The formation of nanocrystalline ZrO(2) nuclei in a Li2O-Al(2)O(3)-SiO(2) glass – a combined XANES and TEM study

The high economic importance of glass ceramics based on Li(2)O/Al(2)O(3)/SiO(2) (LAS) is mainly due to their low coefficients of thermal expansion (CTE), which make these materials suitable candidates for a number of applications. The exact mechanism of the crystallization processes in LAS glasses is still not fully understood. The present work focuses on the formation and development of nanocrystalline ZrO(2) within an LAS base composition which contains only ZrO(2) as nucleating agent. Using a combination of transmission electron microscopy and X-ray absorption spectroscopy techniques, the temporal evolution of the ZrO(2) nanocrystal formation is described. It is found that the formation of ZrO(2) is initiated by liquid-liquid phase separation droplets with high Zr content, which eventually evolve into the nanocrystalline ZrO(2) precipitations. This process is accompanied by a gradual change of the coordination of the tetravalent Zr ions from sixfold in the glass to eightfold in the crystals. The diameters of the ZrO(2) crystals stay well below 4 nm, even at late stages. The degree of crystallization at each step of the crystallization process is deduced, and from that, the Avrami coefficient n is determined to be n ≈ 1, which describes a barrier-limited crystal growth process.