24P090-P
Deuterium Retention of Tungsten with Surface Nanostructures
J. Shia*, M. Yajimab, Y. Hatanoa, T. Saekib, S. Kajitac, and N. OhnobaHydrogen Isotope Research Center, University of Toyama, Toyama 930-8555, Japanb

Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japanc

EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan

Tungsten specimens with nano-morphology (Nano-W) were prepared by exposure to helium plasma in thedivertor plasma simulator NAGDIS-II. For comparison, tungsten specimens with smooth surface (Polished-W)
were also prepared. Surface area of Nano-W was measured by using BET method. Deuterium retention of NanoW and Polished-W was investigated by thermal desorption spectroscopy (TDS) after exposure to D2 gas. It was
found that surface area of Nano-W was significantly larger than that of Polished-W. Deuterium retention wasalso greatly increased due to nanostructure formation.

1. Introduction

3. Surface area measurement

It is widely accepted that tungsten is one of themost promising candidates of plasma-facing
materials in fusion reactors because of its excellent
properties, such as high melting point, low erosion
yield and hydrogen isotope solubility etc. However,tungsten surface is strongly modified by exposure
to He plasma/ions. Blistering occurs at temperature
below 1000K [1], while holes/bubbles [2] andnanostructures [3] form at higher temperatures.
Incident ion energy and surface temperature were
found to be key parameters for the formations of
the nanostructures [4].
Influence of such drastic change in surface
structure on hydrogen isotope retention is necessary
to be investigated. From these viewpoints, thedeuterium (D) retention by nanostructured tungsten
was examined in the present study.
2. Specimen preparation
Tungsten specimens with nano-morphology(Nano-W) were prepared by exposing pure tungsten
sheets to He plasma in the divertor plasma
simulator NAGDIS-II. Two batches of Nano-W(indicated as #1 and #2 Nano-W) were prepared
under slightly different irradiation conditions given
in Table I. As reference, specimens with smoothsurface (Polished-W) were also prepared bypolishing tungsten sheets with diamond powder andcolloidal silica suspension.
Table I. Helium plasma irradiation conditions
Ion energy (eV)-2-1

#1 Nano-W#2 Nano-W

57

59

3.33×1022-2

He Ion fluence (m )4.20×1025

Temperature (K)1180

He Ion flux (m s )

11/

11,

where, P is pressure of Kr gas, P0 is saturation
vapour pressure of Kr at 77 K, V is the volume of
Kr gas adsorbed at given P/ P0, Vm is the volume of
Kr gas to give monolayer coverage and C a
constant related to the heat of adsorption. A linear
relationship was observed between 1/V [(P/P0)-1]
and P/P0 as shown in Fig. 1(b). Hence, Vm wasdetermined from the slope and intercept, andsurface area was calculated by assuming that thearea occupied by single Kr adatom is 18.5 Å2 [5].
Surface areas of #1 and #2 Nano-W were thus
evaluated to be 24 and 12 cm2, respectively. As
reported by Baldwin et al. [6], thickness ofnanostructured lays depends on He ion fluence.
The larger surface area of #1 Nano-W wasascribed to larger fluence of He ions.
Macroscopic surface areas of these specimens
were 2 cm2. Thus, surface area of #1 and #2 NanoW were increased by 12 and 6 times, respectively,
due to nanostructure development.
4. Deuterium retention

21

9.1×101.09×1025
1260

Surface area of Nano-W was measured by BET
method. Namely, after heating in vacuum at 873K,adsorption of Kr gas on the Nano-W was measured
at 77 K as function of pressure. The adsorption
isotherm thus obtained is shown in Fig. 1(a). This
isotherm was replotted according to the BET
equation:

Both Polished-W and Nano-W were exposed toD2 gas at 573 K and 1.2 kPa for 5 hours. Theamount of D2 retained in Polished/Nano-W wasthen measured by thermal desorption spectroscopy(TDS) at ramp rate of 0.5K s-1.

9Nano-size He bubbles inside nano-fibers wereidentified by TEM observation [7]. These preexisted helium bubbles can act as trapping sites ofD. Further investigation is necessary to clarify theroles of He bubbles in D retention.

(a)

8Deuterium released (1017 D/m2s)

Adsorbed amount (10-8mol)

Fig. 3 shows desorption spectrum of D from
Polished-W. The amount of D released from
Polished-W (QP) was calculated to be 6.6×1017 Dm-2. The D desorption spectra from Nano-W areshown in Fig. 4. The amounts of D from #1 NanoW (QN1) and #2 Nano-W (QN2) were 1.5×1019 D m-2
and 8.2×1018 D m-2, respectively.

The correlation between surface area and D
retention is shown in Fig. 4. The amount of retained
D increased almost linearly with surface area. Thus,
the larger D retention by Nano-W was attributed tothe increase in surface area due to nanostructuredevelopment.

765432
100.0

0.1

0.2

0.3

0.4

Relative pressure (P/P0)600(P/P 0)/ V (1- P/P 0)

500

0.05

0.10

0.15

0.20

0.25

0.30

0.35

1417

Qp = 6.6x10 D/m21000

800

1086006400
4Temperature(C)

142

Deuterium release rate (10 D/m s)

Fig. 1 BET measurement results of surface area of #1
Nano-W, (a) Kr adsorption isotherm at 77K,and (b) Plot of 1/V [(P/P0)-1] against P/P0.

200

200500100015002000

2500

3000

3500

Time (s)Fig. 2 D desorption spectrum of Polished-W

14192182

QN1= 1.5x10 D/m
QN2= 8.2x10 D/m

700
600
500

300

400
300

20020010010000500

1000

1500

2000

2500

Time (s)Fig. 3 D desorption spectra of Nano-W

Temperature(C)

500

2Deuterium release rate (10 D/m s)

60
40
200246810 12 14 16 18 20 22 24

5. Summary

Relative pressure (P/P0)080

Fig. 4 Correlation between D retention and surface
area.

100

400

1002(b)2000120

Surface Area (cm )300

12140040000.00

160

Nanostructures formed on tungsten surface
inevitably resulted in huge increase in surface area
comparing with tungsten having smooth surface.
The surface area of nanostructured tungsten wasincreased with fluence of He ions. Deuterium
retention by tungsten under exposure to D2 gas
significantly increased with the development ofnanostructure. The linear correlation was observed
between D retention and surface area.
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