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Effects of implantation conditions on retention behavior of deuterrium in highly oriented pyrolytic graphite
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Fusion Engineering and Design 82 (2007) 1762–1766
Effects of implantation conditions on retention behavior of deuterrium in highly oriented pyrolytic graphite Taichi Suda ∗ , Hideo Miyauchi, Akira Yoshikawa, Hiromi Kimura, Yasuhisa Oya, Kenji Okuno Radiochemistry Research Laboratory, Faculty of Science, Shizuoka University, 836, Ohya, Suruga-ku, Shizuoka 422-8529, Japan Received 31 July 2006; received in revised form 9 April 2007; accepted 10 April 2007 Available online 4 June 2007
Abstract Implantation temperature dependence on chemical behavior of energetic deuterium (D) implanted into highly oriented pyrolytic graphite (HOPG) was investigated by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) in the temperature range 323–873 K. The experimental results showed that the D retention decreased with the increase of the implantation temperature. The -* transition peak of HOPG, which suggests the existence of sp2 - bonding of graphite structure, disappeared after D2 + implantation at 323 K, while the peak recovered when isochronal heating above 773 K was performed, which corresponded to the beginning temperature of the D desorption in the TDS spectrum. Therefore, this suggests that the D desorption could correlate with the recovery of graphite structure disordered by the D2 + implantation. In addition, the implanted D was found to be trapped in two different chemical states that desorbed at 900 K and 1050 K, namely Peak 1 and Peak 2, respectively: the former is a trapping state of D with forming sp3 hybrid orbital type C–D bond and the latter with forming type C–D bond of sp2 hybrid carbon. © 2007 Elsevier B.V. All rights reserved. PACS: 52.40.Hf; 81.05.Uw; 33.60.Fy Keywords: Carbon materials; Tritium; XPS; TDS
1. Introduction Carbon materials have been widely used as plasma facing materials (PFMs) in many plasma-testing ∗ Corresponding author. Tel.: +81 54 238 6436; fax: +81 54 238 3989. E-mail address: [email protected] (T. Suda).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.04.012
devices. In addition, carbon fiber composite (CFC) could be employed as a candidate material for a divertor in international thermonuclear experimental reactor (ITER) because of its ability to withstand high heat flux, its high sublimation temperature [1–5] and being a low-Z material. We have, therefore, investigated chemical behavior of energetic tritium (T) implanted in carbon materials by simulating with energetic deu-
T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
terium ion from the viewpoint of T safety in fusion reactors. In our previous study, it has been reported that there were two desorption stages at around 900 and 1100 K of the D2 thermal desorption spectroscopy (TDS) spectrum for the D2 + implanted pyrolytic graphite (PG) [6]. This suggests that the deuterium (D) implanted into PG exists in two chemical states. However, the implantation temperature dependence on the chemical behaviors of implanted D trapped by the two chemical states in graphite has not been clarified yet, although the temperature range of divertor region could be extremely wide. Therefore, it is necessary to understand how tritium trapped by the two chemical states in graphite behaves in each temperature with respect to tritium safety in fusion reactors. Although some reports have also ever mentioned about the chemical states of D implanted into graphite [6–8], studies for chemical states of D on dependence of implantation temperature has been very limited. It is, however, important from the viewpoint of more realistic estimation of tritium inventory to accumulate data for implantation temperature dependence of the T trapping states in graphite. The present study, therefore, has been conducted to investigate detail implantation temperature dependence of trapping states of implanted hydrogen isotopes into graphite in more realistic operation conditions. In the present study, the D2 + implantation was performed at various implantation temperatures between 323 and 873 K. The sample was a highly oriented pyrolytic graphite (HOPG) because of its simple structure compared with CFC. Thermal desorption behaviors of D implanted into HOPG and the chemical states of D trapped in HOPG were studied by means of the thermal desorption spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively.
2. Experimental The sample used in the present study was HOPG purchased from Pechiney Co. The sample size was 10 × 10 × 1.5t mm3 and its density was 2.3 g/cm3 . The sample was cleaved mechanically and was mounted on a sample holder equipped with a ceramic heater, of which detail was found in Ref. [9]. The sample was preheated at 1473 K for 10 min to remove residual gases
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such as H2 O, O2 and H2 in ultrahigh vacuum less than 10−8 Pa. After the heating treatment, XPS (ESCA 1600 Series, ULVAC-PHI Inc.) measurement using Al K␣ as X-ray source was performed, and it was found that no impurities were observed on the surface. To investigate the dependence of implantation temperature, the D2 + was implanted into HOPG with ion energy of 1.0 keV, the flux of 1.0 × 1018 D+ m−2 s−1 , and the fluence of 6.4 × 1021 D+ m−2 , which corresponded to the saturation fluence at 323–873 K. After D2 + implantation at each temperature, TDS experiment was carried out by heating the sample up to 1473 K with the heating rate of 0.5 K s−1 . In addition, the graphite structural change by D2 + implantation at each temperature was also analyzed by XPS. It should be noted that the XPS, TDS and ion gun equipments were placed under the same vacuum to perform experiments without air exposure [10–15].
3. Results and discussion Figs.1(a) and (b) show the D2 TDS spectrum after D2 + implantation at 323 K with the analysis using the Gaussian distribution function and the C-1s XPS spectra for D implanted HOPG during isochronal heating experiment up to 973 K, respectively. The TDS spectrum is found to consist of two peaks located around 900 and 1050 K, namely Peak 1 and Peak 2. This indicates that the implanted D could exist in two chemical states in HOPG. In Fig. 1(b), it can be seen that a -* transition peak, which indicates the existence of sp2 - bonding of graphite structure, was observed at around 290 eV for the sample after pre-heating treatment. The -* transition peak have disappeared after the D2 + implantation at 323 K, while the peak recovered after isochronal heating above 773 K. This temperature corresponded to the beginning temperature of D desorption in TDS spectrum. Therefore, the D desorption could correlate with the recovery of graphite structure disordered by the D2 + implantation. Fig. 2 shows the typical D2 TDS spectra for HOPG at various implantation temperatures between 323 and 873 K. The ratios of total deuterium to graphite D/C were calculated and plotted as a function of implantation temperatures in Fig. 3. The ion range of deuterium ion in HOPG was obtained by the SRIM code. Based on this result, the amount of carbon within deuterium
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Fig. 1. (a) The TDS spectrum of D2 after deuterium implantation at 323 K. (b) The C-1s XPS spectra (a) during TDS experiments.
Fig. 2. The TDS spectra of D2 for D2 + implanted HOPG at each implantation temperatures.
Fig. 3. The D/C for D2 + implanted HOPG at each temperature.
ion implanted range was estimated. The ratios of D trapped by Peak 1 and Peak 2 were also shown in this figure. It was found that the total D retention in HOPG almost decreased with increasing the implantation temperature. This result was consistent with that in the previous reports [16]: the saturated concentration of hydrogen in PG decreased with increasing the implantation temperature between room temperature and 873 K. The decrease of D retention at the temperatures lower than 573 K was governed by the D2 desorption of Peak 2 because the D retention resulting from Peak 1 was almost constant lower than 573 K and that at the temperature above 573 K mainly depended on the desorption of D trapped by Peak 1. It was found that the D/C of Peak 1 showed no significant dependence on the implantation temperature, while that of Peak 2 depended on the implantation temperature. Fig. 4 shows the C-1s XPS spectra for HOPG with 1.0 keV D2 + implantation at various implantation temperatures between 323 and 873 K. It was found that the C-1s peak was shifted toward higher binding energy side by the D2 + implantation at the implantation temperature of 323 K compared to after pre-heating. This result suggests that the implanted D was bound to carbon with forming C–D bond [17]. For the sample implanted at the implantation temperatures between 473 K and 573 K, however, the peaks were largely shifted toward lower binding energy side. Although, the -* transition peak disappeared after D2 + implantation at 323 K, it became slightly observable at the implantation temperatures above 573 K. On the other hand, comparing with the XPS spectrum obtained for
T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
Fig. 4. The C-1s XPS spectra after 1 keV D2 + implantation at various temperatures.
the sample pre-heated at 1473 K, the almost complete recovery of the -* transition peak was observed and no peak shifts were observed in the C-1s XPS spectrum after the TDS experiment performed up to 950 K, temperature that corresponded to the complete desorption of D desorption from Peak 1. According to the TDS and XPS experimental results, implantation temperature dependence on the chemical behavior of hydrogen isotope in HOPG was discussed. The implantation temperature dependence for Peak 1 was quite different from that for Peak 2. The D2 desorption of Peak 2 clearly depended on the implantation temperature that the D retention resulted from Peak 2 decreased with increasing the implantation temperature. However, the D retention resulting from Peak 1 was almost constant at the implantation temperature lower than 573 K. These facts indicate that the implanted D trapped by Peak 1 would directly react with carbon atoms and trap by a metastable state by the way of hot atomic reactions where trapping behavior would be governed not by thermal reaction kinetics, but by a trapping cross-section depending on kinetics energies of the implanted D ions. It could be thought that the implantation temperature of 573 K would be a threshold temperature for the D trapped by Peak 1. On the other hand, the implanted D trapped by Peak 2 would be trapped and detrapped repeatedly with diffusion under thermal equilibrium and some of them would desorbed by thermal annealing during D2 + implantation. From these observations, the chemical states of D implanted into HOPG were discussed. According to the
1765
recovery of -* transition peak and C-1s peak shift by XPS and the D desorption behavior by TDS, it can be said that the D trapping states for Peak 1 and Peak 2 were attributed to be sp3 hybrid orbital type C–D bond and type C–D bond of sp2 hybrid carbon, respectively [11,12]. Therefore, it can be concluded that T retention decreases with increasing implantation temperatures and the decrease of T retention depends on sp3 hybrid orbital type C–D bond at the implantation temperatures above 573 K, while that mainly depends on type C–D bond of sp2 hybrid carbon lower than 573 K. Therefore, the T trapped by sp3 hybrid orbital type C–D bond would become dominant in the T retention behavior of graphite at operation temperature of a divertor in fusion reactors.
4. Conclusion The dependence of implantation temperature on the chemical behavior of deuterium implanted into HOPG was investigated by TDS and XPS. It was found that D implanted into HOPG was trapped by two chemical states: Peak 1 at 900 K and Peak 2 at 1050 K were attributed to be sp3 hybrid orbital type C–D bond and type C–D bond of sp2 hybrid carbon, respectively. In addition, D trapped by Peak 1 directly reacted with carbon atoms and trapped by metastable state by the way of hot atomic reactions where trapping behavior would be governed not by thermal reaction kinetics, but by a trapping crosssection depended on kinetics energies of the implanted D ions and D trapped by Peak 2 would be trapped and detrapped repeatedly with diffusion under thermal equilibrium and some of them would desorbed by thermal annealing during D2 + implantation. The D retention was also found to decrease with increasing implantation temperature. It was concluded that the T retention would decrease with increasing implantation temperatures and the decrease of T retention would mainly result from the decrease of Peak 1 above the implantation temperature of 573 K, while that below 573 K from the Peak 2 decreasing. Therefore, T retention trapped by sp3 hybrid orbital type C–D bond would be more critical at operation temperature of a divertor in fusion reactors.
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T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
Acknowledgement This study was performed using XPS and TDS devices in the Center of Instrumental Analysis at Shizuoka University.
References [1] R.D. Penzhorn, N. Bekris, W. Hellriegel, H.E. Noppel, W. Nagele, H. Ziegler, et al., Tritium profile in tiles from the first wall of fusion machines and techniques for their detritiation, J. Nucl. Mater. 279 (2000) 139–152. [2] J.P. Adloff, P.P. Gaspar, M. Imamura, A.G. Maddoch, T. Matsuura, H. Sano, K. Yoshihara (Eds.), Handbook of Hot Atom Chemistry, VCH, New York, 1992. [3] W. Moller, P. Borgesen, B.M.U. Scherzer, Thermal and ioninduced release of hydrogen atoms implanted into graphite, Nucl. Instrum. Methods B 19 & 20 (1987) 826–831. [4] A. Miyahara, T. Tanabe, Graphite as plasma facing material, J. Nucl. Mater. 155–157 (1988) 49–57. [5] J. Winter, Carbonization in tokamaks, J. Nucl. Mater. 145–147 (1987) 131–144. [6] Y. Gotoh, FT-IR studies of graphite after keV-energy hydrogen ion irradiation, J. Nucl. Mater. 266–269 (1999) 1051–1058. [7] Frances H. Yang, Ralph T. Yang, Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite: Insight into hydrogen storage in carbon nanotubes, Carbon 40 (2002) 437–444. [8] S.L. Kanashenko, A.E. Gorodetsky, V.N. Chernikov, A.V. Markin, A.P. Zakharov, B.L. Doyle, et al., Hydrogen adsorption on and solubility in graphites, J. Nucl. Mater. 233–237 (1996) 1207–1212.
[9] Y. Morimoto, T. Sugiyama, S. Akahori, H. Kodama, E. Tega, M. Sasaki, et al., Study on energetic ions behavior in plasma facing materials at lower temperature, Phys. Scr. T103 (2003) 117–120. [10] T. Sugiyama, Y. Morimoto, K. Iguchi, K. Okuno, M. Miyamoto, H. Iwakiri, et al., Effects of helium irradiation on chemical behavior of energetic deuterium in SiC, J. Nucl. Mater. 307–311 (2002) 1080–1083. [11] Y. Morimoto, K. Okuno, Correlation between annealing effects of damage and implanted deuterium release from graphite, J. Nucl. Mater. 313–316 (2003) 595–598. [12] M. Sasaki, Y. Morimoto, H. Kimura, K. Takahashi, K. Sakamoto, T. Imai, et al., Energetic deuterium and helium irradiation effects on chemical structure of CVD diamond, J. Nucl. Mater. 329–333 (2004) 899–903. [13] H. Kimura, M. Sasaki, Y. Morimoto, T. Takeda, H. Kodama, A. Yoshikawa, et al., Thermal desorption behavior of deuterium implanted into polycrystalline diamond, J. Nucl. Mater. 337–339 (2005) 614–618. [14] Y. Oya, Y. Onishi, H. Kodama, K. Okuno, S. Tanaka, Dynamic hydrogen isotope behavior and its chemical states in SiC by XPS and TDS technique, J. Nucl. Mater. 337–339 (2005) 595– 599. [15] H. Kimura, Y. Nishikawa, T. Nakahata, M. Oyaidzu, Y. Oya, K. Okuno, Chemical behavior of energetic deuterium implanted into tungsten carbide, Fusion. Eng. Des. 81 (2006) 295– 299. [16] B.L. Doyle, W.R. Wampler, D.K. Brice, Temperature dependence of H saturation and isotope exchange, J. Nucl. Mater. 103 (1981) 513–517. [17] H. Kodama, T. Sugiyama, Y. Morimoto, Y. Oya, K. Okuno, N. Inoue, et al., Thermal annealing effects on chemical states of deuterium implanted into boron coating film, J. Nucl. Mater. 313–316 (2003) 153–157.
Fusion Engineering and Design 82 (2007) 1762–1766
Effects of implantation conditions on retention behavior of deuterrium in highly oriented pyrolytic graphite Taichi Suda ∗ , Hideo Miyauchi, Akira Yoshikawa, Hiromi Kimura, Yasuhisa Oya, Kenji Okuno Radiochemistry Research Laboratory, Faculty of Science, Shizuoka University, 836, Ohya, Suruga-ku, Shizuoka 422-8529, Japan Received 31 July 2006; received in revised form 9 April 2007; accepted 10 April 2007 Available online 4 June 2007
Abstract Implantation temperature dependence on chemical behavior of energetic deuterium (D) implanted into highly oriented pyrolytic graphite (HOPG) was investigated by thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS) in the temperature range 323–873 K. The experimental results showed that the D retention decreased with the increase of the implantation temperature. The -* transition peak of HOPG, which suggests the existence of sp2 - bonding of graphite structure, disappeared after D2 + implantation at 323 K, while the peak recovered when isochronal heating above 773 K was performed, which corresponded to the beginning temperature of the D desorption in the TDS spectrum. Therefore, this suggests that the D desorption could correlate with the recovery of graphite structure disordered by the D2 + implantation. In addition, the implanted D was found to be trapped in two different chemical states that desorbed at 900 K and 1050 K, namely Peak 1 and Peak 2, respectively: the former is a trapping state of D with forming sp3 hybrid orbital type C–D bond and the latter with forming type C–D bond of sp2 hybrid carbon. © 2007 Elsevier B.V. All rights reserved. PACS: 52.40.Hf; 81.05.Uw; 33.60.Fy Keywords: Carbon materials; Tritium; XPS; TDS
1. Introduction Carbon materials have been widely used as plasma facing materials (PFMs) in many plasma-testing ∗ Corresponding author. Tel.: +81 54 238 6436; fax: +81 54 238 3989. E-mail address: [email protected] (T. Suda).
0920-3796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2007.04.012
devices. In addition, carbon fiber composite (CFC) could be employed as a candidate material for a divertor in international thermonuclear experimental reactor (ITER) because of its ability to withstand high heat flux, its high sublimation temperature [1–5] and being a low-Z material. We have, therefore, investigated chemical behavior of energetic tritium (T) implanted in carbon materials by simulating with energetic deu-
T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
terium ion from the viewpoint of T safety in fusion reactors. In our previous study, it has been reported that there were two desorption stages at around 900 and 1100 K of the D2 thermal desorption spectroscopy (TDS) spectrum for the D2 + implanted pyrolytic graphite (PG) [6]. This suggests that the deuterium (D) implanted into PG exists in two chemical states. However, the implantation temperature dependence on the chemical behaviors of implanted D trapped by the two chemical states in graphite has not been clarified yet, although the temperature range of divertor region could be extremely wide. Therefore, it is necessary to understand how tritium trapped by the two chemical states in graphite behaves in each temperature with respect to tritium safety in fusion reactors. Although some reports have also ever mentioned about the chemical states of D implanted into graphite [6–8], studies for chemical states of D on dependence of implantation temperature has been very limited. It is, however, important from the viewpoint of more realistic estimation of tritium inventory to accumulate data for implantation temperature dependence of the T trapping states in graphite. The present study, therefore, has been conducted to investigate detail implantation temperature dependence of trapping states of implanted hydrogen isotopes into graphite in more realistic operation conditions. In the present study, the D2 + implantation was performed at various implantation temperatures between 323 and 873 K. The sample was a highly oriented pyrolytic graphite (HOPG) because of its simple structure compared with CFC. Thermal desorption behaviors of D implanted into HOPG and the chemical states of D trapped in HOPG were studied by means of the thermal desorption spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively.
2. Experimental The sample used in the present study was HOPG purchased from Pechiney Co. The sample size was 10 × 10 × 1.5t mm3 and its density was 2.3 g/cm3 . The sample was cleaved mechanically and was mounted on a sample holder equipped with a ceramic heater, of which detail was found in Ref. [9]. The sample was preheated at 1473 K for 10 min to remove residual gases
1763
such as H2 O, O2 and H2 in ultrahigh vacuum less than 10−8 Pa. After the heating treatment, XPS (ESCA 1600 Series, ULVAC-PHI Inc.) measurement using Al K␣ as X-ray source was performed, and it was found that no impurities were observed on the surface. To investigate the dependence of implantation temperature, the D2 + was implanted into HOPG with ion energy of 1.0 keV, the flux of 1.0 × 1018 D+ m−2 s−1 , and the fluence of 6.4 × 1021 D+ m−2 , which corresponded to the saturation fluence at 323–873 K. After D2 + implantation at each temperature, TDS experiment was carried out by heating the sample up to 1473 K with the heating rate of 0.5 K s−1 . In addition, the graphite structural change by D2 + implantation at each temperature was also analyzed by XPS. It should be noted that the XPS, TDS and ion gun equipments were placed under the same vacuum to perform experiments without air exposure [10–15].
3. Results and discussion Figs.1(a) and (b) show the D2 TDS spectrum after D2 + implantation at 323 K with the analysis using the Gaussian distribution function and the C-1s XPS spectra for D implanted HOPG during isochronal heating experiment up to 973 K, respectively. The TDS spectrum is found to consist of two peaks located around 900 and 1050 K, namely Peak 1 and Peak 2. This indicates that the implanted D could exist in two chemical states in HOPG. In Fig. 1(b), it can be seen that a -* transition peak, which indicates the existence of sp2 - bonding of graphite structure, was observed at around 290 eV for the sample after pre-heating treatment. The -* transition peak have disappeared after the D2 + implantation at 323 K, while the peak recovered after isochronal heating above 773 K. This temperature corresponded to the beginning temperature of D desorption in TDS spectrum. Therefore, the D desorption could correlate with the recovery of graphite structure disordered by the D2 + implantation. Fig. 2 shows the typical D2 TDS spectra for HOPG at various implantation temperatures between 323 and 873 K. The ratios of total deuterium to graphite D/C were calculated and plotted as a function of implantation temperatures in Fig. 3. The ion range of deuterium ion in HOPG was obtained by the SRIM code. Based on this result, the amount of carbon within deuterium
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T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
Fig. 1. (a) The TDS spectrum of D2 after deuterium implantation at 323 K. (b) The C-1s XPS spectra (a) during TDS experiments.
Fig. 2. The TDS spectra of D2 for D2 + implanted HOPG at each implantation temperatures.
Fig. 3. The D/C for D2 + implanted HOPG at each temperature.
ion implanted range was estimated. The ratios of D trapped by Peak 1 and Peak 2 were also shown in this figure. It was found that the total D retention in HOPG almost decreased with increasing the implantation temperature. This result was consistent with that in the previous reports [16]: the saturated concentration of hydrogen in PG decreased with increasing the implantation temperature between room temperature and 873 K. The decrease of D retention at the temperatures lower than 573 K was governed by the D2 desorption of Peak 2 because the D retention resulting from Peak 1 was almost constant lower than 573 K and that at the temperature above 573 K mainly depended on the desorption of D trapped by Peak 1. It was found that the D/C of Peak 1 showed no significant dependence on the implantation temperature, while that of Peak 2 depended on the implantation temperature. Fig. 4 shows the C-1s XPS spectra for HOPG with 1.0 keV D2 + implantation at various implantation temperatures between 323 and 873 K. It was found that the C-1s peak was shifted toward higher binding energy side by the D2 + implantation at the implantation temperature of 323 K compared to after pre-heating. This result suggests that the implanted D was bound to carbon with forming C–D bond [17]. For the sample implanted at the implantation temperatures between 473 K and 573 K, however, the peaks were largely shifted toward lower binding energy side. Although, the -* transition peak disappeared after D2 + implantation at 323 K, it became slightly observable at the implantation temperatures above 573 K. On the other hand, comparing with the XPS spectrum obtained for
T. Suda et al. / Fusion Engineering and Design 82 (2007) 1762–1766
Fig. 4. The C-1s XPS spectra after 1 keV D2 + implantation at various temperatures.
the sample pre-heated at 1473 K, the almost complete recovery of the -* transition peak was observed and no peak shifts were observed in the C-1s XPS spectrum after the TDS experiment performed up to 950 K, temperature that corresponded to the complete desorption of D desorption from Peak 1. According to the TDS and XPS experimental results, implantation temperature dependence on the chemical behavior of hydrogen isotope in HOPG was discussed. The implantation temperature dependence for Peak 1 was quite different from that for Peak 2. The D2 desorption of Peak 2 clearly depended on the implantation temperature that the D retention resulted from Peak 2 decreased with increasing the implantation temperature. However, the D retention resulting from Peak 1 was almost constant at the implantation temperature lower than 573 K. These facts indicate that the implanted D trapped by Peak 1 would directly react with carbon atoms and trap by a metastable state by the way of hot atomic reactions where trapping behavior would be governed not by thermal reaction kinetics, but by a trapping cross-section depending on kinetics energies of the implanted D ions. It could be thought that the implantation temperature of 573 K would be a threshold temperature for the D trapped by Peak 1. On the other hand, the implanted D trapped by Peak 2 would be trapped and detrapped repeatedly with diffusion under thermal equilibrium and some of them would desorbed by thermal annealing during D2 + implantation. From these observations, the chemical states of D implanted into HOPG were discussed. According to the
1765
recovery of -* transition peak and C-1s peak shift by XPS and the D desorption behavior by TDS, it can be said that the D trapping states for Peak 1 and Peak 2 were attributed to be sp3 hybrid orbital type C–D bond and type C–D bond of sp2 hybrid carbon, respectively [11,12]. Therefore, it can be concluded that T retention decreases with increasing implantation temperatures and the decrease of T retention depends on sp3 hybrid orbital type C–D bond at the implantation temperatures above 573 K, while that mainly depends on type C–D bond of sp2 hybrid carbon lower than 573 K. Therefore, the T trapped by sp3 hybrid orbital type C–D bond would become dominant in the T retention behavior of graphite at operation temperature of a divertor in fusion reactors.
4. Conclusion The dependence of implantation temperature on the chemical behavior of deuterium implanted into HOPG was investigated by TDS and XPS. It was found that D implanted into HOPG was trapped by two chemical states: Peak 1 at 900 K and Peak 2 at 1050 K were attributed to be sp3 hybrid orbital type C–D bond and type C–D bond of sp2 hybrid carbon, respectively. In addition, D trapped by Peak 1 directly reacted with carbon atoms and trapped by metastable state by the way of hot atomic reactions where trapping behavior would be governed not by thermal reaction kinetics, but by a trapping crosssection depended on kinetics energies of the implanted D ions and D trapped by Peak 2 would be trapped and detrapped repeatedly with diffusion under thermal equilibrium and some of them would desorbed by thermal annealing during D2 + implantation. The D retention was also found to decrease with increasing implantation temperature. It was concluded that the T retention would decrease with increasing implantation temperatures and the decrease of T retention would mainly result from the decrease of Peak 1 above the implantation temperature of 573 K, while that below 573 K from the Peak 2 decreasing. Therefore, T retention trapped by sp3 hybrid orbital type C–D bond would be more critical at operation temperature of a divertor in fusion reactors.
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Acknowledgement This study was performed using XPS and TDS devices in the Center of Instrumental Analysis at Shizuoka University.
References [1] R.D. Penzhorn, N. Bekris, W. Hellriegel, H.E. Noppel, W. Nagele, H. Ziegler, et al., Tritium profile in tiles from the first wall of fusion machines and techniques for their detritiation, J. Nucl. Mater. 279 (2000) 139–152. [2] J.P. Adloff, P.P. Gaspar, M. Imamura, A.G. Maddoch, T. Matsuura, H. Sano, K. Yoshihara (Eds.), Handbook of Hot Atom Chemistry, VCH, New York, 1992. [3] W. Moller, P. Borgesen, B.M.U. Scherzer, Thermal and ioninduced release of hydrogen atoms implanted into graphite, Nucl. Instrum. Methods B 19 & 20 (1987) 826–831. [4] A. Miyahara, T. Tanabe, Graphite as plasma facing material, J. Nucl. Mater. 155–157 (1988) 49–57. [5] J. Winter, Carbonization in tokamaks, J. Nucl. Mater. 145–147 (1987) 131–144. [6] Y. Gotoh, FT-IR studies of graphite after keV-energy hydrogen ion irradiation, J. Nucl. Mater. 266–269 (1999) 1051–1058. [7] Frances H. Yang, Ralph T. Yang, Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite: Insight into hydrogen storage in carbon nanotubes, Carbon 40 (2002) 437–444. [8] S.L. Kanashenko, A.E. Gorodetsky, V.N. Chernikov, A.V. Markin, A.P. Zakharov, B.L. Doyle, et al., Hydrogen adsorption on and solubility in graphites, J. Nucl. Mater. 233–237 (1996) 1207–1212.
[9] Y. Morimoto, T. Sugiyama, S. Akahori, H. Kodama, E. Tega, M. Sasaki, et al., Study on energetic ions behavior in plasma facing materials at lower temperature, Phys. Scr. T103 (2003) 117–120. [10] T. Sugiyama, Y. Morimoto, K. Iguchi, K. Okuno, M. Miyamoto, H. Iwakiri, et al., Effects of helium irradiation on chemical behavior of energetic deuterium in SiC, J. Nucl. Mater. 307–311 (2002) 1080–1083. [11] Y. Morimoto, K. Okuno, Correlation between annealing effects of damage and implanted deuterium release from graphite, J. Nucl. Mater. 313–316 (2003) 595–598. [12] M. Sasaki, Y. Morimoto, H. Kimura, K. Takahashi, K. Sakamoto, T. Imai, et al., Energetic deuterium and helium irradiation effects on chemical structure of CVD diamond, J. Nucl. Mater. 329–333 (2004) 899–903. [13] H. Kimura, M. Sasaki, Y. Morimoto, T. Takeda, H. Kodama, A. Yoshikawa, et al., Thermal desorption behavior of deuterium implanted into polycrystalline diamond, J. Nucl. Mater. 337–339 (2005) 614–618. [14] Y. Oya, Y. Onishi, H. Kodama, K. Okuno, S. Tanaka, Dynamic hydrogen isotope behavior and its chemical states in SiC by XPS and TDS technique, J. Nucl. Mater. 337–339 (2005) 595– 599. [15] H. Kimura, Y. Nishikawa, T. Nakahata, M. Oyaidzu, Y. Oya, K. Okuno, Chemical behavior of energetic deuterium implanted into tungsten carbide, Fusion. Eng. Des. 81 (2006) 295– 299. [16] B.L. Doyle, W.R. Wampler, D.K. Brice, Temperature dependence of H saturation and isotope exchange, J. Nucl. Mater. 103 (1981) 513–517. [17] H. Kodama, T. Sugiyama, Y. Morimoto, Y. Oya, K. Okuno, N. Inoue, et al., Thermal annealing effects on chemical states of deuterium implanted into boron coating film, J. Nucl. Mater. 313–316 (2003) 153–157.