Structural evolution during Ag+Na+ ion exchange in a sodium silicate glass

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Journal of Non-Crystalline Solids 203 (1996) 268-273

Structural evolution during A g + / N a ÷ ion exchange in a sodium silicate glass Masayuki Yamane *, Shiuichi Shibata, Atsuo Yasumori, Tetsuji Yano, Hiroyasu Takada Department of lnorganic Materials, Tokyo Institute of Technology 2-12-1, Ookayama, Meguro-ku, Tolg,o 152, Japan

Abstract

The structural evolution during Ag+/Na ÷ ion exchange in a sodium silicate glass has been investigated by micro infrared reflectance spectroscopy (IRRS) and 29Si MAS-NMR spectroscopy. The IRRS spectrum varied depending on the Ag concentration in the glass. The spectrum for the original glass had two distinct peaks corresponding to Si-O-Si and Si-O- stretching vibrations in the wavelength region from 1500 to 700 cm- ~. The spectrum at a depth of 0.05 mm from the surface of ion-exchanged glass having Ag20 concentration of about 31 mol% had one peak and two shoulders, whereas the spectrum at a depth of 0.13 mm having AgzO concentration of about 15 mol% had only one peak and the shoulders were not resolved. This change in the IRRS spectra was attributed, from 29Si MAS-NMR spectroscopy, to the structural change induced by the Ag+/Na ÷ ion exchange to from Q4 and Q2 species at the expense of Q3 species, which is responsible for the reduction in glass transition temperature.

1. Introduction

The ion-exchange technique in glass has been widely used for manufacturing various special glasses of high functionality such as chemically tempered glasses [1,2], gradient index glass rods [3,4], planar waveguides [5], etc. The ion exchange, in which a part of the monovalent cations in the glass is replaced by other monovalent cations in a fused salt bath by inter-diffusion, has been conducted to date without paying much attention to the alteration of glass structure induced

* Corresponding author. Tel.: + 81-3 57 34 2522; fax: +81-3 57 34 2877.

during the process. Recently, however, Greaves et al. [6], Houde-Walter et al. [7], and Huang et al. [8], reported that some structural alternation takes place by N a + ~ Ag ÷ ion exchange in silicate glasses. In a recent study, the authors found that some structural evolution, which results in crystallization at a temperature even below the glass transition temperature, Tg, of the original glass, is caused by the exchange of silver for sodium ions in a sodium silicate glass [9]. This unusual crystallization was attributed to a large reduction in Tg, about a 200°C decrease, due to the structural modification induced by the A g + / N a ÷ ion exchange. The micro infrared reflectance spectroscopy (IRRS) spectra of the ionexchanged glass were different from the surface towards the interior of the sample depending on the concentration of Ag ÷ ions.

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 3 5 7 - 2

M. Yamane et al./ Journal of Non-Crystalline Solids 203 (1996) 268-273

This paper reports results for a detailed investigation on the ion exchange induced structural evolution by means of IRRS and magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy.

2. Experiment

2.1. Sample preparation and ion exchange The method of sample preparation and the conditions of ion exchange were essentially the same as those in a previous report [6] except that the glass composition was changed from 25Na20-75SiO 2 (mol%) to 33Na20-67SiO 2 (mol%) so that a comparison with the results for other alkali systems such as lithium silicate or potassium silicate systems is possible in a future study. 0.1 wt% of reagent grade MnO 2 was added to the glass batch consisting of reagent grade Na2CO 3 and SiO 2 in order to enhance relaxation in the MAS-NMR measurement. The glass obtained by melting batch materials in a platinum crucible was pulverized to a size below about 0.5 mm, and remelted at 1500°C for 2 h to improve homogeneity. The 25 X 15 × 0.5 mm platelet sample prepared from the remelted glass was subjected to ion exchange by soaking in 40 g molten AgNO 3 bath contained in a high silica glass crucible at 365°C + 2°C, a temperature which is about 100°C below the glass transition temperature of the remelted glass. The soaking time in the molten salt was varied from 1 to 48 h. Some of the ion-exchanged glasses were further subjected to heat treatment in an ambient atmosphere at the same temperature as the ion-exchange in order to equalize the Ag concentration within the glasses. These glasses are denoted hereafter as Ag-equalized glasses and those without this treatment are denoted as ion-exchanged glasses. The total time for soaking in the molten salt and for heating in air was fixed at 48 h.

2.2. Measurement The concentration profiles of Ag ÷ ions in both ion-exchanged and Ag-equalized glasses were determined from the spot analysis by energy dispersive spectroscopy (EDX) using a scanning electron mi-

269

croscope JSM-T200 with an EDX analyzer. The applied acceleration voltage was 25 kV. The change in the local structure of the glass network with ion exchange was investigated by micro infrared reflectance spectroscopy (IRRS) and 29Si MAS-NMR spectroscopy. IRRS was done using a JIR-6000 FT-IR Spectrophotometer on the ion-exchanged and Agequalized samples ground to various depths of 0.05 mm, 0.15 mm and 0.25 mm from the surface and finished immediately before the measurement with a polishing sheet with diamond powder of 6 txm. The measurement was carried out using a mirror surface of AI metal as a reference. The resolving power was 4 cm-~ and the number of cycles was 40. 29Si MAS-NMR spectroscopy was carried out on the pulverized Ag-equalized sample using a Jouel high resolution FT-NMR spectrometer GX-270. The resonance frequency, pulse width, pulse repeating time, number of cycles, and spinning rate of the sample were 53.54 Hz, 2 ixs, 30 s, 200 × , and 3800 rpm, respectively. Polydimethylsilane was used as the reference material of chemical shift. The chemical shift of the spectrum of this reference material from a standard material, tetramethylsilane, TMS, was determined to be - 3 3 . 8 ppm. Dilatometry and density measurements were also made to obtain supplementary data to the spectroscopy data. The dilatometry was carried out on Ag-equalized glasses cut into platelets of dimension 7 X 2 X 0.5 mm using a Rigaku dilatometer CN8098C1. The sample was heated once to the sag point and slowly cooled to room temperature to remove the effects of thermal strain, and again heated for the measurement at a rate of 5°C/rain to the same temperature. The thermal expansion coeffect of the sample was determined from the dilation between 200-300°C using SiO 2 glass as the standard. The density measurement was made on 0.1-0.2 g of the pulverized sample by the Archimedes method using ethanol as immersion liquid.

3. Results

3.1. Concentration profile of Ag The change in the concentration profiles of Ag ions along the thickness direction with time is shown

M. Yamaneet al./ Journal of Non-Crystalline Solids 203 (1996) 268-273

270

35

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Depth (mm) Fig. 1. Changes in the concentration profiles of Ag ions with ion-exchange time. Lines are drawn as guides for the eye.

in Fig. 1. The Ag concentration near the surface quickly reached 33 mol% (in terms of Ag20) in replacement for 33 mol% Na20 in the original glass. Then the depth of the ion exchanged layer increased with increasing soaking time. It took about 24 h for all of the Na ions to be replaced by Ag ions. All Ag-equalized glasses had uniform Ag concentration profiles, with the particular levels depending on the soaking time as shown in Fig. 2. Both thermal expansion coefficient and Tg of these glasses decrease almost linearly with the increase in A g 2 0 content, while the density conversely increases. There was no distinct peak in the powder X-ray diffraction patterns of Ag-equalized glasses. But the position of the halo of Ag-equalized glasses had shifted from about 24 ° in 2 0 of original glass toward higher angle and almost coincided with the peak position of crystalline Agl0Si4Ol3, i.e. 20--34.5, 35 30

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Fig. 2. Concentration profiles of Ag ions in Ag-equalized glasses after various times of ion-exchange. Lines are drawn as guides for the eye.

1500 1300 1100 900 Wavenumber (cm"1)

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Fig. 3. IRRS at the various depths from the surface of a glass after 3 h of ion-exchange.

when all of the Na ions were replaced by Ag ions by the ion exchange for 24 h.

3.2. Infrared reflectance spectra The infrared reflectance spectra, IRRS, at various depths from the surface of the ion-exchanged glass subjected to 3 h of soaking are shown in Fig. 3. A clear change in the spectrum with Ag concentration is observed. The spectrum at a depth of 0.05 mm from the surface (curve d) has one peak and two shoulders, whereas the spectrum for 0.25 mm depth (curve a) has two distinct peaks. The curves in Fig. 4 are the IRRS spectra at the center of the Ag-equalized glasses of various Ag concentrations. Again a clear change in the spectrum with Ag concentration is observed. It should be noted that the IRRS spectra of the glasses of similar Ag concentration, for example, curve (b) in Fig. 3 and curve (c) in Fig. 4, corresponding to the Ag20 concentrations of 15 and 16 mol%, respectively, are quite similar to each other despite the fact that the former concentration was attained by direct ion exchange and the latter was from the heat treatment for Ag-equalizing. Fig. 5 shows the IRRS spectra of Ag-equalized glass after an ion-exchange of 1 h. Curve (a) represents the spectrum measured at a depth of 0.05 mm

271

M. Yamane et al. / Journal of Non-C~stalline Solids 203 (1996) 268-273 Si-O-Si

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Fig. 4. IRRS at the center of the Ag-equalized glasses of various Ag concentrations.

from the surface where A g concentration originally increased to 33 mol% during the soaking and then decreased to 16 mol% by the heat treatment for Ag-equalized as shown by curve (a) in the insert. Curve (b) shows the spectrum at a depth of 0. l 3 mm from the surface, where the Ag concentration remained at about 16 mol% throughout the treatment except for the first 3 h to reach that concentration by ion-exchange (curve (b) in the insert). Curve (c) is the spectrum at the center of the glass, where the Ag concentration increased monotonically from 0 to 16 mol% by the heat treatment for Ag-equalizing (curve (c) in the insert). These results suggest that the IRRS spectra after the heat treatment for the Ag-equalizing are the same although the spectra were initially different at different positions depending on the Ag concentrations. Similar results were obtained on the Ag-equalized glasses with Ag concentrations of 12 and 30 mol%. 3.3. M A S - N M R s p e c t r a

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Fig. 6 shows 29Si M A S - N M R spectra of the Ag-equalized glasses of various Ag concentrations. The peaks near - 1 1 0 ppm, - 9 0 ppm, - 7 5 ppm and - 6 5 ppm are assigned to Q4, Q3, Q2, and Ql

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....

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Fig. 6. 29Si MAS-NMR spectra of Ag-equalized glasses of various Ag concentrations.

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M. Yamane et al. / Journal of Non-C~stalline Solids 203 (1996) 268-273

species, respectively [10]. It is known from the figure that the initial glass contains only Q3, species, and Q4 and Q2 species are formed at the expense of Q3 species as the N a / A g exchange proceeds. This alteration of the glass structure is completed when all of the Na ions are replaced by Ag ions, leaving a structure consisting of Q4 species alone and one consisting of Q2, and Q~ species, which are the constituents of crystalline Agl0Si4013 , having the main peak of its XRD pattern at nearly the same position as the center of the halo of fully ion exchanged glass.

ioo

i

,

5 10 15 20 25 30 35 Ag.zO(mol%) Fig. 7. Change in the intensity of Q~ peaks with Ag concentration. Lines are drawn as guides for the eye.

4. Discussion It is known from the IRRS spectra shown in Figs. 3-5, that the spectra for the samples of similar Ag content are all similar without regard to the history of the glass. This similarity of the spectra for glasses of similar Ag concentration, particularly the data shown in Fig. 5, suggests that the glass structure once changed by the exchange of Ag for Na can be altered again during the heat treatment for Agequalizing. In other words, structural evolution by the replacement of Ag ÷ for Na ÷ (or vice versa) is reversible, unless the crystal precipitation does not occur by the continuous soaking of Ag ÷ for Na ÷ after the total replacement, suggesting that there is an equilibrium structure of the glass composition. Since the peaks and shoulders in the spectra are assigned to the vibrations of S i - O - S i , S i - O - , O Si-O, etc. [11], the observed changes in the intensity and the position of the peaks suggest that the rearrangement of the glass network took place during both ion-exchange and the heat treatment for Agequalizing. The curves in Fig. 7 show the changes in the intensity of respective peaks assigned to Qn species in the 29Si MAS-NMR spectra with Ag concentration. The increase in the intensity of the peak for Q2 and Q4 species at the expense of Q3 species suggests that the structural evolution with the increase in Ag concentration is accompanied by the development of a silica like structure and Ag-rich phases containing Q2 species as main constituent. The drastic change of Qn in the range between 25 and 33% A g 2 0 may be attributed to the develop-

ment of both silica like structure and Ag-rich phase of AgjoSi40~3 like structure or the precipitation of microcrystallines of Ag ~0Si4013. Since the Tg of glass decreases with the increase in Ag concentration, this silica like structure may be small in size and volume fraction, and may not contribute to building-up of the framework of glass network but is isolated as discrete islands in the Ag-rich glass containing Q2 species as main constituent.

5. Conclusion The structure of N a 2 0 - S i O 2 glass changes by the A g + / N a + ion exchange at a temperature below Tg. The IRRS spectra of the ion-exchange or Agequalized glasses are all similar to each other regardless of the glass, if the A g 2 0 concentration is the same. The structural evolution caused by the increase in A g 2 0 concentration involves the increase in Q4 and Q: species at the expense of Q3 species.

References [1] S.S. Kistler, J. Am. Ceram. Soc. 45 (1962) 59. [2] M.E. Nordberg, E.L. Mochel, H.M. Garfinkel and J.S. OIcott, J. Am. Ceram. Soc. 47 (1964) 215. [3] H. Kita, I. Kitano, T. Uchida and M. Furukawa, J. Am. Ceram. Soc. 54 (1971) 321. [4] S.N. Houde-Walterand B.L. Mcintyre, J. Non-Cryst. Solids 107 (1989) 316.

M. Yamane et al. / Journal of Non-Crystalline Solids 203 (1996) 268-273 [5] Y. Kokubun, S. Suzuki and K. Iga, Appl. Opt. 25 (1986) 3401. [6] G.N. Greaves, S.J. Gurman, C.R.A. Catlow, A.V. Chadwick, S.N. Houde-Walter and C.M.B. Henderson, Philos. Mag. A64 (1991). [7] S.N. Houde-Walter, J.M. Inman, A.J. Dent and G.N. Greaves, J. Phys. Chem. 97 (1993) 9330. [8] C. Huang and A.N. Cormack, in: Physics of Non-Crystalline

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Solids, ed. L.D. Pye, W.C. La Course and H.J. Stevens (Taylor and Francis, London, 1992) p. 31. [9] H. Takada, T. Yano, A. Yasumori, S. Shibata and M. Yamane, Ceram. Trans. 30 (1993) 181. [10] M. Magi, E. Lippnaa, A. Samoson, G. Engelhardt and A.R. Girmmer, J. Phys. Chem. 88 (1984) 1518. [11] J.R. Sweet and W. White, Phys. Chem. Glasses 10 (1969) 246.