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μ-XANES is used to study the modifications in the alkali and alkaline-earth environments induced by the Na+/K+ ion-exchange process in various Na –silicate glasses. The results indicate that the ion-exchange process induces a shortening of the Na O, Ca O and Mg O bond distances. The contraction of the Na O, Ca O and Mg O coordination shell allows a better accommodation of the K+ cations in the glass network and thereby leads to partial relaxation of the stress developed by the Na+/K+ ion-exchange. Nevertheless, despite the stress relaxation process, the K+ environment in the ion-exchanged glass is not equivalent to the one in Na,K–silicate as-melted glasses. Hence, this study clearly shows that the ion swapping forced K+ cations to occupy smaller sites which are not achievable via the melt quench route for glasses with the same K amount.

The effect of the variation in the Al2O3 content on select physical properties of pristine and ion-exchanged glasses in the systems: (75 − x)SiO2mol%-25Na2O-xAl2O3 and (75 − x)SiO2-15Na2O-10CaO-xAl2O3, are reported. The surface compressive stress, surface hardness and K+/Na+ exchange ratio increase, while the depth of the interdiffusion distance decreases, with increasing Al2O3 content. These trends are shown to be consistent with the compositional variation of the glass transition temperature and the molar volume, and their atomistic and thermodynamic bases are discussed.

The possible role of a prior amorphous phase separation on the subsequent crystallization has been the topic of vigorous debates over the last decades and has not yet been clarified, especially regarding the role of the interfaces created by the phase separation. This study proposes to focus on the interplay between a prior amorphous phase separation and the crystallization of fresnoite in the BaO-TiO2-SiO2 system. The crystallization behavior of a non-stoichiometric composition inside the miscibility gap (called APS) is compared with the stoichiometric composition (called FRES) and a non-stoichiometric composition outside the miscibility gap (called NoAPS). The crystallization mechanisms are compared using differential thermal analysis (DTA) by calculating the Avrami parameters and the activation energies as a function of the particle size. The DTA study shows that the two non-stoichiometric compositions exhibit a pronounced surface crystallization behavior whereas FRES undergoes bulk nucleation. This is supported by a multi-scale microstructure characterization. Furthermore, this study demonstrates that the amorphous phase separation and the associated interfaces do not play any significant role in the nucleation step. Moreover, transmission electron microscope (TEM) and local orientation measurements show that the growth of the dendrites is not hindered by the SiO2-rich droplets. The final stage of crystallization of APS is tentatively explained by two composition effects that must be further investigated: the viscosity effect and the formation of a eutectic. (C) 2013 Elsevier B.V. All rights reserved.