engleski

Influence of glass composition on gahnite glass-ceramics crystallization

The influence of glass composition on crystallization path of gahnite glass-ceramics in the system ZnO-Al2O3-SiO2, with TiO2 as nucleating agent, has been investigated. Four glass samples with constant ZnO/Al2O3 ratio and different quantity of SiO2 were prepared by melting the batches at 1570oC for 2 hours. The powder and bulk glass samples (with or without previous nucleation) were studied, and crystal phases formed in the course of controlled thermal treatment were established. The methods used were differential thermal analysis (DTA) and X-ray diffraction (XRD). It was found that in the powder samples first crystallizes zinc-aluminosilicate as the main crystalline phase, accompanied with some amount of gahnite (ZnAl2O4). At higher temperatures the mayor amount of gahnite emerges, and the composition of zinc-aluminosilicate approaches the high-quartz s.s. composition. In the glasses with higher amounts of ZnO and Al2O3 the amount of gahnite crystallizing in the first stage decreases, while that of zinc-aluminosilicate increases. For bulk samples it was determined that, regardless on nucleation, in the course of thermal treatment, gahnite crystallizes as the sole phase. Different crystallization path between powder and bulk samples is due to difference in nucleation mechanisms. For the powder samples the nucleation takes place mainly on the surface of particles, while in the bulk samples the nucleation is random and takes place in the whole glass volume. Glasses with higher amounts of ZnO and Al2O3 crystallize at higher temperatures yielding smaller crystallites. With a proper thermal treatment transparent glass-ceramics is possible to obtain.

glass-ceramics; gahnite; crystallization; differential thermal analysis; X-ray diffraction

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