Effect of fluorine source on crystallization of R2O-MgO-Al2O3-B2O3-SiO2-F (R=K+, Na+) glasses

Two sets of R2O-MgO-Al2O3-B2O3-SiO2-F (R=K+, Na+) glasses were prepared by using NaF and MgF2 as the source of fluorine, respectively. The crystallization behaviors of these glasses were studied by differential thermal analysis, X-ray diffraction, scanning electron spectroscopy, and wave-dispersive spectroscopy. Large dendrite norbergite crystals were found on the surface of the glasses with the fluorine source of NaF due to the strong surface crystallization, but no crystalline phase precipitates in the interior of these glasses were found. On the contrary, the glasses with the fluorine source of MgF2 exhibited a uniform bulk crystallization and, thus, mica-containing glass-ceramic was formed. It is concluded that the presence of fluorine in the R2O-MgO-Al2O3-B2O3-SiO2-F (R=K+, Na+) glasses does not always act as an effective nucleating agent to form mica-containing glass-ceramics. The primary crystalline phase and the crystallization mechanism are strongly dependent on the source of fluorine.