Structure of heat-treated sol-gel SiO 2 glasses containing silver
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nza´lez-Herna´ndez, M.A. Herna´ndez-Landaverde, and Y.V. Vorobiev Centro de Investigacio´n y de Estudios Avanzados del IPN, Unidad Quere´taro, Quere´taro, Qro., Me´xico
F. Ruiza) and J.R. Martínez Facultad de Ciencias, Universidad Auto´noma de San Luis Potosí 78000, San Luis Potosí, S.L.P. Me´xico (Received 27 January 2000; accepted 27 April 2001)
The crystallization of bulk amorphous SiO2 samples, prepared by the sol-gel method, was obtained by heat treatments in air at temperatures as low as 500 °C. This occurs when silver is added to the precursor solutions in an amount such that it forms aggregates embedded in the glass. Another requirement to observe the low-temperature glass crystallization is that the bulk samples must be prepared from precursor solutions with specific compositions. These compositions, have a high H2O/TEOS ratio, which produces an amorphous SiO2 structure with some structural similarities to cristobalite, the phase in which the SiO2 glass crystallizes.
I. INTRODUCTION
Silicon dioxide, SiO2, may occur in several crystalline forms of which quartz is the most stable. However there are other polymorphs that may occur as minerals and/or can be synthesized. The most common SiO2 polymorphs are quartz, cristobalite, and tridymite. They all are classified as tetrahedral framework structures in which the Si is 4-coordinated to the O atoms, which in turn bridges two Si tetrahedron to form a tetrahedral framework with several membered loops. For quartz, the loops have 6 and 12 members, and for cristobalite only 6-membered rings form. All polymorphs have a highand low-temperature form with the same topology, but only the low-temperature form is stable at ambient temperature and pressure.1 Pure fused silica crystallizes into -quartz above 1000 °C. When cooling down, at 573 °C -quartz transforms into ␣-quartz. A drastic structural change occurs at 867 °C when the -quartz transforms into the -tridymite. This is a reconstructive transformation, which requires breaking the Si–O bond. When the -tridymite is heated to 1470 °C, transformation to the -cristobalite occurs. The roomtemperature ␣-cristobalite, a tetragonal phase, forms from the -phase in the temperature range of 200 to 280 °C.
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Address all correspondence to this author. J. Mater. Res., Vol. 16, No. 7, Jul 2001
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The crystallization of silica glasses has been investigated since shortly after the discovery of glass. The rate and mechanism of crystallization vary with the chemical composition of the glass, temperature, and atmosphere. It has been observed that the presence of alkali metal, water, and oxygen accelerates the crystallization of glass.2,3 For instance, low-softening borosilicate glasses crystallize into cristobalite at temperatures of about 800 °C.2 Several works have been published in the last few years, concerning the physical properties of SiO2 glass containing different amounts of silver, prepared by solgel4 –9 and other methods.10,11 However, in none of these works ha
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