Effects of Iron Oxide on the Crystallization of Calcium Alumino-Silicate Glass

Abstract

Different amounts of iron oxide (Fe2O3) were added into a base calcium alumino-silicate glass (CaO 40, Al2O3 9, SiO2 51 wt%). The crystallization behavior of the glassed was investigated by DTA, XRD, SEM and optical microscopy. DTA analysis on the samples with and without the iron additions shows that the iron-containing glasses obviously absorb more heat energy than the base glass. All the glasses show two endothermal peaks correspondent to two transition temperatures, Tg1 and Tg2, indicating the existence of phase separation. Compared with the base glass, an initial addition of 5wt% Fe2O3 does not result in significant changes in Tg1, but a large decrease in Tg2 is observed. Further additions of 10 and 15wt% Fe2O3, both Tg1 and Tg2 decreases. The iron-containing glasses show two crystallization peaks in their DTA curves. Fe2O3 facilitates the crystallization of the calcium alumino-silicate glass indicated by the decreased activation energy and increased Avrami exponent. SEM observations evidenced that the crystallintes in the heat treated samples are nanosized. It is evidenced that the base glass and the glasses containing 5 and 10 wt% Fe2O3 are surface crystallized upon heat-treatment and the main crystalline phase is wollastonite. The surface crystallization layer and the grain size increase with Fe2O3. However, an addition of 15wt% Fe2O3 results in a bulk crystallization leading to the formation of iron-and aluminum-containing phases.