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Tellurium based glasses have been studied for their optical properties in the far infrared region. New glasses, in the system Ge-Te-AgI, present a very good thermal stability. Indeed, for the first time, no obvious crystallization peak is observed in tellurium based glasses. Then, fibers have been drawn showing interesting optical losses and quite large transmission in the mid-infrared. So, these glasses are good candidates for the elaboration of single mode fibers able to detect the CO2 signature at 15µm for the ESA Darwin project.
©2012 Optical Society of America
1. Introduction
Chalcogenide glasses belong to an original family of vitreous materials built from sulphur, selenium and/or tellurium. Due to the large atomic mass of these elements compared to oxygen, chalcogenide glasses are known for their large transparency window extending much further than classical silica based glasses. Indeed, they can be transparent from the visible region up to the mid infrared depending on the glass composition [1–12]. Due to their capability to transmit IR light, chalcogenide glasses are matchless materials for the design of optical sensing devices working in the mid-infrared range which contains the fundamental vibration (stretching and bending) modes of molecules and biomolecules [13]. Selenide glasses can quite easily be shaped into optical fibers, which are used to carry out remote infrared spectrometry in biology or medicine for example [14–19]. At this stage, this technology, called FEWS for Fiber Evanescent Wave Spectroscopy, is sufficiently mature and promising and has given rise to a start-up developing such selenide based sensing devices for medical applications [20]. Nevertheless, the transmission of selenide glass optical fibers is limited to 11 µm. This is a strong and detrimental limitation because numerous chemical or bio-chemical species present absorption bands above 11 µm.
Thus, the development of devices capable of operating up to 20 µm is of primary interest to improve the sensitivity of bio-medical infrared fiber probes and also to reach the very specific CO2 broad absorption band located around 15 µm. The ability to detect CO2 has become increasingly important for the two following reasons. First, in environmental science, the accurate detection of CO2 in the earth’s atmosphere is critical for the study of environmental processes such as global warming. For example, it has been proposed to store CO2 in natural underground geological formations in order to alleviate its detrimental effect. This would require specific monitoring to detect leakages [21]. Second, development of new infrared materials is also relevant in the field of extraterrestrial exploration because CO2 is produced by living organisms and is therefore regarded as one of the markers of potential life on telluric exoplanets [22,23]. Thus, space programs such as the Darwin mission conducted by the European Space Agency ESA or Terrestrial Planet Finder (TPF) under the leadership of the National Aeronautics and Space Administration NASA are currently aiming at deploying novel infrared optical components into space that will permit detection of CO2 on remote planets.
To achieve these societal goals, one needs to develop new glasses fulfilling at least three requirements. First, they need to transmit light far in the infrared to cover the 15 µm range. Second, they have to possess good rheological properties for elaboration of complex devices including optical fibers and molded lenses. Third, they also have to be stable against crystallization to be able to endure the high temperature applied during the shaping process.
To attain the first objective, one has to consider glasses strictly based on tellurium and containing only heavy elements close to tellurium on the periodic chart. However, pure tellurium glasses need an extremely fast quenching such as splat cooling. This has made tellurium glasses very suitable for the fast and reversible glass to crystal transformation required for optical storage applications. In particular, glasses of the ternary system Ge/Sb/Te are widely used in Digital Versatile Disk (DVD) technology [24]. However, this is a major drawback for optical applications that require stable glasses. One way of preventing the tendency of Te to nucleate metallic crystallites in the melt is to reduce the number of free electrons. This strategy has been achieved by grafting halogen atoms along the Te chains as demonstrated in the discovery of the so-called TeX glasses [25]. The main features characterizing these materials are their low thermal and weak mechanical properties due to the low dimensionality of the glassy framework, similar to the chains structure of Te itself. Thus, these glasses are not suitable for the fabrication of optical device working at room temperature. Following the same strategy, the Ge/Ga/Te (known as GGT glasses) and the Ge/Te/I (known as GTI glasses) systems have also been recently investigated [26,27]. Some interesting glass compositions have been identified with better thermal and mechanical properties than for the above TeX glasses thanks to the higher dimensionality of their network provided by GeTe4 tetrahedral structural units. Nevertheless the ΔT values (difference between the vitreous transition temperature Tg and the temperature Tx of the crystallization peak) are still too small to guarantee any devitrification problem when shaping the glasses either by moulding or by drawing optical fibers. Indeed, ΔT is one of the most pertinent criteria to estimate the stability of a glass towards crystallization. In the case of the GGT (ΔT = 113°C) this difference is such that it can be the source of parasitic nucleation phenomena of metallic Te nanoparticles if the glass is heated above Tg. For GTI glasses ΔT = 124°C, which is appropriated for molding but not high enough to ensure the feasibility of optical fiber processing which necessitate to work in the viscous regime of the glass. At this time the only way to obtain optical fibers from a tellurium-rich based glass consists in introducing a small amount of selenium in the composition to mitigate the nucleation process [28,29].
Here, we report the synthesis of a new family of glassy tellurides that fulfills all the requirements mentioned above and could therefore enable the development of a new generation of infrared systems. These glasses result from the combination of the Ge/Te covalent systems with the silver iodine AgI ionic salt. These new tellurium glasses exhibit several promising properties: they possess a large optical window extending from 2µm to 20µm when measured on bulk glasses and do not exhibit any observable crystallization peak on thermal analysis curves. Indeed, it has already been reported that the introduction of the elementary electronegative iodine into the GeTe4 initial vitreous stoichiometry enable to trap free electrons from tellurium thereby helping to form more stable glasses [27]. Moreover, the heavy atomic weight of iodine, which is neighbor to tellurium in the periodic table, allows maintaining the low phonon character of the glassy matrix, therefore retaining the far IR transparency while providing improved rheological properties. However, high-purity synthesis reproducibility is difficult since elementary iodine is a volatile compound. The addition of Ag is also expected to stabilize the glass. Ramesh and al. showed that Ag addition in Ge-Te vitreous systems could improve the glass forming ability because Ag possesses its own substructure which constrains the glassy network [30]. Finally, several additional studies indicate that addition of metal-halide in chalcogenides increases the crystallization temperatures and the optical transmission windows of the different glasses [31,32]. Hence, in order to investigate these potential benefits, samples in the pseudo binary system (GeTe4)100-x(AgI)x with x = 5, 10, 15, 20 and 25 were prepared by the traditional melt-quenching method.
2. Experimental
Raw materials with 5N purity were stored in an oxygen- and water-free glove box and were weighed in the adequate proportion, then introduced in a silica tube which was sealed under vacuum. Silver iodine is a non-volatile powder much easier to weight and to control the quantity than the volatile metal basis iodine used for the synthesis of Ge-Te-I glasses. The ampoules were then placed in a rocking furnace for 10 hours at 750 °C to homogenize the mixture and obtain the melt. The temperature was then decreased to 500 °C before quenching in water and the ampoules where then placed in a preheated furnace at 10°C below Tg, which is the vitreous transition temperature previously determined. Glass rods were then released from the silica ampoules.
Several samples with the composition x = 10 were annealed at Tg + 30°C for 20, 40, 75 and 100h. XRD were performed on powder for each composition samples in order to confirm the amorphous state, and for annealing samples in order to follow the crystallization in the glass, with a Phillips PW3020 diffractometer (Voltage 40 kV, current 30 mA) with CuKα1 radiation. XRD were registered in 2θ scale between 10 and 70°.
Thermal analyses were performed by Differential Scanning Calorimetry (DSC) with a DSC Q20 TA Instruments in order to determinate the difference ΔT between the crystallization temperature Tc and the vitreous transition temperature Tg which is a criteria of glasses stability. A heating rate of 10 °C.min-1 in the range 20-300°C under an argon flow of 70 mL/min is applied.
The IR transparency windows were studied on polished bulk disks with a Bruker Tensor 37 Fourier Transform Infrared (FTIR) spectrometer at room temperature. The optical fiber was drawn from a (GeTe4)90(AgI)10 glass rod with the fiber-drawing tower made in the laboratory. Several meters of the fiber with a diameter of 400 µm were obtained and no coating was applied. the optical losses were determined by the cut-back method with the same Bruker spectrometer. The ability of this fiber to detect the CO2 was demonstrated by detecting the IR light passed through the fiber and a CO2 enriched atmosphere with the same bruker spectrometer.
3. Results and discussion
The results of the thermal analysis are given in Table 1 . To our knowledge, for the first time, a tellurium based glass does not exhibit any exothermic crystallization peak by thermal analysis. For instance, even the tellurium glasses containing Se show some crystallization peaks (see Table 1) [28]. Three of the studied composition, including x = 10, 15 and 20, possess this strong thermal stability.
Figure 1 shows the typical DSC curve of those glasses through the example of the glass composition x = 10. Clearly, no crystallization peak is visible. Note that a recent study reported on some close compositions, but crystallization peaks were observed in all glasses probably because of differences in the synthesis process (raw materials, storing, purification, distillation, …) [33].
Fig. 1 DSC curve of (GeTe4)90(AgI)10 glass. In our knowledge, it is the first tellurium based glass showing no crystallization peak up to 300°C.
In order to confirm its thermal stability, the (GeTe4)90(AgI)10 glass was annealed at 175°C (Tg + 30°C) for increasing times up to 100h. X-Ray Diffraction diagrams have been successively recorded at 0, 20, 40, 75 and 100 h. The first crystalline peaks appeared only after 40h as shown on Fig. 2 . As for GGT or GTI systems, it is the pure Te crystalline phase which appears after this long heating treatment. Overall these experiments confirm the very good stability against crystallization of these new tellurium glasses.
Fig. 2 XRD diagram of (GeTe4)90(AgI)10 annealed glass at 175°C (Tg + 30°C) during 0, 20, 40, 75 and 100h. Crystallization of Tellurium appears only after more than 20h of annealing.
This essential property is obtained by the addition of AgI in the initial GeTe4 stoichiometry, validating the above mentioned benefits of Ag and I addition. As for GGT and GTI glasses, the glass forming domain is located around the composition GeTe4: (GeTe4)xAgI(1-x). By analogy to the Ge/Se and Ge/S systems, the structural model which illustrates the glass forming ability is based on GeTe4 tetrahedra which are 3D connected by sharing Te-Te pairs. Iodine is both an electronegative and monovalent element and tends to break the Te-Te chains because it creates some terminal bonds. So its chemical behavior in such a glass likely prevents tellurium from nucleating and crystallizing. Ag is also known for stabilizing tellurium glasses. The phase diagram of Ag-Te has been investigated by different authors [34,35]. It was reported that multiple Ag-Te crystalline stable phases exist such as Ag2Te, Ag5Te3 and Ag1.9Te5, as well as metastable phases such as AgTe, AgTe4 and Ag3Te8. The competition between numerous crystalline phases is typically beneficial for the glass forming ability of a liquidus because of the availability of multiple path rather than a unique thermodynamically stable configuration, thereby promoting disordered solutions.
Some structural characterization studies are in progress to better understand the role played by AgI in the amorphous network. In particular, it would be interesting to know the oxidation degree of Ag and I in the glassy framework.
Chalcogenide glasses are generally opaque in the visible spectral region, whereas they exhibit wide transmission window in the Infrared spectral region. The IR transmission windows of the different glasses were determinated using polished disks. It can be seen on Fig. 3 that (GeTe4)90(AgI)10 glass presents a large IR windows transparency extending from Near-Infrared (NIR) at 2 µm to Far-Infrared (FIR) at 25 µm. The begin of the cut-off appearing beyond 20µm is correlated to the glassy network phonon energy. This long-wavelength cut-off edge has to be attributed to Ge which is the lightest element. Thus, knowing that Ge-Te covalent bond energy is about twice that of Ge-I (456 versus 212 kJ/mol), it can be concluded that the multiphonon cut-off is due to Ge-Te bonds. This conclusion is corroborated by the two following observations. First, the cut-off edges remain unchanged with the percentage of AgI (from 5% to 20%) in the glass composition. So neither I nor Ag play a significant role on the glass transmission. Second, as shown on Fig. 3, the TGG, TGI and TGAgI glasses present almost the same mid-infrared cut-off, confirming that this limitation is due to the common element of these three Tellurium glass families, that is to say Ge and Te.
Fig. 3 IR transmission window of (GeTe4)90(AgI)10 compared to different rich Tellurium based glasses reported in literature. The inset shows a (GeTe4)90(AgI)10 glass bulk material.
From 3 µm to 20 µm, the transmission percentage is around 50% and no significant absorption peak due to impurity is observed while starting elements were not purified. The percentage of transmission is due to the Fresnel losses and depends on the refractive indices of the glass. For the (GeTe4)100-x(AgI)x glasses (whatever x) the 50% transmission corresponds to a refractive index n estimated to be 3.3, very close to the GGT and GTI glasses. It will therefore be essential to deposit some antireflection coating to decrease the Fresnel losses at the output and input of tellurium glass optical fiber [36,37].
Remote spectroscopy and space optics require materials having a broad optical window extending the optical transmission as far as possible in the infrared. Moreover, optical components are needed in several forms such as lenses, windows, gratings filters and splitters at a reasonable cost. In particular, for exo-planet detection, single mode optical fibers operating from 4 to 16 µm are required. For conventional optics, the current competitive materials for IR technologies are the Ge single crystals as well as the polycrystalline ZnSe ceramics. The main drawback of crystalline Ge and polycrystalline ZnSe is their inability to be easily shaped into complex optical materials such as fibers or molded lenses. Indeed, the main advantage of vitreous materials over single or polycrystalline materials relies on the fact that, above the glass transition temperature, glasses are plastic materials, which can be molded or tapered with high precision. Nevertheless, it is also known that within the plastic regime the risk of crystallites nucleation/growth could prevent the shaping operation. In that context, the (GeTe4)100-x(AgI)x glasses appear as very promising since no crystallization peak is observed. In order to test the potential of these glasses, fiber drawing was implemented from the (GeTe4)90(AgI)10 composition using an home-made fiber-drawing tower suitable for low temperature glasses. The inset in Fig. 4 shows the optical fiber. To our knowledge, it is the first optical fiber obtained from a chalcogenide glass based on tellurium and containing no selenium. The optical losses of this fiber were determined by the cut-back method and the minimum loss is around 0,25 dB.cm−1 at 11 µm. Indeed, the growth of crystallites within the plastic regime could increase the attenuation background due to scattering. Nevertheless, this value is already promising knowing that in the targeted devices, the infrared light will not have to propagate through more than a few centimeters of glass. Among all targets, CO2 is the more difficult to detect because of its absorption band located far in the mid-infrared, around 15µm. Moreover, its monitoring is essential to complete the Darwin or TPF space program. In order to validate the potential of the (GeTe4)90(AgI)10 fibers for carbon dioxyde detection, a transmission experiment with a chamber filled with CO2 was carried out. The gaseous CO2 IR spectrum from an enriched atmosphere was recorded by transmitting the IR light from the source to the detector using this fiber. The collected spectrum is depicted on Fig. 4, the CO2 broad absorption band around 15µm is clearly visible in its entirety. This observation demonstrates the ability of the (GeTe4)90(AgI)10 glasses for designing optical guide operating until 16 µm as required by the ESA or NASA, for space application.
Fig. 4 Gaseous CO2 infrared spectrum recorded thanks to a black body source signal transmitted through a (GeTe4)90(AgI)10 glass fiber. The inset shows the (GeTe4)90(AgI)10 glass fiber.
4. Conclusion
Different compositions in the system (GeTe4)100-x(AgI)x have been synthetized for x = 5, 10, 15, 20 and 25. Glasses with the composition x = 10, 15 and 20 show no obvious crystallization peak on DSC analysis. To our knowledge, this property is observed for the first time for a rich tellurium based glass. An optical fiber from the glass composition (GeTe4)90(AgI)10 has been drawn and its ability to detect the CO2 broad band located around 15 µm has been demonstrated. That is why, glasses from the system (GeTe4)100-x(AgI)x are interesting candidate for space application in the context of the Darwin or TPF mission.
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