A XANES investigation of the network-modifier cations environment before and after the Na+/K+ ion-exchange in silicate glasses
Introduction
The chemical strengthening via ion exchange is a well-known industrial process consisting in replacing smaller Na+ cations near the surface by larger K+, in a molten salt bath at a temperature below the glass transition temperature (Tg) [1]. This process results in the formation of an ion-exchanged region near the glass surface that is under significant compressive stress. While the chemical strengthening process was discovered in the early 1960′s, the structural modifications induced by Na+/K+ ion exchange in silicate glass are still poorly understood. In particular, the question remains as to how the K+ cations are accommodated by the Na+-host modifier cation and other divalent cations in the glass.
Previous studies on structural modification induced by the ion exchange process were carried out using molecular dynamic (MD) simulations [2], [3], [4], nuclear magnetic resonance (NMR) spectroscopy [5] and micro-Raman spectroscopy [6], [7]. These MD studies mainly focus on the silicate network adaptation mechanisms following the Na+/K+ ion exchange and on the difference in potassium‑oxygen coordination number (CN) between the ion-exchanged and the compositionally-equivalent potassium as-melted glass. In the NMR study, the atomistic details of the structural adaptation of the Na+-host modifier cation and the SiO network were analyzed [5]. Hence, no attention has been paid to the environments of network-modifiers divalent cations before and after the ion-exchange process.
The X-ray Absorption Spectroscopy (XAS) technique is a powerful method to study the environment around one specific element in a disordered multicomponent material [8]. Moreover, the use of μ-XANES (X-ray Absorption Near Edge Structure) technique allows following the local environment of desired element at different locations within the diffusion area. Indeed, XANES is highly sensitive to the symmetry and the medium range order around the absorbing atom [9]. However, due to the multiple scattering effects, the interpretation of the XANES spectra is difficult and a comparison with spectra of crystalline references compounds is necessary [9].
Here, we report the results of μ-XANES measurements on Na+, K+, Ca2 + and Mg2 + ions at different location within the interdiffusion area in Na+/K+ ion-exchanged binary Na –silicate and ternary Na,Mg and Na,Ca–silicate glasses. These results were compared with those obtained from Na,K–silicates as-melted glasses compositions.
Section snippets
Glass synthesis
The glass compositions investigated in this study are shown in Table 1. These glasses were synthesized from constituent oxide and carbonate precursors. The constituents were melted in a platinum/rhodium crucible for 2 h between 1400 °C and 1500 °C depending on the composition. The melt was then quenched and the resulting glass was crushed and remixed followed by remelting for two additional hours to improve glass homogeneity. The final melts were finally quenched on a graphite plate and annealed
Interdiffusion behavior
In Fig. 1, the potassium profiles for the SN, SNM and SNC glasses are represented. The depth of interdiffusion ranges between 100 and 160 μm.
Na environment
The Na K-edge spectra of ion-exchanged SN, SNM and SNC glasses at the surface and deeper in the glasses are shown in.
Fig. 2 and compared to the as-melted SN19K and SN14K glasses. The spectra are composed of three features. The first peak, denoted A, corresponds to a pre-edge transition of the Na K-edge. For all the glass composition, this pre-edge appears
Na environment
The Na K-edge spectrum of all the glass samples present a pre-edge that corresponds to 1s–> 3s transition. The main edge is composed of two peaks with the first one that is attributed to 1s–> 3p transition while the origin of the second is under discussion [8].
In order to study the environment of Na, the fingerprint technique is used. Hence, a comparison of the Na K-edge spectra of the as-melted SN14K with different crystalline references is presented in Fig. 3. These crystalline references
Summary
- (1)
The modification of the environment of Na, K, Ca and Mg species at different locations within the Na+/K+ interdiffusion area in Na-, Na,Ca- and Na,Mg- silicate glasses has been studied using μ-XANES technique.
- (2)
The oxygen coordination number of Na is closed to 7 in the various studied Na-silicate glasses. This CN seems not to be affected by the ion-exchange process. Nevertheless, the NaO bond distance is reduced at the surface of the ion-exchanged glass. This reduction is most probably induced by
Acknowledgements
The continuous support of the Walloon region and AGC glass Europe is acknowledged. C. Ragoen thanks the First International Programme for funding her work (convention 1410030). The authors gratefully acknowledge Jean-Claude Boulliard (Minerals Collection, Université Pierre et Marie Curie - Paris 6, France) and Alain Herbosch (Département des Sciences de la terre et de l'environnement, Université Libre de Bruxelles) for providing some specimens used in this study.
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