Conditions for Crystallization of LAS Glass-Ceramics as a Function of Nucleating Agent Amount and Heat Treatment

The effect of nucleating agent content and of the crystallization heat treatment have been explored on a commercial lithium-aluminosilicate (LAS) composition (Eurokera Kerablack). Using X-ray diffraction (XRD) on treated samples and differential scanning calorimetry (DSC), we have observed that crystallization occurs differently in nucleating agent-depleted materials than in standard LAS. In particular, we found larger grain size and β-spodumene content than in KeraBlack. We also assessed the decisive effect of the heating rate: although nucleation can take place under almost arbitrary conditions (except with no nucleating agent), the final materials depend on the crystallization cycle applied. They contain more β-quartz with smaller grain size if the cycles and the soak treatments are short; they consist mainly of β-spodumene with large grains if the ramps and soak times are long. We also observed that a soak heat treatment does not influence the crystallization in the case of low content of nucleating agents, but can be used to obtain crystallization if only ZrO₂ is added. The impact of nucleating agents underlines the importance of heterogeneous nucleation in these glass-ceramics. We conclude that it is not possible to create a fine-grained LAS glass-ceramic without the proper amount of nucleating agents.