Showing papers on "Tridymite published in 2006"

Phase separation and crystallization in the system SiO2–Al2O3–P2O5–B2O3–Na2O glasses

Abstract: The phase separation and crystallization behavior in the system (80 − X )SiO 2 · X (Al 2 O 3 + P 2 O 5 ) · 5B 2 O 3 · 15Na 2 O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al 2 O 3 –P 2 O 5 –SiO 2 and forms a continuous phase. Glasses containing a larger amount of Al 2 O 3 –P 2 O 5 ( X = 40 and 50) readily crystallize and precipitates tridymite type AlPO 4 crystals. It is estimated that the phase separation occurs forming continuous Al 2 O 3 –P 2 O 5 –SiO 2 phase at first, and then tridymite type AlPO 4 crystals precipitate and grow in this phase. Highly transparent glass–ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass–ceramics are 20–30 nm and about 50%, respectively.

Oxidation of silicon carbide and the formation of silica polymorphs

Abstract: The oxidation of both 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.

Ni(3)Si(Al)/a-SiO(x) core-shell nanoparticles: characterization, shell formation, and stability.

Abstract: We have used an electrochemical selective phase dissolution method to extract nanoprecipitates of the Ni3Si-type intermetallic phase from two-phase Ni?Si and Ni?Si?Al alloys by dissolving the matrix phase. The extracted nanoparticles are characterized by transmission electron microscopy, energy-dispersive x-ray spectrometry, x-ray powder diffraction, and electron powder diffraction. It is found that the Ni3Si-type nanoparticles have a core?shell structure. The core maintains the size, the shape, and the crystal structure of the precipitates that existed in the bulk alloys, while the shell is an amorphous phase, containing only Si and O (SiOx). The shell forms around the precipitates during the extraction process. After annealing the nanoparticles in nitrogen at 700??C, the tridymite phase recrystallizes within the shell, which remains partially amorphous. In contrast, on annealing in air at 1000??C, no changes in the composition or the structure of the nanoparticles occur. It is suggested that the shell forms after dealloying of the matrix phase, where Si atoms, the main constituents of the shell, migrate to the surface of the precipitates.

Theoretical Study on Atomic Structures of Thermally Grown Silicon Oxide/Silicon Interfaces

Abstract: The reason why previous researchers have repoterd many different atomic structures such as cristobalite, tridymite, and quartz for the thermally grown silicon oxide/silicon interfaces is studied by following the possible interface structures during the oxidation processes using the first-principles calculations. The results show that the cristobalite-like structures can be easily formed from the silicon diamond structure. Proceeding with the oxidation, compressive strains are accumulated in these cristobalite-like regions. To significantly release these accumulated strains, it is considered that the cristobalite-like regions change into less strained tridymite-like structures and finally more relaxed quartz-like structures. If the transformation occurs without lateral ordering, then the transformed interfacial structure does not have any order and should be amorphous. This explanation provides us with a unified understanding of the interfacial atomic structures. [DOI: 10.1380/ejssnt.2006.584]

Amorphous and partly ordered structures in SiO2 rich volcanic glasses. An ED study

Abstract: Glass structures of obsidian and pumice samples were measured using electron diffraction. Amorphous, partly ordered, and nanocrystalline regions were distinguished and analysed separately. The deconvoluted atomic distances obtained from experimental diffraction patterns through total pair-distribution functions are consistent with distances for ideal SiO 4 tetrahedra. Partly ordered structures in pumices are composed of plate-like fragments of tridymite/cristobalite layers, whereas obsidian contains quartz nanocrystals with abundant moganite-like planar faults. The validity of the structure-model of Goodman for silica glasses is discussed and an alternative interpretation is proposed for silicic volcanic glasses.

Pigmented ceramic element

Abstract: The component partially or wholly comprises pigmented ceramic. The pigment comprises nanoparticles of metal of group IB in periodic table, alkaline metals or their alloy, coated with a layer of silica such as crystalline silica. The crystalline silica is in cristobalite or tridymite form. The metal is gold or gold/silver alloy. The ceramic is zirconia and/or alumina, preferably zirconia. An independent claim is included for manufacture of components of timepiece and jewelry.

The influence of irradiation on the microhardness and photoluminescence of SiO2

Abstract: The microhardness and photoluminescence spectra excited with 337-nm laser radiation in commercial SiO2 glasses (UV windows, substrates with BaTiO3 film coatings) exposed to 60Co gamma radiation and a mixed neutron flux from the reactor are investigated. It is revealed that initial samples contain nanocrystalline phases. An increase in the microhardness and the intensity of the excitonic UV luminescence due to 60Co gamma irradiation and the quenching of the photoluminescence associated with the nonbridging oxygen centers result from healing of Si-O dangling bonds and microcracks in the surface layer at the nanocrystal-glass matrix and substrate-coating interfaces. It is demonstrated that reactor irradiation leads to the phase transformation of SiO2 cristobalite into tridymite and BaO into BO2, as well as to the decomposition of BaTiO3 and BaCO3. This brings about a decrease in the microhardness and photoluminescence quenching.

Refractory and method for producing the same

Abstract: PROBLEM TO BE SOLVED: To provide a tridymite refractory not damaged by abnormal volume expansion and showing improvement in production efficiency and utility efficiency SOLUTION: Main starting materials which is either of used silica brick or rejected fired silica brick in each of which the main mineralogical phase comprises 70 mass% or more tridymite phase and below 30 mass% amorphous phase is compounded with 2 to 5 mass% portland cement and 8 to 22 mass% fumed silica to prepare a compounded raw material Water in an amount of 5 to 10 mass% is added to 100 mass% compounded raw material, and the mixture is molded The molding is dried by irradiation with microwaves at an output of 6 to 20 kW per ton of the molding to obtain a dried product To fire the dried product, it is heat-treated at 1,200°C or higher COPYRIGHT: (C)2007,JPO&INPIT

Phase separation and crystallization in the system SiO 2 -Al 2 O 3 -P 2 O 5 -B 2 O 3 -Na 2 O glasses

Abstract: The phase separation and crystallization behavior in the system (80 � X)SiO2 AE X(Al2O3 +P 2O5) AE 5B2O3 AE 15Na2O (mol%) glasses was investigated. Glasses with X = 20 and 30 phase separated into two phases, one of which is rich in Al2O3–P2O5–SiO2 and forms a continuous phase. Glasses containing a larger amount of Al2O3–P2O5 (X = 40 and 50) readily crystallize and precipitates tridymite type AlPO4 crystals. It is estimated that the phase separation occurs forming continuous Al2O3–P2O5–SiO2 phase at first, and then tridymite type AlPO4 crystals precipitate and grow in this phase. Highly transparent glass–ceramics comparable to glass can be successfully obtained by controlling heat treatment precisely. The crystal size and percent crystallinity of these transparent glass–ceramics are 20– 30 nm and about 50%, respectively.

Mapping of Elemental and Phase Composition in Air-Oxidized Ti3SiC2

Abstract: The oxidation behaviour of Ti3SiC2 in air is not well understood due to mixed results reported in the literature. In this study, the surface composition depth-profiles of air-oxidized Ti3SiC2 have been investigated by synchrotron radiation diffraction (SRD) and secondary ion mass spectroscopy (SIMS) in the temperature range 500-1300°C. Anatase has been observed to form at ~600°C, rutile at ~750°C and tridymite at ~1100°C. Depth-profiling results by SIMS and SRD have revealed a distinct gradation in element and phase composition respectively within the surface oxide layers.

Method of producing silica stone brick for hot repairing and silica stone brick for hot repairing

Abstract: PROBLEM TO BE SOLVED: To obtain silica stone brick for hot repairing which is free from the occurrence of crack or damage due to temperature change by quickly heating and cooling across a temperature zone of about 250°C and can keep excellent thermal shock resistance over a long period. SOLUTION: In a method of producing the silica stone brick for hot repairing using a refractory raw material formulation comprising 30-65 mass% tridymite clinker and 35-70 mass% molten quartz, the tridymite clinker contains 90 mass% tridymite as a mineral phase, wherein the ratio Ic/It of X-ray diffraction intensity Ic of low temperature type cristobalite to X-ray diffraction intensity It of low temperature type tridymite and the ratio Iq/It of X-ray diffraction intensity Iq of low temperature type quartz to X-ray diffraction intensity It of low temperature type tridymite is respectively ≤0.05. Because a tridymite phase having small volume change following the low temperature-high temperature type transition is enriched, the silica stone brick for hot repairing which has excellent thermal shock resistance is produced. COPYRIGHT: (C)2008,JPO&INPIT