One-Step Crystallization of Gahnite Glass-Ceramics in a Wide Thermal Gradient

Abstract

The glass crystallization regime plays a crucial role in the fabrication of glass ceramics: it affects both phase composition and microstructure, and thus the properties of the final product. In the search for new glass-ceramic materials, the development of a proper heat-treatment schedule involves the utilization of numerous glass samples that need to be thermally treated and then investigated to determine the values of the target characteristics. In this study, we evaluated the effect of crystallization temperature on the glass structure, phase composition, and hardness of glass ceramics in the ZnO-MgO-Al2O3-SiO2 system containing TiO2 and ZrO2 as nucleators. To maximize the number of heat treatments, we performed polythermal crystallization of the glass in a wide temperature range with the help of a gradient furnace. Using X-ray diffraction, Raman spectroscopy, and transmission electron microscopy, we showed the precipitation of gahnite nanocrystals as the main phase in the bulk of a single glass sample and observed a gradual change in its microstructure, transparency, and hardness. The dependence of Vickers hardness values on heat treatment temperature was found to follow a non-linear trend, revealing the optimal thermal range for glass crystallization.

Tuning the Coefficient of Thermal Expansion of Transparent Lithium Aluminosilicate Glass-Ceramics by a Two-Stage Heat Treatment

“…Thus, the tuning of glass-ceramics' CTE is required, which can help to partially compensate for the change in the refractive index within the temperature. In line with that, the optical transparency of glass-ceramics must remain high, because light scattering can lead to increased optical losses [35].…”

“…The glass block was cut into ~10 mm × 10 mm × 2 mm plates and subsequently heat-treated in a muffle furnace with different regimes to produce the glass-ceramics. Polythermal crystallization of the bulk glass samples was also performed using the technique described previously [35].…”

“…A polythermal analysis in the gradient furnace was performed for glass samples to obtain a preliminary view of the crystallization behavior. The main aim of this analysis was to identify the temperature range at which the glass-ceramics maintain their transparency after crystal growth [35]. The hot end temperature of the gradient furnace was varied between 800 and 1000 • C, while the holding times used were 1 or 6 h. It was observed that at temperatures above 900 • C, intense crystallization took place, resulting in the complete opacity of the glass-ceramics.…”