Showing papers on "Tridymite published in 1975"
TL;DR: In this paper, the tridymite primary phase field of the MgO-Al2O3-SiO2 system was studied by DTA, X-ray diffraction, and other techniques.
Abstract: Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2O3-SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2. Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2Ti2O4 (Si2O7). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2, but no crystalline TiO2 was detected. In the presence of CeO2, TiO2 was a more effective catalyst than ZrO2. Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.
120 citations
TL;DR: In this paper, a transmission electron microscope was used to determine the precipitate phases and their crystallographic and morphological relationships to the host. But oxygen diffusion does not appear to be the rate controlling process for precipitate nucleation and growth as precipitation kinetics in fresh olivine oxidized at 900 ° C would indicate an oxygen diffusivity of 10−8 cm2/sec, a value 10 orders of magnitude faster than determined previously.
Abstract: Iron-bearing olivine grains naturally altered by oxidation were examined in the transmission electron microscope to determine the precipitate phases and their crystallographic and morphological relationships to the host. Precipitate complexes heterogeneously nucleated on dislocations were composed of Si-rich, Mg-rich/Si-rich, and Fe-rich regions corresponding to α- tridymite, enstatite, and magnetite and/or hematite, respectively. The tridymite and magnetite (hematite) occurred as rod-like interleaved fingers, while the enstatite was more equidimensional. The crystal orientations of the precipitate phases with respect to the host structure, listed in Table 2, were well defined, but, in general, could not be simply related to the close packing of oxygen planes. Iron-rich (001) planar precipitates occasionally nucleated homogeneously in the host as well as heterogeneously on dislocations. Oxygen diffusion does not appear to be the rate-controlling process for precipitate nucleation and growth as precipitation kinetics in fresh olivine oxidized at 900 ° C would indicate an oxygen diffusivity of 10−8 cm2/sec, a value 10 orders of magnitude faster than determined previously.
59 citations
Patent•
25 Sep 1975
TL;DR: In this paper, the authors show that when exposed to elevated temperatures, the nuclei promote the formation of the outer layer of uniformly fine crystalline silica which imparts thermal dimensional stability for extended periods of use at elevated temperatures.
Abstract: Quartz glass element, such as a diffusion tube useful in the production of semiconductor elements, capable of forming an outer layer of uniformly fine crystalline silica such as cristobalite or tridymite when heated to a temperature at which such crystalline silica forms containing crystallization promoting nuclei having a rate of diffusion in quartz glass less than that of sodium at elevated temperatures. Such nuclei are preferably present in the outer half of the element wall. When the quartz glass element is exposed to elevated temperatures, the nuclei promotes the formation of the outer layer of uniformly fine crystalline silica which imparts thermal dimensional stability for extended periods of use at elevated temperatures.
21 citations
TL;DR: Evidence indicates that the high-temperature forms of cristobalite and tridymite have been found occurring in a Middle Eocene, radiolarian-rich claystone formed at low temperature and were chemical precipitates.
Abstract: The high-temperature forms of cristobalite and tridymite have been found occurring in a Middle Eocene, radiolarian-rich claystone. Evidence indicates that these crystals formed at low temperature and were chemical precipitates. Scanning electron microscopy, energy-dispersive x-ray analysis (microprobe), and x-ray diffraction provided the data upon which the determinative mineralogy was based.
9 citations
TL;DR: In this paper, the tridymite primary phase field of the MgO-Al2O3-SiO2 system was studied by DTA, X-ray diffraction, and other techniques.
Abstract: Crystallization sequences of glasses with compositions in the tridymite primary phase field of the MgO-Al2O3-SiO2 system were studied by DTA, X-ray diffraction, and other techniques. Crystallization was catalyzed by the addition of 7 wt% of either ZrO2 or TiO2. Up to 10 wt% CeO2 was also added to some glasses. Metastable solid solutions with the high-quartz structure exhibiting varying lattice parameters commonly occurred at low temperatures, transforming into a high cordierite at higher temperatures. Depending on the composition and heat treatment, other phases also appeared, e.g. Ce2Ti2O4 (Si2O7). The rate of crystallization was markedly dependent on the catalyst. Colloidal precipitation of the catalyst accompanied by bulk crystallization of the glass was observed with ZrO2, but no crystalline TiO2 was detected. In the presence of CeO2, TiO2 was a more effective catalyst than ZrO2. Although CeO2 lowered the melting temperatures of the glass-ceramics, it increased the stability of the glasses and inhibited volume nucleation, causing coarse structures to form on crystallization.
4 citations