Examining the role of nucleating agents within glass-ceramic systems

Nucleation is one of the primary contributions to the microstructure of glass-ceramics. However, the nucleation process within glass-ceramics is not understood across all glass systems, nor is the specific role of nucleating agents. The present article proposes a comprehensive definition of a nucleating agent: “a nucleating agent is a minority component of the glass composition that leads to increased internal nucleation rates or precipitation and control of desired crystal phases, either by lowering the thermodynamic or the kinetic barrier for nucleation, or some combination thereof”. We also examine the role that five important nucleating agents (TiO₂, ZrO₂, MoO₃, P₂O₅, and OH⁻) play in different systems, particularly in their effect on the thermodynamics and kinetics of the nucleation process. We argue that nucleating agents help control the nucleation process either by decreasing the thermodynamic or kinetic barrier or some combination of the two and provide a method to distinguish their effect.