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Fabrication of highly homogeneous As2Se3 glass under argon flow
Journal of Non-Crystalline Solids 357 (2011) 2897–2902
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Fabrication of highly homogeneous As2Se3 glass under argon flow Erwan Guillevic a,⁎, Xianghua Zhang a, Jean-Luc Adam a, Hongli Ma a, Jacques Lucas a, Hugues Tariel b a b
Laboratoire Verres & Céramiques, UMR CNRS 6226, Université de Rennes 1, Campus Beaulieu, 35042 Rennes Cedex, France Umicore IR Glass, Le Boulais, 35690 Acigné, France
a r t i c l e
i n f o
Article history: Received 26 January 2011 Received in revised form 22 March 2011 Available online 14 April 2011 Keywords: 42.70.Km; 81.05.Kf; 42.70.Ce; As2Se3; Glass; Synthesis
a b s t r a c t The present study relates to a new method for the synthesis of As2Se3 glass in a controlled atmosphere. The advantage of this technique is that it does not require sealing of the silica reaction container and therefore makes it likely to substitute the current industrial batch by batch synthesis which actually needs very expensive single-use sealed silica vessels. An experimental device has been developed for these purposes. It is equipped with a stirring mechanism to homogenize the molten bath. In order to avoid contamination by oxygen and moisture, the synthesis is carried out under argon flow (pressure of 1 bar). Material losses during synthesis can be reduced to less than 2% when temperature is progressively increased up to 430 °C. Bulk glass ingots are finally obtained according to a two-step annealing process. Their chemical composition is analyzed by EDS and shows a variation range of less than 0.2%. The excellent reproducibility of the given method is also confirmed by the refractive indexes, that do not differ for more than 1 · 10−3 from one another. Adverse absorption bands due to oxygen do not occur in the 8–12 μm spectral region when 1000 ppm of Mg is added. As no distilling operation has been carried out until now, the magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past, infrared systems were very expensive and thus they only found military applications. The invention of uncooled infrared focal plane detectors recently gave a boost to their commercial use, especially in the field of thermal imaging [1]. Night vision enhancement systems for car drivers are especially helpful when visibility is reduced during the night or in foggy weather. So it is not a surprise that sales of this type of infrared camera steadily increase [2]. Unfortunately, many infrared camera lenses are still made from germanium, which is rather expensive and raises the final cost of the cameras. Cost reduction of infrared imaging systems – as far as it concerns optics – may be made in using cheaper materials as well as in exploring less expensive production processes. Nowadays seleniumbased glasses are commonly used in infrared optical systems because of their excellent transmission in the 8–12 μm spectral range and their relatively low production costs [3–6]. Chalcogenide glasses are perfectly suitable for mass production since they can be easily shaped by molding into optical devices with highly accurate profiles [7,8]. Nevertheless, the currently available chalcogenide glasses still contain significant amounts of germanium, but “germanium-free” compositions for thermal imaging devices are under investigation [9].
⁎ Corresponding author. E-mail address: [email protected] (E. Guillevic). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.03.030
During synthesis, chalcogenide glasses are very sensitive to moisture and oxygen, leading to formation of chemical bonds which exhibit strong absorption peaks in the glasses' transparency window [10]. Therefore, industrial chalcogenide glass production is made batch by batch in vacuum sealed silica ampoules. This fabrication technique also prevents the loss of reacting elements, especially selenium, which are prone to evaporate out of the reaction melt due to their high vapor pressures. Large silica ampoules are expensive and cannot be reused what means that they represent an important part of the fabrication cost of chalcogenide glasses. In consequence, an ideal reaction container for industrial chalcogenide glass synthesis has to be reusable. As it is impossible to vacuum-seal such a reactor in order to avoid contamination (by air or moisture), the chemical reaction must be carried out in an inert atmosphere. Possible material losses during synthesis have to be taken into account. The present study aims to evaluate the feasibility of As2Se3 glass synthesis in a controlled atmosphere. As2Se3 composition has been chosen for its transparency window which extends beyond 12 μm and for this reason covers the 8–12 μm atmospheric transparency window. Despite its lower thermo-mechanical performances in comparison with germanium containing glasses, its large transmission window taken together with its low thermo-optic coefficient make it convenient for long-wave (8–12 μm) applications (automotive ones, for example) where small optical devices are required [9,11]. Furthermore, As2Se3 glass can be obtained at relatively low temperatures (less than 600 °C) whereas germanium glass synthesis needs temperatures above 750 °C.
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For laboratory use, As2Se3 glass is usually synthesized from elemental arsenic and selenium placed in a vacuum-sealed silica ampoule and heated in a rocking furnace. Small amounts of metallic magnesium may be added to the melt bath, this strongly reductive element acting as an oxygen getter. Eventually, to get rid of magnesium oxide and remaining magnesium, the glass is distilled [6,12]. To our knowledge, the synthesis of As2Se3 glass in an inert atmosphere has not been reported yet. As2Se3 glass synthesis in such a way is only of interest if material losses remain at an acceptably low level, reproducibility of the process being the second key condition. Preliminary investigations concerning the behavior of elemental selenium as well as of As2Se3 glass under various argon pressures and at different temperatures have been carried out in the intention to find out if As2Se3 glass synthesis is actually possible in this view. For the purposes, a reaction system including a silica container placed under argon flow, a furnace and a stirring mechanism has been specially developed. Properties of the obtained glass rods will be discussed. 2. Experimental The behavior of elemental selenium and As2Se3 glass at different temperatures (200 to 700 °C) and under various argon pressures (up to 0.9 bar) has been evaluated. Experiments have been carried out on bulk As2Se3 glass pieces and on elemental selenium pellets. Commercial selenium pellets (Umicore) used for these experiments had 5 N purity. Tested As2Se3 glass pieces had been synthesized beforehand via the classical route as described hereafter: Highly pure (5 N) raw elements arsenic and selenium were weighed out with a precision of 1 mg and introduced into a vacuum-sealed silica ampoule. Synthesis has taken place in a rocking furnace at 650 °C for 5 h. The glass has then been quenched in water at room temperature and further annealed at 170 °C for 2 h. 3 g pieces were cut from the fabricated glass rods for the evaporation experiments. To carry out preliminary studies on evaporation of elemental selenium and of As2Se3 glass, a simple experimental device has been developed. It is presented in Fig. 1. The outer silica tube is 300 mm high and has an inner diameter of 16 mm allowing to introduce a small silica crucible (10 mm diameter and 40 mm height) which contains a 3 g sample of bulk As2Se3 glass or 3 g of selenium pellets (diameter of 3 mm). Once the experimental device is assembled, air is evacuated (10−2 mbar) and replaced by argon gas. The silica tube containing the crucible and the sample is introduced into a furnace and heated up to a given temperature. As soon as the target temperature is reached, argon pressure inside the vessel is adjusted to 0.5 or 0.9 bar. Then, gas pressure and temperature are maintained for 1 h. Eventually, the system is removed from the furnace and cooled down to room temperature in freeair. The weight loss of the sample during the experiment is determined
Fig. 1. Experimental system to evaluate weight losses.
by weighing out the crucible with the sample before and after each test, the weighting precision being of ±0.5 mg. The uncertainties of the weight loss measurements have been estimated by repeating each test for five times, the largest uncertainty was determined to be of ±0.3 wt.%. To synthesize As2Se3 glass under continuous argon flow starting from elemental arsenic and selenium, a second experimental setup, shown in Fig. 2, has been developed. It is equipped with a stirring mechanism and has been designed to prepare glass pieces being over 500 g of weight, their diameter measuring 60 mm. The stirring paddle is introduced into the tube at first. Then, the highly pure (5 N) raw materials arsenic and selenium are poured inside. Selenium is introduced in form of pellets (diameter of 3 mm) whereas arsenic is ground to powder (with a particle size inferior to 0.5 mm), solid–liquid reactions being easier to carry out when contact surfaces are large. According to data given in the literature concerning the preparation of highly pure As2Se3 [12], the glass synthesis under argon flow has been performed with 1000 ppm of magnesium (metallic particles) added to the mixture of raw materials. The crucible-tube set is put into the furnace and vacuum pumped in order to eliminate oxygen and moisture from the reaction chamber. Then it is filled up with argon. During the experiment, argon flow (50 mL/min.) is permanently maintained while a simple bubbler keeps the pressure inside the vessel at 1 bar. A typical thermal process is shown in Fig. 3. At the beginning of the experiment, the temperature of the reaction chamber is set to 250 °C and maintained for 30 min to allow the selenium to melt. Once it is molten, stirring can start. Stirring speed is adjustable from 4 to 170 rpm. During the second heating step, temperature increases slowly up to 430 °C so that arsenic and selenium can react progressively. Temperature is then maintained at 430 °C for a given period of time to ensure that the chemical reaction completes and the glass homogenizes. The reaction time depends on the heating rates chosen for the synthesis and was determined by trial and error. The rotation speed during the synthesis and the form of the paddle have both been optimized to eliminate bubbles in the glass. The glass is quenched by blowing compressed air on the crucibletube set until it unsticks from the inner silica surface. The glass rod is further annealed (under argon) at 150 °C and subsequently extracted from the silica tube. It's then possible to anneal the glass rod a second time close to its glass transition temperature (170 °C) to further increase its mechanical strength. This second annealing step is carried out in air.
Fig. 2. Experimental device with stirring system developed for the synthesis of glass.
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Fig. 3. Heating process applied for the synthesis.
Eventually, the glass rod is cut down to pieces having adequate sizes for thermal analysis, transmission and refractive index measurement as well as composition analysis. To evaluate the reproducibility of glass synthesis under argon flow, the process was carried out three times under exactly the same experimental conditions. Starting materials were composed of 38 mol% As and 62 mol% Se, with a total weight of 400 g. 1000 ppm of magnesium were added. The thermal process applied during the syntheses is described in Fig. 3. The product of the first experiment was named A, the next one was named B and the third one C. To monitor differences of chemical composition and refractive index within each fabricated glass ingot, two samples were cut out from every single rod A, B and C, one from the lower part and another from the upper part. Thermal analysis was performed with a DSC2010CE differential scanning calorimeter (TA Instruments) to determine the glass transition temperatures of the prepared ingots. In an argon atmosphere, 10 mg samples were heated at a 10 °C/min rate. The glass transition temperature is measured at the inflection point of the endothermic peak corresponding to the transition. The experimental error for Tg is about ±2 °C. Visible and near-infrared optical transmission of the glasses was measured with a Cary 5 double-beam spectrophotometer (Varian) which operates in the 200–3300 nm optical range. A VECTOR 22 (Bruker) FTIR spectrophotometer was used to carry out the near-tofar infrared measurements (3–25 μm). 5 mm-thick samples were polished on both sides for transmittance measurements. To check the homogeneity of the chemical composition, elemental analysis of the prepared glasses A, B and C has been carried out on a Jeol JSM 6400 Scanning Electron Microscope (SEM) equipped with an Oxford Link Inca EDS analyzer. Each sample has a 10 × 15 mm² surface and is 5 mm thick. Precision of measurement is ±1%. For refractive index measurements, 2 mm-thick samples cut out from the A, B and C ingots were polished on both sides. Measurements were made at 1.55 and 1.31 μm on a Metricon using the so-called M-lines technique. To obtain the precision of Metricon measurements, the refractive index of a piece of GASIR1® glass has been measured on the same equipment. Refractive index of GASIR1® glass had been precisely measured by using the minimum deviation technique performed on a prism (precision of measurement ± 1 · 10−5). Differences between the minimum deviation measurements and the Metricon are as low as ±1 · 10−3, thus the precision of the M-lines technique is assumed to be of ±1 · 10−3.
It appears that above 450 °C selenium losses increase dramatically. However, below 450 °C, selenium remains remarkably stable, with losses inferior to 0.5 wt.%. Results obtained with As2Se3 glass are presented in Table 1. As2Se3 sample weight losses strongly increase as a function of temperature. However, as shown in Fig. 5, they also tend to decrease significantly when the argon pressure during the experiment is raised. As an example, at 700 °C, weight losses are divided by four when argon pressure increases from 0.5 to 0.9 bar. Data presented in Table 1 further indicate that under a 0.9 bar argon pressure at 500 °C, weight losses are very low (b1%).
3.2. Glass synthesis under argon flow Three glass ingots named A, B and C were prepared under exactly the same experimental conditions from a mixture containing 38 mol% of arsenic and 62 mol% of selenium, as described above. Altogether the starting materials weighed 400 g. A typical glass ingot obtained with this technique is shown in Fig. 6. The three glass ingots A, B and C were weighed out and the obtained values compared to the amount of starting elements. In all three cases, weight losses were evaluated at 1.5 ± 0.1%. Samples were taken from the glass rods A, B, C and analyzed by differential calorimetry. The glass transition temperatures measured on glasses A, B and C are collected in Table 2. Since the precision of the transition temperature measurement is about ±2 °C, it is reasonable to consider that the three glasses have the same glass transition temperature. Fig. 7 shows two DSC curves, the first one corresponding to glass C whereas the second one was obtained with an As38Se62 glass synthesized in a vacuum sealed silica ampoule. This As38Se62 glass is a reference shown for comparison.
3. Results 3.1. Preliminary study of evaporation In the first step, the behavior of elemental selenium at temperatures varying from 230 to 600 °C under an argon pressure of 0.9 bar has been evaluated. Results are shown in Fig. 4.
Fig. 4. Selenium evaporation from 3 g samples as a function of temperature under 0.9 bar of argon pressure. Uncertainty of weight losses is ± 0.3%. The line is drawn as a guide to the eyes.
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E. Guillevic et al. / Journal of Non-Crystalline Solids 357 (2011) 2897–2902 Table 1 As2Se3 weight losses at various temperatures under 0.9 bar of argon pressure. Temperature (°C)
Weight loss (%)
500 600 700
0.06 ± 0.05 0.9 ± 0.3 2.9 ± 0.3
Glass C, synthesized under argon flow, exhibits a slightly lower glass transition temperature than the reference glass. Thus, glass C and the reference glass must have different chemical compositions. Indeed, as we will see later, glasses A, B and C do not exactly have an As38Se62 composition, a fact due to evaporation of the elements. Composition analysis results of the six samples coming from the glasses A, B and C are gathered in Table 3. Arsenic and selenium content of each sample has been measured. Precision of composition analysis is about ±1%. Considering the results gathered in Table 3 and the precision of EDS measurements, it can be assumed that the glasses A, B and C are of the same chemical composition. However, two objections may arise: Firstly, lower parts of the glass rods tend to have a slightly higher arsenic content than the upper parts, even if the difference remains way below the measurement precision. Secondly, the chemical compositions of the glasses A, B and C are close to As36Se64 although the amounts of starting elements had been weighed out to obtain As38Se62 glasses. Refractive index values measured on the glass samples coming from ingots A, B and C are gathered in Table 4. Between the lower and upper parts of a same rod, refractive index differences are smaller than 1 · 10−3. These differences are not significant because they are inferior to the precision of the measurement (±1 · 10−3). Thus, we can conclude that the refractive index is homogeneous within each ingot. From batch to batch, the refractive index of batch A is slightly higher than that of the other glasses, with a difference of about 1 · 10−2. The last two batches have very close refractive indexes with a reproducibility around 1 · 10−3. Optical transmission spectra are presented in Fig. 8. The first transmission spectrum (blue curve) corresponds to As40Se60 glass synthesized in a sealed silica ampoule and has been added as a reference for comparison. The two other transmission spectra were recorded respectively with unpurified As38Se62 glass (without addition of magnesium) prepared in the open crucible (red curve) and with As38Se62 glass prepared in the open crucible in presence of magnesium as an oxygen getter (green curve). One can notice on Fig. 8 that the addition of 1000 ppm of magnesium helps eliminate oxide traces in the glasses synthesized under
Fig. 6. As38Se62 glass rod obtained after two-step annealing.
argon flow. When magnesium is added to the mixture, absorption peaks generated by oxygen bonds in the far infrared spectral range are removed and the transparency window of the glass completely covers the atmospheric transparency window from 8 to 12 μm. However, samples prepared with magnesium scatter light at short wavelengths, whereas those prepared without magnesium do not. An absorption peak due to Se\H bonds is also still present at 4.6 μm. A subsidiary remark can be made about the step above 15 μm observed on the transmission spectrum of As38Se62 glass: its optical transmission in this spectral range is lower than the As40Se60 glass. Fig. 9 shows pictures of a piece of a glass ingot prepared under argon and taken at different wavelengths. Three cameras working in different spectral ranges (visible, 3–5 μm and 8–12 μm respectively) were used to take these pictures. It is obvious that this glass is free from bubbles and without any visible filament. 4. Discussion The first step of this study consisted in figuring out if As2Se3 glass synthesis under argon is actually feasible in spite of the evaporation of arsenic and selenium. At low temperatures (b450 °C), selenium vapor pressure is higher than that of arsenic [13]. Consequently, the temperature should be lower than 450 °C until the chemical reaction of selenium with arsenic is completed, but high enough for allowing selenium to react with arsenic. Preliminary investigations about the behavior of elemental selenium have shown that for temperatures up to 450 °C, evaporation is low. Furthermore, if placed at 500 °C under 0.9 bar of argon pressure for 1 h, material losses from a bulk piece of As2Se3 glass are inferior to 1%. For the synthesis of As2Se3 glass, a reaction temperature of 450 °C is sufficient, thus it was decided to keep on the development of the synthesis process of binary arsenic selenium glasses under argon. Under a pressure of 1 bar, arsenic does not melt, but sublimates at temperatures above 614 °C [13]. The reaction of arsenic with selenium therefore is a solid–liquid reaction. In consequence, the synthesis of As2Se3 glass under argon requires a stirring system to homogenize the molten bath. Thus, glass synthesis was not possible in the
Table 2 Glass transition temperatures of glasses A, B and C, and control glass synthesized in vacuum sealed silica ampoule.
Fig. 5. As2Se3 weight losses as a function of argon pressure at various temperatures. Maximum uncertainty of weight losses is ±0.3%. The lines are drawn as guides to the eyes.
Glass
Tg (°C) at inflection point (± 2 °C)
A B C Synthesized in sealed ampoule
181 177 178 186
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Table 4 Refractive indexes of glasses A, B and C at 1.55 and 1.31 μm. Glass
A B C
Fig. 7. DSC curves of two glasses with the As38Se62 composition, prepared by different techniques.
experimental device used to carry out the preliminary investigations, and the second setup was created. To keep things simple and avoid vapor leakage from the silica vessel, it was decided to carry out the experiments under an argon pressure of 1 bar. Maintaining a continuous inert gas flow during the whole process ensures that no contamination of the reaction chamber by air is possible. Since the reaction of arsenic with selenium is a solid–liquid one, the molten bath contains more and more arsenic and tends to behave like AsxSe100-x glasses with an arsenic content growing throughout the experiment. Glass transition temperatures of AsxSe100-x glasses with x b 40 increase regularly as a function of arsenic content [14]. The temperature ramp thus follows the increase of arsenic amount in the molten bath. Maximum reaction temperature for the homogenization of the glass has been set experimentally to 430 °C in order to minimize material losses. The main problem to face during the study consisted in the mechanical weakness of the prepared glass ingots. Experiments carried out in our laboratory have shown that As2Se3 glass always has a tendency to break apart (even if synthesized in a sealed silica ampoule). The exact reason for this behavior is still unknown. As soon as arsenic content starts to exceed 40%, weakness of the glass increases dramatically. But on the other hand, various tests show that a selenium content higher than the stoechiometric composition (for example in As38Se62), makes it much easier to obtain massive glass samples without cracks. Contrary to As2Se3, the glassy structure of As38Se62 contains small selenium chains probably leading to a more flexible structure [14]. Thus, for the glass synthesis under argon flow, 38 mol% of arsenic and 62 mol% of selenium were used as starting materials. The annealing process is also known to have much influence on the mechanical properties of a glass. For As38Se62 glass synthesized under argon, we have chosen a particular two-step annealing process. During the first step, the glass is placed in a furnace at 150 °C, which means 20 °C below the usual annealing temperature for As2Se3, in order to avoid sticking. Annealed at a higher temperature, the glass rod would stick against the silica reaction container. During the cooling process, this would generate strong local stresses in the glass and lead to cracking. In a second step, the glass rod is removed from the silica tube and annealed again. This second annealing step is performed at a higher temperature (170 °C) to eliminate remaining stresses, conferring that way a better mechanical strength to the glass.
Index at 1.55 μm (± 0.001)
Index at 1.31 μm (± 0.001)
Lower sample
Upper sample
Lower sample
Upper sample
2.820 2.811 2.810
2.820 2.812 2.810
2.844 2.834 2.833
2.844 2.833 2.833
Results of the elemental composition analysis are in good agreement with the differential calorimetry measurements. Since glasses A, B and C are of the same chemical composition (As36Se64), it is logical that they present identical glass transition temperatures. All three glasses however contain a higher amount of selenium than the As38Se62 reference glass and thus have lower glass transition temperatures. Indeed, starting from As40Se60, glass transition temperatures of As–Se compositions decrease as a function of selenium content [14,15]. It seems however surprising to get glasses with less arsenic content than expected. The explanation according to the literature is that under atmospheric pressure, arsenic does not melt, but sublimates at 614 °C [13]. In reality, its partial pressure is higher than that of Se at temperatures above 430 °C. According to EDS measurements, the lower parts of the ingots A, B and C have an arsenic content which is 0.5 mol% higher than in the upper parts. This value is however way below the precision of the measurement which is ±1 mol%. Refractive index values measured on glass samples A, B and C indicate that between the lower and upper parts of a same rod, refractive index differences are smaller than 1 · 10−3. These differences are not significant since they are inferior to the precision of the measurement (±1 · 10−3) and we can conclude that the refractive index is homogeneous inside the same ingot. From batch to batch, the refractive index of batch A is slightly higher than that of the other glasses, with a difference of about 1 · 10−2. The refractive indexes of the last two batches are very close, with a reproducibility around 1 · 10−3. This type of reproducibility is comparable to the specifications of manufacturers of commercial chalcogenide glasses. The refractive index of a material is known to vary strongly as a function of its chemical composition. As the refractive indexes of glasses A, B and C are very similar, we assume that the reproducibility of the synthesis under argon in terms of chemical composition is good. Concerning the variations of arsenic content within each ingot, as revealed by the elemental analysis, they most likely rely on the measurement inaccuracy of EDS.
Table 3 Results of composition analysis on glasses A, B and C. Glass
at.% As (± 1 at.%)
at.% Se (± 1 at.%)
Lower sample
Upper sample
Lower sample
Upper sample
A B C
36.1 36.2 36.2
35.8 35.8 35.7
63.9 63.8 63.8
64.2 64.2 64.3
Fig. 8. Transmission spectra of various 5 mm-thick glass samples: unpurified As38Se62 glass prepared in the open crucible (red curve); As38Se62 glass prepared in the open crucible in presence of magnesium as an oxygen getter (green curve). A transmission spectrum of As40Se60 glass synthesized in a sealed silica ampoule (blue curve) has been added for comparison.
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The picture presented in Fig. 9 shows a piece of a glass ingot synthesized under argon flow which is highly homogenous. There is absolutely no bubble or filament visible inside the sample. Although homogeneity is very difficult to obtain in chalcogenide glass fabrication with the traditional method using sealed silica vessels, the stirring system developed for the synthesis under argon leads to highly uniform glass ingots.
5. Conclusion
Fig. 9. As38Se62 glass in visible light (top), through a NIR camera (middle) and through a FIR camera (bottom).
Reproducibility of the synthesis is a very important criterion for the fabrication of optical glasses, especially as far as its optical properties are concerned. If chalcogenide glass is synthesized in an inert atmosphere, material losses during the process cannot be completely avoided. But this is not a major problem if the amounts of lost elements use to be always in the same range. In this case, they become predictable, and a glass with given composition and required properties can still be obtained by carefully adjusting the quantities of the raw materials. The presence of oxides in chalcogenide glasses introduces additional absorption peaks and decreases the optical window in the long wavelength range. Fig. 8 clearly shows the necessity to remove oxide traces during the synthesis. Magnesium is a strongly reducing agent and traps oxygen atoms from surface oxides of arsenic and selenium pieces, forming a very stable oxide. Magnesium and its oxide have lower densities than arsenic and selenium and therefore should float on the surface of the molten bath. However, magnesium oxide is obviously still present in the glass, inducing scattering losses in the short wavelength range. This hypothesis is corroborated by the fact that samples prepared without magnesium do not scatter light at short wavelengths. Contrary to the stoechiometric As2Se3 compound, As34Se64 and As38Se62 glasses contain small selenium chains, and thus Se\Se bonds, which generate multiphonon absorption around 15 μm [3]. The glasses prepared under argon flow therefore have a bit lower optical transmission in this wavelength area compared to As2Se3.
Synthesis of chalcogenide glasses is quite tricky because of the volatility of the starting elements and the presence of impurities that can create strong adverse absorption bands in the transparency windows of interest. For these reasons, chalcogenide glasses are usually prepared in vacuum sealed silica ampoules which unfortunately cannot be reused. So this method is appropriate for laboratory tests, but the large-sized ampoules necessary for mass production make it very expensive on an industrial scale. Therefore, without doubt, a more economical procedure constitutes a remarkable improvement. This study precisely deals with a new route to synthesize As2Se3 glass under argon flow which does not require sealing of the reaction container. Experiments demonstrated that glasses prepared with this so-called “open crucible method” show interesting reproducible properties: Refractive indexes are almost identical (they differ in less than 1 · 10−3 from one another). Weight losses do not exceed 2%. Magnesium is an efficient oxygen getter purifying glasses so that no absorption bands in the 8–12 μm range can occur. As no distillation of the glass has been carried out until now, the resulting magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths. The magnesium content will therefore have to be optimized according to the purity of the starting elements.
Acknowledgements This work has been partially supported by the Région Bretagne through the Pôle de compétitivité “ID4Cars” as well as by the French Ministry of Defense/DGA under the contract No. 2009.34.0019.
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Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Fabrication of highly homogeneous As2Se3 glass under argon flow Erwan Guillevic a,⁎, Xianghua Zhang a, Jean-Luc Adam a, Hongli Ma a, Jacques Lucas a, Hugues Tariel b a b
Laboratoire Verres & Céramiques, UMR CNRS 6226, Université de Rennes 1, Campus Beaulieu, 35042 Rennes Cedex, France Umicore IR Glass, Le Boulais, 35690 Acigné, France
a r t i c l e
i n f o
Article history: Received 26 January 2011 Received in revised form 22 March 2011 Available online 14 April 2011 Keywords: 42.70.Km; 81.05.Kf; 42.70.Ce; As2Se3; Glass; Synthesis
a b s t r a c t The present study relates to a new method for the synthesis of As2Se3 glass in a controlled atmosphere. The advantage of this technique is that it does not require sealing of the silica reaction container and therefore makes it likely to substitute the current industrial batch by batch synthesis which actually needs very expensive single-use sealed silica vessels. An experimental device has been developed for these purposes. It is equipped with a stirring mechanism to homogenize the molten bath. In order to avoid contamination by oxygen and moisture, the synthesis is carried out under argon flow (pressure of 1 bar). Material losses during synthesis can be reduced to less than 2% when temperature is progressively increased up to 430 °C. Bulk glass ingots are finally obtained according to a two-step annealing process. Their chemical composition is analyzed by EDS and shows a variation range of less than 0.2%. The excellent reproducibility of the given method is also confirmed by the refractive indexes, that do not differ for more than 1 · 10−3 from one another. Adverse absorption bands due to oxygen do not occur in the 8–12 μm spectral region when 1000 ppm of Mg is added. As no distilling operation has been carried out until now, the magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In the past, infrared systems were very expensive and thus they only found military applications. The invention of uncooled infrared focal plane detectors recently gave a boost to their commercial use, especially in the field of thermal imaging [1]. Night vision enhancement systems for car drivers are especially helpful when visibility is reduced during the night or in foggy weather. So it is not a surprise that sales of this type of infrared camera steadily increase [2]. Unfortunately, many infrared camera lenses are still made from germanium, which is rather expensive and raises the final cost of the cameras. Cost reduction of infrared imaging systems – as far as it concerns optics – may be made in using cheaper materials as well as in exploring less expensive production processes. Nowadays seleniumbased glasses are commonly used in infrared optical systems because of their excellent transmission in the 8–12 μm spectral range and their relatively low production costs [3–6]. Chalcogenide glasses are perfectly suitable for mass production since they can be easily shaped by molding into optical devices with highly accurate profiles [7,8]. Nevertheless, the currently available chalcogenide glasses still contain significant amounts of germanium, but “germanium-free” compositions for thermal imaging devices are under investigation [9].
⁎ Corresponding author. E-mail address: [email protected] (E. Guillevic). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.03.030
During synthesis, chalcogenide glasses are very sensitive to moisture and oxygen, leading to formation of chemical bonds which exhibit strong absorption peaks in the glasses' transparency window [10]. Therefore, industrial chalcogenide glass production is made batch by batch in vacuum sealed silica ampoules. This fabrication technique also prevents the loss of reacting elements, especially selenium, which are prone to evaporate out of the reaction melt due to their high vapor pressures. Large silica ampoules are expensive and cannot be reused what means that they represent an important part of the fabrication cost of chalcogenide glasses. In consequence, an ideal reaction container for industrial chalcogenide glass synthesis has to be reusable. As it is impossible to vacuum-seal such a reactor in order to avoid contamination (by air or moisture), the chemical reaction must be carried out in an inert atmosphere. Possible material losses during synthesis have to be taken into account. The present study aims to evaluate the feasibility of As2Se3 glass synthesis in a controlled atmosphere. As2Se3 composition has been chosen for its transparency window which extends beyond 12 μm and for this reason covers the 8–12 μm atmospheric transparency window. Despite its lower thermo-mechanical performances in comparison with germanium containing glasses, its large transmission window taken together with its low thermo-optic coefficient make it convenient for long-wave (8–12 μm) applications (automotive ones, for example) where small optical devices are required [9,11]. Furthermore, As2Se3 glass can be obtained at relatively low temperatures (less than 600 °C) whereas germanium glass synthesis needs temperatures above 750 °C.
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For laboratory use, As2Se3 glass is usually synthesized from elemental arsenic and selenium placed in a vacuum-sealed silica ampoule and heated in a rocking furnace. Small amounts of metallic magnesium may be added to the melt bath, this strongly reductive element acting as an oxygen getter. Eventually, to get rid of magnesium oxide and remaining magnesium, the glass is distilled [6,12]. To our knowledge, the synthesis of As2Se3 glass in an inert atmosphere has not been reported yet. As2Se3 glass synthesis in such a way is only of interest if material losses remain at an acceptably low level, reproducibility of the process being the second key condition. Preliminary investigations concerning the behavior of elemental selenium as well as of As2Se3 glass under various argon pressures and at different temperatures have been carried out in the intention to find out if As2Se3 glass synthesis is actually possible in this view. For the purposes, a reaction system including a silica container placed under argon flow, a furnace and a stirring mechanism has been specially developed. Properties of the obtained glass rods will be discussed. 2. Experimental The behavior of elemental selenium and As2Se3 glass at different temperatures (200 to 700 °C) and under various argon pressures (up to 0.9 bar) has been evaluated. Experiments have been carried out on bulk As2Se3 glass pieces and on elemental selenium pellets. Commercial selenium pellets (Umicore) used for these experiments had 5 N purity. Tested As2Se3 glass pieces had been synthesized beforehand via the classical route as described hereafter: Highly pure (5 N) raw elements arsenic and selenium were weighed out with a precision of 1 mg and introduced into a vacuum-sealed silica ampoule. Synthesis has taken place in a rocking furnace at 650 °C for 5 h. The glass has then been quenched in water at room temperature and further annealed at 170 °C for 2 h. 3 g pieces were cut from the fabricated glass rods for the evaporation experiments. To carry out preliminary studies on evaporation of elemental selenium and of As2Se3 glass, a simple experimental device has been developed. It is presented in Fig. 1. The outer silica tube is 300 mm high and has an inner diameter of 16 mm allowing to introduce a small silica crucible (10 mm diameter and 40 mm height) which contains a 3 g sample of bulk As2Se3 glass or 3 g of selenium pellets (diameter of 3 mm). Once the experimental device is assembled, air is evacuated (10−2 mbar) and replaced by argon gas. The silica tube containing the crucible and the sample is introduced into a furnace and heated up to a given temperature. As soon as the target temperature is reached, argon pressure inside the vessel is adjusted to 0.5 or 0.9 bar. Then, gas pressure and temperature are maintained for 1 h. Eventually, the system is removed from the furnace and cooled down to room temperature in freeair. The weight loss of the sample during the experiment is determined
Fig. 1. Experimental system to evaluate weight losses.
by weighing out the crucible with the sample before and after each test, the weighting precision being of ±0.5 mg. The uncertainties of the weight loss measurements have been estimated by repeating each test for five times, the largest uncertainty was determined to be of ±0.3 wt.%. To synthesize As2Se3 glass under continuous argon flow starting from elemental arsenic and selenium, a second experimental setup, shown in Fig. 2, has been developed. It is equipped with a stirring mechanism and has been designed to prepare glass pieces being over 500 g of weight, their diameter measuring 60 mm. The stirring paddle is introduced into the tube at first. Then, the highly pure (5 N) raw materials arsenic and selenium are poured inside. Selenium is introduced in form of pellets (diameter of 3 mm) whereas arsenic is ground to powder (with a particle size inferior to 0.5 mm), solid–liquid reactions being easier to carry out when contact surfaces are large. According to data given in the literature concerning the preparation of highly pure As2Se3 [12], the glass synthesis under argon flow has been performed with 1000 ppm of magnesium (metallic particles) added to the mixture of raw materials. The crucible-tube set is put into the furnace and vacuum pumped in order to eliminate oxygen and moisture from the reaction chamber. Then it is filled up with argon. During the experiment, argon flow (50 mL/min.) is permanently maintained while a simple bubbler keeps the pressure inside the vessel at 1 bar. A typical thermal process is shown in Fig. 3. At the beginning of the experiment, the temperature of the reaction chamber is set to 250 °C and maintained for 30 min to allow the selenium to melt. Once it is molten, stirring can start. Stirring speed is adjustable from 4 to 170 rpm. During the second heating step, temperature increases slowly up to 430 °C so that arsenic and selenium can react progressively. Temperature is then maintained at 430 °C for a given period of time to ensure that the chemical reaction completes and the glass homogenizes. The reaction time depends on the heating rates chosen for the synthesis and was determined by trial and error. The rotation speed during the synthesis and the form of the paddle have both been optimized to eliminate bubbles in the glass. The glass is quenched by blowing compressed air on the crucibletube set until it unsticks from the inner silica surface. The glass rod is further annealed (under argon) at 150 °C and subsequently extracted from the silica tube. It's then possible to anneal the glass rod a second time close to its glass transition temperature (170 °C) to further increase its mechanical strength. This second annealing step is carried out in air.
Fig. 2. Experimental device with stirring system developed for the synthesis of glass.
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Fig. 3. Heating process applied for the synthesis.
Eventually, the glass rod is cut down to pieces having adequate sizes for thermal analysis, transmission and refractive index measurement as well as composition analysis. To evaluate the reproducibility of glass synthesis under argon flow, the process was carried out three times under exactly the same experimental conditions. Starting materials were composed of 38 mol% As and 62 mol% Se, with a total weight of 400 g. 1000 ppm of magnesium were added. The thermal process applied during the syntheses is described in Fig. 3. The product of the first experiment was named A, the next one was named B and the third one C. To monitor differences of chemical composition and refractive index within each fabricated glass ingot, two samples were cut out from every single rod A, B and C, one from the lower part and another from the upper part. Thermal analysis was performed with a DSC2010CE differential scanning calorimeter (TA Instruments) to determine the glass transition temperatures of the prepared ingots. In an argon atmosphere, 10 mg samples were heated at a 10 °C/min rate. The glass transition temperature is measured at the inflection point of the endothermic peak corresponding to the transition. The experimental error for Tg is about ±2 °C. Visible and near-infrared optical transmission of the glasses was measured with a Cary 5 double-beam spectrophotometer (Varian) which operates in the 200–3300 nm optical range. A VECTOR 22 (Bruker) FTIR spectrophotometer was used to carry out the near-tofar infrared measurements (3–25 μm). 5 mm-thick samples were polished on both sides for transmittance measurements. To check the homogeneity of the chemical composition, elemental analysis of the prepared glasses A, B and C has been carried out on a Jeol JSM 6400 Scanning Electron Microscope (SEM) equipped with an Oxford Link Inca EDS analyzer. Each sample has a 10 × 15 mm² surface and is 5 mm thick. Precision of measurement is ±1%. For refractive index measurements, 2 mm-thick samples cut out from the A, B and C ingots were polished on both sides. Measurements were made at 1.55 and 1.31 μm on a Metricon using the so-called M-lines technique. To obtain the precision of Metricon measurements, the refractive index of a piece of GASIR1® glass has been measured on the same equipment. Refractive index of GASIR1® glass had been precisely measured by using the minimum deviation technique performed on a prism (precision of measurement ± 1 · 10−5). Differences between the minimum deviation measurements and the Metricon are as low as ±1 · 10−3, thus the precision of the M-lines technique is assumed to be of ±1 · 10−3.
It appears that above 450 °C selenium losses increase dramatically. However, below 450 °C, selenium remains remarkably stable, with losses inferior to 0.5 wt.%. Results obtained with As2Se3 glass are presented in Table 1. As2Se3 sample weight losses strongly increase as a function of temperature. However, as shown in Fig. 5, they also tend to decrease significantly when the argon pressure during the experiment is raised. As an example, at 700 °C, weight losses are divided by four when argon pressure increases from 0.5 to 0.9 bar. Data presented in Table 1 further indicate that under a 0.9 bar argon pressure at 500 °C, weight losses are very low (b1%).
3.2. Glass synthesis under argon flow Three glass ingots named A, B and C were prepared under exactly the same experimental conditions from a mixture containing 38 mol% of arsenic and 62 mol% of selenium, as described above. Altogether the starting materials weighed 400 g. A typical glass ingot obtained with this technique is shown in Fig. 6. The three glass ingots A, B and C were weighed out and the obtained values compared to the amount of starting elements. In all three cases, weight losses were evaluated at 1.5 ± 0.1%. Samples were taken from the glass rods A, B, C and analyzed by differential calorimetry. The glass transition temperatures measured on glasses A, B and C are collected in Table 2. Since the precision of the transition temperature measurement is about ±2 °C, it is reasonable to consider that the three glasses have the same glass transition temperature. Fig. 7 shows two DSC curves, the first one corresponding to glass C whereas the second one was obtained with an As38Se62 glass synthesized in a vacuum sealed silica ampoule. This As38Se62 glass is a reference shown for comparison.
3. Results 3.1. Preliminary study of evaporation In the first step, the behavior of elemental selenium at temperatures varying from 230 to 600 °C under an argon pressure of 0.9 bar has been evaluated. Results are shown in Fig. 4.
Fig. 4. Selenium evaporation from 3 g samples as a function of temperature under 0.9 bar of argon pressure. Uncertainty of weight losses is ± 0.3%. The line is drawn as a guide to the eyes.
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Weight loss (%)
500 600 700
0.06 ± 0.05 0.9 ± 0.3 2.9 ± 0.3
Glass C, synthesized under argon flow, exhibits a slightly lower glass transition temperature than the reference glass. Thus, glass C and the reference glass must have different chemical compositions. Indeed, as we will see later, glasses A, B and C do not exactly have an As38Se62 composition, a fact due to evaporation of the elements. Composition analysis results of the six samples coming from the glasses A, B and C are gathered in Table 3. Arsenic and selenium content of each sample has been measured. Precision of composition analysis is about ±1%. Considering the results gathered in Table 3 and the precision of EDS measurements, it can be assumed that the glasses A, B and C are of the same chemical composition. However, two objections may arise: Firstly, lower parts of the glass rods tend to have a slightly higher arsenic content than the upper parts, even if the difference remains way below the measurement precision. Secondly, the chemical compositions of the glasses A, B and C are close to As36Se64 although the amounts of starting elements had been weighed out to obtain As38Se62 glasses. Refractive index values measured on the glass samples coming from ingots A, B and C are gathered in Table 4. Between the lower and upper parts of a same rod, refractive index differences are smaller than 1 · 10−3. These differences are not significant because they are inferior to the precision of the measurement (±1 · 10−3). Thus, we can conclude that the refractive index is homogeneous within each ingot. From batch to batch, the refractive index of batch A is slightly higher than that of the other glasses, with a difference of about 1 · 10−2. The last two batches have very close refractive indexes with a reproducibility around 1 · 10−3. Optical transmission spectra are presented in Fig. 8. The first transmission spectrum (blue curve) corresponds to As40Se60 glass synthesized in a sealed silica ampoule and has been added as a reference for comparison. The two other transmission spectra were recorded respectively with unpurified As38Se62 glass (without addition of magnesium) prepared in the open crucible (red curve) and with As38Se62 glass prepared in the open crucible in presence of magnesium as an oxygen getter (green curve). One can notice on Fig. 8 that the addition of 1000 ppm of magnesium helps eliminate oxide traces in the glasses synthesized under
Fig. 6. As38Se62 glass rod obtained after two-step annealing.
argon flow. When magnesium is added to the mixture, absorption peaks generated by oxygen bonds in the far infrared spectral range are removed and the transparency window of the glass completely covers the atmospheric transparency window from 8 to 12 μm. However, samples prepared with magnesium scatter light at short wavelengths, whereas those prepared without magnesium do not. An absorption peak due to Se\H bonds is also still present at 4.6 μm. A subsidiary remark can be made about the step above 15 μm observed on the transmission spectrum of As38Se62 glass: its optical transmission in this spectral range is lower than the As40Se60 glass. Fig. 9 shows pictures of a piece of a glass ingot prepared under argon and taken at different wavelengths. Three cameras working in different spectral ranges (visible, 3–5 μm and 8–12 μm respectively) were used to take these pictures. It is obvious that this glass is free from bubbles and without any visible filament. 4. Discussion The first step of this study consisted in figuring out if As2Se3 glass synthesis under argon is actually feasible in spite of the evaporation of arsenic and selenium. At low temperatures (b450 °C), selenium vapor pressure is higher than that of arsenic [13]. Consequently, the temperature should be lower than 450 °C until the chemical reaction of selenium with arsenic is completed, but high enough for allowing selenium to react with arsenic. Preliminary investigations about the behavior of elemental selenium have shown that for temperatures up to 450 °C, evaporation is low. Furthermore, if placed at 500 °C under 0.9 bar of argon pressure for 1 h, material losses from a bulk piece of As2Se3 glass are inferior to 1%. For the synthesis of As2Se3 glass, a reaction temperature of 450 °C is sufficient, thus it was decided to keep on the development of the synthesis process of binary arsenic selenium glasses under argon. Under a pressure of 1 bar, arsenic does not melt, but sublimates at temperatures above 614 °C [13]. The reaction of arsenic with selenium therefore is a solid–liquid reaction. In consequence, the synthesis of As2Se3 glass under argon requires a stirring system to homogenize the molten bath. Thus, glass synthesis was not possible in the
Table 2 Glass transition temperatures of glasses A, B and C, and control glass synthesized in vacuum sealed silica ampoule.
Fig. 5. As2Se3 weight losses as a function of argon pressure at various temperatures. Maximum uncertainty of weight losses is ±0.3%. The lines are drawn as guides to the eyes.
Glass
Tg (°C) at inflection point (± 2 °C)
A B C Synthesized in sealed ampoule
181 177 178 186
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Table 4 Refractive indexes of glasses A, B and C at 1.55 and 1.31 μm. Glass
A B C
Fig. 7. DSC curves of two glasses with the As38Se62 composition, prepared by different techniques.
experimental device used to carry out the preliminary investigations, and the second setup was created. To keep things simple and avoid vapor leakage from the silica vessel, it was decided to carry out the experiments under an argon pressure of 1 bar. Maintaining a continuous inert gas flow during the whole process ensures that no contamination of the reaction chamber by air is possible. Since the reaction of arsenic with selenium is a solid–liquid one, the molten bath contains more and more arsenic and tends to behave like AsxSe100-x glasses with an arsenic content growing throughout the experiment. Glass transition temperatures of AsxSe100-x glasses with x b 40 increase regularly as a function of arsenic content [14]. The temperature ramp thus follows the increase of arsenic amount in the molten bath. Maximum reaction temperature for the homogenization of the glass has been set experimentally to 430 °C in order to minimize material losses. The main problem to face during the study consisted in the mechanical weakness of the prepared glass ingots. Experiments carried out in our laboratory have shown that As2Se3 glass always has a tendency to break apart (even if synthesized in a sealed silica ampoule). The exact reason for this behavior is still unknown. As soon as arsenic content starts to exceed 40%, weakness of the glass increases dramatically. But on the other hand, various tests show that a selenium content higher than the stoechiometric composition (for example in As38Se62), makes it much easier to obtain massive glass samples without cracks. Contrary to As2Se3, the glassy structure of As38Se62 contains small selenium chains probably leading to a more flexible structure [14]. Thus, for the glass synthesis under argon flow, 38 mol% of arsenic and 62 mol% of selenium were used as starting materials. The annealing process is also known to have much influence on the mechanical properties of a glass. For As38Se62 glass synthesized under argon, we have chosen a particular two-step annealing process. During the first step, the glass is placed in a furnace at 150 °C, which means 20 °C below the usual annealing temperature for As2Se3, in order to avoid sticking. Annealed at a higher temperature, the glass rod would stick against the silica reaction container. During the cooling process, this would generate strong local stresses in the glass and lead to cracking. In a second step, the glass rod is removed from the silica tube and annealed again. This second annealing step is performed at a higher temperature (170 °C) to eliminate remaining stresses, conferring that way a better mechanical strength to the glass.
Index at 1.55 μm (± 0.001)
Index at 1.31 μm (± 0.001)
Lower sample
Upper sample
Lower sample
Upper sample
2.820 2.811 2.810
2.820 2.812 2.810
2.844 2.834 2.833
2.844 2.833 2.833
Results of the elemental composition analysis are in good agreement with the differential calorimetry measurements. Since glasses A, B and C are of the same chemical composition (As36Se64), it is logical that they present identical glass transition temperatures. All three glasses however contain a higher amount of selenium than the As38Se62 reference glass and thus have lower glass transition temperatures. Indeed, starting from As40Se60, glass transition temperatures of As–Se compositions decrease as a function of selenium content [14,15]. It seems however surprising to get glasses with less arsenic content than expected. The explanation according to the literature is that under atmospheric pressure, arsenic does not melt, but sublimates at 614 °C [13]. In reality, its partial pressure is higher than that of Se at temperatures above 430 °C. According to EDS measurements, the lower parts of the ingots A, B and C have an arsenic content which is 0.5 mol% higher than in the upper parts. This value is however way below the precision of the measurement which is ±1 mol%. Refractive index values measured on glass samples A, B and C indicate that between the lower and upper parts of a same rod, refractive index differences are smaller than 1 · 10−3. These differences are not significant since they are inferior to the precision of the measurement (±1 · 10−3) and we can conclude that the refractive index is homogeneous inside the same ingot. From batch to batch, the refractive index of batch A is slightly higher than that of the other glasses, with a difference of about 1 · 10−2. The refractive indexes of the last two batches are very close, with a reproducibility around 1 · 10−3. This type of reproducibility is comparable to the specifications of manufacturers of commercial chalcogenide glasses. The refractive index of a material is known to vary strongly as a function of its chemical composition. As the refractive indexes of glasses A, B and C are very similar, we assume that the reproducibility of the synthesis under argon in terms of chemical composition is good. Concerning the variations of arsenic content within each ingot, as revealed by the elemental analysis, they most likely rely on the measurement inaccuracy of EDS.
Table 3 Results of composition analysis on glasses A, B and C. Glass
at.% As (± 1 at.%)
at.% Se (± 1 at.%)
Lower sample
Upper sample
Lower sample
Upper sample
A B C
36.1 36.2 36.2
35.8 35.8 35.7
63.9 63.8 63.8
64.2 64.2 64.3
Fig. 8. Transmission spectra of various 5 mm-thick glass samples: unpurified As38Se62 glass prepared in the open crucible (red curve); As38Se62 glass prepared in the open crucible in presence of magnesium as an oxygen getter (green curve). A transmission spectrum of As40Se60 glass synthesized in a sealed silica ampoule (blue curve) has been added for comparison.
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The picture presented in Fig. 9 shows a piece of a glass ingot synthesized under argon flow which is highly homogenous. There is absolutely no bubble or filament visible inside the sample. Although homogeneity is very difficult to obtain in chalcogenide glass fabrication with the traditional method using sealed silica vessels, the stirring system developed for the synthesis under argon leads to highly uniform glass ingots.
5. Conclusion
Fig. 9. As38Se62 glass in visible light (top), through a NIR camera (middle) and through a FIR camera (bottom).
Reproducibility of the synthesis is a very important criterion for the fabrication of optical glasses, especially as far as its optical properties are concerned. If chalcogenide glass is synthesized in an inert atmosphere, material losses during the process cannot be completely avoided. But this is not a major problem if the amounts of lost elements use to be always in the same range. In this case, they become predictable, and a glass with given composition and required properties can still be obtained by carefully adjusting the quantities of the raw materials. The presence of oxides in chalcogenide glasses introduces additional absorption peaks and decreases the optical window in the long wavelength range. Fig. 8 clearly shows the necessity to remove oxide traces during the synthesis. Magnesium is a strongly reducing agent and traps oxygen atoms from surface oxides of arsenic and selenium pieces, forming a very stable oxide. Magnesium and its oxide have lower densities than arsenic and selenium and therefore should float on the surface of the molten bath. However, magnesium oxide is obviously still present in the glass, inducing scattering losses in the short wavelength range. This hypothesis is corroborated by the fact that samples prepared without magnesium do not scatter light at short wavelengths. Contrary to the stoechiometric As2Se3 compound, As34Se64 and As38Se62 glasses contain small selenium chains, and thus Se\Se bonds, which generate multiphonon absorption around 15 μm [3]. The glasses prepared under argon flow therefore have a bit lower optical transmission in this wavelength area compared to As2Se3.
Synthesis of chalcogenide glasses is quite tricky because of the volatility of the starting elements and the presence of impurities that can create strong adverse absorption bands in the transparency windows of interest. For these reasons, chalcogenide glasses are usually prepared in vacuum sealed silica ampoules which unfortunately cannot be reused. So this method is appropriate for laboratory tests, but the large-sized ampoules necessary for mass production make it very expensive on an industrial scale. Therefore, without doubt, a more economical procedure constitutes a remarkable improvement. This study precisely deals with a new route to synthesize As2Se3 glass under argon flow which does not require sealing of the reaction container. Experiments demonstrated that glasses prepared with this so-called “open crucible method” show interesting reproducible properties: Refractive indexes are almost identical (they differ in less than 1 · 10−3 from one another). Weight losses do not exceed 2%. Magnesium is an efficient oxygen getter purifying glasses so that no absorption bands in the 8–12 μm range can occur. As no distillation of the glass has been carried out until now, the resulting magnesium oxide partially keeps staying in the glass and leads to scattering losses at short wavelengths. The magnesium content will therefore have to be optimized according to the purity of the starting elements.
Acknowledgements This work has been partially supported by the Région Bretagne through the Pôle de compétitivité “ID4Cars” as well as by the French Ministry of Defense/DGA under the contract No. 2009.34.0019.
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