CRYSTALLIZATION BEHAVIOR OF NONALKALI GLASSES IN THE SIO2-CAO-AL2O3-ZNO-TIO2-B2O3 SYSTEM

The crystallization behavior of non-alkali glass in the SiO2-CaO-Al2O3-ZnO-TiO2-B2O3 system was investigated. Differential thermal analysis data of the base glass showed double exothermic peaks, one at a lower temperature with precipitation of anorthite (CaO.Al2O3.2SiO2) and gehlenite (2CaO.Al2O3.SiO2), the other at a higher temperature with precipitation of willemite (2ZnO.SiO2) and titanite (CaO.TiO2.SiO2). Phase separation of the glass into an Al2O3-rich glass phase and a TiO2 + ZnO-rich glass phase prior to crystallization was confirmed. The effect of additives and water vapor soaking on surface crystallization of the glass were tested to find a best way to promote the crystallization of the glass. Wollastonite (CaO.SiO2) addition proved most effective, shifting the double peaks of DTA curve into a single one. A similar effect was observed with water vapor exposure of the glass. It was revealed from SEM analyses that the glass-ceramics body consisted of four types of crystals of small grain size.