The environment around Zr4+ is studied at the Zr K-edge in a MgO-Al2O3-SiO2 glass-ceramics heat treated along its very first crystallization steps. The heat treatments were calibrated by differential scanning calorimetry. ZrO2 acts as a nucleating agent, adding a nucleation/crystallization event around 920 degrees C, less than 100 degrees C above the glass transition temperature (830 degrees C) and below the massive crystallization of the parent-glass (similar to 1015 degrees C). It also lowers crystallization temperatures of the matrix as compared with a Zr-free sample. High-resolution transmission electron microscopy imaging allows an unambiguous attribution to tetragonal ZrO2 for, at least, some of the very first detectable crystallites. Zr K-edge extended X ray absorption fine structure analysis shows a peculiar surrounding around Zr4+ in the parent glass, composed of Si (Mg,Al) and Zr sites linked by edge sharing. This indicates direct linkages between Zr sites, pre-existing in the glass structure that play a key role for promoting nucleation. This analysis is in agreement with only a small part of Zr4+ evolving during the first steps of nucleation toward the formation of few nano-crystals of tetragonal (ZrO2). (C) 2010 Elsevier B.V. All rights reserved.
Dargaud, Olivier Cormier, Laurent Menguy, Nicolas Galoisy, Laurence Calas, Georges Papin, Sophie Querel, Gilles Olivi, Luca 9th International Symposium on Crystallization in Glasses and Liquids SEP 10-13, 2009 Foz do Iguacu, BRAZIL Sp. Iss. SI