Showing papers on "Tridymite published in 2001"

Phase compositions and microstructure of plasma sprayed wollastonite coating

Abstract: The phase compositions and microstructure of wollastonite coatings deposited onto Ti–6Al–4V substrates by plasma spraying were studied using scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The plasma sprayed wollastonite coating was not homogeneous and had a rough surface. Some pores and microcracks existed in the coatings. The primary crystalline phase in the coating was triclinic wollastonite. Some crystalline wollastonite grains had dislocation. The glassy phase was also discovered with some small size crystals in the coating. In addition, tridymite (SiO2) and CaO were also found in the coating.

Coupled experimental and thermodynamic study of the Zn-Fe-Si-O system

Abstract: Experimental and thermodynamic modeling studies have been carried out on the Zn-Fe-Si-O system. This research is part of a wider program to characterize zinc/lead industrial slags and sinters in the PbO-ZnO-SiO2-CaO-FeO-Fe2O3 system. Experimental investigations involve high-temperature equilibration and quenching techniques followed by electron probe X-ray microanalysis (EPMA). Liquidus temperatures and solid solubilities of the crystalline phases were measured in the temperature range from 1200 °C to 1450 °C (1473 to 1723 K) in the zinc ferrite, zincite, willemite, and tridymite primary-phase fields in the Zn-Fe-Si-O system in air. These equilibrium data for the Zn-Fe-Si-O system in air, combined with previously reported data for this system, were used to obtain an optimized self-consistent set of parameters of thermodynamic models for all phases.

Ptc composite material

Abstract: PROBLEM TO BE SOLVED: To provide a reusable PTC composite material which has a low room- temperature resistivity, a transition temperature of <200 deg.C, and a high heat resistance and causes large resistivity jumping at the transition temperature and a low power loss. SOLUTION: This PTC composite material contains; (i) a matrix of a ceramic material having either one of a cristobalite crystal structure to which with the oxide of at least element selected from among Be, B, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, and W and a tridymite crystal structure, and (ii) a conductive phase containing at least one kind of metal, silicide, nitride, carbide, or boride and scattered through the matrix.

2 – Petrographic and Technological Methods for Evaluation of Concrete Aggregates

Abstract: Aggregates are defined as “gravel, sand, slag, crushed rock or similar inert materials.” However, this description is not entirely accurate for concrete aggregates because in the highly alkaline medium of the pore solution in concrete (pH >13), most minerals are at least partly unstable. Chemical tests to determine the potential alkali-silica reactivity of aggregates include standard chemical whole rock analyses and a variety of specialized chemical tests developed specifically to evaluate the suitability of aggregates for use in concrete. The phosphoric acid extraction technique is used to preferentially dissolve silicate minerals in rocks leaving the silica minerals; for example, quartz, tridymite, or cristobalite, behind. The porosity of an aggregate determines its water absorption capacity, which in turn affects the water to cement ratio of the concrete. Porosity is also related to the drying shrinkage of aggregates, which can lead to excessive shrinkage and damage to concrete. X-ray diffraction can be used in the normal way to identify potential alkali-silica reactive minerals that may not readily be identifiable in thin sections viewed under a petrographic microscope either because the particles are too fine or because they are present in too small a quantity, or a combination of both. Staining is typically used to distinguish between potassium feldspars and plagioclase, which is of great significance for petrographers.

X-ray powder diffraction study on the modulated high temperature forms of SiO 2 tridymite between 110 and 220°C

Abstract: The crystal structure of intermediate incommensurate tridymite was refined at 150 °C from powder data Upon cooling from above 220 °C, the basic structure with space group symmetry C2221 is gradually distorted from orthorhombic to monoclinic symmetry With decreasing temperature, the monoclinic angle γ smoothly opens up to 903°, while a displacive modulation with temperature-dependent wavelength develops The 3 + 1 dimensional superspace group of the incommensurate phase is C1121(αβ0) The modulation mainly consists of two sinusoidal transverse displacement waves for the silicon atoms coupled to rotations of the rigid SiO4/2 tetrahedra The wave vector is r=01192(1)a* − 00043(1)b* at 150 °C Below 150 °C tridymite discontinuously transforms to another orthorhombic phase and the modulation partially locks in at the wave vector r 1=1/3a* Simultaneously, an additional incommensurate modulation with r 2= 00395(1)b* − 03882(1)c* is formed The two-dimensional modulation does not vary significantly with the temperature

Hexagonal high-temperature form of aluminium phosphate tridymite from X-ray powder data.

Abstract: Similar to silica tridymite, AlPO4 tridymite shows a sequence of displacive phase transitions resulting in a dynamically disordered hexagonal high-temperature modification. Rietveld refinement reveals that the thermal motions of the tetrahedra can be described either by strongly anisotropic displacement parameters for oxy­gen or by split O atoms. Due to the ordered distribution of aluminium and phospho­rus over alternating tetrahedra, the space group symmetry of high-temperature AlPO4 tridymite is reduced with respect to SiO2 tridymite from P63/mmc to P63mc.

Carbon Dioxide at High Pressure and Temperature

Abstract: We present a review of recent studies of carbon dioxide (CO 2 ) at high pressures and temperatures and describe the experimental evidence for a dipole interacting bent CO 2 phase IV and for a non-molecular extended phase V. Phase IV is stabilized above 12 GPa and 1000 K; the Raman activation of the internal bending mode (v 2 ) indicates a loss of molecular inversion symmetry in this new phase. Above 35 GPa, and 1800 K. the CO 2 molecules associate to form a covalent solid (V) with a structure based on singly bonded CO 4 tetrahedra, similar to that of SiO 2 tridymite, We discuss the stability and properties of this polymeric form of CO 2 .

Crystal Structures of Silica Polymorphs and their Phase Transitions

Abstract: Silica occurs as minerals involving more than 20 different phases. Most silica mineral structures can persist up to high temperatures, where thermal expansion coefficients are nearly null or rather negative. Quartz, well known by its unparalleled applications of piezoelectricity and, of course, as the most common silica mineral, shows the α-β structure transition, which was found by Le Chatelier in the end of the 19th century and has continuously drawn attention of a wide variety of researchers. However, the structural problems such as high temperature structures and structure transformations in many silica minerals have been yet established. Tridymite, for example, shows successive displacive-structure transitions with varying temperature among disordered states, but the theoretical aspect of its mechanism has remained unknown so far. In this article, we review the recent structural problems in quartz, tridymite and cristobalite, all low-pressure polymorphs, and then introduce an example of successful application of molecular dynamics simulation to quartz, stressing the importance of unified application of diffraction crystallography and computer simulation to the complicated problems in silicas.

Optimisation of processing parameters for making pyrophyllite based ceramic tiles using di-sodium hydrogen phosphate binder

Abstract: The processing parameters for making tiles using pyrophyllite with di-sodium hydrogen phosphate binder have been optimised. Infrared studies on sintered tiles prepared under optimised conditions indicate the presence of Si–O linkages and the formation of AlPO4. Using X-ray powder diffraction the various phases formed during sintering were identified as the tridymite form of aluminium phosphate (T-AlPO4), dehydroxylated pyrophyllite, and sodium silicate. Thermal changes taking place during sintering were studied using thermal analysis. Scanning electron micrographs revealed the presence of acicular T-AlPO4 crystals and dehydroxylated pyrophyllite with platelet morphology.

Positive temperature resistor coefficient composite material

Abstract: A PTC composite material including a matrix of ceramic material having one of a cristobalite crystal structure and a tridymite crystal structure, each doped with an oxide of at least one of Be, B, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge and W, and a conductive phase dispersed throughout the matrix. The conductive phase includes at least one of a metal, silicide, nitride, carbide and boride. The material can also contain a polymer to make it more flexible.

Sintering and crystallisation of a P2O5-added Li2O-ZrO2-SiO2 glass powder system

Abstract: Sintering and crystallisation of a 11.5 wt % Li2O, 22.8 wt % ZrO2, 65.7 wt % SiO2 glass powder with P2O5 added were investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and was completed in a very short temperature interval (ΔT ≈ 100°C) in less than 30 min. Crystallisation took place just after completion of densification and was almost completed at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallisation stage. The glass powder compacts first crystallised into lithium metasilicate (Li2SiO3) and/or zircon (ZrSiO4) and tridymite (SiO2) which transformed and/or grew into lithium disilicate (Li2Si2O5), zircon and tridymite after the crystallisation process was essentially complete, so that, a crystallinity degree between 52.4 ± 2.0 and 68.5 ± 3.2 wt % was obtained. P2O5 doping little affected the densification. However, adding P2O5 remarkably enhanced the zircon and tridymite crystallisation while delaying the Li2SiO3 to Li2Si2O5 transformation. The microstructure is characterised by fine crystals uniformly distributed arbitrarily oriented throughout the residual glass phase.

Board for magnetic disk and magnetic disk

Abstract: PROBLEM TO BE SOLVED: To make fine crystal particles, to decrease the coefficient of thermal expansion and to prevent elution of lithium from the glass in a SiO2-Al2O3-Li2O crystallized glass for a magnetic disk board having lithium disilicate, β- cristobalite and tridymite phases. SOLUTION: The main crystal phase of the crystallized glass is a petalite phase and a lithium disilicate phase, and the subcrystal phase is a β-cristobalite and a tridymite phase. The quantitative proportion of each crystal phase measured by Rietvelt quantitative analysis ranges as follows: 30 wt.%<= lithium disilicate phase <=45 wt.%, 20 wt.%<= petalite phase <=35 wt.%, 3 wt.%<= β- cristobalite phase <=10 wt.%, and 3 wt.% <= tridymite phase <=10 wt.%.

Cs and Sr Behaviors in Hydrothermal Alteration of Li- or Na-Borosilicate Glasses.

Abstract: In order to assess the long-term stability of high-level radioactive waste form, various compositions of lithium and sodium borosilicate glasses containing cesium and strontium have been treated under a hydrothermal condition (200 C and 1.54 MPa) for maximum 6 months. The formed phases are tridymite, low quartz, Li2SiO3, Li2Si2O 5 · xH2O, Li2Si2O5, Li42O5, searlesite (NaBSi2O5(OH) 2), and veatchite (Sr2[B5O8(OH)] 2[B(OH)3] · H2O) including some unidentified phases. The behaviors of cesium and strontium are quite different on the hydrothermal process. Cesium is not included in any of the solid phases. In contrast, strontium is an essential constituent of veatchite. Natural veatchite occurs usually as a constituent of evaporite, and hence the mineral is inferred not to be stable, when groundwater flows into a disposal site. The results suggest, accordingly, that in order to incorporate cesium and strontium into some of forming minerals, another element must be added to the glass composition.

Vapor-Phase Garnet at Yucca Mountain, Nevada: Geochemistry and Oxygen-Isotope Thermometry

Abstract: About 20 vapor-phase garnets were studied in two samples of the Topopah Spring Tuff from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350-m-thick, devitrified, moderately to densely welded ash flow that is compositionally zoned from high-silica rhyolite to quartz latite. During cooling of the tuff, escaping vapor produced lithophysae (former gas cavities) lined with an assemblage of tridymite, cristobalite, alkali feldspar, and locally, hematite and/or garnet. Vapor-phase topaz and economic deposits (such as porphyry molybdenum-tungsten) commonly associated with topaz-bearing rhyolites (characteristically enriched in fluorine) were not found in the Topopah Spring Tuff at Yucca Mountain. The garnets are not primary igneous phenocrysts, but rather crystals that grew from a fluorine-poor magma-derived vapor trapped during emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter, and fractured. The garnets also contain inclusions of tridymite. Electron-microprobe analyses of the garnets reveal that they are almandine-spessartine (48.0 and 47.9 mol percent, respectively), have an average chemical formula of (Fe{sub 1.46}, Mn{sub 1.45}, Mg{sub 0.03}, Ca{sub 0.10}) (Al{sub 1.93}, TiO{sub 0.02}) Si{sub 3.01}O{sub 12}, and are homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have {delta}{sup 18}O values of 7.2 and 7.4{per_thousand}. The coexisting tridymite, however, has {delta}{sup 18}O values of 17.4 and 17.6{per_thousand} values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a {delta}{sup 18}O of 11.1{per_thousand} which, when coupled with the garnet {delta}{sup 18}O values in a quartz-garnet fractionation equation, indicates vapor-phase crystallization at temperatures of almost 600 C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50-70 C) of calcite, quartz, and opal secondary mineralization, formed from percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Molding sand for jewelry casting

Abstract: investment casting of jewelry from alloys of precious and nonferrous metals. SUBSTANCE: powder mixture from preparation of mold mixed with water in molding contains, wt.%: cristobalite, 48-78; high-strength gypsum (alpha+beta), 20-50; potassium nitrate, 0.5-2.0; potassium dihydrophosphate, the balance. Cristobalite is used with admixture of tridymite and amorphous quartz in amount of not more 20 or 40%, and alkali metal oxides, not more 2.0%. EFFECT: higher strength characteristics of molding sand ensuring high quality of castings. 3 cl, 6 tbl