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Abstract
The use of a 3D-printing additive manufacturing process is reported for the first time for the extrusion of chalcogenide glasses by using a filament feed. Several challenges were overcome: preparation of chalcogenide glass filaments by the crucible technique, optimization of extrusion temperature or even filament feeding. The As40S60 chalcogenide glass was selected for its low glass transition temperature (Tg = 188°C) and ease of synthesis and processing. It was extruded using a commercial 3D-printer at a temperature around 140°C above the glass transition temperature. 3D-printed glass specimens were then characterized and no significant difference was observed in comparison with the bulk precursor glass in terms of chemical and thermal properties. This first report of additive manufacturing of chalcogenide glass complex shapes paves the way for the development of novel specialty optical components that could not be produced by conventional methods, including the fabrication of multimaterial optical fiber preforms.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The development of multimaterial fibers and related components for optical, optoelectronic or other functional applications is generating considerable interest in many sectors such as health and medicine, telecommunications, defence, environment, energy, etc. [1–3]. Amongst the materials considered for designing such innovative technological devices, chalcogenide glasses are closely studied because of their unique optical transmission in the infrared range, for both passive [4–7] and active applications (e.g. rare-earth ion photoluminescence [7–9], supercontinuum generation [10–14], Raman gain [15], etc.). The conventional fabrication methods employed to produce fiber preforms rely on the rod-in-tube technique [6], preform extrusion [6,16], crucible technique [17], stack-and-draw [18,19], thin-film rolling [1,2] or their combination. A novel approach currently explored for the fabrication of optical inorganic glass components [20,21] that could be advantageously used to produce multimaterial fiber preforms is additive manufacturing (AM). AM is a three-dimensional layer-by-layer deposition technique that is currently employed in many fields for industrial applications [22], in medicine for implants [23], for rapid prototyping [24], etc. Compared to conventional fabrication techniques, it uses a digital model providing several advantages such as design versatility, low cost, rapid realization or a high degree of repeatability due to the automated process. Additive manufacturing methods are thus opening novel avenues for the production of glass optical components, including fiber preforms with complex geometries and/or different material associations that could not be possible with the traditional techniques.
AM of optical materials, which has attracted interest in the past few years, includes several processes for instance Selective Laser Sintering (SLS) [25,26], inkjet printing [27,28] and an extrusion technique such as Fused Deposition Modeling (FDM) [29–32]. Among those techniques, the FDM approach is showing more promise for producing dense and bubble free volumes of material for optical applications. FDM is a rapid prototyping technology using a continuous filament material extruded layer-by-layer to produce a three-dimensional scaffold. Most studies of optical material additive manufacturing center on polymers owing to their ease of synthesis and low temperature processing. For example, 3D-printed polymer glass preforms for optical fiber drawing have been already reported in the literature [29–32]. However, alternative materials to polymers should also be considered depending on the targeted applications and requirements in terms of optical transmission range, temperature resistance or chemical/mechanical durability, etc. Furthermore, in the past few years, additive manufacturing of inorganic glasses (soda-lime glass [26,33,34], fused quartz [35] and borosilicate [36]) has been investigated for their attractive properties. To our knowledge, there is no study to date on additive manufacturing of chalcogenide glasses although their capability to be extruded through dies is well-known [37–40]. Recently, important progress has also been reported in solution processing of chalcogenide glasses for the development of planar optical devices [41,42]. The possibility of processing chalcogenide glass materials in such different ways points to its strong potential for being used for 3D-printing of infrared optical materials.
In this paper, we report the feasibility of three-dimensional chalcogenide glass printing by fused deposition modeling. Because of its low glass transition temperature [43] and ease of processing [17], chalcogenide glass with the nominal composition As40S60 was selected. We aim at studying the chemical and physical properties of extruded chalcogenide glasses and demonstrating that 3D-printing may be a suitable technique for the fabrication of chalcogenide glass complex optical components, including multimaterial fiber preforms (e.g. with polymers) with geometries or structures than cannot be produced by conventional techniques.
2. Experimental procedures
2.1 Glass preform synthesis and filament drawing
Chalcogenide glasses with the nominal composition As40S60 was prepared using the conventional melting and quenching method [44] starting from high purity (5N) commercial elements (Arsenic and Sulfur). Elements were weighed in appropriate amounts and inserted in silica ampoules which were then sealed under vacuum (10−5 Torr). The elements were melted in a rocking furnace at 750°C and then quenched in water. Finally, the resulting glasses were annealed during 8 hours in a furnace at a temperature close to their glass transition temperature (Tg - 10°C) to remove the internal stress. Resulting chalcogenide glass preforms with a 25 mm diameter and around 10 cm long were obtained and used for drawing filaments with a diameter of 1.75 mm using the crucible method (Tfurnace = 340°C). Unpolished slices from the glass preform about 2 mm thick were also used as substrates for the 3D-printing process.
2.2 Extrusion of chalcogenide glasses
To allow 3D-printing of chalcogenide glasses, some modifications were made to a commercial 3D-printer (Creality, ENDER-4, represented in Fig. 1). It was equipped with two E3DV6 extruders close to each other in order to increase the heating zone length. Extruders 1 and 2 (Fig. 1) have respectively one and two heating elements to better adjust glass viscosity for its extrusion through the nozzle. Moreover, this printer is designed for flexible filament printing, i.e. polymeric materials, and is therefore equipped with drive wheels. The latter have been modified to increase load and enable glass filament feeding. To obtain the appropriate As40S60 chalcogenide glass viscosity for extrusion, the temperature of both extruders were manually adjusted by varying the operating tension and current of the tension controller. The filament feeding speed and the print bed displacement (lateral movements X and Y and vertical motion Z) were driven by an automatic process. The nozzle diameter used during printing was 0.4 mm. To limit thermal stresses during 3D-printing, it is preferable to use as a substrate a material with a similar thermal expansion coefficient. Several materials such as brass or stainless steel were tested, showing very weak adhesion of the printed material to the substrate, which is essential to ensure progress of the printing process. The As40S60 chalcogenide glass itself actually appeared to be the best substrate to sustain 3D-printing of arsenic-sulfide glass. Moreover, in order to reduce the thermal gradients [45], the substrate was installed on a heating bed during deposition. Thus, As40S60 substrate was fixed on the building plate and preheated at T = 80°C. After 3D-printing of the chalcogenide glasses, the resulting printed specimens were annealed in ambient air during a few hours at T = 180°C, a temperature close to their glass transition temperature Tg.
Fig. 1. Illustration (a) and photograph (b) of the experimental setup used for additive manufacturing of chalcogenide glasses (see Visualization 1 in Supplementary Material). The 3D-printer is installed inside a chemical fume hood to protect the operator from toxic vapor generated during the process.
The 3D-printer was installed inside a chemical fume hood to protect the operator from any toxic vapor generated during the process. Chalcogenide glasses are indeed well known to possess high vapor pressure at temperatures above their glass transition temperature. Such property results in material evaporation during the printing process at ambient pressure and therefore requires special precautions like working under a chemical fume hood and using a gas tight protection enclosure.
2.3 Material characterization
Glass transition temperatures Tg of filament and printed glasses were determined by differential scanning calorimetry (DSC) using a Netzsch DSC Pegasus 404F3 apparatus. Chalcogenide glass pieces (few mg) were inserted and sealed in Al pans, and heated up to 350°C at 10°C.min-1. The chemical compositions of filament and printed glass were measured using a scanning electron microscope (SEM) with an energy-dispersive X-ray analyzer (EDS) (QUANTA-3D-FEG). Atomic percentage of each element (accuracy ± 1%) was extracted exploiting the K lines of As and S working at 20 kV. The infrared absorbance spectrum of a printed As40S60 chalcogenide glass (about 2 mm thick) was recorded using a FTIR Perkin Elmer Frontier spectrometer in the 1.5-12 µm spectral range. The densities ρ of printed and bulk glasses were determined using the Archimedes method with a Mettler Toledo XSE204 analytical balance. The obtained values correspond to the average of 5 weight measurements taken on samples in air and deionized water. Special attention was exercised to ensure the absence of air bubbles during weighing in deionized water, in particular for the 3D-printed samples. Raman spectra were recorded on a Renishaw inVia spectrometer coupled to a Leica DM2700 microscope and a Renishaw CCD camera detector. The laser beam at 785 nm was focused on the sample with a 50x long working distance objective. All spectra were recorded in the same conditions of exposition. No photoinduced effect was observed after the acquisitions. The obtained spectra were then baseline corrected and normalized.
3. Results and discussion
3.1 Temperature extrusion
Temperature distribution in the setup during As40S60 chalcogenide glass extrusion was monitored using a thermal camera (HypIR, Telops Inc.). Infrared images were acquired during the As40S60 chalcogenide glass printing process. The temperature was probed in the initial extruder (1), between both extruders (2), in the additional extruder (3 and 4) and in the nozzle (5) (Fig. 2). The temperature distribution was perfectly homogeneous throughout the extruding setup as one can see in Fig. 2. In fact, the temperature in both extruders and in the nozzle output were ∼330°C which can be ascribed to the temperature of As40S60 chalcogenide glass printing. Note that this temperature is about 70°C above the maximum working temperature of conventional polymeric material 3D-printers. This relatively high temperature for a low-Tg glass provides suitable viscosity for stability and adhesion of chalcogenide glass to the substrate during processing. According to the temperature-dependent viscosity reported for chalcogenide glasses in [46], the viscosity of As2S3 glass for 3D-printing process is ∼9.105 Poise whereas As2S3 glass fiber is usually drawn from a preform at ∼300°C (viscosity of ∼106 Poise) or by the crucible technique at ∼340°C (viscosity of ∼4.104 Poise).
Fig. 2. Temperature distribution observed with thermal camera and measured at specific locations including the nozzle and both extruders 1 and 2 during 3D-printing of As40S60 chalcogenide glass.
3.2 Morphology of printed chalcogenide glass
Two different samples were designed and printed. Figure 3(a) and Fig. 3(b) present samples with a 100% density of infill whereas Fig. 3(c) shows a 20% density of infill of print. At the end of chalcogenide 3D-printing, the resulting As40S60 specimen was easily removed from the chalcogenide substrate as soon as the printed material was cooled down. It is interesting to note that although the adhesion of printed material to the substrate is essential to complete the printing process (to avoid breaking), it appears to be very weak. Due to the relatively high temperature process (∼330°C), a yellowish sulfur vapor deposition was observed on printed specimens. Figure 3 shows ripples on the chalcogenide printed glass surface which is characteristic of 3D-printing. Due to their fragile character and despite their annealing process after printing, it was not possible to achieve optical polishing of the printed samples without inducing cracks. SEM images of samples are presented in Fig. 4. Using a 0.4 mm nozzle diameter, the width of the printed chalcogenide glass was approximately 0.6 mm (Fig. 4(a)). The COPL acronym (for Center for Optics, Photonics and Lasers research center) was also 3D-printed as illustrated in Fig. 3(d), demonstrating the feasibility of printing complex shapes.
Fig. 3. (a) Photograph of As40S60 printed glass samples with 100% density of infill printed, sample on left and right sides were fractured intentionally after processing for better observation. The fracture zone of the sample on the left is also magnified in (b), showing absence of bubbles. (c) Photographs of a sample with 20% density of infill printed (c), and of COPL research center acronym (d).
Bubble formation is also a common problem in additive manufacturing especially for glass printing [36,47], that can deteriorate the material’s optical properties. Luo et al. showed that bubble formation in additive manufacturing of soda-lime glass using laser processing was mainly caused by the reboil [48] of the glass melt. Here, no evidence of bubble formation was observed in the printed As40S60 glass samples, as can be seen in Fig. 3 (b) which presents a sample fractured after processing, or in Fig. 4 where no evidence of bubbles was observed during the SEM analyses. The result of chalcogenide extrusion is a bubble-free glass with no cracks formed during printing. An automatic process was used to repeat the geometry layer-by-layer. Figure 4 also shows a precise superposition and a good contact between layers. Likewise, there is no bubble at the interface between layers. The fabrication of the 20% density of infill sample (Fig. 3(b)) allows to produce different angle designs (Fig. 4(b)). Effective superposition of chalcogenide layers was also obtained. However, some irregularities were observed due to inhomogeneous glass flow of the feeding filament in particular between two filaments (end of the filament and beginning of the new filament). It has to be noted that these interface irregularities between layers can lead to a loss of optical properties such as light scattering.
3.3. Characterization of printed glass specimens
The chemical composition and glass transition temperature Tg of printed glass specimens were measured and compared to the filament. The chemical composition of the printed chalcogenide glass estimated by energy-dispersive X-ray spectroscopy was found to be As41S59; these results are in excellent agreement with the theoretical As40S60 and the filament As41S59 chemical compositions considering a measurement uncertainty of about ± 1%. Thus there is no modification of chemical composition during the 3D-printing process. In the same way, we note that the glass transition temperature Tg obtained for 3D-printed glass, i.e. 188°C, is identical to that of the filament glass.
The densities measured for the bulk and printed glass samples are 3.104 and 2.830 g.cm-3, respectively. The slightly lower value obtained for the printed glass may be explained by the complex stacked structure (as observed in Fig. 4) whose interstices between layers may trap some air bubbles when the sample is weighed while immersed in water, thus resulting in an underestimation of its weight.
The infrared absorbance spectrum of a printed glass specimen of about 2 mm thick was recorded in the 1.5–12 µm range. As mentioned above, the sample was not optically polished. The absorbance spectrum recorded on the precursor glass used to draw the filament for 3D-printing is also presented in Fig. 5 for comparison. The latter sample, which was not prepared with any particular precaution in terms of purification, was optically polished for this measurement. Note that each spectrum is presented with its own absorbance axis for better readability. As it can be noted in Fig. 5, the absorbance spectrum of the printed sample is similar to that of the bulk precursor glass, with similar optical transmission in the infrared up to 9-10 µm and the presence of absorption bands related to the presence of impurities. The relatively large difference in the background intensity level between the two spectra can be explained first by the absence of polishing for the printed sample, resulting in high scattering losses. Additional scattering losses may also come from the stacking structure (as observed in Fig. 4). Last, as it can be observed in the photographs in Fig. 3, the printed specimens show some darker regions that could possibly result from contamination during the process. The absorption band at 2.9 µm is attributed to –OH group, at 4.0 µm to –S-H, at 4.3 µm to CO2 molecules, at 6.3 µm to H2O, at 7.6 µm, 9.5 µm and 10.1 µm to arsenic oxides, at 8.6 µm to S-O [7]. Moreover, when comparing the scale of the respective absorbance axes in Fig. 5, one can notice that the intensity of those absorption bands is more than twice greater for the printed glass. This can be easily explained by the experimental conditions of glass 3D-printing. Indeed, additive manufacturing of As40S60 chalcogenide glass was performed under ambient atmosphere, and the post-printing annealing too. Moreover, one cannot exclude some glass contamination during the contact with metal extruders at high temperature, despite no observed visible damage on their surface. To limit this contamination, future 3D-printing of chalcogenide glass will be performed under inert atmosphere.
Fig. 5. Absorbance spectra of an unpolished As40S60 printed chalcogenide glass sample (with thickness of about 2 mm) and of a polished slice of the As40S60 precursor glass used to prepare the filaments (thickness is 2.3 mm). Note that each spectrum has its own y-axis for better reading.
The Raman spectra recorded on the bulk precursor glass, the glass filament used for 3D-printing and the printed glass are presented in Fig. 6. The spectra were acquired in strictly the same conditions. Printed glass specimens were analyzed at different places but no variation of the recorded Raman spectra was observed. The main band located at 340 cm-1 is ascribed to As-S vibrations in As-S3/2 pyramidal sites whereas the bands around 200 cm-1 and 500 cm-1 are ascribed to As-As homopolar bonds and S-S bonds, respectively [49]. More importantly, no significant difference can be observed from the Raman spectra, indicating the glass structure of the printed arsenic sulfide glass is globally the same as the precursor glass. Nevertheless, additional structural investigation is required to better understand the local changes of coloration.
Fig. 6. Raman spectra of As40S60 chalcogenide glass, filament and printed samples. Spectra were baseline corrected, normalized and vertically translated for better reading.
Future developments of the chalcogenide glass 3D-printing system will focus first on a better control of the extruders’ surrounding atmosphere, by using a specific gas-tight enclosure with inert atmosphere. In addition to the safety concerns, such enclosure should help us in terms of thermal management during processing. The rather strong gas aspiration inside a chemical fume hood may indeed affect the 3D-printing process, in particular the material cooling after extrusion.
Besides a better control of the printer’s surrounding atmosphere, future developments are directed towards improving the 3D-printer design in terms of thermal management to adequately anneal and cool to room temperature the printed glass specimens. This is a prerequisite to limit the formation of internal stress during processing that inevitably results in mechanically fragile pieces. The addition of a second extrusion set to enable co-printing with other materials like polymers or low-Tg glasses is also currently being tested. Development of both the co-printing approach and the thermal management of specimens during the printing process will allow the fabrication of multimaterial components, including preforms of complex geometries, paving the way for additive manufacturing of unique optical fiber components.
4. Conclusion
By demonstrating the feasibility of additive manufacturing of As2S3 glass, this work explored a new way to produce chalcogenide fiber preforms. This original approach has strong potential for the fabrication of complex preform geometries and structures, including combinations not only with other inorganic glass compositions but also with other printable materials like polymers to offer multiple functionalities. The aim of this present work was to demonstrate the feasibility of additive manufacturing of chalcogenide glass by defining the most effective printing parameters. A three-dimensional chalcogenide glass with the chemical composition As40S60 was successfully manufactured using fused deposition modeling. The effects of the additive manufacturing process on the morphological, chemical and physical properties of As40S60 printed chalcogenide glass were studied. Results revealed that there is no modification of chemical composition and thermal properties of printed glass properties compared to bulk glass. Future works are focused on improving thermal management and process conditions to increase mechanical robustness and optical quality of 3D-printed specimens to produce glass preforms for fiber drawing.
Funding
Canada Excellence Research Chairs, Government of Canada (CERC); Natural Sciences and Engineering Research Council of Canada (NSERC); Canada Foundation for Innovation (CFI); Fonds de Recherche du Québec - Nature et Technologies (FRQNT).
Acknowledgments
This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) within the framework of the Canada – France (ANR) international joint project entitled PROTEus (PRinting of exOTic multi-maTErials fibers). The authors are also grateful to the Canada Foundation for Innovation (CFI), the Fonds de recherche Québec – Nature et technologies (FRQNT) and the Canada Excellence Research Chair in Photonic Innovation (CERCP) for their financial support. M. El-Amraoui and A. Douaud are acknowledged for their help in preparing the glass samples and the Raman spectroscopy characterization, respectively.
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