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Thermal annealing and density fluctuations in silica glass
Journal of Non-Crystalline Solids 293±295 (2001) 366±369
www.elsevier.com/locate/jnoncrysol
Thermal annealing and density ¯uctuations in silica glass R. Le Parc a, B. Champagnon a,*, Ph. Guenot b, S. Dubois b a
Laboratoire de Physico-Chimie des Mat eriaux Luminescents, Universit e Lyon 1, Campus La Doua, UMR 5620 CNRS , 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne cedex, France b Alcatel, R & D Fibres Optiques, 53 rue Jean Broutin, 78700 Con¯ans Sainte Honorine, France
Abstract Silica glasses of a same composition with dierent annealing times above and below Tg are characterized by infra-red and Raman spectroscopy in order to determine their ®ctive temperature. The Rayleigh scattering of these dierent samples is shown to decrease when the ®ctive temperature decreases in agreement with a decrease of density ¯uctuations. Some quantitative discrepancies are however shown to occur for long annealing at 950 °C. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
2. Samples and experiments
Vitreous silica although extensively studied for many years still has a poorly de®ned structure [1]. This is usually described in term of short, medium and long range order and each of these is explored by dierent scattering techniques, neutron, X-rays, light, associated with dierent characteristic wavelengths. Among the dierent structural units forming a simple glass, like SiO2 , the existence of density ¯uctuations has been postulated for a long time but due to lack of direct observation, little is known about them. In this paper we will describe how these density ¯uctuations are dependent of the thermal history of the glass and correlate them with the Rayleigh scattering [2].
Samples considered in this paper are fusion glasses, type I with low OH and Al content. The samples considered are as received (sample A), heat treated at 1500°C (sample B) or 1300°C (sample C) for 105 min and then quenched in water. Sample D and E are heat treated for 1 h and 60 h respectively at 1200 and 1100 °C and quenched on a metallic plate. Sample F was heat-treated for 11 days at 950 °C and just removed from the oven. The samples have been optically polished after heat treatment in order to prevent spurious light scattering. Low frequency or elastic light scattering experiments are made with a ®ve stage monochromator Dilor Z40 with an Ar ion laser using the 514 nm line. Scattered light is detected in a direction perpendicular to the incident laser line. Raman scattering in the region 10±600 cm 1 is analysed with a Dilor XY with a CCD camera micro-Raman spectrometer in a direction perpendicular to an Ar ion laser beam. Both incident and scattered light are parallel (VV
* Corresponding author. Tel.: +33-4 72 44 83 34; fax: +33-4 72 44 84 42. E-mail address: [email protected] (B. Champagnon).
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 3 5 - 3
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
367
polarisation). Infrared absorption is determined from a Perkin±Elmer GXII and Auto-image microscope.
3. Results The eect of the thermal treatments, fast quenching or long annealing, can be quanti®ed considering the observed change in the glass structure in infrared or Raman experiments. If the quenching rate of the high temperature samples (B and C) is fast enough to preserve the high temperature structure we can attribute to these samples `®ctive' temperatures, TF , of 1500 and 1300 °C respectively. The plot of the position of the maximum of the infrared absorption band at 2260 cm 1 is then made as function of the inverse of the maximum temperature [3] for the samples B, C and of the temperature of the annealing for samples D and E (Fig. 1). The good correlation obtained for these four samples can be described by the relation m cm 1 40871:4=TF 2229:7:
1
It allows us to determine TF in K for the unknown samples A and F. For sample A we found that TF 1380 40 °C and for sample F we determine TF 1155 40 °C. The Si±O±Si Raman band at 430 cm 1 also shifts linearly with the ®ctive temperature and gives a similar result. Another way to determine the ®ctive temperature of a glass is the correlation of the intensities of the D1 and D2 lines in Raman scattering [4] as discussed in previous papers [5,6]. Using the same procedure (Fig. 2) as for infrared we found the value of TF0 1421 40 °C for the sample A and TF0 1135 40 °C for sample F. The Rayleigh and Brillouin scatterings of the dierent samples are also shown on Fig. 3. The intensity of the elastic scattering decreases as the annealing temperature of the dierent samples decreases. The inset represents the Landau±Placzek ratio IR =2IB of the Rayleigh scattering IR to the Brillouin scattering IB and function of the annealing temperature.
Fig. 1. Plot of the 2260 cm 1 infra-red overtone as function of the inverse of the ®ctive temperature (in K).
4. Discussion Density ¯uctuations are considered as thermodynamic ¯uctuations in the liquid state frozen at the glass transition temperature Tg . However it is well known that a higher temperature structure can be frozen in the solid state by a fast quenching at room temperature, the concept of ®ctive temperature being a way to describe this property of the quenched glass. Low OH and Al content silica glasses, type I in Br uckner notation [7], have a 1013 Poise viscosity for a temperature close to 1200 °C, this value being strongly dependent on the impurity level. Our samples are above and below this temperature. Our experimental results compared
368
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
Fig. 3. Rayleigh scattering intensity for the dierent samples. The arrows at 0.8 cm 1 point the Brillouin lines. The insert represents the Landau±Placzek ratio RLP as function of T.
Fig. 2. Logarithm plot of the intensity of the D1 and D2 Raman lines as function of the inverse of the ®ctive temperature (in K).
two ways to de®ne the ®ctive temperature of the samples : both test the vibrational properties of the material but whereas the infrared considers the Si±O±Si stretching vibrations overtone at 2260 cm 1 , the Raman spectra consider the amount of three and four member rings, D2 and D1 lines, respectively. The values for the unknown samples A and F are slightly dierent but still within the experimental error. Nevertheless it must be pointed out that the ®ctive temperature as determined by the spectroscopic technique probes into one particular part of the atomic structure, Si±O±Si vibration, or three or four members rings. These structure are of course correlated to the relaxation of the glass but not necessarily in a similar way.
The above results show clearly that the sample F has not reached its equilibrium temperature during the heat treatment. The temperature, 950 °C, and the annealing time (264 h) were chosen by reference to the previous paper of Todoroki and Sakagushi [8] where a ®ctive temperature lower than 1000 °C was attained after 156 h. The ®ctive temperature of our sample F (TF 1155 °C) determined in the same way than in [8] is more than 100 °C above although the annealing time was longer. This result illustrates the tremendous importance of the impurity in the relaxation behaviour of pure silica. This is well known for OH content but our sample F contains less than 20 ppm OH and has however a larger ®ctive temperature than the 30 ppm OH sample of the [8]. The small fraction of other impurities like Al which are also contained in these types of silica [3] is supposed to be responsible of for this dierence [9]. A striking eect is observed for Rayleigh scattering (Fig. 3). We observe, as previously shown [5], a decrease of the elastic scattering with the ®ctive temperature except for the F sample which gives the smallest Rayleigh intensity although its ®ctive temperature is higher than that of the E sample. The Rayleigh scattering dependence on TF is usually described by IRayleigh / hDq2 i /
8P3 kB 8 2 n p K T TF ; 3k4
2
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
where dependence of the index n and of the isothermal compressibility KT on TF are neglected. A recent paper of Haken et al. [10] is devoted to the in¯uence of ®ctive temperature on the refractive index of silica for glasses containing dierent concentrations of OH, Cl, F, Al impurities. Even considering that the density ¯uctuations varies as the eighth power of the refractive index, n8 , these variations are still negligible as dn=dT are close to 10 6 K 1 . The dependence of KT on the temperature was analysed by Saito et al. [11]: it shows a constant value above Tg and a discontinuity to reach a value ®ve times smaller below. This discontinuity is likely due to the high cooling rate used in these experiment (100 K/h) which does not allow our accurate observation of the variation of KT near Tg in the region concerned by the ®ctive temperature frozen in the glass. We can suppose that KT decreases continuously in this region and that the frozen value at TF is dierent from that in the supercooled state [12]. These considerations show that the dependance of the Rayleigh intensity considering only a linear dependance on TF (2) is not fully satisfying if very long annealing below Tg are considered. Apart from the above discussion on the formula (2) we must also point out that the ®ctive temperature determined from the spectroscopic methods re¯ects the in¯uence of the relaxation on the short range order of the glass whereas the Rayleigh scattering is due to inhomogeneities on dierent length scales. Relaxation of the glass at these dierent scales are likely not characterized by the same dynamic. 5. Conclusion Density ¯uctuations are dependent on the ®ctive temperature of the silica glass but the deter-
369
mination of this parameter is still a matter of question. Spectroscopic methods, infrared Si±O±Si overtone and D1 and D2 Raman lines lead to determinations in good agreement but the results on long annealed samples show a discrepancy with the observed Rayleigh scattering. Further experimental studies are necessary to ascertain this point of view, ®rst to con®rm the experimental results on the F sample, and by preparing samples in intermediate conditions to establish the law of the deviation from the linear behaviour. The full understanding of these eects is related to the knowledge of the relaxation of the structure of glasses at the dierent scales from the short and medium range order. References [1] A.C. Wright, J. Non-Cryst. Solids 179 (1994) 84. [2] S. Sakaguchi, S. Tadoroki, T. Murata, J. Non-Cryst. Solids 220 (1997) 178. [3] A. Agarwal, K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1995) 191. [4] F.L. Galeener, A.E. Geissberger, Phys. Rev. 28 (1983) 3266. [5] B. Champagnon, C. Chemarin, E. Duval, R. Le Parc, J. Non-Cryst. Solids 274 (2000) 81. [6] C. Chemarin, B. Champagnon, E. Duval, Philos. Mag. 79 (1999) 2057. [7] R. Br uckner, J. Non-Cryst. Solids 5 (1970) 123. [8] S. Todoroki, S. Sakaguchi, J. Ceram. Soc. Jpn. 105 (1997) 377. [9] K. Saito, N. Ogawa, A.J. Ikushima, Y. Tsurita, K. Yamahara, J. Non-Cryst. Solids 270 (2000) 60. [10] U. Haken, O. Humbach, S. Ortner, H. Fabian, J. NonCryst. Solids 265 (2000) 9. [11] K. Saito, H. Kakiuchida, A.J. Ikushima, J. Non-Cryst. Solids 222 (1997) 329. [12] K. Saito, H. Kakiuchida, A.J. Ikushima, J. Appl. Phys. 84 (1998) 3107.
www.elsevier.com/locate/jnoncrysol
Thermal annealing and density ¯uctuations in silica glass R. Le Parc a, B. Champagnon a,*, Ph. Guenot b, S. Dubois b a
Laboratoire de Physico-Chimie des Mat eriaux Luminescents, Universit e Lyon 1, Campus La Doua, UMR 5620 CNRS , 43 Boulevard du 11 Novembre 1918, 69622 Villeurbanne cedex, France b Alcatel, R & D Fibres Optiques, 53 rue Jean Broutin, 78700 Con¯ans Sainte Honorine, France
Abstract Silica glasses of a same composition with dierent annealing times above and below Tg are characterized by infra-red and Raman spectroscopy in order to determine their ®ctive temperature. The Rayleigh scattering of these dierent samples is shown to decrease when the ®ctive temperature decreases in agreement with a decrease of density ¯uctuations. Some quantitative discrepancies are however shown to occur for long annealing at 950 °C. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
2. Samples and experiments
Vitreous silica although extensively studied for many years still has a poorly de®ned structure [1]. This is usually described in term of short, medium and long range order and each of these is explored by dierent scattering techniques, neutron, X-rays, light, associated with dierent characteristic wavelengths. Among the dierent structural units forming a simple glass, like SiO2 , the existence of density ¯uctuations has been postulated for a long time but due to lack of direct observation, little is known about them. In this paper we will describe how these density ¯uctuations are dependent of the thermal history of the glass and correlate them with the Rayleigh scattering [2].
Samples considered in this paper are fusion glasses, type I with low OH and Al content. The samples considered are as received (sample A), heat treated at 1500°C (sample B) or 1300°C (sample C) for 105 min and then quenched in water. Sample D and E are heat treated for 1 h and 60 h respectively at 1200 and 1100 °C and quenched on a metallic plate. Sample F was heat-treated for 11 days at 950 °C and just removed from the oven. The samples have been optically polished after heat treatment in order to prevent spurious light scattering. Low frequency or elastic light scattering experiments are made with a ®ve stage monochromator Dilor Z40 with an Ar ion laser using the 514 nm line. Scattered light is detected in a direction perpendicular to the incident laser line. Raman scattering in the region 10±600 cm 1 is analysed with a Dilor XY with a CCD camera micro-Raman spectrometer in a direction perpendicular to an Ar ion laser beam. Both incident and scattered light are parallel (VV
* Corresponding author. Tel.: +33-4 72 44 83 34; fax: +33-4 72 44 84 42. E-mail address: [email protected] (B. Champagnon).
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 3 5 - 3
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
367
polarisation). Infrared absorption is determined from a Perkin±Elmer GXII and Auto-image microscope.
3. Results The eect of the thermal treatments, fast quenching or long annealing, can be quanti®ed considering the observed change in the glass structure in infrared or Raman experiments. If the quenching rate of the high temperature samples (B and C) is fast enough to preserve the high temperature structure we can attribute to these samples `®ctive' temperatures, TF , of 1500 and 1300 °C respectively. The plot of the position of the maximum of the infrared absorption band at 2260 cm 1 is then made as function of the inverse of the maximum temperature [3] for the samples B, C and of the temperature of the annealing for samples D and E (Fig. 1). The good correlation obtained for these four samples can be described by the relation m cm 1 40871:4=TF 2229:7:
1
It allows us to determine TF in K for the unknown samples A and F. For sample A we found that TF 1380 40 °C and for sample F we determine TF 1155 40 °C. The Si±O±Si Raman band at 430 cm 1 also shifts linearly with the ®ctive temperature and gives a similar result. Another way to determine the ®ctive temperature of a glass is the correlation of the intensities of the D1 and D2 lines in Raman scattering [4] as discussed in previous papers [5,6]. Using the same procedure (Fig. 2) as for infrared we found the value of TF0 1421 40 °C for the sample A and TF0 1135 40 °C for sample F. The Rayleigh and Brillouin scatterings of the dierent samples are also shown on Fig. 3. The intensity of the elastic scattering decreases as the annealing temperature of the dierent samples decreases. The inset represents the Landau±Placzek ratio IR =2IB of the Rayleigh scattering IR to the Brillouin scattering IB and function of the annealing temperature.
Fig. 1. Plot of the 2260 cm 1 infra-red overtone as function of the inverse of the ®ctive temperature (in K).
4. Discussion Density ¯uctuations are considered as thermodynamic ¯uctuations in the liquid state frozen at the glass transition temperature Tg . However it is well known that a higher temperature structure can be frozen in the solid state by a fast quenching at room temperature, the concept of ®ctive temperature being a way to describe this property of the quenched glass. Low OH and Al content silica glasses, type I in Br uckner notation [7], have a 1013 Poise viscosity for a temperature close to 1200 °C, this value being strongly dependent on the impurity level. Our samples are above and below this temperature. Our experimental results compared
368
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
Fig. 3. Rayleigh scattering intensity for the dierent samples. The arrows at 0.8 cm 1 point the Brillouin lines. The insert represents the Landau±Placzek ratio RLP as function of T.
Fig. 2. Logarithm plot of the intensity of the D1 and D2 Raman lines as function of the inverse of the ®ctive temperature (in K).
two ways to de®ne the ®ctive temperature of the samples : both test the vibrational properties of the material but whereas the infrared considers the Si±O±Si stretching vibrations overtone at 2260 cm 1 , the Raman spectra consider the amount of three and four member rings, D2 and D1 lines, respectively. The values for the unknown samples A and F are slightly dierent but still within the experimental error. Nevertheless it must be pointed out that the ®ctive temperature as determined by the spectroscopic technique probes into one particular part of the atomic structure, Si±O±Si vibration, or three or four members rings. These structure are of course correlated to the relaxation of the glass but not necessarily in a similar way.
The above results show clearly that the sample F has not reached its equilibrium temperature during the heat treatment. The temperature, 950 °C, and the annealing time (264 h) were chosen by reference to the previous paper of Todoroki and Sakagushi [8] where a ®ctive temperature lower than 1000 °C was attained after 156 h. The ®ctive temperature of our sample F (TF 1155 °C) determined in the same way than in [8] is more than 100 °C above although the annealing time was longer. This result illustrates the tremendous importance of the impurity in the relaxation behaviour of pure silica. This is well known for OH content but our sample F contains less than 20 ppm OH and has however a larger ®ctive temperature than the 30 ppm OH sample of the [8]. The small fraction of other impurities like Al which are also contained in these types of silica [3] is supposed to be responsible of for this dierence [9]. A striking eect is observed for Rayleigh scattering (Fig. 3). We observe, as previously shown [5], a decrease of the elastic scattering with the ®ctive temperature except for the F sample which gives the smallest Rayleigh intensity although its ®ctive temperature is higher than that of the E sample. The Rayleigh scattering dependence on TF is usually described by IRayleigh / hDq2 i /
8P3 kB 8 2 n p K T TF ; 3k4
2
R. Le Parc et al. / Journal of Non-Crystalline Solids 293±295 (2001) 366±369
where dependence of the index n and of the isothermal compressibility KT on TF are neglected. A recent paper of Haken et al. [10] is devoted to the in¯uence of ®ctive temperature on the refractive index of silica for glasses containing dierent concentrations of OH, Cl, F, Al impurities. Even considering that the density ¯uctuations varies as the eighth power of the refractive index, n8 , these variations are still negligible as dn=dT are close to 10 6 K 1 . The dependence of KT on the temperature was analysed by Saito et al. [11]: it shows a constant value above Tg and a discontinuity to reach a value ®ve times smaller below. This discontinuity is likely due to the high cooling rate used in these experiment (100 K/h) which does not allow our accurate observation of the variation of KT near Tg in the region concerned by the ®ctive temperature frozen in the glass. We can suppose that KT decreases continuously in this region and that the frozen value at TF is dierent from that in the supercooled state [12]. These considerations show that the dependance of the Rayleigh intensity considering only a linear dependance on TF (2) is not fully satisfying if very long annealing below Tg are considered. Apart from the above discussion on the formula (2) we must also point out that the ®ctive temperature determined from the spectroscopic methods re¯ects the in¯uence of the relaxation on the short range order of the glass whereas the Rayleigh scattering is due to inhomogeneities on dierent length scales. Relaxation of the glass at these dierent scales are likely not characterized by the same dynamic. 5. Conclusion Density ¯uctuations are dependent on the ®ctive temperature of the silica glass but the deter-
369
mination of this parameter is still a matter of question. Spectroscopic methods, infrared Si±O±Si overtone and D1 and D2 Raman lines lead to determinations in good agreement but the results on long annealed samples show a discrepancy with the observed Rayleigh scattering. Further experimental studies are necessary to ascertain this point of view, ®rst to con®rm the experimental results on the F sample, and by preparing samples in intermediate conditions to establish the law of the deviation from the linear behaviour. The full understanding of these eects is related to the knowledge of the relaxation of the structure of glasses at the dierent scales from the short and medium range order. References [1] A.C. Wright, J. Non-Cryst. Solids 179 (1994) 84. [2] S. Sakaguchi, S. Tadoroki, T. Murata, J. Non-Cryst. Solids 220 (1997) 178. [3] A. Agarwal, K.M. Davis, M. Tomozawa, J. Non-Cryst. Solids 185 (1995) 191. [4] F.L. Galeener, A.E. Geissberger, Phys. Rev. 28 (1983) 3266. [5] B. Champagnon, C. Chemarin, E. Duval, R. Le Parc, J. Non-Cryst. Solids 274 (2000) 81. [6] C. Chemarin, B. Champagnon, E. Duval, Philos. Mag. 79 (1999) 2057. [7] R. Br uckner, J. Non-Cryst. Solids 5 (1970) 123. [8] S. Todoroki, S. Sakaguchi, J. Ceram. Soc. Jpn. 105 (1997) 377. [9] K. Saito, N. Ogawa, A.J. Ikushima, Y. Tsurita, K. Yamahara, J. Non-Cryst. Solids 270 (2000) 60. [10] U. Haken, O. Humbach, S. Ortner, H. Fabian, J. NonCryst. Solids 265 (2000) 9. [11] K. Saito, H. Kakiuchida, A.J. Ikushima, J. Non-Cryst. Solids 222 (1997) 329. [12] K. Saito, H. Kakiuchida, A.J. Ikushima, J. Appl. Phys. 84 (1998) 3107.