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Fluoride glasses in the InF3–GaF3–YF3–PbF2–CaF2–ZnF2 system
Journal of Physics and Chemistry of Solids 63 (2002) 2129–2133 www.elsevier.com/locate/jpcs
Fluoride glasses in the InF3 – GaF3 –YF3 –PbF2 –CaF2 –ZnF2 system Ahmed Boutarfaiaa,b,*, Marcel Poulaina,b a Laboratoire de Chimie Applique´e, Universite´ de Biskra, PB. 145, RP Biskra 07000, Algeria Laboratoire de Mate´riaux Photoniques, Centre d’Etude des Mate´riaux Avances, Universite´ de Rennes, Campus de Beaulieu 1, 35042 Rennes, France
b
Received 11 December 2001; received in revised form 27 March 2002; accepted 28 March 2002
Abstract New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35 2 x ) InF3 –xGaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. Tg lies between 260 and 296 8C while melting starts around 480 8C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glasses; C. Differential scanning calorimetry
1. Introduction Fluoroindate glasses have been extensively studied because some of their optical properties go beyond the range of the standard fluorozirconate glasses [1– 4]. In particular, they offer an extended infrared transmission range, which directly derives from their lower phonon energy. In addition, their intrinsic optical losses should be smaller than those of the standard ZBLAN glass in the midIR. However, practical applications require very homogeneous samples and fibers. The control of the defects and the impurities in the bulk glass is possible only for compositions stable enough against devitrification. In most vitreous systems based on indium fluoride nucleation and crystallization problems are far from being under control, which often results in high scattering losses. Systematic studies have been carried out using the IZBS glass composition, 40InF3 – 20ZnF2 – 20BaF2 – 20SrF2, as a starting point [2,3]. Glasses based on InF3 and PbF2 offer * Corresponding author. Address: Laboratoire de Mate´riaux Photoniques, Centre d’Etude des Mate´riaux Avances, Universite´ de Rennes, Campus de Beaulieu 1, 35042 Rennes, France. Tel.: þ 33-2-23-23-62-63; fax: þ 33-2-23-23-69-72. E-mail address: [email protected] (M. Poulain).
another alternative. This paper reports attempts to optimize the glass stability in a multicomponent system based on the fluorides of In, Y, Pb, Ca, Zn and Ga. The influence of the Ga/In substitution on glass stability and physical properties has also been studied.
2. Experimental Glass samples have been prepared in open crucibles at room atmosphere using ammonium bifluoride to prevent hydrolysis. Starting materials are the oxides In2O3 3N from Preussag, Y2O3 3N from Rhone Poulenc and the fluorides BaF2, SrF2, PbF2, CdF2 and NaF which have been supplied by Merck. Batches, usually 6 g in weight, were introduced and heated in platinum crucibles using the ammonium bifluoride processing. Fluorination was carried out for 1 h at 300 8C and 30 min at 450 8C. Then the batch was gradually heated up to melting temperature before the fining, cooling and casting steps. Processing included a 20 min fining stage at 800– 900 8C after melting. In a final stage glass samples were annealed at a temperature lower than glass transition temperature Tg and cooled slowly to room temperature. For
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 2 6 3 - 9
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A. Boutarfaia, M. Poulain / Journal of Physics and Chemistry of Solids 63 (2002) 2129–2133
an Oxford Linkisis analysis equipment. Concentration measurements were deduced from data processing with calibrated standards.
3. Results and discussion
Fig. 1. Evolution of stability parameters DT and S versus GaF3 content in (35 2 x ) InF 3 – xGaF3 – 10YF 3 – 25PbF2 – 15CaF2 – 15ZnF2 glasses.
new compositions, annealing temperature was estimated from semi-empirical criteria. Sample size and optical quality depend on the cooling rate of the melt because large samples will crack if they are cooled too rapidly. On the other hand, glass formation requires that melt is solidified before crystallization occurs. Therefore, there is a direct correlation between maximum sample thickness, cooling rate and glass forming ability. As a practical consequence, the compositional limits for glass formation extend when quenching rate increases. The vitreous areas which are reported later correspond to the quenching of the melt between two brass plates. The corresponding quenching rate is close to 1000 K s21 and sample thickness ranges from 0.3 to 0.5 mm. Large samples may be prepared only from vitreous compositions stable enough against devitrification. Such samples are required for optical measurements. They are obtained by pouring the melt onto a hot metal mould. The characteristic temperatures of the glasses were measured by differential scanning calorimetry (DSC) with a heating rate of 10 K/min (D.S.C. 220 SEIKO Instruments). Glass forming ability was estimated using the following numerical factors: thermal stability range: DT ¼ ðTx 2 Tg Þ [5] and stability criterion S ¼ ðTx 2 Tg ÞðTp 2 Tx Þ/Tg [6], where Tg, Tx and Tp are, respectively, the temperature in Kelvin for glass transition, onset of crystallization and exotherm maximum. As usually, the absence of crystals is checked by visual inspection. This simple method is much more sensitive than X-ray diffraction: light scattering may be perceived when crystalline fraction exceeds 1024 and microscopic observation is still more accurate. Refractive index is measured using an Abbe refractometer and thermal expansion from a Seiko TMA/SS 120. Estimated accuracy is ^ 1 8C for temperatures, ^0.0005 for refractive index and ^ 5 £ 1027 K21 for thermal expansion. An archimedean method was used for density is expected to lead to an error margin of ^ 0.01. Observation in scanning electron microscopy (SEM) were made using a JEOL 6400 set-up working at 20 kV with
Optimization of glass compositions is a necessary step for the preparation of the optical fibers and the homogenous glass samples required for physical measurements. Devitrification rate at cooling is probably the most critical phenomenon. It encompasses the classical steps of nucleation and crystal growth. The first condition for glass formation is the occurrence of a supercooled liquid, which means that it is free of nuclei. Processing conditions must be adjusted to minimize—and hopefully avoid—heterogeneous nucleation arising from crucible walls, dust and impurities. Homogenous nucleation depends on melt composition, time and temperature. Insofar as melt must cross the entire temperature range between liquidus temperature and glass transition temperature, only time— i.e. cooling rate—and chemical composition can be adjusted. Both anionic and cationic composition must be considered. In practice, fluoride glasses are hardly free of anionic impurities, especially anionic oxygen which has a significant influence on glass stability [7]. While residual oxygen may decrease the devitrification tendency of the melt [7], the reverse effect is predominant. Anionic oxygen originates from several sources: (i) oxides in starting materials; (ii) hydrolysis; (iii) contamination or incomplete fluorination during synthesis. The last item is critical when oxides are used as starting materials with the ammonium fluoride processing. The key point is that water generated in the fluorination process must be removed at low temperature to avoid further hydrolysis. Time and temperature must be adjusted as a function of batch composition. When fluorination is incomplete, crystalline white phase is formed upon melt cooling, and a similar effect is obtained when indium or gallium oxide is incorporated into the fluoride melt. Our experimental observations show that fluorination of gallium and indium oxides is more difficult than that of zirconium oxide. As a consequence, longer heating time is required for the fluorination stage: 1 h at 300 8C for a 6 g batch. In spite of the influence of cationic and anionic composition on glass stability no systematic analytic control has been implemented in current works on fluoride glass systems. The first step of this work has been to ensure that synthesis conditions were reproducible and lead to clear glass samples when applied to standard fluoroindate glasses such as IZBS. In these conditions the amount of residual
A. Boutarfaia, M. Poulain / Journal of Physics and Chemistry of Solids 63 (2002) 2129–2133
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Fig. 3. Evolution of refractive index, nD versus GaF3 content. Fig. 2. Influence of GaF3 on the value of the characteristic temperatures Tg, Tx and Tf for glass transition, onset of crystallization and melting.
oxygen is assumed to be close to that of current fluoride glasses, that is a few hundreds ppm. In a second step we have investigated the influence of cationic composition on glass formation and stability. Previous works aiming at the optimization of the InF3 – YF3 – PbF2 ternary glasses showed that more stable compositions could be obtained incorporating calcium and indium fluorides as stabilizers [8]. A typical composition was 40InF3 –5YF3 – 25PbF2 – 15CaF2 – 15ZnF2. In the first step, we have substituted indium by gallium, which is known to decrease devitrification rate in fluoroindate glasses. Glass samples were prepared with the general chemical formula: (35 2 x )InF3 – xGaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. The evolution of the stability criteria versus gallium content is reported in Fig. 1. This suggests that optimum composition corresponds to 15 – 30 mol% GaF3. In other words, fluorogallate glasses appear more stable in this multicomponent system than fluoroindates. When InF3 is replaced by GaF3, glass transition temperature increases from 260 to 296 8C. The evolution of the other characteristic temperatures—Tx for onset of crystallization and Tf for the beginning of the fusion—is reported in Fig. 2. In order to assess the real glass composition, several chemical analysis have been carried out on glass samples from the nominal composition 15InF3 – 20GaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. These analysis were made using EDAX coupled to SEM and corresponding results are given in Table 1. The agreement between nominal and
experimental compositions seems fairly good with respect to the limited accuracy of this method for a multicomponent sample. Residual oxygen was analyzed in a few glass samples using a LECO set-up. While a few thousands ppm oxygen were detected in these samples, this concentration level was close to that of pure powdered fluorides for which the X-ray diffraction pattern showed only the pure phase. Density and refractive index decrease, while thermal expansion increases, when GaF3 replaces InF3. This evolution is exemplified by Figs. 3 – 5. These glasses are stable at room atmosphere and their chemical durability may be compared to that of fluorozirconate glasses. The influence of the other glass components on glass stability and physical properties was also investigated. Various samples were prepared according to the general formulas: 15InF3 – 10YF3 – ð40 2 xÞPbF2 – 15ZnF2 – 20GaF3 – xCaF2 15InF3 – xYF3 – ð35 2 xÞPbF2 – 15ZnF2 – 20GaF3 – 15CaF2 15InF3 – 10YF3 – ð40 2 xÞPbF2 – xZnF2 – 20GaF3 – 15CaF2 The effect of the Pb/Y, Pb/Zn and Pb/Ca substitutions may be seen from Figs. 6 – 9. Thermal stability range may reach 120 8C, which is generally sufficient for casting large bulk samples. The evolution of the physical properties versus chemical composition is consistent with previous
Table 1 Comparison between nominal and analyzed cationic compositions Elements
In
Ga
Pb
Y
Ca
Zn
Nominal Analysis 1 Analysis 2 Analysis 3
15 14.1 15.4 15.8
20 23.9 22.5 22.6
25 23.4 24.4 23.4
10 9.9 10.2 9.8
15 14.1 14.4 14.7
15 13.2 13.1 13.7
Fig. 4. Evolution of density versus GaF3 content.
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Fig. 5. Thermal expansion coefficient, a, as a function of GaF3.
knowledge of fluoride glasses: refractive index and density are correlated with lead content. These results show that more stable glasses could be obtained by convenient adjustments of chemical composition. It may be viewed as the illustration of the ‘confusion principle’ which expressed that the stability of a multicomponent glass versus devitrification generally increases as the number of glass components increases. While numerous examples support this rule, current experience shows that does not apply automatically. There is an optimum concentration range which cannot be predicted from theoretical considerations. In addition, processing has also to be adjusted in order to minimize the factors promoting heterogeneous nucleation. It is widely admitted that the diffusion processes involved for crystallization are more difficult when chemical species are more numerous. However, this is only one of the factors ruling the whole devitrification process, and some glass forming melts may devitrify more rapidly when even one additional compound is incorporated. Correlating glass forming ability to chemical com-
Fig. 6. Thermal stability range DT versus composition in the glasses: 15InF 3 – 10YF 3 – (40 2 x )(PbF2 – 15ZnF2 – 20GaF3 – xCaF2 ; 15InF 3 – x YF3 – (35 2 x )PbF 2 – 15ZnF 2 – 20GaF3 – 15CaF2 ; 15InF3 –10YF3 –(40 2 x )PbF2 –x ZnF2 –20GaF3 –15CaF2.
Fig. 7. Influence of Pb/Y, Pb/Ca and Pb/Zn substitutions on glass transition temperature Tg.
position and bonding has been a constant concern for glass scientists, and Zachariasen’s rules [9] make the most famous—and perhaps the most successful—attempt in this way. Unfortunately most approaches fail on the point that devitrification first arises from nucleation and crystal growth which are not clearly influenced by chemical bonding. It is often assumed that any glass consists in a vitreous network which has a predominant covalent character. One may be tempted to conclude that glass stability will increase if network covalency is enhanced. This would explain why the substitution of indium fluoride by gallium fluoride, which is more covalent, improves glass formation; but it is contradictory with the fact that indium incorporation has a similar effect in fluorogallate glasses [10]. In a more general way, glass formation occurs in chemical systems having all types of bonding: covalent, ionic and metallic, and there is only one example of nitride glass—nitrides are covalent— while there are thousands of fluoride—and predominantly ionic—glasses. As a consequence, glass science still remains largely empirical in this area of glass formation. Fortunately, it is largely predictable as far as physical properties are concerned. The changes in the physical properties of the
Fig. 8. Refractive index, nD, versus ZnF2, CaF2 and YF3 content.
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4. Conclusions Numerous glass compositions have been investigated in the multicomponent systems based on the fluorides of In, Ga, Pb, Ca, Zn, Y. The influence of the composition on glass stability and physical properties has been investigated. Some of these glasses are stable enough against devitrification to be cast as thick samples from which preforms for fiber drawing experiments could be made.
References Fig. 9. Density versus ZnF2, CaF2 and YF3 content.
modified fluoroindate glasses are consistent with the observations in other fluoride systems: lead fluoride is known to increase density and refractive index and to decrease glass transition temperature. The respective influences of calcium and zinc fluorides appear different for glass transition. This could be correlated to their different coordination numbers in glass structure. Fluoroindate glasses may find practical applications to extend the possibilities of the standard fluorozirconate glasses [11] such as ZBLAN because they may transmit IR light at longer wavelength [12,13]. As an example, one may quote thermometry, remote spectroscopy and CO laser transmission. However, as most of these applications require optical fibers, the level of scattering and absorption losses should be reduced. Finally, active fibers made from fluoroindate glasses may have higher efficiency for various laser transitions, and also for optical amplification at 1.3 mm [14]. The fluoroindate glasses from this study have a high refractive index. Their association with lead-free glasses makes possible the design of optical fibers with a very large numerical aperture.
[1] J. Nishii, Y. Kaite, T. Yamagishi, Phys. Chem. Glasses 30 (1989) 55. [2] M. Poulain, M. Poulain, Y. Messaddeq, A. Soufiane, in: A.J. Bruce, B.V. Hiremath (Eds.), Solid State Optical Materials, Fluoroindate Glasses, vol. 28, American Ceramic Society, Westerville, USA, 1992, p. 381. [3] Y. Messaddeq, A. Delben, M.A. Aegerter, A. Soufiane, M. Poulain, J. Mater. Res. 8 (1993) 885. [4] A. Boutarfaia, M. Poulain, M. Poulain, S.E. Bouaoud, J. NonCryst. Solids 213–214 (1997) 36. [5] A. Dietzel, Glastech. Ber. 22 (1968) 41. [6] M. Saad, M. Poulain, Mater. Sci. Forum 19–20 (1987) 11. [7] S. Mitachi, P.A. Tick, Mater. Sci. Forum 32–33 (1988) 197. [8] A. Boutarfaia, The`se de Doctorat d’Etat, Constantine, 1997. [9] W.H. Zachariasen, J. Chem. Soc. 54 (1932) 3841. [10] A. Soufiane, M. Poulain, J. Non-Cryst. Solids 161 (1993) 206. [11] G. Maze´, in: A. Comyns (Ed.), Fluoride Glasses, Applications and Prospects, Wiley, Chichester, 1989, p. 201. [12] M. Poulain, Proceedings of the XVIII International Congress on Glass, 1998, p. 1. [13] M. Poulain, Proc. SPIE 3416 (2–12) (1998). [14] G. Zhang, J. Jiang, M. Poulain, G. Maze, B. Kinsmann, E. Taylor, B.N. Samson, A. Jha, Proceedings of the XVIII International Congress on Glass, 1998, p. 31.
Fluoride glasses in the InF3 – GaF3 –YF3 –PbF2 –CaF2 –ZnF2 system Ahmed Boutarfaiaa,b,*, Marcel Poulaina,b a Laboratoire de Chimie Applique´e, Universite´ de Biskra, PB. 145, RP Biskra 07000, Algeria Laboratoire de Mate´riaux Photoniques, Centre d’Etude des Mate´riaux Avances, Universite´ de Rennes, Campus de Beaulieu 1, 35042 Rennes, France
b
Received 11 December 2001; received in revised form 27 March 2002; accepted 28 March 2002
Abstract New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35 2 x ) InF3 –xGaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. Tg lies between 260 and 296 8C while melting starts around 480 8C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Glasses; C. Differential scanning calorimetry
1. Introduction Fluoroindate glasses have been extensively studied because some of their optical properties go beyond the range of the standard fluorozirconate glasses [1– 4]. In particular, they offer an extended infrared transmission range, which directly derives from their lower phonon energy. In addition, their intrinsic optical losses should be smaller than those of the standard ZBLAN glass in the midIR. However, practical applications require very homogeneous samples and fibers. The control of the defects and the impurities in the bulk glass is possible only for compositions stable enough against devitrification. In most vitreous systems based on indium fluoride nucleation and crystallization problems are far from being under control, which often results in high scattering losses. Systematic studies have been carried out using the IZBS glass composition, 40InF3 – 20ZnF2 – 20BaF2 – 20SrF2, as a starting point [2,3]. Glasses based on InF3 and PbF2 offer * Corresponding author. Address: Laboratoire de Mate´riaux Photoniques, Centre d’Etude des Mate´riaux Avances, Universite´ de Rennes, Campus de Beaulieu 1, 35042 Rennes, France. Tel.: þ 33-2-23-23-62-63; fax: þ 33-2-23-23-69-72. E-mail address: [email protected] (M. Poulain).
another alternative. This paper reports attempts to optimize the glass stability in a multicomponent system based on the fluorides of In, Y, Pb, Ca, Zn and Ga. The influence of the Ga/In substitution on glass stability and physical properties has also been studied.
2. Experimental Glass samples have been prepared in open crucibles at room atmosphere using ammonium bifluoride to prevent hydrolysis. Starting materials are the oxides In2O3 3N from Preussag, Y2O3 3N from Rhone Poulenc and the fluorides BaF2, SrF2, PbF2, CdF2 and NaF which have been supplied by Merck. Batches, usually 6 g in weight, were introduced and heated in platinum crucibles using the ammonium bifluoride processing. Fluorination was carried out for 1 h at 300 8C and 30 min at 450 8C. Then the batch was gradually heated up to melting temperature before the fining, cooling and casting steps. Processing included a 20 min fining stage at 800– 900 8C after melting. In a final stage glass samples were annealed at a temperature lower than glass transition temperature Tg and cooled slowly to room temperature. For
0022-3697/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 3 6 9 7 ( 0 2 ) 0 0 2 6 3 - 9
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A. Boutarfaia, M. Poulain / Journal of Physics and Chemistry of Solids 63 (2002) 2129–2133
an Oxford Linkisis analysis equipment. Concentration measurements were deduced from data processing with calibrated standards.
3. Results and discussion
Fig. 1. Evolution of stability parameters DT and S versus GaF3 content in (35 2 x ) InF 3 – xGaF3 – 10YF 3 – 25PbF2 – 15CaF2 – 15ZnF2 glasses.
new compositions, annealing temperature was estimated from semi-empirical criteria. Sample size and optical quality depend on the cooling rate of the melt because large samples will crack if they are cooled too rapidly. On the other hand, glass formation requires that melt is solidified before crystallization occurs. Therefore, there is a direct correlation between maximum sample thickness, cooling rate and glass forming ability. As a practical consequence, the compositional limits for glass formation extend when quenching rate increases. The vitreous areas which are reported later correspond to the quenching of the melt between two brass plates. The corresponding quenching rate is close to 1000 K s21 and sample thickness ranges from 0.3 to 0.5 mm. Large samples may be prepared only from vitreous compositions stable enough against devitrification. Such samples are required for optical measurements. They are obtained by pouring the melt onto a hot metal mould. The characteristic temperatures of the glasses were measured by differential scanning calorimetry (DSC) with a heating rate of 10 K/min (D.S.C. 220 SEIKO Instruments). Glass forming ability was estimated using the following numerical factors: thermal stability range: DT ¼ ðTx 2 Tg Þ [5] and stability criterion S ¼ ðTx 2 Tg ÞðTp 2 Tx Þ/Tg [6], where Tg, Tx and Tp are, respectively, the temperature in Kelvin for glass transition, onset of crystallization and exotherm maximum. As usually, the absence of crystals is checked by visual inspection. This simple method is much more sensitive than X-ray diffraction: light scattering may be perceived when crystalline fraction exceeds 1024 and microscopic observation is still more accurate. Refractive index is measured using an Abbe refractometer and thermal expansion from a Seiko TMA/SS 120. Estimated accuracy is ^ 1 8C for temperatures, ^0.0005 for refractive index and ^ 5 £ 1027 K21 for thermal expansion. An archimedean method was used for density is expected to lead to an error margin of ^ 0.01. Observation in scanning electron microscopy (SEM) were made using a JEOL 6400 set-up working at 20 kV with
Optimization of glass compositions is a necessary step for the preparation of the optical fibers and the homogenous glass samples required for physical measurements. Devitrification rate at cooling is probably the most critical phenomenon. It encompasses the classical steps of nucleation and crystal growth. The first condition for glass formation is the occurrence of a supercooled liquid, which means that it is free of nuclei. Processing conditions must be adjusted to minimize—and hopefully avoid—heterogeneous nucleation arising from crucible walls, dust and impurities. Homogenous nucleation depends on melt composition, time and temperature. Insofar as melt must cross the entire temperature range between liquidus temperature and glass transition temperature, only time— i.e. cooling rate—and chemical composition can be adjusted. Both anionic and cationic composition must be considered. In practice, fluoride glasses are hardly free of anionic impurities, especially anionic oxygen which has a significant influence on glass stability [7]. While residual oxygen may decrease the devitrification tendency of the melt [7], the reverse effect is predominant. Anionic oxygen originates from several sources: (i) oxides in starting materials; (ii) hydrolysis; (iii) contamination or incomplete fluorination during synthesis. The last item is critical when oxides are used as starting materials with the ammonium fluoride processing. The key point is that water generated in the fluorination process must be removed at low temperature to avoid further hydrolysis. Time and temperature must be adjusted as a function of batch composition. When fluorination is incomplete, crystalline white phase is formed upon melt cooling, and a similar effect is obtained when indium or gallium oxide is incorporated into the fluoride melt. Our experimental observations show that fluorination of gallium and indium oxides is more difficult than that of zirconium oxide. As a consequence, longer heating time is required for the fluorination stage: 1 h at 300 8C for a 6 g batch. In spite of the influence of cationic and anionic composition on glass stability no systematic analytic control has been implemented in current works on fluoride glass systems. The first step of this work has been to ensure that synthesis conditions were reproducible and lead to clear glass samples when applied to standard fluoroindate glasses such as IZBS. In these conditions the amount of residual
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Fig. 3. Evolution of refractive index, nD versus GaF3 content. Fig. 2. Influence of GaF3 on the value of the characteristic temperatures Tg, Tx and Tf for glass transition, onset of crystallization and melting.
oxygen is assumed to be close to that of current fluoride glasses, that is a few hundreds ppm. In a second step we have investigated the influence of cationic composition on glass formation and stability. Previous works aiming at the optimization of the InF3 – YF3 – PbF2 ternary glasses showed that more stable compositions could be obtained incorporating calcium and indium fluorides as stabilizers [8]. A typical composition was 40InF3 –5YF3 – 25PbF2 – 15CaF2 – 15ZnF2. In the first step, we have substituted indium by gallium, which is known to decrease devitrification rate in fluoroindate glasses. Glass samples were prepared with the general chemical formula: (35 2 x )InF3 – xGaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. The evolution of the stability criteria versus gallium content is reported in Fig. 1. This suggests that optimum composition corresponds to 15 – 30 mol% GaF3. In other words, fluorogallate glasses appear more stable in this multicomponent system than fluoroindates. When InF3 is replaced by GaF3, glass transition temperature increases from 260 to 296 8C. The evolution of the other characteristic temperatures—Tx for onset of crystallization and Tf for the beginning of the fusion—is reported in Fig. 2. In order to assess the real glass composition, several chemical analysis have been carried out on glass samples from the nominal composition 15InF3 – 20GaF3 – 10YF3 – 25PbF2 – 15CaF2 – 15ZnF2. These analysis were made using EDAX coupled to SEM and corresponding results are given in Table 1. The agreement between nominal and
experimental compositions seems fairly good with respect to the limited accuracy of this method for a multicomponent sample. Residual oxygen was analyzed in a few glass samples using a LECO set-up. While a few thousands ppm oxygen were detected in these samples, this concentration level was close to that of pure powdered fluorides for which the X-ray diffraction pattern showed only the pure phase. Density and refractive index decrease, while thermal expansion increases, when GaF3 replaces InF3. This evolution is exemplified by Figs. 3 – 5. These glasses are stable at room atmosphere and their chemical durability may be compared to that of fluorozirconate glasses. The influence of the other glass components on glass stability and physical properties was also investigated. Various samples were prepared according to the general formulas: 15InF3 – 10YF3 – ð40 2 xÞPbF2 – 15ZnF2 – 20GaF3 – xCaF2 15InF3 – xYF3 – ð35 2 xÞPbF2 – 15ZnF2 – 20GaF3 – 15CaF2 15InF3 – 10YF3 – ð40 2 xÞPbF2 – xZnF2 – 20GaF3 – 15CaF2 The effect of the Pb/Y, Pb/Zn and Pb/Ca substitutions may be seen from Figs. 6 – 9. Thermal stability range may reach 120 8C, which is generally sufficient for casting large bulk samples. The evolution of the physical properties versus chemical composition is consistent with previous
Table 1 Comparison between nominal and analyzed cationic compositions Elements
In
Ga
Pb
Y
Ca
Zn
Nominal Analysis 1 Analysis 2 Analysis 3
15 14.1 15.4 15.8
20 23.9 22.5 22.6
25 23.4 24.4 23.4
10 9.9 10.2 9.8
15 14.1 14.4 14.7
15 13.2 13.1 13.7
Fig. 4. Evolution of density versus GaF3 content.
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Fig. 5. Thermal expansion coefficient, a, as a function of GaF3.
knowledge of fluoride glasses: refractive index and density are correlated with lead content. These results show that more stable glasses could be obtained by convenient adjustments of chemical composition. It may be viewed as the illustration of the ‘confusion principle’ which expressed that the stability of a multicomponent glass versus devitrification generally increases as the number of glass components increases. While numerous examples support this rule, current experience shows that does not apply automatically. There is an optimum concentration range which cannot be predicted from theoretical considerations. In addition, processing has also to be adjusted in order to minimize the factors promoting heterogeneous nucleation. It is widely admitted that the diffusion processes involved for crystallization are more difficult when chemical species are more numerous. However, this is only one of the factors ruling the whole devitrification process, and some glass forming melts may devitrify more rapidly when even one additional compound is incorporated. Correlating glass forming ability to chemical com-
Fig. 6. Thermal stability range DT versus composition in the glasses: 15InF 3 – 10YF 3 – (40 2 x )(PbF2 – 15ZnF2 – 20GaF3 – xCaF2 ; 15InF 3 – x YF3 – (35 2 x )PbF 2 – 15ZnF 2 – 20GaF3 – 15CaF2 ; 15InF3 –10YF3 –(40 2 x )PbF2 –x ZnF2 –20GaF3 –15CaF2.
Fig. 7. Influence of Pb/Y, Pb/Ca and Pb/Zn substitutions on glass transition temperature Tg.
position and bonding has been a constant concern for glass scientists, and Zachariasen’s rules [9] make the most famous—and perhaps the most successful—attempt in this way. Unfortunately most approaches fail on the point that devitrification first arises from nucleation and crystal growth which are not clearly influenced by chemical bonding. It is often assumed that any glass consists in a vitreous network which has a predominant covalent character. One may be tempted to conclude that glass stability will increase if network covalency is enhanced. This would explain why the substitution of indium fluoride by gallium fluoride, which is more covalent, improves glass formation; but it is contradictory with the fact that indium incorporation has a similar effect in fluorogallate glasses [10]. In a more general way, glass formation occurs in chemical systems having all types of bonding: covalent, ionic and metallic, and there is only one example of nitride glass—nitrides are covalent— while there are thousands of fluoride—and predominantly ionic—glasses. As a consequence, glass science still remains largely empirical in this area of glass formation. Fortunately, it is largely predictable as far as physical properties are concerned. The changes in the physical properties of the
Fig. 8. Refractive index, nD, versus ZnF2, CaF2 and YF3 content.
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4. Conclusions Numerous glass compositions have been investigated in the multicomponent systems based on the fluorides of In, Ga, Pb, Ca, Zn, Y. The influence of the composition on glass stability and physical properties has been investigated. Some of these glasses are stable enough against devitrification to be cast as thick samples from which preforms for fiber drawing experiments could be made.
References Fig. 9. Density versus ZnF2, CaF2 and YF3 content.
modified fluoroindate glasses are consistent with the observations in other fluoride systems: lead fluoride is known to increase density and refractive index and to decrease glass transition temperature. The respective influences of calcium and zinc fluorides appear different for glass transition. This could be correlated to their different coordination numbers in glass structure. Fluoroindate glasses may find practical applications to extend the possibilities of the standard fluorozirconate glasses [11] such as ZBLAN because they may transmit IR light at longer wavelength [12,13]. As an example, one may quote thermometry, remote spectroscopy and CO laser transmission. However, as most of these applications require optical fibers, the level of scattering and absorption losses should be reduced. Finally, active fibers made from fluoroindate glasses may have higher efficiency for various laser transitions, and also for optical amplification at 1.3 mm [14]. The fluoroindate glasses from this study have a high refractive index. Their association with lead-free glasses makes possible the design of optical fibers with a very large numerical aperture.
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