Showing papers on "Tridymite published in 2000"

Steps toward interstellar silicate mineralogy. V. Thermal Evolution of Amorphous Magnesium Silicates and Silica

Abstract: The thermally induced amorphous-to-crystalline transition has been studied for bulk sheets and micrometre-sized particles of magnesium silicate glass (MgSiO3), nanometresized amorphous magnesium silicate (MgSiO3 and Mg2SiO4 smokes) and amorphous silica particles (SiO2) Silicate glass was produced by the shock-quenching of melts Samples of nanometre-sized smoke particles have been obtained by the laser ablation technique Both the MgSiO3 and the Mg2SiO4 smokes have been found to consist of two particle species; particles of smaller size ranging in diametre from 10 nm to about 100 nm and bigger size ranging from a few 100 nm to almost 3 micrometres in diametre Nanometre-sized particles have been shown to be depleted in magnesium whereas the micrometre-sized particles were found to be enriched in Mg Generally, the particles are composed of nonstoichiometric magnesium silicates with compositions varying even inside of the particles Frequently, the particles contained internal voids that are assumed to have been formed by thermal shrinkage or outgassing of the particles’ interior during cooling Annealing at 1000 K transformed the magnesium silicate smokes into crystalline forsterite (c-Mg2SiO4), tridymite (a crystalline modification of SiO2) and amorphous silica (a-SiO2) according to the initial Mg/Si-ratio of the smoke Crystallization took place within a few hours for the Mg2SiO4 smoke and within one day for the MgSiO3 smoke The MgSiO3 glass evolved more slowly because crystallization started at the sample surface It has been annealed at temperatures ranging from 1000 to 1165 K In contrast to the smoke samples, MgSiO3 glass crystallized as orthoenstatite (MgSiO3) Only after 50 hours of annealing at 1000 K, weak indications of forsterite and tridymite formation have been found in the X-ray diffraction spectra At a temperature of 1000 K, amorphous silica nanoparticles showed distinctly lower rates of thermal evolution compared with the magnesium silicates At 1220 K, the timescale of crystallization of a-SiO2 into cristobalite and tridymite amounts to 45 h Send offprint requests to: D Fabian (fabian@astrouni-jenade) From the experiments, crystallization parameters have been obtained: activation energy and velocity of crystal growth The spectra shown in this study will be made publically available in the electronic database (http://wwwastrounijenade)

V. Thermal evolution of amorphous magnesium silicates and silica

Abstract: The thermally induced amorphous-to-crystalline transition has been studied for bulk sheets and micrometre-sized particles of magnesium silicate glass (MgSiO3), nanometre- sized amorphous magnesium silicate (MgSiO3 and Mg2SiO4 smokes) and amorphous silica particles (SiO2). Silicate glass was produced by the shock-quenching of melts. Samples of nanometre-sized smoke particles have been obtained by the laser ablation technique. Both the MgSiO3 and the Mg2SiO4 smokes have been found to consist of two particle species; particles of smaller size ranging in diametre from 10 nm to about 100 nm and big- ger size ranging from a few 100 nm to almost 3 micrometres in diametre. Nanometre-sized particles have been shown to be depleted in magnesium whereas the micrometre-sized particles were found to be enriched in Mg. Generally, the particles are composed of nonstoichiometric magnesium silicates with com- positions varying even inside of the particles. Frequently, the particles contained internal voids that are assumed to have been formed by thermal shrinkage or outgassing of the particles' in- terior during cooling. Annealing at 1000 K transformed the magnesium silicate smokes into crystalline forsterite (c-Mg2SiO4), tridymite (a crystalline modification of SiO 2) and amorphous silica (a-SiO2) according to the initial Mg/Si-ratio of the smoke. Crystalliza- tion took place within a few hours for the Mg2SiO4 smoke and within one day for the MgSiO3 smoke. The MgSiO3 glass evolved more slowly because crystal- lization started at the sample surface. It has been annealed at temperatures ranging from 1000 to 1165 K. In contrast to the smoke samples, MgSiO3 glass crystallized as orthoenstatite (MgSiO3). Only after 50 hours of annealing at 1000 K, weak in- dications of forsterite and tridymite formation have been found in the X-ray diffraction spectra. At a temperature of 1000 K, amorphous silica nanoparticles showed distinctly lower rates of thermal evolution compared with the magnesium silicates. At 1220 K, the timescale of crys- tallization of a-SiO2 into cristobalite and tridymite amounts to 4.5 h.

Direct measurement of the thermal expansion of the Si-O bond by neutron total scattering

Abstract: The coefficient of thermal expansion of the Si-O bond has been obtained from neutron total scattering measurements of five different phases of silica, with value (2.2±0.4)×10-6 K-1. This value is smaller than values obtained by conventional x-ray diffraction measurements corrected for rigid-body thermal motion. Three of the datasets used in this study, tridymite and two zeolite structures, are completely new.

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

Abstract: The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl2O4 was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6322, a≈ 5.22 A, c ≈ 8.8 A) and of the FE phase (space group P63, a= 10.4469(1)A, c = 8.7927(1)A) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translat...

Phase transitions in tridymite studied using ‘Rigid Unit Mode’ theory, Reverse Monte Carlo methods and molecular dynamics simulations

Abstract: The phase transitions in tridymite and the nature of the high-temperature phase are investigated using a combination of Rigid Unit Mode theory, neutron total scattering measurements analysed using the Reverse Monte Carlo method, and molecular dynamics simulations. The unusually large number of phase transitions in tridymite can be explained within the Rigid Unit Mode theory. The Rigid Unit Mode theory also gives an interpretation of the disordered high-temperature phase as revealed by the neutron scattering data and the molecular dynamics simulations. There is a close correspondence between the structure of the disordered high-temperature phase of tridymite and that of β-cristobalite.

Two forms of aluminium phosphate tridymite from X-ray powder data

Abstract: Monoclinic and triclinic (pseudo-orthorhombic) AlPO4 tridymites have been refined from X-ray powder diffraction data using the silica analogues as starting models. The framework structures of both forms of tridymite are made up of six-membered rings of tetrahedra which differ in the distortion patterns of the ring shapes. Ordered occupation of alternate tetrahedra by Al and P leads to a doubling of the a lattice parameter for monoclinic AlPO4 tridymite (space group Pc) and loss of the C-centring with respect to the isotypic silica tridymite (space group Cc). Triclinic AlPO4 tridymite was refined in the same space group (F1) as the SiO2 analogue.

Crystallization of cristobalite and tridymite in the presence of vanadium

Abstract: The formation of cristobalite and tridymite compounds is described in the system SiO 2 -V 2 O 5 -M 2 CO 3 (M = Na, K). Syntheses were performed with various vanadium concentrations at annealing temperatures between room temperature and 1000°C. Crystallization of cristobalite occurs at much lower temperatures (about 800°C) in the presence of vanadium than observed in the pure silica system, which remains amorphous in this temperature range. With increasing amounts of vanadium, the crystallization of NaVO 3 or K 3 V 5 O 14 is observed along with the formation of cristobalite and tridymite. XRD, IR, and NMR indicate that cristobalite is formed by catalytic processes in the presence of vanadium without measurable V-substitution in its structure. Variations in the thermal behavior of cristobalite are attributed to different states of crystallinity.

Topology and topological disorder in silica

Abstract: In this contribution, we define silica generically as comprising all compounds of silicon and oxygen with the composition SiO2. These compounds are among the most abundant on the earth’s surface and adopt a large number of possible polymorphic forms, among them the seven compact crystalline polymorphs cristobalite, tridymite, moganite, keatite, quartz, coesite and stishovite (the last two high-pressure forms); a family of porous framework structures (e.g. silicalite); vitreous silica obtained by cooling molten silica without crystallization through a glass transition; aperiodic metamict silicas formed by radiation-induced disordering; and other non-crystalline forms produced by application of pressure, oxidation of silicon, vapor deposition or dehydration of gels [1]. All of these solid forms of silica share a common composition, a common chemistry and even (with the exception of stishovite) a common structural element: a substantially covalent [SiO4] tetrahedral unit; but they are all structurally very different (Table 1).

Defect structure of the low temperature α-cristobalite phase and the cristobalite tridymite transformation in (Si-Ge)O 2

Abstract: 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.

Rapid stress release caused by polymorphic transformation during the experimental deformation of quartz

Abstract: Cylindrical samples of polycrystalline quartz were fabricated by hot pressing of crushed Brazilian quartz at 1100 and 1200°C, then deformed in extension at 1200°C at a confining pressure of 300 or 350 MPa. After initial plastic yield, samples failed with sudden and total offloading. Samples could be reloaded in compression and deformed plastically to large strains. Microscopic examination showed extensional failure had occurred with the formation of ‘veins’, oriented normal to the extension direction, containing fibers of silica with their length and c crystal axes parallel to the extension direction. From X-ray and microstructural observations, these are inferred to have formed as mixed tridymite/cristobalite reaction products, now reverted to quartz, accommodating offloading owing to the 38% extensional strain parallel to c. Such rapid transformation and offloading can be compared to processes invoked to explain deep-focus earthquakes in appropriate rock types, but also to the slower formation of oriented, localized reaction products in crustal shear zones.

Thermoluminescent Properties of Tridymite SiO2 : Al3+, Eu3+

Abstract: SiO2 crystals have been used in electroluminescence devices and thermoluminescence (TL) dosimeters. However, their emission mechanisms have not yet been clearly explained. Recently, it has become possible to obtain amorphous, highly pure, SiO2 prepared by the sol-gel method. The emission mechanism of TL was investigated using Al3+ and/or Eu3+-doped SiO2 crystalline samples prepared by heat-treating under much lower temperature than the melting point of SiO2. The TL spectrum of Eu3+-doped sample had main peaks due to the electron transitions from 5D2 to 7F5 (ca. 570 nm, yellow peak) and from 5D0 to 7F2 (ca. 610 nm, red peak). The yellow peak intensity has a maximum value in the SiO2 doped with near 1 mmol% of Eu2O3, while the red peak intensity was almost constant. These facts suggest that bright yellow emission of SiO2TL phosphor is synthesized by the diffusion of Eu3+ ion in SiO2 matrix prepared by sol-gel method.

Influence of porosity on the high temperature oxidation of a SiC-reinforced Si3N4 ceramic composite

Abstract: In this work the oxidation kinetics of a porous (21.5% porosity) nitride bonded silicon carbide containing 20% SiC was investigated in the temperature range of 1100–1400 °C. Two oxidation stages were found which corresponded to (i) a rapid parabolic oxidation rate during short term oxidation exposure (0–10 hours), and (ii) a parabolic scale growth after some exposure time. During short term oxidation a continuous oxide film developed, but it was unable to block internal oxidation at the pore channels. An experimental activation energy of 55 kJ/mol was obtained which was attributed to intially rapid external and internal oxidation through the open pore network. After 1–10 hours depending on the actual temperature, closure of the surface pores through oxidation lead to a transition where a continuous SiO2 scale grew on the outer sample surface through diffusion. An activation energy of 132 kJ/mol was associated with this parabolic growth suggesting that inward oxygen diffusion through the SiO2 scale was rate limiting. Metallographic observations indicated severe cracking of the scale developed. This was attributed to the relatively large shrinkage (≈1%) associated with the β–α cristobalite transformation occurring at temperatures below 250 °C. Moreover, X-ray diffraction indicated the presence of cristobalite and tridymite, but it was unable to identify a discontinuous phase developed beneath the SiO2 scale during oxidation.

Liminous oxide having stuffed tridymite type or kaliphilite type structure and oxide luminophor

Abstract: PROBLEM TO BE SOLVED: To obtain a luminous oxide which shows excellent heat resistance and chemical stability, is useful for a luminophor of a high luminance capable of emitting light of colors from blue to yellow and shows afterglow by using a compound oxide having a specific composition SOLUTION: An oxide to be used has a stuffed tridymite type structure having a composition of M1M22O4 or a kaliphilite type structure having a composition of M1M32O4 (wherein M1 is an alkaline earth metal element selected from Ca, Sr and Ba; M2 is a metal element selected from Al and Ga; and M3 is Si, Mg or Zn) Further, the alkaline earth metal element M1 of the oxide is substituted by a divalent ion of europium or a trivalent ion of other rare earth metal element provided that the substitution ratio is not more than 30 mol% of the sum of the alkaline earth metal element ions

Postmortem analyses of salvaged conventional silica bricks from glass production furnaces

Abstract: The microstructure, phase content, and thermal conductivity of salvaged conventional silica bricks from float glass and TV-panel glass production furnaces were examined as a function of position through the brick, and compared with the original, unaltered brick materials. The silica brick from the float glass furnace was in service for approximately 10 years while that for the TV-panel glass furnace was for approximately 6.5 years. The microstructure and phase content in both salvaged bricks showed gradients, from tridymite at the bricks' cold-face ends, to cristobalite at their hot-face end even though both bricks were an initial mixture of tridymite and cristobalite to begin with. The thermal conductivity of both bricks had increased as a consequence of these phase and microstructural changes. A thermal analysis model predicted that such changes would result in an increase in the bricks' cold-face temperature and heat content during service. The initially-produced temperature gradients and environment caused microstructural changes in the silica brick; however, the cause-and-effect relationship between temperature/environment and microstructural changes in the brick likely became mutually reversible once the microstructural changes initiated and the thermal conductivity of the brick started to change as a consequence.

Initial Dissolution Rate of Quartz Rods in Silicon Melts: Influence of Quartz Surface Conditions

Abstract: The influence of quartz surface conditions on the quartz dissolution rate in silicon melts has been investigated. As experimental samples with different surface conditions, raw fused amorphous quartz and fused quartz pretreated in silicon melts at 1550°C and 1500°C for 300 min were prepared, respectively. In the case of quartz pretreated in silicon melts at 1550°C, the initial dissolution rate in silicon melts at 1430°C was approximately one-third that of raw fused amorphous quartz or quartz pretreated in silicon melts at 1500°C.

X-ray diffractometric determination of tridymite in silica refractories

Abstract: L'article est une etude critique des possibilites d'estimer la teneur en tridymite dans les refractaires de silice par diffraction des rayons X (XRD). A l'occasion de l'examen du polymorphisme de SiO 2 dans des briques de silice ayant servi pendant environ 25 ans dans les parois d'un four a coke, on a montre que des briques de silice ayant les memes teneurs en polymorphes de SiO 2 (a savoir : quartz, cristobalite et verre de silice) conduisent a des taux de comptage differents pour la tridymite. L'observation microscopique des briques montre que l'intensite diffractee est liee, plus ou moins directement, a la taille des cristaux de tridymite, une taille qui depend des conditions de temperature lors de la formation de tridymite. Cela signifie que des echantillons de reference (des standards) pour la tridymite doivent etre prepares selon les memes conditions que celles subies par les echantillons etudies, faute de quoi l'estimation par XRD de la teneur en tridymite peut etre erronee.

TEM of Phase Transitions in Tridymite and Cristobalite Based Materials.

Abstract: We have determined the structure of the paraelectric phase of BaAl2O4, which is a stuffed tridymite, by different TEM techniques and we will describe the phase transition between the ferroelectric room temperature phase and the paraelectric high temperature phase. We have also obtained HREM images of the higly radiation sensitive acristobalite phase of (Si0,9 Ge0,1)O2 and analysed the extended defects in this material.The stuffed tridymite BaAl2O4 is ferroelectric at room temperature and undergoes a paraelectric-ferroelectric (PEFE) phase transition. The transition is reversible, takes place over a wide temperature range (400K-670K) and has a dynamical character. BaAl2O4 is easily obtained by solid state reaction of BaCO3, and A12O4,. The stoichiometric amounts of the initial reagents were mixed, grinded in an agate mortar under acetone and pressed into a pellet. The pellet was annealed in alumina crucibles at 1000 °C and 1300 °C for 40 h in air and furnace cooled.

OH—F Disorder in Non‐Centrosymmetric Zn2(BO3)(OH)0.75F0.25: Ab initio Structure Determination and NMR Study; Comparison with Tridymite and Fluoride Borates.

Abstract: The crystal structure of Zn 2 (BO 3 )(OH) 0.75 F 0.25 is determined ab initio from X-ray and neutron powder data. The unit cell is monoclinic, space group P2 1 (no. 4), Z =2, with a =6.8738(4) A, b =4.9178(2) A, c =5.7018(3) A, β =98.829(3) °, V =190.46(3) A 3 (neutron powder data). Alternating up and down ZnO 3 (OH,F) tetrahedra and triangular BO 3 3− groups built corrugated layers. Their connection by hydroxyl ions gives a 3D network similar to that of tridymite. The statistical distribution of fluorine and hydroxyl in infinite chains is evidenced by magical angle spinning-NMR spectroscopy.

Method for producing cristobalite and cladding articles made thereof

Abstract: The present invention pertains to the production of cristobalite and of building materials made from cristobalite. This invention can be used in the production of flat or three-dimensional architectural building articles and structural members having large dimensions. The method of the present invention essentially involves producing cristobalite having an alkaline metal oxide content of 0.1 to 2.0 mol % by baking quartz sand. To this end, the quartz sand is moisturised using an aqueous solution of an alkaline metal compound selected from the group comprising sodium and/or potassium carbonate and hydrate. The baking process is carried out at a temperature of 1000 to 1550 DEG C until the quartz is entirely converted into an X-ray-amorphous intermediate phase and into a metastable cristobalite having a tridymite impurity content not exceeding 20 wt. %. The method further involves producing the cladding decorative articles and preparing a binder containing alkaline silica by mixing the cristobalite thus obtained with finely ground quartz sand, with a filler and with a solvent liquid consisting of an aqueous solution of an alkaline metal compound. The articles are then moulded from the freshly prepared raw mixture and processed in an autoclave. The solvent liquid consists of an aqueous solution of sodium hydroxide while the finely ground quartz sand has a specific surface of at least 4500 to 7000 cm /g. The articles are moulded so as to include a dual layer, which is achieved by providing in the mould a lower finishing layer of the raw mixture which is arranged in a "face downward" position and has a thickness not exceeding 5 to 10 mm.

PTC 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, and Ge, and a conductive phase dispersed throughout the matrix. The conductive phase includes at least one of a metal, silicide, nitride, carbide and boride.