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In-situ depth-profiling of deuterium in nickel exposed to RF plasma
319
Journal of Nuclear Materials 179-181 (1991) 319-321 North-Holland
In-situ depth-profiling Ikuji Takagi, Department
Masato
Matsuoka,
of Nuclear Engineering,
of deuterium in nickel exposed to RF plasma Haruyuki
Kyoto University,
Fujita,
Kazuo
Yoshida, Sakyo-ku,
Shin and Kunio
Higashi
Kyoto 606, Japan
Depth-profiling of deuterium near the surface of nickel membranes was performed by using the D(3He, p)4He nuclear reaction, and the permeation rate of deuterium was measured at the same time. When the surface was exposed to RF plasma, remarkable PDP (plasma driven permeation) was observed. The depth profile consists of a high and sharp peak and a flat plateau, which correspond to the surface and the bulk concentration of deuterium, respectively. The permeation rate was proportional to the square of the bulk concentration under our experimental conditions. As an application of the depth-profiling, values of recombination coefficient of the downstream side were determined in case of GDP (gas driven permeation). It was shown that depth-profiling near the surface exposed to plasma was a useful method for understanding hydrogen recycling for fusion reactors 1. Introduction The permeation and retention of hydrogen isotopes are significant factors in the fueling for fusion reactors [l]. Superpermeation and superinventory [2] may occur in the first wall facing the edge plasma. From this point of view, plasma driven permeation (PDP) is of great concern. We measured the profile of deuterium concentrations on the upstream side of nickel membranes during PDP by using the technique of nuclear reaction analysis [3]. The relationship between the permeation rate and the deuterium concentration in bulk was examined. Recombination coefficients on the downstream side were also obtained in the case of gas driven permeation (GDP). 2. Experimental A schematic view of the experimental set-up is shown in fig. 1. The essential part consists of two vacuum chambers 1 and 2 separated by a 0.1 mm thick nickel membrane. The nickel membrane was mechanically
polished and heated up to about 700 K for several hours before the experiment. The 1.3 MeV 3He probing beam was injected into the membrane at an angle of 45”. Protons produced by the D(3He, p)4He reaction were detected by a surfacebarrier type solid-state detector (SSD) at an angle of 174.3’ to the 3He beam. Since the beam current was less than 200 nA ( - 7.0 X lo’* 3He/cm2 s), no temperature change of the membrane was observed during the irradiation. Deconvolutions of the energy spectra of protons were performed to obtain the depth profile of deuterium [4]. In the case of PDP, deuterium gas was continuously fed to a discharge tube made of Pyrex glass [5] and flowed out from a narrow spout. When RF was applied to the tube, the deuterium gas was weakly ionized and deuterium ions and atoms impinged on the membrane. The flux of neutral deuterium was diffucult to measure, so only the pressure in the reservoir and RF-power output were monitored. Since the pressure in chamber 1 was kept as low as allowable, it was not necessary to take account of the effect of the scattering and reaction of ‘He with deuterium in chamber 1. The permeation rate of deuterium was measured by a QMA (quadrupole mass analyzer) in chamber 2. When the steady-state permeation rate was achieved, the ‘He beam was injected into the membrane. Most of the procedure for GDP was the same as in PDP, except that deuterium gas was fed to chamber 2 at a desired pressure without evacuation and the permeation rate was measured by a QMA in chamber 1. 3. Results and discussions 3.1. Depth profiles
thermocouple TMP
Fig. 1. A schematic view of the experimental apparatus. 0022-3115/91/$03.50
0 1991 - Elsevier Science Publishers
in PDP
Fig. 2 shows deuterium depth profiles on the upstream side of nickel during steady-state PDP at a temperature of 523 K. Experimental conditions are summarized in table 1. When RF was not applied (in B.V. (North-Holland)
320
I. Tagaki et al. / In-situ depth-profiling of deuterwn
in Ni
experiment, we did not check the surface state of our specimen. An oxide layer probably remained on the membrane surface. The permeation rate J in table 1 is proportional to the square of the bulk concentration C, and no significant gradient of C, against depth can be seen in fig. 2. The latter suggests that permeation is surface-limited based on the theoretical work by Waelbroeck [2]. The relationship between the bulk concentration C, and the surface density S is difficult to explain but it is clear that S increases with CU. 3.2. Determination case of GDP
0.5 16 Depth ( pm) Fig. 2. Depth profiles of deuterium on the upstream side during PDP experiment. Curves are guides for eyes. ”
0.0
of the recombination
coefficient
in the
GDP (gas driven permeation) at the steady state was measured in the pressure range of 17 to 1700 Pa and in the temperature range of 483 to 673 K. The high and sharp peak and flat plateau were also observed on depth profiles. But the relationship between the surface density and the bulk concentration was different from that in PDP. Most surface sites seemed to be occupied by deuterium because the surface density was constant and independent of the bulk concentration. The bulk concentration was very small (1.0 X 10’s atoms/cm’ for a maximum value). These observations are well reproduced by the Richards model [lo]. According to Richards’ model [lo], the permeation rate J (atoms/cm2 s) and the bulk concentration C, (atoms/cm3) near the surface is related by eq. (l),
the case of GDP at low pressure), the permeation rate was 3 x 10” atoms/cm’ s which was less than onethousandth of that of Run 1407 in PDP under the same pressure. Remarkable permeation and a dramatic increase in the deuterium concentration in the membrane exposed to RF plasma were observed. A special feature which is seen in fig. 2 is that the depth profile consists of a high and sharp peak and a flat plateau. These two components were always observed throughout all the experiments in both GDP and PDP. The observed FWHM (full width half maximum) of the peak agreed well with the estimated resolution of the measuring system, so the peak was judged be due to the accumulation of deuterium near the surface. The plateau shows the bulk concentration. The experimental results are listed in table 1. The surface density S (atoms/cm2) is the integrated deuterium concentration up to 0.1 pm thickness and the bulk concentration C, (atoms/cm3) is the average value over the region of 0.2 to 0.8 urn depth. The accumulation of hydrogen isotopes at the surface of metals is considered to be the result of hydride formation. trapping by radiation damage and chemisorption at surface sites [6]. Among them, chemisorption at surface sites is the most probable. Another possible reason is the existence of an oxide layer which repressed the release of hydrogen from the surface [7-91. In our
J = k,(C,)‘,
(1)
where k, (cm4/s) is a phenomenological constant called the recombination coefficient. Both J and C, were measured, thus one can obtain the values of k,. The same approach was adopted by Braun et al. [II] to estimate the k, value. Fig. 3 shows values of k, determined in our GDP experiment, together with published values determined experimentally [ll-181. The data of Baskes [19] and Pick et al. [20] which are theoretical values for hydrogen are also shown. Our experimental results are in good agreement with those of Rota [18] and Winter [17] but are smaller than
Table 1
Experimental conditions and results of PDP. P,, membrane deuterium Run
No.
temperature.
respectively.
P”
ERF
(pa)
W)
1405
2.1
6.5
1406
2.9
9.0
1407 1408
5.2 13
14 35
J, C and
E,,
and T are deuterium pressure in the reservoir,
S are the permeation
rate,
J (D/cm2
523
7.2 x lOI 1.5 x 10’4 3.0 x 10’4 6.6X1014
523
concentration
C”
fK)
523 523
the bulk
s)
and
RF-power output and the surface
s
(D/cm’)
(D/cm’)
1.0x10” 1.8 x 10” 2.2x10” 4.0 x 10”
4.8x10’4 8.7x10’4 9.4x lOI 1.3 x 10’5
density
of
I. Tagaki et al. / In-situ depth-profiling of deuterium
in Ni
321
tion coefficient of the upstream side in the case of PDP is necessary in order to elucidate the mechanism of PDP. In-situ depth-profiling of deuterium on metals exposed to plasma is a useful method for examining hydrogen recycling for fusion reactors. References
PI K.L. Wilson, J. Nucl. Mater. 103 & 104 (1981) 453. in collaboration with P. Wienhold, PI F. Waelbroeck
I
-Jzo.5 Fig. 3. Arrhenius nickel determined
I 1.0
t 1.5 1000/T
I 2.0 (l/K)
diagrams of recombination experimentally (symbols) (solid lines).
I 2.5
1 3.0
coefficient k, of and theoretically
those of Nagasaki [15] and Yamaguchi 1161. As was shown by Wampler [g], the membrane surface with oxides tends to suppress the release of hydrogen. The rtiombination coefficient will become larger if oxides are effectively removed. It is concluded that dam-prof~ng is an effective method for determining the recombination coefficient, 4. Conclusion
As a result of in-situ depth-profiling on nickel exposed to m plasma, permeation rates were proportional to the square of the bulk concentration near the surface and the accumulation of deuterium at the surface was observed. Values of the recombination coefficient of nickel during GDP were obtained and they agreed well with other experimental results. Determining the recombina-
J. Winter, E. Rota and T. Bammo, Influence of Bulk and Surface Phenomena on the Hydrogen Permeation through Metals, Kemforschungsanlage Jiilich GmbH, Institut fir Plasmaphysik, Association Euratom-KFA, Jiil-1966 (December 1984). ]31 P.P. Pronko and J.G. Pronko, Phys. Rev. B9 (1974) 2870. ]41 K. Higashi, Y. Matsuno, H. Sakamoto, Y. Matsui and H. Fujita, Memoirs of Faculty of Engineering, Kyoto University, XLIV (1982) 396. I. Takagi, T. Komori, H. Fujita and K. Higashi, J. Nucl. Mater. 136 (1985) 287. S.M. Myers, P.M. Richards, W.R. Wampler and F. Besenbacher, J. Nucl. Mater. 165 (1989) 9. T.S. Elleman and K. Verghese, J. Nucl. Mater. 53 (1974) 299. W.R. Wampler, J. Nucl. Mater. 145-147 (1987) 313. P.M. Richards, SM. Myers, W.R. Wampler and D.M. Follstaedt, J. Appl. Phys. 65 (1989) 180. P.M. Richards, J. Nuci. Mater. 152 (1988) 246. M. Braun, B. Emoth, F. Waelbroeck and P. Wienhold, J. Nucl. Mater. 93 & 94 (1980) 861. R.A. Causey and M.I. Baskes, J. Nucl. Mater. 145-147 (1987) 284. [13] P. Presinger, P. Borgesen, W. Moller and B.M.W. Schemer, Nucl. fnstr. and Meth. B9 (1985) 270. [14] P. Bergesen, B.M.U. Schemer and W. MBller, Nucl. Instr. and Meth. B9 (1985) 33. [15] T. Nagasaki, R. Yamada, M. Saidoh and H. Katsuta, J. Nucl. Mater. 151 (1988) 189. [16] K. Yamaguchi, T. Namba and M. Yamawaki, J. Nucl. Sci. Technot. 24 (1987) 915. 1171 J. Winter, F. Waelbroeck, P. Wienhold, E. Rota and T. Banno, J. Vat. Sci. Technol. A2 (1984) 679. [18] E. Rota, F. Waelbroeck, P. Wienhold and J. Winter, J. Nucl. Mater. 111 & 112 (1982) 233. [19] M.I. Baskes, J. Nucl. Mater. 92 (1980) 318. [ZO] M.A. Pick and K. Sonnenberg, J. Nuci. Mater. 131 (1985) 208.
Journal of Nuclear Materials 179-181 (1991) 319-321 North-Holland
In-situ depth-profiling Ikuji Takagi, Department
Masato
Matsuoka,
of Nuclear Engineering,
of deuterium in nickel exposed to RF plasma Haruyuki
Kyoto University,
Fujita,
Kazuo
Yoshida, Sakyo-ku,
Shin and Kunio
Higashi
Kyoto 606, Japan
Depth-profiling of deuterium near the surface of nickel membranes was performed by using the D(3He, p)4He nuclear reaction, and the permeation rate of deuterium was measured at the same time. When the surface was exposed to RF plasma, remarkable PDP (plasma driven permeation) was observed. The depth profile consists of a high and sharp peak and a flat plateau, which correspond to the surface and the bulk concentration of deuterium, respectively. The permeation rate was proportional to the square of the bulk concentration under our experimental conditions. As an application of the depth-profiling, values of recombination coefficient of the downstream side were determined in case of GDP (gas driven permeation). It was shown that depth-profiling near the surface exposed to plasma was a useful method for understanding hydrogen recycling for fusion reactors 1. Introduction The permeation and retention of hydrogen isotopes are significant factors in the fueling for fusion reactors [l]. Superpermeation and superinventory [2] may occur in the first wall facing the edge plasma. From this point of view, plasma driven permeation (PDP) is of great concern. We measured the profile of deuterium concentrations on the upstream side of nickel membranes during PDP by using the technique of nuclear reaction analysis [3]. The relationship between the permeation rate and the deuterium concentration in bulk was examined. Recombination coefficients on the downstream side were also obtained in the case of gas driven permeation (GDP). 2. Experimental A schematic view of the experimental set-up is shown in fig. 1. The essential part consists of two vacuum chambers 1 and 2 separated by a 0.1 mm thick nickel membrane. The nickel membrane was mechanically
polished and heated up to about 700 K for several hours before the experiment. The 1.3 MeV 3He probing beam was injected into the membrane at an angle of 45”. Protons produced by the D(3He, p)4He reaction were detected by a surfacebarrier type solid-state detector (SSD) at an angle of 174.3’ to the 3He beam. Since the beam current was less than 200 nA ( - 7.0 X lo’* 3He/cm2 s), no temperature change of the membrane was observed during the irradiation. Deconvolutions of the energy spectra of protons were performed to obtain the depth profile of deuterium [4]. In the case of PDP, deuterium gas was continuously fed to a discharge tube made of Pyrex glass [5] and flowed out from a narrow spout. When RF was applied to the tube, the deuterium gas was weakly ionized and deuterium ions and atoms impinged on the membrane. The flux of neutral deuterium was diffucult to measure, so only the pressure in the reservoir and RF-power output were monitored. Since the pressure in chamber 1 was kept as low as allowable, it was not necessary to take account of the effect of the scattering and reaction of ‘He with deuterium in chamber 1. The permeation rate of deuterium was measured by a QMA (quadrupole mass analyzer) in chamber 2. When the steady-state permeation rate was achieved, the ‘He beam was injected into the membrane. Most of the procedure for GDP was the same as in PDP, except that deuterium gas was fed to chamber 2 at a desired pressure without evacuation and the permeation rate was measured by a QMA in chamber 1. 3. Results and discussions 3.1. Depth profiles
thermocouple TMP
Fig. 1. A schematic view of the experimental apparatus. 0022-3115/91/$03.50
0 1991 - Elsevier Science Publishers
in PDP
Fig. 2 shows deuterium depth profiles on the upstream side of nickel during steady-state PDP at a temperature of 523 K. Experimental conditions are summarized in table 1. When RF was not applied (in B.V. (North-Holland)
320
I. Tagaki et al. / In-situ depth-profiling of deuterwn
in Ni
experiment, we did not check the surface state of our specimen. An oxide layer probably remained on the membrane surface. The permeation rate J in table 1 is proportional to the square of the bulk concentration C, and no significant gradient of C, against depth can be seen in fig. 2. The latter suggests that permeation is surface-limited based on the theoretical work by Waelbroeck [2]. The relationship between the bulk concentration C, and the surface density S is difficult to explain but it is clear that S increases with CU. 3.2. Determination case of GDP
0.5 16 Depth ( pm) Fig. 2. Depth profiles of deuterium on the upstream side during PDP experiment. Curves are guides for eyes. ”
0.0
of the recombination
coefficient
in the
GDP (gas driven permeation) at the steady state was measured in the pressure range of 17 to 1700 Pa and in the temperature range of 483 to 673 K. The high and sharp peak and flat plateau were also observed on depth profiles. But the relationship between the surface density and the bulk concentration was different from that in PDP. Most surface sites seemed to be occupied by deuterium because the surface density was constant and independent of the bulk concentration. The bulk concentration was very small (1.0 X 10’s atoms/cm’ for a maximum value). These observations are well reproduced by the Richards model [lo]. According to Richards’ model [lo], the permeation rate J (atoms/cm2 s) and the bulk concentration C, (atoms/cm3) near the surface is related by eq. (l),
the case of GDP at low pressure), the permeation rate was 3 x 10” atoms/cm’ s which was less than onethousandth of that of Run 1407 in PDP under the same pressure. Remarkable permeation and a dramatic increase in the deuterium concentration in the membrane exposed to RF plasma were observed. A special feature which is seen in fig. 2 is that the depth profile consists of a high and sharp peak and a flat plateau. These two components were always observed throughout all the experiments in both GDP and PDP. The observed FWHM (full width half maximum) of the peak agreed well with the estimated resolution of the measuring system, so the peak was judged be due to the accumulation of deuterium near the surface. The plateau shows the bulk concentration. The experimental results are listed in table 1. The surface density S (atoms/cm2) is the integrated deuterium concentration up to 0.1 pm thickness and the bulk concentration C, (atoms/cm3) is the average value over the region of 0.2 to 0.8 urn depth. The accumulation of hydrogen isotopes at the surface of metals is considered to be the result of hydride formation. trapping by radiation damage and chemisorption at surface sites [6]. Among them, chemisorption at surface sites is the most probable. Another possible reason is the existence of an oxide layer which repressed the release of hydrogen from the surface [7-91. In our
J = k,(C,)‘,
(1)
where k, (cm4/s) is a phenomenological constant called the recombination coefficient. Both J and C, were measured, thus one can obtain the values of k,. The same approach was adopted by Braun et al. [II] to estimate the k, value. Fig. 3 shows values of k, determined in our GDP experiment, together with published values determined experimentally [ll-181. The data of Baskes [19] and Pick et al. [20] which are theoretical values for hydrogen are also shown. Our experimental results are in good agreement with those of Rota [18] and Winter [17] but are smaller than
Table 1
Experimental conditions and results of PDP. P,, membrane deuterium Run
No.
temperature.
respectively.
P”
ERF
(pa)
W)
1405
2.1
6.5
1406
2.9
9.0
1407 1408
5.2 13
14 35
J, C and
E,,
and T are deuterium pressure in the reservoir,
S are the permeation
rate,
J (D/cm2
523
7.2 x lOI 1.5 x 10’4 3.0 x 10’4 6.6X1014
523
concentration
C”
fK)
523 523
the bulk
s)
and
RF-power output and the surface
s
(D/cm’)
(D/cm’)
1.0x10” 1.8 x 10” 2.2x10” 4.0 x 10”
4.8x10’4 8.7x10’4 9.4x lOI 1.3 x 10’5
density
of
I. Tagaki et al. / In-situ depth-profiling of deuterium
in Ni
321
tion coefficient of the upstream side in the case of PDP is necessary in order to elucidate the mechanism of PDP. In-situ depth-profiling of deuterium on metals exposed to plasma is a useful method for examining hydrogen recycling for fusion reactors. References
PI K.L. Wilson, J. Nucl. Mater. 103 & 104 (1981) 453. in collaboration with P. Wienhold, PI F. Waelbroeck
I
-Jzo.5 Fig. 3. Arrhenius nickel determined
I 1.0
t 1.5 1000/T
I 2.0 (l/K)
diagrams of recombination experimentally (symbols) (solid lines).
I 2.5
1 3.0
coefficient k, of and theoretically
those of Nagasaki [15] and Yamaguchi 1161. As was shown by Wampler [g], the membrane surface with oxides tends to suppress the release of hydrogen. The rtiombination coefficient will become larger if oxides are effectively removed. It is concluded that dam-prof~ng is an effective method for determining the recombination coefficient, 4. Conclusion
As a result of in-situ depth-profiling on nickel exposed to m plasma, permeation rates were proportional to the square of the bulk concentration near the surface and the accumulation of deuterium at the surface was observed. Values of the recombination coefficient of nickel during GDP were obtained and they agreed well with other experimental results. Determining the recombina-
J. Winter, E. Rota and T. Bammo, Influence of Bulk and Surface Phenomena on the Hydrogen Permeation through Metals, Kemforschungsanlage Jiilich GmbH, Institut fir Plasmaphysik, Association Euratom-KFA, Jiil-1966 (December 1984). ]31 P.P. Pronko and J.G. Pronko, Phys. Rev. B9 (1974) 2870. ]41 K. Higashi, Y. Matsuno, H. Sakamoto, Y. Matsui and H. Fujita, Memoirs of Faculty of Engineering, Kyoto University, XLIV (1982) 396. I. Takagi, T. Komori, H. Fujita and K. Higashi, J. Nucl. Mater. 136 (1985) 287. S.M. Myers, P.M. Richards, W.R. Wampler and F. Besenbacher, J. Nucl. Mater. 165 (1989) 9. T.S. Elleman and K. Verghese, J. Nucl. Mater. 53 (1974) 299. W.R. Wampler, J. Nucl. Mater. 145-147 (1987) 313. P.M. Richards, SM. Myers, W.R. Wampler and D.M. Follstaedt, J. Appl. Phys. 65 (1989) 180. P.M. Richards, J. Nuci. Mater. 152 (1988) 246. M. Braun, B. Emoth, F. Waelbroeck and P. Wienhold, J. Nucl. Mater. 93 & 94 (1980) 861. R.A. Causey and M.I. Baskes, J. Nucl. Mater. 145-147 (1987) 284. [13] P. Presinger, P. Borgesen, W. Moller and B.M.W. Schemer, Nucl. fnstr. and Meth. B9 (1985) 270. [14] P. Bergesen, B.M.U. Schemer and W. MBller, Nucl. Instr. and Meth. B9 (1985) 33. [15] T. Nagasaki, R. Yamada, M. Saidoh and H. Katsuta, J. Nucl. Mater. 151 (1988) 189. [16] K. Yamaguchi, T. Namba and M. Yamawaki, J. Nucl. Sci. Technot. 24 (1987) 915. 1171 J. Winter, F. Waelbroeck, P. Wienhold, E. Rota and T. Banno, J. Vat. Sci. Technol. A2 (1984) 679. [18] E. Rota, F. Waelbroeck, P. Wienhold and J. Winter, J. Nucl. Mater. 111 & 112 (1982) 233. [19] M.I. Baskes, J. Nucl. Mater. 92 (1980) 318. [ZO] M.A. Pick and K. Sonnenberg, J. Nuci. Mater. 131 (1985) 208.