富山大学水素同位体科学研究センター研究報告 32：9－20，2012.

論文
照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす
Pd 表面被覆の影響
Vladimir Kh. Alimov1, 波多野雄治 1, 杉山一慶 2, 高木郁二 3, 松山政夫 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
Max-Planck-Institut für Plasmaphysik, EURATOM Association,
D-85748 Garching, Germany
3
京都大学大学院工学研究科原子核工学専攻〒606-8501 京都市左京区吉田本町

Deuterium Retention in Self-Damaged Tungsten with and without
Deposited Pd Layer
Vladimir Kh. Alimov1, Yuji Hatano1, Kazuyoshi Sugiyama2, Ikuji Takagi3,
Masao Matsuyama1
1

Hydrogen Isotope Research Center, University of Toyama,
Toyama 930-8555, Japan
2
Max-Planck-Institut für Plasmaphysik, EURATOM Association,
D-85748 Garching, Germany
3
Department of Nuclear Engineering, Kyoto University
Kyoto 606-8501, Japan(Received January 31, 2013; accepted May 7, 2013)
Abstract

Influence of Pd deposition on deuterium (D) retention in W samples irradiatedwith W self-ions was examined to understand the correlation between the probability ofoccupation of radiation-induced traps by D and the concentration of D in a solid
solution state under plasma exposure. W samples were irradiated with 4.8 MeV W ions

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V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫

to 0.65 and 12 dpa, and a thin Pd layer was deposited on the sample with the higher
damage level by a technique of sputter-deposition. Then, the samples were exposed to
DC glow-discharge D plasma at 403 K. In spite of the higher damage level, theconcentration of trapped D in the damaged zone of the Pd-covered sample was far
smaller than that in the non-covered one. The small D retention in the Pd-covered
sample was explained by a decrease in the D concentration in the solution state owing to
the enhanced recombinative release by Pd and the consequent reduction in theoccupation probability of radiation-induced traps. These observations indicated that
tritium inventory in neutron-irradiated W materials can be significantly reduced byenhancement of tritium reemission by surface modifications.

1. Introduction
Tungsten (W) and its alloys are recognized as candidates of plasma-facing
materials of future fusion reactors. Because defects such as vacancies and voids act asstrong traps for hydrogen isotopes, tritium (T) inventory in neutron-irradiated W
materials is an important problem in safety assessment of fusion reactors. Indeed,significant increase in deuterium (D) retention in W after neutron irradiation in a fission
reactor was reported in Refs. [1–6]. It is also known that irradiation effects by neutrons
can be simulated by irradiation with high energy ions [7].
Under exposure to hydrogen isotope plasma, a majority of hydrogen particles
impinging on the W surface are emitted back to the plasma through recombination intomolecules on the surface and subsequent desorption. A minor portion of impinging
hydrogen isotopes settles in interstitial sites in the bcc lattice, diffuses in the lattice, andthen gets trapped when they encounter unoccupied defects. Under exposure to high flux10

照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす Pd 表面被覆の影響

plasma, formation of bubbles and blisters takes place in the near-surface regions by
precipitation of molecular hydrogen isotopes in cavities. The hydrogen isotope retention
in blisters and bubbles increases practically infinitely with increase in internal pressure,and these defects can be called as unsaturable traps. On the other hand, in the bulk ofmaterials where chemical potential is far lower, hydrogen isotopes are trapped in atomic
form in defects such as dislocations, vacancies, and vacancies clusters with finite
capacity. These defects are often referred as saturable traps. In this study, the attention
is focused on the trapping effects in the bulk of materials, and hence only saturable traps
are considered below. Inventory of hydrogen isotopes increases with increase in thefraction of occupied traps. Based on the model proposed by Beshers [8], the fraction ofoccupied traps t at temperature T is expressed as

t,i/(1 −t,i) = L exp(Ebin,i/kT),(1)

where L is the fraction of occupied interstitial sites, Ebin,i is the binding energy betweenhydrogen isotope and defect, i is the type of defect and k is the Boltzmann constant.
This equation suggests that the fraction of occupied trap sites and consequently
inventory of hydrogen isotopes increases with an increase in the concentration ofhydrogen isotopes in the solid solution state CSS because CSS is proportional to L. Even
under the conditions where Ebin,i/kT >> 1 (i.e. t ≈ 1), the inventory of hydrogen isotopes
depends on CSS if a discharge pulses are short enough to keep the penetration depth ofhydrogen isotopes smaller than the thickness of W materials [9]. If Ebin,i/kT >> 1, clear
interface between filled zone (t ≈ 1) and empty zone (t ≈ 0) can be observed, and thevelocity of the interface is
dx/dt = DLCSS-S/xNt,

(2)

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V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫

where x is the depth from the surface, t is time, DL is the diffusion coefficient ofhydrogen isotopes in the lattice of W, and Nt is the trap density [10]. Here, CSS-S is theconcentration of hydrogen isotopes in the solution state just beneath the plasma-facing
surface. Although CSS including CSS-S is far smaller than the concentration of trappedhydrogen isotopes, the variation in CSS has strong influence on total inventory ofhydrogen isotopes. Indeed, such correlation between CSS and inventory of D wasdemonstrated by a gas absorption technique under equilibrium conditions in ourprevious papers [5,11]; CSS was determined as a function of pressure of D2 gas andtemperature. However, the correlation under non-equilibrium conditions, such as underplasma exposure, has not been proven.
The objective of the present study is to demonstrate the effects of surface
modification of W on D inventory in MeV-range W-ion-irradiated W. A thin Pd layer
was prepared on the surface of the W-ion-irradiated W sample, and then the samples
with and without Pd layers were exposed to weakly-ionized D plasma created by DC
glow discharge. Weakly-ionized plasma is suitable to examine the interaction betweenhydrogen isotopes and saturable traps because such low-flux plasma does not form
blisters. As previously reported [11], the W-ion-induced defects in the sample without
Pd layer become fully occupied during plasma exposure of surface to a depth dependingon implantation fluence (or time) and are empty at greater depths; the D depth profiles
are consistent with saturable traps being filled by D atoms diffusing from the surface.
The mass balance at the plasma-facing surface can be expressed as in = krCSS-S2 where is the sticking coefficient of impinging particles, in is incident flux and kr is thesurface recombination rate constant. Hence, CSS-S = (in/kr)1/2. Although kr for a clean
W surface is very high (kr ≥10−11 m4s−1 at ≤ 1000 K [7]), such surface state is available

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照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす Pd 表面被覆の影響

only in an ultra-high vacuum. The values of kr reported for W with ion-implantation[12] and linear plasma machines [5] are just kr ≥ 10−20 m4s−1 owing to contamination by
impurities such as oxygen and carbon. On the other hand, Pd is a noble metal and its
surface is less sensitive to contamination by impurities. In addition, solubility ofhydrogen isotopes in Pd is significantly larger than that in W, and Pd overlayers would
provide high CSS-S together with low CSS in the W substrate.

2. Experimental
Polycrystalline mechanically-deformed W from A.L.M.T. Co., Japan, with apurity of 99.99 mass%, was used in this work. Two W disc-type samples, 6 mm indiameter and 0.2 mm in thickness, were prepared by cutting a polycrystalline tungsten
rod annealed at 1173 K for 3.6 ks in a hydrogen atmosphere to relieve internal stresses
occurred in the manufacturing process. The samples were mechanically polished to amirror finish with diamond powders (9- and 3-m-sized grains) and colloidal silica
suspension (40-nm-sized grains). After cleaning in an ultra-sonic bath with acetone,
surfactant and de-ionized water for 5 min each, the samples were annealed at 1173 K for
1.8 ks in vacuum (∼10−6 Pa). The grains are elongated along the direction normal to thesurface, which is similar to ITER-grade W.
The first W sample (sample #1) was irradiated with 4.8 MeV W ions attemperature of 573 K to damage levels of 0.65 displacements per atom (dpa) at thedamage peak, whereas the second sample (sample #2) was irradiated with 4.8 MeV W
ions at 300 K to 12 dpa. The damage profiles were calculated using the program SRIM
2008.03 [13], “full cascade option”, with the displacement energy of Ed = 90 eV as themore reliable value for the W lattice [14,15]. Note that for irradiation with 4.8 MeV W

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V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫

ions, the damage peak situated at a depth of 0.23 µm.
The damaged sides of sample #1 were exposed to D plasma formed by DC glow
discharge at temperature of 403 K [11]. The sample was set on a holder equipped withan ohmic heater and a thermocouple served as anode, whereas W disc located at a
distance of about 10 cm from the sample holder was used as a cathode. Deuteriumpressure in the chamber was maintained at 1 Pa, DC discharge voltage was 400 V, anddischarge current averaged about 0.18 A. Because the sample was set on an anode, themain impinging particles are D atoms and molecules (D neutrals) in addition toelectrons. The energies of atoms were in the range from few eV (atoms originated in theglow discharge) to ~150 eV (atoms reflected from the W cathode). The flux of
implanted deuterium was estimated to be about 21018 D/m2s with the use of Ti probe
exposed in the DC glow discharge at 300 K for 10 and 60 min, while the D retention in
the Ti probe was determined by thermal desorption spectroscopy [11]. The implantation
fluence was about 61022 D m−2.
Sample #2 was exposed to the D plasma in a similar manner at 403 K to theimplantation fluence of about 61022 D/m2, but with Pd cathode instead of W one.
Owing to far larger sputtering yield for Pd than W by 400 eV D ions, the surface ofsample #2 was covered by sputtered Pd particles. Here, the sputtering yield for W is
3×10−3 [16]. The sputtering yield for Pd is not available, but that for Ag locating next to
Pd in the periodic table is 0.1 [16]. Thickness of the deposited Pd layer on the sample
was determined by means of Rutherford backscattering spectroscopy (RBS) at 3He ion
energies of 2.8 MeV and evaluated with SIMNRA program [17]. The backscattered 3He
was energy-analyzed at a scattering angle of 165° by a small-angle surface barrier
detector. It was found that after D plasma exposure for 30 ks (that corresponded to the14

照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす Pd 表面被覆の影響

implantation fluence of about 61022 D/m2), a thickness of Pd layer was around 270 nm.
After the RBS measurement, the Pd layer on sample #2 was thinned by sputtering with
Ar ions in a commercial glow-discharge optical emission spectroscopy (GDOES)
device (Horiba Jobin Yvon, GD-Profiler 2). Repeated RBS measurement performed
after that showed that a thickness of the Pd layer was reduced to 58 nm (Fig. 1). Sample#2 with 58 nm Pd layer was exposed to the D plasma with the use of the W cathode at
403 K to the implantation fluence of about 61022 D/m2. Namely, sample #2 wassequentially exposed to D plasma 2 times with the in-situ Pd deposition up to 270 nm

RBS, 1650 Detector
Analysing beam: 2.8 MeV 3He+

Counts

8000

4000

W
RBS experimental data
simulation with SIMNRA
58 nm Pd layer on W
RBS experimental data
simulation with SIMNRA

0
1600

2000
Energy (keV)

2400

Fig. 1 Rutherford backscattering (RBS) spectra for 2.8 MeV 3He ions incident on tungsten
with and without 58 nm thick Pd layer deposited on the W surface. The symbols and solidlines represent respectively experimentally measured and calculated RBS spectra. The
RBS spectra were calculated by the SIMNRA program [17].

15V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫

and with the pre-prepared thin Pd layer (58 nm).
The deuterium profiles in samples #1 and #2 after exposure to the D plasma were
determined by nuclear reaction analysis (NRA) at IPP, Garching. The D(3He,p)4He
reaction was utilized, and both the  particles and protons were analyzed. To determine
the D concentration at larger depths, an analyzing beam of 3He ions with energies varied
from 0.69 to 4.0 MeV was used. The proton yields measured at different 3He ion
energies allow measuring the D depth profile at depths of up to 6 μm.

3. Results and discussion
Generation of displacement damage in the sample #1 at 573 K and subsequent
exposure to D plasma at temperature of 403 K leads to accumulation of deuterium at
depths up to 2 µm (i.e., in the damage zone) to concentrations of about 0.6 at.% asshown in Fig. 2. Note that after exposure to D2 gas at pressure of 1 Pa (the same
deuterium pressure was maintained under DC glow discharge) and temperature of
403 K for 30 ks (i.e., for time period of the D fluence), the D concentration in thedamage zone does not exceed 10−3 at.%. It should be noted that, in undamaged W
exposed to the D plasma, the D concentration at depths  0.1 µm is below the NRA
detection limit of 510−4 at.%.
In the sample #2 exposed to the D plasma with the Pd cathode, deuterium isaccumulated mainly in the 270 nm thick Pd layer (Fig. 2). However, after thinning of
the Pd layer to 58 nm and second D plasma exposure with the W cathode, the Dconcentration in the W damage zone is significantly lower than that for the W sample#1 without Pd film (Fig. 2), in spite of higher defect concentration. Note that in thesample #2 irradiated with 4.8 MeV W ions at 300 K to the damage level of 12 dpa, a

16

照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす Pd 表面被覆の影響

010

D concentration (at.%)20

W (0.65 dpa @ 573 K) (sample #1)
270 nm Pd / W (12 dpa @ 300 K)(sample #2, after the first exposure)
58 nm Pd / W (12 dpa @ 300 K)(sample #2, after the second exposure)

-1

10

10

-1

10

damage profile, 0.65 dpa

Pd

W

-2

-2

10

10

-3

10

Damage (dpa)22

D plasma (403 K @ 6 x10 D/m )  W
damaged with 4.8 MeV W ions

-3

0.0

0.4

0.8

1.2
1.6
Depth (m)

2.0

2.4

10

Fig. 2 Depth profiles of D retained in the sample #1 and sample #2 after the first andsecond D plasma exposures. Thickness of the Pd layer on the sample #2 after the firstand second D plasma exposures is indicated in the legend. A boundary between the Pdlayer and the W substrate corresponds to a depth of 0 µm. Damage profile is plotted in alogarithmic scale on the right ordinate axis.

concentration of defects responsible for trapping of diffusing D atoms is slightly higher
than that for W-ion irradiation at 573 K to 0.65 dpa [5].
Obviously, the deposited Pd film on the W surface serves as a storage for
implanted D and reduces flux of diffusing D atoms penetrating into the damage zone byincreasing krCSS-S2. Note that the diffusivity of D in Pd is 4.5 × 10−10 m2s−1 at 403 K [18].
Hence, the mean diffusion length of D during the exposure to D plasma for 30 ks isevaluated to be 5 mm. This value of mean diffusion length is far larger than the
thickness of Pd layer (58 nm). It means that the low concentration of trapped D in W
damage zone in sample #2 was not owing to slow diffusion in the Pd layer. The D

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V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫

concentration in sample #2 beneath the surface (Pd region) was ca. 1 at.% (6.9 × 1026 Dm−3). Since in under exposure to D plasma was about 21018 D m−2s−1, the kr was
evaluated to be 4 × 10−36 m4s−1. This value is in reasonable agreement with that reported
by Pick and Sonnenberg [19]. The entropy and enthalpy of H solution in Pd are−60 J mol−1K−1 and −10 kJ mol−1, respectively [18]. Hence, the above-mentioned D
concentration (1 at.% at 403 K) corresponds to equilibrium pressure of 31 kPa. On theother hand, the concentration of D in a solid solution state in W at this pressure (31 kPa)
and temperature (403 K) can be evaluated from the solubility data reported by
Frauenfelder [20] to be 2.5 × 1013 D m−3 (3.9 × 10−16 in [D]/[W]). According to Eq. (1),at this fraction of occupied interstitial sites,L, the fraction of occupied traps,t,
becomes equal to or below 0.1 if Ebin ≤ 1.2 eV. Since the activation energy for diffusionof H in W is 0.39 eV, this Ebin corresponds to the activation energy for desorption of ca.
1.6 eV. This means that occupation probability of mono-vacancies [21] and dislocations[22] by D is negligibly small. Namely, low concentration of trapped D in sample #2 canbe explained by low occupation probability of defects with small Ebin owing to low D
concentration in the solid solution state. In other words, under plasma exposure, theretention of hydrogen isotopes in irradiated W can be reduced by reduction in the Dconcentration in the solid solution state by enhancement of recombinative release.

4. Summary
Preparation of a Pd layer on an MeV-range W-ion-irradiated W sample resulted
in significant reduction in D retention after exposure to DC glow-discharge D plasma at
403 K. The reduced D retention was explained by decrease in D concentration in a solid
solution state caused by enhancement of recombinative D release by the Pd layer and18

照射損傷を受けたタングステン中の重水素滞留挙動に及ぼす Pd 表面被覆の影響

consequent reduction in occupation probability of radiation-induced traps. Theseobservations indicated that tritium inventory in neutron-irradiated W materials can besignificantly reduced by enhancement of tritium reemission by surface modifications.

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V. Kh. Alimov・波多野雄治・杉山一慶・高木郁二・松山政夫[13] J.F. Ziegler, SRIM - The Stopping and Range of Ions in Matter, ver.
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