Showing papers on "Tridymite published in 1999"

Crystal structure of carbon dioxide at high pressure : "Superhard" polymeric carbon dioxide

Abstract: The crystal structures of two molecular phases (I and III) and a polymeric phase (V) of CO2 have been investigated to 60 GPa. CO2-1 (Pa3) transforms to CO2-III (Cmca) at 12 GPa with almost no change of density. Although CO2-III persists in Cmca to at least 60 GPa at ambient temperature, it transforms when heated to 1800 K above 40 GPa to tridymite (P212121) CO2-V with 15.3% volume change. Each carbon atom of CO2-V is tetrahedrally bonded to four oxygen atoms. CO2-V is likely superhard with low compressibility B0 = 365 GPa, similar to cubic BN.

Dehydration of 2‐methylbutanal to isoprene using aluminium phosphate catalysts

Abstract: The synthesis of isoprene from the dehydration of 2‐methylbutanal is described using aluminium phosphates as catalysts. Two samples of aluminium phosphate are studied prepared from the reaction of phosphoric acid with aluminium chloride or sulphate. The chloride route gives a mixed cristobalite/tridymite AlPO4 and this is shown to be more active than a catalyst containing only the tridymite form of AlPO4 formed from the sulphate route. The AlPO4 catalysts are also shown to be active catalysts for the synthesis of isoprene from 3‐methylbutan‐2‐one, which is the major by‐product formed from the reaction of 2‐methylbutanal. The AlPO4 catalysts are deactivated due to the deposition of coke in addition to loss of phosphorus from the surface. Catalytic activity can be totally restored by a simple calcination procedure at 800°C.

A powder diffraction study of BaCaGa4O8

Abstract: The results of the structural determination of barium calcium tetragallium octaoxide, BaCaGa 4 O 8 , from X-ray (Cu Kα) powder diffraction data and refinement by the Rietveld technique are presented. In BaCaGa 4 O 8 , GaO 4 tetrahedra are arranged in a tridymite-like framework. The arrangement of GaO 4 tetrahedra has not previously been found among compounds with the stuffed tridymite structure.

Very-dense silica minerals in the Shergotty SNC meteorite: Evidence for extreme shock pressures

Abstract: Electron and X-ray diffraction confirm that SiO2 in Shergotty consists of two very dense SiO 2 polymorphs in addition to stishovite. All three high-density structures represent quench phases from post-stishovite polymorphs that are stable from 47 to > 80 GPa. Introduction. Shock metamorphism of meteorites and terrestrial samples is characterized by deformation microstructures and high-pressure minerals and glasses[1, 2]. Quartz is a important indicator of shock metamorphism because it forms planar deformation features (PDFs) [1-6] and transforms into the high-density polymorphs coesite and stishovite [7-8, 1-2, 6]. Experimental [9-14] and theoretical [14-18] studies of SiO2 at high pressure have shown that there are several “post-stishovite” structures that are more dense than stishovite. Natural examples of dense SiO2 phases were discovered in Shergotty and interpreted as “post-stishovite” structures that were produced during the shock metamorphism of Shergotty [19-21]. The dense SiO2 was initially thought to have the αPbO2 structure [20], but subsequent diffraction experiments have shown more complex structures [19, 21]. Here we combine field-emission scanning electron microscopy (FESEM), powder X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED) to characterize two new SiO2 structures and discuss these polymorphs in terms of the peak shock pressure experienced by Shergotty. Petrography. Silica in Shergotty mostly occur as large (>150 μm) wedge-shaped grains typical of β-tridymite. They are embedded in clinopyroxene or between clinopyroxene, mesostacis, and "maskelynite". Each grain is surrounded by radiating cracks that initiate at the surfaces of the silica grains and penetrate deep (up to 600 μm) in the Shergotty matrix (Fig. 1). The radiating cracks are similar to those reported from ultra-high pressure metamorphic rocks around coesite grains [22] and are indicative of a large volume increase after decompression. The individual silica grains consist of mosaics of many of domains (10-60 μm), many displaying an orthogonal intergrowth of two or more sets of lamellae with different brightness in back-scattered electron images (Fig. 1b). Electron microprobe analyses show that the lamellae and lamellae-free areas are almost pure SiO2 with minor amounts of Na2O (0.40 wt. %) and Al2O3 (1.14 wt. %). A 120 μm disc containing a large silica (> 60 μm) grain was cored out with a high-precision diamond micro-drill for successive X-ray and TEM investigation. Electron diffraction. Our initial electron diffraction data fit a post-stishovite structure similar to an α-PbO2 –like structure[20]. Since the initial investigation we have obtained diffraction patterns from three distinct zone axes, providing seven diffraction vectors to constrain the structure of this SiO2 polymorph (Table 1). The corresponding d-spacings cannot be indexed using any structures of low-pressure SiO2 polymorphs, including tridymite. Similarly, the diffraction data are inconsistent Figure 1. BSE images of SiO2 showing radiating cracks in surrounding maskelynite and cpx, (a). FESEM BSE images (b), show a fine lamellae microstructure in the SiO2 grains.

Thermal stability of V-loaded AlPO4-5 materials

Abstract: A progressive sintering of the porosity of both V-free and V-loaded AlPO4-5 samples is observed upon calcination in air at increasing temperatures. This sintering leads to the coexistence of porous AlPO4-5 and dense tridymite. As established by cristallinity and pore volume measurements. the presence of vanadium decreases by 100 to 300°C the threshold temperature at which this sintering begins, in spite of the low V amount in the samples (V/(V+Al+P) atomic ratio of 0.5–1%). This threshold temperature depends on the method used to introduce V (by hydrothermal synthesis, under static conditions or agitation, and/or by grinding vanadia with a V-free AlPO4-5) and on the morphology of the AlPO4-5 crystals as observed by scanning electron microscopy. These factors are related to the dispersion of the V species within the porosity. Since only very small amounts of intraporosity VIV species (VIV/VT<1%) are detected by ESR, whatever the extent of the Sintering which generates tridymite, it is concluded that the thermal stability of V-loaded AlPO4-5 is determined by the dispersion of Vv species within the porosity. This conclusion is in line with earlier propositions made to explain the thermal behaviour of V-loaded zeolites and V-contamined FCC catalysts.

Structures and Electronic States of Si/SiO2 Interface

Abstract: Stable structures and electronic states of Si(100)/SiO2 interface are investigated using the first-principles molecular dynamics method. Quartz, tridymite, and pseudo beta-cristobalite are employed as the initial structures of the SiO2 at the interface to find the stable ones by the structural optimization. It is found that the optimized tridymite-type SiO2 structure on Si is the most stable for thin (about 7A) SiO2 layer. For the thicker (about 15A) layer, however, this structure becomes less stable and the optimized quartz-type SiO2 structure is the most stable. The band gap variation along the direction perpendicular to the interface is also investigated for the optimized structures. In the SiO2 region within 1A from the structural interface, the band gap remains as narrow as that of silicon. The drastic change of the band gap takes place in the SiO2 region between 1 and 4A.

Mechanisms of silica refractory corrosion in glass-melting furnaces: Equilibrium predictions

Abstract: Corrosion of refractory silica brick used to line the roof or crown of many-glass-melting furnaces is a serious problem in furnaces using oxygen-fuel rather than air-fuel. In this work, the authors report equilibrium calculations for the Na{sub 2}O-SiO{sub 2} system that predict the formation of a variable-composition liquid-solution phase as a function of key furnace variables. Since thermodynamic data for the relevant liquid phases are unavailable in standard compilations, new data generated using the associate species model are included in the calculations. The calculations indicate that gas-phase NaOH concentrations less than {approximately}15 ppm will not react with the silica refractory under either air-fired or oxy-fired conditions, since this is the smallest equilibrium NaOH partial pressure in a system containing crystalline SiO{sub 2} (either cristobalite or tridymite) in equilibrium with a variable-composition sodium-silicate liquid phase at refractory temperatures in the range 1,400--1,700 C. The high water content ({approximately}65%) of oxygen-fired furnaces results in measured NaOH(g) concentrations as high as 300 ppm, which greatly exceeds the 1,600 C maximum of 68 ppm NaOH(g) for oxy-fired equilibrium with a liquid-SiO{sub 2} (crystalline) system. This indicates that there is a thermodynamic driving force for NaOH(g) to react with silica refractories in oxy-fired furnaces. The results of the calculations are used to define a critical temperature, above which corrosion is not expected to occur for a given NaOH(g) partial pressure.

A Powder Diffraction Study of BaCaGa4O8.

Abstract: The results of the structural determination of barium calcium tetragallium octaoxide, BaCaGa 4 O 8 , from X-ray (Cu Kα) powder diffraction data and refinement by the Rietveld technique are presented. In BaCaGa 4 O 8 , GaO 4 tetrahedra are arranged in a tridymite-like framework. The arrangement of GaO 4 tetrahedra has not previously been found among compounds with the stuffed tridymite structure.