Comparison of the Effects of Titania and Tantalum Oxide Nucleating Agents on the Crystallization of Li 2 O · Al 2 O 3 · 6SiO 2 Glasses

The crystallization mechanism in Li 2 O‐Al 2 O 3 ‐SiO 2 (1 : 1 : 6) glasses was investigated with TiO 2 or Ta 2 O 5 as nucleation agents, by differential thermal analysis, X‐ray diffractometry, and transmission electron microscopy. Phase separation occurred prior to crystallization in all of the glasses studied. Both TiO 2 and Ta 2 O 5 additives promoted nucleation and crystal growth in the bulk glasses by causing the precipitation of crystalline precursor phases dispersed within phaseseparated glass, which acted as heterogeneous nucleation sites for β‐quartz(ss) (solid solution) formation. Upon reheating glasses with Ta 2 O 5 as the nucleating agent, phase separation occurred, followed by LiTa 3 O 8 and Ta 2 O 5 nucleation and limited growth as the first‐formed crystalline phases, and then crystallization of β‐quartz structured solid solution, which nucleated and grew from the LiTa 3 O 8 /Ta 2 O 5 crystallites. Tantalum was rejected from the growing β‐quartz(ss), further precipitating Ta 2 O 5 . Eventually, β‐quartz(ss) transformed to β‐spodumene(ss) with increasing temperature. Upon reheating glasses nucleated with TiO 2 additive, they initially phase separated, followed by the formation of crystalline precursors, in turn followed by the near simultaneous crystallization of β‐quartz(ss) and β‐spodumene(ss), transforming to solely β‐spodumene(ss) with increasing temperature [Key words: glass, titania, tantalum oxide, nucleation, crystallization.]