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.