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Te-rich Ge–Te–Se glass for the CO2 infrared detection at 15 μm
Journal of Non-Crystalline Solids 355 (2009) 2074–2078
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Te-rich Ge–Te–Se glass for the CO2 infrared detection at 15 lm S. Maurugeon a, B. Bureau a,*, C. Boussard-Plédel a, A.J. Faber b, X.H. Zhang a, W. Geliesen b, J. Lucas a a b
Laboratoire des Verres et Céramiques, UMR-CNRS 6226, Sciences chimiques de Rennes, Campus de Beaulieu, Université de Rennes1, 35042 Rennes Cedex, France TNO Science and Industry, P.O. Box 6235, 6500 HE Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Available online 16 July 2009 PACS: 61.43.Fs 65.60.+a 42.70.Km 42.81. i Keywords: Glass transition Chalcogenides Optical spectroscopy Absorption Infrared properties Glass transition
a b s t r a c t Te80 xGe20Sex glasses have been prepared along the GeSe4–GeTe4 axis using the classical method in silica tube under vacuum. A phase separation domain appears for composition around Te40Ge20Se40. Our attention was turned toward the Te-rich compositions corresponding to 1 < x < 5 at.%. These glasses are transparent from 4 to about 20 lm without any purification of the starting elements. Furthermore the difference DT between the crystallization temperature Tx and the vitreous transition temperature Tg lies at about 110 °C that is to say 30 °C higher than for the GeTe4 reference glass. Finally the introduction of a few percentages of Se makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers for example. Taking into account their transparency window, encompassing the CO2 absorption band around 15 lm, the Te80 xGe20Sex with 1 < x < 5 at.% could become matchless composition for the CO2 infrared detection as planed by the Darwin mission of the European Space Agency. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The search for infrared transmitting glasses has been largely motivated by the existence of two atmospheric transparent optical windows located in the 3–5 and 8–12 lm spectral regions just in between the water and carbon dioxide absorption bands [1–5]. Since several decades those windows have been used for designing thermal imaging systems such as IR cameras. One of the bottlenecks in the development of low cost night vision systems for car driving assistance for example is in the fabrication of IR glass moulded optics. Recent progresses in IR fiber evanescent sensors as well as a growing interest for the spectral exploration of the universe from space telescopes located out of the atmosphere have reactivated the search for glasses transmitting as far as possible in the infrared [6–9]. The typical working domain of the current fiber evanescent sensors, based on a Te/As/Se glass, is limited in the IR region to the 11 lm edge due to phonon absorption. Extending the transparency beyond this limit would of importance for analyzing the vibrational modes of chemical bonds involving heavy atoms such as halogens. Indeed many molecules which are concerned in soil pollution have their main infrared signatures beyond 11 lm [10–14]. Since the evidence of the role played by carbon dioxide in global warming because of its strong absorption in the * Corresponding author. Fax: +33 (0)2 23 56 11. E-mail address: [email protected] (B. Bureau). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.01.059
15 lm region it is clearly of interest to develop transparent vitreous optics which will covered this spectral domain for allowing the direct spectral analysis of CO2 in several conditions including for instance its storage in geological reservoir [15]. The European Space Agency (ESA) Darwin mission aims at directly analyzing extra solar earth like planets in order to detect signs life in these exoplanets. The main biological markers of life are the molecules of water, ozone and carbon dioxide which have their infrared signatures in the 6, 10, and 15 lm region and development of IR optics operating in the 4–20 lm region is critical for space exploration [16,17]. The transparency of a chalcogen based glass in the infrared region is governed by phonon absorption and the atomic weight of the chalcogen is directly correlated to the position of the IR cutoff. For instance the IR edge for a S based glass is located in the 12 lm region while for a Se based materials the edge is shifted towards the 16 lm region. To extend the transparency towards longer wavelength it is necessary to use Te the heaviest chalcogen but with the disadvantage that Te based glasses are known to be prone to easy devitrification. We already demonstrated that optical glasses could be fabricated in combining Te either with Ge and Ga [6] or Ge and I [7]. During these investigations it was clear that the addition of Ga or I to the binary Ge/Te glasses was beneficial to extend the separation DT between the crystallization temperature Tx and the glass transition temperature Tg. In this contribution the strategy is to analyse the impact of the small addition of Se to the
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binary Te–Ge glasses both on the glass stability as well as on the IR transmission. The Te–Se–Ge system has been already investigated [18–20] mainly from the point of view of selecting glasses for phase change materials than for developing optical glasses. The region of interest in our investigation corresponds to vitreous materials having the lowest content in Se since the addition of Se could be detrimental to phonon absorption as well as an excess of Se is known to provoke phase separation. In order to validate the glasses useful for optical applications the three following criteria will be investigated and discussed: (a) the influence of the Se addition on the transmission spectra especially in the phonon region, (b) the effect of Se addition on the stability of the glass versus devitrification, (c) the aptitude of bulk glass to be moulded in order to produce high quality optical elements. 2. Experimental procedure Te80 xGe20Sex glasses (with x from 0 to 80) preparation is conducted following a procedure which consists in mixing the commercial high purity Te (5 N), Se (5 N) and Ge (5 N) in the exact appropriate stoichiometry and putting the as-obtained elements into a silica tube to be sealed under vacuum. The initial chemicals were not further purified before being introduced into the silica set-up. The ampoules were then placed into a rocking furnace to prepare the melt at 750 °C (9 h). Then the batch is homogenized at 450 °C during 4 h before quenching in water, annealing at 150 °C (3 h) and, finally, cooling down slowly to the room temperature. The amorphous character of the Te80 xGe20Sex glasses was verified by X-ray diffraction. Thermal analyses were performed on a DSC 2010 TA Instruments applying a heating rate of 10 °C/min.
The differences DT between the crystallization temperature Tx and the vitreous transition temperature Tg were measured to estimate the stability of a glass toward crystallization for each composition. After cutting and polishing disks of 8 mm thickness, the IR transparency windows were studied on a Bruker Vector 22 Fourier Transform Infra Red spectrometer. The same disks were then polished a second time in the same conditions to obtain a 2 ± 0.05 mm thickness disk and to study their transparency in order to estimate the attenuation versus wavelength on bulk. 3. Results The obtained Te80 xGe20Sex materials according to the experimental protocol described previously conduced to amorphous glassy matrix except for the composition where Te and Se are in same proportions, corresponding to the Te40Ge20Se40 composition which presents incapacity for vitrification. This particular point on the GeTe4/GeSe4 axis reflects a total non-mixing zone on the vitreous diagram depicted in Fig. 1. Concerning the richest compositions in Se (50 6 x 6 80 at.%), glasses are easy to prepare and very stable against crystallization due to the excellent glass forming ability of pure-Se. Thus, these glasses do not present any crystallization peak, except for the Te30Ge20Se50 composition for which Tx = 337 °C. Counterbalancing extended thermal stabilities of the materials, the IR transmission window is limited up to 15 lm on bulk due to Se–Se and Ge–Se chemical bonds. As expected, the introduction of Te, up to 30 at.% in the initial composition, does not show any benefit in optical behaviour, keeping it close to the GeSe4 one as shown on Fig. 2. Generally speaking, a strong discrepancy is visible between the optical behaviour of Te-rich glasses compared to the Se rich glasses.
Ge 0 10
100 90
20
80
30
70
40
60
50
50
60
40
Non-mixing zone
GeSe4
GeTe4
70
30 Zone of interest
80
20
90
10
100
Se 0
0 10
20
30
40
50
60
70
80
90
100
Fig. 1. Ternary Te–Se–Ge diagram and picture of a 2 ± 0.05 mm thick Te77Ge20Se3 polished glass.
Te
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Fig. 2. Te80 xGe20Sex 2 ± 0.05 mm thickness disks IR transmission for some selected glasses corresponding to x = 0, 3, 4, 5, 10, 50 and 80.
Firstly, the gap between the transmission percentages for the both families of composition is due to reflections of Fresnel losses and depends on the refractive indices of the glasses. For the rich Se glass the 65% transmission corresponds to a refractive index n close to 2.5 while for the Te-rich glass n is estimated to be close to 3.2 in the range from 4 to 16 lm. Secondly, the cut-off appears beyond 18 lm for the Te-rich compositions, while the GeSe4 like glasses become opaque in the 14 lm region. In both cases, a typical absorption shoulder is observed around 21 lm and 17 lm, respectively. It is likely that these bands are harmonic intrinsic vibrations and correspond to a two phonons absorption mechanism. Nevertheless, for this preliminary study, the glasses were not yet totally purified and one can hopefully improve again their transmission which is clearly affected by some absorption bands around 13 lm which can be attributed to Ge–O impurities. The intensity of the Ge–O absorption band depends directly on the quality of the starting elements. Knowing that no special purification has been carried out, it is then not possible to correlate the intensity of this band and the glass compositions. A special focus is proposed here on the zone close to GeTe4 where x 6 10 at.%. Indeed, glasses belonging to this domain present an interesting compromise between an extended IR transmission window and a thermal stability permitting future fiber drawing. Table 1 lists the thermal characteristics of the glasses focused on.
Table 1 Glass transition Tg, crystallization temperature Tx and thermal stability DT = Tx–Tg for Te80 xGe20Sex glasses belonging to the zone of interest, with x = 0–10. Tg and Tx values are measured with an uncertainty of ±2 °C. Glass
Tg (°C)
Tx (°C)
DT
Te80Ge20 Te79Ge20Se1 Te78Ge20Se2 Te77Ge20Se3 Te76Ge20Se4 Te75Ge20Se5 Te70Ge20Se10
154 155 153 155 156 157 156
233 235 250 261 262 269 236
79 80 97 106 106 112 80
4. Discussion The initial GeTe4 glass exhibits a wide transparency in the infrared but is characterized by a DT = 79 °C. The DT measurement is one of the most pertinent criteria to estimate the stability of a glass towards devitrification, particularly during the shaping of the glass into optical devices. In the case of a pure tellurium glass, this difference is such that it can be the source of parasitic nucleation phenomena of metallic Te nanoparticles. From this point of view, the drawing process of the glass rod into an optical fiber is a very risky operation because the glass is heated above Tg, in its viscous regime. The presence of such Te nanoparticles can seriously compromise transmission, especially when light propagation on planar or fiber waveguide is concerned. Starting from a GeTe4 matrix, the gradual replacement of Te by Se (up to 5 at.%) conducts to an increase of the thermal stability which is more suitable with the fiber drawing needs. Particularly, compositions with 3–5 at.%, present DT values of 106, 106 and 112 °C respectively, and reveal to have the most effective Se contents. For the lowest percentages of Se, the DT increases almost regularly within the error bares. Then, in case of the Te70Ge20Se10 composition, the thermal stability decrease can be correlated to the proximity of the non-mixing zone. From a structural point of view, in the pure tellurium glass such as GeTe4, the Ge–Te bonds are extended in the network, engaging connections between GeTe4 tetrahedra. In term of infrared transmission, optical window is consequently limited by a multiphonon cut-off of very low energy due to vibrational modes associated with Ge–Te covalent bonds. Then, for the Te80 xGe20Sex glasses with x = 3, 4 or 5 at.%, the introduced selenium is a minor component and it can be assumed that the phonon edge remains mainly associated with Ge–Te vibrations. This effect is described in Fig. 2, which compares the optical window of the Te80 xGe20Sex glasses (thickness 2 ± 0.05 mm) with GeSe4 and GeTe4. As expected, it is observed that the transmission of the Te80 xGe20Sex glasses extend far in the IR with a cut-off close to GeTe4 for 0 < x < 5 at.%. The substitution of Te by low at.% of Se does not affect significantly the IR cut-off on bulk until x = 3 at.%. Then, for the 5 and
S. Maurugeon et al. / Journal of Non-Crystalline Solids 355 (2009) 2074–2078
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Fig. 3. Evaluation of the optical attenuation of the Te77Ge20Se3 glass measured by division of the transmission between two bulks of 8 ± 0.05 mm and a 2 ± 0.05 mm thicknesses. The comparison with CO2 absorption spectra shows that such glass could be the good candidate for the CO2 detection in space.
10 at.% compositions, a slight shift is observable on Fig. 3 which will be harmful to the propagation of light into guides fabricated with these glasses. So, the Te77Ge20Se3 glass constitutes the best compromise between the thermal stability against crystallization on one hand, and the largest extended IR transmission window on the other hand. Finally, in view of its interesting properties and considering the easiness to prepare the rods, this glass is to be declared as one of the most suitable candidate, in the same way as pure tellurium glasses in the TeGeGa and the TeGeI glass families, for applications in the infrared such as the Darwin ESA program. Considering the potential of the glass, it is interesting to estimate the optical losses through the material. For this purpose, the glass has been purified using metallic Al to trap the remaining oxygen in order to reduce the oxide absorption bands. Deduced from the 2 IR scans recorded on a 8 ± 0.05 mm and a 2 ± 0.05 mm thickness bulks, Fig. 3 shows the attenuation curve of the Te77Ge20Se3 glass. Attenuation values, given in dB/m, are more indicative than revealing real potential of the material because of the short optical path. Nevertheless, the main information is the glass potential to conduct IR light with an extended transmission window up to 16 lm. In order to validate the potential of the Te80 xGe20Sex glasses for carbon dioxide spectral characterization, the vibrational spectra of CO2 gas is also shown in Fig. 3. The two bands around 4 lm due to the C–O vibrations and around 15 lm due to bending and rotation of the molecules are located within the high transparency region of the glass. This observation demonstrates the good potential of Te77Ge20Se3 for designing planar waveguides or optical glass fibers operating at these wavelengths. Clearly, this glass composition suits well with the requirements of the European Darwin mission. 5. Conclusions Glasses along the GeSe4–GeTe4 axis were studied in the Ge–Te– Se ternary diagram. A special attention has been paid to the compositions rich in Te. The replacement of a few percentages of Te by Se gives rise to stable glasses against crystallization. The DT in-
creases up to 106, 106 and 112 °C, for 3, 4 and 5 at.%, respectively. That corresponds to an improvement of about 30 °C compared to the GeTe4 reference glass. Among them, the Te77Ge20Se3 composition appears as the best compromise between thermal stability and optical transparency. Its optical cut-off extends beyond 18 lm, not so far from the GeTe4 glass, and encompasses the CO2 wide absorption band around 15 lm. In order to enlarge again the optical window and to lower the attenuation level, further purification operations are currently ongoing. The determinations of the best parameters of drawing for this glass are also under investigation. These achievements could hopefully qualify this glass composition for the CO2 absorption band detection as requested by the ESA in the framework of the Darwin mission. Acknowledgements The authors thank the European Space Agency for financial support: Single Mode Waveguide, European Spatial Agency contract A0/ 1-S262/07/NL/PM (2007–2009). References [1] X.H. Zhang, B. Bureau, C. Boussard, H.L. Ma, J. Lucas, Chemistry 14 (2008) 432. [2] B. Bureau, Jean Luc Adam, in: G. Meyer, D. Nauman, L. Weseman (Eds.), Inorg. Chem. Highlights, vol. 2, Wiley-VCH, Weiheim, 2005, pp. 365–392. [3] V.F. Kokorina, Glasses for Infrared Optics, in: M.J. Weber (Ed.), Laser and Optical Science and Technology Series, CRC Press, 1996. [4] M.A. Popescu, Non-crystalline Chalcogenides, Kluwer Academic, 2000. [5] J.A. Savage, J. Non-Cryst. Solids 11 (1972) 121. [6] S. Danto, P. Houizot, C. Boussard-Plédel, X.H. Zhang, F. Smektala, J. Lucas, Adv. Funct. Mat. 16 (14) (2006) 1847. [7] A. Wilhelm, C. Boussard-Plédel, Q. Coulombier, J. Lucas, B. Bureau, P. Lucas, Adv. Mat. 19 (2007) 3796. [8] B. Bureau, S. Danto, H.L. Ma, C. Boussard-Pledel, X.H. Zhang, J. Lucas, Solid State Sci. 10 (4) (2008) 427. [9] C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J. Non-Cryst. Solids 353 (2007) 1388. [10] S. Hocde, C. Boussard-Plédel, G. Fonteneau, J. Lucas, Solid State Sci. 3 (2001) 279. [11] B. Bureau, X.H. Zhang, F. Smektala, J.L. Adam, J. Troles, H.L. Ma, C. BoussardPledel, J. Lucas, P. Lucas, D Le. Coq, M.R. Riley, J.H. Simmons, J. Non-Cryst. Solids 345 (2004) 276.
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[12] H. Steiner, M. Jakush, Kraft, B. Mizaïkov, M. Kerlowatz, B. Mizaïkov, T. Baumann, R. Niessner, Y. Reichlin, A. Katzir, W. Konz, A. Brandenburg, N. Fleischmann, K. Staubmann, R. Allabashi, K. Michel, C. Boussard, B. Bureau, J. Lucas, J.M. Bayona, Appl. Spect. 57 (6) (2003) 607. [13] D. Le Coq, K. Michel, J. Keirsse, C. Boussard-Plédel, G. Fonteneau, B. Bureau, J.M. Le Quéré, O. Sire, J. Lucas, C.R. Chim. 5 (2002) 907. [14] K. Michel, B. Bureau, C. Boussard-Plédel, T. Jouan, J.L. Adam, K. Staubmann, T. Baumann, Sensor Actuat. B 101 (2004) 252. [15] F. Charpentier, B. Bureau, J. Troles, C. Boussard-Plédel, K. Michel-Le Pierrès, F. Smektala, J.-L. Adam, Opt. Mater. 31 (3) (2009) 496.
[16] C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J.E. Broquin, L. Labadie, P. Kern, Appl. Phys. Lett. 90 (2007) 1. [17] P. Houizot, C. Boussard-Plédel, A.J. Faber, L.K. Cheng, B. Bureau, P.A. Van Nijnatten, W.L.M. Gielesen, J. Pereira do Carmo, J. Lucas, Opt. Exp. 15 (19) (2007) 12529. [18] T. Katsuyama, H. Matsumura, Appl. Phys. Lett. 49 (1) (1986). [19] T. Katsuyama, H. Matsumura, J. Appl. Phys. 75 (6) (1994). [20] T. Katsuyama, H. Matsumura, J. Appl. Phys. 76 (4) (1994).
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Te-rich Ge–Te–Se glass for the CO2 infrared detection at 15 lm S. Maurugeon a, B. Bureau a,*, C. Boussard-Plédel a, A.J. Faber b, X.H. Zhang a, W. Geliesen b, J. Lucas a a b
Laboratoire des Verres et Céramiques, UMR-CNRS 6226, Sciences chimiques de Rennes, Campus de Beaulieu, Université de Rennes1, 35042 Rennes Cedex, France TNO Science and Industry, P.O. Box 6235, 6500 HE Eindhoven, The Netherlands
a r t i c l e
i n f o
Article history: Available online 16 July 2009 PACS: 61.43.Fs 65.60.+a 42.70.Km 42.81. i Keywords: Glass transition Chalcogenides Optical spectroscopy Absorption Infrared properties Glass transition
a b s t r a c t Te80 xGe20Sex glasses have been prepared along the GeSe4–GeTe4 axis using the classical method in silica tube under vacuum. A phase separation domain appears for composition around Te40Ge20Se40. Our attention was turned toward the Te-rich compositions corresponding to 1 < x < 5 at.%. These glasses are transparent from 4 to about 20 lm without any purification of the starting elements. Furthermore the difference DT between the crystallization temperature Tx and the vitreous transition temperature Tg lies at about 110 °C that is to say 30 °C higher than for the GeTe4 reference glass. Finally the introduction of a few percentages of Se makes the glasses much easier to prepare and more stable against crystallization, making them drawable as optical fibers for example. Taking into account their transparency window, encompassing the CO2 absorption band around 15 lm, the Te80 xGe20Sex with 1 < x < 5 at.% could become matchless composition for the CO2 infrared detection as planed by the Darwin mission of the European Space Agency. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The search for infrared transmitting glasses has been largely motivated by the existence of two atmospheric transparent optical windows located in the 3–5 and 8–12 lm spectral regions just in between the water and carbon dioxide absorption bands [1–5]. Since several decades those windows have been used for designing thermal imaging systems such as IR cameras. One of the bottlenecks in the development of low cost night vision systems for car driving assistance for example is in the fabrication of IR glass moulded optics. Recent progresses in IR fiber evanescent sensors as well as a growing interest for the spectral exploration of the universe from space telescopes located out of the atmosphere have reactivated the search for glasses transmitting as far as possible in the infrared [6–9]. The typical working domain of the current fiber evanescent sensors, based on a Te/As/Se glass, is limited in the IR region to the 11 lm edge due to phonon absorption. Extending the transparency beyond this limit would of importance for analyzing the vibrational modes of chemical bonds involving heavy atoms such as halogens. Indeed many molecules which are concerned in soil pollution have their main infrared signatures beyond 11 lm [10–14]. Since the evidence of the role played by carbon dioxide in global warming because of its strong absorption in the * Corresponding author. Fax: +33 (0)2 23 56 11. E-mail address: [email protected] (B. Bureau). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.01.059
15 lm region it is clearly of interest to develop transparent vitreous optics which will covered this spectral domain for allowing the direct spectral analysis of CO2 in several conditions including for instance its storage in geological reservoir [15]. The European Space Agency (ESA) Darwin mission aims at directly analyzing extra solar earth like planets in order to detect signs life in these exoplanets. The main biological markers of life are the molecules of water, ozone and carbon dioxide which have their infrared signatures in the 6, 10, and 15 lm region and development of IR optics operating in the 4–20 lm region is critical for space exploration [16,17]. The transparency of a chalcogen based glass in the infrared region is governed by phonon absorption and the atomic weight of the chalcogen is directly correlated to the position of the IR cutoff. For instance the IR edge for a S based glass is located in the 12 lm region while for a Se based materials the edge is shifted towards the 16 lm region. To extend the transparency towards longer wavelength it is necessary to use Te the heaviest chalcogen but with the disadvantage that Te based glasses are known to be prone to easy devitrification. We already demonstrated that optical glasses could be fabricated in combining Te either with Ge and Ga [6] or Ge and I [7]. During these investigations it was clear that the addition of Ga or I to the binary Ge/Te glasses was beneficial to extend the separation DT between the crystallization temperature Tx and the glass transition temperature Tg. In this contribution the strategy is to analyse the impact of the small addition of Se to the
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binary Te–Ge glasses both on the glass stability as well as on the IR transmission. The Te–Se–Ge system has been already investigated [18–20] mainly from the point of view of selecting glasses for phase change materials than for developing optical glasses. The region of interest in our investigation corresponds to vitreous materials having the lowest content in Se since the addition of Se could be detrimental to phonon absorption as well as an excess of Se is known to provoke phase separation. In order to validate the glasses useful for optical applications the three following criteria will be investigated and discussed: (a) the influence of the Se addition on the transmission spectra especially in the phonon region, (b) the effect of Se addition on the stability of the glass versus devitrification, (c) the aptitude of bulk glass to be moulded in order to produce high quality optical elements. 2. Experimental procedure Te80 xGe20Sex glasses (with x from 0 to 80) preparation is conducted following a procedure which consists in mixing the commercial high purity Te (5 N), Se (5 N) and Ge (5 N) in the exact appropriate stoichiometry and putting the as-obtained elements into a silica tube to be sealed under vacuum. The initial chemicals were not further purified before being introduced into the silica set-up. The ampoules were then placed into a rocking furnace to prepare the melt at 750 °C (9 h). Then the batch is homogenized at 450 °C during 4 h before quenching in water, annealing at 150 °C (3 h) and, finally, cooling down slowly to the room temperature. The amorphous character of the Te80 xGe20Sex glasses was verified by X-ray diffraction. Thermal analyses were performed on a DSC 2010 TA Instruments applying a heating rate of 10 °C/min.
The differences DT between the crystallization temperature Tx and the vitreous transition temperature Tg were measured to estimate the stability of a glass toward crystallization for each composition. After cutting and polishing disks of 8 mm thickness, the IR transparency windows were studied on a Bruker Vector 22 Fourier Transform Infra Red spectrometer. The same disks were then polished a second time in the same conditions to obtain a 2 ± 0.05 mm thickness disk and to study their transparency in order to estimate the attenuation versus wavelength on bulk. 3. Results The obtained Te80 xGe20Sex materials according to the experimental protocol described previously conduced to amorphous glassy matrix except for the composition where Te and Se are in same proportions, corresponding to the Te40Ge20Se40 composition which presents incapacity for vitrification. This particular point on the GeTe4/GeSe4 axis reflects a total non-mixing zone on the vitreous diagram depicted in Fig. 1. Concerning the richest compositions in Se (50 6 x 6 80 at.%), glasses are easy to prepare and very stable against crystallization due to the excellent glass forming ability of pure-Se. Thus, these glasses do not present any crystallization peak, except for the Te30Ge20Se50 composition for which Tx = 337 °C. Counterbalancing extended thermal stabilities of the materials, the IR transmission window is limited up to 15 lm on bulk due to Se–Se and Ge–Se chemical bonds. As expected, the introduction of Te, up to 30 at.% in the initial composition, does not show any benefit in optical behaviour, keeping it close to the GeSe4 one as shown on Fig. 2. Generally speaking, a strong discrepancy is visible between the optical behaviour of Te-rich glasses compared to the Se rich glasses.
Ge 0 10
100 90
20
80
30
70
40
60
50
50
60
40
Non-mixing zone
GeSe4
GeTe4
70
30 Zone of interest
80
20
90
10
100
Se 0
0 10
20
30
40
50
60
70
80
90
100
Fig. 1. Ternary Te–Se–Ge diagram and picture of a 2 ± 0.05 mm thick Te77Ge20Se3 polished glass.
Te
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Fig. 2. Te80 xGe20Sex 2 ± 0.05 mm thickness disks IR transmission for some selected glasses corresponding to x = 0, 3, 4, 5, 10, 50 and 80.
Firstly, the gap between the transmission percentages for the both families of composition is due to reflections of Fresnel losses and depends on the refractive indices of the glasses. For the rich Se glass the 65% transmission corresponds to a refractive index n close to 2.5 while for the Te-rich glass n is estimated to be close to 3.2 in the range from 4 to 16 lm. Secondly, the cut-off appears beyond 18 lm for the Te-rich compositions, while the GeSe4 like glasses become opaque in the 14 lm region. In both cases, a typical absorption shoulder is observed around 21 lm and 17 lm, respectively. It is likely that these bands are harmonic intrinsic vibrations and correspond to a two phonons absorption mechanism. Nevertheless, for this preliminary study, the glasses were not yet totally purified and one can hopefully improve again their transmission which is clearly affected by some absorption bands around 13 lm which can be attributed to Ge–O impurities. The intensity of the Ge–O absorption band depends directly on the quality of the starting elements. Knowing that no special purification has been carried out, it is then not possible to correlate the intensity of this band and the glass compositions. A special focus is proposed here on the zone close to GeTe4 where x 6 10 at.%. Indeed, glasses belonging to this domain present an interesting compromise between an extended IR transmission window and a thermal stability permitting future fiber drawing. Table 1 lists the thermal characteristics of the glasses focused on.
Table 1 Glass transition Tg, crystallization temperature Tx and thermal stability DT = Tx–Tg for Te80 xGe20Sex glasses belonging to the zone of interest, with x = 0–10. Tg and Tx values are measured with an uncertainty of ±2 °C. Glass
Tg (°C)
Tx (°C)
DT
Te80Ge20 Te79Ge20Se1 Te78Ge20Se2 Te77Ge20Se3 Te76Ge20Se4 Te75Ge20Se5 Te70Ge20Se10
154 155 153 155 156 157 156
233 235 250 261 262 269 236
79 80 97 106 106 112 80
4. Discussion The initial GeTe4 glass exhibits a wide transparency in the infrared but is characterized by a DT = 79 °C. The DT measurement is one of the most pertinent criteria to estimate the stability of a glass towards devitrification, particularly during the shaping of the glass into optical devices. In the case of a pure tellurium glass, this difference is such that it can be the source of parasitic nucleation phenomena of metallic Te nanoparticles. From this point of view, the drawing process of the glass rod into an optical fiber is a very risky operation because the glass is heated above Tg, in its viscous regime. The presence of such Te nanoparticles can seriously compromise transmission, especially when light propagation on planar or fiber waveguide is concerned. Starting from a GeTe4 matrix, the gradual replacement of Te by Se (up to 5 at.%) conducts to an increase of the thermal stability which is more suitable with the fiber drawing needs. Particularly, compositions with 3–5 at.%, present DT values of 106, 106 and 112 °C respectively, and reveal to have the most effective Se contents. For the lowest percentages of Se, the DT increases almost regularly within the error bares. Then, in case of the Te70Ge20Se10 composition, the thermal stability decrease can be correlated to the proximity of the non-mixing zone. From a structural point of view, in the pure tellurium glass such as GeTe4, the Ge–Te bonds are extended in the network, engaging connections between GeTe4 tetrahedra. In term of infrared transmission, optical window is consequently limited by a multiphonon cut-off of very low energy due to vibrational modes associated with Ge–Te covalent bonds. Then, for the Te80 xGe20Sex glasses with x = 3, 4 or 5 at.%, the introduced selenium is a minor component and it can be assumed that the phonon edge remains mainly associated with Ge–Te vibrations. This effect is described in Fig. 2, which compares the optical window of the Te80 xGe20Sex glasses (thickness 2 ± 0.05 mm) with GeSe4 and GeTe4. As expected, it is observed that the transmission of the Te80 xGe20Sex glasses extend far in the IR with a cut-off close to GeTe4 for 0 < x < 5 at.%. The substitution of Te by low at.% of Se does not affect significantly the IR cut-off on bulk until x = 3 at.%. Then, for the 5 and
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Fig. 3. Evaluation of the optical attenuation of the Te77Ge20Se3 glass measured by division of the transmission between two bulks of 8 ± 0.05 mm and a 2 ± 0.05 mm thicknesses. The comparison with CO2 absorption spectra shows that such glass could be the good candidate for the CO2 detection in space.
10 at.% compositions, a slight shift is observable on Fig. 3 which will be harmful to the propagation of light into guides fabricated with these glasses. So, the Te77Ge20Se3 glass constitutes the best compromise between the thermal stability against crystallization on one hand, and the largest extended IR transmission window on the other hand. Finally, in view of its interesting properties and considering the easiness to prepare the rods, this glass is to be declared as one of the most suitable candidate, in the same way as pure tellurium glasses in the TeGeGa and the TeGeI glass families, for applications in the infrared such as the Darwin ESA program. Considering the potential of the glass, it is interesting to estimate the optical losses through the material. For this purpose, the glass has been purified using metallic Al to trap the remaining oxygen in order to reduce the oxide absorption bands. Deduced from the 2 IR scans recorded on a 8 ± 0.05 mm and a 2 ± 0.05 mm thickness bulks, Fig. 3 shows the attenuation curve of the Te77Ge20Se3 glass. Attenuation values, given in dB/m, are more indicative than revealing real potential of the material because of the short optical path. Nevertheless, the main information is the glass potential to conduct IR light with an extended transmission window up to 16 lm. In order to validate the potential of the Te80 xGe20Sex glasses for carbon dioxide spectral characterization, the vibrational spectra of CO2 gas is also shown in Fig. 3. The two bands around 4 lm due to the C–O vibrations and around 15 lm due to bending and rotation of the molecules are located within the high transparency region of the glass. This observation demonstrates the good potential of Te77Ge20Se3 for designing planar waveguides or optical glass fibers operating at these wavelengths. Clearly, this glass composition suits well with the requirements of the European Darwin mission. 5. Conclusions Glasses along the GeSe4–GeTe4 axis were studied in the Ge–Te– Se ternary diagram. A special attention has been paid to the compositions rich in Te. The replacement of a few percentages of Te by Se gives rise to stable glasses against crystallization. The DT in-
creases up to 106, 106 and 112 °C, for 3, 4 and 5 at.%, respectively. That corresponds to an improvement of about 30 °C compared to the GeTe4 reference glass. Among them, the Te77Ge20Se3 composition appears as the best compromise between thermal stability and optical transparency. Its optical cut-off extends beyond 18 lm, not so far from the GeTe4 glass, and encompasses the CO2 wide absorption band around 15 lm. In order to enlarge again the optical window and to lower the attenuation level, further purification operations are currently ongoing. The determinations of the best parameters of drawing for this glass are also under investigation. These achievements could hopefully qualify this glass composition for the CO2 absorption band detection as requested by the ESA in the framework of the Darwin mission. Acknowledgements The authors thank the European Space Agency for financial support: Single Mode Waveguide, European Spatial Agency contract A0/ 1-S262/07/NL/PM (2007–2009). References [1] X.H. Zhang, B. Bureau, C. Boussard, H.L. Ma, J. Lucas, Chemistry 14 (2008) 432. [2] B. Bureau, Jean Luc Adam, in: G. Meyer, D. Nauman, L. Weseman (Eds.), Inorg. Chem. Highlights, vol. 2, Wiley-VCH, Weiheim, 2005, pp. 365–392. [3] V.F. Kokorina, Glasses for Infrared Optics, in: M.J. Weber (Ed.), Laser and Optical Science and Technology Series, CRC Press, 1996. [4] M.A. Popescu, Non-crystalline Chalcogenides, Kluwer Academic, 2000. [5] J.A. Savage, J. Non-Cryst. Solids 11 (1972) 121. [6] S. Danto, P. Houizot, C. Boussard-Plédel, X.H. Zhang, F. Smektala, J. Lucas, Adv. Funct. Mat. 16 (14) (2006) 1847. [7] A. Wilhelm, C. Boussard-Plédel, Q. Coulombier, J. Lucas, B. Bureau, P. Lucas, Adv. Mat. 19 (2007) 3796. [8] B. Bureau, S. Danto, H.L. Ma, C. Boussard-Pledel, X.H. Zhang, J. Lucas, Solid State Sci. 10 (4) (2008) 427. [9] C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J. Non-Cryst. Solids 353 (2007) 1388. [10] S. Hocde, C. Boussard-Plédel, G. Fonteneau, J. Lucas, Solid State Sci. 3 (2001) 279. [11] B. Bureau, X.H. Zhang, F. Smektala, J.L. Adam, J. Troles, H.L. Ma, C. BoussardPledel, J. Lucas, P. Lucas, D Le. Coq, M.R. Riley, J.H. Simmons, J. Non-Cryst. Solids 345 (2004) 276.
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S. Maurugeon et al. / Journal of Non-Crystalline Solids 355 (2009) 2074–2078
[12] H. Steiner, M. Jakush, Kraft, B. Mizaïkov, M. Kerlowatz, B. Mizaïkov, T. Baumann, R. Niessner, Y. Reichlin, A. Katzir, W. Konz, A. Brandenburg, N. Fleischmann, K. Staubmann, R. Allabashi, K. Michel, C. Boussard, B. Bureau, J. Lucas, J.M. Bayona, Appl. Spect. 57 (6) (2003) 607. [13] D. Le Coq, K. Michel, J. Keirsse, C. Boussard-Plédel, G. Fonteneau, B. Bureau, J.M. Le Quéré, O. Sire, J. Lucas, C.R. Chim. 5 (2002) 907. [14] K. Michel, B. Bureau, C. Boussard-Plédel, T. Jouan, J.L. Adam, K. Staubmann, T. Baumann, Sensor Actuat. B 101 (2004) 252. [15] F. Charpentier, B. Bureau, J. Troles, C. Boussard-Plédel, K. Michel-Le Pierrès, F. Smektala, J.-L. Adam, Opt. Mater. 31 (3) (2009) 496.
[16] C. Vigreux-Bercovici, E. Bonhomme, A. Pradel, J.E. Broquin, L. Labadie, P. Kern, Appl. Phys. Lett. 90 (2007) 1. [17] P. Houizot, C. Boussard-Plédel, A.J. Faber, L.K. Cheng, B. Bureau, P.A. Van Nijnatten, W.L.M. Gielesen, J. Pereira do Carmo, J. Lucas, Opt. Exp. 15 (19) (2007) 12529. [18] T. Katsuyama, H. Matsumura, Appl. Phys. Lett. 49 (1) (1986). [19] T. Katsuyama, H. Matsumura, J. Appl. Phys. 75 (6) (1994). [20] T. Katsuyama, H. Matsumura, J. Appl. Phys. 76 (4) (1994).