ABSORPTION OF ATOMIC HYDROGEN BY VANADIUM

ABSORPTION OF ATOMIC HYDROGEN BY VANADIUM

Yuji Hatano1, Andrei Busnyuk2, Alexander Livshits2, Yukio Nakamura3, Masao Matsuyama1 1

2

University of Toyama, Gofuku 3190, Toyam 930-8555, Japan Bonch-Bruyevich University, 61 Moika, St. Petersberg 191186, Russia 3 National Institute for Fusion Science, Toki 509-5292, Japan * [email protected]

In order to understand the capability of vanadium panels and membranes for fuel particle pumping at relatively low temperatures, absorption of neutral hydrogen atoms by vanadium sheet was examined at/below 350 ºC under wide variety of experimental conditions. A niobium sheet kept at high temperature (420 ºC) was used as a reference specimen. Sufficiently high absorption rates were obtained even at around room temperature in the range of incident fluxes from 1017 to 1021 m-2s-1. No noticeable reduction in absorption rates was observed up to the H retention level of 0.1 at%. The influence of CO and water vapor was negligibly small up to an exposure of 1023 m-2. Significant reduction in the absorption rate was observed only when an oxide film was formed on the surface by exposure to O2 to 1020 m-2 and to H2O over 1023 m-2 at room temperature.

I. INTRODUCTION Hydrogen permeable membranes and absorption panels made of group 5 metals (V, Nb and Ta) are promising means for fuel particle pumping in edge plasmas in fusion devices.1-3 The membranes are suitable for steady-state operation in fusion reactors, whereas periodically-regenerated panels can be used for intermittent operations in existing devices. From these viewpoints, Nakamura et al.3 have examined the absorption of hydrogen atoms by a Nb panel in detail and observed sufficiently large absorption rate (1.3 × 1020 H m-2s) at panel temperatures above 200 ºC. In the lower temperature region, however, the absorption rate sharply decreased with decreasing panel temperature; the absorption rate observed at 90 ºC was a half of the value at 200 ºC, and that at room temperature was evaluated to be 1/10 by extrapolating the obtained data. The membranes and panels could be used at sufficiently high temperatures in future fusion reactors. The operation tests in existing fusion devices with cold walls, however, can be seriously disturbed by such reduction in the absorption rate at low temperatures. FUSION SCIENCE AND TECHNOLOGY

VOL. 52

OCT. 2007

Significant reduction in absorption rate of H atoms with decreasing temperature has been also observed for Ni by Livshits4 in the temperature region below 400 ºC. He measured atom-driven permeation of hydrogen at different incident flux of H atoms and reported that the extent of reduction increased with incident flux. This observation was explained by the enhancement of recombinative desorption from upstream surface due to increase in hydrogen concentrations on the surface and in the bulk. Krenn et al.5 have examined the absorption of hydrogen atoms with a single crystal specimen of V in the temperature range from 90 to 700 K. They also observed the reduction in absorption rate below 200 ºC, but the extent of reduction was moderate in comparison with the observations of Nakamura et al.3 and Livshits4. Namely, the absorption rate at around room temperature was about 2/3 of the value at 200 ºC. The flux dependence of absorption rate, however, was not examined in their study. Not only hydrogen but also the coverage by impurities such as water vapor can increase with decreasing surface temperature. In addition, the growth of oxide film could be possible at low temperatures, whereas continuous dissolution of oxygen into the bulk prevents the growth of such oxide layer at high temperatures.6,7 Surface analysis, however, was not possible in the apparatus used by Nakamura et al.3 and Livshits4. In the present paper, the absorption rate of atomic hydrogen by vanadium sheet was measured at relatively low temperatures ( 350 ºC) in wide ranges of incident flux (from 1017 to 1021 H m-2s-1) and hydrogen concentration in the bulk (from 10-4 to 0.1 at%). The influence of exposure to H2O, CO and O2 gases on the absorption rate was also examined in combination with surface analysis by means of X-ray photoelectron spectroscopy (XPS).

II. EXPERIMENTAL Atom-driven hydrogen absorption experiments were carried out in an ultra-high vacuum apparatus evacuated

613


with a magnetically-suspended turbo molecular pump (TMP) and oil-free scroll pump (Fig. 1). The base pressure was lower than 1×10-7 Pa. This apparatus was equipped with Mg-KD X-ray source (1253.6 eV) and a concentric hemispherical analyzer (CHA) for X-ray photoelectron spectroscopy (XPS). A quadrulepole mass spectrometer was also installed for partial pressure measurements. Hydrogen gas was introduced through a variable-leak valve as well as H2O, CO and O2 gases. Atomic hydrogen was generated by incandescent filament made of Ta. A shield made of Mo was installed in front of the filament to avoid the deposition of Ta onto the surfaces of specimens. A vanadium specimen used was 313×7.15×0.1 mm in size and 99.9% in purity. The specimen was electrically isolated from the chamber and heated ohmically. The temperature of the specimen was measured with a thermocouple spot-welded on the edge of the specimen. A sheet of Nb (99.9% purity) which was also heated ohmically was installed as a reference specimen. Absorption coefficient of H atoms, DH, by V specimen was measured in the following manner. Here DH is defined as the probability that H atom is absorbed by the specimen at a single collision to the surface. First, surface conditioning of the V specimen was carried out by heating at 500 ºC in vacuum to remove socalled natural oxide film through the dissolution of oxygen into the bulk. The photoelectron spectra obtained after this heat treatment is shown in Fig. 2. The binding energies of V 2p electrons were close to the values reported by Schiechl and Winlker6 for V(100) surface covered by monolayer of oxygen [(5×1)-O superstructure]. The temperature of V specimen was strictly controlled not to exceed 500 ºC to prevent the modification in surface state due to segregation of sulfur.8 The Nb reference specimen was also subjected to surface conditioning by heating in vacuum to 1000 ºC. It is known that the surface with monolayer oxygen coverage can be prepared by this heat treatment. 7 Then both V specimen and Nb reference specimen were exposed to hydrogen atoms. The incident flux of H atoms was controlled by adjusting H2 pressures in a range from 4×10-5 to 0.3 Pa. The temperature of V specimen was varied from room temperature to 350 ºC; the specimen temperature where atom-driven absorption experiments were carried out is hereafter denoted as Tab. In the low temperature region, it was impossible to maintain constant specimen temperature because of radiation from Ta filament. For example, the specimen temperature increased to 70 ºC during atom-driven absorption when initial temperature was 30 ºC. The specimen temperature in such conditions is hereafter denoted as Tab = 30-70 ºC. In the case of the Nb reference specimen, the temperature was kept at 420 ºC. It is known

614

X-ray source

H2, O2, CO, H2O

CHA

V specimen Ta filament Nb reference specimen

TMP

Mo shield

Scroll pump

Fig. 1 Schematic description of apparatus used for atom-driven absorption experiments.

Photoelectron intensity (arb. unit)

Hatano et al.

㩷

V LMM

V 2p

V 2p3/2

O 1s

V 2p1/2

V 2s 528

524

520

516

512

508

㩷

V 3p V 3s 800

600

400

200

0

Binding energy (eV) Fig. 2 Photoelectron spectrum of V specimen after heat treatment at 500 oC in vacuum. Inserted figure is the enlarged spectrum of V 2p region. that the H atom absorption coefficient of Nb is a very stable value (around 0.25) at this temperature.7 After exposure to H atoms for a given period of time, the amounts of hydrogen absorbed by V specimen and Nb reference specimen were measured in turn by a technique of thermal desorption; the V specimen and the Nb reference specimen were heated in vacuum to 500 and 1000 ºC, respectively. Then the amount of H atoms absorbed by V specimen was compared with that by Nb reference specimen to cancel the influence of uncontrollable change in H atom flux due to modification in surface states of Ta filament and chamber walls. The temperatures at which the specimens were heated for desorption were the same as the temperatures of surface conditionings. Therefore, the surface states of specimens

FUSION SCIENCE AND TECHNOLOGY

VOL. 52

OCT. 2007

0.25

DH

0.20 0.15

㩷

were automatically “initialized” for subsequent experiments. In the present study, the incident flux of H atoms was not measured directly. It is, however, known that DH of Nb is stable at the value around 0.25 as described above.7 Hence, the flux of H atoms was roughly evaluated from the hydrogen concentration in Nb reference specimen by assuming DH = 0.25. The incident flux is overestimated if the real value of DH for Nb is larger than 0.25. The possible extent of overestimation, however, is 4 (= 1/0.25) times even in the maximum case and not in orders of magnitude. The surface of V specimen was analyzed by means of XPS at room temperature before and after each absorption-desorption experiment. The chemical composition of the surface was evaluated with the relative sensitivity factors recommended by Briggs and Seah9 by assuming that the value of sensitivity factor is independent of the chemical state of the corresponding element. Influences of CO, H2O and O2 gases were examined by exposing the specimen to these gases at room temperature. The pressures of these gases ranged from 4×10-6 to 1×10-2 Pa. The duration of exposure was 3.6 ks for CO and H2O, and 600 s for O2. The extent of exposure was controlled by adjusting the pressure of these gases. After the exposure, the surface of V specimen was analyzed by means of XPS, and then atom-driven absorption and desorption experiments were carried out in the above-mentioned manner. The Nb reference specimen was heated in vacuum to 1000 ºC prior to the absorptiondesorption experiments to eliminate the influence of exposure to the impurity gases.


0.10 0.05 0.00

0

50 100 150 200 250 300 350 400 o Tab (㩷 C)

Fig. 3 Dependence of absorption coefficient of hydrogen atoms, DH , on temperature of V specimen. 㩷

0.25 0.20 0.15 DH

Hatano et al.

0.10 Tab = 30 - 70 qC

0.05 0.00

17

10

18

19

20

21

10 10 10 10 -2 -1 Hydrogen atom flux (H m s )

Fig. 4 Correlation between DH and incident flux of hydrogen atoms. 㩷

0.25

III. RESULTS AND DISCUSSION III. A Dependence of DH on Incident Flux and Bulk Hydrogen Concentration


VOL. 52

OCT. 2007

㩷

DH

0.15

㩷

Fig. 3 shows the temperature dependence of DH for V obtained at incident flux of 1×1017 Hm-2. In the low temperature region where Tab increased during absorption due to the radiation from the filament for atomization, DH is plotted against the maximum value of Tab: e. g. 70 ºC for Tab = 30-70 ºC. The values of DH for V were determined by assuming that DH for the Nb reference specimen was 0.25 as mentioned above. The absorption coefficient decreased with decreasing temperature. Interestingly, the extent of reduction was significantly smaller than the previous observations for Nb3 and Ni4, and comparable to that of Krenn et al.5 for V(100) specimen covered by carbon (0.6 monolayer, ML) and oxygen (1.3 ML). The mechanism underlying this reduction is discussed later.

0.20

0.10

Tab = 30 - 70 qC

0.05 0.00 -4 10

-3

-2

-1

10 10 10 H concentration (at. %)

0

10

Fig. 5 Correlation between DH and hydrogen concentration in the bulk of V specimen. The dependence of DH on incident flux at Tab = 30-70 ºC is shown in Fig. 4. Although the incident flux was varied over 4 orders of magnitude, DH showed no significant change. In contrast to the present results, the

615

Hatano et al.

ABSORPTION OF ATOMIC HYDROGEN BY VANADIUM 㩷

III. B Influence of Impurities The change in DH by exposure to impurity gases is shown in Fig. 6. No significant effect of CO gas was observed up to the exposure of 4×1023 m-2. In the case of water vapor, gradual reduction was observed up to 1×1023 m-2, and it was followed by sharp drop at higher exposure. The influence of O2 gas was much stronger; DH started to drop even at the exposure of 1020 m-2. Such large difference between water vapor and O2 gas, however, was not observed in the plot ofDH against surface oxygen coverage (Fig. 7); the comparable values of DH were obtained at the same oxygen coverage. The difference between water vapor and O2 gas shown in Fig. 6 was attributed to that in reaction probabilities between these species on oxygen-covered V surface at room temperature. Fig. 8 shows V 2p photoelectron spectra after exposure to O2 gas up to 4×1020 m-2 and that to water vapor up to 5×1024 m-2 together with the spectrum before

616

㩷

H 2O

O2

0.05 0.00 19 20 21 22 23 24 25 10 10 10 10 10 -2 10 10 Exposure (m ) Fig. 6 Change in DH with exposure to impurity gasesat room temperature. 0.25

Tab = 30 - 70 qC

0.20

(2)

where TH is the surface coverage of hydrogen, CH is the concentration in the bulk, K0 is the entropy factor, and 'Hseg is the heat of surface segregation. In principle, surface hydrogen coverage should increase with increasing bulk hydrogen concentration and decreasing temperature. In fact, reduction in DH with accumulation of hydrogen was clearly observed by Krenn et al.5 at 90 K. The independence of DH on the bulk H concentration at 30-70 ºC shown in Fig. 5 indicates that 'Hseg of hydrogen on oxygen-covered V surface is rather small.

CO

0.10

0.15 DH

'Hseg CH K0 exp( ), RTS

0.15

0.10 Oxidation in O2 Oxidation in H2O

0.05 0.00 0.0

0.2 0.4 0.6 0.8 1.0 Oxygen coverage ([O]/[V])

1.2

Fig. 7 Correlation between DH and surface oxygen coverage. Photoelectron intensity (arb. unit)

TH 1 TH

Tab = 30 - 70 qC

0.20

DH

flux dependence in DH was observed for Ni as described above.4 The observations in Fig. 4 clearly indicate that the mechanism underlying the reduction in DH with decreasing temperature shown in Fig. 3 was different from that proposed for Ni: the enhancement of recombinative desorption due to increase in hydrogen concentrations on the surface and in the bulk.4 One possible explanation for this difference between V and Ni is the lower activation barrier for hydrogen migration in V than in Ni10 which mitigates the accumulation of hydrogen on/in the surface/subsurface. In Fig. 5, DH at Tab = 30-70 ºC is plotted against the bulk hydrogen concentration evaluated from the total amount of desorbed hydrogen. The absorption coefficient for hydrogen atoms showed no systematic change with accumulation of hydrogen in the bulk. According to the conventional model of surface segregation, correlation between surface hydrogen coverage and bulk concentration is expressed as follows (Langmuir-McLean equation11):

0.25

V 2p1/2

V 2p3/2

(a) (b) (c) 528

524

520 516 512 Binding energy (eV)

508

Fig. 8 Change in V 2p photoelectron spectra by exposure to O2 gas or water vapor at room temperature; (a) before exposure, (b) 4×1020 O2 m-2, and (c) 5×1024 H2O m-2. exposure. After exposure to O2 gas or water vapor, V 2p3/2 peak shifted from 512.4 to 512.9 eV, indicating formation


VOL. 52

OCT. 2007

Hatano et al.

of VO.12 In addition, shoulders were observed at the binding energy corresponding to V2O3 (515.0-515.9 eV12,13). Hence, it was concluded that the sharp drop in DH shown in Figs. 6 and 7 is due to the growth of the oxide layer (VO and V2O3) which prevents the penetration of hydrogen atoms into the bulk. Such reduction in DH with the growth of oxide layer was also observed by Schiechl and Winlker6. Since the concentration of O2 in the residual gas was negligibly small, and the effects of H2O and CO were rather weak (Fig. 6), it was difficult to ascribe the reduction in DH with decreasing temperature shown in Fig. 3 to the influence of residual gases. Krenn et al.5 and the present authors attempted to explain such temperature dependence by the activation barrier for migration of hydrogen atom from the surface to the bulk, but it was not successful. Further investigation is necessary to understand the mechanism underlying this phenomenon. The reason for the difference between V and Nb3 has not been fully clarified, either. Preliminary experiments, however, have been carried out for V and Nb under the same conditions by using the apparatus employed in the previous study.3 Smaller extent of DH reduction with decreasing temperature was observed for V also in this case. The present results clearly indicate that no severe reduction in DH would takes place for V unless the oxide layer is grown. It is known that partial pressure of water vapor in fusion machines can be reduced by wall conditioning such as Ti evaporation and boronization.14 The operation tests of membrane/panel at low temperatures in existing fusion devices could be possible by such wall conditioning and/or periodical surface conditioning.


ACKNOWLEDGMENTS This study has been supported in part by the NIFS LHD Project Research Collaboration, NIFS04KOBR001.

REFERENCES 1.

2.

3.

4. 5.

6. 7.

8.

9.

IV. CONCLUSIONS The hydrogen atom pumping by vanadium was examined at relatively low temperatures ( 350 ºC) by using Nb reference specimen kept at 420 oC. The absorption coefficient of H atoms, DH, showed slight reduction with decreasing temperature. The extent of reduction was comparable with the study of Krenn et al.5, and significantly smaller than the previous observations for Nb3 and Ni4. The value of DH around room temperature was independent of the incident flux in the range from 1017 to 1021 m-2s-1 and bulk hydrogen concentration up to the level of 0.1 at%. The exposure to CO and water vapor at room temperature resulted in no significant change in DH up to 1023 m-2. Serious reduction in DH was observed only when the oxide film was formed on the vanadium surface by exposure to O2 to 1020 m-2 and to H2O over 1023 m-2.


VOL. 52

OCT. 2007

10.

11. 12.

13. 14.

A. I. LIVSHITS, F. SUBE, M. N. SOLOVYEV, M. E. NOTKIN, and M. BACAL, J. Appl. Phys., 84, 2558 (1998). Y. NAKAMURA, S. SENGOKU, Y. NAKAHARA, N. SUZUKI, H. SUZUKI, N. OHYABU, A. BUSNYUK, M. NOTKIN, A. LIVSHITS, J. Nucl. Mater., 278, 312 (2000). Y. NAKAMURA. I. LIVSHITS, Y. NAKAHARA, Y. HATANO, A. BUSNYUK and N. OHYABU, J. Nucl. Mater., 337-339, 461 (2005). A. I. LIVSHITS, Sov. Tech. Phys. Lett., 3, 236 (1977). G. KRENN, C. EIBL, W. MAURITSCH, E. L. D. HEBENSTREIT, P. VARGA and A. WINKLER, Surf. Sci., 445, 343 (2000). H. SCHIECHL and A. WINKLER, Fresenius J. Anal. Chem., 371, 342 (2001). Y. HATANO, A. LIVSHITS, Y. NAKAMURA, A. BUSNYUK, V. ALIMOV, C. HIROMI, N. OHYABU and K. WATANABE, Fusion Eng. Design, 81, 771 (2006). R. HAYAKAWA, K. NISHINO, Y. HATANO, V. ALIMOV, A. I. LIVSHITS, S. IKENO, Y. NAKAMURA, N. OHYABU and K. WATANABE, Annual Report of Hydrogen Isotope Research Center, University of Toyama, 24, 9 (2004). D. BRIGGS and M. P. SEAH, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy (Japanese edition), p. 267, Agne, Tokyo (1990). J. VÖLKL and G. ALEFELD, Hydrogen in Metals I, p. 326 and 333, Springer-Verlag, Berlin Heidelberg (1978). D. McLEAN, Grain Boundaries in Metals, p. 116, Clarendon Press, Oxford (1957). C. N. R. RAO, D. D. SARMA, S. VASUDEVAN and M. S. HEGDE, Proc. R. Soc. Lond. A, 367, 239 (1979). G. A. SAWATZKY and D. POST, Phys. Rev. B, 20, 1546 (1979). W. POSCHENRIEDER, K. DESINGER, THE ASDEX TEAM, J. Nucl. Mater., 176&177, 381 (1990).

617