- Email: [email protected]
An optical hydrogen sensor based on a Pd-capped Mg thin film wedge
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
An optical hydrogen sensor based on a Pd-capped Mg thin film wedge V. Palmisano a,*, M. Filippi a,b, A. Baldi a, M. Slaman b,a, H. Schreuders a, B. Dam a a
Department of Chemical Engineering, Technical University Delft, Julianalaan 136, 2628 BL Delft, The Netherlands Department of Physics and Astronomy, Condensed Matter Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands b
article info
abstract
Article history:
We report the proof of concept of a thin film device with a one-to-one relationship between
Received 22 June 2010
the H2 partial pressure and the lateral progression of a visible optical change along a thin
Received in revised form
film multilayer 70 mm long. The device basically consists of a sensing Mg layer with
1 September 2010
a thickness gradient. It exploits the thickness dependence of the hydrogenation thermo-
Accepted 1 September 2010
dynamics of Pd-capped Mg thin films. The optical change of the Mg layer during the
Available online 22 September 2010
metalehydride transition can be detected both in reflection and in transmission. This optical sensor allows a continuous measurement of hydrogen partial pressure in the range
Keywords: Optical hydrogen sensor
between 200 and 4000 Pa. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Hydrogen safety Thin film Switchable mirror
1.
Introduction
The development of high-performance hydrogen sensors is crucial for the successful implementation of hydrogen as a sustainable energy carrier. Hydrogen is characterized by a wide flammability range (4e75 vol%) and a relatively low ignition energy (only 0.017 mJ at room temperature), while its presence is undetectable by human senses. Conventional sensors [1e3], i.e. catalytic, electrochemical and semiconducting metal-oxide sensors use electrical leads which may induce sparks and provide a cause for ignition in hazardous atmospheres. Optical sensors based on fiber optics allow working in an explosive environment thanks to the possibility of separating the sensing point from the electrical readout. Various strategies have been proposed
which basically infer the hydrogen concentration from a change in the optical response, e.g. the interference pattern [4], the frequency of the optical signal [5], or in the intensity of the optical signal [6,7]. Following the discovery that due to the absorption of hydrogen some metals experience a metaleinsulator transition characterized by a large change in optical reflectance or transmittance [8,9], our group in Amsterdam developed a fiber optic hydrogen detector based on a sensing layer made of Mg0.70Ti0.30 [10]. This detector satisfies the requirements of having a high contrast, small dimensions, low cost and being safe to operate. However, it only indicates whether the hydrogen pressure is above or below a certain threshold level, which is given by the equilibrium pressure of the Mg0.7Ti0.3 detection layer.
* Corresponding author. Tel.: þ31 15278 2676; fax: þ31 15278 7421. E-mail address: [email protected] (V. Palmisano). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.001
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
12575
Fig. 1 e (a) Sample design: a wedge of Mg with varying thickness along the x direction is grown on a glass substrate 70 3 5 3 1 mm3. The Mg wedge is sandwiched between a Ti-adhesion layer 3 nm thick and a Pd-cap layer 40 nm thick. (b) Simulation generated through the optical simulation tool SCOUT of the optical transmittance of the sample before (circles) and after (squares) the hydrogenation of the Mg layer at various thicknesses: lines are guide to the eyes.
A quantitative detector can be designed exploiting the recent discovery of the fact that the thermodynamics of Mg thin films (<40 nm) is affected by the Pd-cap layer [11]. This layer is usually applied to prevent the oxidation of the Mg layer and to promote the hydrogen absorption. Due to the elastic coupling between the Pd and Mg layer the hydrogenation pressure p is extremely sensitive to the Mg thickness d, showing an exponential dependence given by the formula: p ¼ p0 exp
1 : a,d þ b
(1)
The parameters a and b contain the elastic properties (Young’s modulus and Poisson’s ratio) of the cap layer and the clamping
layer respectively, while p0 represents the equilibrium pressure of the hydrogenation reaction of Mg in the non-capped situation: Mg þ H2 0 MgH2 [11]. Based on this effect we report in this paper on a thin film optical device, which is able to measure the hydrogen concentration in Ar atmosphere within a hydrogen partial pressure range varying from 200 Pa to 4000 Pa (we recall here that 4000 Pa represents the lower flammability level of hydrogen in air). The sensor is made of a wedge of Pd-capped Mg, where the Mg thickness varies from 5 to 32 nm along a 70 mm glass substrate (Fig. 1(a)). Because the equilibrium pressure of magnesium hydride depends on the layer thickness, the length over which the sample becomes transparent after exposure to hydrogen is a measure for the hydrogen pressure. In reflectance this effect can be observed through the substrate as a progressive lateral change in reflectance from a shiny metallic to a dark, light absorbing appearance.
2.
Fig. 2 e PTIs measured at 333 K corresponding to different Mg thicknesses: increasing Mg thickness results in an increase of the optical contrast and a decrease of the plateau pressure. We do not report PTIs corresponding to thicknesses lower than 9.5 nm for which the experimental pressure ramp is not sufficient to complete the metalhydride transition.
Materials and methods
The sample is deposited in a UHV chamber (background pressure is 5 107 Pa and sputter pressure 0.3 Pa of argon) by RF/ DC magnetron sputtering of Ti (99.99%), Mg (99.95%), and Pd (99.98%) respectively. First, we deposit a 3 nm Ti-adhesion layer on a rotating glass substrate. The Ti thickness was minimized in order to maximize the change in reflection upon loading of the Mg layer. To obtain the desired Mg wedge geometry we stop the substrate rotation and tilt the Mg source away from its optimal direction, i.e. the direction pointing to the center of the substrate. Finally, we deposit a uniform Pdcap layer resulting in the sample geometry shown in Fig. 1(a): substrate (1 mm) þ Ti (3 nm) þ Mg (5e32 nm) þ Pd (40 nm). The expected optical response of such a sensor is calculated using the optical simulation software tool SCOUT [12]. Fig. 1(b) shows the simulation of the optical transmittance, averaged over the visible spectrum (1.1 < hn < 3.3 eV), of the layer stack as
12576
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
Fig. 3 e Sample snapshots taken at different times while increasing the H2 partial pressure from 12 to 4000 Pa. The Mg thickness along the sample and the corresponding loading pressure are reported on top. The inset shows the plateau pressure behavior as a function of the Mg effective thickness: the fit with Eq. (1) is also reported.
a function of the Mg thickness in the metallic and the hydrogenated state. We find a high optical contrast, i.e. difference in the logarithm of transmittance before and after the hydrogenation of Mg, for Mg thickness down to 10 nm.
Fig. 4 e Color map of the optical reflectivity as a function of pressure and time measured along the wedge-shaped sample at room temperature. The transparency front progresses laterally (the x axis) with pressure; the H2 pressure is increased stepwise from 100 to 2000 Pa (as shown in the vertical axis). The scale of intensities goes from orange (high reflectance) to a scale of gray (with minimum reflectance in black). Black closed circles indicate the time when the H2 pressure is suddenly increased and the corresponding position of the sample where Mg undergoes the metaleinsulator transition.
3.
Results and discussion
To explore the behavior of the stack as a sensing device, we use hydrogenography, an optical technique which allows the measurement of pressure-optical transmission isotherms (PTIs) [13]. A 150 W diffuse white light source illuminates the sample through the substrate while a 3-channel (RGB) CCD camera monitors the transmitted light. In order to distinguish various pressure plateaus we increase the hydrogen partial pressure near the sample from 10 to 4000 Pa in 20 h with a logarithmic function of time. The measured PTIs along the sample at various nominal Mg thicknesses are illustrated in Fig. 2. The optical response here is given by the logarithm of the optical transmission normalized to the optical transmission in the initial metallic state. We observe that the plateau pressure increases when the Mg gets thinner while the optical contrast given by the plateau width decreases, which is in agreement with the optical simulations shown in Fig. 1(b). Snapshots of the sample are regularly taken while measuring the PTIs. Fig. 3 shows some of these snapshots taken at different H2 pressures. The loading results in a progressive brightening of the device along the x direction (the reference system is shown in Fig. 1(a)); the position on the sample of the bright front gives the readout of the pressure. The bright spots in the picture are due to pinholes which serve as additional markers of the position on the wedge. The H2 partial pressure can hence be scaled with the length of the more transparent region, as shown by the upper abscissa in Fig. 3. Thus, the thin film strip represents a sort of macroscopic barometer for H2 partial pressure. At present the scale is non-linear with the H2 partial pressure, but it is possible to make the Mg wedge with a geometry such that the equilibrium pressure can be varied linearly with the distance. As shown in the inset of Fig. 3 the plateau pressure depends exponentially on the Mg thickness as predicted by the elastic clamping model [11].
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
We studied the behavior of the optical reflectivity of our sample observing the film from the substrate side. This configuration, used in the fiber optics hydrogen detector [10], could be in principle extended to a multifiber hydrogen sensor where on each fiber a sensing layer is grown with a different thickness and therefore a different plateau pressure. The optical reflection of the sample is measured through a 3-channel (RGB) CCD camera in the presence of a homogeneous white light source. Fig. 4 shows the change in the optical reflection measured through the substrate as a function of time at room temperature. While increasing stepwise the hydrogen pressure from 100 to 2000 Pa in 7 h, the decrease of the reflectivity as a function of the hydrogen pressure shows again a thickness dependent behavior. The readout of the pressure is given by the position of the front demarcating the highly reflective state from the dark, light absorbing state. From the picture we also observe that 80% of the response is realized within 500 s, giving an indication of the time response of this sensor. The measured H2 partial pressure in this configuration is different from the presented transmittance measurements due to the increase the plateau pressure with the temperature [14]. This implies that, in a realistic device, the sample temperature should be kept constant or measured separately in order to infer the correct hydrogen pressure. In the latter case the scale of the measured pressure can be converted by software and the optical response should be read by electronics: the multifiber configuration has the advantage of allowing a simultaneous measurement of the temperature through interferometric or fiber Bragg grating methods [15]. The physical background of this device [11] allows for an extension of the operating range, by choosing e.g. clamping materials with a higher Young’s modulus. In addition, the size of the device can be adjusted by changing the steepness of the wedge in Mg thicknesses. We should also mention that cycling results in a continuous decrease of the optical contrast as hydrogenation promotes the alloying between Mg and Pd [16], resulting in a lack of stability and a very slow desorption kinetics. Thus, the device considered here can only be exploited as a single-use sensor. In our current research we investigate other clamping materials and new layer geometries to improve the reproducibility of our device on cycling. The use of a protective coating, for example, prevents the degradation of the catalytic properties of Pd due to moisture and pollutants, allowing an increase of the lifetime of the device [17]. The device proves to work in inert atmospheres, where the interaction between the balance gas and the cap layer plays a negligible role. If the gas mixture contains components with a high affinity for hydrogen, as in the case of oxygen, a shift in the range of the measured hydrogen pressures towards higher values is expected, due to the recombination processes (e.g. H2O formation reaction) at the Pd surface. Nevertheless, the working principle of the device is still valid as we found the shift to be reproducible in a single layer detector [10]. Moreover the use of an appropriate cap layer would improve the selectivity to hydrogen of the device [18].
4.
Conclusions
In conclusion, we have reported the proof of concept of an optical hydrogen sensor characterized by a one-to-one
12577
relationship between the H2 partial pressure and the optical response in a single gradient thin film sample. Both in transmission and reflection we are able to read what fraction of the wedge has undergone the metal-insulator transition, which is a direct measure of the applied hydrogen pressure. The high contrast in reflectance between the metal and metal-hydride states allows to use the sensor for visual inspection purposes or in a multifiber configuration. The thickness dependence of the hydrogen equilibrium pressure in constrained Mg thin films is exploited to make a simple hydrogen barometer which is small and cost effective.
Acknowledgments This work is supported by the Technologiestichting STW, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) through the Sustainable Hydrogen Programme of Advanced Chemical Technologies for Sustainability (ACTS).
references
[1] Boon-Brett L, Bousek J, Black G, Moretto P, Castello P, Hubert H, et al. Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications. Int J Hydrogen Energy 2010;35:373e84. [2] Boon-Brett L, Bousek J, Moretto P. Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: part II e selected sensor test results. Int J Hydrogen Energy 2010;34:562e71. [3] Boon-Brett L, Bousek J, Salyk O, Harskamp F, Aldea L, Tinaut F. Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: part I e testing facility and methodologies. Int J Hydrogen Energy 2008;33:7648e57. [4] Butler MA. Optical fiber hydrogen sensor. Appl Phys Lett 1984;45:1007e9. [5] Sutapun T, Tabib-Azar M, Kazemi A. Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing. Sens Actuators B 1999;60:27e34. [6] Tabib-Azar M, Sutapun B, Petrick R, Kazemi A. Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions. Sens Actuators B 1999;56:158e63. [7] Be´venot X, Trouillet A, Veillas C, Gagnaire H, Cle´ment M. Surface plasmon resonance hydrogen sensor using optical fibre. Meas Sci Technol 2002;13:118e24. [8] Huiberts JN, Griessen R, Rector JH, Wijngaarden RJ, Dekker JP, de Groot DG, et al. Yttrium and lanthanum hydride films with switchable optical properties. Nature 1996;380:231e4. [9] Borsa DM, Baldi A, Pasturel M, Schreuders H, Dam B, Griessen R, et al. MgeTi thin films for smart solar collectors. Appl Phys Lett 2006;88:241910. [10] Slaman M, Dam B, Schreuders H, Griessen R. Fiber optic hydrogen detectors containing Mg-based metal hydrides. Sens Actuators B 2007;123:538e45. [11] Baldi A, Gonzalez-Silveira M, Palmisano V, Dam B, Griessen R. Destabilization of the Mg-H system through elastic constraints. Phys Rev Lett 2009;102:226102. [12] Theiss W. SCOUT thin film analysis software handbook, hard and software, www.mtheiss.com; 2000. Aachen. [13] Gremaud R, Broedersz C, Borsa DM, Borgschulte A, Mauron P, Schreuders H, et al. Hydrogenography: an optical
12578
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
combinatorial method to find new light-weight hydrogenstorage materials. Adv Mater 2007;19:2813e7. [14] Griessen R, Riester T. Topics in applied physics: heat of formation models, in hydrogen in intermetallic compounds I, vol. 63. Berlin, Heidelberg: Springer Verlag; 1988. [15] James SW, Tatam RP. Optical fibre long period grating sensors: characteristics and application. Meas Sci Technol 2003;14:49e61.
[16] Baldi A, Palmisano V, Gonzalez-Silveira M, Pivak Y, Slaman M, Schreuders H, et al. Quasi-free Mg-H thin films. Appl Phys Lett 2009;95:071903. [17] Dam B, Schreuders H, Slaman M, Pasturel M. WO2007126313, NL1031708, EP2010960, AU2007244044 (P6007119NL); 2006. [18] Slaman M, Dam B, Schreuders H, Griessen R. Optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst. Int J Hydrogen Energy 2008;33:1084e9.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Technical Communication
An optical hydrogen sensor based on a Pd-capped Mg thin film wedge V. Palmisano a,*, M. Filippi a,b, A. Baldi a, M. Slaman b,a, H. Schreuders a, B. Dam a a
Department of Chemical Engineering, Technical University Delft, Julianalaan 136, 2628 BL Delft, The Netherlands Department of Physics and Astronomy, Condensed Matter Physics, Vrije Universiteit, De Boelelaan 1081, 1081 HV Amsterdam, The Netherlands b
article info
abstract
Article history:
We report the proof of concept of a thin film device with a one-to-one relationship between
Received 22 June 2010
the H2 partial pressure and the lateral progression of a visible optical change along a thin
Received in revised form
film multilayer 70 mm long. The device basically consists of a sensing Mg layer with
1 September 2010
a thickness gradient. It exploits the thickness dependence of the hydrogenation thermo-
Accepted 1 September 2010
dynamics of Pd-capped Mg thin films. The optical change of the Mg layer during the
Available online 22 September 2010
metalehydride transition can be detected both in reflection and in transmission. This optical sensor allows a continuous measurement of hydrogen partial pressure in the range
Keywords: Optical hydrogen sensor
between 200 and 4000 Pa. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Hydrogen safety Thin film Switchable mirror
1.
Introduction
The development of high-performance hydrogen sensors is crucial for the successful implementation of hydrogen as a sustainable energy carrier. Hydrogen is characterized by a wide flammability range (4e75 vol%) and a relatively low ignition energy (only 0.017 mJ at room temperature), while its presence is undetectable by human senses. Conventional sensors [1e3], i.e. catalytic, electrochemical and semiconducting metal-oxide sensors use electrical leads which may induce sparks and provide a cause for ignition in hazardous atmospheres. Optical sensors based on fiber optics allow working in an explosive environment thanks to the possibility of separating the sensing point from the electrical readout. Various strategies have been proposed
which basically infer the hydrogen concentration from a change in the optical response, e.g. the interference pattern [4], the frequency of the optical signal [5], or in the intensity of the optical signal [6,7]. Following the discovery that due to the absorption of hydrogen some metals experience a metaleinsulator transition characterized by a large change in optical reflectance or transmittance [8,9], our group in Amsterdam developed a fiber optic hydrogen detector based on a sensing layer made of Mg0.70Ti0.30 [10]. This detector satisfies the requirements of having a high contrast, small dimensions, low cost and being safe to operate. However, it only indicates whether the hydrogen pressure is above or below a certain threshold level, which is given by the equilibrium pressure of the Mg0.7Ti0.3 detection layer.
* Corresponding author. Tel.: þ31 15278 2676; fax: þ31 15278 7421. E-mail address: [email protected] (V. Palmisano). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.09.001
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
12575
Fig. 1 e (a) Sample design: a wedge of Mg with varying thickness along the x direction is grown on a glass substrate 70 3 5 3 1 mm3. The Mg wedge is sandwiched between a Ti-adhesion layer 3 nm thick and a Pd-cap layer 40 nm thick. (b) Simulation generated through the optical simulation tool SCOUT of the optical transmittance of the sample before (circles) and after (squares) the hydrogenation of the Mg layer at various thicknesses: lines are guide to the eyes.
A quantitative detector can be designed exploiting the recent discovery of the fact that the thermodynamics of Mg thin films (<40 nm) is affected by the Pd-cap layer [11]. This layer is usually applied to prevent the oxidation of the Mg layer and to promote the hydrogen absorption. Due to the elastic coupling between the Pd and Mg layer the hydrogenation pressure p is extremely sensitive to the Mg thickness d, showing an exponential dependence given by the formula: p ¼ p0 exp
1 : a,d þ b
(1)
The parameters a and b contain the elastic properties (Young’s modulus and Poisson’s ratio) of the cap layer and the clamping
layer respectively, while p0 represents the equilibrium pressure of the hydrogenation reaction of Mg in the non-capped situation: Mg þ H2 0 MgH2 [11]. Based on this effect we report in this paper on a thin film optical device, which is able to measure the hydrogen concentration in Ar atmosphere within a hydrogen partial pressure range varying from 200 Pa to 4000 Pa (we recall here that 4000 Pa represents the lower flammability level of hydrogen in air). The sensor is made of a wedge of Pd-capped Mg, where the Mg thickness varies from 5 to 32 nm along a 70 mm glass substrate (Fig. 1(a)). Because the equilibrium pressure of magnesium hydride depends on the layer thickness, the length over which the sample becomes transparent after exposure to hydrogen is a measure for the hydrogen pressure. In reflectance this effect can be observed through the substrate as a progressive lateral change in reflectance from a shiny metallic to a dark, light absorbing appearance.
2.
Fig. 2 e PTIs measured at 333 K corresponding to different Mg thicknesses: increasing Mg thickness results in an increase of the optical contrast and a decrease of the plateau pressure. We do not report PTIs corresponding to thicknesses lower than 9.5 nm for which the experimental pressure ramp is not sufficient to complete the metalhydride transition.
Materials and methods
The sample is deposited in a UHV chamber (background pressure is 5 107 Pa and sputter pressure 0.3 Pa of argon) by RF/ DC magnetron sputtering of Ti (99.99%), Mg (99.95%), and Pd (99.98%) respectively. First, we deposit a 3 nm Ti-adhesion layer on a rotating glass substrate. The Ti thickness was minimized in order to maximize the change in reflection upon loading of the Mg layer. To obtain the desired Mg wedge geometry we stop the substrate rotation and tilt the Mg source away from its optimal direction, i.e. the direction pointing to the center of the substrate. Finally, we deposit a uniform Pdcap layer resulting in the sample geometry shown in Fig. 1(a): substrate (1 mm) þ Ti (3 nm) þ Mg (5e32 nm) þ Pd (40 nm). The expected optical response of such a sensor is calculated using the optical simulation software tool SCOUT [12]. Fig. 1(b) shows the simulation of the optical transmittance, averaged over the visible spectrum (1.1 < hn < 3.3 eV), of the layer stack as
12576
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
Fig. 3 e Sample snapshots taken at different times while increasing the H2 partial pressure from 12 to 4000 Pa. The Mg thickness along the sample and the corresponding loading pressure are reported on top. The inset shows the plateau pressure behavior as a function of the Mg effective thickness: the fit with Eq. (1) is also reported.
a function of the Mg thickness in the metallic and the hydrogenated state. We find a high optical contrast, i.e. difference in the logarithm of transmittance before and after the hydrogenation of Mg, for Mg thickness down to 10 nm.
Fig. 4 e Color map of the optical reflectivity as a function of pressure and time measured along the wedge-shaped sample at room temperature. The transparency front progresses laterally (the x axis) with pressure; the H2 pressure is increased stepwise from 100 to 2000 Pa (as shown in the vertical axis). The scale of intensities goes from orange (high reflectance) to a scale of gray (with minimum reflectance in black). Black closed circles indicate the time when the H2 pressure is suddenly increased and the corresponding position of the sample where Mg undergoes the metaleinsulator transition.
3.
Results and discussion
To explore the behavior of the stack as a sensing device, we use hydrogenography, an optical technique which allows the measurement of pressure-optical transmission isotherms (PTIs) [13]. A 150 W diffuse white light source illuminates the sample through the substrate while a 3-channel (RGB) CCD camera monitors the transmitted light. In order to distinguish various pressure plateaus we increase the hydrogen partial pressure near the sample from 10 to 4000 Pa in 20 h with a logarithmic function of time. The measured PTIs along the sample at various nominal Mg thicknesses are illustrated in Fig. 2. The optical response here is given by the logarithm of the optical transmission normalized to the optical transmission in the initial metallic state. We observe that the plateau pressure increases when the Mg gets thinner while the optical contrast given by the plateau width decreases, which is in agreement with the optical simulations shown in Fig. 1(b). Snapshots of the sample are regularly taken while measuring the PTIs. Fig. 3 shows some of these snapshots taken at different H2 pressures. The loading results in a progressive brightening of the device along the x direction (the reference system is shown in Fig. 1(a)); the position on the sample of the bright front gives the readout of the pressure. The bright spots in the picture are due to pinholes which serve as additional markers of the position on the wedge. The H2 partial pressure can hence be scaled with the length of the more transparent region, as shown by the upper abscissa in Fig. 3. Thus, the thin film strip represents a sort of macroscopic barometer for H2 partial pressure. At present the scale is non-linear with the H2 partial pressure, but it is possible to make the Mg wedge with a geometry such that the equilibrium pressure can be varied linearly with the distance. As shown in the inset of Fig. 3 the plateau pressure depends exponentially on the Mg thickness as predicted by the elastic clamping model [11].
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
We studied the behavior of the optical reflectivity of our sample observing the film from the substrate side. This configuration, used in the fiber optics hydrogen detector [10], could be in principle extended to a multifiber hydrogen sensor where on each fiber a sensing layer is grown with a different thickness and therefore a different plateau pressure. The optical reflection of the sample is measured through a 3-channel (RGB) CCD camera in the presence of a homogeneous white light source. Fig. 4 shows the change in the optical reflection measured through the substrate as a function of time at room temperature. While increasing stepwise the hydrogen pressure from 100 to 2000 Pa in 7 h, the decrease of the reflectivity as a function of the hydrogen pressure shows again a thickness dependent behavior. The readout of the pressure is given by the position of the front demarcating the highly reflective state from the dark, light absorbing state. From the picture we also observe that 80% of the response is realized within 500 s, giving an indication of the time response of this sensor. The measured H2 partial pressure in this configuration is different from the presented transmittance measurements due to the increase the plateau pressure with the temperature [14]. This implies that, in a realistic device, the sample temperature should be kept constant or measured separately in order to infer the correct hydrogen pressure. In the latter case the scale of the measured pressure can be converted by software and the optical response should be read by electronics: the multifiber configuration has the advantage of allowing a simultaneous measurement of the temperature through interferometric or fiber Bragg grating methods [15]. The physical background of this device [11] allows for an extension of the operating range, by choosing e.g. clamping materials with a higher Young’s modulus. In addition, the size of the device can be adjusted by changing the steepness of the wedge in Mg thicknesses. We should also mention that cycling results in a continuous decrease of the optical contrast as hydrogenation promotes the alloying between Mg and Pd [16], resulting in a lack of stability and a very slow desorption kinetics. Thus, the device considered here can only be exploited as a single-use sensor. In our current research we investigate other clamping materials and new layer geometries to improve the reproducibility of our device on cycling. The use of a protective coating, for example, prevents the degradation of the catalytic properties of Pd due to moisture and pollutants, allowing an increase of the lifetime of the device [17]. The device proves to work in inert atmospheres, where the interaction between the balance gas and the cap layer plays a negligible role. If the gas mixture contains components with a high affinity for hydrogen, as in the case of oxygen, a shift in the range of the measured hydrogen pressures towards higher values is expected, due to the recombination processes (e.g. H2O formation reaction) at the Pd surface. Nevertheless, the working principle of the device is still valid as we found the shift to be reproducible in a single layer detector [10]. Moreover the use of an appropriate cap layer would improve the selectivity to hydrogen of the device [18].
4.
Conclusions
In conclusion, we have reported the proof of concept of an optical hydrogen sensor characterized by a one-to-one
12577
relationship between the H2 partial pressure and the optical response in a single gradient thin film sample. Both in transmission and reflection we are able to read what fraction of the wedge has undergone the metal-insulator transition, which is a direct measure of the applied hydrogen pressure. The high contrast in reflectance between the metal and metal-hydride states allows to use the sensor for visual inspection purposes or in a multifiber configuration. The thickness dependence of the hydrogen equilibrium pressure in constrained Mg thin films is exploited to make a simple hydrogen barometer which is small and cost effective.
Acknowledgments This work is supported by the Technologiestichting STW, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) through the Sustainable Hydrogen Programme of Advanced Chemical Technologies for Sustainability (ACTS).
references
[1] Boon-Brett L, Bousek J, Black G, Moretto P, Castello P, Hubert H, et al. Identifying performance gaps in hydrogen safety sensor technology for automotive and stationary applications. Int J Hydrogen Energy 2010;35:373e84. [2] Boon-Brett L, Bousek J, Moretto P. Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: part II e selected sensor test results. Int J Hydrogen Energy 2010;34:562e71. [3] Boon-Brett L, Bousek J, Salyk O, Harskamp F, Aldea L, Tinaut F. Reliability of commercially available hydrogen sensors for detection of hydrogen at critical concentrations: part I e testing facility and methodologies. Int J Hydrogen Energy 2008;33:7648e57. [4] Butler MA. Optical fiber hydrogen sensor. Appl Phys Lett 1984;45:1007e9. [5] Sutapun T, Tabib-Azar M, Kazemi A. Pd-coated elastooptic fiber optic Bragg grating sensors for multiplexed hydrogen sensing. Sens Actuators B 1999;60:27e34. [6] Tabib-Azar M, Sutapun B, Petrick R, Kazemi A. Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions. Sens Actuators B 1999;56:158e63. [7] Be´venot X, Trouillet A, Veillas C, Gagnaire H, Cle´ment M. Surface plasmon resonance hydrogen sensor using optical fibre. Meas Sci Technol 2002;13:118e24. [8] Huiberts JN, Griessen R, Rector JH, Wijngaarden RJ, Dekker JP, de Groot DG, et al. Yttrium and lanthanum hydride films with switchable optical properties. Nature 1996;380:231e4. [9] Borsa DM, Baldi A, Pasturel M, Schreuders H, Dam B, Griessen R, et al. MgeTi thin films for smart solar collectors. Appl Phys Lett 2006;88:241910. [10] Slaman M, Dam B, Schreuders H, Griessen R. Fiber optic hydrogen detectors containing Mg-based metal hydrides. Sens Actuators B 2007;123:538e45. [11] Baldi A, Gonzalez-Silveira M, Palmisano V, Dam B, Griessen R. Destabilization of the Mg-H system through elastic constraints. Phys Rev Lett 2009;102:226102. [12] Theiss W. SCOUT thin film analysis software handbook, hard and software, www.mtheiss.com; 2000. Aachen. [13] Gremaud R, Broedersz C, Borsa DM, Borgschulte A, Mauron P, Schreuders H, et al. Hydrogenography: an optical
12578
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 2 5 7 4 e1 2 5 7 8
combinatorial method to find new light-weight hydrogenstorage materials. Adv Mater 2007;19:2813e7. [14] Griessen R, Riester T. Topics in applied physics: heat of formation models, in hydrogen in intermetallic compounds I, vol. 63. Berlin, Heidelberg: Springer Verlag; 1988. [15] James SW, Tatam RP. Optical fibre long period grating sensors: characteristics and application. Meas Sci Technol 2003;14:49e61.
[16] Baldi A, Palmisano V, Gonzalez-Silveira M, Pivak Y, Slaman M, Schreuders H, et al. Quasi-free Mg-H thin films. Appl Phys Lett 2009;95:071903. [17] Dam B, Schreuders H, Slaman M, Pasturel M. WO2007126313, NL1031708, EP2010960, AU2007244044 (P6007119NL); 2006. [18] Slaman M, Dam B, Schreuders H, Griessen R. Optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst. Int J Hydrogen Energy 2008;33:1084e9.