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K+ ion exchange in silicate glasses: Results from 17O 3QMAS NMR
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the editor
Na+/K+ ion exchange in silicate glasses: Results from
17
O 3QMAS NMR
C. Ragoena,⁎, M.A.T. Marpleb, S. Senb, F. Bolandc, S. Godeta a b c
4MAT Department, CP165/63, Université Libre de Bruxelles, Avenue F. Roosevelt 50, B-1050 Brussels, Belgium Dept. of Materials Science & Engineering, University of California at Davis, Davis, CA 95616, USA AGC Glass Europe, Technovation Centre, Rue Louis Blériot 12, B-6041 Gosselies, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Glass Ion exchange Alkali distribution Mixed-alkali effect NMR
The effect of Na+/K+ ion exchange on the distribution of the modifier ions and on their interaction with the oxygen atoms in the network is investigated in ternary Na,Mg- and Na,Ca- silicate glasses using 17O triplequantum magic-angle-spinning nuclear magnetic resonance spectroscopy. The results indicate that the K+ ions preferentially exchange with the Na+ ions present in {Na,Ca}-NBO and {Na,Mg}-NBO environments, while the Na-NBO environment is not significantly affected. Remelting of the ion-exchanged glasses leads to a homogenization of the modifier ion distribution around the NBO sites. The dynamical site preference for the Na+/K+ ion exchange has far-reaching implications in our understanding of the alkali ion transport and more specifically of the mixed-alkali effect in silicate glasses.
1. Introduction The technological applications of glasses are often limited by their poor mechanical strength [1]. Chemical strengthening via exchange of smaller alkali ions (e.g. Na+) with larger ones (e.g. K+) has proved to be an efficient method to increase the mechanical strength of glass against brittle fracture, thereby enabling a diverse set of applications ranging from autoinjector cartridges for drug delivery to touch screens for electronic devices [2]. Although the ion-exchange is carried out at temperatures well below the glass transition, the structural network undergoes partial relaxation during the exchange process, which lowers the compressive stress from its maximum ideal value predicted from asmelted glasses of same composition [1,3,4]. Unfortunately, the atomistic nature of this structural relaxation remains poorly understood to date. 17O nuclear magnetic resonance (NMR) spectroscopy can be used to study the structural relaxation of the network as it is a powerful tool for probing the local environment around both the non-bridging and the bridging (NBO and BO) oxygen atoms in silicate glasses [5]. Moreover, enhancement in the resolution of the 17O NMR signals from multiple coordination environments can be obtained by using the triplequantum magic-angle-spinning (3QMAS) NMR spectroscopic technique. This two-dimensional NMR spectroscopic technique averages out the second-order quadrupolar line broadening effects for quadrupolar nuclides such as 17O to generate an isotropic spectrum in one dimension, correlated with the single-quantum MAS central transition spectrum in the second dimension. The high-resolution spectrum in the isotropic dimension is only broadened by the distributions of chemical
⁎
shift and the isotropic quadrupolar shift [5,6]. Here, we report the results of a 17O 3QMAS NMR spectroscopic study of ternary Na,Mg- and Na,Ca- silicate glasses before and after the Na+/K+ ion exchange. It should be noted here that a major drawback of NMR spectroscopy is its relatively low sensitivity that imposes severe limits on its application in studying phenomena such as ion exchange which is typically limited to a depth of a few microns near the surface. Therefore, in order to circumvent this problem, the ion exchange in the present study is performed on glass powder with grain size of ~200–300 μm (see below) to maximize the volume fraction of the ion exchanged region. The goal of this study is to investigate the effects of ion-exchange on the distribution and interaction of the modifier alkali and alkaline-earth cations with the NBO and BO atoms in the silicate network. The corresponding implications on the ionic transport properties are also discussed. 2. Experimental methods 2.1. Glass synthesis The glass compositions investigated in this study are presented in Table 1. These glasses were synthesized by typical melt-quench method from constituent oxide and carbonate precursors including 17O-enriched SiO2, and were doped with 0.1 wt% CoO to enhance the 17O spin-lattice relaxation rate to shorten the time required for 17O NMR data collection. The 17O-enriched SiO2 was prepared by hydrolyzing SiCl4 with 35% enriched H217O. The batches were first decarbonated
Corresponding author. E-mail address: [email protected] (C. Ragoen).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.003 Received 23 June 2017; Received in revised form 22 August 2017; Accepted 1 September 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Ragoen, C., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.003
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
C. Ragoen et al.
optimized to 5.2 μs and 1.8 μs, respectively. The pulse length of the πselective pulses was 25 μs and the filter delay was set to 20 μs. At least 2880 to up to 4032 FIDs were acquired for each of the 48 t1 increments with a recycle delay of 1 s. The 17O chemical shift was externally referenced to the signal of neat H2O (δiso = 0 ppm).
Table 1 Chemical compositions of glass samples ( ± 2 mol%) from XRF measurements. Sample
SiO2
Na2O
CaO
MgO
SNC SNM
72 74
17 16
11 0
0 9
3. Results for three hours at 800 °C and then melted for 30 min at 1450 °C in a high-purity Ar atmosphere. The compositions of the resulting glasses were analyzed by X-ray fluorescence (XRF, Siemens Bruker S4 PIONEER, Cu Kα radiation). For each composition, a plate-shaped sample was prepared for measurement of the K interdiffusion profile.
3.1. Na-Ca silicate glass Fig. 2 shows the 17O 3QMAS NMR spectra for the as-cast, the ion exchanged and the ion exchanged and subsequently remelted SNC glass samples. These spectra are comparable to those reported by Lee and Stebbins [6] in a previous 17O NMR spectroscopic study of the structure of (NaxCa1 − x).3SiO2 glasses and clearly display the presence of bridging oxygen (BO) and two different types of non-bridging oxygen (NBO) resonances. Following Lee and Stebbins [6], the NBO resonances can be assigned to those primarily bonded to Na (Na-NBO) and a mixed {Na,Ca}-NBO environment (Fig. 2). The Na-NBO environment seems to remain unaffected by the Na/K ion-exchange process, while the {Na,Ca}-NBO resonance displays growth of additional intensity towards higher frequency, consistent with the replacement of Na by K in this environment upon ion-exchange. This change can also be observed clearly in the total isotropic projection of the 17O 3QMAS spectra for the as-cast and ion-exchanged SNC glasses (Fig. 3). Remelting of this ionexchanged glass greatly diminishes the intensity of the Na-NBO resonance (Figs. 2, 3), implying major redistribution of the alkali ions in the glass structure induced by thermodynamic equilibration upon melting.
2.2. Ion-exchange The glasses were crushed to particle sizes ranging between 200 and 300 μm and were placed in stainless steel bags with mesh size ~ 200 μm, which were immersed in a KNO3 molten salt bath for 66 h at 400 °C. The ion-exchanged samples were finally washed in water. The plate-shaped samples were ion-exchanged under identical conditions. 2.3. Interdiffusion characterization The ion-exchanged plate-shaped samples were cut and the cross section was embedded in resin, polished down to 3 μm using abrasive polishing pads and cerium oxide. In order to avoid charging effect, these samples were coated with a thin carbon coating. The K profiles were obtained by performing line scans from the surface to a desired depth in the material using an energy dispersive x-ray spectrometer in a scanning electron microscope (FEG-SEM, Hitachi SU 70). The measurements were performed with an accelerating voltage of 15 kV and a dwell time of 1 s, using a 1 cm2 solid-state detector. The K profiles for the SNC and SNM glasses are shown in Fig. 1. The depth of interdiffusion ranges between ~ 30 and 60 μm. Hence, under the approximation of spherical particle shape, the volume fraction of the ion-exchanged region can be estimated for the particulate samples. For the SNC glass, the volume of the ion-exchanged shell is about the same as that of the unexchanged core of these particles, while the exchanged volume fraction is still higher for the SNM glass. 2.4.
3.2. Na-Mg silicate glass The 17O 3QMAS NMR spectra for the as-cast, the ion-exchanged and the ion exchanged and subsequently remelted SNM glass samples are shown in Fig. 4. The resonances corresponding to the BO and NBO species can be readily identified in these spectra. A single resonance corresponding to the NBO sites may be indicative of a mixed {Na,Mg}-O environment. The apparent absence of an Na-O type NBO environment with Na as the exclusive nearest-neighbor charge balancing modifier ion may seem surprising, especially considering the significantly higher concentration of Na compared to that of Mg in the glass. However, the possibility of the presence of a Na-NBO environment with strongly overlapping signal cannot be ignored since the 17O chemical shifts for the Na-NBO and Mg-NBO environments are known to be rather similar [7]. On the other hand, Allwardt and Stebbins [8] have shown that in mixed K-Mg silicate glasses there is a strong preference for the NBO to be linked to the higher field strength (FS) Mg2 + (FS ~ 0.46) while K+ (FS ~ 0.12) is predominantly linked to BO. Here FS is defined as Z/r2, where Z is the charge and r is the cation‑oxygen distance [9]. A similar argument can also be made for the SNM glass since the field strength of Mg2 + is also significantly higher than that of Na+ (FS ~ 0.18). Similar to the SNC glass, ion-exchange results in an increase in the chemical shift dispersion for the NBO towards higher frequencies (higher ppm) indicating the appearance of a K-NBO interaction [8]. These changes become particularly clear in the total isotropic projection of the 17O 3QMAS spectra for these glasses as shown in Fig. 5. No clear change in the shape or peak position of the BO contribution is observed between the different glasses. However, in the ion-exchanged SNM glass the center of gravity of the NBO peak shifts to higher ppm and becomes bimodal, indicating the incorporation of K (via replacement of Na) in the NBO coordination sphere. Remelting of this ion-exchanged glass results in removal of this bimodality, although the center of gravity of the NBO peak remains unchanged. This result implies a homogenization of the modifier ion environment around the NBO sites upon remelting of the ion-exchanged glass.
17
O NMR measurements
The 17O triple-quantum magic-angle-spinning (3QMAS) NMR measurements were carried out using a Bruker Avance500 (11.7 T) spectrometer operating at a Larmor frequency of 67. 8 MHz. Crushed glass powder was packed into a 4 mm ZrO2 rotor and spun at ~ 15 kHz. The spectra were collected using a hypercomplex 3QMAS pulse sequence with a Z filter. The high-power excitation and conversion pulses were 12 SNC
Atom % of K
10
SNM 8 6 4 2 0 0
20
40
60
80
100
Distance (µm) Fig. 1. K concentration as a function of depth in ion-exchanged SNC and SNM glasses.
2
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Fig. 2.
17
O 3QMAS NMR spectra of the SNC glasses: as-cast (top left), ion-exchanged (bottom left), and ion-exchanged and remelted (top right) samples.
(FS ~ 0.36) ions are expected to have a stronger interaction with the NBOs compared to the Na+ cations in the {Na,Ca}-NBO environments. This effect would result in significant coordination of Na+ with BO and therefore the Na-O coordination sphere for the {Na,Ca}-NBO environments would be larger than that for the Na-NBO environment. The combination of longer Na-O distance and weaker interaction with the NBO would make this Na site more suitable for an exchange with the larger K+ ion, in comparison with that for the Na-NBO environments. After the ion exchange process, the K+ cations are therefore likely to be present in dissimilar {K,Ca}-NBO pairs. It may be argued that the formation of K-NBO in this glass upon ion-exchange is somewhat unlikely in view of the significantly higher FS of Ca2 + compared to that of K+. However, the possibility of the formation of some K-NBO sites cannot be completely ruled out solely on the basis of the 17O NMR results presented here, due to the potential overlap of the 17O NMR signal of KNBO with that of the {K,Ca}-NBO sites in the 17O 3QMAS spectra in Fig. 2 [8]. As evidenced by a clear decrease in the concentration of the Na-NBO sites, the remelting of the ion-exchanged glass results in a redistribution and homogenization of these alkali cations in the glass structure presumably in an effort to lower the network stress. Unfortunately, as discussed earlier, potential overlap of the 17O signal from Na-NBO and the {Na,Mg}-NBO sites in the spectra of the SNM glasses precludes the possibility of a clear observation of any preferential site exchange for K in the structure. Nevertheless, the change in the NBO line shape in the isotropic projection of the 17O 3QMAS spectra between the ion exchanged SNM glasses before and after remelting corroborates with similar observations made for the SNC glasses and suggests that the K+ ions after ion-exchange do not occupy positions that correspond to local minima in the energy landscape. This scenario of strong site preference for alkali ions in an otherwise rigid silicate network, that emerges from the 17O 3QMAS spectra of the SNC glasses, has remarkable implications in our understanding of alkali transport in silicate glasses including the mixed-alkali effect where the
Fig. 3. Isotropic projection of the 17O 3QMAS spectra in Fig. 2 for the SNC glasses: as-cast (black), ion-exchanged (blue) and ion-exchanged and remelted (red) samples. Note the diminished intensity of the small peak near ~ 40 ppm for the ion-exchanged and remelted glass. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Discussion The 17O 3QMAS spectra of the SNC glasses (Fig. 2), when taken together, indicate a preferential exchange of the Na+ cations presents in mixed {Na,Ca}-NBO environments by K, while the Na-NBO environment is not significantly affected. This observation can be explained by considering the difference in the Na+ environment between the two Na sites in the glass structure. Owing to its higher FS, the Ca2 + 3
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Fig. 4.
17
O 3QMAS NMR spectra of the SNM glasses: as-cast (top left), ion-exchanged (bottom left), and ion-exchanged and remelted (top right) samples.
other. However, such a preference for hopping cannot be deduced directly from the static structural measurements. This dynamical preference is directly demonstrated for the first time, to the best of our knowledge, in the present study where the structural characterization of ion-exchanged glasses provides clear evidence in favor of a preferential Na+/K+ site exchange in the glass structure. Moreover, the dynamic preference for any one alkali to hop to its own characteristic site can also explain the increasing mobility of alkali ions with concentration in glasses [14]. As the concentration of an alkali increases, so does the number of its characteristic sites in the structure, which increases the probability of a successful hop and hence, the mobility of the ion [15,16]. 5. Summary The 17O 3QMAS NMR spectroscopic results presented here indicate the presence of at least two distinct Na environments in the Na,Casilicate glass. During ion exchange K+ displays a strong preference for Na+ in the mixed {Na,Ca}-NBO environments over those in the NaNBO environment. The direct observation of this dynamical site preference, governed by both steric constraint and Coulombic interaction, has important implications in understanding the concentration dependence of the mobility of alkali ions as well as the mixed-alkali effect. Strong overlap of 17O resonances from the Na-NBO and Mg-NBO environments precludes the observation of similar site preference for Na+/K+ ion exchange in Na,Mg-silicate glasses. However, for both compositions, remelting of the ion-exchanged glasses results in a significant redistribution of the modifier cations around the NBO atoms.
Fig. 5. Isotropic projection of the 17O 3QMAS spectra in Fig. 4 for the SNM glasses: as-cast (black), ion-exchanged (blue) and ion-exchanged and remelted (red) samples. Note the change in the position and the shape of the NBO resonance upon ion-exchange and subsequent remelting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
mobility of one alkali drastically decreases upon addition of a second alkali ion [10–13]. Previous 17O NMR spectroscopic studies of silicate glasses [6,8] demonstrated that alkali ions in silicate glasses occupy distinct sites. This static structural attribute can be linked to the dynamic phenomena such as the mixed-alkali effect if each alkali prefers to only hop to its own characteristic site and therefore cannot switch sites with the second alkali, as this would result in a blocking effect which would lower the mobility of each cation in the presence of the
Acknowledgment The continuous support of the Walloon region and AGC Glass Europe (1410030) is acknowledged. C. Ragoen thanks the First International Programme for funding her work. The authors thank Dr. Laurent Cormier from the Université Pierre et Marie Curie for his help with the synthesis of the 17O-enriched glasses. 4
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References [9] [1] A. Tandia, D. Vargheese, J. Mauro, A. Varshneya, Atomistic understanding of the network dilation anomaly in ion-exchanged glass, J. Non-Cryst. Solids 358 (2012) 316–320. [2] A. Varshneya, Chemical strengthening of glass: lessons learned and yet to be learned, Int. J. Appl. Glas. Sci. 1 (2) (2010) 131–142. [3] P. Kreski, A. Varshneya, A. Cormack, Investigation of ion-exchange ‘stuffed’ glass structures by molecular dynamics simulation, J. Non-Cryst. Solids 358 (2012) 3539–3545. [4] A. Varshneya, G. Olson, P. Kreski, P. Gupta, Buildup and relaxation of stress in chemically strengthened glass, J. Non-Cryst. Solids 427 (2015) 91–97. [5] J.F. Stebbins, J.V. Oglesby, S.K. Lee, Oxygen sites in silicate glasses: a new view from oxygen-17 NMR, Chem. Geol. 174 (2001) 63–75. [6] S.K. Lee, J. Stebbins, Nature of cation mixing and ordering in Na-Ca silicate glasses and melts, J. Phys. Chem. B 107 (2003) 3141–3148. [7] T.M. Clarck, P.J. Grandinetti, P. Florian, J.F. Stebbins, An 17O NMR investigation of crystalline sodium metasilicate: implications for the determination of local structure in alkali silicates, J. Phys. Chem. B 105 (2001) 12257–12265. [8] J. Allwardt, J. Stebbins, Ca-Mg and K-Mg mixing around non-bridging O atoms in
[10] [11] [12] [13] [14]
[15] [16]
5
silicate glasses: an investigation using 17O MAS and 3QMAS NMR, Am. Mineral. 89 (2004) 777–784. G.E. Brown, F. Farges, G. Calas, X-ray scattering and X-ray spectroscopy studies of silicate melts, Rev. Mineral. 32 (1995) 317–410. J.O. Isard, The mixed alkali effect in glass, J. Non-Cryst. Solids 1 (1969) 235–261. D. Day, Mixed alkali glasses – their properties and uses, J. Non-Cryst. Solids 21 (1976) 343–372. H. Lammert, A. Heuer, Contributions to the mixed-alkali effect in molecular dynamics simulations of alkali silicate glasses, Phys. Rev. B 72 (2005) 214202. H. Jain, H.L. Downing, N.L. Peterson, The Mixed Alkali Effect in Lithium-Sodium Borate Glasses, 64 (1984), pp. 335–349. B. Roling, M. Ingram, Determination of divalent cation mobilities in glass dynamic mechanical thermal analysis (DMTA): evidence for cation coupling effect, Solid State Ionics 105 (1998) 47–53. P. Maass, A. Bunde, M.D. Ingram, Ion-transport anomalies in glasses, Phys. Rev. Lett. 68 (1992) 3064–3067. M. Ingram, B. Roling, The concept of matrix-mediated coupling: a new interpretation of mixed-cation effects in glass, J. Phys. Condens. Matter 15 (2003) S1595–S1605.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Letter to the editor
Na+/K+ ion exchange in silicate glasses: Results from
17
O 3QMAS NMR
C. Ragoena,⁎, M.A.T. Marpleb, S. Senb, F. Bolandc, S. Godeta a b c
4MAT Department, CP165/63, Université Libre de Bruxelles, Avenue F. Roosevelt 50, B-1050 Brussels, Belgium Dept. of Materials Science & Engineering, University of California at Davis, Davis, CA 95616, USA AGC Glass Europe, Technovation Centre, Rue Louis Blériot 12, B-6041 Gosselies, Belgium
A R T I C L E I N F O
A B S T R A C T
Keywords: Glass Ion exchange Alkali distribution Mixed-alkali effect NMR
The effect of Na+/K+ ion exchange on the distribution of the modifier ions and on their interaction with the oxygen atoms in the network is investigated in ternary Na,Mg- and Na,Ca- silicate glasses using 17O triplequantum magic-angle-spinning nuclear magnetic resonance spectroscopy. The results indicate that the K+ ions preferentially exchange with the Na+ ions present in {Na,Ca}-NBO and {Na,Mg}-NBO environments, while the Na-NBO environment is not significantly affected. Remelting of the ion-exchanged glasses leads to a homogenization of the modifier ion distribution around the NBO sites. The dynamical site preference for the Na+/K+ ion exchange has far-reaching implications in our understanding of the alkali ion transport and more specifically of the mixed-alkali effect in silicate glasses.
1. Introduction The technological applications of glasses are often limited by their poor mechanical strength [1]. Chemical strengthening via exchange of smaller alkali ions (e.g. Na+) with larger ones (e.g. K+) has proved to be an efficient method to increase the mechanical strength of glass against brittle fracture, thereby enabling a diverse set of applications ranging from autoinjector cartridges for drug delivery to touch screens for electronic devices [2]. Although the ion-exchange is carried out at temperatures well below the glass transition, the structural network undergoes partial relaxation during the exchange process, which lowers the compressive stress from its maximum ideal value predicted from asmelted glasses of same composition [1,3,4]. Unfortunately, the atomistic nature of this structural relaxation remains poorly understood to date. 17O nuclear magnetic resonance (NMR) spectroscopy can be used to study the structural relaxation of the network as it is a powerful tool for probing the local environment around both the non-bridging and the bridging (NBO and BO) oxygen atoms in silicate glasses [5]. Moreover, enhancement in the resolution of the 17O NMR signals from multiple coordination environments can be obtained by using the triplequantum magic-angle-spinning (3QMAS) NMR spectroscopic technique. This two-dimensional NMR spectroscopic technique averages out the second-order quadrupolar line broadening effects for quadrupolar nuclides such as 17O to generate an isotropic spectrum in one dimension, correlated with the single-quantum MAS central transition spectrum in the second dimension. The high-resolution spectrum in the isotropic dimension is only broadened by the distributions of chemical
⁎
shift and the isotropic quadrupolar shift [5,6]. Here, we report the results of a 17O 3QMAS NMR spectroscopic study of ternary Na,Mg- and Na,Ca- silicate glasses before and after the Na+/K+ ion exchange. It should be noted here that a major drawback of NMR spectroscopy is its relatively low sensitivity that imposes severe limits on its application in studying phenomena such as ion exchange which is typically limited to a depth of a few microns near the surface. Therefore, in order to circumvent this problem, the ion exchange in the present study is performed on glass powder with grain size of ~200–300 μm (see below) to maximize the volume fraction of the ion exchanged region. The goal of this study is to investigate the effects of ion-exchange on the distribution and interaction of the modifier alkali and alkaline-earth cations with the NBO and BO atoms in the silicate network. The corresponding implications on the ionic transport properties are also discussed. 2. Experimental methods 2.1. Glass synthesis The glass compositions investigated in this study are presented in Table 1. These glasses were synthesized by typical melt-quench method from constituent oxide and carbonate precursors including 17O-enriched SiO2, and were doped with 0.1 wt% CoO to enhance the 17O spin-lattice relaxation rate to shorten the time required for 17O NMR data collection. The 17O-enriched SiO2 was prepared by hydrolyzing SiCl4 with 35% enriched H217O. The batches were first decarbonated
Corresponding author. E-mail address: [email protected] (C. Ragoen).
http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.003 Received 23 June 2017; Received in revised form 22 August 2017; Accepted 1 September 2017 0022-3093/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Ragoen, C., Journal of Non-Crystalline Solids (2017), http://dx.doi.org/10.1016/j.jnoncrysol.2017.09.003
Journal of Non-Crystalline Solids xxx (xxxx) xxx–xxx
C. Ragoen et al.
optimized to 5.2 μs and 1.8 μs, respectively. The pulse length of the πselective pulses was 25 μs and the filter delay was set to 20 μs. At least 2880 to up to 4032 FIDs were acquired for each of the 48 t1 increments with a recycle delay of 1 s. The 17O chemical shift was externally referenced to the signal of neat H2O (δiso = 0 ppm).
Table 1 Chemical compositions of glass samples ( ± 2 mol%) from XRF measurements. Sample
SiO2
Na2O
CaO
MgO
SNC SNM
72 74
17 16
11 0
0 9
3. Results for three hours at 800 °C and then melted for 30 min at 1450 °C in a high-purity Ar atmosphere. The compositions of the resulting glasses were analyzed by X-ray fluorescence (XRF, Siemens Bruker S4 PIONEER, Cu Kα radiation). For each composition, a plate-shaped sample was prepared for measurement of the K interdiffusion profile.
3.1. Na-Ca silicate glass Fig. 2 shows the 17O 3QMAS NMR spectra for the as-cast, the ion exchanged and the ion exchanged and subsequently remelted SNC glass samples. These spectra are comparable to those reported by Lee and Stebbins [6] in a previous 17O NMR spectroscopic study of the structure of (NaxCa1 − x).3SiO2 glasses and clearly display the presence of bridging oxygen (BO) and two different types of non-bridging oxygen (NBO) resonances. Following Lee and Stebbins [6], the NBO resonances can be assigned to those primarily bonded to Na (Na-NBO) and a mixed {Na,Ca}-NBO environment (Fig. 2). The Na-NBO environment seems to remain unaffected by the Na/K ion-exchange process, while the {Na,Ca}-NBO resonance displays growth of additional intensity towards higher frequency, consistent with the replacement of Na by K in this environment upon ion-exchange. This change can also be observed clearly in the total isotropic projection of the 17O 3QMAS spectra for the as-cast and ion-exchanged SNC glasses (Fig. 3). Remelting of this ionexchanged glass greatly diminishes the intensity of the Na-NBO resonance (Figs. 2, 3), implying major redistribution of the alkali ions in the glass structure induced by thermodynamic equilibration upon melting.
2.2. Ion-exchange The glasses were crushed to particle sizes ranging between 200 and 300 μm and were placed in stainless steel bags with mesh size ~ 200 μm, which were immersed in a KNO3 molten salt bath for 66 h at 400 °C. The ion-exchanged samples were finally washed in water. The plate-shaped samples were ion-exchanged under identical conditions. 2.3. Interdiffusion characterization The ion-exchanged plate-shaped samples were cut and the cross section was embedded in resin, polished down to 3 μm using abrasive polishing pads and cerium oxide. In order to avoid charging effect, these samples were coated with a thin carbon coating. The K profiles were obtained by performing line scans from the surface to a desired depth in the material using an energy dispersive x-ray spectrometer in a scanning electron microscope (FEG-SEM, Hitachi SU 70). The measurements were performed with an accelerating voltage of 15 kV and a dwell time of 1 s, using a 1 cm2 solid-state detector. The K profiles for the SNC and SNM glasses are shown in Fig. 1. The depth of interdiffusion ranges between ~ 30 and 60 μm. Hence, under the approximation of spherical particle shape, the volume fraction of the ion-exchanged region can be estimated for the particulate samples. For the SNC glass, the volume of the ion-exchanged shell is about the same as that of the unexchanged core of these particles, while the exchanged volume fraction is still higher for the SNM glass. 2.4.
3.2. Na-Mg silicate glass The 17O 3QMAS NMR spectra for the as-cast, the ion-exchanged and the ion exchanged and subsequently remelted SNM glass samples are shown in Fig. 4. The resonances corresponding to the BO and NBO species can be readily identified in these spectra. A single resonance corresponding to the NBO sites may be indicative of a mixed {Na,Mg}-O environment. The apparent absence of an Na-O type NBO environment with Na as the exclusive nearest-neighbor charge balancing modifier ion may seem surprising, especially considering the significantly higher concentration of Na compared to that of Mg in the glass. However, the possibility of the presence of a Na-NBO environment with strongly overlapping signal cannot be ignored since the 17O chemical shifts for the Na-NBO and Mg-NBO environments are known to be rather similar [7]. On the other hand, Allwardt and Stebbins [8] have shown that in mixed K-Mg silicate glasses there is a strong preference for the NBO to be linked to the higher field strength (FS) Mg2 + (FS ~ 0.46) while K+ (FS ~ 0.12) is predominantly linked to BO. Here FS is defined as Z/r2, where Z is the charge and r is the cation‑oxygen distance [9]. A similar argument can also be made for the SNM glass since the field strength of Mg2 + is also significantly higher than that of Na+ (FS ~ 0.18). Similar to the SNC glass, ion-exchange results in an increase in the chemical shift dispersion for the NBO towards higher frequencies (higher ppm) indicating the appearance of a K-NBO interaction [8]. These changes become particularly clear in the total isotropic projection of the 17O 3QMAS spectra for these glasses as shown in Fig. 5. No clear change in the shape or peak position of the BO contribution is observed between the different glasses. However, in the ion-exchanged SNM glass the center of gravity of the NBO peak shifts to higher ppm and becomes bimodal, indicating the incorporation of K (via replacement of Na) in the NBO coordination sphere. Remelting of this ion-exchanged glass results in removal of this bimodality, although the center of gravity of the NBO peak remains unchanged. This result implies a homogenization of the modifier ion environment around the NBO sites upon remelting of the ion-exchanged glass.
17
O NMR measurements
The 17O triple-quantum magic-angle-spinning (3QMAS) NMR measurements were carried out using a Bruker Avance500 (11.7 T) spectrometer operating at a Larmor frequency of 67. 8 MHz. Crushed glass powder was packed into a 4 mm ZrO2 rotor and spun at ~ 15 kHz. The spectra were collected using a hypercomplex 3QMAS pulse sequence with a Z filter. The high-power excitation and conversion pulses were 12 SNC
Atom % of K
10
SNM 8 6 4 2 0 0
20
40
60
80
100
Distance (µm) Fig. 1. K concentration as a function of depth in ion-exchanged SNC and SNM glasses.
2
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Fig. 2.
17
O 3QMAS NMR spectra of the SNC glasses: as-cast (top left), ion-exchanged (bottom left), and ion-exchanged and remelted (top right) samples.
(FS ~ 0.36) ions are expected to have a stronger interaction with the NBOs compared to the Na+ cations in the {Na,Ca}-NBO environments. This effect would result in significant coordination of Na+ with BO and therefore the Na-O coordination sphere for the {Na,Ca}-NBO environments would be larger than that for the Na-NBO environment. The combination of longer Na-O distance and weaker interaction with the NBO would make this Na site more suitable for an exchange with the larger K+ ion, in comparison with that for the Na-NBO environments. After the ion exchange process, the K+ cations are therefore likely to be present in dissimilar {K,Ca}-NBO pairs. It may be argued that the formation of K-NBO in this glass upon ion-exchange is somewhat unlikely in view of the significantly higher FS of Ca2 + compared to that of K+. However, the possibility of the formation of some K-NBO sites cannot be completely ruled out solely on the basis of the 17O NMR results presented here, due to the potential overlap of the 17O NMR signal of KNBO with that of the {K,Ca}-NBO sites in the 17O 3QMAS spectra in Fig. 2 [8]. As evidenced by a clear decrease in the concentration of the Na-NBO sites, the remelting of the ion-exchanged glass results in a redistribution and homogenization of these alkali cations in the glass structure presumably in an effort to lower the network stress. Unfortunately, as discussed earlier, potential overlap of the 17O signal from Na-NBO and the {Na,Mg}-NBO sites in the spectra of the SNM glasses precludes the possibility of a clear observation of any preferential site exchange for K in the structure. Nevertheless, the change in the NBO line shape in the isotropic projection of the 17O 3QMAS spectra between the ion exchanged SNM glasses before and after remelting corroborates with similar observations made for the SNC glasses and suggests that the K+ ions after ion-exchange do not occupy positions that correspond to local minima in the energy landscape. This scenario of strong site preference for alkali ions in an otherwise rigid silicate network, that emerges from the 17O 3QMAS spectra of the SNC glasses, has remarkable implications in our understanding of alkali transport in silicate glasses including the mixed-alkali effect where the
Fig. 3. Isotropic projection of the 17O 3QMAS spectra in Fig. 2 for the SNC glasses: as-cast (black), ion-exchanged (blue) and ion-exchanged and remelted (red) samples. Note the diminished intensity of the small peak near ~ 40 ppm for the ion-exchanged and remelted glass. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Discussion The 17O 3QMAS spectra of the SNC glasses (Fig. 2), when taken together, indicate a preferential exchange of the Na+ cations presents in mixed {Na,Ca}-NBO environments by K, while the Na-NBO environment is not significantly affected. This observation can be explained by considering the difference in the Na+ environment between the two Na sites in the glass structure. Owing to its higher FS, the Ca2 + 3
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Fig. 4.
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O 3QMAS NMR spectra of the SNM glasses: as-cast (top left), ion-exchanged (bottom left), and ion-exchanged and remelted (top right) samples.
other. However, such a preference for hopping cannot be deduced directly from the static structural measurements. This dynamical preference is directly demonstrated for the first time, to the best of our knowledge, in the present study where the structural characterization of ion-exchanged glasses provides clear evidence in favor of a preferential Na+/K+ site exchange in the glass structure. Moreover, the dynamic preference for any one alkali to hop to its own characteristic site can also explain the increasing mobility of alkali ions with concentration in glasses [14]. As the concentration of an alkali increases, so does the number of its characteristic sites in the structure, which increases the probability of a successful hop and hence, the mobility of the ion [15,16]. 5. Summary The 17O 3QMAS NMR spectroscopic results presented here indicate the presence of at least two distinct Na environments in the Na,Casilicate glass. During ion exchange K+ displays a strong preference for Na+ in the mixed {Na,Ca}-NBO environments over those in the NaNBO environment. The direct observation of this dynamical site preference, governed by both steric constraint and Coulombic interaction, has important implications in understanding the concentration dependence of the mobility of alkali ions as well as the mixed-alkali effect. Strong overlap of 17O resonances from the Na-NBO and Mg-NBO environments precludes the observation of similar site preference for Na+/K+ ion exchange in Na,Mg-silicate glasses. However, for both compositions, remelting of the ion-exchanged glasses results in a significant redistribution of the modifier cations around the NBO atoms.
Fig. 5. Isotropic projection of the 17O 3QMAS spectra in Fig. 4 for the SNM glasses: as-cast (black), ion-exchanged (blue) and ion-exchanged and remelted (red) samples. Note the change in the position and the shape of the NBO resonance upon ion-exchange and subsequent remelting. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
mobility of one alkali drastically decreases upon addition of a second alkali ion [10–13]. Previous 17O NMR spectroscopic studies of silicate glasses [6,8] demonstrated that alkali ions in silicate glasses occupy distinct sites. This static structural attribute can be linked to the dynamic phenomena such as the mixed-alkali effect if each alkali prefers to only hop to its own characteristic site and therefore cannot switch sites with the second alkali, as this would result in a blocking effect which would lower the mobility of each cation in the presence of the
Acknowledgment The continuous support of the Walloon region and AGC Glass Europe (1410030) is acknowledged. C. Ragoen thanks the First International Programme for funding her work. The authors thank Dr. Laurent Cormier from the Université Pierre et Marie Curie for his help with the synthesis of the 17O-enriched glasses. 4
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