Journal of Nuclear and Radiochemical Sciences, Vol. 18, pp. 9-12, 2018

ArticlesRadiation exposure effect on deuterium retention in SiCYasuhisa Oyaa,*, Kenta Yuyamaa, Keisuke Azumaa, Shodai Sakuradaa, Hiroe Fujitaa,
Yuki Uemuraa, Hiroto Matsuurab, Masafumi Akiyoshib, Sosuke Kondoc, Tatsuya Hinokid andTakumi Chikadaa
aShizuoka University, 836 Ohya, Suruga-ku Shizuoka 422-8529, Japan
Osaka Prefecture University, 1-2 Gakuen-cho, Sakai, Osaka 599-8570, Japan
cTohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980-8577, JapandKyoto University, Gokasho, Uji, Kyoto 611-0011, Japanb

Received March 23, 2018; Accepted June 25, 2018: Published online July 23, 2018
Heavy ion (Fe3+) irradiation or gamma-ray irradiation of silicon carbide (β-SiC) was done to evaluate the radiation
effect on hydrogen isotope retention in SiC. Thereafter, 1 keV D2+ implantation or D2 gas exposure was performed, and their D retentions were evaluated by thermal desorption spectroscopy (TDS). The D2 TDS spectra
consisted of two desorption stages, namely the desorption of D bound to Si as the Si-D bond and C as the C-Dbond. The D retention for Fe3+ irradiated SiC at the lowest D2+ fluence of 0.1 × 1022 D+ m-2 was lower than that forunirradiated SiC. It was expected that the displaced C atoms would be accumulated near the surface region byFe3+ irradiation, leading to their dynamic desorption in methane molecules. For gamma-ray irradiation, C or Si
with dangling bonds were formed, which enhanced D trapping. It was concluded that the collision process caused
during Fe3+ irradiation induced the dissociation of SiC matrix and C aggregates near the surface region, but theelectron excitation by gamma-ray irradiation dissociated the single Si-C bond and the quick D trapping by dangling bonds enhanced D retention in SiC.

1. Introduction
Silicon carbide (SiC), in the form of SiC/SiC composites, isbeing considered for use in the blanket modules for the hightemperature gas-cooling system in D-T fusion reactors [1-3].
During D-T fusion operation, SiC would be exposed to severeradiation conditions by neutrons and gamma-rays from transmuted materials. These forms of radiation induce various
types of damages by a collision process and an electron excitation process, leading to the retention enhancement of hydrogenisotopes including tritium. In previous studies by the authors[4-6], hydrogen isotope behavior in SiC was extensively studied and it was found that deuterium was preferentially trappedby C and, after saturation of the C-D bond formation, it wastrapped by Si in SiC. The deuterium desorption was found to
consist of two desorption stages, namely first, desorption ofdeuterium bound to Si and then, desorption of deuterium
bound to C. The trapping mechanisms of the two stages wereinf luenced by the damage structures produced by the D 2+
implantation. In addition, the deuterium retention in SiC attemperatures above 700 K was higher than that in graphite,
where the retention is defined as the area concentration (in theunit of D m-2) of D atoms trapped in material, indicating thattritium retention in SiC may be high.
In ceramics like SiC, two fundamental mechanisms, a cascade collision process and an electron excitation process, arethought to introduce the damage caused by 14 MeV neutrons
by DT fusion reaction and energetically exchange charged
particles in the SiC [7]. In the fusion environment, both processes will induce irradiation damage in SiC. To elucidate theD trapping mechanism for SiC, it is quite important to separate
these two processes. With gamma-ray irradiation, only theelectron excitation will be induced. Therefore, the comparisonof D retention behavior for heavy-ion irradiated tungsten (W)*Corresponding author. E-mail: oya.yasuhisa@shizuoka.ac.jp

and gamma-ray irradiated SiC is used quite often. These facts
motivated the authors to explore deuterium retention behavior
in radiation damaged SiC.
In this study, Fe3+ irradiation was done to simulate the cascade collision by neutron irradiation of SiC to avoid the activation of SiC by nuclear reactions induced with neutron
irradiation. The gamma-ray irradiation was also done using
the cobalt-60 gamma-ray irradiation facility at Shizuoka
University. Deuterium retention behavior for damaged SiC
was discussed based on the identified damage introduction
process.
2. Experimental
Disk-shaped polycrystalline β-SiC (10 mm in diameter andca. 0.5 mm in thickness) was purchased from Asahi Glass Co.
Ltd. and used as sample material. After annealing at 1173 K,
6.4 MeV Fe3+ irradiation with a damage level of 0.2 displacement per atom (dpa) was carried out at room temperature using
the tandem accelerator at the Dual-Beam Facility for Energy
Science and Technology (DuET) in Kyoto University [8-9].
The implantation depth of Fe3+ was about 2.3 μm. Cobalt-60
gamma-ray irradiation was performed for other samples at thedose of 100 or 400 kGy at Shizuoka University. After Fe3+irradiation or gamma-ray irradiation, 1 keV D2+ was implantedwith a flux of 1.0 × 1018 D+ m-2 s-1 up to a fluence of 1.0 × 1022
D+ m-2 at room temperature at triple ion implantation system[10]. The implantation depth was 14 nm, which was less than
the implantation depth of Fe3+.
The XPS analysis was performed to evaluate the chemical
states of SiC using the ESCA1600 system (ULVAC-PHI Inc.,Chigasaki, Japan) equipped with an Mg-kα X-ray source(1253.6 eV) and a hemispherical electron analyzer. In addition, a D2 gas exposure experiment was also performed at 100
kPa for 20 hours at room temperature. The D retention wasevaluated by thermal desorption spectroscopy (TDS), with the© 2018 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/23/2018

J. Nucl. Radiochem. Sci., Vol. 18, 2018

Oya-1

heating rate of 0.5 K s up to 1273 K. The desorbed species
were analyzed by a quadruple mass spectrometer (MKSMicrovision Plus), whose sensitivity was calibrated using theD2 standard leak.
3. Results and discussion

D2 desorption rate / 1018 D2 m-2 s-1

Figure 1 shows D2 TDS spectra for unirradiated SiC andFe3+ irradiated SiC. D2 desorption consisted of two stages
occurring at 890 K and 1080 K. These were attributed to thedesorption of D bound to Si as a Si-D bond and that bound toC as a C-D bond, respectively. For unirradiated SiC, after saturation of the C-D bond formation at 1080 K as described previously [4], the Si-D bond became a major trapping state inSiC. However, by Fe3+ irradiation, the retentions of D as Si-Dand C-D bonds decreased together even at the D + fluence of0.1 × 1022 D + m -2 . For the f luence of 1.0 × 1022 D + m -2 , theextent of desorption of D trapped by Si as Si-D bonds wasalmost equivalent to that for unirradiated SiC.
Total D retention and retentions of D as Si-D and C-D bonds
as a function of D + fluence are summarized in Figure 2. At
the D + fluence of 0.1 × 1022 D + m-2, total D retention for Fe3+
irradiated SiC was half that for unirradiated SiC. The retention of D bound to C as the C-D bond for unirradiated SiC wasconstant. However, for the Fe3+ irradiation case, the D retention increased as the D+ fluence increased. At the fluence of
1.0 × 1022 D+ m-2, total D retention was the same as that for that
Un-irradiated SiC(1022 D+ m-2)
1
0.5
0.1
Fe3+ irradiated SiC(1022 D+ m-2)
1
0.5
0.1

0.6

0.4

for unirradiated SiC. To evaluate the chemical interactionwith D, C 1s XPS analysis was performed as a function of D+f luence and the results are shown in Figure 3. Before D2+implantation, two major chemical moieties were found, namely
the C-O bond with the binding energy of 286.5 eV and the C-Cbond with the binding energy of 284.5 eV. The formation ofthe C-O bond would be caused by exposure to air after Fe3+
irradiation. The existence of the C-C bond would indicate thatFe3+ irradiation induced the dissociation of the Si-C bond by
the collision process and C was aggregated near the surface
region. This fact indicated that Si was dispersed in the SiCmatrix, which would induce the D trapping by Si even in theinitial stage of D2+ implantation. After the D2+ implantation,
the negative peak shift of the C 1s XPS spectrum was found to
be due to chemical sputtering of surface C by the formation ofhydrocarbons and the full width at half maximum wasincreased due to the co-existence of C-D and C-Si bonds.
These behavio rs were almost in agreement with the TDS
results as shown in Figure 1.
To compare the D retention behavior for Fe3+ irradiated SiC
with that for gamma-ray irradiated SiC, 100 kGy or 400 kGy
gamma-ray irradiation was performed. 1 keV D 2 + was
implanted with three fluences of 0.1 × 1022, 0.5 × 1022 and 1.0× 1022 D + m-2 at room temperature. Figure 4 shows D2 TDS
spectra for 100 kGy or 400 kGy gamma-ray irradiated SiC. In

1.0C 1s
After D2 + imp.

0.8 (1022D+m-2 )Normalized / -10

0.6
0.4
0.2

0.2

0.0
292
0.0
400

600

800
1000
Temperature / K

1200
3+D retention / 1021 D+ m-2

0.6

0.4

Unirradiation
Si-DC-D
Total
Fe3+ irradiation
Si-DC-D
Total

0.6

0.4

282280Gamma ray irradiation(1022 D+ m-2)
100 kGy
10.5
0.1
400 kGy
10.5
0.1

0.2

0.2

0.0
0.0288286284
Binding energy / eV

Figure 3. C 1s XPS spectra for Fe3+ irradiated SiC as a function ofD+ fluence.

D2 desorption rate / 1018 D2 m-2 s-1

Figure 1. D2 TDS spectra for unirradiated SiC and Fe irradiated
SiC as a function of D+ fluence.

0.8

290

0.2

0.4
0.6
0.8
D+ fluence / 1022 D+ m-2

1.0

Figure 2. D retention for unirradiated SiC and Fe3+ irradiated SiC asa function of D+ fluence.

0.0400

600

800
1000
Temperature / K

1200

Figure 4. D2 TDS spectra for 100 kGy or 400 kGy gamma-ray irradiated SiC as a function of D+ fluence.

J. Nucl. Radiochem. Sci., Vol. 18, 2018

Radiation exposure effect on deuterium retention in SiC

the case of the lowest D+ fluence, 0.1 × 1022 D+ m-2, it was clearthat the shape of the TDS spectra and the extent of the D retention were almost the same as those for Fe3+ irradiated SiC.
However, two clear desorption stages were found at the highest
D+ fluence of 1.0 × 1022 D+ m-2 for both gamma-ray irradiation
cases. Their D retentions are summarized in Figure 5 as afunction of D+ fluence. Large enhancement for the retentionof D as the C-D bond was found at 1.0 × 1022 D+ m-2, although
D retention as the Si-D bond was almost directly proportional
to the D+ fluence. C 1s XPS spectra for 400 kGy gamma-rayirradiated SiC as a function of D + f luence are shown inFigure 6. Major chemical states of C were assigned as C-Si
(282.5 eV) and C-C bonds [11]. It is known that the accumulation of D induces the positive peak shift of C 1s XPS spectrafrom 284 to 285 eV. After D2+ implantation, peak intensity for
the C-C bond sharply decreased and the highest peak wasshifted toward the higher binding energy side due to formationof the C-D bond; the binding energy reached 284 eV. After theTDS measurement, the peak had largely shifted toward thelower binding energy side due to D desorption, but the C-C
bond still existed. This might be caused by some of the C witha dangling bond remaining in the SiC bulk and emerging on
the surface due to heating.
To evaluate dynamic trapping by the formation of damage
structures by gamma-ray irradiation, 88 kGy gamma-ray irra-

100 kGy gammaray irradiation
Total
Si-D
C-D
400kGy gamma ray irradiation
Total
Si-D
C-D

0.8
0.6

diation was performed under the D2 gas exposure condition(in-situ) and their trapping behavior was compared with D2 gas
exposed SiC after gamma-ray irradiation (ex-situ). D2 gas
exposure conditions were the same for in-situ and ex-situ: 100
kPa for 20 hours at room temperature. Figure 7 shows D2 TDS
spectra for in-situ D 2 gas exposed SiC and ex-situ D 2 gas
exposed SiC. The spectrum for D2 gas exposed SiC without
gamma-ray irradiation is also shown in this figure. The D2
desorption was concentrated on the lower temperature side,
indicating that D was accumulated near the surface region
with a lower trapping energy. Much D desorption was foundat temperatures between 600 and 800 K and the D desorption
was larger for in-situ D2 gas exposed SiC than ex-situ D2 gas
exposed SiC, indicating that rapid D trapping by the damage
structures produced by gamma-ray irradiation was proceeding. In addition, in Figure 7, no D trapping for SiC without
gamma-ray irradiation was found. The D desorption at low
temperature would be caused by the adsorption of D at the surface with low trapping energy. From Figure 8, it was clear thatD retention was almost doubled compared with that for ex-situ
D2 gas exposed SiC. From these results, it can be concluded
that the co-existence of radiation and hydrogen isotopes likefusion environment would enhance D trapping in SiC.
Based on these experimental results, D trapping behavior inSiC is discussed. With Fe3+ irradiation, dissociation of the3.0
D2 desorption rate / 1014 D2 m-2 s-1

D retention / 10 21 D m-2

1.0

2.5

Un-irradiation
88 kGy (4.4 kGy h-1)
Ex-situ D2 gas exposure

2.0

In-situ D2 gas exposure

1.5

0.4

1.0
0.5

0.2

0.0

0.0
0.0

0.2

0.40.6
0.8D+ fluence / 10 22 D m-2

1.0Figure 5. D retention for gamma-ray irradiated SiC with the dose of
100 kGy or 400 kGy as a function of D+ fluence.

1.0400600
800
Temperature / K1000

Figure 7. D2 TDS spectra for D2 gas exposed SiC during gamma-rayirradiation (in-situ) and after gamma-ray irradiation (ex-situ).

1.4
C 1s

1.2
D retention / 1017 D m-2

After TDS+AfterD2 imp.
0.8

Normalized / -11(10 22 D+ m-2 )
10.5
0.6
0.1
Before D2+ imp0.4
0.2
0.0292

1.0
0.8
0.6
0.4
0.2

290288286284
Binding energy / eV282

280

Figure 6. C 1s XPS spectra for 400 kGy gamma-ray irradiated SiC
as a function of D+ fluence.

0.0Unirradiation

Ex-situIn-situ

Figure 8. Comparison of D retentions for D2 gas exposed SiC without gamma-ray irradiation or, ex-situ or in-situ gamma-ray irradiation.

12J. Nucl. Radiochem. Sci., Vol. 18, 2018

Si-C bond was induced, especially near the surface region,
leading to the aggregation of C. This indicates that somehydrocarbons were formed and desorbed from SiC by D2+
implantation, which is consistent with previous reports that
only methane is desorbed from SiC, although silane is notformed [12, 13]. Carbon vacancies were introduced and Si
with dangling bonds remained in SiC, which would be themajor D trapping sites in SiC. This behavior can also beassumed for the gamma-ray irradiation case. The ratio of C
aggregation on the surface would be quite small because theelectron excitation process induced the dissociation of a single
Si-C bond while another Si-C bond still remained. However,these Si and C atoms with the dangling bond could be potential hydrogen isotope trapping sites. This can be explained by
the fact that the extent of desorption of D as the C-D bond as
seen in the TDS spectra was higher than that for Fe3+ irradiated SiC. In addition, the in-situ D 2 gas exposure during
gamma-ray irradiation clearly showed that some of the dangling bonds quickly recombined and the D present quickly
interacted with these dangling bonds, leading to the enhancement of D retention. It can be concluded that the collision process induces the dissociation of SiC matrix and C aggregatesnear the surface region, but the electron excitation process candissociate a single Si-C bond and D trapping by dangling
bonds can enhance D retention in SiC.
4. Conclusions
Two different radiation techniques, Fe3+ irradiation andgamma-ray irradiation, were utilized to evaluate the radiation
effect on hydrogen isotope retention in SiC. It was found thatthe D2 TDS spectra included two desorption stages, namely
the desorption of D bound to Si as a Si-D bond and C as a C-Dbond. It was found that the damage introduced by Fe3+ irradiation produced displaced carbon atoms in SiC, which led to thedynamic D desorption. For gamma-ray irradiation, C or Si
with the dangling bond was formed uniformly throughout theSiC matrix. It was presumed that the electron excitation process by gamma-ray irradiation dissociated a single Si-C bondand D trapping by dangling bonds enhanced D retention.
However, the collision process by Fe3+ irradiation could induce
the dissociation and aggregation of C near the surface region,
leading to less D retention.

OyaAcknowledgments
This study was supported by the Joint Usage/Research
Program on Zero-Emission Energy Research, Institute ofAdvanced Energy, Kyoto University (ZE28A-17, ZE29A-8)
and a collaborative program with Osaka Prefecture University.
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