Showing papers on "Tridymite published in 1988"

Molecular dynamics of water: Adsorption of water on β-tridymite

Abstract: Methods of numerical simulation of molecular dynamics have been employed to investigate some problems of adsorption of water on a hydroxylated surface of the face (0001) of β-tridymite We have used an experimentally verified model which consists in a boundary layer of tridymite corresponding to a completely hydroxylated state of the surface (when there is one hydroxyl group per one surface Si atom) Calculations by methods of molecular dynamics have shown that rotational mobility of hydroxyl groups is an important factor in the process of adsorption of water on hydroxylated surfaces of silicas Adsorbed water molecules have been shown to have preferential orientation with “hydrogens down” at the crystal surface of β-tridymite Mutual orientation of water molecules and hydroxyl groups prevents the adsorption of two water molecules simultaneously It has been shown that one molecule of water is adsorbed, on the average, on one silanol group

Transformation mechanisms of tridymite to cristobalite studied by transmission electron microscopy

Abstract: Available kinetic data on the transformation of tridymite to cristobalite imply that several different mechanisms can operate. Optical microscopy, and transmission electron microscopy (TEM), are here used to establish the mechanisms in specimens of industrial refractory brick heated experimentally at 1545° C. Two mechanisms are distinguished, in both of which cristobalite grows directly, in the solid state, from well-ordered tridymite. Cristobalite with lamellar morphology develops topotactically, with strict crystallographic orientation derived from tridymite. In competition with this mechanism, a massive transformation produces cristobalite with no crystallographic relation to the tridymite. Massive cristobalite nucleates at grain boundaries whereas lamellar cristobalite nucleates within tridymite grains. Together with a third mechanism, involving melting as an intermediate step, these processes can account for the different transformation-rate behaviours of different tridymite specimens.

Silica brick and process for producing same

Abstract: Silica brick with a quartz grain structure that can be detected in the matrix and is converted to cristobalite and/or tridymite, where the quartz grain structures are surrounded by a thin layer of tridymite formed from a silica gel and/or silica sol and consisting of fine tridymite crystals matted together.

Silica brick and process for its production

Abstract: Silikastein mit den in der Matrix erkennbaren, in Cristobalit und/oder Tridymit umgewandelten Quarzkornstrukturen, wobei die Quarzkornstrukturen von einem aus einem Kieselgel und/oder Kieselsol gebildeten dunnen Tridymitsaum umgeben sind, der aus feinen, miteinander verfilzten Tridymitkristallen besteht. Silica brick with the recognizable in the matrix, in cristobalite and / or tridymite converted quartz grain structures, wherein the quartz grain structures are surrounded by a carrier formed from a silica gel and / or silica sol thin Tridymitsaum, which consists of fine tridymite crystals matted together.

Silica brick and manufacture

Abstract: Silica brick with a quartz grain structure that can be detected in the matrix and is converted to cristobalite and/or tridymite, where the quartz grain structures are surrounded by a thin layer of tridymite formed from a silica gel and/or silica sol and consisting of fine tridymite crystals matted together.

Incommensurate Phase of Quartz: Microscopic Origin and Interaction with Defects

Abstract: Silicon dioxide is well known for its extensive polymorphism1: in addition to about 20 crystalline phases, it is also easily obtained in an amorphous state which is a prototype of glass structure. With the exception of Stishovite, these phases consist of three dimensional frameworks of corner sharing SiO4 tetrahedra, giving structures with different topological connections. Furthermore the low pressure phases (quartz, cristobalite, tridymite) present displacive transitions produced by small displacements of the SiO4 tetrahedra, without breaking any atomic bond. In this way quartz at 846 K transforms from the low temperature α phase of trigonal symmetry to the high temperature β phase of hexagonal symmetry. Although this transition has been studied for nearly a century, it was only in 1980 that Bachheimer discovered that the α-β transition was not direct2 but occured through a new intermediate phase, later characterized as an incommensurate (inc) phase3. In an inc structure some property (atomic position, electronic or spin density …) is modulated with a period λ which is not commensurate with the lattice period a. In a diffraction experiment satellite peaks are observed in addition to the usual lattice reflections. Indeed in the inc phase of quartz satellites have been observed by diffraction experiments with neutrons3, X rays4 and electrons5: satellites are observed along the 6 equivalent directions of the hexagonal reciprocal lattice at a small distance q ≃ 0.03 a * from the Bragg peaks.