Impact of surface phenomena in metals on hydrogen isotope permeation

ELSEVIER

Fusion Engineering and Design 28 (1995) 125 130

Fusion Engineering and Design

Impact of surface phenomena in metals on hydrogen isotope permeation M. Yamawaki, N. Chitose, V. Bandurko, K. Yamaguchi Nuclear Engineering Research Laboratory, University of Tokyo, Tokai-mura, Ibaraki-ken, Japan

Abstract

The ion and gas driven deuterium permeation behavior in niobium (Nb) was investigated. The GDP (gas driven permeation) rate (flux) was measured to be proportional to the D2 gas pressure and to increase with the temperature, whereas the IDP (ion driven permeation) rate (flux) was 0.2-0.3 times the incident ion flux and was almost independent of the temperature. Based on the diffusion-recombination limited hydrogen transport model, the "phenomenological" recombination rate coefficient, kR, was calculated for both upstream-side and downstream-side from the IDP experiment and the obtained result agreed well with the GDP result. Furthermore, in-situ surface analysis by Auger electron spectroscopy (AES) revealed that the presence of carbon suppressed the GDP rate to a larger extent than that of oxygen. These newly obtained results on Nb enable comparison of the hydrogen transport characteristics of various metals which are of interest for fusion applications in terms of the so-called "recycling constant", D/2kr~ (D is the diffusivity). It is shown that Nb, together with vanadium, has larger values of D/2kR.

1. Introduction

The recycling of hydrogen isotopes (H, D and T) from the first wall in a fusion device, as well as its tritium inventory and permeation characteristics will depend to a large extent on the state of the surface and on the hydrogen uptake and release properties of the material under consideration. The surface of the first wall of early fusion experiments was typically composed of metals such as stainless steel and nickel (Ni, the main constituent of Inconel) because of their good outgassing properties as vacuum vessel components [1]. Then graphite [2,3] and beryllium (Be) [4] were used due to their low Z and favorable plasma compatibility properties. Presently, high Z refractory metals, such as molybdenum (Mo) and tungsten (W), are also being tested in

some devices and the hope is to control the plasma edge conditions and to reduce sufficiently the energy of the incident plasma particles to prevent erosion [5]. Vanadium (V) and its alloys are also of interest because of their low activation and are considered for DEMO. Finally, niobium (Nb) and tantalum (Ta) are potentially suitable for superpermeable membrane for separation and purification of hydrogen isotopes in the exhaust gas [6,7]. Therefore, it is clear that a very wide range of materials is under investigation and their interactions with hydrogen isotopes vary substantially. It is widely accepted that the tritium inventory and permeation are strongly affected by the state of surface, due to the importance of the surface recombination processes. Livshits et al. maintain that surface recombination lim-

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M. Yamawaki et al. / Fusion Engineering and Design 28 (1995) 125-130

ited transport of hydrogen is essential for realization of "superpermeation" [6,7]. In the first part of the present study, new experimental results on the ion and gas driven permeation behavior of deuterium in Nb, which is known to have large hydrogen diffusivity and solubility, are presented. The dependence of the permeation rate on the D2 gas pressure or temperature allowed to determine the rate-limiting step and then to evaluate the phenomenological "recombination rate coefficient", kR. In addition, by using in-situ Auger electron spectroscopy, the effect of surface impurities such as carbon and oxygen on the results is discussed. We have shown in the past that these contaminants affect kR by several orders of magnitude for Ni [8], stainless steel [9] and V [8]. Furthermore, using these laboratory data on kk as well as the compiled data on hydrogen diffusivity (D) and solubility (Ks), the "recycling constant", D]2kR [10], is evaluated and compared for these materials.

2. Deuterium permeation behavior in niobium 2.1. Preliminary experiment The niobium sample employed in the experiment was supplied by Daido Steel Co. Ltd. Its purity was 99.9% and the thickness, Xo, was 0.1 mm. It was cleaned ultrasonically in acetone and was annealed in vacuo at 1173 K for about 6 h prior to the first experiment. Both G D P (gas driven permeation) and IDP (ion driven permeation) experiments were performed. The pressure, PI, of D2 gas in the upstream-side was varied between 10-4 and 1 Pa in the G D P experiment, while in the IDP experiment the incident deuterium ion flux, ~bp was fixed at 9.4 x 1016 D m -2 s 1 in the presence of residual D2 molecules (Pt =7.5 x 10 -3 Pa) leaking from the ion gun. The deuterium permeation flux, qbp (D2 m -2 s - l ) , was measured with a quadrupole mass spectrometer (Anelva Co.; AQA-360). The surface composition was analyzed with Auger electron spectrometer (Anelva Co.; AAS-200). Further details of the apparatus are given in a previous paper [11]. The first experiment was initiated by introducing D 2 gas in the upstream-side of the as-received specimen at 873 K. The permeation rate of deuterium through the membrane is shown by the circles in Fig. 1 as a function of D2 pressure (indicated in the figure as GDP1). While the open circles denote the result corresponding to the specimen whose surfaces were saturated with sulfur (S), the closed circles denote the sulfur-free case. It appears that S suppresses deuterium permeation, the details of which is to be discussed in the following sections. The

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Fig. 1. Dependence of gas driven permeation (GDP) rate, (O)P)G, on the D 2 pressure, P1, in the upstream-side. GDP1 and GDP2 are obtained in the present study, which are compared with the result of previous study [ 12] denoted by the rectangles. magnitude of the permeation rate was such that, though it was comparable to that of previous study [ 12] it reached ~ 1 0 is D 2 m - 2 s l a t P l = 1 0 2 p a , which was more than an order of magnitude larger than the maximum IDP rate, that is, 4.7 x 10 t6 D2~m 2 s - t . Then oxygen (02) gas (~< 10 .4 Pa) was introduced in the upstream-side at 473 K prior to the permeation experiment (GDP2). As a result, the permeation rate decreased more than two orders of magnitude. 2.2. Results o f ion and gas driven permeation experiments In the GDP2 experiment, the gas driven deuterium permeation rate (flux), (q)p)a, was measured as a function of the D2 pressure, P~, and of the temperature, T, the result of which is shown in Fig. 2. It is shown that (~e)G is proportional to P1. The IDP experiments, on the other hand, were performed as follows. First, the temperature is kept constant in the temperature range between 873 and 1173 K, and then the upstream-side was filled with D 2 gas (P1 = 7.5 x 10 -3 Pa) . The gas gradually permeates the membrane and reaches steady state at q)p = (q)p)G,i after several hours. Then the deuterium ions (3 keV D2 ÷) is implanted and q% increases further to its steady state value, (q)p)~. When ion implantation is terminated ~p decreases to (~P)G,f- The result of IDP experiments is summarized in Fig. 3, where the permeation rate (q~p)i - (q~p)G,fis plotted as a function of inverse temperature, T - 1, in order to compensate for the contribution of GDP. The data for 1173 K is scattered due to large (q)p)a.f at this temperature (see Fig. 2). Except for this temperature, it is shown

M. Yamawaki et al. / Fusion Engineering and Design 28 (1995) 125-130 I

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nation are considered to be the rate-limiting steps [13]. In steady state the particle balance of hydrogen is given by; ~bin= 2(kR)lG 2 + 2(kR)2 C22, where (kR) 1 and (kR)2 are the recombination rate coefficients for upstreamside and downstream-side, respectively, and CI and C2 are the bulk hydrogen concentrations just beneath the upstream-side and downstream-side surface, respectively. The ~bi~is the incident atomic flux, which is equal to 2 (kR)l KS 2 P1 for the G D P case, and is equal to ~bp for the IDP case, where Ks is the hydrogen solubility. If the hydrogen transport is strongly limited by recombination at both surfaces, hydrogen concentration profile becomes uniform, so that C1 ~ C> Therefore, the theoretical expressions for G D P and IDP rate are given as follows: ((])P)GDP ~- (kR)2C2 2

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Fig. 3. Temperature dependence of ion driven permeation (IDP) rate, (q)ph- (@P)~.r, through niobium, where the ion flux (q~e) and pressure (P0 of deuterium were 9.4 x 1016 D 2 m 2 s - l and 7.5 × 10- ~ Pa, respectively. The dotted line is drawn just to guide the eye. The permeation probability is defined as a ratio of permeation flux (D m -2 s- 1) to incident flux (absorbed flux, to be exact; D m -z s-~). The result of permeation experiment after Ar + bombardment is denoted by the closed diamond, whereas that after the second O2 admission by the closed triangle. that the permeation rate is almost independent of temperature. The magnitude of the permeation rate, on the other hand, was such that it reached almost 20 to 30% of the incident ion flux, qSe. This is, according to the definition by Livshits et al. [6], within the range of " superpermeation ".

According to the model of steady-state hydrogen transport in metals, bulk diffusion and surface recombi-

(]CR)I(kR)2 K, 2 p

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This particular mechanism is often referred to as " R R regime" irate-determined by the Recombination process at both surfaces). It can be seen that the above equations agree well with the observed results. How the obtained results were sensitive to the surface state of the specimen was observed by the following experiments. After the completion of GDP2 (Fig. 2) and the simultaneous IDP + GDP experiments (open circles in Fig. 3), the upstreamside surface was bombarded by Ar + ions at room temperature to a fluence of 3 × 102o A r m -2, and the IDP ( + GDP) experiment was performed at 1073 K. As a result, ( ~ p ) i - (~p)G,r was decreased by a factor of about 2 (shown by the closed diamond in Fig. 3). When the succeeding permeation experiment at 1073 K was performed after exposing the upstream-side surface to 02 at r.t. (second 02 admission), the permeation rate (dosed triangle) increased to the same magnitude as that before Ar + bombardment (open circles). It should be noted that Eqs. (1) and (2) indicate that the experimental result may also be sensitive to the surface state of the downstream-side. Since the surface analysis by AES was limited to the upstream-side in the present study, care was taken so as not to modify the downstream-side surface. 2.3. Evaluation o f recombination rate coefficient

By equating the experimental result of (qbe)G,f to ( 0 p ) G D P in Eq. (1) and (@P)1 - (0p)G,r to ((I)p)iDP in Eq.

(2), the "phenomenological" recombination rate co-

128

M. Yamawaki et al. / Fusion Engineering and Design 28 (1995) 125 130 n

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Fig. 4. Recombination rate coefficient, kR, of niobium for upstream-side ((kR)l; closed symbols) and downstream-side ((kR)2; open symbols).

where (Ks)o is the pre-exponential coefficient for hydrogen solubility, Es = - 34 kJ t o o l - ~ is the heat of solution taken from the literature [15], and Ec is the activation energy for dissociative adsorption. The data can be explained fairly well by just one fitting parameter; that is, 2(Ec)1 ~ 7 0 kJ mol J for (kR)l and 2(Ec)2 ~ 80 kJ m o l - 1 for (kR)2. The 2Ec values are also obtained for other metals; e.g. 2Ec ~ 70 and 50 kJ mol - I for sulfur covered surface of nickel (Ni) and vanadium (V), respectively [9], and 2Ec ~ 60 kJ m o l for oxygen covered surface of stainless steel (304 SS) [16]. The obtained values of kR are then employed to calculate G D P rate using Eq. (1), the result of which is shown by the straight lines in Fig. 2. An agreement between calculation and the experiment is good, confirming the consistency of the results of two different permeation experiments.

3. Impact of surface phenomena on hydrogen permeation behavior

It is the opinion of the present authors that nonmetallic impurity elements such as sulfur (S), carbon

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efficients, (kR)~ and (kR)2 can be evaluated. The result of the calculation is shown in Fig. 4. In the figure (kR) ~ is denoted by the closed circles, while (kR)2 is denoted by the open circles. In addition, straight lines indicate the calculated result using the formula of Pick and Sonnenberg [14], whose expression for deuterium is given by [8] 1.30 x 1024 2(E s - E c ) k R - (Ks)0ex/~ exp RT

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Fig. 5. Correlation between the gas dri-,en deuterium permeation rates, (~e)o,i and (~e)o,f, which are observed in the simultaneous IDP + GDP experiments, and surface impurity composition at 1073 K: (a) carbon (C) and (b) oxygen (O). For the definition of the symbols and further explanation, see text. (C) and oxygen (O) are the origin of the surface barrier, 2 E o in Eq. (3). Such a mechanism has also been presented by Livshits and co-workers [6,7]. They have demonstrated that the permeation probability for 2 keV H + ions incident on Nb approached unity by admitting oxygen of the pressure > 10 -3 Pa. They did not, however, perform any surface analysis, so that the role of oxygen on the observed phenomenon remained unanswered. By using in-situ AES technique, we investigated the correlation between the permeation rate and surface impurity composition. The AES analysis revealed that the specimen surface is covered by S, C and O, with a trace level of nitrogen (N). Moreover, the depth profile of the Nb surface showed that S is strongly segregated to the surface. No S can be observed in the region deeper than 1 nm while C and O are found to extend deeper in the bulk. Fig. 5 shows correlation between the GDP rate (flux) of deuterium

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M. Yamawaki et al. / Fusion Engineering and Design 28 (1995) 125-130

through Nb observed in the IDP + G D P experiments and the surface impurity composition at upstream-side surface. The initial value of G D P rate, (~p)o,~, of an experimental run is denoted by the open symbols, while that of (q)~,)o,r by the closed ones. (qsp)Q,i and ((I)p)~.f belonging to the same experimental run are connected by the straight line. In addition, the result of permeation experiment after Ar + bombardment is denoted by the diamonds, whereas that after the second 02 admission by the triangles. According to the figure, it is shown that C on the surface suppresses deuterium permeation rate (a), which is consistent with the result of previous studies by the present authors [8,9,11,16-18]. Similar trend was observed for S, although not as clear as C. However, quite an opposite result was obtained for oxygen (b). Although, as mentioned above, the depth profiles of C and O are similar, it is C that plays a dominant role at the surface, instead of O. Prolonged bombardment of the surface by Ar + to a fluence of 3x1020 Ar m 2 res dted in a preferential sputtering of C, as shown in Fi~;. 5. Hence, C may be more attracted to the surface than O. On the contrary, the role of O is not clear, although it was the 02 admission (first admission) to the upstream-side surface which caused the drastic reduction of G D P rate, as shown in Fig. 1. Moreover, the correlation observed in Figs. 5(a) and (b) appears to hold only within the particular series of permeation experiments between the first and second 02 admission. When the permeation experiment was performed after second 02 admission to the upstream-side surface, the result shown by the triangles in Fig. 5 does not follow the same trend. Surprisingly, O2 admission did not result in the complete coverage of surface by O. It may have formed an oxide layer in the subsurface region, which is beyond the detection range of AES. This is due to the fact that when the specimen is heated to elevated temperatures where the permeation experiments were performed, O may easily diffuse in the bulk of Nb. Further investigation is thus required to clarify the role of 02 admission on the reduction of the G D P rate. The surface composition in the present study was merely the consequence of ion sputtering during IDP experiments and thermal annealing between the IDP experiments. In this sense, the surface state was by no means prepared intentionally. From the engineering standpoint it is important for the successful application of Nb as a superpermeable membrane that the surface state is controllable. The control of surface composition can be achieved when the impurities form a segregated layer. Then, combination of sputter cleaning and thermal annealing will prepare surface with desired compositions [19].

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Fig. 6. "Recycling constant", D/2ka, calculated for various metals. See text for explanation. Finally, it is of interest to examine how these metals behave in the event of transient wall pumping, hydrogen recycling or permeation. In this sense, the so-called "recycling constant" [10] defined as D/2k R is calculated for Ni, 304 SS, Cu, V and Nb, and the results are plotted in Fig. 6 as a function of inverse temperature, T-~. All the diffusivity data are taken from the literature. The data for kR, on the other hand, are mostly taken from the work of the present authors; Ni [8,17], 304 SS [9], V [18] and Cu [20]. Two cases of data are shown for Nb, labeled as Nb-1 and Nb-2, where in the former kR = (kR)l and in the latter kR = (kR)2 is assumed. Furthermore, the vertical lines for Ni and 304 SS indicate the extent to which the calculated result may depend on the surface impurity compositions. The recycling constant when divided by the incident flux, qbp, in the case of the present study, gives a measure of the characteristic time needed for the recycling flux to become comparable to qSp. When permeation is negligible, it should be directly related to hydrogen inventory. According to the figure, the metals considered here can be separated into two groups; whereas Ni, 304 SS and Cu have small D/2kR values, those of V and Nb are considerably large. This is at least in qualitative agreement with the experimental observations; i.e. in the case of latter materials it took more than several hours ( > 104 s) for the permeation rate to achieve steady state.

4. Concluding remarks Deuterium permeation behavior in niobium was investigated experimentally. In this particular series of experiment, the surface of the specimen was exposed to

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M. Yamawaki et al. / Fusion Engineering and Design 28 (1995) 125 130

oxygen gas at the beginning of the series. The results revealed that the GDP (Gas Driven Permeation) rate was proportional to D 2 gas pressure and increased with temperature, whereas the IDP (Ion Driven Permeation) rate was 0.2-0.3 times the incident ion flux and was almost independent on the temperature. These results were characteristics of "RR regime", so that the recombination rate coefficient, k R, was obtained for both upstream-side and downstream-side from the simultaneous IDP + G D P experiment. The obtained result when applied to the GDP result showed good agreement. Furthermore, in-situ surface analysis by Auger electron spectroscopy revealed that, rather surprisingly, the presence of carbon suppressed the GDP rate, not of oxygen. Finally, using these newly obtained results on Nb, the "recycling constant", D/2kR, was calculated and was compared with that of Ni, 304 SS, Cu and V. The result showed that Nb, together with vanadium, had largest values of D / 2 k R. Before concluding, several remarks are worth making. The fact that the surface composition was not controllable would throw doubt on the reproducibility of the experimental results. In fact, as mentioned previously, the reproducibility and consistency of the obtained results were confirmed only within the particular series of experiments which was preceded and followed by 02 admission to the upstream-side surface. Moreover, the slow transient characteristic of deuterium permeation made determination of steady state level rather difficult. If the permeation rates, q)~ and Ocx, were larger than were determined in the present study, then the obtained recombination rate coefficients, (kR)2 at least, might have been underestimated. In these respects, the authors would like to present aforementioned results as "reference" for future investigations which must be performed under well-characterized surface conditions.

Acknowledgment This study was partly supported by a Grant-in Aid for general research, the Ministry of Education, Science and Culture.

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[4] K.L. Wilson, R.A. Causey, W.L. Hsu, B.E. Mills, M.F. Smith and J.B. Whitley, Beryllium: a better tokamak plasma-facing material?, J. Vac. Sci. Technol. A8(3) (1990) 1750. [5] T. Tanabe, N. Noda and H. Nakamura, Review of high Z materials for PSI applications, J. Nucl. Mater. 196-198 (1992) 11. [6] A.I. Livshits, M.E. Notkin and A.A. Samartsev, Physicochemical origin of superpermeability: large scale effects of surface chemistry on "hot" hydrogen permeation and absorption in metals, J. Nucl. Mater. 170 (1990) 79. [7] A.I. Livshits, M.E. Notkin, A.A. Samartsev and I.P. Grigoriadi, Large-scale effects of H20 and 02 on the absorption and permeation in Nb of energetic hydrogen particles, J. Nucl. Mater. 178 (1991) 1. [8] M. Yamawaki, T. Namba, K. Yamaguchi and T. Kiyoshi, Surface modification of first wall material due to ion implantation and thermal annealing and its effect on hydrogen permeation behavior, Nucl. Instrum. Meth. B23 (1987) 498. [9] M. Yamawaki, K. Yamaguchi, S. Tanaka, T. Namba, T. Kiyoshi and Y. Takahashi, Effect of surface impurities on the hydrogen recombination coefficientof first wall materials, J. Nucl. Mater. 162-164 (1989) 1071. [10] F. Waelbroeck, P. Wienhold and J. Winter, Thermally activated processes in hydrogen recycling, J. Nucl. Mater. 111/112 (1982) 185. [11] M. Yamawaki, T. Namba, T. Kiyoshi, T. Yoneoka and M. Kanno, Hydrogen permeation of vanadium and in situ surface analysis, J. Nucl. Mater. 133/134 (1985) 292. [12] M. Yamawaki, T. Namba, T. Kiyoshi and M. Kanno, Surface effects on hydrogen permeation through niobium, J. Nucl. Mater. 122/123 (1984) 1573. [13] B.L. Doyle, A simple theory for maximum H inventory and release: a new transport parameter, J. Nucl. Mater. 111/112 (1982) 628. [14] M.A. Pick and K. Sonnenberg, A model for atomic hydrogen-metal interactions: application to recycling, recombination and permeation, J. Nucl. Mater. 131 (1985) 208. [15] Y. Fukai, Hydrogen in metals, Bull. Jpn. Inst. Met. 24 (1985) 597. [ 16] K. Yamaguchi, unpublished data. [17] K. Yamaguchi, T. Namba and M. Yamawaki, Surface effects in simultaneous ion- and gas-driven hydrogen isotope permeation of nickel, J. Nucl. Sci. Technol. 24(11) (1987) 915. [18] K. Yamaguchi, S. Tanaka and M. Yamawaki, The modeling of hydrogen transport in metals and its application to the evaluation of hydrogen permeation and inventories, J. Nucl. Mater. 179-181 (1991) 325. [19] V. Bandurko, T. Nagasaki, K. Yamaguchi, and M. Yamawaki, presented at l lth Int. Conf. Plasma-Surface Interactions, 23-27 May 1994, Mito, Japan. [20] T. Nagasaki, R. Yamada and H. Ohno, Recombination coefficients of deuterium on metal surfaces evaluated from ion-driven permeation, J. Nuel. Mater. 191-194 (1992) 258.