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Absorption of hydrogen gas (H2) in contact with palladium (Pd) makes Pd a material of choice for numerous H2 sensors. In this paper, we present the fabrication of optical fibers with embedded Pd particles in the silica cladding of the fibers. Fiber preforms prepared with a powder mixture of silica and palladium oxide (PdO) are heat-treated under specific conditions to reduce PdO to metallic Pd particles, dispersed in the silica matrix. Optical fibers with different topologies have been fabricated with lengths of several hundred meters and PdO concentration ranging from 0.01% to 5% mol (in addition to silica). Oxidation state, homogeneity, shape and size distribution of the particles embedded in the cladding of the preform and the fiber samples are studied with structural and micro-structural characterizations. Optical properties of the fibers are finally studied for evaluating the potential of this proof-of-concept work.
© 2015 Optical Society of America
1. Introduction
Hydrogen sensing is an important issue driving many industrial applications such as fuel cells, geothermal wells or nuclear power plants. Hydrogen concentration must be controlled and monitored because it forms explosive mixtures when combined with air for a large concentration range (from 4 to 75% vol in air [1]). Conventional hydrogen sensors usually rely on a sensitive material, such as tin or tungsten oxide (SnO2 and WO3), or metals like palladium (Pd) or platinum (Pt), whose resistive behavior is modified in contact with H2 gas. Optical fiber sensors are well known to be attractive components for harsh and explosive conditions because they allow remote sensing interrogation devices far from the measurement area without any ignition risk from electrical sparks. With appropriate dopants (such as Ge and F) and primary coatings (such as polyimide), silica optical fibers are also resistant to extreme temperature and gamma ray radiations [2]. This is the reason why many types of H2 optical fiber sensors have been studied for several years, based on H2-sensitive materials such as WO3 [3], yttrium oxide Y2O3 [4] or Pd compounds [5–8].
In contact with Pd, H2 gas dissociates on surface of the host metal before absorption leading to a variation of both the material refractive index [9] and lattice cell volume [5,9]. Efficient hydrogen sensing was achieved by using standard optical fibers (like G652 fibers used for telecommunications) with i) Pd films deposited on the fiber cross-section [1], ii) laid around the fiber [5,6], iii) around the core [7] or iiii) around tapered fiber [8]. However, these sensors suffer from untimely deterioration in harsh environments and poor robustness [10]. They are mainly dedicated to local gas detection only. More recently, frequency shift of the Brillouin backscattered peak resulting from H2 diffusion into a standard G652 fiber tested at 1.55 µm has been demonstrated [11], allowing distributed monitoring of H2 concentration. It is noteworthy that the diffusion kinetics of H2 gas in silica restricts the use of this sensor to monitor slow H2 leakage, in the order or hours, not millisecond.
We propose to introduce Pd particles into the silica cladding of optical fibers in order to protect the sensing metal from harsh environments and for enabling distributed sensing of H2 gas along long lengths [12]. Embedding Pd into fibers might therefore improve the sensitivity and the response time of the distributed fiber gas sensor, by exploiting the mechanical strain induced by the crystal lattice expansion of Pd particles in contact with H2 gas. The envisioned system relies on Brillouin scattering measurements paired with Pd-fibers. As demonstrated in literature with surface -Pd coatings, we expect that the palladium expansion (in contact with H2) creates strain inside the fiber, thus modifies the acoustic velocity and makes the Brillouin frequency shifting. This process should enhance the Brillouin sensitivity to H2 described in [11] with standard fiber. What is more, since Pd particles are located in the cladding material, closer to the atmosphere to sense, we also expect to detect H2 while diffusing through the optical fiber before it reaches the core, thus before standard fiber response. This assumption is currently being quantified, thanks to a model of H2 diffusion kinetics into silica and its impact on the both refractive index and acoustic velocity [13].
In this paper, we present the fabrication route of optical fibers with embedded Pd particles. We took advantage of the powder-based technology for managing fabrication constraints induced by mixing glassy and metallic materials with different thermo-mechanical properties and for obtaining non oxidized Pd particles embedded into the silica matrix. Fabrication process, structural and optical characterizations of optical fibers with embedded Pd particles are presented.
2. Fabrication of fibers with palladium particles embedded into the cladding material
2.1. Preform preparation
Traditional techniques used to fabricate optical fibers such as Chemical Vapor Deposition (CVD) methods or Rod-in-Tube are limited by the selection of the materials and/or the fiber topologies that can be fabricated. We chose the Modified Powder-in-Tube technique (MPIT) to fabricate these original optical fibers. The MPIT is a non-conventional technique based on the use of powdered materials for fabricating fiber preforms [14]. It is well adapted for managing fabrication constraints induced by different materials (large thermo-mechanical different properties), for controlling and modifying the oxidation state of powder materials inside the preform, and for fabricating fibers with complex topologies by associating it with others fabrication processes such as the stack-and-draw technique.
As shown in Fig. 1, the preform is realized by filling a silica tube with a powder mixture of silica (SiO2) and palladium oxide (PdO), around a cane positioned in the center. The cane composed of a core and an optical cladding ensures light guiding conditions. It could be fabricated by CVD and / or stack-and-draw processes depending on the complexity of the fiber topology studied. Prior to the preform assembly, powders, canes and tubes are dried at 350°C to remove adsorbed water from ambient humidity and also to avoid high OH content.
Fig. 1 Illustration of the MPIT based process developed for fabricating preforms with Pd particles embedded into the cladding material.
Powder mixture with compositions ranging from 0.01 to 5% mol of PdO in addition to SiO2 is used as the cladding material. Silica and palladium oxide precursor powders (99.995% purity, ACROS ORGANICS) with similar particle size distribution are weighted carefully and progressively mixed using a planetary ball milling equipment (RETSCH PM100) to form homogeneous SiO2-PdO mixtures. Precursor’s particle size distribution, with an initial median diameter around 5 and 15 µm for SiO2 and PdO respectively, was controlled using a MASTERSIZER 2000 analyzer (MALVERN Instruments Ltd).
The conditions for reducing PdO, from + II (Pd2+ in palladium oxide PdIIO) to 0 (Pd0 metallic particles), can be defined by the Ellingham’s diagram of the PdO/Pd redox couple. In the graph shown in Fig. 2, Gibbs free energy variation (ΔrG0) of the reversible Pd0 + ½O2↔PdIIO oxidation reaction is plotted versus the temperature [15]. It describes the thermodynamic driving force for which an oxidation-reduction reaction can occur.
The oxygen equilibrium energy (when the driving forces for the reaction are null) calculated by the empirical relation ΔG = R.T.ln(P(O2)/P) is plotted in Fig. 2 at different pressures, where R is the ideal gas constant, P(O2) and P are the oxygen partial pressure and total pressure respectively (in bars) and T is the temperature (in Kelvins). The intersection with the Gibbs free energy curve corresponds to the temperature of the redox equilibrium above which PdO is spontaneously reduced to metallic Pd.
From this diagram, PdO/Pd equilibrium occurs under ambient atmosphere at 865°C which is lower than the transition temperature of silica (TG~1200°C). Such heat-treatment leads to color variation of the SiO2-PdO powder mixture from grey to dark.
Furthermore, heating powdered preforms at a high temperature contributes to the elimination of residual air trapped into the porous materials as well as the consolidation of the powder mixture in the cladding. This allows more stable drawing campaigns.
2.2. Palladium-based fibers fabrication
Canes with various topologies have been fabricated and then used for realizing fiber preforms. After the heat-treatment, the preforms are drawn down to optical fibers with lengths of several hundred meters and a PdO concentration ranging from 0.01% to 5% mol (in addition to SiO2).
A Scanning Electron Microscopy (SEM) photograph of the cross section of a step-index fiber – the simplest topology - composed of a Ge-doped core (∆n = 9.10−3, dcore = 7.5 µm) with an external diameter of 170 µm is presented in Fig. 3(a). The fiber shown in Fig. 3(b) with an external diameter of 200 µm, is composed of a 12.8 µm diameter pure silica core surrounded by a photonic crystal cladding of air holes (dhole = 4.5 µm) spaced by 8.6 µm. A fiber with a more complex topology is shown in Fig. 3(c). It is composed of a Ge-doped step-index core (∆n = 8.10−3, dcore = 5.7 µm) in a silica inner cladding surrounded by a ring of 14 large air holes (dhole = 10.3 x 17.1 µm, d/Λ = 0.94) separated by thin silica struts (around 700 nm).
Fig. 3 SEM (in the backscattered configuration) photographs of cross-sections of three optical fibers with Pd particles embedded into the silica cladding fabricated by the MPIT process. (a) a SiO2-GeO2 core step index fiber, (b) a pure silica core microstructured fiber, (c) a SiO2-GeO2 step index core with a microstructured cladding fiber and (d) zoom-in photograph of the cladding region of the fiber shown in (c).
Bright elements in the external cladding are observed on the SEM pictures, where the SiO2-PdO powder mixture has been inserted. A zoom-in photograph of this region (c.f. Fig. 3(d)) illustrates clearly sub-micronic particles, while black spots are remaining air inclusions from powder interstices.
3. Structural and optical characterizations of fabricated optical fibers
3.1. Material characterizations
Structural and microstructural characterizations of the preform and fiber samples were realized to evaluate the oxidation state of Pd during the fabrication process, and then for studying homogeneity, geometry and size distribution of the particles embedded in the cladding of the optical fibers. In the following most of the characterizations are realized on the fiber with a more complex topology (c.f. Fig. 3(c)) and its corresponding preform.
Elemental analysis
SEM photographs (measured in backscattered electron configuration) indicate that the bright elements have larger molecular weight than silica (darker background). A similar SEM image was obtained by measuring a portion of the external cladding of a preform after the heat-treatment (c.f. Fig. 4(a)). As expected, Si, O and Pd elements have been identified in this portion by Energy Dispersive X-ray Spectroscopy (SEM-EDS) elemental analysis (c.f. Fig. 4(b)). The individual element mappings of Si and Pd (c.f. Fig. 4(c)) associated to the SEM picture clearly show that the bright particles are composed of Pd without Si (particles are replaced by dark spots in the cartography of Si). Nevertheless, these characterizations are not sufficient for discriminating between Pd and PdO.
Fig. 4 Elemental analyses of a preform portion after the heat-treatment. (a) SEM photograph (in the backscattered configuration), (b) SEM-EDS elemental analysis and (c) SEM-EDS mapping of Pd and Si individual elements (detection of the L and K lines, respectively).
X-Ray Powder Diffraction (XRD) was performed to estimate the oxidation state of Pd into the silica cladding at the different stages of the fiber fabrication process.
Portions of preforms before and after the heat-treatment and the corresponding fiber have been characterized with a BRUKER D8 diffractometer (with CuKα radiation λ = 1.5406Å). XRD patterns were collected from 10° to 60° in 2-theta angles (c.f. Fig. 5) and can be indexed with database patterns of Pd and PdO (JCPDS n°46-1043 and JCPDS n°41-1107, respectively). PdO is only detected into the non-heat-treated preform sample and metallic Pd is identified in the heat-treated preform and fiber samples without any trace of PdO. PdO is thus entirely reduced during the preform heat-treatment to form metallic palladium particles into the silica cladding.
Fig. 5 Comparison of XRD diagrams of palladium-based fiber and untreated and heat-treated preform samples (the large diffraction peak centered to 2θ ~22° is due to the silica amorphous network and is typical to glass structures).
Distribution and shape of Pd particles
As observed in the SEM pictures of the fiber cross-section in Fig. 3, Pd particles are spread randomly and concentrically in the external cladding material. We have realized several SEM pictures of the same fiber at different locations along a total length of nearby 130 m. As shown on the three SEM pictures presented in Fig. 6, the distribution of Pd particles is uniform along the fiber length. These observations demonstrate that the fabrication process yields homogenous long lengths of fibers.
Fig. 6 Comparison of SEM pictures of fibers from the same batch, taken at different locations of the drawing processing of the fiber presented in Fig. 3(c).
The particle size measured from SEM pictures ranges from hundreds of nanometers to few micrometers. According to the Debye-Scherrer approximation (applied on Pd peaks in the XRD patterns of the heat-treated preform and the optical fiber (c.f. Fig. 5)), the mean diameter of the Pd particles is estimated to be around 50 nm; this suggests that some particles may be aggregated.
To fully characterize the shape of the Pd particles, we have polished some fiber samples in the longitudinal direction (over 1 cm). A SEM picture of the longitudinal section of a polished fiber is shown in Fig. 7(b). We observed Pd wires with length up to 50 µm, all wires having their long axis in the longitudinal direction of the fiber. In conclusion, spherical Pd particles in the preform have been stretched during the drawing process to form longitudinal metallic wires dispersed into the silica cladding. Particles stretching phenomenon might be enhanced by the low viscosity of Pd at the drawing temperature (nearby 1850°C) since liquid metallic palladium occurs at nearby 1550°C.
Fig. 7 SEM photographs of (a) the fiber cross section and (b) longitudinal section of the fiber, polished until the core.
3.2. Optical characterizations
Transmission spectrum of the fiber presented in Fig. 3(c) was measured by using a broadband light source (SM-20, LEUKOS, spectral range 350-2000 nm, output average power 80 mW) and an optical spectrum analyzer (ANDO AQ-6315A) with a spectral resolution of 5 nm. Transmission spectrum of the light source transmitted through 10.2 meters long length of the fiber is shown in Fig. 8(a), with the transmission spectra of different fiber lengths measured after cutting the fiber (with the same launching conditions). Light is guided over the wavelength range from 700 nm to 1700 nm without any absorption band. The optical attenuation coefficient is estimated around 3 dB/m (at the wavelength of 1.55 µm) by comparing the transmission spectra (cutback technique).
Fig. 8 (a). Transmission spectra of the light source transmitted through different lengths of the fiber (shown in Fig. 3(c)), with the same launching conditions, and (b) Intensity distribution (two dimensional pattern and profile) at the fiber output of the propagated light (filtered around 1.55 µm).
The near-field pattern at the fiber output of the guided light was monitored with an In:Ga:As camera (FLIR Indigo Alpha NIR camera 900-1700 nm) and a band-pass filter (at 1550 nm +/− 12 nm). As shown in Fig. 8(b), light is confined in the Ge-doped core with a Gaussian-like shape of the intensity distribution. Higher order modes were not successfully observed even under different light-coupling conditions, confirming single mode guidance regime. However, the diameter of the Ge-doped core is rather small (dcore = 5.7 µm) leading to weaker light confinement in the core, which is observed by the pedestal of the intensity distribution in the inner silica cladding (c.f. Fig. 8(b)). The normalized frequency of this fiber at 1.55µm is around 1.75 against 2.06 for a standard silica fiber (typically G652). This weaker confinement has a strong impact on the large attenuation coefficient at this wavelength. Furthermore, it is noteworthy that no intensive cleaning process has been applied during the fabrication process.
The mode field diameter of the fundamental mode estimated around 8.2 µm (at 1.55µm) is closed to the one of a G652 fiber (10.4 µm at 1.55µm) allowing straightforward integration of the fiber in all-fiber systems. We have obtained splicing losses lower than 1 dB at 1.55µm by using an ERICSSON FSU995 arc fusion splicer with some optimizations of the splicing parameters.
Finally, functionalized samples composed of a 2-meter-long length of the fiber spliced at each end with a single-mode E2000/APC (or FC/APC) pigtail, have been realized. Their sensitivity to H2 gas is currently tested with a low spatial resolution backscattered Rayleigh and/or Brillouin instrument.
4. Conclusion
We have demonstrated, for the first time to our knowledge, the fabrication of optical fibers with metallic Pd particles embedded in the silica cladding. Pd particles are formed from PdO powder by heating the preform above 865° C at atmospheric pressure, following the reduction conditions from the PdO/Pd redox couple Ellingham diagram. We have taken advantages of the MPIT based process for fabricating optical fibers with different topology complexities. Structural and microstructural characterizations of the preforms and fibers demonstrate that the fabrication process yields homogenous long lengths of fibers with metallic Pd particles randomly spread concentrically in the external cladding. We have observed a stretching behavior of the particles during the drawing stage to form tens of micrometers long length metallic wires. Even if optical properties of the fibers were not primary considered in this work, fiber samples could be inserted in all fiber systems for investigating their performances as distributed H2 gas sensor.
These proof-of-concept fabrications pave the way to the development of inherently functionalized optical fibers with Pd particles or other materials with compatible reduction conditions.
References and links
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