded zones; and vapor phase, pervasively devitrified, spherulite and lithophysae zones. The vapor phase, pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones. At constant thickness (20±3 m), and over a distance of 1-3 km, nonrheomorphic sections can cary between two extremes: (a) entirely vitric sections grading from nonwelded to incipiently welded; and (b) highly zoned sections. Highly zoned sections have a basal non- to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae-dominated zone to a zone of pervasive devitrification, which, in turn, is overlain by a zone of vapor-phase crystallization and is capped by partially welded vitric tuff. A three-dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections. Strong local variations are interpreted to be the result of threshold-governed welding and crystallization controlled by residence time above a critical temperature, which is achieved through differences in thickness and accumulation rate. [ABSTRACT FROM AUTHOR]

The corrosion behaviour of SiC and Si3N4 prepared by chemical vapour deposition (CVD) technique was investigated in molten Na2SO4 salt under argon and argon–oxygen mixture gases. The CVD-SiC was more attacked than the CVD-Si3N4 in molten Na2SO4 salt. This was in good agreement with the results of the thermodynamic equilibrium calculations. The corrosion of both materials in argon–oxygen mixture was less severe compared to that in argon. This was attributed to the formation of amorphous SiO2 acting as a protective film against the corrosion. The apparent activation energies of the CVD-SiC and CVD-Si3N4 in molten Na2SO4 under argon gas were 167 kJ mol-1 and 595 kJ mol-1, respectively. © 1998 Chapman & Hall [ABSTRACT FROM AUTHOR]

The oxidation of single-crystal and relatively pure polycrystalline silicon carbide, between 973 and 2053 K, resulted in the formation of cristobalite, quartz, or tridymite, which are the stable crystalline polymorphs of silica (SiO2) at ambient pressure. The oxide scales were found to be pure SiO2 with no contamination resulting from the oxidizing environment. The only variable affecting the occurrence of a specific polymorph was the oxidation temperature. Cristobalite was formed at temperatures ?1673 K, tridymite between 1073 and 1573 K, and quartz formed at 973 K. The polymorphs were determined using electron diffraction in a transmission electron microscope. These results were further confirmed using infrared and Raman spectroscopies. Cristobalite was observed to grow in a spherulitic fashion from amorphous silica. This was not the case for tridymite and quartz, which appeared to grow as oriented crystalline films. The presence of a thin silicon oxycarbide interlayer was detected at the interface between the SiC substrate and the crystalline silica using x-ray photoelectron spectroscopy. [ABSTRACT FROM AUTHOR]