The crystallization of bulk amorphous SiO 2 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 H 2 O/TEOS ratio, which produces an amorphous SiO 2 structure with some structural similarities to cristobalite, the phase in which the SiO 2 glass crystallizes.
Sol-gel thin films and microcrystalline powders of SiO 2 doped with Ag and Cu were obtained under environmental conditions of pressure and temperature. The precursor material was a mixture of water, tetraethyl-orthosilicate, ethanol and aqueous solutions of Ag and Cu salts, the components were taken in ratios known to provide SiO 2 matrix material with good structure. The annealing was performed in oxidizing and reducing atmosphere, the temperature varied in the range of 100 -800 °C. Formation of metal particles of 100 nm order of magnitude was confirmed by X-ray, optical and SEM analysis. In presence of metal particles, the SiO 2 matrix was crystallized in α-cristobalite at annealing temperature as low as 500 °C, and in quartz form at 800 °C. A simple model treating the metal-induced low-temperature crystallization is presented. The material obtained is promising for the use as optical filter, gas sensing element and antibacterial agent.
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