Plasma and Fusion Research: Regular Articles

Volume 13, 3406065 (2018)

Nanostructure Growth on Rhodium/Ruthenium by the Exposure
to He Plasma∗)Tomohiro NOJIMA, Shin KAJITA1) , Naoaki YOSHIDA2) , Shota KAWAGUCHI, Noriyasu OHNO,
Hirohiko TANAKA, Masayuki TOKITANI3) , Daisuke NAGATA3) , Tsuyoshi AKIYAMA3)
and Takashi YAGI4)
1)

Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Institute of Materials and Systems for Sustainability, Nagoya University, Nagoya 464-8603, Japan2)
Research Institute for Applied Mechanics, Kyushu University, Fukuoka 819-0395, Japan3)
National Institute for Fusion Science, Toki 509-5292, Japan
4)
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8563, Japan(Received 28 December 2017 / Accepted 18 April 2018)

Rhodium and ruthenium thin film coatings were conducted on tungsten samples by using a magnetron sputtering device; then, we irradiated He plasma to the samples in the linear plasma device NAGDIS-II (NagoyaDivertor Simulator). Fuzzy nanostructures were formed on rhodium and ruthenium samples when the surface
temperatures were ∼950 and ∼1200 K, respectively. When the surface temperature was high, i.e., >1200 K, it
was found that tungsten atoms diﬀused across the rhodium film and reached the film surface, and tungsten-fuzz
was formed over the rhodium layer. From TEM analysis of ruthenium fibers, it was identified that there were thinparts in the fibers, and some fibers had no He bubbles. It was likely that fiber growth mechanism on ruthenium
was diﬀerent from the other metals.
c 2018 The Japan Society of Plasma Science and Nuclear Fusion Research

Keywords: rhodium, ruthenium, fuzz, percolation, morphology change
DOI: 10.1585/pfr.13.3406065

1. Introduction
Fiber-form nanostructure (fuzz) growth is one of thekey issues in plasma-wall interactions, because it may beformed on tungsten divertor under fusion relevant conditions [1]. It is concerned that fuzz formed on divertor
wall induces the formation of impurities and deteriorates
confinement performance in fusion reactors. On the otherhand, fuzz is expected to be applied to enhance catalysis
activity by increasing surface area [2].
In order to advance studies concerning fuzz, it is necessary to make the formation mechanism clear. Althoughsome formation models have been suggested [3–5], thefuzz growth mechanism has yet to be explained well, as
well as nano tendril bundle (NTB) [6] and large scalenanostructures [7].
The viscoelastic model proposed by S. Krasheninnikov suggested that shear stress induced by embedded
He atoms could be a driving force of tungsten (W) fuzz
growth [3]. And a molecular dynamics (MD) simulation showed that yield strength of W fiber reduces significantly when He atoms were embedded [8]. Following these studies, the relationship between fuzz formationand shear modulus was discussed [9]; the fuzz was easily
formed on high shear modulus metals such as W, Molybauthor’s e-mail: nojima-tomohiro@ees.nagoya-u.ac.jp∗) This article is based on the presentation at the 26th International Toki
Conference (ITC26).

denum (Mo) and rhenium (Re), [10–12]. However, morphology changes induced by He plasma irradiation were
not well-researched in other high shear modulus metals including rhodium (Rh) and ruthenium (Ru), which are facecentered cubic (FCC) and hexagonal close-packed (HCP),
respectively. The shear modulus of Rh and Ru is 152 and
173 GPa at room temperature (R.T) [13, 14], respectively,
which are comparable or higher than that of W (160 GPaat R.T).
In this study, we show the morphology changes of Rh
and Ru surfaces by the He plasma irradiation and discuss
the mechanism of fuzz formation.

2. Experimental Setup
Rh and Ru thin film coatings were conducted on W
samples using a magnetron sputtering device. The thickness of Rh and Ru was approximately 1 and 2 µm, respectively. The He plasma irradiations were conductedin the linear plasma device NAGDIS-II (Nagoya Divertor Simulator) [15]. High density plasmas, typically 1018 1019 m−3 , can be produced in steady state. The incident ion
energy was controlled by changing the biasing of the samples, and the sample temperature was measured by a radiation pyrometer. The emissivity of Rh and Ru at 1.6 µm wasassumed to be 0.18 and 0.20, respectively. When the samples were irradiated at temperature lower than 1000 K, they3406065-1c 2018 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 13, 3406065 (2018)

were mounted on a water cooling stage. When the surface
temperature was higher than 1000 K, samples were spotwelded to an electrode. In those cases, the temperature
was determined by the balance between the heat flux fromthe plasma and the radiation from the sample. After the He
plasma irradiation, the samples were analyzed by scanningelectron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray spectrometry (EDS)
and thermal desorption spectroscopy (TDS).

3. Results and Discussion
3.1Ruthenium (Ru)He plasma irradiation was conducted at the incident
ion energy of 45 eV as changing the surface temperaturefrom 1123 to 1523 K, as shown in Table 1. The He fluence
was on the order of 1025 m−2 .
Figures 1 (a-c) show the SEM micrographs of Ru sample exposed to He plasmas when the surface temperatures were 1193, 1383 and 1523 K, respectively. Fuzz
was formed on the Ru sample when the surface temperature was 1193 K. As shown in Fig. 1 (b), rough surface
with fibers was observed when the surface temperature was1373 K. It was likely that fuzz was diﬃcult to be formed onRu surface at this temperature. A large scale wavy surface
was observed when the surface temperature was 1523 K.
Figures 2 (a-c) show the EDS and TEM analysis of Ru
sample exposed to He plasma when the surface temperature was 1193 K. Figure 2 (a) shows distributions of Ru andW atoms with green and red colored maps, respectively. Itwas found that W atoms diﬀused to Ru layer during theplasma irradiation. The thickness of the Ru layer was originally 2 µm and W atoms diﬀused to the half bottom of thelayer. On the top of the layer and the fibers, the ratio of
the number of Ru atoms to that of W atoms was about 10.
Therefore, it can be said that Ru comprised the fibers.
Figures 2 (b, c) show the TEM micrographs of fiber
region. It was identified that Ru fibers grew linearly different from W fuzz [16]. On the other hand, He bubbles
were observed in thick regions of fibers and node parts.
The formation of He bubbles was commonly observed infibers after He plasma irradiation [16]. Elliptical He bubbles were observed in Ru fibers, as shown in the inset ofFig. 2 (b) which have not been observed in W. On the otherhand, no He bubbles were observed in narrow parts of Ru
fibers.
Linear fiber growth was also observed on Re surface

Table 1 Summary of the condition of the irradiation experiments and results of Ru.

Fig. 2 (a) TEM micrographs and EDS mappings of Ru (green)
and W (red), and (b, c) TEM micrographs of Ru sample
exposed to He plasma at surface temperature was 1193 K.

Fig. 1 SEM micrographs of Ru sample exposed to He plasmas at the surface temperature of (a) 1193, (b) 1373 and (c) 1523 K.

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Table 2 Summary of the condition of the irradiation experiments and results of Rh.

Fig. 3 TDS spectrums from Rh (red line) and Ru samples (blueline) exposed to the He plasmas. The irradiation conditions of Rh and Ru were the followings: surface temperature was 923 K and 1383 K, fluence was 1.6 × 1025
and 8.9 × 1025 m−2 , respectively. Incident ion energy was
45 eV. Heating rates of Ru and Rh samples were 0.26 and0.17 K/s, respectively.

exposed to He plasma at high surface temperature [9]. It isnoted that the crystal structure of Ru and Re is HCP. All themetals on which fuzz formation was confirmed have body
centered cubic (BCC) or FCC other than Ru and Re, as
was shown in [17]. In addition, although the relationshipbetween fuzz formation and shear modulus was updated in[17], linear fiber growth was not identified on matals with
BCC or FCC crystal structures. Thus, the results indicatedthat linear fiber growth was related to crystal structure; it
is of interest to investigate the crystal orientation similar tothe analysis recently reported on W [18].
A blue curve in Fig. 3 shows the TDS spectrum ofHe desorption from Ru samples exposed to He plasma. AHe desorption peak was observed at 1100 K. On W, there
were two TDS peaks around the lower and upper temperature limits for the fuzz growth [19]; the results suggested
that He migration was deeply related with the morphology changes. Similarly, it was likely that He migration
was activated around this temperature and fuzz was observed at 1123 - 1193 K. Although we conducted the TDS
up to 1300 K, it is likely that another peak will appear
above 1300 K considering the fact that the structure became greater probably due to significant He migration.

3.2

Rhodium (Rh)

Table 2 summarizes the irradiation condition for Rh
irradiation experiments. The incident ion energy was fixedat 45 eV, and the surface temperature was changed from613 to 1633 K; the fluence was higher than 1025 m−2 .
Significant surface morphology changes were not observed from SEM analysis when the surface temperature
was 613 K. From TEM analysis, however, nano bubbles
and dislocation loops were observed in the surface region,
as shown in Fig. 4. Surface roughness seems to be inducedby sputtering of incident He atoms. Recently, He plasma
irradiation to Rh sample at 510 - 550 K was conducted byFig. 4 TEM micrograph of Rh sample exposed to He plasma at
the temperature of 613 K.

Litnovsky et al. [20], and slight reduction in the optical reflectance was identified. It was likely that reduction in the
optical reflectance became greater as increasing the surface
temperature because of an enhancement of roughness.
Figures 5 (a-c) present an SEM and TEM micrographs, and EDS mapping of Rh samples exposed to the He
plasma at the surface temperature of 973 K, respectively.
Fuzz was formed on Rh sample. The distributions of Wand Rh atoms are shown as green and red images, respectively, in Fig. 5 (b). It is seen that fuzz formed at 973 K
mainly consists of Rh and some W atoms diﬀused fromthe substrate to original Rh layer. The ratio of Rh atoms
and W ones in the bottom part of Rh layer was 2 to 1; itis likely that an intermediate layer (ε-layer), diﬀerent from
BCC in W and FCC in Rh, was formed from Rh-W phase
diagram [21]. On the other hand, the fraction of W in Rh
fiber was negligible. From Fig. 5 (c), it is seen that elliptical He bubbles were observed in addition to round shaped
He bubbles. The elliptical shaped bubbles in fibers were always axially suppressed and radially elongated. It is notedthat elliptical shaped bubbles have not been identified in Wcases.
In Fig. 3, TDS spectrum for Rh sample was alsoshown. The gas desorption was terminated around 1150 K.
This indicated that He atoms were suﬃciently desorbed
from Rh sample at about 1150 K and that the fuzz growth
only occurs when the surface temperature was below∼ 1100 K on Rh.

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4. Conclusion
He plasma irradiations were conducted using the linear plasma device NAGDIS-II to Rh and Ru films deposited on tungsten sample. Fiber-form nanostructure(fuzz) was formed on Ru and Rh when the surface temperate was in the range of 1100 - 1200 and 900 - 1000 K,
respectively. In comparison with fuzz formation temperature of W, which was in the range of 0.27 < T s /T m (T s ; surface temperature, T m ; melting point) < 0.54 [10], the fuzz
formation condition was within the same range, but was
narrower (0.42 - 0.46 for Ru and 0.40 - 0.44 for Rh).
There were no significant morphology changes on Rh
surface exposed to He plasma at the surface temperature of613 K. Percolation of W atoms was observed on Rh samples when the surface temperature was higher than 1323 Kor higher.
It was noted that both metals have high shear modulus.
Because fuzzy structure was formed on both metals easily
with the He fluence of the order of 1025 m−2 , the present
results support the fact that fuzzy structure can be easily
grown on high shear modulus metals. On Ru surface, linear fiber growth was observed. It was likely that crystal
structure is related to linear fiber growth. On metals withHCP crystal structure, He plasma induced fibers could begrown linearly diﬀerent from BCC or FCC metals.
Fig. 5 (a) SEM, (b, c) TEM/EDS micrographs of Rh samples exposed to He plasma at the surface temperature of 973 K.

Acknowledgments
This work was supported in part by a Grant-in-Aidfor Scientific Research (B) 15H04229, a Grant-in-Aid for
Exploratory Research 16K13917 from the Japan Society
for the Promotion of Science (JSPS).

Fig. 6 (a) SEM and (b) TEM of Rh samples exposed to He
plasma at the surface temperature of 1323 K.

SEM and TEM micrographs with EDS mappings forthe sample exposed to the He plasma at a higher surface
temperature of 1323 K were shown in Figs. 6 (a) and (b),respectively. Microstructure similar to the sample shownin Fig. 6 (a) was formed on the surface. From EDS analysis
shown in Fig. 6 (b), it mainly consisted of W, not Rh atom,indicating that W atoms in the substrate actively diﬀused
at 1323 K and percolated on the Rh layer, the thicknessof which was about 1 µm. Percolation was also observed
when the surface temperature was 1633 K, and the fluence
was 5.6 × 1025 He/m2 . Percolation was observed when thesurface temperature was higher than 1300 K. Probably an
increase of the diﬀusion coeﬃcient of W with increasing
the temperature has led to this phenomenon.

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