Crystallization Behavior of Multi-Component Non-Alkali Silicate Glasses

Abstract

Crystallization behavior of multi-component non-alkali silicate glasses has been studied by DTA and SEM observation. The experimental results obtained in the present study are summarized as follows. (1) On the DTA curve of SiO2-Al2O3-CaO-ZnO-TiO2-B2O3 glass two or three exothermic peaks emerged, one (Tc1) is in the temperature range between 840° and 880°C, responsible for precipitation of anorthite (CaO⋅Al2O3⋅2SiO2) and gehlenite (2CaO⋅Al2O3⋅SiO2), and the others at Tc2=850°-920°C range, responsible for precipitation of willemite (2ZnO⋅SiO2) and titanite (CaO⋅TiO2⋅SiO2). (2) Heating at a fixed temperature induced the crystallization of anorthite, followd by precipitation of gehlenite, willemite and titanite phases. (3) SEM observation showed surface crystallization on glass specimens. The crystallization peak temperatures on the DTA curve shifted downward as the specific surface area increased. (4) A crystallization process was proposed, which includes three steps; (a) a phase separation of glass into a SiO2-Al2O3-CaO rich glass phase and a SiO2-CaO-ZnO-TiO2 rich glass phase, (b) precipitation of anorthite and gehlenite from the former, and (c) precipitation of willemite and titanite from the latter. (5) The possible roles of glass constituents playing in the crystallization process were speculated in terms of viscosity change (Tg, Td) and compositional difference (ΔSf) between the primary phase to be precipitated and the bulk glass.