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Crystallization behavior of glass-ceramics in the BaO-TiO2-SiO2 system

par Boulay, Emilie ;Couturiaux, Gaëlle;Idrissi, H.;Godet, Stéphane
Référence ICG (2013: Prague, Czech Rep.)
Publication Non publié, 2013

Communication à un colloque

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Résumé :

Glass ceramics are of growing interest due to their enhanced properties compared to the base glasses. The control of microstructures is consequently a major challenge in those systems. A glass with the stoichiometric composition of fresnoite (Ba2TiSi2O8) has been reported by Cabral et al. to show a very large nucleation rate leading to the formation of nanometric crystals. Komatsu et al. reported that transparent nanocrystallized glasses exhibit blue/white photoluminescence by UV excitation, demonstrating that fresnoite has a significant potential as luminescent material. Furthermore, it was recently reported by Hijiya et al. that non stoichiometric compositions included inside the miscibility gap in the phase diagram allow enhancing the photoluminescence properties; the heterogeneous crystallization is finer, providing an enhancement of the fluorescence effect. The possible role of a prior Amorphous Phase Separation (APS) on the subsequent crystallization has been the topic of vigorous debates over the last decades and has not yet been clarified. This study proposes to focus on the interplay between APS and the crystallization of fresnoite by comparing the crystallization behavior of the stoichiometric composition with two non-stoichiometric ones, one outside the miscibility gap and one inside. The crystallization mechanism is studied using Differential Scanning Calorimetry. It reveals that interfaces created by the APS have no effect on the crystallization mechanism since both non-stoichiometric compositions exhibit surface crystallization. The evolution of the microstructure is investigated by conventional Scanning Electron Microscopy (SEM). It confirms the surface crystallisation mechanism and the very limited role of APS. However, the final microstructures depend on the compositions investigated. The size of the crystals become finer when the silica content is increased and the composition falls inside the miscibility gap. Two possible scenarios are proposed to explain the formation of oriented but finer microstructures: the presence of amorphous silica-rich droplets or a higher viscosity, both disturbing the dendritic growth of fresnoite observed for the non-stoichiometric composition. In order to explore those hypotheses, the microstructures of bulk samples with prior APS are investigated more deeply by coupling SEM to local crystallographic orientation mapping using Electron BackScatterred Diffraction techniques to scrutinize the local orientation around the droplets at the early and final stages of crystallization. High Resolution Transmission Electron Microscope coupled with Electron Energy Loss Spectroscopy is also used in order to characterize the nanoscale composition gradients at the interface between the matrix and the droplets both in the amorphous state and after the crystallization of fresnoite. In order to minimize the effect of composition and the associated viscosity change, two specific glass compositions are further investigated. They are chosen such that they are very close but fall respectively inside and outside the miscibility gap. Their final microstructures are compared.