Direct Evidence of Al-Rich Layers around Nanosized ZrTiO₄ in Glass: Putting the Role of Nucleation Agents in Perspective

Abstract

In the glass industry, nucleation agents are broadly used to obtain homogeneously dispersed and nanograined crystals during heat treatment leading to glass ceramics. In this paper, the role of zirconia and titania used as nucleation agents in a lithium alumosilicate glass is studied on the nanoscale using cutting edge analytical and imaging techniques performed in the transmission electron microscope. It is found that the formation of ZrTiO4 nanocrystals upon heat treatment is accompanied by the formation of a diffusion barrier surrounding these precipitates. At the interface between ZrTiO4 nanocrystals and the residual glass, an enrichment of alumina is observed, making the glass network more rigid and eventually preventing further growth or ripening of the nanosized crystals. Within the compositional gradient formed around the homogeneously distributed ZrTiO4 crystallites, nucleation of the secondary crystalline phase, a lithium alumosilicate high-quartz solid solution, is likely to be much facilitated rather than the never doubtlessly reported on epitaxial overgrowth assumed to act so far.

Crystallization, Microstructure, and Viscosity Evolutions in Lithium Aluminosilicate Glass-Ceramics

“…When a proper amount of TiO2 was added to the Li2O-Al2O3-SiO2 glass, phase separation was developed at the nucleation stage, which led to the formation of β-quartz ss (Figures 3 and 4). This is consistent with previous reports that the addition of TiO2 produced a TiO2-rich phase and serves as a nucleation phase for the formation of β-quartz ss (Doherty et al, 1967;Barbieri et al, 1997b,c;Nordmann and Cheng, 1997;Bhattacharyya et al, 2009). In contract, the TiO2-free glass remained amorphous at this nucleation stage (Figure 3).…”

Design of oxy-fluoride glass-ceramics containing NaLaF4 nano-crystals

“…This effect has recently been reported for other glasses with BaF 2 and ZrTiO 4 nano-crystals [35][36][37]. Energy-filtered TEM has been revealed as an elegant way to map elemental distributions [38] of silicon and aluminium.…”

Multi-scale structuration of glasses: Observations of phase separation and nanoscale heterogeneities in glasses by Z-contrast scanning electron transmission microscopy

“…Indeed, Al atoms present in the SiO 2 -rich regions, corresponding to the dark field regions in HAAFD, will not have an environment sensitive to the presence of Zr/Zn atoms. Such Al segregation in the glass could also explain that Al forms a rich layer around ZrTiO 4 crystals after glass nucleation [27].…”