- Email: [email protected]
Effect of thin copper and palladium films on hydrogen permeation through iron
732
Journal of the Less-Common
Metals, 172-l 74 (1991) 732-739
Effect of thin copper and palladium films on hydrogen permeation through iron P. L. Andrew and A. A. Haasz University of Toronto, Institute for Aerospace Ontario M3H 5T6 (Canada)
Studies, 4925 Dufferin Street, North York,
Abstract Adsorption and desorption processes at the vacuum-metal interface can have a strong effect on the rate of hydrogen permeation through metal membranes. A test specimen was prepared by vapour-depositing palladium onto one side of an iron membrane to enhance the rate of hydrogen transport across that surface. The other surface, which could be monitored in situ by Auger electron spectroscopy, was sputter cleaned with argon ions and coated with a thin layer of copper, less than 0.1 pm. Hydrogen incident as molecules (10 ‘-lo4 Pa) was used to determine the membrane permeability and the overall surface-limited permeation rate constant in the temperature range 300-600 K. Atomic hydrogen (10’5-101g Ho mm2 s-*) incident on either side of the membrane was used to determine the contribution of each surface to the overall surface-limited permeation rate. The copper layer had no measurable effect on the diffusion-limited permeation, but reduced the surface-limited permeation rate, and affected its temperature dependence. Contamination of the copper surface, achieved by exposure to atmosphere, resulted in a further decrease by a factor of 10 in the surface-limited permeation rate.
1. Introduction The rate of hydrogen permeation through metal membranes depends not only on hydrogen transport within the metal bulk, but also on the rate of gas uptake and release at the vacuum-metal interfaces [l]. These latter processes can be strongly affected by the chemical composition of the surfaces. Because hydrogen dissolves atomically in metals, the mathematical dependence of the permeation rate on incident flux can be used to determine whether bulk or surface processes are rate controlling. The rates of adsorption and desorption measured using membrane techniques are, however, often many orders of magnitude lower than expected from surface physics experiments. While this is sometimes viewed as an inadequacy of conventional adsorption models in describing permeation, it is also possibly due to the difficulty in achieving impurity-free surfaces in a permeation experiment. Permeation rate-controlling surface processes are exploited in schemes for hydrogen pumps and compressors [2-41 which couple a metal membrane with a source of hydrogen atoms or ions. (Unlike molecules, hydrogen in these forms has a high probability of surface penetration, even for contaminated surfaces.) It is desirable that, once hydrogen atoms penetrate the
0 Elsevier Sequoia/Printed in The Netherlands
733
upstream surface, they preferentially recombine on the downstream surface so that the permeating hydrogen flux approaches the upstream incident flux. It has been suggested [5, 61 that such a membrane could be realized by applying a thin layer of gold or copper (known to have a small molecular hydrogen adsorption probability) to a metal with suitable bulk properties. In this investigation the rate of hydrogen permeation was measured for an iron membrane coated with copper on one side. A thin coating of palladium was used to increase the rate of hydrogen transport across the opposite surface [7], facilitating the interpretation of the data.
2. Experimental
details
The experimental facility used in this investigation has been described elsewhere [ 8,9] ; only a brief description is given here. The iron membrane was mounted on a heating assembly, separating two ultrahigh vacuum vessels. Hydrogen gas could be introduced into either chamber, and the permeation rate was determined from measurements of the downstream pressure and system pumping speed. Tungsten filaments on either side of the membrane were used to dissociate H, molecules for HO-driven permeation measurements. Argon ion sputtering for surface cleaning and evaporation of copper atoms were accomplished in situ for one of the membrane surfaces. This same surface was also accessible to Auger electron spectroscopy (AES) for in. situ surface analysis. Evaporation of copper was accomplished by resistive heating of a tungsten filament (approximately 1200 K) onto which copper strips (Johnson Matthey Chemicals, 99.9985% pure on a metal basis) had been spot welded. For a variety of surface states described below, the rate of hydrogen permeation, for hydrogen incident as H,, was measured at several pressures for a constant temperature, and repeated for different temperatures. The total time at which the copper-coated membrane temperature was held above 600 K was kept under 24 h to minimize the possibility of diffusion of copper into the iron substrate. In most cases, permeation for hydrogen incident as atoms was also measured, but this could only be done for temperatures above 400 K owing to radiative heating of the membrane by the Ho-producing tungsten filament. The polycrystalline iron membrane (0.1 mm thick, 30 mm diameter) had been used in a previous investigation [8]. The surface not facing the AES had been argon ion sputter cleaned and vapour coated with palladium. The AES-facing surface was argon ion sputter cleaned and then exposed to atmosphere at room temperature in an attempt to introduce surface impurities. The membrane, together with the whole vacuum system, was then baked at 470 K under vacuum for 40 h. Then, while heating the membrane at 600 K, molecular hydrogen was used to remove most of the surface oxygen, leaving a surface with a high level of carbon contamination. This surface state will be referred to as state “A”. Subsequent surface states were achieved by cleaning the iron surface, depositing copper atoms, exposure to atmosphere,
734
TABLE 1 AES surface analysis: elemental State
concentration
in the near-surface
(first few monolayers)
% Fe
% c
%O
% s
Others
Membrane conditioning
49 82
40 11
10 4
1 1
< 1% 2%W
82
14
2
1
1
< 1%
88
9
1
1
1
< 1%
61
8
30
1
< 1%
75
11
10
1
3
< 1%
See text Ho cleaning Ar + sputter cleaning Cu evaporation ( z 0.01 pm) substrate at 620 K Cu evaporation ( z 0.04 pm) substrate at 620 K Atmospheric exposure (3 weeks), bakeout (470 K) H, cleaning (600 K) Ho cleaning
% cu
and finally cleaning the copper surface. Actual surface concentrations, as measured by AES, for the various surface states are listed in Table 1, together with the method of preparation. Copper coatings of two different thicknesses ( w 0.01 and 0.04 ,am) were prepared. The thickness was estimated from the copper deposition time and rate, assuming the latter to be constant. In turn, the deposition rate was determined from the time taken to produce a copper thickness of N 3 monolayers as measured by AES [lo]. The AES iron signal was larger than expected for these copper thicknesses, indicative of a possible non-uniformity in the coating or interdiffusion of copper and iron. However, AES spectra for various positions on the membrane indicated no significant macroscopic non-uniformity.
3. Results and discussion Typical results for hydrogen incident as H, are shown in Fig. 1. Measurements conducted in the opposite flow direction gave similar results. At low upstream pressures P, the permeation flux &, of hydrogen atoms approaches direct proportionality with P, while at higher pressures 4, approaches a square root of P dependence., A general curve [l,111 is fitted to each set of data, yielding proportionality constants for the asymptotic values of 4,/P and &,/fi. The latter is given by [ll]
where xi, Di and Si, are, respectively, the thickness, hydrogen diffusivity and hydrogen solubility of the ith material (Le. copper, iron, palladium). The term for palladium can be neglected outright since (DS),, $ (DS),, [ 12, 131
<
I
r lo’6:
0
: ,”
-6
. different surfaces, same temperature 10’5 z-: E 0 585 + 10 K C 0 600 D n 600 F A 600 1014j ',,',I' ",,,,'
10-X
IO"
IO“
different temperatures same surface state (E)
,,,,,,,
0 420 2 10 K 0 585
"' 1,' j I
,,,'I' IO
',,',,' IO'
IO3
IO4
PAPal Fig. 1.Typical Ha-driven permeation isotherms. General permeation curve of Waelbroeck et al. [l]is shown for 0 data. Two different thicknesses of copper ( n , 0) give similar curves. The surface-limited permeation rate (low pressure regime) is reduced by atmospheric exposure ( :I), but recovers following Ho-cleaning (a). The different surface states, labelled C to F, are described in Table 1.
and xPd 6 xFe. Although xcU < xFe, (DS),, < (DS),,, especially at lower temperatures. It is not clear from published values of (DS),, [14] whether the copper term in eqn. (1) will be dominant below about 400 K, although it can be neglected above about 500 K. This is due to the dependence of (DS),, on crystal structure at low temperatures. For the present set of results (DS/x) was found to be in good agreement with (DS/x),+ [8] for all cases, implying negligible contribution from (DS),, . When the driving pressure is sufficiently low, surface processes become rate controlling and the permeation is described by [ll]
4
$ (small (2akrS2) =
(2)
where gi and kri are, respectively, the surface roughness factor and the hydrogen recombination coefficient of the materials at the upstream and downstream surfaces. The contribution of each surface to the overall surfacelimited rate constant, (2ak,S2), can be determined from the relative permeation probability of Ho incident on opposite surfaces, assuming the sticking probability for Ho is the same for each surface [ 111. Typical results for HO-driven permeation are shown in Fig. 2. The values of 2ok,S2 for each of the various surface states facing the AES are shown in Fig. 3, along with numerical fits to an Arrhenius type relationship.
100
1 o-1
1 o-2 State B
1 o-3
Pd
Cu
•J 1 o-4 1015
101s
1017
1 O’s
10’9
1020
Incident HOFlux &/(H/mQ) Fig. 2. HO-driven permeation isotherms for T= 520 f 10 K. At low incident fluxes, relative values of &,/I$~ for the same surface, but different flow directions ( n and 0) give the relative value of 2&S’ for the two surfaces (i.e. in this case (2ak,SZ)pd/(2uk,S2),, z 3.6). For a given incident flux, the permeating flux is greater for Ho incident on a clean copper-coated surface ( 0) than for a bare iron surface (0). Lines of constant permeating flux &, are indicated by dashed lines.
Evaporation of copper onto the sputter-cleaned iron surface led to a lower value of 2ak,S2 with an increase in activation energy. Results were the same for the two different coating thicknesses. The effect of atmospheric exposure of this surface was to lower 2ok,S2 by about an order of magnitude (same activation energy). Following exposure to Ho, the value of 2ak,S2 for a temperature of 585 K recovered its level prior to contamination. Assuming that incoming H, undergoes a conventional dissociative chemisorption process prior to absorption of hydrogen atoms in the metal, the ratio of 2ak,S2 to 2(2nm,,kT) -112, i.e. the ratio of adsorbing flux to incident flux per unit pressure, can be interpreted as an H, sticking probability [15]. For the present results this gives an activation energy of 0.22 eV for the sticking probability of H, on the evaporated copper surfaces. The apparent activation energy for 2ak,S2 (Fig. 3) is lower by approximately kT/2 owing to the temperature dependence of the incident flux per unit pressure. The magnitudes of the sticking probability at 500 K for the clean and contaminated surfaces are approximately 10m6 and 1O-7 respectively. An independent measure of (2ak,S2),, was made to ensure that the surface-limited permeation rate was not dominated by a small percentage of
L
L
J
d2amhkT
Present Results 0 contaminated Fe 0 sputter-cleaned Fe + A Cu overlayer (thin) x v Cu overlayer (thick) w contaminated Cu + Ho-cleaned Cu
lo** I-
1020r sputter-cleaned Fe with evaporated Fe [Sl
--__ --_
I
4
1-_
-1
lo’6
sputter-cleaned Fe [81 ---o__ T
2.4 x 1Ol9exp(-0.2OeV/kT)
lOI -
I
1.3 x 10” expt-O.lSeV/kT) --/ I I I, I
1.2
1.6
2.0
I
2.4
(IOOO/T)
I
2.8
I
I
3.2
I
3.6
/(K-‘1
Fig. 3. Surface-limited permeation parameter 2&S” us. l/T for various surface states. The + and x data were obtained from high flux Ho permeation, while low pressure Hz-driven permeation was used for all the rest. Solid lines are linear fits to data determined by the latter method. Short dashes represent data from ref. [S]. Long dashes show the maximum theoretical value, i.e. 2 atoms absorbed for every incident molecule.
the membrane area which may have been shadowed from copper coverage. When the incident Ho flux is sufficiently large, the permeation rate depends on both (DS/x) (which was obtained from high flux Hz-driven permeation experiments) and the upstream 2ak,S2 [ll]. In this regime, which is characterized by a square root dependence of 4P on the incident Ho flux, a given hydrogen atom in the metal will recombine to form a gas molecule before lateral diffusion has become much greater than a membrane thickness, so
738
that a small percentage of the membrane area will only have an equally small effect on the permeation rate. The values of (2ak,S2)c. obtained this way are in good agreement with those discussed above (see Fig. 3). Although it is generally agreed that adsorption of H, on copper, unlike most metals, is a thermally activated process [16-191, experimentally determined activation energies span a large range of values. Measurements of the angular distribution of desorbed hydrogen from polycrystalline and singlecrystal copper surfaces [ 20, 211 as well as molecular beam experiments [ 181 on copper single crystals suggest an activation barrier of adsorption comparable with that deduced here. In contrast, an activation energy of approximately 1.2 eV was found by applying a permeation technique to supposedly clean copper membranes by Kompaniets et al. [19, 221. Whether a copper layer is a useful addition to a hydrogen permeation pump or compressor depends on how low a value of 2ak,S2 is attainable on the upstream surface. The present results show that a clean copper coating might offer an improvement over a sputter-cleaned iron surface. Although both the contaminated iron and contaminated copper surfaces resulted in still lower 2ak,S2, they may not be stable (i.e. they may become clean as a result of hydrogen exposure) and therefore would not be practical surfaces for pump applications. In Fig. 2, lines of constant permeating flux show how the throughput is increased for a given incident flux after applying the copper overlayer (compare 0 and 0 symbols in Fig. 2). For a given value of 2ak,S2, the value of & at which &,/&J~ starts to decrease (resulting in a reduced pumping speed) with increasing $, depends on the value of (DS/x). Future experiments are directed at increasing this critical flux by replacing the iron substrate with palladium, which has a much higher value of DS. 4. Conclusions The effect of an evaporated copper layer on the permeation of hydrogen through an iron membrane was measured. At high pressures, the permeation rate was unaffected. At low pressure, where the permeation rate is controlled by adsorption of H,, the effect of evaporating copper onto the sputter-cleaned iron surface was to lower the permeation rate. From this, an activation energy of approximately 0.22 eV was estimated for the sticking probability of the surface by exposure to on copper. The effect of contaminating H2 atmosphere was to decrease the surface-limited permeation rate by about an order of magnitude. The conditions prior to atmospheric exposure were easily recovered following membrane exposure to atomic hydrogen, Acknowledgments This work was supported by the Canadian Fusion Fuels Technology Project and the Natural Sciences and Engineering Research Council of Canada.
739
References 1 F. Waelbroeck, P. Wienhold, J. Winter, E. Rota and T. Banno, Kernforschungsanlage, Jiilich, Germany, Rep. No. Jiil. 1966, 1984. 2 A. I. Livshitz, Vacuum, 29 (1979) 103. 3 A. I. Livshitz, M. E. Notkin, Yu. M. Pustovoit and A. A. Samartsev, Vacuum, 29 (1979) 113. 4 H. Hackfort, K. Bosche, F. Waelbroeck, J. Winter and P. Wienhold, J. Nucl. Mater., 144 (1987) 10. 5 W. T. Shmayda, F. Waelbroeck, J. Winter, P. Wienhold, T. Banno and N. P. Kherani, Fusion Technol., 8 (1985) 2285. 6 A. I. Livshitz, M. E. Notkin and A. A. Samartsev, J. Nucl. Mater., 170 (1990) 79. 7 A. J. Kumnick and H. H. Johnson, Metall. Trans. A, 6 (1975) 1087. 8 P. L. Andrew and A. A. Haasz, J. Vuc. Sci. Z’echnol. A,8 (1990) 1807. 9 A. B. Antoniazzi, Ph.D. Thesis, University of Toronto, 1987. 10 P. H. Holloway, J. Vat. Sci. Z’echnol., 12 (1975) 1418. 11 A. A. Haasz, P. L. Andrew and A. B. Antoniazzi, J. Vat. Sci. Z’echnol. A, 7 (1989) 1042. 12 S. A. Koffler, J. B. Hudson and G. S. Ansell, Trans. Metall. Sot. AZME, 245 (1969) 1735. 13 R. F. Miller, J. B. Hudson and G. S. Ansell, Metall. Trans. A, 6 (1975) 117. 14 J. L. Maienschein, F. E. McMurphy and V. I,. Duval, Fusion Technol., 14 (1988) 701, and references therein. 15 M. A. Pick and K. Sonnenberg, J. Nucl. Mater., 131 (1985) 208. 16 J. Pritchard and F. C. Tompkins, Trans. Faraday Sot., 56 (1960) 540. 17 C. S. Alexander and J. Pritchard, J. Chem. Sot., Faraday Trans. I, 68 (1972) 202. 18 M. Balooch, M. J. Cardillo, D. R. Miller and R. E. Stickney, Surf. Sci., 46 (1974) 358. 19 T. N. Kompaniets and A. A. Kurdyumov, Progr. Surf. Sci., 17 (1984) 75. 20 T. L. Bradley and R. E. Stickney, Surf. Sci., 38 (1973) 313. 21 M. Balooch and R. E. Stickney, Surf. Sci., 44 (1974) 310. 22 T. N. Kompaniets, V. A. Kurakin, A. A. Kurdyumov and V. N. Lyasnikov, Sou. Phys. Solid State, 20 (12) (1978) 2043.
Journal of the Less-Common
Metals, 172-l 74 (1991) 732-739
Effect of thin copper and palladium films on hydrogen permeation through iron P. L. Andrew and A. A. Haasz University of Toronto, Institute for Aerospace Ontario M3H 5T6 (Canada)
Studies, 4925 Dufferin Street, North York,
Abstract Adsorption and desorption processes at the vacuum-metal interface can have a strong effect on the rate of hydrogen permeation through metal membranes. A test specimen was prepared by vapour-depositing palladium onto one side of an iron membrane to enhance the rate of hydrogen transport across that surface. The other surface, which could be monitored in situ by Auger electron spectroscopy, was sputter cleaned with argon ions and coated with a thin layer of copper, less than 0.1 pm. Hydrogen incident as molecules (10 ‘-lo4 Pa) was used to determine the membrane permeability and the overall surface-limited permeation rate constant in the temperature range 300-600 K. Atomic hydrogen (10’5-101g Ho mm2 s-*) incident on either side of the membrane was used to determine the contribution of each surface to the overall surface-limited permeation rate. The copper layer had no measurable effect on the diffusion-limited permeation, but reduced the surface-limited permeation rate, and affected its temperature dependence. Contamination of the copper surface, achieved by exposure to atmosphere, resulted in a further decrease by a factor of 10 in the surface-limited permeation rate.
1. Introduction The rate of hydrogen permeation through metal membranes depends not only on hydrogen transport within the metal bulk, but also on the rate of gas uptake and release at the vacuum-metal interfaces [l]. These latter processes can be strongly affected by the chemical composition of the surfaces. Because hydrogen dissolves atomically in metals, the mathematical dependence of the permeation rate on incident flux can be used to determine whether bulk or surface processes are rate controlling. The rates of adsorption and desorption measured using membrane techniques are, however, often many orders of magnitude lower than expected from surface physics experiments. While this is sometimes viewed as an inadequacy of conventional adsorption models in describing permeation, it is also possibly due to the difficulty in achieving impurity-free surfaces in a permeation experiment. Permeation rate-controlling surface processes are exploited in schemes for hydrogen pumps and compressors [2-41 which couple a metal membrane with a source of hydrogen atoms or ions. (Unlike molecules, hydrogen in these forms has a high probability of surface penetration, even for contaminated surfaces.) It is desirable that, once hydrogen atoms penetrate the
0 Elsevier Sequoia/Printed in The Netherlands
733
upstream surface, they preferentially recombine on the downstream surface so that the permeating hydrogen flux approaches the upstream incident flux. It has been suggested [5, 61 that such a membrane could be realized by applying a thin layer of gold or copper (known to have a small molecular hydrogen adsorption probability) to a metal with suitable bulk properties. In this investigation the rate of hydrogen permeation was measured for an iron membrane coated with copper on one side. A thin coating of palladium was used to increase the rate of hydrogen transport across the opposite surface [7], facilitating the interpretation of the data.
2. Experimental
details
The experimental facility used in this investigation has been described elsewhere [ 8,9] ; only a brief description is given here. The iron membrane was mounted on a heating assembly, separating two ultrahigh vacuum vessels. Hydrogen gas could be introduced into either chamber, and the permeation rate was determined from measurements of the downstream pressure and system pumping speed. Tungsten filaments on either side of the membrane were used to dissociate H, molecules for HO-driven permeation measurements. Argon ion sputtering for surface cleaning and evaporation of copper atoms were accomplished in situ for one of the membrane surfaces. This same surface was also accessible to Auger electron spectroscopy (AES) for in. situ surface analysis. Evaporation of copper was accomplished by resistive heating of a tungsten filament (approximately 1200 K) onto which copper strips (Johnson Matthey Chemicals, 99.9985% pure on a metal basis) had been spot welded. For a variety of surface states described below, the rate of hydrogen permeation, for hydrogen incident as H,, was measured at several pressures for a constant temperature, and repeated for different temperatures. The total time at which the copper-coated membrane temperature was held above 600 K was kept under 24 h to minimize the possibility of diffusion of copper into the iron substrate. In most cases, permeation for hydrogen incident as atoms was also measured, but this could only be done for temperatures above 400 K owing to radiative heating of the membrane by the Ho-producing tungsten filament. The polycrystalline iron membrane (0.1 mm thick, 30 mm diameter) had been used in a previous investigation [8]. The surface not facing the AES had been argon ion sputter cleaned and vapour coated with palladium. The AES-facing surface was argon ion sputter cleaned and then exposed to atmosphere at room temperature in an attempt to introduce surface impurities. The membrane, together with the whole vacuum system, was then baked at 470 K under vacuum for 40 h. Then, while heating the membrane at 600 K, molecular hydrogen was used to remove most of the surface oxygen, leaving a surface with a high level of carbon contamination. This surface state will be referred to as state “A”. Subsequent surface states were achieved by cleaning the iron surface, depositing copper atoms, exposure to atmosphere,
734
TABLE 1 AES surface analysis: elemental State
concentration
in the near-surface
(first few monolayers)
% Fe
% c
%O
% s
Others
Membrane conditioning
49 82
40 11
10 4
1 1
< 1% 2%W
82
14
2
1
1
< 1%
88
9
1
1
1
< 1%
61
8
30
1
< 1%
75
11
10
1
3
< 1%
See text Ho cleaning Ar + sputter cleaning Cu evaporation ( z 0.01 pm) substrate at 620 K Cu evaporation ( z 0.04 pm) substrate at 620 K Atmospheric exposure (3 weeks), bakeout (470 K) H, cleaning (600 K) Ho cleaning
% cu
and finally cleaning the copper surface. Actual surface concentrations, as measured by AES, for the various surface states are listed in Table 1, together with the method of preparation. Copper coatings of two different thicknesses ( w 0.01 and 0.04 ,am) were prepared. The thickness was estimated from the copper deposition time and rate, assuming the latter to be constant. In turn, the deposition rate was determined from the time taken to produce a copper thickness of N 3 monolayers as measured by AES [lo]. The AES iron signal was larger than expected for these copper thicknesses, indicative of a possible non-uniformity in the coating or interdiffusion of copper and iron. However, AES spectra for various positions on the membrane indicated no significant macroscopic non-uniformity.
3. Results and discussion Typical results for hydrogen incident as H, are shown in Fig. 1. Measurements conducted in the opposite flow direction gave similar results. At low upstream pressures P, the permeation flux &, of hydrogen atoms approaches direct proportionality with P, while at higher pressures 4, approaches a square root of P dependence., A general curve [l,111 is fitted to each set of data, yielding proportionality constants for the asymptotic values of 4,/P and &,/fi. The latter is given by [ll]
where xi, Di and Si, are, respectively, the thickness, hydrogen diffusivity and hydrogen solubility of the ith material (Le. copper, iron, palladium). The term for palladium can be neglected outright since (DS),, $ (DS),, [ 12, 131
<
I
r lo’6:
0
: ,”
-6
. different surfaces, same temperature 10’5 z-: E 0 585 + 10 K C 0 600 D n 600 F A 600 1014j ',,',I' ",,,,'
10-X
IO"
IO“
different temperatures same surface state (E)
,,,,,,,
0 420 2 10 K 0 585
"' 1,' j I
,,,'I' IO
',,',,' IO'
IO3
IO4
PAPal Fig. 1.Typical Ha-driven permeation isotherms. General permeation curve of Waelbroeck et al. [l]is shown for 0 data. Two different thicknesses of copper ( n , 0) give similar curves. The surface-limited permeation rate (low pressure regime) is reduced by atmospheric exposure ( :I), but recovers following Ho-cleaning (a). The different surface states, labelled C to F, are described in Table 1.
and xPd 6 xFe. Although xcU < xFe, (DS),, < (DS),,, especially at lower temperatures. It is not clear from published values of (DS),, [14] whether the copper term in eqn. (1) will be dominant below about 400 K, although it can be neglected above about 500 K. This is due to the dependence of (DS),, on crystal structure at low temperatures. For the present set of results (DS/x) was found to be in good agreement with (DS/x),+ [8] for all cases, implying negligible contribution from (DS),, . When the driving pressure is sufficiently low, surface processes become rate controlling and the permeation is described by [ll]
4
$ (small (2akrS2) =
(2)
where gi and kri are, respectively, the surface roughness factor and the hydrogen recombination coefficient of the materials at the upstream and downstream surfaces. The contribution of each surface to the overall surfacelimited rate constant, (2ak,S2), can be determined from the relative permeation probability of Ho incident on opposite surfaces, assuming the sticking probability for Ho is the same for each surface [ 111. Typical results for HO-driven permeation are shown in Fig. 2. The values of 2ok,S2 for each of the various surface states facing the AES are shown in Fig. 3, along with numerical fits to an Arrhenius type relationship.
100
1 o-1
1 o-2 State B
1 o-3
Pd
Cu
•J 1 o-4 1015
101s
1017
1 O’s
10’9
1020
Incident HOFlux &/(H/mQ) Fig. 2. HO-driven permeation isotherms for T= 520 f 10 K. At low incident fluxes, relative values of &,/I$~ for the same surface, but different flow directions ( n and 0) give the relative value of 2&S’ for the two surfaces (i.e. in this case (2ak,SZ)pd/(2uk,S2),, z 3.6). For a given incident flux, the permeating flux is greater for Ho incident on a clean copper-coated surface ( 0) than for a bare iron surface (0). Lines of constant permeating flux &, are indicated by dashed lines.
Evaporation of copper onto the sputter-cleaned iron surface led to a lower value of 2ak,S2 with an increase in activation energy. Results were the same for the two different coating thicknesses. The effect of atmospheric exposure of this surface was to lower 2ok,S2 by about an order of magnitude (same activation energy). Following exposure to Ho, the value of 2ak,S2 for a temperature of 585 K recovered its level prior to contamination. Assuming that incoming H, undergoes a conventional dissociative chemisorption process prior to absorption of hydrogen atoms in the metal, the ratio of 2ak,S2 to 2(2nm,,kT) -112, i.e. the ratio of adsorbing flux to incident flux per unit pressure, can be interpreted as an H, sticking probability [15]. For the present results this gives an activation energy of 0.22 eV for the sticking probability of H, on the evaporated copper surfaces. The apparent activation energy for 2ak,S2 (Fig. 3) is lower by approximately kT/2 owing to the temperature dependence of the incident flux per unit pressure. The magnitudes of the sticking probability at 500 K for the clean and contaminated surfaces are approximately 10m6 and 1O-7 respectively. An independent measure of (2ak,S2),, was made to ensure that the surface-limited permeation rate was not dominated by a small percentage of
L
L
J
d2amhkT
Present Results 0 contaminated Fe 0 sputter-cleaned Fe + A Cu overlayer (thin) x v Cu overlayer (thick) w contaminated Cu + Ho-cleaned Cu
lo** I-
1020r sputter-cleaned Fe with evaporated Fe [Sl
--__ --_
I
4
1-_
-1
lo’6
sputter-cleaned Fe [81 ---o__ T
2.4 x 1Ol9exp(-0.2OeV/kT)
lOI -
I
1.3 x 10” expt-O.lSeV/kT) --/ I I I, I
1.2
1.6
2.0
I
2.4
(IOOO/T)
I
2.8
I
I
3.2
I
3.6
/(K-‘1
Fig. 3. Surface-limited permeation parameter 2&S” us. l/T for various surface states. The + and x data were obtained from high flux Ho permeation, while low pressure Hz-driven permeation was used for all the rest. Solid lines are linear fits to data determined by the latter method. Short dashes represent data from ref. [S]. Long dashes show the maximum theoretical value, i.e. 2 atoms absorbed for every incident molecule.
the membrane area which may have been shadowed from copper coverage. When the incident Ho flux is sufficiently large, the permeation rate depends on both (DS/x) (which was obtained from high flux Hz-driven permeation experiments) and the upstream 2ak,S2 [ll]. In this regime, which is characterized by a square root dependence of 4P on the incident Ho flux, a given hydrogen atom in the metal will recombine to form a gas molecule before lateral diffusion has become much greater than a membrane thickness, so
738
that a small percentage of the membrane area will only have an equally small effect on the permeation rate. The values of (2ak,S2)c. obtained this way are in good agreement with those discussed above (see Fig. 3). Although it is generally agreed that adsorption of H, on copper, unlike most metals, is a thermally activated process [16-191, experimentally determined activation energies span a large range of values. Measurements of the angular distribution of desorbed hydrogen from polycrystalline and singlecrystal copper surfaces [ 20, 211 as well as molecular beam experiments [ 181 on copper single crystals suggest an activation barrier of adsorption comparable with that deduced here. In contrast, an activation energy of approximately 1.2 eV was found by applying a permeation technique to supposedly clean copper membranes by Kompaniets et al. [19, 221. Whether a copper layer is a useful addition to a hydrogen permeation pump or compressor depends on how low a value of 2ak,S2 is attainable on the upstream surface. The present results show that a clean copper coating might offer an improvement over a sputter-cleaned iron surface. Although both the contaminated iron and contaminated copper surfaces resulted in still lower 2ak,S2, they may not be stable (i.e. they may become clean as a result of hydrogen exposure) and therefore would not be practical surfaces for pump applications. In Fig. 2, lines of constant permeating flux show how the throughput is increased for a given incident flux after applying the copper overlayer (compare 0 and 0 symbols in Fig. 2). For a given value of 2ak,S2, the value of & at which &,/&J~ starts to decrease (resulting in a reduced pumping speed) with increasing $, depends on the value of (DS/x). Future experiments are directed at increasing this critical flux by replacing the iron substrate with palladium, which has a much higher value of DS. 4. Conclusions The effect of an evaporated copper layer on the permeation of hydrogen through an iron membrane was measured. At high pressures, the permeation rate was unaffected. At low pressure, where the permeation rate is controlled by adsorption of H,, the effect of evaporating copper onto the sputter-cleaned iron surface was to lower the permeation rate. From this, an activation energy of approximately 0.22 eV was estimated for the sticking probability of the surface by exposure to on copper. The effect of contaminating H2 atmosphere was to decrease the surface-limited permeation rate by about an order of magnitude. The conditions prior to atmospheric exposure were easily recovered following membrane exposure to atomic hydrogen, Acknowledgments This work was supported by the Canadian Fusion Fuels Technology Project and the Natural Sciences and Engineering Research Council of Canada.
739
References 1 F. Waelbroeck, P. Wienhold, J. Winter, E. Rota and T. Banno, Kernforschungsanlage, Jiilich, Germany, Rep. No. Jiil. 1966, 1984. 2 A. I. Livshitz, Vacuum, 29 (1979) 103. 3 A. I. Livshitz, M. E. Notkin, Yu. M. Pustovoit and A. A. Samartsev, Vacuum, 29 (1979) 113. 4 H. Hackfort, K. Bosche, F. Waelbroeck, J. Winter and P. Wienhold, J. Nucl. Mater., 144 (1987) 10. 5 W. T. Shmayda, F. Waelbroeck, J. Winter, P. Wienhold, T. Banno and N. P. Kherani, Fusion Technol., 8 (1985) 2285. 6 A. I. Livshitz, M. E. Notkin and A. A. Samartsev, J. Nucl. Mater., 170 (1990) 79. 7 A. J. Kumnick and H. H. Johnson, Metall. Trans. A, 6 (1975) 1087. 8 P. L. Andrew and A. A. Haasz, J. Vuc. Sci. Z’echnol. A,8 (1990) 1807. 9 A. B. Antoniazzi, Ph.D. Thesis, University of Toronto, 1987. 10 P. H. Holloway, J. Vat. Sci. Z’echnol., 12 (1975) 1418. 11 A. A. Haasz, P. L. Andrew and A. B. Antoniazzi, J. Vat. Sci. Z’echnol. A, 7 (1989) 1042. 12 S. A. Koffler, J. B. Hudson and G. S. Ansell, Trans. Metall. Sot. AZME, 245 (1969) 1735. 13 R. F. Miller, J. B. Hudson and G. S. Ansell, Metall. Trans. A, 6 (1975) 117. 14 J. L. Maienschein, F. E. McMurphy and V. I,. Duval, Fusion Technol., 14 (1988) 701, and references therein. 15 M. A. Pick and K. Sonnenberg, J. Nucl. Mater., 131 (1985) 208. 16 J. Pritchard and F. C. Tompkins, Trans. Faraday Sot., 56 (1960) 540. 17 C. S. Alexander and J. Pritchard, J. Chem. Sot., Faraday Trans. I, 68 (1972) 202. 18 M. Balooch, M. J. Cardillo, D. R. Miller and R. E. Stickney, Surf. Sci., 46 (1974) 358. 19 T. N. Kompaniets and A. A. Kurdyumov, Progr. Surf. Sci., 17 (1984) 75. 20 T. L. Bradley and R. E. Stickney, Surf. Sci., 38 (1973) 313. 21 M. Balooch and R. E. Stickney, Surf. Sci., 44 (1974) 310. 22 T. N. Kompaniets, V. A. Kurakin, A. A. Kurdyumov and V. N. Lyasnikov, Sou. Phys. Solid State, 20 (12) (1978) 2043.