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Chemical stability of ZrF4- and AlF3-based heavy metal fluoride glasses in water
Journal of Non-Crystalline Solids 284 (2001) 105±109
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Chemical stability of ZrF4- and AlF3-based heavy metal ¯uoride glasses in water G.H. Frischat *, B. Hueber 1, B. Ramdohr 2 Institut f ur Nichtmetallische Werkstoe, Technische Universitat Clausthal, Zehntnerstraûe 2A, Clausthal-Zellerfeld D-38678, Germany
Abstract Glasses of the compositions 50ZrF4 ±33BaF2 ±10YF3 ±7AlF3 and 40AlF3 ±12BaF2 ±22CaF2 ±16YF3 ±10SrF2 (mol%), respectively, were corroded in H2 O for dierent times at 20±80°C. The cationic species Zr, Al, Ba, Y, etc. were measured by secondary neutral mass spectrometry, whereas H was measured by nuclear reaction analysis. In the case of the ZrF4 based glass the reaction depths p follow at 20°C a dependence proportional to t, the attack time. This dependence changed with temperature as the t at 80°C. The H reaction pro®les ®t well into this scheme. In the case of the AlF3 p based glass also a t-dependence is obtained at 80°C. Whereas the reaction layer peels o for the ZrF4 -based glass for longer times (>1 h), it is relatively stable on the AlF3 -based glass and protects it against further attack. The AlF3 -based glass is about 10 times more stable in water than the ZrF4 -based glass. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The heavy metal ¯uoride glass family belongs to one of the most investigated materials [1,2]. The application as optical components, such as ®bers, sensors, waveguides or lasers, is restricted by their chemical reactions in aqueous surroundings [1]. Thus, for example, ZrF4 -based glasses are in the same stability regime as Na2 O±SiO2 glasses [3]. Numerous studies of the chemical stability on a variety of glass compositions have been performed in the past [4±6], demonstrating a hierarchy of stabilities, with the AlF3 -based glasses as the most stable ones [3]. Most of the investigations refer* Corresponding author. Tel.: +49-5323 722 463; fax: +495323 723 119. E-mail address: [email protected] (G.H. Frischat). 1 Present address: Schott-Rohrglas, Bayreuth, Germany. 2 Present address: Schott Glas, Mainz, Germany.
enced were made using commonly available methods, e.g. by monitoring changes in surface morphology, weight, corrosion solution or infrared transmission. During such long time (>1 h) corrosion processes dierent mechanisms may overlap, e.g. OH () F ion exchange [4], incongruent dissolution [4], layer formation by redeposition from the solution [5], and crack formation on the surface [5]. Only recently at the nanometer scale initial corrosion stages of a ZrF4 based heavy metal ¯uoride glass in water have been studied [7]. Dierent surface-sensitive analysis methods with nm depth resolution were used and a modi®ed corrosion model could be set up, explaining the ongoing processes in the dierent reaction zones. This study considers further aspects of the short time (<1 h) corrosion of this ZrF4 -based glass and shows a comparison with a more stable AlF3 -based glass. Both similarities and dissimilarities between the two glasses determine their performance.
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 3 8 7 - 8
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G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
2. Experimental The composition of the ZrF4 -based glass was 50ZrF4 ±33BaF2 ±10YF3 ±7AlF3 (mol%). Details on that glass can be found in [7]. The composition of the AlF3 -based glass was 40AlF3 ±12BaF2 ± 22CaF2 ±16YF3 ±10SrF2 (mol%). The ¯uoride raw materials were melted in 30 g batches for 2.5 h at 1200°C in a Pt crucible in air atmosphere. The glass liquid was cooled within 1 h to 950°C and poured then into a brass mould, held at 380°C. After cooling to room temperature within 12 h, 2 mm thick 7 7 mm2 glass slabs were prepared by sawing, grinding and polishing. To overcome possible surface de®ciencies originating from sample preparation and/or weathering during storage, the 7 7 mm2 glass slabs were sputtered immediately before the corrosion experiment by Kr ion bombardment in the secondary neutral mass spectrometry (SNMS) apparatus (see below) to a depth of 6350 nm [8]. Samples had a concentration distribution of all components close to that of the interior of the glass. The sputtered surfaces were plane and there was no evidence of any eect of the sputtering on the corrosion process. The short time corrosion on these samples with the sputtered areas was carried out in 250 ml of distilled water in polyethylene vessels. Corrosion temperatures were between 20°C and 80°C for times between 1 and 50 min (ZrF4 -based glass) and at 80°C for times between 3 and 21 min (AlF3 based glass). The glass samples were then rinsed in acetone (reagent grade) and placed either in the vacuum transfer system 5 10 7 mbar or in a desiccator with silica gel as a desiccant. The corrosion and rinse procedures were the same for all experiments. As described in the previous work [7], the depth pro®ling of the cations was performed using an SNMS apparatus (Ions and Neutrals Analyzer, INA 3, Leybold AG, K oln, Germany), operated in the high frequency mode (HFM, Specs GmbH, Berlin, Germany). The intensity±time pro®les obtained were subsequently converted into concentration±depth pro®les. From the depth of the sputter craters sputter rates of 0:35 0:05 nm=s (ZrF4 -based glass) and 0:30 0:03 nm=s (AlF3 -
based glass) were calculated, which were used to convert the sputter times into depths. The quanti®cation of the intensities was done as described earlier [7] on the basis of the molar compounds ZrX4 , BaX2 , YX3 , AlX3 , CaX2 and SrX2 , respectively, where X aF bOH c 1=2O2 , with the partition factors, a; b and c. In the interior of the glass a 1 and b c 0. A change in sample thickness by the corrosion process could not be detected. 3. Results Fig. 1 shows typical SNMS reaction pro®les after corrosion of the ZrF4 -based glass for 1 min at 80°C. The statistical signi®cance of the data is given by the fact that for a time of 300 s about 50 data points were gathered. In a zone 50 nm near the glass surface a relative depletion of Ba was observed, whereas Zr is enriched there. Going deeper into the glass, Zr is depleted, in company with a corresponding enrichment in Ba. Similar depth pro®les were obtained for other times and temperatures. To be able to compare the data appropriately, a reaction depth is de®ned; see the curve construction for ZrX4 in Fig. 1. Fig. 2 shows a compilation of these reaction depths for the dierent components and dierent times at 20°C, whereas Fig. 3 shows a similar dependence at
Fig. 1. Concentration±distance pro®les of the ZrF4 -based glass after corrosion for 1 min at 80°C in water (the lines in this and the following ®gures are drawn as a guide to the eye).
G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
Fig. 2. Reaction depths of the ZrF4 -based glass as a function of corrosion time at 20°C.
Fig. 3. Reaction depths of the ZrF4 -based glass as a function of corrosion time at 80°C.
80°C. The reaction depths of the dierent components do not dier much within the error, only the reaction depth of YX3 seems to be less than those of the other constituents. Fig. 2 displays an approximate t-dependence for most of the data at the lower temperature, whereas p the data in Fig. 3 display an approximate t-dependence at the higher reaction temperature. The data at 40°C and 60°C are in between these two dependencies [8]. Fig. 4 shows typical reaction pro®les after corrosion of the AlF3 -based glass for 15 min at 80°C. Again there is a relative change in concentration near the surface. Whereas Al is enriched, all other components seem to be depleted. At a greater depth in the glass, these dependencies are reversed.
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Fig. 4. Concentration±distance pro®les of the AlF3 -based glass after corrosion for 15 min at 80°C in water.
Fig. 5. Reaction depths of the AlF3 -based glass as a function of corrosion time at 80°C.
The component AlX3 has a minimum, whereas the other components display smaller amplitudes. Introducing a reaction depth (see the curve construction for AlX3 in Fig. 4), results in data as shown in Fig. 5. As in the case p of the ZrF4 -based glass there is an approximate t-dependence of the data to be seen for the high temperature of 80°C.
4. Discussion As already shown for the ZrF4 -based glass, the components at the minimum of ZrX4 (see Fig. 1), which is in the main corrosion layer,
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reached a practically constant ®nal composition of 43 2ZrF4 ± 39 3BaF2 ± 11 1YF3 ± 5 1 AlF3 (mol%), irrespective of corrosion temperature and time [7]. This composition approached the glass forming boundary of the system [9] and was also not far from the composition of the stable crystal phase BaZrF6 . A similar change is also found for the AlF3 -based glasses with a limiting composition of 31 2AlF3 ± 13 1BaF2 ± 25 1CaF2 ± 20 2YF3 ± 12 1SrF2 (mol%) This composition is also near the glass forming boundary [10], but it is not yet known whether it also approaches a stable crystal phase composition. This question has not been investigated so far. However, there is also a dissimilarity in the water corrosion process of both glasses. Whereas the corrosion layer tends to peel o in the case of the ZrF4 -based glass, e.g. after 20 min at 80°C, initiating the corrosion process at a fresh glass surface again, the corrosion layer remains stable in the case of the AlF3 -based glass, even after 12 h at 80°C. Although scanning electron microscopy shows that the microstructure in the corrosion zone becomes inhomogeneous with increasing attack time, this layer does not peel o but stabilizes the whole system against further aqueous attack. Moreover, if we compare the reaction depths at 80°C in Figs. 3 and 5 for both glasses, we ®nd that the AlF3 -based glass is about 10 times more stable than the ZrF4 -based glass. This stability dierence holds both for the short time and the long time water attack processes, which is in line with earlier ®ndings considering the overall stabilities of both glass systems [11]. Figs. 2, 3 and 5 display the dependence of the reaction depths for the two glasses as a function of attack time. It is interesting to us to note that for both glasses this reaction p depth increases approx imately proportional to t at 80°C, the higher corrosion temperature. In the case of the ZrF4 based glass this dependence had been explained by a Ba2 diusion process from the surface into the interior [7], which is in line with the enrichment of this ion in the main corrosion zone, see also Fig. 1. Possibly a similar Al3 diusion mechanism from the surface to the interior could also hold for the AlF3 -based glass, where except ± for AlX3 ± all other components have a maximum in that zone.
Note that the data for CaX2 have not been included in Fig. 4 because this species generally has two maxima, making the de®nition of a simple reaction depth more complicated. Contrary to the data at 80°C, those at the lower reaction temperature of 20°C have a direct t-dependence, see Fig. 2. This dependence had been explained by a mechanism where H2 O molecules split the glass network at the surface and remove a part of the Ba2 ions as Ba OH2 into the leach solution [7]. With increasing attack temperature from 20°C, 40°C, 60°C, to 80°C, the diusion mechanism dominates the network dissolution mechanism. Whether such a change in mechanisms also holds for the AlF3 -based glass we cannot decide yet. During the reaction of the glasses with water there is not only an alteration in the concentration of the cationic species like Zr, Al, Y, etc., we also ®nd O- or H-bearing species from the water in the glasses. Using 15 N nuclear reaction analysis (NRA) and Rutherford backscattering (RBS) spectroscopies, both the pro®les of H and O could be measured [7,8]. Fig. 3 displays that pthe penetration depths of the hydrogen also has a t-dependence at 80°C and are in reasonable agreement with the other components. This dependence could be an indication of an OH () F ion exchange. There is only one exception, the data for 20 min. There the corrosion layer had been peeled o during sample transport so that the H content in this glass was comparable to that of a non-corroded sample. 5. Conclusions ZrF4 - and AlF3 -based sputtered heavy metal ¯uoride glasses were corroded in H2 O for dierent times at temperatures between 20°C and 80°C. The cationic species Zr, Ba, Y, Al (ZrF4 -based glass) and Al, Ba, Ca, Y, Sr (AlF3 -based glass) were measured by SNMS, H was measured by NRA. We have proven that · a plot of the reaction depths against attack times results in a t-dependence for the ZrF4 -based glass at 20°C, · a similar plot at 80°C results in a dependence for both the ZrF4 and the AlF3 based glasses,
G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
p · the hydrogen data ®t the t-dependence at 80°C well and · the AlF3 -based glass is 10 times more stable than the ZrF4 -based glass, whose corrosion layer peels o, whereas that of the AlF3 -based glass remains stable and protects the glass against further attack. Acknowledgements The authors gratefully acknowledge ®nancial support of this work by the Deutsche Forschungsgemeinschaft (DFG), Bonn. They also thank Professor Dr F. Rauch, Universit at Frankfurt/Main, for the hydrogen measurements of the glasses. References [1] A.E. Comyns, Fluoride Glasses, Critical Reports on Applied Chemistry, vol. 27, Wiley, New York, 1989. [2] I.D. Aggarwal, G. Lu, in: Fluoride Glass Fiber Optics, Academic Press, New York, 1991.
109
[3] A.B. Seddon, in: A.E. Comyns (Ed.), Fluoride Glasses, Critical Reports on Applied Chemistry, vol. 27, Wiley, New York, 1989, p. 157. [4] G.H. Frischat, I. Overbeck, J. Am. Ceram. Soc. 67 (1984) C238. [5] C.J. Simmons, J.H. Simmons, J. Am. Ceram. Soc. 69 (1986) 661. [6] Y. Dai, K. Takahashi, I. Yamaguchi, J. Am. Ceram. Soc. 78 (1995) 182. [7] B. Hueber, G.H. Frischat, A. Maldener, O. Dersch, F. Rauch, J. Non-Cryst. Solids 256&257 (1999) 130. [8] B. Hueber, PhD dissertation, TU Clausthal, 1998. [9] A. Lecoq, M. Poulain, J. Non-Cryst. Solids 41 (1980) 209. [10] T. Kanamori, K. Oikawa, S. Shibata, Jpn. J. Appl. Phys. 20 (1981) L326. [11] T. Iqbal, M.R. Shahriari, G. Weitz, G.H. Sigel, J. NonCryst. Solids 184 (1995) 190.
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Chemical stability of ZrF4- and AlF3-based heavy metal ¯uoride glasses in water G.H. Frischat *, B. Hueber 1, B. Ramdohr 2 Institut f ur Nichtmetallische Werkstoe, Technische Universitat Clausthal, Zehntnerstraûe 2A, Clausthal-Zellerfeld D-38678, Germany
Abstract Glasses of the compositions 50ZrF4 ±33BaF2 ±10YF3 ±7AlF3 and 40AlF3 ±12BaF2 ±22CaF2 ±16YF3 ±10SrF2 (mol%), respectively, were corroded in H2 O for dierent times at 20±80°C. The cationic species Zr, Al, Ba, Y, etc. were measured by secondary neutral mass spectrometry, whereas H was measured by nuclear reaction analysis. In the case of the ZrF4 based glass the reaction depths p follow at 20°C a dependence proportional to t, the attack time. This dependence changed with temperature as the t at 80°C. The H reaction pro®les ®t well into this scheme. In the case of the AlF3 p based glass also a t-dependence is obtained at 80°C. Whereas the reaction layer peels o for the ZrF4 -based glass for longer times (>1 h), it is relatively stable on the AlF3 -based glass and protects it against further attack. The AlF3 -based glass is about 10 times more stable in water than the ZrF4 -based glass. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction The heavy metal ¯uoride glass family belongs to one of the most investigated materials [1,2]. The application as optical components, such as ®bers, sensors, waveguides or lasers, is restricted by their chemical reactions in aqueous surroundings [1]. Thus, for example, ZrF4 -based glasses are in the same stability regime as Na2 O±SiO2 glasses [3]. Numerous studies of the chemical stability on a variety of glass compositions have been performed in the past [4±6], demonstrating a hierarchy of stabilities, with the AlF3 -based glasses as the most stable ones [3]. Most of the investigations refer* Corresponding author. Tel.: +49-5323 722 463; fax: +495323 723 119. E-mail address: [email protected] (G.H. Frischat). 1 Present address: Schott-Rohrglas, Bayreuth, Germany. 2 Present address: Schott Glas, Mainz, Germany.
enced were made using commonly available methods, e.g. by monitoring changes in surface morphology, weight, corrosion solution or infrared transmission. During such long time (>1 h) corrosion processes dierent mechanisms may overlap, e.g. OH () F ion exchange [4], incongruent dissolution [4], layer formation by redeposition from the solution [5], and crack formation on the surface [5]. Only recently at the nanometer scale initial corrosion stages of a ZrF4 based heavy metal ¯uoride glass in water have been studied [7]. Dierent surface-sensitive analysis methods with nm depth resolution were used and a modi®ed corrosion model could be set up, explaining the ongoing processes in the dierent reaction zones. This study considers further aspects of the short time (<1 h) corrosion of this ZrF4 -based glass and shows a comparison with a more stable AlF3 -based glass. Both similarities and dissimilarities between the two glasses determine their performance.
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 3 8 7 - 8
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G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
2. Experimental The composition of the ZrF4 -based glass was 50ZrF4 ±33BaF2 ±10YF3 ±7AlF3 (mol%). Details on that glass can be found in [7]. The composition of the AlF3 -based glass was 40AlF3 ±12BaF2 ± 22CaF2 ±16YF3 ±10SrF2 (mol%). The ¯uoride raw materials were melted in 30 g batches for 2.5 h at 1200°C in a Pt crucible in air atmosphere. The glass liquid was cooled within 1 h to 950°C and poured then into a brass mould, held at 380°C. After cooling to room temperature within 12 h, 2 mm thick 7 7 mm2 glass slabs were prepared by sawing, grinding and polishing. To overcome possible surface de®ciencies originating from sample preparation and/or weathering during storage, the 7 7 mm2 glass slabs were sputtered immediately before the corrosion experiment by Kr ion bombardment in the secondary neutral mass spectrometry (SNMS) apparatus (see below) to a depth of 6350 nm [8]. Samples had a concentration distribution of all components close to that of the interior of the glass. The sputtered surfaces were plane and there was no evidence of any eect of the sputtering on the corrosion process. The short time corrosion on these samples with the sputtered areas was carried out in 250 ml of distilled water in polyethylene vessels. Corrosion temperatures were between 20°C and 80°C for times between 1 and 50 min (ZrF4 -based glass) and at 80°C for times between 3 and 21 min (AlF3 based glass). The glass samples were then rinsed in acetone (reagent grade) and placed either in the vacuum transfer system 5 10 7 mbar or in a desiccator with silica gel as a desiccant. The corrosion and rinse procedures were the same for all experiments. As described in the previous work [7], the depth pro®ling of the cations was performed using an SNMS apparatus (Ions and Neutrals Analyzer, INA 3, Leybold AG, K oln, Germany), operated in the high frequency mode (HFM, Specs GmbH, Berlin, Germany). The intensity±time pro®les obtained were subsequently converted into concentration±depth pro®les. From the depth of the sputter craters sputter rates of 0:35 0:05 nm=s (ZrF4 -based glass) and 0:30 0:03 nm=s (AlF3 -
based glass) were calculated, which were used to convert the sputter times into depths. The quanti®cation of the intensities was done as described earlier [7] on the basis of the molar compounds ZrX4 , BaX2 , YX3 , AlX3 , CaX2 and SrX2 , respectively, where X aF bOH c 1=2O2 , with the partition factors, a; b and c. In the interior of the glass a 1 and b c 0. A change in sample thickness by the corrosion process could not be detected. 3. Results Fig. 1 shows typical SNMS reaction pro®les after corrosion of the ZrF4 -based glass for 1 min at 80°C. The statistical signi®cance of the data is given by the fact that for a time of 300 s about 50 data points were gathered. In a zone 50 nm near the glass surface a relative depletion of Ba was observed, whereas Zr is enriched there. Going deeper into the glass, Zr is depleted, in company with a corresponding enrichment in Ba. Similar depth pro®les were obtained for other times and temperatures. To be able to compare the data appropriately, a reaction depth is de®ned; see the curve construction for ZrX4 in Fig. 1. Fig. 2 shows a compilation of these reaction depths for the dierent components and dierent times at 20°C, whereas Fig. 3 shows a similar dependence at
Fig. 1. Concentration±distance pro®les of the ZrF4 -based glass after corrosion for 1 min at 80°C in water (the lines in this and the following ®gures are drawn as a guide to the eye).
G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
Fig. 2. Reaction depths of the ZrF4 -based glass as a function of corrosion time at 20°C.
Fig. 3. Reaction depths of the ZrF4 -based glass as a function of corrosion time at 80°C.
80°C. The reaction depths of the dierent components do not dier much within the error, only the reaction depth of YX3 seems to be less than those of the other constituents. Fig. 2 displays an approximate t-dependence for most of the data at the lower temperature, whereas p the data in Fig. 3 display an approximate t-dependence at the higher reaction temperature. The data at 40°C and 60°C are in between these two dependencies [8]. Fig. 4 shows typical reaction pro®les after corrosion of the AlF3 -based glass for 15 min at 80°C. Again there is a relative change in concentration near the surface. Whereas Al is enriched, all other components seem to be depleted. At a greater depth in the glass, these dependencies are reversed.
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Fig. 4. Concentration±distance pro®les of the AlF3 -based glass after corrosion for 15 min at 80°C in water.
Fig. 5. Reaction depths of the AlF3 -based glass as a function of corrosion time at 80°C.
The component AlX3 has a minimum, whereas the other components display smaller amplitudes. Introducing a reaction depth (see the curve construction for AlX3 in Fig. 4), results in data as shown in Fig. 5. As in the case p of the ZrF4 -based glass there is an approximate t-dependence of the data to be seen for the high temperature of 80°C.
4. Discussion As already shown for the ZrF4 -based glass, the components at the minimum of ZrX4 (see Fig. 1), which is in the main corrosion layer,
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G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
reached a practically constant ®nal composition of 43 2ZrF4 ± 39 3BaF2 ± 11 1YF3 ± 5 1 AlF3 (mol%), irrespective of corrosion temperature and time [7]. This composition approached the glass forming boundary of the system [9] and was also not far from the composition of the stable crystal phase BaZrF6 . A similar change is also found for the AlF3 -based glasses with a limiting composition of 31 2AlF3 ± 13 1BaF2 ± 25 1CaF2 ± 20 2YF3 ± 12 1SrF2 (mol%) This composition is also near the glass forming boundary [10], but it is not yet known whether it also approaches a stable crystal phase composition. This question has not been investigated so far. However, there is also a dissimilarity in the water corrosion process of both glasses. Whereas the corrosion layer tends to peel o in the case of the ZrF4 -based glass, e.g. after 20 min at 80°C, initiating the corrosion process at a fresh glass surface again, the corrosion layer remains stable in the case of the AlF3 -based glass, even after 12 h at 80°C. Although scanning electron microscopy shows that the microstructure in the corrosion zone becomes inhomogeneous with increasing attack time, this layer does not peel o but stabilizes the whole system against further aqueous attack. Moreover, if we compare the reaction depths at 80°C in Figs. 3 and 5 for both glasses, we ®nd that the AlF3 -based glass is about 10 times more stable than the ZrF4 -based glass. This stability dierence holds both for the short time and the long time water attack processes, which is in line with earlier ®ndings considering the overall stabilities of both glass systems [11]. Figs. 2, 3 and 5 display the dependence of the reaction depths for the two glasses as a function of attack time. It is interesting to us to note that for both glasses this reaction p depth increases approx imately proportional to t at 80°C, the higher corrosion temperature. In the case of the ZrF4 based glass this dependence had been explained by a Ba2 diusion process from the surface into the interior [7], which is in line with the enrichment of this ion in the main corrosion zone, see also Fig. 1. Possibly a similar Al3 diusion mechanism from the surface to the interior could also hold for the AlF3 -based glass, where except ± for AlX3 ± all other components have a maximum in that zone.
Note that the data for CaX2 have not been included in Fig. 4 because this species generally has two maxima, making the de®nition of a simple reaction depth more complicated. Contrary to the data at 80°C, those at the lower reaction temperature of 20°C have a direct t-dependence, see Fig. 2. This dependence had been explained by a mechanism where H2 O molecules split the glass network at the surface and remove a part of the Ba2 ions as Ba OH2 into the leach solution [7]. With increasing attack temperature from 20°C, 40°C, 60°C, to 80°C, the diusion mechanism dominates the network dissolution mechanism. Whether such a change in mechanisms also holds for the AlF3 -based glass we cannot decide yet. During the reaction of the glasses with water there is not only an alteration in the concentration of the cationic species like Zr, Al, Y, etc., we also ®nd O- or H-bearing species from the water in the glasses. Using 15 N nuclear reaction analysis (NRA) and Rutherford backscattering (RBS) spectroscopies, both the pro®les of H and O could be measured [7,8]. Fig. 3 displays that pthe penetration depths of the hydrogen also has a t-dependence at 80°C and are in reasonable agreement with the other components. This dependence could be an indication of an OH () F ion exchange. There is only one exception, the data for 20 min. There the corrosion layer had been peeled o during sample transport so that the H content in this glass was comparable to that of a non-corroded sample. 5. Conclusions ZrF4 - and AlF3 -based sputtered heavy metal ¯uoride glasses were corroded in H2 O for dierent times at temperatures between 20°C and 80°C. The cationic species Zr, Ba, Y, Al (ZrF4 -based glass) and Al, Ba, Ca, Y, Sr (AlF3 -based glass) were measured by SNMS, H was measured by NRA. We have proven that · a plot of the reaction depths against attack times results in a t-dependence for the ZrF4 -based glass at 20°C, · a similar plot at 80°C results in a dependence for both the ZrF4 and the AlF3 based glasses,
G.H. Frischat et al. / Journal of Non-Crystalline Solids 284 (2001) 105±109
p · the hydrogen data ®t the t-dependence at 80°C well and · the AlF3 -based glass is 10 times more stable than the ZrF4 -based glass, whose corrosion layer peels o, whereas that of the AlF3 -based glass remains stable and protects the glass against further attack. Acknowledgements The authors gratefully acknowledge ®nancial support of this work by the Deutsche Forschungsgemeinschaft (DFG), Bonn. They also thank Professor Dr F. Rauch, Universit at Frankfurt/Main, for the hydrogen measurements of the glasses. References [1] A.E. Comyns, Fluoride Glasses, Critical Reports on Applied Chemistry, vol. 27, Wiley, New York, 1989. [2] I.D. Aggarwal, G. Lu, in: Fluoride Glass Fiber Optics, Academic Press, New York, 1991.
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[3] A.B. Seddon, in: A.E. Comyns (Ed.), Fluoride Glasses, Critical Reports on Applied Chemistry, vol. 27, Wiley, New York, 1989, p. 157. [4] G.H. Frischat, I. Overbeck, J. Am. Ceram. Soc. 67 (1984) C238. [5] C.J. Simmons, J.H. Simmons, J. Am. Ceram. Soc. 69 (1986) 661. [6] Y. Dai, K. Takahashi, I. Yamaguchi, J. Am. Ceram. Soc. 78 (1995) 182. [7] B. Hueber, G.H. Frischat, A. Maldener, O. Dersch, F. Rauch, J. Non-Cryst. Solids 256&257 (1999) 130. [8] B. Hueber, PhD dissertation, TU Clausthal, 1998. [9] A. Lecoq, M. Poulain, J. Non-Cryst. Solids 41 (1980) 209. [10] T. Kanamori, K. Oikawa, S. Shibata, Jpn. J. Appl. Phys. 20 (1981) L326. [11] T. Iqbal, M.R. Shahriari, G. Weitz, G.H. Sigel, J. NonCryst. Solids 184 (1995) 190.