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Hydrogen storage in the bubbles formed by high-flux ion implantation in thin Al films
Journal of Alloys and Compounds 398 (2005) 203–207
Hydrogen storage in the bubbles formed by high-flux ion implantation in thin Al films D. Milcius a, ∗ , L.L. Pranevicius a, b, ∗ , C. Templier c a
Lithuanian Energy Institute, Laboratory of Materials Research, 3 Breslaujos St., 44403 Kaunas, Lithuania b Vytautas Magnus University, Physics Department, 8 Vileikos St., 44404 Kaunas, Lithuania c Laboratoire de Metallurgie Physique, Universite de Poitiers, 86960 Futuroscope, France Received 23 December 2004; received in revised form 31 January 2005; accepted 1 February 2005 Available online 23 March 2005
Abstract The storage of implanted hydrogen in 2–5 m thick Al films on stainless steel substrates was investigated in this work. Plasma immersion 1 keV H2 + ion implantation was used to load hydrogen into the Al film. The correlation between the effusion of the implanted hydrogen and the evolution of surface morphology was studied by thermal desorption spectroscopy and scanning electron microscopy. It has been found that as-implanted hydrogen at temperatures below 320 K is associated with defects and is chemically bonded at the grain boundaries of nanocrystallites. At higher temperatures, the released hydrogen is accommodated in bubbles. The major part of hydrogen effuses at ∼630 K and the effusion process is controlled by the migration of hydrogen through the surface oxide layer. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Aluminum; Implantation; Effusion
1. Introduction Due to its low density, one of the main obstacles to the widespread use of hydrogen in the energy sector is an efficient storage technology. Hydrogen densification is required for many applications. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are considered as possible hydrogen-storage methods [1]. Recently, progress has been made in efforts to advance the room temperature hydrogen gas storage capacity of traditional hydrides, where high gravimetric hydrogen-storage density is required and where hydrogen must be liberated at temperatures compatible with the waste heat of the fuel-cell (<100 ◦ C) [2,3].
∗
Corresponding authors. Tel.: +370 37 401904; fax: +370 37 351271. E-mail address: [email protected] (D. Milcius).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.003
It becomes clear that the development of new materials for hydrogen storage requires overcoming many thermodynamic and kinetic limitations. Metastable materials are of great interest. A proper engineering of the alloy composition, surface properties, microstructure, grain size, etc., and control of thermodynamic functions that determine the equilibrium state of metal–hydrogen systems are required. Thin film hydride storage is an emerging area of research [4,5]. Physical vapor deposition technologies activated with plasma/ion beam irradiation allow the formation of thermodynamically non-stable materials, such as amorphous alloys and nanocrystalline metastable phases, although very often exhibiting stable and reproducing behavior in practical applications. These materials contain many grain boundaries, defects, impurities, disorder and strain and their hydrogen adsorption/desorption properties cannot be fully described by the basic thermodynamic function. It is known [6] that the number of available sites for hydrogen in nanocrystalline materials is higher than in crystalline, and the unrelaxed material is much easier to hydrogenate than the relaxed one.
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D. Milcius et al. / Journal of Alloys and Compounds 398 (2005) 203–207
Although there are plenty of investigations on hydrogen absorption in thin metal films under equilibrium conditions to be found in the literature, few publications are concerned with the influence of the very surface on the hydriding behavior under non-equilibrium conditions using external irradiation for the modification of properties and structure of surface and near-surface layers. The concentrated beams of photons, electrons, protons and ions are used to modify surface properties during adsorption (in situ). In this paper, an attempt is made to store hydrogen in the bubbles which, as is well known [7,8], are formed in metals under high-fluence low-energy hydrogen ion irradiation. The aluminum has been selected as possible candidate. Aluminum dissolves hydrogen in only very small quantities, approximately 1 ppm near the melting point at atmospheric pressure [9]. When the hydrogen content in the aluminum exceeds the limit of its solubility, hydrogen atoms are rejected from the Al lattice and agglomerate to form bubbles. Hydrogen atoms trapped on the vacancy type defects are released at temperatures higher than 340 K [10]. In the present work, hydrogen plasma ion implantation is used. As the consequence of high-flux ion implantation process, the near-surface layer of aluminum becomes highly activated and thermodynamically unstable. The physics of H atoms becomes strongly related to the nature of defects. The idea of the research is to fill available sites of activated aluminum film with hydrogen under high-flux ion irradiation, to thermally release hydrogen from the trap sites and to accommodate it in the bubbles containing molecular hydrogen and finally to effuse hydrogen from the bubbles into vacuum.
was performed keeping the hydrogen working gas pressure equal to 10 Pa. The ultimate vacuum was 10−4 Pa and the partial pressure of oxygen was about 10−3 Pa. In plasma immersion ion implantation, the sample is immersed in highdensity plasma generated independently and biased with a series of 1 kV negative voltage pulses. The pulse repetition rate was kept fixed at 20 kHz. Monte Carlo simulations using the TRIM computer code [11] yielded for 1 keV H+ in Al an average projected range close to 20 nm and a straggling of 12 nm. The incident energetic ions passed through the barrier layer of natural oxide and stopped in the Al film. The dose of implanted hydrogen was about 1018 H2 + cm−2 . The sputtering yield of Al2 O3 by incident 1 keV H2 + is equal to about 10−3 . The hydrogen effusion kinetics was studied by plasma desorption spectroscopy and the evolution of the sample morphology during thermal treatment by scanning electron microscopy (SEM). Thermal desorption spectroscopy (TDS) was performed in a vacuum stainless steel cell. The Ar gas carrier moved the released hydrogen from the dehydrogenation cell into the plasma chamber which was equipped with high resolution spectroscopy Spectrumat1000 and pumped by turbomolecular pump. The surface morphology of the deposited, hydrogenated and thermally treated samples was studied by scanning electron microscopy (JEOL JSM-6300 Scanning Microscope). The structure of samples was determined using room-temperature X-ray diffraction (XRD) measurements with the 2θ angle in the range 20–90◦ with Cu K␣ radiation. The identification of phases has been performed using Crystallographic Search-Match Program based on Powder Diffraction Data. The grain sizes were estimated by Debye–Scherrer analysis of the XRD spectra [12].
2. Experimental 3. Results Two-micrometer thick Al thin films were deposited at room temperature on 2 mm thick stainless steel Alloy 600 substrates by dc-magnetron sputtering (nominal target purity better than 99.99 at.%) in a vacuum chamber with an argon gas pressure of 0.2 Pa. After deposition the thin Al films were hydrogenated in the same vacuum chamber without exposure to air. The plasma immersion hydrogen ion implantation
SEM micrographs of H2 ion implanted Al thin films are presented in Fig. 1a and b. Fig. 1a is typical of an as-hydrogenated sample with a temperature at the end of hydrogenation of about 320 K. Fig. 1b is typical of the hydrogenated sample, following thermal treatment at 450 K for 15 min in air. In the as-hydrogenated sample at 370 K
Fig. 1. The SEM surface views of as-hydrogenated Al film with 320 K at the end of the procedure (a), and after subsequent thermal annealing at 450 K for 15 min in air (b).
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Fig. 3. The cross-sectional SEM view of plasma hydrogenated Al film on stainless steel substrate with the maximum temperature 550 K at the end of procedure.
Fig. 2. The diffractograms of Al films on stainless steel substrate after hydrogenation with the temperature equal to 320 K (curve 1), 450 K (curve 2) and 550 K (curve 3) at the end of procedure.
we see the smooth surface topography with small randomly distributed bubbles with diameter 10–20 m. After the annealing at ∼450 K there is an increase in size and density of bubbles. The size of bubbles reaches 150–200 m, see Fig. 1b. Fig. 2 includes XRD diffractograms of the hydrogenated Al films with different temperatures at the end of hydrogenation. Curve 1 corresponds to as-hydrogenated Al film with a temperature equal to 320 K at the end of the procedure, curve 2 – 450 K, and curve 3 – 550 K. It is seen that the ashydrogenated Al film at around 320 K consists of two characteristic peaks of the substrate at 43.7 and 50.9◦ and three peaks indicating the presence of hydrogen bonded with Al atoms as AlH at 42.2◦ , AlH3 at 42.7 and 44.4◦ . After hydrogenation at 450 K (curve 2, Fig. 2), the AlH peaks at 42.2◦ and AlH3 at 42.7◦ disappear and the AlH3 peak at 44.4◦ decreases. This indicates that the concentration of Al–H bonds decreases in the film. At the same time, the characteristic c-Al(1 1 1) peak at 38.6◦ appears. This indicates that as-implanted hydrogen at temperatures below 320 K, when the concentration significantly exceeds the limit of hydrogen solubility in Al, is associated with defects and is located at grain boundaries forming Al–H bonds. At temperatures above 340 K hydrogen becomes mobile [10]. The concentration of Al–H bonds decreases and restructuring of the Al film takes place. After hydrogenation at 550 K, the sample diffractogram (curve 3, Fig. 2) shows only Al peaks. The surface oxide barrier layer inhibits the effusion of the released hydrogen which nucleates on defects forming bubbles. In Fig. 3, the SEM cross-sectional view of a 2 m thick as-deposited Al film on the stainless steel substrate reveals the distribution and size of bubbles across the hydrogenated Al film thickness. The maximum treatment temperature was 550 K. It is seen that hydrogen released from defect trap sites and boundaries of nanocrystallites is accommodated in bubbles randomly distributed in the bulk.
Fig. 4. The hydrogen effusion intensity as a function of the dehydrogenation time/temperature: curve 1, the effusion intensity vs. time; and curve 2, the sample temperature vs. time.
The time dependences of the hydrogen effusion intensity and sample temperature during thermal desorption of the hydrogenated Al thin film are shown in Fig. 4 (curves 1 and 2, respectively). During desorption experiments, the heating of sample was carried out with a nearly linear temperature ramp of 0.3 K s−1 up to 620 K. The heating procedure ended at 710 K. The desorption of accumulated hydrogen presents single broad effusion peak with a sharp initial release of hydrogen beginning at the temperature 625 K with a maximum at 640 K and decreasing hydrogen effusion in the range of temperatures 640–710 K. Fig. 5 illustrates the typical SEM surface view of single bubble after the effusion procedure.
Fig. 5. The SEM surface view of a single bubble after the hydrogen effusion procedure.
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4. Discussion The experimental results direct us to the following conclusions: (i) the plasma immersion hydrogen ion implantation technique is an efficient method for hydrogenation of Al films, (ii) the as-implanted hydrogen at temperatures around 320 K is associated with different trap sites of the highly disordered film structure and accommodated at the grain boundaries with the formation of Al–H bonds, and (iii) the efficient hydrogen effusion starts at around 630 K. The high efficiency of the plasma immersion hydrogen ion implantation technique follows from the consideration of classical theory of the efficiency of retention of gases in solids [13]. For the maximum efficiency of retention, the following conditions have to be fulfilled. The first condition stems from the necessity that incident hydrogen atoms have to pass the surface barrier, and the second condition is related to the preservation of the surface layer for the retention of the implanted hydrogen. Monatomic non-metallic elements (O, C, N, etc.) are responsible for the surface potential barrier. The hydrogenation process was performed in the H2 working gas which included some oxygen as residual gas. The partial pressure of oxygen was about 10−3 Pa. At this pressure, a monolayer of oxygen on the Al film is formed in 10−1 –10−2 s. Under these conditions the Al film is covered by 2–4 nm thick of dense natural Al2 O3 oxide layer. This layer is transparent for incident energetic hydrogen ions extracted from the plasma, yet hinders the effusion of implanted hydrogen. We thus suggest the following model for the plasma hydrogenation of Al films. In the plasma immersion ion implantation technique, the Al film is immersed in the plasma and biased with the negative 1 kV voltage. The hydrogen ions are thus accelerated from the plasma towards the Al film where they are implanted through the surface barrier. The surface barrier during implantation is not destroyed because the sputtering yield is low (∼10−3 ) for 1 keV H+ . Additionally, the partial pressure of residual oxygen in the working gas is about 10−3 Pa, sufficient for the compensation of losses of oxygen. This oxide layer which exists during implantation process inhibits release of the implanted hydrogen. In the asimplanted Al film, hydrogen is contained atomically in interstitial sites, at the grain boundaries of the nanocrystalline Al film and as H2 molecules in bubbles. This is because the concentration of implanted hydrogen significantly exceeds the solubility limit of hydrogen atoms in Al. At temperatures higher than 320 K, hydrogen atoms diffuse and precipitate on nucleation sites to form bubbles. The formation of new bubbles stops when the concentration of hydrogen in the aluminum lattice reaches its solubility limit. This mechanism explains the XRD and SEM observations. We estimated that if all implanted hydrogen is stored in the bubbles of a 2 m thick Al film for an implantation fluence equal to 1018 cm−2 , this corresponds to about 10 at.% H in the Al film.
At temperatures higher than 600 K the surface oxide layer becomes permeable [7] to the hydrogen accommodated in bubbles and it effuses, thus producing the strong and repeatable effusion peak at ∼630 K temperature (Fig. 4). Analysis of the effusion peak at ∼640 K through the reaction rate theory indicates effusion activation energy of 1.85 eV, which is in very good agreement with the value 1.9 eV of the activation energy for hydrogen diffusion measured in ␥-Al2 O3 [14]. The effusion temperature 630 K is too high for many practical applications. Other methods of the release of hydrogen contained in bubbles are needed. Preliminary results show that the covers of bubbles are fragile and can be broken by surface mechanical waves [15,16].
5. Conclusions Two- to five-micrometer thick Al films were deposited by dc magnetron sputter deposition and hydrogenated without exposure to air employing plasma immersion ion implantation. H fluences up to 1018 cm−2 were achieved in an H2 working gas with the partial pressure of oxygen equal to about 10−3 Pa. The implanted hydrogen was accommodated in bubbles with diameters up 200 m at temperatures above 320 K. At lower temperatures, hydrogen was mainly associated to defects, distributed at the grain boundaries and contained in bubbles with diameter 10–20 m. The large part of the accommodated hydrogen effuses starting at 630 K. The dominant role in the mechanism of retention and release of hydrogen is attributed to the surface barrier Al2 O3 layer.
Acknowledgements The work was performed under support from the Hydrogen Program of the US Department of Energy and the Sandia National Laboratories. Authors express special gratitude to Mr. A. Vasys (USA) for his encouragement and help in the administration of the research. We are also grateful to Drs. George Thomas and Gary Sandrock, consultants to Sandia, for their technical advice and encouragement.
References [1] http://hydpark.ca.sandia.gov. [2] G. Sandrock, K. Gross, G. Thomas, J. Alloys Comp. 339 (2002) 299. [3] K.J. Gross, E.H. Majzoub, S.W. Spangler, J. Alloys Comp. 356–357 (2003) 423. [4] K. Higuchi, H. Kajioka, K. Toiyama, H. Fujii, Y. Kikuchi, J. Alloys Comp. 293 (1999) 484. [5] K. Higuchi, K. Yamamoto, H. Kajioka, K. Toiyama, M. Honda, S. Orimo, H. Fuji, J. Alloys Comp. 330–332 (2002) 526. [6] N. Hoon Goo, K. Sub Lee, Int. J. Hydrogen Energy 27 (2002) 433. [7] R. Checchetto, L.M. Graton, A. Miotello, Surf. Coat. Technol. 158–159 (2002) 356.
D. Milcius et al. / Journal of Alloys and Compounds 398 (2005) 203–207 [8] J.B. Condon, T. Schober, J. Nucl. Mater. 207 (1993) 1. [9] M. Ichimura, Y. Sasajima, M. Imabayashi, H. Katsuta, J. Phys. Chem. Sol. 49 (1988) 1259. [10] S. Linderoth, Phil. Mag. Lett. 57 (1988) 229. [11] J.F. Ziegler, J.P. Biersack, U. Littmark, SRIM/TRIM (http://www. srim.org). [12] R. Checchetto, C. Tosello, A. Miotello, G. Principi, J. Phys. Condens. Matt. 13 (2001) 811.
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[13] A.I. Livshits, F. Sube, M. Solovyev, M. Notkin, M. Bacal, J. Appl. Phys. 84 (1998) 2558. [14] P. Jung, in: R.P. Agarwala (Ed.), Diffusion Processes in Nuclear Materials, Amsterdam, North Holland, 1991, p. 251. [15] S. Kelling, N. Saito, Y. Inoue, D.A. King, Appl. Surf. Sci. 150 (1999) 47. [16] S. Joneliunas, L. Pranevicius, R. Valatka, Nucl. Instrum. Methods 182–183 (1981) 761.
Hydrogen storage in the bubbles formed by high-flux ion implantation in thin Al films D. Milcius a, ∗ , L.L. Pranevicius a, b, ∗ , C. Templier c a
Lithuanian Energy Institute, Laboratory of Materials Research, 3 Breslaujos St., 44403 Kaunas, Lithuania b Vytautas Magnus University, Physics Department, 8 Vileikos St., 44404 Kaunas, Lithuania c Laboratoire de Metallurgie Physique, Universite de Poitiers, 86960 Futuroscope, France Received 23 December 2004; received in revised form 31 January 2005; accepted 1 February 2005 Available online 23 March 2005
Abstract The storage of implanted hydrogen in 2–5 m thick Al films on stainless steel substrates was investigated in this work. Plasma immersion 1 keV H2 + ion implantation was used to load hydrogen into the Al film. The correlation between the effusion of the implanted hydrogen and the evolution of surface morphology was studied by thermal desorption spectroscopy and scanning electron microscopy. It has been found that as-implanted hydrogen at temperatures below 320 K is associated with defects and is chemically bonded at the grain boundaries of nanocrystallites. At higher temperatures, the released hydrogen is accommodated in bubbles. The major part of hydrogen effuses at ∼630 K and the effusion process is controlled by the migration of hydrogen through the surface oxide layer. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen; Aluminum; Implantation; Effusion
1. Introduction Due to its low density, one of the main obstacles to the widespread use of hydrogen in the energy sector is an efficient storage technology. Hydrogen densification is required for many applications. Besides conventional storage methods, i.e. high pressure gas cylinders and liquid hydrogen, the physisorption of hydrogen on materials with a high specific surface area, hydrogen intercalation in metals and complex hydrides, and storage of hydrogen based on metals and water are considered as possible hydrogen-storage methods [1]. Recently, progress has been made in efforts to advance the room temperature hydrogen gas storage capacity of traditional hydrides, where high gravimetric hydrogen-storage density is required and where hydrogen must be liberated at temperatures compatible with the waste heat of the fuel-cell (<100 ◦ C) [2,3].
∗
Corresponding authors. Tel.: +370 37 401904; fax: +370 37 351271. E-mail address: [email protected] (D. Milcius).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.02.003
It becomes clear that the development of new materials for hydrogen storage requires overcoming many thermodynamic and kinetic limitations. Metastable materials are of great interest. A proper engineering of the alloy composition, surface properties, microstructure, grain size, etc., and control of thermodynamic functions that determine the equilibrium state of metal–hydrogen systems are required. Thin film hydride storage is an emerging area of research [4,5]. Physical vapor deposition technologies activated with plasma/ion beam irradiation allow the formation of thermodynamically non-stable materials, such as amorphous alloys and nanocrystalline metastable phases, although very often exhibiting stable and reproducing behavior in practical applications. These materials contain many grain boundaries, defects, impurities, disorder and strain and their hydrogen adsorption/desorption properties cannot be fully described by the basic thermodynamic function. It is known [6] that the number of available sites for hydrogen in nanocrystalline materials is higher than in crystalline, and the unrelaxed material is much easier to hydrogenate than the relaxed one.
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D. Milcius et al. / Journal of Alloys and Compounds 398 (2005) 203–207
Although there are plenty of investigations on hydrogen absorption in thin metal films under equilibrium conditions to be found in the literature, few publications are concerned with the influence of the very surface on the hydriding behavior under non-equilibrium conditions using external irradiation for the modification of properties and structure of surface and near-surface layers. The concentrated beams of photons, electrons, protons and ions are used to modify surface properties during adsorption (in situ). In this paper, an attempt is made to store hydrogen in the bubbles which, as is well known [7,8], are formed in metals under high-fluence low-energy hydrogen ion irradiation. The aluminum has been selected as possible candidate. Aluminum dissolves hydrogen in only very small quantities, approximately 1 ppm near the melting point at atmospheric pressure [9]. When the hydrogen content in the aluminum exceeds the limit of its solubility, hydrogen atoms are rejected from the Al lattice and agglomerate to form bubbles. Hydrogen atoms trapped on the vacancy type defects are released at temperatures higher than 340 K [10]. In the present work, hydrogen plasma ion implantation is used. As the consequence of high-flux ion implantation process, the near-surface layer of aluminum becomes highly activated and thermodynamically unstable. The physics of H atoms becomes strongly related to the nature of defects. The idea of the research is to fill available sites of activated aluminum film with hydrogen under high-flux ion irradiation, to thermally release hydrogen from the trap sites and to accommodate it in the bubbles containing molecular hydrogen and finally to effuse hydrogen from the bubbles into vacuum.
was performed keeping the hydrogen working gas pressure equal to 10 Pa. The ultimate vacuum was 10−4 Pa and the partial pressure of oxygen was about 10−3 Pa. In plasma immersion ion implantation, the sample is immersed in highdensity plasma generated independently and biased with a series of 1 kV negative voltage pulses. The pulse repetition rate was kept fixed at 20 kHz. Monte Carlo simulations using the TRIM computer code [11] yielded for 1 keV H+ in Al an average projected range close to 20 nm and a straggling of 12 nm. The incident energetic ions passed through the barrier layer of natural oxide and stopped in the Al film. The dose of implanted hydrogen was about 1018 H2 + cm−2 . The sputtering yield of Al2 O3 by incident 1 keV H2 + is equal to about 10−3 . The hydrogen effusion kinetics was studied by plasma desorption spectroscopy and the evolution of the sample morphology during thermal treatment by scanning electron microscopy (SEM). Thermal desorption spectroscopy (TDS) was performed in a vacuum stainless steel cell. The Ar gas carrier moved the released hydrogen from the dehydrogenation cell into the plasma chamber which was equipped with high resolution spectroscopy Spectrumat1000 and pumped by turbomolecular pump. The surface morphology of the deposited, hydrogenated and thermally treated samples was studied by scanning electron microscopy (JEOL JSM-6300 Scanning Microscope). The structure of samples was determined using room-temperature X-ray diffraction (XRD) measurements with the 2θ angle in the range 20–90◦ with Cu K␣ radiation. The identification of phases has been performed using Crystallographic Search-Match Program based on Powder Diffraction Data. The grain sizes were estimated by Debye–Scherrer analysis of the XRD spectra [12].
2. Experimental 3. Results Two-micrometer thick Al thin films were deposited at room temperature on 2 mm thick stainless steel Alloy 600 substrates by dc-magnetron sputtering (nominal target purity better than 99.99 at.%) in a vacuum chamber with an argon gas pressure of 0.2 Pa. After deposition the thin Al films were hydrogenated in the same vacuum chamber without exposure to air. The plasma immersion hydrogen ion implantation
SEM micrographs of H2 ion implanted Al thin films are presented in Fig. 1a and b. Fig. 1a is typical of an as-hydrogenated sample with a temperature at the end of hydrogenation of about 320 K. Fig. 1b is typical of the hydrogenated sample, following thermal treatment at 450 K for 15 min in air. In the as-hydrogenated sample at 370 K
Fig. 1. The SEM surface views of as-hydrogenated Al film with 320 K at the end of the procedure (a), and after subsequent thermal annealing at 450 K for 15 min in air (b).
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Fig. 3. The cross-sectional SEM view of plasma hydrogenated Al film on stainless steel substrate with the maximum temperature 550 K at the end of procedure.
Fig. 2. The diffractograms of Al films on stainless steel substrate after hydrogenation with the temperature equal to 320 K (curve 1), 450 K (curve 2) and 550 K (curve 3) at the end of procedure.
we see the smooth surface topography with small randomly distributed bubbles with diameter 10–20 m. After the annealing at ∼450 K there is an increase in size and density of bubbles. The size of bubbles reaches 150–200 m, see Fig. 1b. Fig. 2 includes XRD diffractograms of the hydrogenated Al films with different temperatures at the end of hydrogenation. Curve 1 corresponds to as-hydrogenated Al film with a temperature equal to 320 K at the end of the procedure, curve 2 – 450 K, and curve 3 – 550 K. It is seen that the ashydrogenated Al film at around 320 K consists of two characteristic peaks of the substrate at 43.7 and 50.9◦ and three peaks indicating the presence of hydrogen bonded with Al atoms as AlH at 42.2◦ , AlH3 at 42.7 and 44.4◦ . After hydrogenation at 450 K (curve 2, Fig. 2), the AlH peaks at 42.2◦ and AlH3 at 42.7◦ disappear and the AlH3 peak at 44.4◦ decreases. This indicates that the concentration of Al–H bonds decreases in the film. At the same time, the characteristic c-Al(1 1 1) peak at 38.6◦ appears. This indicates that as-implanted hydrogen at temperatures below 320 K, when the concentration significantly exceeds the limit of hydrogen solubility in Al, is associated with defects and is located at grain boundaries forming Al–H bonds. At temperatures above 340 K hydrogen becomes mobile [10]. The concentration of Al–H bonds decreases and restructuring of the Al film takes place. After hydrogenation at 550 K, the sample diffractogram (curve 3, Fig. 2) shows only Al peaks. The surface oxide barrier layer inhibits the effusion of the released hydrogen which nucleates on defects forming bubbles. In Fig. 3, the SEM cross-sectional view of a 2 m thick as-deposited Al film on the stainless steel substrate reveals the distribution and size of bubbles across the hydrogenated Al film thickness. The maximum treatment temperature was 550 K. It is seen that hydrogen released from defect trap sites and boundaries of nanocrystallites is accommodated in bubbles randomly distributed in the bulk.
Fig. 4. The hydrogen effusion intensity as a function of the dehydrogenation time/temperature: curve 1, the effusion intensity vs. time; and curve 2, the sample temperature vs. time.
The time dependences of the hydrogen effusion intensity and sample temperature during thermal desorption of the hydrogenated Al thin film are shown in Fig. 4 (curves 1 and 2, respectively). During desorption experiments, the heating of sample was carried out with a nearly linear temperature ramp of 0.3 K s−1 up to 620 K. The heating procedure ended at 710 K. The desorption of accumulated hydrogen presents single broad effusion peak with a sharp initial release of hydrogen beginning at the temperature 625 K with a maximum at 640 K and decreasing hydrogen effusion in the range of temperatures 640–710 K. Fig. 5 illustrates the typical SEM surface view of single bubble after the effusion procedure.
Fig. 5. The SEM surface view of a single bubble after the hydrogen effusion procedure.
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4. Discussion The experimental results direct us to the following conclusions: (i) the plasma immersion hydrogen ion implantation technique is an efficient method for hydrogenation of Al films, (ii) the as-implanted hydrogen at temperatures around 320 K is associated with different trap sites of the highly disordered film structure and accommodated at the grain boundaries with the formation of Al–H bonds, and (iii) the efficient hydrogen effusion starts at around 630 K. The high efficiency of the plasma immersion hydrogen ion implantation technique follows from the consideration of classical theory of the efficiency of retention of gases in solids [13]. For the maximum efficiency of retention, the following conditions have to be fulfilled. The first condition stems from the necessity that incident hydrogen atoms have to pass the surface barrier, and the second condition is related to the preservation of the surface layer for the retention of the implanted hydrogen. Monatomic non-metallic elements (O, C, N, etc.) are responsible for the surface potential barrier. The hydrogenation process was performed in the H2 working gas which included some oxygen as residual gas. The partial pressure of oxygen was about 10−3 Pa. At this pressure, a monolayer of oxygen on the Al film is formed in 10−1 –10−2 s. Under these conditions the Al film is covered by 2–4 nm thick of dense natural Al2 O3 oxide layer. This layer is transparent for incident energetic hydrogen ions extracted from the plasma, yet hinders the effusion of implanted hydrogen. We thus suggest the following model for the plasma hydrogenation of Al films. In the plasma immersion ion implantation technique, the Al film is immersed in the plasma and biased with the negative 1 kV voltage. The hydrogen ions are thus accelerated from the plasma towards the Al film where they are implanted through the surface barrier. The surface barrier during implantation is not destroyed because the sputtering yield is low (∼10−3 ) for 1 keV H+ . Additionally, the partial pressure of residual oxygen in the working gas is about 10−3 Pa, sufficient for the compensation of losses of oxygen. This oxide layer which exists during implantation process inhibits release of the implanted hydrogen. In the asimplanted Al film, hydrogen is contained atomically in interstitial sites, at the grain boundaries of the nanocrystalline Al film and as H2 molecules in bubbles. This is because the concentration of implanted hydrogen significantly exceeds the solubility limit of hydrogen atoms in Al. At temperatures higher than 320 K, hydrogen atoms diffuse and precipitate on nucleation sites to form bubbles. The formation of new bubbles stops when the concentration of hydrogen in the aluminum lattice reaches its solubility limit. This mechanism explains the XRD and SEM observations. We estimated that if all implanted hydrogen is stored in the bubbles of a 2 m thick Al film for an implantation fluence equal to 1018 cm−2 , this corresponds to about 10 at.% H in the Al film.
At temperatures higher than 600 K the surface oxide layer becomes permeable [7] to the hydrogen accommodated in bubbles and it effuses, thus producing the strong and repeatable effusion peak at ∼630 K temperature (Fig. 4). Analysis of the effusion peak at ∼640 K through the reaction rate theory indicates effusion activation energy of 1.85 eV, which is in very good agreement with the value 1.9 eV of the activation energy for hydrogen diffusion measured in ␥-Al2 O3 [14]. The effusion temperature 630 K is too high for many practical applications. Other methods of the release of hydrogen contained in bubbles are needed. Preliminary results show that the covers of bubbles are fragile and can be broken by surface mechanical waves [15,16].
5. Conclusions Two- to five-micrometer thick Al films were deposited by dc magnetron sputter deposition and hydrogenated without exposure to air employing plasma immersion ion implantation. H fluences up to 1018 cm−2 were achieved in an H2 working gas with the partial pressure of oxygen equal to about 10−3 Pa. The implanted hydrogen was accommodated in bubbles with diameters up 200 m at temperatures above 320 K. At lower temperatures, hydrogen was mainly associated to defects, distributed at the grain boundaries and contained in bubbles with diameter 10–20 m. The large part of the accommodated hydrogen effuses starting at 630 K. The dominant role in the mechanism of retention and release of hydrogen is attributed to the surface barrier Al2 O3 layer.
Acknowledgements The work was performed under support from the Hydrogen Program of the US Department of Energy and the Sandia National Laboratories. Authors express special gratitude to Mr. A. Vasys (USA) for his encouragement and help in the administration of the research. We are also grateful to Drs. George Thomas and Gary Sandrock, consultants to Sandia, for their technical advice and encouragement.
References [1] http://hydpark.ca.sandia.gov. [2] G. Sandrock, K. Gross, G. Thomas, J. Alloys Comp. 339 (2002) 299. [3] K.J. Gross, E.H. Majzoub, S.W. Spangler, J. Alloys Comp. 356–357 (2003) 423. [4] K. Higuchi, H. Kajioka, K. Toiyama, H. Fujii, Y. Kikuchi, J. Alloys Comp. 293 (1999) 484. [5] K. Higuchi, K. Yamamoto, H. Kajioka, K. Toiyama, M. Honda, S. Orimo, H. Fuji, J. Alloys Comp. 330–332 (2002) 526. [6] N. Hoon Goo, K. Sub Lee, Int. J. Hydrogen Energy 27 (2002) 433. [7] R. Checchetto, L.M. Graton, A. Miotello, Surf. Coat. Technol. 158–159 (2002) 356.
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