- Email: [email protected]
Preparation of high purity, low water content fused silica glass
Journal of Non-Crystalline Solids 296 (2001) 102±106
www.elsevier.com/locate/jnoncrysol
Preparation of high purity, low water content fused silica glass Minoru Tomozawa a
a,*
, Dong-Lae Kim
a,1
, Victor Lou
b
Materials Science and Engineering Department, Rensselaer Polytechnic Institute, 110 8th street, Troy, NY 12180-3590, USA b General Electric Corporate Research and Development Center, Schenectady, NY 12345, USA Received 18 May 2001
Abstract High purity, low water content silica glasses were prepared by melting crystallized, sol±gel-derived powders. Sol±gelderived amorphous powders have been used to produce high purity silica glass, but a small quantity of water (hydroxyl) tends to remain in the resulting silica glass. High purity crystalline silica powders were prepared by heattreating the sol±gel-derived amorphous powders in a silica crucible. Both amorphous silica powders and crystallized silica powders were melted in vacuum at 1800 °C for 1 h. The water content of the glass from amorphous silica powders was 44 ppm while that of the glass from the crystallized silica powder was less than 0.1 ppm. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction High purity silica glasses are widely used in both the electronics and in optical communications. Impurities are often sources of poor electrical, optical and mechanical characteristics as well as the origins of the premature degradation of thermal, chemical and mechanical characteristics. Often, conventional silica glasses contain impurities such as Al2 O3 , TiO2 , alkalis oxides, water (hydroxyl, ±OH) and chlorine. These impurities are either impurities in the raw materials (e.g., Al2 O3 , TiO2 ) in the case of fused silica or added deliberately to eliminate more detrimental impurities (e.g., Cl addition to re* Corresponding author. Tel.: +1-518 276 6659; fax: +1-518 276 8554. E-mail address: [email protected] (M. Tomozawa). 1 Present address: Kumgang Korea Chemical Co., Ltd., 85 Mabook-Ri, Koosung-Eub, Yongin-Si, Kyungki-Do 449-910, South Korea.
move hydroxyl) in the case of SiCl4 -derived silica glass. In many cases, it is desirable to have no impurities in silica glasses to achieve the highest quality. Sol±gel processing can yield high purity amorphous powders, which can be used as a raw material for glass melting; Mitsubishi Chemical Corporation in Japan [1] produces such materials. It is reported that silica powder made by this method has much lower concentration of impurities compared with natural quartz powder commonly used for silica glass melting. Furthermore, the silica glass prepared by melting the sol±gel-derived silica powder had neither optical absorption at 240 nm nor corresponding photoluminescence which are due to oxygen vacancy [2,3]. The sol±gel-derived silica glasses are clearly purer and have superior optical properties. One diculty of this method is the high water (hydroxyl) concentration remaining in the glass products. Even when the glass melting is conducted in vacuum, it was reported [1], hy-
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 7 7 - 8
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
droxyl concentration in the range 5±30 ppm remains in the glass. Melting of the crystalline silica powder, e.g., quartz powder, using a similar process can produce a silica glass with much lower hydroxyl concentration. Apparently it is easier to remove water from crystalline silica powder than from amorphous silica powder. Water has detrimental eects on many properties of glasses, including silica glasses. For example, it is well known that water increases the optical transmission loss [4]. Additionally, water causes the quenching of the laser eect of rare-earth elements in glasses and decreases their photoluminescence lifetime [5±7]. This impairs the characteristics of the optical ®ber ampli®ers. Furthermore, water causes deterioration of thermal, chemical and mechanical properties [8]. Recently, it was found that the density of brownish rings formed at the interface of a silica glass crucible and silicon melt increased drastically with increasing water content in silica glass [9]. Thus, it is essential to minimize the hydroxyl concentration in high quality silica glass products. Amorphous silica, including those made by the sol±gel process, can be transformed into a crystalline phase, usually cristobalite, by an appropriate heat-treatment especially when a small quantity of alkali is added [10]. Taking advantage of this feature, and knowing that water removal is easier from crystalline silica powder, Seki et al. [11] successfully produced silica glasses with low hydroxyl concentration by melting crystallized silica powders, which were prepared by heat-treating amorphous silica powders intentionally mixed with a small amount of the crystallization-promoting agent such as alkali. However, the complete removal of the added alkali from the silica glass product is dicult. It is desirable to crystallize amorphous silica powder without introducing impurities in order to use the crystalline powder as raw material for high purity silica glass. Although the process of crystallization of pure silica is slow, once small quantity of crystalline silica is obtained this can readily cause the crystallization of the amorphous powders.
103
2. Experimental procedure 2.1. Preparation and characterization of crystalline silica powders Amorphous silica glass powders made by a sol± gel process were provided by Mitsubishi Chemical Corporation. The average grain size of the powder was approximately 200 lm and the water content was reported to be 36 ppm. First, a small quantity (10 g) of the silica glass powder was placed into a platinum crucible (4.5 cm diameter, 4.5 cm height) with a platinum cover and was heated to 1400 °C for 48 h using a box furnace made by CM Inc. This procedure produced crystallized silica powder by the surface crystallization nucleated by platinum [12]. The crystallized silica powder was mixed with the fresh amorphous silica powder such that the crystallized powder constitutes approximately 15% of the mixture. The mixture silica glass powder was placed into a silica crucible (4.5 cm diameter, 5 cm height) and covered with a silica plate to avoid contamination from the furnace. The powders were heat-treated in a box furnace at 1350 °C in air for 24 h, air-quenched and crushed with a silica rod to break up the weakly sintered particles. The heattreated powder surface was slightly opaque because of surface devitri®cation and cracked during cooling presumably due to the b-cristobalite to a-cristobalite transformation at about 250 °C. The powders were reheat-treated for an additional 24 h at 1350 °C and air-quenched. The reheat-treated powders were not sintered this time because the grain surface transformed to cristobalite during the ®rst heat-treatment. To complete the crystallization, the powders were heated at 1400 °C in air for 24 h and air-quenched. This last step heat-treatment at 1400 °C for 24 h was repeated twice more. Thus the total sequence of the heat-treatment of the powder was: 1350 °C, 24 h; 1350 °C, 24 h; 1400 °C, 24 h; 1400 °C, 24 h and 1400 °C, 24 h. During the crystallization, water can accumulate in glass ahead of crystallization front [13], but the cracks can help release the water from glass to air. The crystalline phase was analyzed using the X-ray diractometer. The hydroxyl content in the powder was determined by measuring IR absorbance of the powder immersed in a reagent-grade, index
104
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
matching liquid, CCl4 . A weighted amount of the powders was placed, together with CCl4 , in a sample holder made of fused silica and IR absorbance was measured. The cell length of the sample holder was 1 mm and the absorbance of the sample holder was below the detection limit. The eective thickness of the sample was calculated from the mass of the sample powders used and the density of silica powders. The water content in the silica powders was calculated from the absorbance at 3670 cm 1 , using the extinction coecient of 77.5 l (of glass)/mol (OH)-cm [14] and the eective thickness of the sample. 2.2. Melting and characterization of high purity, low water content silica glass The powder, both amorphous and crystallized, was melted using silica crucibles in vacuum better than 10 4 torr at approximately 1800 °C for 60 min. The glass samples were cut into approximately 4.5 mm thick pieces and polished and their IR spectra were obtained. The water content was again determined from the absorption coecient at 3670 cm 1 using the extinction coecient of 77.5 l (of glass)/mol (OH)-cm [14].
3. Results
Fig. 2. IR spectra of SiO2 powders. Solid line: IR absorbance of sol±gel-derived amorphous powder with the eective thickness of 0.67 mm. Dotted line: IR absorbance of crystallized powder with the eective thickness of 0.58 mm.
and after the heat-treatment for crystallization. The original powders are amorphous while the heat-treated samples are cristobalite. Fig. 2 shows the IR absorbance of the powder in the range of hydroxyl band. The as-received powders contained 52 ppm of hydroxyl (OH) while the crystallized powders contained 12 ppm of hydroxyl. Fig. 3 shows the IR spectra of the glass samples with approximate thickness of 4.5 mm. Water content in the glass in terms of hydroxyl was 44 ppm for the sample derived from the amorphous sol±gelderived powder while it was less than the detection limit for the sample derived from the crystallized
Fig. 1 shows the X-ray diraction patterns for sol±gel-derived high purity silica powder before
Fig. 1. X-ray diraction pattern, intensity vs. diraction angle, 2h, of silica powder before and after the heat-treatment for crystallization. Top: sol±gel-derived amorphous powder. Bottom: power heat-treated at 1350 °C for 48 h, Middle: powder heat-treated at 1400 °C for 48 h. The three patterns are vertically displaced from each other by 100 units.
Fig. 3. IR spectra of SiO2 glass samples with thickness 4.5 mm, melted in vacuum at 1800 °C for 60 min. Solid line: IR absorbance for SiO2 glass melted using sol±gel-derived amorphous powder. Dotted line: IR absorbance for SiO2 glass melted using crystallized powder.
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
powder. A thicker specimen with approximately 43 mm thick was cut from the glass derived from the crystallized powder and its IR spectrum was obtained in an attempt to determine the water content. The water content was less than 0.1 ppm. The result clearly shows that the glass sample prepared from crystalline powders has much lower hydroxyl content than the glass sample prepared from amorphous powders. 4. Discussion Low water content of the heat-treated powder is due to the crystallization rather than a simple heating of the powder. The equilibrium water content in amorphous silica at 1300±1400 °C is approximately 150 ppm water when heat-treated in air with the water vapor pressure of 10 torr. If the powder remained amorphous during the heattreatment, the water content in the powder would increase from the initial value of 52 ppm, towards the equilibrium value. The observed water content of 12 ppm in the heat-treated powder is clearly lower than the equilibrium value; therefore, the lower water solubility of crystalline powder compared with the amorphous powder must be the cause of the lower water content in the treated powder. The reason why it is easier to remove water from crystalline silica powder than from amorphous silica powder during the melting in vacuum is not clear at present, although several mechanisms are conceivable. The ®rst mechanism is that amorphous powder tends to fuse together before complete dehydration, trapping the remaining water inside the material while crystalline powder is less likely to do so. The second mechanism is that water solubility is lower in crystalline material than in amorphous material as was mentioned above and the third mechanism is that water diffusion coecient is higher in crystalline material than in amorphous material. Water in silica glass can exist both as molecular water H2 O and hydroxyl (±OH) and these two species are in equilibrium at high temperature by Si±O±Si H2 O¡2SiOH. Usually the concentration of hydroxyl dominates in
105
glasses. The equilibrium constant, K, of the re2 action is given by K ±OH =H2 O, where [±OH] and H2 O are concentrations of hydroxyl and molecular water, respectively, and the concentration of silica is considered to be unity. The molecular water is similar to other gaseous species, and its solubility in crystal is expected to be lower than in glass [15]. Thus if the equilibrium constant, K, is similar for both crystalline material and amorphous material, the solubility of water should be lower in crystalline silica than in amorphous silica. Water diusion in silica glasses is believed to involve the motion of the molecular water H2 O and its reaction with the silica network to form immobile hydroxyl. Under such condition, the eective water diusion coecient, Deff , in silica glass is given by Deff 4DH2 O OH=K, where DH2 O is the diusion coecient of molecular water [16]. This diusion equation appears to agree with the experimental data [16,17] and the water diusion coecient in silica glasses was observed to decrease with decreasing hydroxyl water concentration [18]. If this equation holds to the low hydroxyl concentration, the eective diusion coecient approaches zero as the water (or hydroxyl) content becomes small. This would explain why the removal of the trace amount of water from glasses is dicult. While the similar water diusion mechanism was suggested for crystalline silica [19], a dierent behavior has been observed. Schaer et al. [20], for example, observed that the water diusion in crystalline quartz increased with decreasing water concentration. In this case, the water diusion coecient in crystalline silica remains ®nite even at low water concentration, making it possible to remove the trace amount of water form crystalline silica. 5. Conclusion High purity, low water content fused silica glass was prepared by using pre-crystallized powder of the sol±gel-derived high purity silica powder. The water content in the product silica glass was less than 0.1 ppm.
106
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
Acknowledgements This research was supported by the US Department of Energy, under Grant No. DE-FG3601GO11013. The authors appreciate Mr Keiji Yamahara of Mitsubishi Chemical Corporation for providing the sol±gel-derived high purity silica powder. References [1] K. Shima, A. Utsunomiya, Ceramics 33 (1998) 39 (in Japanese). [2] D.L. Griscom, J. Ceram. Soc., Japan 99 (1991) 923. [3] M. Kohketsu, K. Awazu, H. Kawazoe, M. Yamane, Jpn. J. Appl. Phys. 28 (1989) 615. [4] O. Humfbach, H. Fabian, U. Grezesik, U. Haken, W. Heitmann, J. Non-Cryst. Solids 203 (1996) 19. [5] N.E. Alekseev, V.P. Gapontsev, A.K. Gromov, S.A. Zelentsova, A.A. Izyneev, V.B. Kravchenko, N.A. Paramonova, Yu.P. Rudnitskii, Inorg. Mater. 11 (1975) 270. [6] A.J. Bruce, W.A. Reed, A.E. Neeves, L.R. Copeland, W.H. Crodkiewicz, A. Ligard, in: MRS Symp. Proc., vol. 244, 1992, p. 157. [7] Y. Yan, A.J. Faber, H. deWaal, J. Non-Cryst. Solids 181 (1995) 283.
[8] W.-T. Han, M. Tomozawa, J. Non-Cryst. Solids 127 (1991) 97. [9] A. Ikari, S. Matsuo, K. Terashima, S. Kimura, Jpn. J. Appl. Phys. 35 (1996) 3547. [10] J. Phalippou, M. Prassas, J. Zarzycki, J. Non-Cryst. Solids 48 (1982) 17. [11] H. Seki, H. Morishita, K. Ohno, H. Yokota, Manufacturing method of transparent silica glasses, Japanese Patent 1864078, 1993. [12] P.W. McMillan, Glass-ceramics, 2nd ed., Academic Press, New York, 1979. [13] D.-L. Kim, M. Tomozawa, J. Non-Cryst. Solids 279 (2000) 179. [14] K.M. Davis, A. Agarwal, M. Tomozawa, K. Hirao, J. Non-Cryst. Solids 203 (1996) 27. [15] J.E. Shelby, Handbook of Gas Diusion in Solids and Melts, ASM International, Materials Park, OH, 1996. [16] R.H. Doremus, in: J.W. Mitchell, R.C. DeVries, R.W. Roberts, P. Cannon (Eds.), Reactivity of Solids, Wiley, New York, 1969, p. 667. [17] K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1995) 203. [18] G.J. Roberts, J.P. Roberts, Phys. Chem. Glasses 5 (1964) 26. [19] R.H. Doremus, Earth Planet. Sci. Lett. 163 (1998) 43. [20] E.W. Schaer, J. Shi-LanSang, A.R. Cooper, A.H. Heuer, in: A.W. Hofmann, B.J. Gilleti, H.S. Yoder Jr., A.R. Yund (Eds.), Geochemical Transport Kinetics, vol. 634, Carnegie Institute Publication, Washington, DC, 1974, p. 131.
www.elsevier.com/locate/jnoncrysol
Preparation of high purity, low water content fused silica glass Minoru Tomozawa a
a,*
, Dong-Lae Kim
a,1
, Victor Lou
b
Materials Science and Engineering Department, Rensselaer Polytechnic Institute, 110 8th street, Troy, NY 12180-3590, USA b General Electric Corporate Research and Development Center, Schenectady, NY 12345, USA Received 18 May 2001
Abstract High purity, low water content silica glasses were prepared by melting crystallized, sol±gel-derived powders. Sol±gelderived amorphous powders have been used to produce high purity silica glass, but a small quantity of water (hydroxyl) tends to remain in the resulting silica glass. High purity crystalline silica powders were prepared by heattreating the sol±gel-derived amorphous powders in a silica crucible. Both amorphous silica powders and crystallized silica powders were melted in vacuum at 1800 °C for 1 h. The water content of the glass from amorphous silica powders was 44 ppm while that of the glass from the crystallized silica powder was less than 0.1 ppm. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction High purity silica glasses are widely used in both the electronics and in optical communications. Impurities are often sources of poor electrical, optical and mechanical characteristics as well as the origins of the premature degradation of thermal, chemical and mechanical characteristics. Often, conventional silica glasses contain impurities such as Al2 O3 , TiO2 , alkalis oxides, water (hydroxyl, ±OH) and chlorine. These impurities are either impurities in the raw materials (e.g., Al2 O3 , TiO2 ) in the case of fused silica or added deliberately to eliminate more detrimental impurities (e.g., Cl addition to re* Corresponding author. Tel.: +1-518 276 6659; fax: +1-518 276 8554. E-mail address: [email protected] (M. Tomozawa). 1 Present address: Kumgang Korea Chemical Co., Ltd., 85 Mabook-Ri, Koosung-Eub, Yongin-Si, Kyungki-Do 449-910, South Korea.
move hydroxyl) in the case of SiCl4 -derived silica glass. In many cases, it is desirable to have no impurities in silica glasses to achieve the highest quality. Sol±gel processing can yield high purity amorphous powders, which can be used as a raw material for glass melting; Mitsubishi Chemical Corporation in Japan [1] produces such materials. It is reported that silica powder made by this method has much lower concentration of impurities compared with natural quartz powder commonly used for silica glass melting. Furthermore, the silica glass prepared by melting the sol±gel-derived silica powder had neither optical absorption at 240 nm nor corresponding photoluminescence which are due to oxygen vacancy [2,3]. The sol±gel-derived silica glasses are clearly purer and have superior optical properties. One diculty of this method is the high water (hydroxyl) concentration remaining in the glass products. Even when the glass melting is conducted in vacuum, it was reported [1], hy-
0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 8 7 7 - 8
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
droxyl concentration in the range 5±30 ppm remains in the glass. Melting of the crystalline silica powder, e.g., quartz powder, using a similar process can produce a silica glass with much lower hydroxyl concentration. Apparently it is easier to remove water from crystalline silica powder than from amorphous silica powder. Water has detrimental eects on many properties of glasses, including silica glasses. For example, it is well known that water increases the optical transmission loss [4]. Additionally, water causes the quenching of the laser eect of rare-earth elements in glasses and decreases their photoluminescence lifetime [5±7]. This impairs the characteristics of the optical ®ber ampli®ers. Furthermore, water causes deterioration of thermal, chemical and mechanical properties [8]. Recently, it was found that the density of brownish rings formed at the interface of a silica glass crucible and silicon melt increased drastically with increasing water content in silica glass [9]. Thus, it is essential to minimize the hydroxyl concentration in high quality silica glass products. Amorphous silica, including those made by the sol±gel process, can be transformed into a crystalline phase, usually cristobalite, by an appropriate heat-treatment especially when a small quantity of alkali is added [10]. Taking advantage of this feature, and knowing that water removal is easier from crystalline silica powder, Seki et al. [11] successfully produced silica glasses with low hydroxyl concentration by melting crystallized silica powders, which were prepared by heat-treating amorphous silica powders intentionally mixed with a small amount of the crystallization-promoting agent such as alkali. However, the complete removal of the added alkali from the silica glass product is dicult. It is desirable to crystallize amorphous silica powder without introducing impurities in order to use the crystalline powder as raw material for high purity silica glass. Although the process of crystallization of pure silica is slow, once small quantity of crystalline silica is obtained this can readily cause the crystallization of the amorphous powders.
103
2. Experimental procedure 2.1. Preparation and characterization of crystalline silica powders Amorphous silica glass powders made by a sol± gel process were provided by Mitsubishi Chemical Corporation. The average grain size of the powder was approximately 200 lm and the water content was reported to be 36 ppm. First, a small quantity (10 g) of the silica glass powder was placed into a platinum crucible (4.5 cm diameter, 4.5 cm height) with a platinum cover and was heated to 1400 °C for 48 h using a box furnace made by CM Inc. This procedure produced crystallized silica powder by the surface crystallization nucleated by platinum [12]. The crystallized silica powder was mixed with the fresh amorphous silica powder such that the crystallized powder constitutes approximately 15% of the mixture. The mixture silica glass powder was placed into a silica crucible (4.5 cm diameter, 5 cm height) and covered with a silica plate to avoid contamination from the furnace. The powders were heat-treated in a box furnace at 1350 °C in air for 24 h, air-quenched and crushed with a silica rod to break up the weakly sintered particles. The heattreated powder surface was slightly opaque because of surface devitri®cation and cracked during cooling presumably due to the b-cristobalite to a-cristobalite transformation at about 250 °C. The powders were reheat-treated for an additional 24 h at 1350 °C and air-quenched. The reheat-treated powders were not sintered this time because the grain surface transformed to cristobalite during the ®rst heat-treatment. To complete the crystallization, the powders were heated at 1400 °C in air for 24 h and air-quenched. This last step heat-treatment at 1400 °C for 24 h was repeated twice more. Thus the total sequence of the heat-treatment of the powder was: 1350 °C, 24 h; 1350 °C, 24 h; 1400 °C, 24 h; 1400 °C, 24 h and 1400 °C, 24 h. During the crystallization, water can accumulate in glass ahead of crystallization front [13], but the cracks can help release the water from glass to air. The crystalline phase was analyzed using the X-ray diractometer. The hydroxyl content in the powder was determined by measuring IR absorbance of the powder immersed in a reagent-grade, index
104
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
matching liquid, CCl4 . A weighted amount of the powders was placed, together with CCl4 , in a sample holder made of fused silica and IR absorbance was measured. The cell length of the sample holder was 1 mm and the absorbance of the sample holder was below the detection limit. The eective thickness of the sample was calculated from the mass of the sample powders used and the density of silica powders. The water content in the silica powders was calculated from the absorbance at 3670 cm 1 , using the extinction coecient of 77.5 l (of glass)/mol (OH)-cm [14] and the eective thickness of the sample. 2.2. Melting and characterization of high purity, low water content silica glass The powder, both amorphous and crystallized, was melted using silica crucibles in vacuum better than 10 4 torr at approximately 1800 °C for 60 min. The glass samples were cut into approximately 4.5 mm thick pieces and polished and their IR spectra were obtained. The water content was again determined from the absorption coecient at 3670 cm 1 using the extinction coecient of 77.5 l (of glass)/mol (OH)-cm [14].
3. Results
Fig. 2. IR spectra of SiO2 powders. Solid line: IR absorbance of sol±gel-derived amorphous powder with the eective thickness of 0.67 mm. Dotted line: IR absorbance of crystallized powder with the eective thickness of 0.58 mm.
and after the heat-treatment for crystallization. The original powders are amorphous while the heat-treated samples are cristobalite. Fig. 2 shows the IR absorbance of the powder in the range of hydroxyl band. The as-received powders contained 52 ppm of hydroxyl (OH) while the crystallized powders contained 12 ppm of hydroxyl. Fig. 3 shows the IR spectra of the glass samples with approximate thickness of 4.5 mm. Water content in the glass in terms of hydroxyl was 44 ppm for the sample derived from the amorphous sol±gelderived powder while it was less than the detection limit for the sample derived from the crystallized
Fig. 1 shows the X-ray diraction patterns for sol±gel-derived high purity silica powder before
Fig. 1. X-ray diraction pattern, intensity vs. diraction angle, 2h, of silica powder before and after the heat-treatment for crystallization. Top: sol±gel-derived amorphous powder. Bottom: power heat-treated at 1350 °C for 48 h, Middle: powder heat-treated at 1400 °C for 48 h. The three patterns are vertically displaced from each other by 100 units.
Fig. 3. IR spectra of SiO2 glass samples with thickness 4.5 mm, melted in vacuum at 1800 °C for 60 min. Solid line: IR absorbance for SiO2 glass melted using sol±gel-derived amorphous powder. Dotted line: IR absorbance for SiO2 glass melted using crystallized powder.
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
powder. A thicker specimen with approximately 43 mm thick was cut from the glass derived from the crystallized powder and its IR spectrum was obtained in an attempt to determine the water content. The water content was less than 0.1 ppm. The result clearly shows that the glass sample prepared from crystalline powders has much lower hydroxyl content than the glass sample prepared from amorphous powders. 4. Discussion Low water content of the heat-treated powder is due to the crystallization rather than a simple heating of the powder. The equilibrium water content in amorphous silica at 1300±1400 °C is approximately 150 ppm water when heat-treated in air with the water vapor pressure of 10 torr. If the powder remained amorphous during the heattreatment, the water content in the powder would increase from the initial value of 52 ppm, towards the equilibrium value. The observed water content of 12 ppm in the heat-treated powder is clearly lower than the equilibrium value; therefore, the lower water solubility of crystalline powder compared with the amorphous powder must be the cause of the lower water content in the treated powder. The reason why it is easier to remove water from crystalline silica powder than from amorphous silica powder during the melting in vacuum is not clear at present, although several mechanisms are conceivable. The ®rst mechanism is that amorphous powder tends to fuse together before complete dehydration, trapping the remaining water inside the material while crystalline powder is less likely to do so. The second mechanism is that water solubility is lower in crystalline material than in amorphous material as was mentioned above and the third mechanism is that water diffusion coecient is higher in crystalline material than in amorphous material. Water in silica glass can exist both as molecular water H2 O and hydroxyl (±OH) and these two species are in equilibrium at high temperature by Si±O±Si H2 O¡2SiOH. Usually the concentration of hydroxyl dominates in
105
glasses. The equilibrium constant, K, of the re2 action is given by K ±OH =H2 O, where [±OH] and H2 O are concentrations of hydroxyl and molecular water, respectively, and the concentration of silica is considered to be unity. The molecular water is similar to other gaseous species, and its solubility in crystal is expected to be lower than in glass [15]. Thus if the equilibrium constant, K, is similar for both crystalline material and amorphous material, the solubility of water should be lower in crystalline silica than in amorphous silica. Water diusion in silica glasses is believed to involve the motion of the molecular water H2 O and its reaction with the silica network to form immobile hydroxyl. Under such condition, the eective water diusion coecient, Deff , in silica glass is given by Deff 4DH2 O OH=K, where DH2 O is the diusion coecient of molecular water [16]. This diusion equation appears to agree with the experimental data [16,17] and the water diusion coecient in silica glasses was observed to decrease with decreasing hydroxyl water concentration [18]. If this equation holds to the low hydroxyl concentration, the eective diusion coecient approaches zero as the water (or hydroxyl) content becomes small. This would explain why the removal of the trace amount of water from glasses is dicult. While the similar water diusion mechanism was suggested for crystalline silica [19], a dierent behavior has been observed. Schaer et al. [20], for example, observed that the water diusion in crystalline quartz increased with decreasing water concentration. In this case, the water diusion coecient in crystalline silica remains ®nite even at low water concentration, making it possible to remove the trace amount of water form crystalline silica. 5. Conclusion High purity, low water content fused silica glass was prepared by using pre-crystallized powder of the sol±gel-derived high purity silica powder. The water content in the product silica glass was less than 0.1 ppm.
106
M. Tomozawa et al. / Journal of Non-Crystalline Solids 296 (2001) 102±106
Acknowledgements This research was supported by the US Department of Energy, under Grant No. DE-FG3601GO11013. The authors appreciate Mr Keiji Yamahara of Mitsubishi Chemical Corporation for providing the sol±gel-derived high purity silica powder. References [1] K. Shima, A. Utsunomiya, Ceramics 33 (1998) 39 (in Japanese). [2] D.L. Griscom, J. Ceram. Soc., Japan 99 (1991) 923. [3] M. Kohketsu, K. Awazu, H. Kawazoe, M. Yamane, Jpn. J. Appl. Phys. 28 (1989) 615. [4] O. Humfbach, H. Fabian, U. Grezesik, U. Haken, W. Heitmann, J. Non-Cryst. Solids 203 (1996) 19. [5] N.E. Alekseev, V.P. Gapontsev, A.K. Gromov, S.A. Zelentsova, A.A. Izyneev, V.B. Kravchenko, N.A. Paramonova, Yu.P. Rudnitskii, Inorg. Mater. 11 (1975) 270. [6] A.J. Bruce, W.A. Reed, A.E. Neeves, L.R. Copeland, W.H. Crodkiewicz, A. Ligard, in: MRS Symp. Proc., vol. 244, 1992, p. 157. [7] Y. Yan, A.J. Faber, H. deWaal, J. Non-Cryst. Solids 181 (1995) 283.
[8] W.-T. Han, M. Tomozawa, J. Non-Cryst. Solids 127 (1991) 97. [9] A. Ikari, S. Matsuo, K. Terashima, S. Kimura, Jpn. J. Appl. Phys. 35 (1996) 3547. [10] J. Phalippou, M. Prassas, J. Zarzycki, J. Non-Cryst. Solids 48 (1982) 17. [11] H. Seki, H. Morishita, K. Ohno, H. Yokota, Manufacturing method of transparent silica glasses, Japanese Patent 1864078, 1993. [12] P.W. McMillan, Glass-ceramics, 2nd ed., Academic Press, New York, 1979. [13] D.-L. Kim, M. Tomozawa, J. Non-Cryst. Solids 279 (2000) 179. [14] K.M. Davis, A. Agarwal, M. Tomozawa, K. Hirao, J. Non-Cryst. Solids 203 (1996) 27. [15] J.E. Shelby, Handbook of Gas Diusion in Solids and Melts, ASM International, Materials Park, OH, 1996. [16] R.H. Doremus, in: J.W. Mitchell, R.C. DeVries, R.W. Roberts, P. Cannon (Eds.), Reactivity of Solids, Wiley, New York, 1969, p. 667. [17] K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1995) 203. [18] G.J. Roberts, J.P. Roberts, Phys. Chem. Glasses 5 (1964) 26. [19] R.H. Doremus, Earth Planet. Sci. Lett. 163 (1998) 43. [20] E.W. Schaer, J. Shi-LanSang, A.R. Cooper, A.H. Heuer, in: A.W. Hofmann, B.J. Gilleti, H.S. Yoder Jr., A.R. Yund (Eds.), Geochemical Transport Kinetics, vol. 634, Carnegie Institute Publication, Washington, DC, 1974, p. 131.