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

Abstract Two sets of R 2 O-MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -F (R=K + , Na + ) glasses were prepared by using NaF and MgF 2 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 MgF 2 exhibited a uniform bulk crystallization and, thus, mica-containing glass-ceramic was formed. It is concluded that the presence of fluorine in the R 2 O-MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -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.