The role of titania and titania mixtures in the nucleation and crystallization of spodumene—willemite—diopside glasses

Abstract

The nucleation and crystallization processes in glasses and melts of different compositions within the system spodumene—willemite—diopside, using different proportions of TiO 2 and mixtures of TiO 2+ZrO 2 and TiO 2+ZrO 2+F as nucleating agents, have been studied by employing a combination of DTA, X-ray diffraction, and optical and scanning electron microscopy. Incorporation of titania or its mixtures in the base composition modified the crystallization kinetics, as expressed by variations observed on the peak temperatures and spans of the DTA exotherms of crystallization. These nucleants also influenced the type and relative proportions of the crystallized phases, transformation temperatures, degree of metastability of β-Zn 2SiO 4 and β-eucryptite ss 1 1 ss=solid solution; β-eucryptite ss was also named β-quartz ss in the literature. , and the texture of the resultant glass-ceramic. TiO 2 additions lowered the onset crystallization temperatures and favoured the crystallizability. TiO 2 catalyzed the pyroxene and β-eucryptite ss formation and also the transformation of the latter into β-spodumene ss. TiO 2+ZrO 2 mixtures enhanced the crystallization but retarded phase transformation. Addition of F to TiO 2+ZrO 2 showed a synergistic effect to the action of TiO 2 and also greatly facilitated willemite formation. The proposed nucleation mechanism of titania and its mixtures involves the enhancement of liquid-liquid phase separation followed by the separation of a zinc titanate phase which heterogeneously nucleates the crystallization of the glass bulk.