Behavior of water in glass during crystallization

Behavior of water in glass during crystallization

Journal of Non-Crystalline Solids 320 (2003) 56–63 www.elsevier.com/locate/jnoncrysol Behavior of water in glass during crystallization S. Fujita b ...

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Journal of Non-Crystalline Solids 320 (2003) 56–63 www.elsevier.com/locate/jnoncrysol

Behavior of water in glass during crystallization S. Fujita b

a,b

, A. Sakamoto a, M. Tomozawa

b,*

a Technical Division, Nippon Electric Glass Co., Ltd., Otsu, Shiga 520-8639, Japan Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180-3590, USA

Received 14 May 2002; received in revised form 21 November 2002

Abstract The presence of water in glass has a large influence on the rate of crystallization. Furthermore, water can diffuse into or out of a glass during the heat-treatment for crystallization and influence the crystallization behavior of the glass surface. Water diffusion in a lithium alumino-silicate (LAS) glass and the corresponding LAS based transparent glassceramic was investigated using IR spectroscopy. IR spectra of water-related species changed drastically during crystallization, from an exclusively hydroxyl peak to a peak possibly containing molecular water. Both water solubility and diffusion coefficient were lower in the glass-ceramic than in the glass. These results were attributed to the expulsion of water from the crystalline phase and segregation into the remaining glassy phase, resulting in a high water concentration in the glassy phase. Ó 2003 Elsevier Science B.V. All rights reserved.

1. Introduction Glasses usually contain a small amount of water, incorporated during glass melting. Water in glass is known to have a significant influence on the crystallization kinetics. Both the nucleation rate [1] and the crystal growth rate [2,3] in a given glass system usually increase with increasing water content and the phenomena are normally attributed to the reduction of viscosity of the glass by water. However, the water concentration in the glass during crystallization does not remain constant even when the glass and the crystal have the same composition, since the solubility of water in *

Corresponding author. Tel.: +1-518 276 6659; fax: +1-518 276 8554. E-mail address: [email protected] (M. Tomozawa).

crystals is usually much lower than that in glasses. In fact, during the crystallization of some glass systems, water was found to accumulate into the adjacent glass phase [4,5]. Furthermore, water can diffuse into or out of the glass during the heattreatment for crystallization and influence the crystallization kinetics of the surface layer. It is important, therefore, to investigate the behavior of water in glass during crystallization. In the present work, water diffusion and water solubility as well as the structure of water-related species in a lithium alumino-silicate (LAS) glass and the corresponding LAS based transparent glass-ceramic were investigated using IR spectroscopy. The transparent nature of the glass-ceramic makes it possible to investigate the behavior of water by IR spectroscopy even after crystallization has occurred.

0022-3093/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-3093(03)00077-2

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

2. Experimental

3000

LAS system glass and glass-ceramic were used as samples. This glass is commercially used to produce heat-resistant, transparent glass-ceramic plates and contains approximately 70 mol% SiO2 , 15 mol% Al2 O3 and 10 mol% Li2 O as the main components and 3 mol% of TiO2 plus ZrO2 as nucleation agents and 2 mol% of other alkali and alkali earth oxides. The glass was converted to a transparent glass-ceramic by a two-stage heattreatment in air, the schedule of which is schematically shown in Fig. 1. The heat-treatment schedule included a nucleation stage at 780 °C for 2 h and a crystal growth stage at 870 °C for 1 h. The final glass-ceramic product consisted of approximately 80 wt% b-quartz solid solution (Li2 O  Al2 O3  5.7SiO2 ) phase and 20 wt% glass phase. The variation of crystallinity during the heat-treatment expressed in terms of the X-ray diffraction peak intensity for the crystalline phase is shown in Fig. 2. The glass-ceramic samples are transparent with 85% transmittance in the visible range for a 4 mm thick specimen. Both the glass and glass-ceramic samples were cut into 10 mm  10 mm squares with thicknesses of approximately 1 mm and were polished using abrasives of various sizes starting with 600 grit SiC polishing paper and ending with a 0.06 lm cerium oxide slurry to obtain optically smooth surfaces.

2500

(101)

crystal growth 870 °C -1hr

nucleation

4

780 °C -2hr

2

1

57

3

5 2.3 °C / min.

6

Fig. 1. The schematic diagram of the heat-treatment schedule used for crystallization. Vertical direction indicates the temperature and horizontal direction indicates the heat-treatment time. Nucleation stage: 750 °C for 2 h; crystal growth stage: 850 °C for 1 h.

cps

2000 (112)

1500

(100) 6 5 4 3 2 1

1000 500 0 15

20

25

30

35

40

45

50



Fig. 2. The X-ray diffraction patterns of the LAS system at various stages of the crystallization. The number on the righthand side of the figure corresponds to the number in Fig. 1.

First, the water diffusion behavior at various temperatures under wet atmosphere was determined. The maximum heat-treatment temperature for the water diffusion study was limited to 780 °C in order to avoid crystallization during the heattreatment. The wet atmosphere had a water vapor pressure of 355 Torr (4.75  104 Pa) and was created by bubbling air through a hot water bath kept at 80 °C and into the furnace where the samples were heat-treated. Samples were taken out of the furnace periodically and the water content in the sample was determined using IR spectroscopy. The amount of water was expressed in terms of absorbance units of water-related absorption peak, measured using a Nicolet Model Magna 560 FTIR spectrometer. Each measurement consisted of 256 scans at a data spacing of 0.482 cm1 with a beam size approximately 4 mm in diameter. The IR absorption spectra centered around the waterrelated peaks for the glass and glass-ceramic are shown in Fig. 3; each sample was 1 mm thick. The absorbance of each peak was used as a measure of the water content of the respective specimen. Since the peak shape of the water-related band remained unchanged during water diffusion, same water content is obtained by the use of integrated area of the peak or absorbance. Here absorbance was used since it is simpler. Absorbance is equal to Ced, where C is the average concentration of water, e is the extinction coefficient, and d is the specimen thickness. The value of the extinction coefficient, e,

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

Cs0 ¼ Cs  C0 . The diffusion coefficient was evaluated using the following relation of water uptake versus heat-treatment time [7],  1=2 Dt Mt ¼ 4Cs0  ; ð1Þ p

absorbance

glass-ceramic

glass

4000

3500

3000

wavenumber / cm

2500

-1

Fig. 3. IR absorbance spectra of water-related species, for glass and glass-ceramic. Both specimens were 1 mm thick.

for this glass is known to be 74 (lglass /molH2O cm) [6]. The corresponding value for the glass-ceramic was estimated from the comparison of the absorbances of the two peaks shown in Fig. 3. Since the changes in the total water content as well as the specimen thickness due to the crystallization are small, the extinction coefficient, e, is considered to be proportional to the absorbance. This assumption yielded 119 (lglass-ceramic /molH2O cm) as the extinction coefficient, e, for the glass-ceramic. After the water uptake was measured as a function of heat-treatment time at various temperatures, the surface concentrations of water in the samples had to be determined to obtain the diffusion coefficient. For this purpose, one side of the specimen surface was successively polished off, while maintaining the optically smooth surface, several lm at a time, and the residual absorbance of the IR peak was measured. The depth removed by polishing was estimated from the measured weight loss, surface area, and density of the specimen. Then, the concentration of water at the surface, Cs , was calculated from the slope of the residual absorbance versus depth relation at the surface. Since the sample initially contained a uniform concentration of water, C0 , this amount was subtracted from the measured surface concentration, Cs , to obtain the surface concentration corresponding to water diffusion, Cs0 , i.e.

where Mt (absorbance) is the amount of water absorbed from both surfaces of the sample, expressed in terms of absorbance, Cs0 (absorbance/ cm) is the increased surface concentration of water during heat-treatment expressed in terms of the absorbance per unit specimen thickness, D (cm2 /s) is the diffusion coefficient, and t (s) is the heattreatment time. This equation was derived under the boundary condition of a constant surface concentration, constant diffusion coefficient and semi-infinite media with two surfaces. Physical meanings of various parameters are explained schematically in Fig. 4. Water diffusion during dehydration was also measured by heat-treating samples in a dry atmosphere. The dry atmosphere was created by passing air through a cold trap, kept at approximately )150 °C, and then into the furnace. The water vapor pressure in the dry atmosphere was estimated to be 0.7 Torr from a water uptake measurement of water-free silica glass. The corresponding water solubility or surface concentration

Water concentration

58

Cs Mt / 2 Cs’

C0 Distance surface

Mt : amount of water absorbed from both surfaces Cs : surface water concentration at the surface after heat-treatment C0 : original water concentration Cs’ : increased water concentration during heat-treatment

Fig. 4. Schematic diagram of the water (hydroxyl) concentration profile on one side of the specimen, after a diffusion heattreatment for time, t.

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

in the specimen was estimated using the surface concentration in the wet atmosphere and assuming that the water solubility is proportional to the square root of the water vapor pressure. When the samples were heat-treated for dehydration, the water content decreased, yet Eq. (1) can still be used to evaluate the water diffusion coefficient. In this case, however, Mt represents the amount of water removed and the surface concentration Cs0 is replaced by the initial water concentration, C0 , minus the residual concentration, Cr , the latter being the water solubility in dry atmosphere. IR and near IR absorption spectra were obtained at each heat-treatment stage of crystallization shown in Fig. 1 in order to investigate the changing structure of water in the samples. Samples 1 mm thick having optically smooth surfaces were used. The error in the IR absorbance measurement was estimated to be approximately 0.003 (absorbance unit) from the measurements of several samples treated in the same manner. The error range in the water concentration and diffusion coefficient were estimated using the error in the absorbance measurement.

59

Fig. 5. Absorbance increase of the water-related IR peak plotted against the square root of heat-treatment time at various temperatures. The values for the glass-ceramic were divided by 1.67 to normalize their water contents to the same scale as those for the glass.

3. Results The time dependence of the absorbance of the water-related peak at various heat-treatment temperatures is shown in Fig. 5, plotted against the square root of the heat-treatment time. The absorbance values for the glass-ceramic samples were normalized to the absorbance values of the glass samples by dividing the absorbance change by 1.67, the ratio of the extinction coefficients. It is then possible to directly compare the water uptake in the glass and glass-ceramic. The absorbance change for the glass was greater than that of the glass-ceramic, by a factor of three or more, under any given experimental condition. The residual absorbance depth profiles are shown in Fig. 6 for selected glass samples. The depth profile near the surface was fitted with straight lines, as shown in the figure, and the surface concentrations were obtained by taking the negative values of these slopes. The corresponding

Fig. 6. Depth profile of the residual absorbance after heattreatment at selected temperature and time under 355 Torr (4.75  104 Pa) of water vapor. The dashed line indicates the background residual absorbance, which includes both the amount of water originally existed in the glass sample and the amount of water diffused into the specimen on the opposite side, which is not polished. The solid line represents residual absorbance depth profile of original, unheat-treated glass.

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S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

background residual absorbance is also shown as a dashed line in the Fig. 6 for comparison. The background includes the amount of water existed in the sample prior to the diffusion heat-treatment as well as the amount diffused into the specimen on the opposite side of the surface, which is not removed by the polishing process. The residual absorbance depth profile of original, unheat-treated glass is also shown as a solid line. The obtained surface concentrations, Cs , were used as the solubility. Fig. 7 shows the surface water concentration, Cs , in ppm of water against reciprocal heat-treatment temperature under a wet atmosphere with a water vapor pressure of 355 Torr (4.75  104 Pa). The original amount of water in the glass, which is the same for a glass-ceramic, is shown in Fig. 7 as a dashed line. For the glass, the surface water concentration decreased with increasing temperature. A similar trend was observed for silica glass in 550–850 °C temperature range [8]. In the glassceramic, the surface water concentration after the heat-treatment in the wet atmosphere was almost the same as the initial value. Since the calculation of the diffusion coefficient by Eq. (1) requires a difference between the surface concentration and the initial concentration, this made it impossible to

Fig. 7. Surface water concentration against reciprocal heattreatment temperature under a water vapor pressure of 355 Torr (4.75  104 Pa). The dashed line indicates the original amount of water in the glasses, as well as in the glass-ceramics.

obtain a reliable value of the diffusion coefficient for the glass-ceramic by the present method. Therefore, only the water diffusion coefficient in the glass could be obtained during hydration. The calculated diffusion coefficients in the wet atmosphere (hydration) for the glass and in the dry atmosphere (dehydration) for the glass and glassceramic are shown in Fig. 8. The diffusion coefficient of water during hydration was higher than that during dehydration for the glass, with the activation energy being approximately 179 kJ/mol. The activation energy for water diffusion during dehydration appears the same although the number of data points is limited. A similar trend has been observed for silica glass, with the diffusion coefficient during hydration being higher than during dehydration and the activation energy being nearly the same for both processes [9]. The diffusion coefficients of water in the glass-ceramic were smaller than those in the glass, at least during dehydration. IR and near IR spectra at various stages of crystallization are shown in Fig. 9. The effect of water diffusion during the crystallization heattreatment on these spectra is negligible since the estimated water diffusion distance is less than 20 lm while the specimen thickness is approximately 1 mm. The spectral changes observed here, therefore, reflect the structural changes in the bulk of the specimen. The numbers adjacent to the spectra

Fig. 8. Diffusion coefficient of water in glass and glass-ceramics as a function of temperature.

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

appears that the amount of hydroxyl decreases with the progress of crystallization. The other peak observed around 3900 cm1 increases with crystallization. These two peaks changed most conspicuously between stage 3 and stage 4 of the crystallization. Additionally, there is an increase in the broad background in the wavenumber range of 5000–6000 cm1 .

absorbance / thickness (cm-1)

7 4~6 6 5 4

1~3

3 2

4. Discussion

1 0 6000

5000

4000

3000

2000

-1

Wavenumber ( cm ) Fig. 9. IR and near IR absorption spectra at each stage of the crystallization process described in Fig. 1.

indicated in Fig. 9 correspond to the numbers given in Fig. 1. The main water-related peak around 3400–3500 cm1 shifted with the progress of crystallization. The greatest spectral change was observed between stages 3 and 4. The peak position shifted towards lower wavenumber and the shape of the spectra became sharper. The higher wavenumber portion of Fig. 9 was magnified and is shown in Fig. 10. The peak observed around 4500 cm1 corresponds to the OH band [10,11]. It

0.15 OH 0.10 glass-ceramic

-1

absorbance / thickness (cm )

61

0.05

4~ 6

0.00 6000

5500

5000

4500

4000

3500

4000

3500

0.15 OH

0.10 glass 1~ 3 0.05

0.00 6000

5500

5000

4500 -1

wavenumber (cm )

Fig. 10. Magnified spectra of Fig. 9 in the wavenumber range from 3500 to 6000 cm1 .

It can be seen in Fig. 5 that the amount of water entry into the glass-ceramic is much less than that into the glass. The glass-ceramic is estimated to have only 20 wt% glassy matrix phase with the rest being a crystalline phase. Since, in general, the water solubility in the crystalline phase is much less than that in the glassy phase [4,5], water is expected to accumulate in the glassy matrix phase of the glass-ceramic. During the crystallization heat-treatment, the diffusion distance of water is estimated to be 10–20 lm, and most of the water that initially existed in the glass sample would remain; the total water content of the specimen remains essentially unchanged but a lot of water has accumulated in the minor glassy phase. If all of the original water in the glass, 345 ppm, accumulated in the glassy phase of the glass-ceramic, the water concentration would become 1725 ppm on average. This high water concentration makes it difficult for additional water to enter into the glassceramic specimen even under the high water vapor pressure condition of 355 Torr (4.75  104 Pa). The structure of water in the glass appears to change during crystallization. From the data in Fig. 10 it is clear that the peak around 4500 cm1 , representative of the hydroxyl content, decreases during crystallization, especially between stages 3 and 4 of crystallization. Fig. 2 showed that the greatest change in crystallinity took place at the same time. Therefore, the change in the FTIR spectra between stages 3 and 4 appears to be related to the change in % crystallinity. The decrease in the 4500 cm1 peak is accompanied by an increase in the 3900 cm1 peak. The origin of this peak at 3900 cm1 is not clear at present but a peak at a similar position, 3850 cm1 , was

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

observed as a shoulder in a silica glass containing comparatively high concentrations of hydroxyl [12,13] as well as on the surface of aerosil [14] and in porous silica glass [15]. This band in bulk silica glass is attributed to a combination band between OH and SiO2 vibrations [12], while the peak on the areosil surface is attributed to the combination of the hydroxyl stretching and torsion vibrations [14,15]. At any rate, these peaks are observed only when the water content is high. In general water in silica glasses undergoes the following reaction:

concentration of [OH] or [H2O]

62

H2 O

OH

BSiAOASiB þ H2 O $ BSiOH þ HOSiB; where a short bar indicates a chemical bond. The equilibrium constant, K, of the reaction is given approximately by

total water concentration Fig. 11. Schematic variation of hydroxyl and molecular water concentrations with the total water content in a glass.

2

K ¼ ½OH =½H2 O ; where [OH] and [H2 O] are the activities of hydroxyl and molecular water, respectively, and the activity of silica is taken to be unity when the water content is small. A similar reaction is believed to take place for silicate glasses [16]. When the total water content is increased while maintaining a constant equilibrium constant, K, the fraction of molecular water increases. The expected trend of the change of hydroxyl and molecular water concentration with increasing total water content is shown schematically in Fig. 11 [16]. When the water content is low, most of the water is in the form of hydroxyl [OH] and the molecular water is below the detection limit of currently available IR spectrometers. This is the case with most commercial glasses, including the present glass. However, in glasses containing a high concentration of water, molecular water has been observed [10,11]. The present results can be explained in terms of water expulsion from the crystalline phase and accumulation into the remaining glassy phase; resulting in a high water concentration in the glassy phase. The peak shift shown in Fig. 3 is also consistent with this interpretation. In silica glass, the IR peak at 3670 cm1 is attributed to hydroxyl and the peak near 3400 cm1 is attributed to the all types of molecular water while the peak at 3200 cm1 is attributed to free molecular water [17]. The IR peak shift due to

crystallization observed in the present system appears to support the view that the amount of molecular water increases at the expense of hydroxyl, although IR peak wavenumber of water bands can also be affected by the matrix glass composition. Molecular water, however, is known to have an absorption peak around 5200 cm1 [10,11] but this peak was not clearly visible in the IR spectra. Instead, a broad spectral feature was observed at this wavenumber. In this wavenumber range, there are many vibrational modes related to molecular water and related structures [18]. This might suggest that the OH changed to a H2 O-like structure during crystallization but was not completely converted to molecular water, H2 O.

5. Conclusion The diffusion of water, as well as its structural change, during the crystallization of a LAS glass system has been investigated. It was found that both water uptake and water diffusion were greater in the glass than in the glass-ceramic. It was also found that the OH in the glass seems to change its structure to a H2 O-like condition during crystallization. This was attributed to the low water solubility of the crystalline phase and the segregation and accumulation of water into the minor glassy phase during crystallization.

S. Fujita et al. / Journal of Non-Crystalline Solids 320 (2003) 56–63

Acknowledgements This research was supported by Nippon Electric Glass Co., Ltd. Careful reading of the manuscript by Dr Robert Hepburn of Rensselaer is greatly appreciated.

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