A. Sagara

K. Nishimura

M. Tokitani

N. Ashikawa

N. Matsunami

N. Ohno

S. Masuzaki

T. Sogawa

Y. Sakuma

National Institute for Fusion Science (NIFS-Repository)

   Stainless steel (SUS) is widely employed for the nuclear fusion devices. When O2-plasma-discharge is performed to remove tritium [1], layers of iron-oxide such as Fe2O3 will be formed on SUS surface [2]. Thus, deuterium (D) retention in the oxide layers is important for designing the devices. In this work, we have studied D-retention in Fe2O3 under D2 plasma exposure [3], as well as in SUS and Fe for comparison.   Fe2O3 layers were prepared by oxidation of Fe deposited on SiO2-glass substrates and Fe sheet in air. According to X-ray diffraction (XRD), the oxide layers on SiO2 oxidized at 400 to 500 ºC for more than 5 min are polycrystalline hematite (hexagonal �-Fe2O3) while those on Fe oxidized at 500 ºC are mixture of �-Fe2O3 and cubic �-Fe2O3 (maghemite). Oxide-layer thickness is evaluated by means of Rutherford backscattering spectroscopy (RBS) of 1.8 MeV He+ and H+, using the stopping power [4] and density of 3.96x1022 Fe cm-3 (5.25 g cm-3). For analysis of ion beam data, e.g., RBS, SUS is assumed to consist of Fe only with the density of Fe (8.48x1022 cm-3 or 7.86 g cm-3). The samples were exposed to D2-plasma under AC discharge of 1.5 kV in D2 gas pressure of 0.4 Torr for 15 min at room temperature [3]. The amount of D was analyzed by using nuclear reaction analysis (NRA), D(3He,�)H with 3He-energy of 1.0 MeV. The NRA cross section was taken after [5]. The depth resolution of the NRA at very near surface is estimated to be ~0.24 and 0.19 �m for Fe2O3 and Fe. Also, the probing depth is roughly estimated to be 1.3 and 1�m for Fe2O3 and Fe, based on the calculated path length [4] where the 3He beam slows down to 0.25 MeV so that the NRA cross section is reduced to one-fifth of the maximum value.   The results are summarized in Table 1. Firstly, D-retention in �-Fe2O3 appears to be independent of the oxide layer thickness of 0.06 to 0.86 �m, indicating low  diffusivity of D in �-Fe2O3 (<4x1014 cm2s-1). Secondly, D-retention in �-Fe2O3 is found to be larger by a factor of at least 1.7 than that in �-Fe2O3. An explanation is that in �-Fe2O3, non-negligible fraction of vacancies exists in Fe site (defect-spinel-type-structure, e.g., Fe2/3Fe2O4 or one third of vacancies at Mg site in the spinel to keep Fe2O3 composition) [6]. Therefore it is suggested that D’s occupy the vacancy sites in �-Fe2O3, enhancing D-retention. Another explanation is that the D-diffusivity in �-Fe2O3 is larger than that in �-Fe2O3, giving rise to larger D-retention. Measurement of D-depth-distribution, estimation of the ratio of �- to �-Fe2O3 in these mixed layers, and preparation of only �-Fe2O3 layers followed by D-retention measurement would be fruitful. In this study, the density of �-Fe2O3 is assumed to be the same as that of �-Fe2O3. Due to the presence of vacancies as described above, the density of �-Fe2O3 is smaller by several % than that of �-Fe2O3, which is insignificant in the D-retention evaluation.     D-retention in Fe appears to be the smallest in this study and a half of that in �-Fe2O3, in spite of a large diffusivity of H and D in Fe [7]. Surprisingly, D-retention in SUS is larger than that in Fe and �-Fe2O3. The amount of oxygen near surface of Fe and SUS is obtained to be ~13x1016 cm-2, by using 16O(d, �)14N with 1.2 MeV d. This indicates the existence of oxide layers near surface and a contribution of the oxide layers to D-retention.    Table 1 Summary of D-retention in Fe2O3, Fe and SUS (an estimated error of 15 %). Thickness of oxide layers, Fe and SUS are given in the parenthesis.     Sample D-retention    (1015 cm-2)    �-Fe2O3/SiO2 (0.06, 0.18, 0.86 �m)  21    (�+�)-Fe2O3/Fe (>2�m) 36   Fe (0.2 mm) 12    SUS (0.25, 2 mm) 27     References 1. R. E. H. Clark, D. H.  Reiter, eds. Nuclear Fusion   Research, Springer, 2004.    2. C. Anandan, K.S. Rajam, Appl. Surf. Sci. 253(2007)   6854.   3. T. Sogawa, N. Matsunami, N. Ohno, M. Tokitani,  S. Masuzaki, CP1231 Ion Implantation technology     2010, eds. J. Matsuo et. Al., AIP 2010 and Int. Conf.  PFMC-13/FEMS-1, Germany, 2011. 4. J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping   and Range of Ions in Solids, Pergamon Press,   New York, 1985. 5. W. Moller, F. Besenbacher, Nucl. Instrum. Meth.   168(1980)111.    6. M. Lie, H. Fjellvag, A. Kjekshus, Thin Solid Films   488(2005)74.   7. H. Hagi, Y. Hayashi, Trans. Japan Institute Metals   28 (1987)368.   109§2. Deuterium Retention in Iron Oxide (Fe2O3) under Low-Energy Deuterium ExposureMatsunami, N., Sogawa, T., Sakuma, Y. (Nagoya Univ. EcoTopia), Ohno, N. (Nagoya Univ. Engineering), Masuzaki, S., Tokitani, M.,  Ashikawa, N., Sagara, A., Nishimura, K.

Deuterium Retention in Iron Oxide (Fe2O3) under Low-Energy Deuterium Exposure

Deuterium Retention in Iron Oxide (Fe2O3) under Low-Energy Deuterium Exposure

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