Crystallization of R2O-MgO-Al2O3-B2O3-SiO2-F (R = K+, Na+) glasses with different fluorine source

The effect of fluorine source on the crystallization behaviors of the R2O-MgO-Al2O3-B2O3-SiO2-F (R = K+, Na+) glasses is studied by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrum. Two glasses with the same composition were prepared by using NaF and MgF2 as the source of fluorine, respectively. The glass with the fluorine source of NaF is quite stable during heat-treatment. Only large dendrite crystals form on the surface of this glass, while the interior of the glass still remains amorphous. In contrast, the glass with the fluorine source of MgF2 exhibits uniform bulk crystallization and forms mica-containing glass-ceramics after heat-treatment. The structure difference between these two glasses was investigated by Raman spectrum. The glass with the fluorine source of NaF shows a simple Raman spectrum. Boroxol rings are found to be the dominant unit in this glass. In contrast, multiple peaks appear on the Raman spectrum of the glass with the fluorine source of MgF2, and these peaks are resolved to the vibrations of three- or four-coordinated boron units, such as chain-type metaborate groups, pentaborate groups, diborate groups, highly changed orthoborate groups, pyroborate groups, as well as boroxol rings. The stability of the glass with the NaF fluorine source is attributed to the high stability of the boroxol rings in this glass. (C) 2001 Elsevier Science B.V. All rights reserved.