Effect of Alkaline and Alkaline-Earth Metal Oxides Addition on the Glass Formation and Crystallization of ZnO-Al<sub>2</sub>O<sub>3</sub>-SiO<sub>2</sub> Glasses
Keywords:Transparent glass-ceramics, nucleating agent, gahnite, crystallization.
The effect of alkaline and alkaline-earth metal oxides addition to the ZnO-Al2O3-SiO2 system on the glass formation has been studied and the crystallization behaviour of these glasses was investigated by DTA and XRD. The addition of alkaline metal oxides tends to hinder the glass formation and only the system containing Li2O forms glass. The addition of Li2O changes the crystallization behaviour and the precipitation of LiAlSi3O8 is caused together with gahnite which is a typical precipitation phase in the ZnO-Al2O3-SiO2 system. TiO2 added as a nucleating agent tends to form a compound with ZnO when extra ZnO exists. All glasses could be formed when alkaline-earth metal oxides were added and the colour was changed to be brown by the addition of TiO2. In case of no TiO2 addition, glasses changed the form and melted before finish DTA because of low viscosity or high nucleation rate. However, crystallization peaks were clearer after added TiO2 due to the effect of nucleating agent. To conclusion, some glasses could not be formed when alkaline metal oxide was added because crystal growth rate was higher than alkaline-earth metal oxide addition; however, transparent glasses could not be obtained in all samples.
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