富山大学水素同位体科学研究センター研究報告25 : 25‑39, 2005.

論文
清浄Nb表面‑の水素の吸着と偏析波多野雄治1), vasilyAlimov2),AlexanderI.Livshits2),中村松山幸男3),大薮

修義3),

政夫1)

1)富山大学水素同位体科学研究センター〒930‑8555

富山市五福3190

2) Bonch‑Bruyevich University of Telecommunications

61 Moika, St. Petersberg 191186, Russia

3)核融合科学研究所〒509‑5292岐阜県土岐市下石町322‑6

Adsorption and Segregation of Hydrogen on Clean Nb Surface

Yuji Hatano 1), vasily Alimov 2), Alexander Livshits 2), Yukio Nakamura 3),

Nobuyoshi Ohyabu 3)and Masao Matsuyamal)l) Hydrogen Isotope Research Center, University onbyama
Gofuku 3 190, Toyama 93018555, Japan

2) Bonch‑Bruyevich Universlty Of Telecommunications
61 Moika, St. Petersberg 191186, Russia
3) National lnstitute f♭r Fusion Science

Oroshi‑cho, Toki 509‑5292, Japan(Received January 6, 2006; accepted March 30, 2006)

Abstract

Sticking coefficient of H2, a, On a CleanNb surface was measured as afunction of
specimen temperature (25 ‑ 477 oC)and hydrogen uptake. The initial value of sticking
coefficient at negligible hydrogen uptake was ca. 0.25and was independent of surface

temperature. Atany temperature examined, a showed a significant reductionwith increase inhydrogen uptake, although the extent of reduction decreased wi血increaslng temperature.

Such reduction in α was ascribed to the increase inthe surface coverage of hydrogen due to25

波多野雄治･ vasilyAlimov ･ AlexanderI. Livshits ･中村幸男･大薮修義･松山政夫

surface segregationand was explained by a model assumlng equilibrium partitioning Of
hydrogen between the surfaceand the solid solution phase in the bulk.

I. Introduction

Adsorption of hydrogen on a clean Nb surface has been examined by several

researchers l1‑8]. The results obtained by these researchers, however, were notfullyinterpreted. Jolmson et al. ll] measured the sticking coefficient for dissociative adsorption of
H2, a, On a Cleansurface ofpolycrystalline Nb wire in a temperature range from 377 to 925 K･They reported that the initial value or α at negligible hydrogen coverage was 0.13 and was

independent of the specimen temperature l1]. On the other hand, Pick l7] who measured a at

ca. 340 ‑ 500 K on athin Nbfoil preferentially having (110) plane on its surface observed
significant temperature dependence of initial sticking coefrlCientand reported that there is apotential bamier (5.32 kJ/moI H) against dissociative sticking. Besides, Smith l5,6]andStrongin et al. 【8] examined hydrogen adsorption on Nb (110) planes by means ofphotoemission spectroscopy and reported that the growths of hydrogen‑induced photoelectron

peaks were fasterthanthat expected fTrom a simple model assumlng equilibrium partitionlng
of hydrogen between the surfaceand the solid solution phase inthe bulk. In order to interpret
such results, these authors proposed more complicated models assumlng fb‑ations of"near‑surface" distribution of hydrogen l6] or surface hydride l8]. The validities of thesemodels, however, were not fully conflrmed.
Niobiunand other group5

metals (Vand Ta) are suitable materials fTor

superpemeable membranes which can be used f♭r particle control in edge plasma and tritium

recovery in fusion devices l9]. From this viewpoint, the present authors have examined theinteraction of atomicand molecular hydrogen with Nb l10‑13]. In these studies, the surfacesofspecimens

werecoveredby

non一metallic

impurities

such

as

oxygen,becausethe

suppression or reemission by such impurities is essential fb∫ superpe‑eation under exposure

to suprathermal hydrogen particles such as atomsand ions. The teclmiques developed in these26

清浄Nb表面‑の水素の吸着と偏析

experiments, however, can be applied fわr studies of cleansurfTaces.

In this paper, the sticking of H2 molecules on a cleansurface of Nb ribbon

preferentially having (100) plane was examined in a temperature range from 25 to 477 0C by
measuring the speed of H2 Pumping by the specimen･ The initial sticking coefrlCient showed
no slgnificant temperature dependence; the dissociative sticking of H2 Was Virtually

non‑activated process in the temperature range examined･ The kinetics of hydrogen uptake

was well described by a simple model assumlng equilibriumpartitionlng Of hydrogen between
the su血ce and the solid solution phase in the bulk･ The heat of su血ce segregation ofhydrogenand that of adsorptlOn Were evaluated fTrom the dependence of α On the specimen

temperature and hydrogen uptake; the obtained value of the heat of adsorption was close tothat reported in literatures l2,7].

2. Experimental

A dbbon of polycrystalline Nb (7.2
･ 370 × 0.1 mm3) was used as a specimen. In

order to obtain unifbm temperature
distribution during ohmic heating described

below, two Mo sheets (7.3 × 46 × 0.2 mm3)

were spot welded to the both ends of thespecimen ribbon as shownFig. 1. After
polishing the su血ce with abrasive p叩erS,

the specimen was cleaned with acetone in aultrasonic bath and then installed in an

auxiliary chamber ofanultra‑high vacuum
apparatus described elsewhere [10] With anelectric feedthrough as shownin Fig. 2. The
Fig. 1 Schematic description of specimen
pressure of residual gases in the chamber

setup ･

27

波多野雄治･ VasilyAlimov ･ Alexander I. Livshits ･中村幸男･大薮修養･松山政夫

was below I x 1017 pa,and the main component was H2･

The specimen was heated ohmically. Measurements of voltage‑current (V‑I)
characteristics allowed the evaluation of specimen temperature fTrom (1) resistivity l14],and(2) emissivity 【14] by assuming that the inputted power was balanced against the emission･

The values evaluated by these two methods were slightly different fTrom each other below

500 oC as shownin Fig. 3, although fairly good agreement was obtained in higher temperature
reg10n. Hence, in the region below 500 oC, the specimen temperature was evaluated from thedashed line cormecting data point for room temperature ( V‑ I ‑ 0)and that fわr 500 oC･

A cleansurface was prepared by repeated heating of the specimen in ultra‑highvacuum up to 2100 0C fわr durations of 10‑30 S; the total heating time was ca. 10 ks. Such heat

treatment in ultra‑high vacuum is commonly employed to obtain a clean Nb su血ce [1 5]･ The

M

Specimen eW )rt＼▲ /ariable aphragm

I
l

<

H2

QMS

I

I

暮l

1･p
Fig. 2

Schematic descriptlOn Ofvacuum chamber; TMP: turbo‑molecular pump,and

QMS: quadrupole mass spectrometer.

28

清浄Nb表面への水素の吸着と偏析
exposure of prepared clean(uo)Sl'31n7!13dutauP3dsJou!tmpt

丘V

5

0

00

0

4

30

00

0

surfTace to the residual gases led tono significant change in the

sticking coefficient of H2; theinfluence o f surfhc econtamination by residual gases

was negligibly small under thepresent conditions.

つム一 l

0

00 0 0The sticking coefficientof H2, a, Was measured byabsorption experiment in thetemperaturereglOn

fTrom

0

25

2

4

6

8

10

′(A)
(controlled room temperature) toFig. 3477 oC in thefollowlng maJmer.
First, pumplng Of the chambers

Correlation between electric current, I,and

specimen temperature evaluated&omresistivity( △ )andemissivity ( ● ).

was stopped by closing valves,

and the surface was deactivated by the exposure to residual gases at rather high pressure(10 4 pa) at room temperature. Then the chamber was evacuated to ultra‑high vacuum again

by a sputter‑ion pump (IP),and H2 gas Was introduced through a variable‑leak valve (VLV) at

knownnow rates (10‑8 ‑ 1017 pa･m3/S) to pressures in the order of lO 6 pa. After theestablishment of stable pressure, the H2 introduction was stopped by pumplng the backside of

the VLV, in which the openlng Of VLV was kept constant. Then, the cleansurface wasprepared by heating the specimen fわr a short period of time (activation). Hydrogen gas wasintroduced into the chamber at the sameflOw rate as befわre but in stepwise by supplying H2

gas of the same pressure to the backside of VLV. The pressure of H2 in the chamber did notreach the previous value due to H2 absorption (i. e. pumping) by the specimen. The sticking
coefficient α was estimated from this pressure difrTerence.

29

波多野雄治･ vasilyAlimov ･ Alexander I. Livshits ･中村幸男･大薮修義･松山政夫

3. Results and discussion
Theanalyses by means of X‑ray di飴action (XRD)and electron backscatter

diffraction (EBSD) showed that the specimen surface mainly consisted of Nb(100) plane(〜90%) inclined at 0.8‑1.90 to the surfaces,and also of Nb(211) (〜10%) plane inclined at‑2.20. Hence, the results described below mainly indicate the characteristics of Nb(100)plane.

A typical result of absorptlOn experiments obtained at 25 0C is shown in Fig. 4. When
H2 gas Was introduced into the chamber aRer the activation treatment, the pressure of H2
immediately reached to 6･2 x 10‑7 pa, while that before the activation was 4･O x 10‑6 pa at thesame flow rate ofH2 gas. This difference in H2 Pressure Was due to pumping by the specimen.
The sticking coefficient α was evaluated from this extent of pressure difference with thefollowlng equation:

sp冶蒜p. ‑p2
α=

(1)

両｢｢

where Sp is the pumplng speed by IP typically controlled to be 0.12 m3/s by adjusting theposition of variable diaphragm shownin Fig. 2, m is the molar mass ofH2 molecules, A is the

54i壬 beforeactivation 剿ﾂ B ﾓ

ﾓg(cJ9･0‑):JPuCtJ

ﾂ

1■

兮

(250C)‑

l afteractivation 剪

3 つ山一

3r

白

/I

T‑ 梯 #都 2 ﾖ F柳ﾖ "＼%(250C)‑0.27

0

500

ll

1 000

剪

1 500

FW&ﾖ

2000

唯

2500

7

V6蒙6諞V

3000

3500

J(〜)

Fig. 430

Typical result ofabsorpt10n experiment at 25 oC.

F匁r

ﾂ

4000

4500

清浄Nb表面‑の水素の吸着と偏析
surface area of the specimen (5400 mm2), R is the gas constant, Tg is the gas temperature, Pl

is the H2 Pressure befTore the activationand P2 is that aRer the activation. The initial value of
sticking coefficient a immediately after the stepwise introduction of H2 Was evaluated to beo･27 in this case･ The pressure of H2 gradually lnCreaSed to 2･4 × 10‑6 pa with time,∫, asshown in Fig. 4. Such increase in H2 pressure COuld be ascribed to either of(1) reduction in α

due to saturation of surface sites,and (2) increase in reemission rate due to accumulation ofhydrogen in the specimen. In order to understand the mechanism of this pressure increase, thespecimen was heated to 297 oC at ∫ ‑ 3450 S. The increase in specimen temperature resulted in

the reduction in H2 pressure tO l･0 × 10‑6 pa･ This reduction in H2 pressure Was ascribed to theincrease in α due to reduction in surface hydrogen coverage caused by the enhancement ofdissolution of hydrogen atoms fTrom the surface into the bulk. Namely, this observation
indicates that the increase in H2 1iressure with t was due to the reduction in αand not causedby the increase in reemission. The mechanism underlying the pressure increase with ∫ was

also examined by stopping the H2 gas introduction (not shownin the figure). The pressure ofH2 immediately dropped to the background level aRer the interruptlOn Of H2 introduction,
indicating no significant hydrogen release from the specimen. The value of a corresponding

to p2 0f2.4 x l0‑6 pa was evaluated to be 0.037 by Eq. (1). Namely, the value of a was
reduced by an order of magnitude.

Such change in a with i was measured in the same manner up to 200 oC. At higher
temperatures, slgnificant reemission of absorbed hydrogen was observed after accumulationof hydrogen in the bulk. Namely, H2 pressure did not reach the background level when the
introduction of H2 gas Was interrupted. Therefわre, only initial values of α were obtained

above 200 oC. The values of initial sticking coefficient, 〜, thus obtained are plotted against

the specimen temperature, Ts, in Fig. 5. The value at the highest Ts (1520 oC) was evaluatedfrom the rate of dissociation (atomization) [10]. Interestingly,物Showed no significant

dependence on Ts in a verywide range of Ts. This observation agreed with the results
obtained by Jolmson et al. ll] in a narrower temperature range (377 ‑ 925 K). Namely, in

31波多野雄治･ ⅦsilyAlimov ･ AlexanderI.Livshits ･中村幸男･大薮修義･松山政夫㌔ (oC)
1500

500400300

200

100

25

100

■
■
･‑日日11‑I‑I‑‑‑‑i‑‑‑‑‑‑‑.‑‑

d> 10‑I

0.5

1.0

1.5

2.0

2.5

3.0

3.5

1000 / Ts (K‑I)

Fig･ 5Temperature dependence of initial sticking coefrlCient,句･Contrast tothe study of Pick l7], no evidence for the presence of activation barrier wasobserved. The average value of q) was calculated to be O･25and was comparable to the value
reported by Johnson et al･, 0･13 [1]･
In Fig･ 4, the pressure ofH2 Was reStOred丘om l･0 × 10･6 pa to the previous value,

2.4 × 10‑6 pa, within 600 s a鮎r the temination of specimen heating, while it took more than

2700 s in the initial stage (i. e. from 250 to 3000 S). Namely, the duration of time required forthe pressure increase was slgnifiCantly reduced by the accumulation of hydrogen in thespecimen bulk･ This observation indicates that the surface concentration of hydrogen wasdetermined by the surface segregation from the bulk and not by the incident flux of H2fromthe gas phase. According to the data on diffusion coefficient of hydrogen in Nb l16], theabsorbed hydrogen atoms difnlSe along a distance comparable to the specimen thickness
within 1 s. Hence, it is appropriate to consider that the equilibrium is readily attained betweenhydrogen atoms on the surfaceand those in the bulk･32

清浄Nb表面‑の水素の吸着と偏析

When the equilibrium is attained, and the bulk hydrogen concentration is enough low,
the correlation between hydrogen concentration on the surfaceand that in the bulk canbe
expressed as fわllows:

A‑ cHKo exp,普),
トβH

(2)

where免is the surface coverage of hydrogen, CH is the bulk concentration, Ko is the entropy

factor, and AHseg is the heat of surface segregation･ By assumlng that two adjacenthydrogen‑fTree surface sites act as the active center for H2 adsorptlOn, α On hydrogen‑Covered

surface can be described asα‑喝(1‑恥)2.

(3)

Hence, the value of & can be obtained as
1

免‑ ll(ヱ)ラ.

(4)

αo

The specific amount of hydrogen taken up perunit surface area (H atoms/m2), qut, isdescribed as

qut‑jh免+A,CH

X

d/2,

(5)

in which /奄and ih are areal density (9.2 × 1018 atoms/m2 for (100) plane)and volume density(5.6 x 1028 atoms/m3) ofNb,and d is the specimen thickness. The value ofqu. Canbe easily
evaluated from Fig. 4 with the followlng equation:

qut

‑忍KPl

‑P2(i,,dt･(6,

Typical examples of changes in qut,免and CH With t at 25 0C thus evaluated areshownFigs. 6and 7. Although the pressure ofH2 Was COmParable, the development of蝕wasmuch s一ower thanthat shownby Strongin et al. (Fig. 1 in Ref. [8]), in which免reachedthemaximum value within 200 s at 2 × lol阜 To,, (2.7 × 10‑6 pa). Two possible mechanisms canbe

proposed f♭r this discrepancy; (1) the intensity of hydrogen‑induced photoelectron peakexamined in their study is not simply proportional t0 % or (2) the characteristics of(100)and33

波多野雄治･ vaSilyAlimov ･Alexanderl. Livshits ･中村幸男･大薮修義･松山政夫(Ntl/SuO"6TOT)Pb
rム一

l
●

●

0

5

0

200

400

600

g(S)

Fig･ 6

Change inhydrogenuptake qut with time t at 25 oC･

0.7

0.6

(uddt!)HD

0.5
よV

0.4
､‑

0.3

0.2

0.1

0.0
0

1 00

200

300

400

500

ペS)
Fig. 734

Changesin免andCHWithtat25oC･

600

4

清浄Nb表面‑の水素の吸着と偏析(1 10) planes are diffTerent fTrom each other. Final conclusion, however, has not been derived.

According to the present model, the rate of hydrogen uptake, dqut / dt, Canbe
expressed as fわllows:dqut

2ao(1‑OH)2

dt

P2(t) ･

J両

(7)

Namely, the rate of hydrogen uptake, dqut / dt, should be proportional to (11%)2･ p2(t). In
order to check the validity of the present model, (dqu. / dt) is plotted against (I‑a)2･ p2(t) in
Fig. 8 which clearly shows that (dqu. / dt) is in proportion to (1一触)2･ p2(t). It was therefore

concluded that the sticking of H2 COuld be described by the above一mentioned simple model

assumlng the equilibrium partitionlng Of hydrogen between the surface and the solid solution

phase in the bulk under the present conditions.

一uノ(S/∩ u/H,.OT)7P/'nbp

8 7 /hV

5 4

3

2
7

8

9

10

11 12

13

14

15

16

17

18

(1‑eH)2 p2(i) (10‑7 pa)

Fig. 8Correlation between the rate of hydrogen uptake, qu. / dt,and (1‑a)2･p2(t) at 25 oC.

35波多野雄治･ VasilyAlimov ･ AlexanderI. Livshits ･中村幸男･大薮修義･松山政夫

Here, Eqs. (2)and (5) yield‑::完..:'≠ チ

AHseg‑RTs x ln

1,]

･

(8,

[
The values of AHseg evaluated with Eqs･ (4)and (7) by assuming K0 ‑ 1 are plotted against ain Fig･ 9; AHseg was determined to be 29 kJ/mol at 25 0C and to be ca･ 38 kJ/mol at higher

temperatures. slight reduction in AHseg with increasing a was observed at elevatedtemperatures, while no significant dependence on a was observed at 25 oC･ Such distinct
tendency obseⅣed at 25 0c may be due to the su血ce reconstmction such as ordering ofadsorbed hydrogen atoms, but the mechanism underlying this difference has not been fully
clarified.

Figure 10 shows the correlation between Wand qut at 25 oC as an example･ The
sticking coeaicient α started to drop at qut ‑ 1018 H/m2and became 1/10 0finitial value at 2 ×

1019 H/m2. According to Eq. (4), ab/α becomes 0.1 at蝕‑ 0･684 and consequently at Fb免‑

50 40

ll 1､㌔欄間.心.. ●.+.●●●●●‑●●‑●一一 L ■■■ql<.PM‑塾狩野抱.‑L岱祈 .〟‑,.‑.‑rTγ;r<:PrLに̲て迅野SET,TTL茄茄正郡空室璽聖和佃=協 免ﾂ OｩF 亊 ?

闔ｨ而闔｢ 耳耳簽薑2ﾓ

s 2ﾅ ﾃﾓr縢 ﾃ ﾓg

ﾃﾓG

(10uJrq)3 SHVI‑

3 勺ム一 0

0

Eニ正L7(T‑‑TTTLT:'TTT=r■●Ts‑250C,P1‑3.7×10‑6pa

oTs=25○C,P1‑7.6×10‑6pa 貞G3ﾓ
･Ts‑117○C,P.‑5.3×10‑6pa ll 貞G2ﾓ

To.o

o.2

c#

0.4

2ﾅ

0.6

s
ﾃﾓB

2ﾅ
ﾃ

ﾓ2纓
ﾖ

0.8

ﾆﾂ

1.0

eH

Fig. 936

Correlation between AHseg and q obtained under various conditions･清浄Nb表面への水素の吸着と偏析

0.40
0.35

0.30
0.25
eS

0.20

0.15

0.10
0.05
0.00

1017

1018

1019

qut (H atoms / m2)

Fig. 10

Change inqutwith蝕at25 oC･

6.3 × 1018 H/m2. Hence, ihCH X d/ 2 ‑ 1.4 × 1019 H/m2 (Eq. (5)) where CH is detemined tobe 5 appm. On the other hand, according to Eq･ (2),蝕becomes O･684 at CH = 5 appm when

AHseg ‑ 32 kJ/mol. This value of AH,eg is close to that determined by Eq･ (8), i･ e･ 29 kJ/mol･This consistency also shows the validity of the present modeL

The heat of surface segregation AHseg corresponds to the enthalpy difference between
adsorptlOn State On the surface sitesand dissolution state in the bulk interstitial sites･ On theother hand, the heat of solution AHs.1 Which is reported to be 34 kJ/molfor H‑Nb system l17]
is defined to be the enthalpy difference between hydrogen atoms in the state of H2 moleculeand the dissolution state. Therefore, the heat of adsorption AHad COrreSPOnding to the enthalpy
difference between the state of H2 mOleculeand the adsorption state is obtained as AHad =AHs.I. AHseg ‑ 72 kJ/moI H (elevated temperatures) or 63 kJ/moI H (25 oC)･ These values arecomparable to that reported by Hagenand Donaldson l2] (56 kJ/moI H),and Pick l7] (56･6
kJ/moI H).

37

波多野雄治･ VasilyAlimov ･ Alexander I.Livshits ･中村幸男･大薮修義.松山政夫

4. Conclusions(I) Initial value of sticking coefficient of H2,物, On a CleanNb surface obtained at gas

temperature of 25 oCand specimen temperature of 25 ‑ 280 0C was about 0.25 and was
independent of surface temperature; no evidence for the presence of activation barrier

agalnSt Sticking was obseⅣed.

(2) The sticking coefficient, a, significantly decreased with increasing hydrogen uptake,and
this reduction in α was well described by a simple model assumlng the equilibrium

partitionlng Of hydrogen between the surfaceand the bulk･(3) The heat of surface segregation of hydrogen was evaluated to be 29 kJ/mol at 25 oCand
38 kJ/mol at elevated temperatures.

(4) The value of heat of adsorption of hydrogen obtained fTrom the heat of surface segregationand that of solution agreed with a value reported in literatures l1,7]･

Acknowledgements
This work has been supported in part by a Grant‑in‑Aid for Young Scientists (A) (No･14702068)and the NIFS LHD Project Research Collaboration (NIFSO4KOBROOl)･ The

authors express their sincere thanks to Professor S. Ikenoand Professor K. Matsuda ofFaculty of Engineenng, Universlty Of Tbyama f♭r the use of EBSD device･

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[2] D. I. HagenandE. E. Donaldson, Surf. Sci., 45 (1974) 61.

[3] S. M.KoandL.D. Sclmidt, Surf. Sci.,42 (1974)508.
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286.
[5] R.∫. Smith,Phys.Rev. B,21 (1980)3131.

[6] 良. ∫. Smith,Phys. Rev. Lett., 45 (1980) 1277.

38

清浄Nb表面‑の水素の吸着と偏析17] M.A. Pick, Phys. Rev. B, 24 (1981) 4287.
[8] M･ Strongin, J･ Colbert, G･ J･ Dienes,and D. 0. Welch, Phys. Rev. B, 26 (1982) 2715.
[9] A･ Ⅰ･ Livshits, Y･ Hatano and K. Watanabe, Fusion S°i. Techno1., 41 (2002) 882.

[10] Y･ Hatano, M･ Nomura, K･ Watanabe, A. I. Livshits, A. 0. Busnyuk, Y. Nakamuraand N.

Ohyabu,Am･ Rep･ Hydrogen Isotope Res. Ctr. Toyama Univ., 21 (2001) 13.
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(2003)113.

[12] Y･ Hatano, A･ Livshits, A･ Busnyuk, M. Nomura, K. Hashizume, M. Sugisaki, Y.

Nakamura, N･ Ohyabu and K. Watanabe, Phys. Scr., TIO8 (2004) 14.
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and K･ Watanabe, Fusion Eng. Design, 81 (2006) 771.

[14] W･ Espe, Werkstofrkunde der Hochvakuumteclmik, VEB Deutscher Verlag derWissenscha氏en, Berlin, 1 959.

[15] H｡ H･ Farrelland M. Strongin, Surf. Sci., 38 (1973) 18.

t16] J･ V61kland G･ Alefeld, "Hydrogen in Metals I" (Edited by G. Alefeldand J. V61kl)(Springer, Berlin, 1978), p.329.

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39