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Temperature dependence of retention of energetic deuterium and carbon simultaneously implanted into tungsten
Journal of Nuclear Materials 417 (2011) 555–558
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Temperature dependence of retention of energetic deuterium and carbon simultaneously implanted into tungsten Wanjing Wang a,e,⇑, Makoto Kobayashi a, Rie Kurata a, Sachiko Suzuki a, Naoko Ashikawa b, Akio Sagara b, Naoaki Yoshida c, Yuji Hatano d, Guang-Nan Luo e, Yasuhisa Oya a, Kenji Okuno a a
Radioscience Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan National Institute for Fusion Science, Gifu, Japan Research Institute for Applied Mechanics, Kyushu University, Kyushu, Japan d Hydrogen Isotope Research Center, University of Toyama, Toyama, Japan e Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China b c
a r t i c l e
i n f o
Article history: Available online 24 December 2010
a b s t r a c t The simultaneous implantation of 10 keV C+ and 3.0 keV Dþ 2 into tungsten was carried out at elevated temperatures to establish the retention mechanism of energetic deuterium and carbon. Thermal desorption spectroscopy showed that the deuterium retention obviously decreased as the implantation temperature increased. All the desorption stages disappeared when the implantation was performed at 673 K, indicating no deuterium trapping as C–D bonds was processed in this temperature. The results of X-ray photoelectron spectroscopy and glow discharge-optical emission spectroscopy showed that a mixed carbon layer had been formed during the implantation, resulting from the carbon deposition and accumulation near the surface of tungsten during the implantation. The carbon layer would enhance the chemical sputtering with deuterium and reduce the deuterium retention, as the implantation temperature increases. During discussion, a simple retention mechanism has been proposed, which shows the importance of implantation temperature. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Tungsten and graphite have been selected as the divertor materials in ITER, which will be exposed to a high flux plasma [1,2]. Under these conditions, graphite could be sputtered as C+ and hydrocarbons, and implanted into tungsten along with hydrogen isotopes, including tritium. Therefore, mixed layers are thought to be formed on the surface of tungsten dynamically during discharges, which complicates hydrogen retention and recovery processes of damage [3–9]. In our previous studies [9], sequential and simultaneous C+ and Dþ 2 implantations into tungsten were carried out and it was found that the chemical sputtering of deuterium with carbon could reduce deuterium retention. In addition, the temperature dependences of the tungsten sputtering and the accumulation of implanted carbon in tungsten under only C+ and simultaneous C+ + and Dþ 2 irradiations were reported [8]. For only C irradiation, the accumulation of carbon layer was occurred, indicative of the C deposition regime, which would protect the underlying tungsten from further sputtering. The C deposition regime decreased and transition to a tungsten sputtering regime occurred with increasing irradiation ⇑ Corresponding author at: Radioscience Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan. Tel.: +81 54 238 4752; fax: +81 54 238 3989. E-mail address: [email protected] (W. Wang).
temperature. For simultaneous C+ and Dþ 2 irradiation, the transition from the C deposition regime to a tungsten sputtering regime was influenced by the implantation temperature and C fraction in the total implantation flux contributed to chemical erosion in the tungsten sputtering regime. However, deuterium retention during the sputtering and the accumulation of carbon has not been discussed. This motivated us to investigate further the retention mechanism of energetic deuterium and carbon in tungsten. This study was focused on the deuterium trapping and desorption processes during the implantation as a function of implantation temperature and investigate the role of carbon on the deuterium retention. In this work, the simultaneous implantation of C+ and Dþ 2 were carried out at temperatures ranging from room temperature to 673 K. The thermal desorption processes of implanted deuterium and the chemical states of implanted carbon were studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS), respectively. The glow discharge-optical emission spectroscopy (GD-OES) was also performed to estimate depth profile of deuterium and carbon in the mixed layer.
2. Experimental Disk-shaped samples, 10 mm diameter and 0.5 mm thickness, were cut from a rod of stress-relieved tungsten supplied by Allied
0022-3115/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.111
W. Wang et al. / Journal of Nuclear Materials 417 (2011) 555–558
Materials Co. Ltd. The samples were mechanically polished and heated to 1173 K for 10 min in vacuum to remove surface impurities and damages introduced by the polishing process. CO2 was used as a C+ source gas to minimize hydrogen impurities. An E B mass separator was located at the head of the C+ ion gun to remove all impurity ion species. To evaluate the interaction mechanism of energetic deuterium and carbon in tungsten, it is important to maintain the same implantation depth for C+ and Dþ 2 . Therefore the ion energies of C+ and Dþ 2 were fixed to be 10 keV and 3 keV, respectively, which correspond to implantation depths of 11 nm [10]. In order to evaluate the temperature dependence, the simultaneous implantations of C+ and Dþ 2 were performed at selected temperatures ranging from room temperature to 673 K. The ion flux and fluence of C+, was 0.2 1018 C+ m2 s1 and 0.2 1022 C+ m2, 18 + respectively, and those of Dþ D m2 s1 and 1.0 2 , 1.0 10 1022 D+ m2, for a flux ratio C+/D+ of 0.2. After implantation, the chemical states of tungsten and carbon were evaluated by XPS (ESCA1600 system, ULVAC-PHI Inc.) using an Al Ka X-ray source (1486.6 eV) and a hemispherical electron analyzer [9]. The desorption and retention behavior of deuterium was evaluated by TDS with a heating rate of 0.5 K s1 up to 1173 K. Measurements of GD-OES were also carried out at University of Toyama using Ar+ plasma to obtain the depth profiles of deuterium and carbon in tungsten. 3. Results and discussion
7.5
D2 TDS Spectrum 6.0
Peak 1 Peak 2
17
Desorption rate /10 D2m s
(a)
-2 -1
Fig. 1a shows the typical D2 TDS spectrum for the simultaneous C+ and Dþ 2 implanted tungsten at room temperature. The TDS spectrum was consisted of two desorption stages, the peaks of which were located around 410 and 580 K, named Peaks 1 and 2,
4.5 3.0 1.5 0.0 300
500
700
900
1100
respectively. According to the previous study [11,12], Peak 1 was assigned to the desorption of deuterium trapped in intrinsic defects in tungsten, such as grain boundaries and dislocations, and Peak 2 was corresponded to the deuterium retained in some other ion-induced defects. The D2 TDS spectra for various implantation temperatures between R.T. and 673 K was shown in Fig. 1b. It was clear that the total deuterium retention decreased as implantation temperature increased. For the sample irradiated at 373 K, Peak 1 in the D2 TDS spectrum decreased and shifted to higher temperature side. Then all thermal desorption peaks have disappeared for 673 K implantation. These results indicated that as the implantation temperature was elevated, the trapped deuterium would be detrapped and desorbed. In Fig. 2, the decrease of deuterium retention as a function of implantation temperature, including the fraction of Peaks 1 and 2, has been shown clearly. In Ref. [11], for WC sample a deuterium desorption stage above 700 K has been observed, indicating that C–D bonds have formed. However, here we have not found these desorption stage. In the following, we will discuss the deposition of carbon on the tungsten and its influence on the deuterium retention. Fig. 3a shows the XPS spectra for the binding energy of 1s shell electron in carbon atom (C-1s) on the surface of C+ and Dþ 2 implanted tungsten [13]. The C-1s XPS spectra were divided into two peaks: the lower peak located at 282.7 eV was attributed to C–W bonds and the higher one at 284.6 eV was attributed to C–C bonds [9,13]. The areas of these peaks as a function of implantation temperature are summarized in Fig. 3b. This figure shows that the ratio of C–W bond slightly decreased as the implantation temperature increased. However, the amount of C–C bond was almost constant among these implantation temperatures, indicating that the carbon with forming C–C bond would be accumulated near the surface of the tungsten during the implantation. The XPS spectra just provided the chemical state of carbon on the surface. In order to obtain more information on the accumulation of carbon and deuterium near the tungsten surface, GD-OES measurements were carried out. Fig. 4 shows the GD-OES spectra for deuterium and carbon after the simultaneous implantation in tungsten at various temperatures. The GD-OES spectra for deuterium (Fig. 4a) show that the deuterium retention decreased quickly as the implantation temperature increased. This behavior is almost consistent with the TDS result as shown in Fig. 2. The GD-OES spectra for carbon (Fig. 4b) showed that the carbon is mainly aggregated near the implanted tungsten surface, within 10 nm of the surface, which is the same as the expected implantation depth calculated by SRIM [10]. The GD-OES spectra of carbon also
6
6.0 4.5 3.0 1.5
0.55
-2
Total(TDS) Peak 1(TDS) Peak 2(TDS) GD-OES
4
0.50
20
C+D imp. at RT C+D imp. at 373K C+D imp. at 473K C+D imp. at 673K
D retention / 10 D2 m
7.5
17
Desorption rate /10 D2m s
(b)
-2 -1
Temperature /K
0.45
2 0.40
0 0.0
D2 peak area in GD-OES / -
556
0.35
300
500
700
900
1100
Temperature /K Fig. 1. (a) Peak analysis of D2 TDS spectra for the simultaneous C+ and D+ implanted tungsten at room temperatures. (b) D2 TDS spectra for the simultaneous C+ and D+ implanted tungsten at various implantation temperatures.
RT
473K
673K
Implantation temperatures Fig. 2. Decrease of deuterium retention as a function of implantation temperature (retention in TDS and peak area in GD-OES). The fraction of each peak has also been shown.
W. Wang et al. / Journal of Nuclear Materials 417 (2011) 555–558
C-C bond
(a) 1.0
C-W bond
Intensity / -
0.8 673K
0.6
473K
0.4 R.T.
0.2 0.0 290
285
280
275
Binding energy /eV
Peak area of C-1s / -
(b) 1.0 0.8 0.6 0.4 0.2
C-C bond C-W bond
0.0 RT
473K
673K
Implantation temperature Fig. 3. (a) XPS spectra of C-1s at various implantation temperatures. (b) Peak area for C–C and C–W bonds.
3.0
R.T. 473 K 673 K
(a) Deuterium
Count / -
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
50
Depth /nm 3.0
Count / -
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
showed that the retention of carbon decreased as the implantation temperature increased, especially large reduction of carbon aggregation near the surface has observed, indicating that the dynamic desorption during the implantation has occurred at higher implantation temperature, which can be explained by the increase in C sputtering with increasing temperature [8]. Based on the above experimental results and previous studies, we can summarize the deuterium retention mechanism simply as follows: During the simultaneous implantation of C+ and Dþ 2, most of the energetic ions are reflected [14,15] and the other part of them is implanted into the tungsten. The implanted carbon and deuterium would diffuse and be trapped by grain boundaries, interstitial sites and vacancies in tungsten, if the sample temperature is not so high that not all of traps are inactive. In addition, during the ion implantation, some ion-induced defects will be produced by the energetic C+ and Dþ 2 and these defects will also trap the implanted deuterium [12]. Various types of trapping sites have different activation energies [12]. When the sample temperature is elevated, the deuterium trapped in sites with low activation energy can be detrapped. In addition, some implanted deuterium will diffuse back to the surface and can be sputtered by energetic ions implanted sequentially [8]. The sample temperature also influences the sputtering process [14]. When the sample temperature is high, the trapped deuterium would diffuse to surface and be sputtered more easily [8]. The signal of QMS during simultaneous implantation at various temperature showed the enhancement of CDx desorption with increasing implantation temperature, meaning that chemical sputtering has occurred during implantation and this chemical sputtering would increase as temperature increases [8]. In all, on one hand the energetic ions, such as C+ and Dþ 2 , would like to implant, diffuse and be trapped near the surface of tungsten during implantation; on the other hand they also are like to be detrapped, desorbed and sputtered. In this processes, the temperature plays a dependent role. In summary, the implanted carbon would create many defects in the bulk tungsten [10], which can trap the implanted deuterium. Therefore, more deuterium would be retained in the simultaneous implantation of deuterium and carbon. However, the mixed layer formed near the surface of tungsten will react with the implanted deuterium and the chemical sputtering will occur. As a result, the deuterium retention will decrease and the concentration of carbon close to the surface would decrease. As the implantation temperature increases, the implanted deuterium would be detrapped and diffuse back to surface where the deuterium atoms recombine into molecules for the desorption, accompanied with the sputtering including chemical sputtering. So only a few of deuterium has been trapped in the tungsten. As shown in Figs. 2 and 4a, most of the implanted deuterium was thermal desorbed when the implantation were performed above 473 K. 4. Conclusions
R.T. 473K 673K
(b) Carbon
557
50
Depth /nm Fig. 4. GD-OES spectra for (a) deuterium and (b) carbon at various implantation temperatures.
The temperature dependence of retention of energetic deuterium and carbon implanted in tungsten was investigated by TDS and XPS. The TDS and GD-OES spectra shown that the deuterium retention decreased obviously with increasing implantation temperature. The experimental results also showed that a mixed tungsten–carbon layer has formed near the surface of implanted tungsten, resulting from the trapping and deposition during implantation. The mixed layer would react and chemical sputter with deuterium and then reduce the deuterium retention. During the discussion of the effects of implantation temperature and carbon layer, a simple deuterium retention mechanism has been proposed. However, no C–D bond has been observed in this experiment, which is different from the deuterium retention in tungsten carbide (WC) implanted by deuterium [11]. Therefore, further studies are required to determine
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W. Wang et al. / Journal of Nuclear Materials 417 (2011) 555–558
the role of the mixed layer on the deuterium trapping and detrapping. Acknowledgements This study has been supported by NIFS collaboration Program No. NIFS07KOBA020, JSPS Kakenhi Nos. 19686055 and 19055002 from MEXT, Japan, and the Center for Instrumental Analysis at Shizuoka University. This work was supported partially by the National Natural Science Foundation of China under Contract No. 10905070. We also would like to acknowledge the Hydrogen Isotope Research Center, University of Toyama, for the measurements of GD-OES. References [1] R. Tivey, T. Andoa, A. Antipenkova, V. Barabasha, S. Chiocchioa, G. Federicia, C. Ibbotta, R. Jakemana, G. aneschitza, R. Raffraya, M. Akibab, I. Mazulc, H. Pacherd, M. Ulricksone, G. Vieder, Fusion Eng. Des. 46 (1999) 207. [2] G. Federici, P. Andrew, P. Barabaschi, J. Brooks, R. Doerner, A. Geier, A. Herrmann, G. Janeschitz, K. Krieger, A. Kukushkin, A. Loarte, R. Neu, G. Saibene, M. Shimada, G. Strohmayer, M. Sugihara, J. Nucl. Mater. 313–316 (2003) 11.
[3] T. Shimada, Y. Ueda, M. Nishikawa, Fusion Eng. Des. 66–68 (2003) 247. [4] I. Bizyukov, K. Krieger, N. Azarenkov, Ch. Linsmeier, S. Levchuk, J. Nucl. Mater. 363–365 (2007) 1184–1189. [5] V. Kh. Alimov, Phys. Scripta T108 (2004) 46–56. [6] V.Kh. Alimov, J. Roth, R.A. Causey, D.A. Komarov, Ch. Linsmeier, A. Wiltner, F. Kost, S. Lindig, J. Nucl. Mater. 375 (2008) 192. [7] W.M. Shu, M. Nakamichi, V.Kh. Alimov, G.-N. Luo, K. Isobe, T. Yamanishi, J. Nucl. Mater. 390–391 (2009) 1017–1021. [8] H.T. Lee, K. Krieger, J. Nucl. Mater. 390–391 (2009) 971–974. [9] Y. Oya, Y. Inagaki, S. Suzuki, H. Ishikawa, Y. Kikuchi, A. Yoshikawa, H. Iwakiri, N. Ashikawa, A. Sagara, N. Yoshida, K. Okuno, J. Nucl. Mater. 390–391 (2009) 622– 625. [10] J.F. Ziegler, SRIM code (2006).
Contents lists available at ScienceDirect
Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Temperature dependence of retention of energetic deuterium and carbon simultaneously implanted into tungsten Wanjing Wang a,e,⇑, Makoto Kobayashi a, Rie Kurata a, Sachiko Suzuki a, Naoko Ashikawa b, Akio Sagara b, Naoaki Yoshida c, Yuji Hatano d, Guang-Nan Luo e, Yasuhisa Oya a, Kenji Okuno a a
Radioscience Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan National Institute for Fusion Science, Gifu, Japan Research Institute for Applied Mechanics, Kyushu University, Kyushu, Japan d Hydrogen Isotope Research Center, University of Toyama, Toyama, Japan e Institute of Plasma Physics, Chinese Academy of Sciences, Hefei, China b c
a r t i c l e
i n f o
Article history: Available online 24 December 2010
a b s t r a c t The simultaneous implantation of 10 keV C+ and 3.0 keV Dþ 2 into tungsten was carried out at elevated temperatures to establish the retention mechanism of energetic deuterium and carbon. Thermal desorption spectroscopy showed that the deuterium retention obviously decreased as the implantation temperature increased. All the desorption stages disappeared when the implantation was performed at 673 K, indicating no deuterium trapping as C–D bonds was processed in this temperature. The results of X-ray photoelectron spectroscopy and glow discharge-optical emission spectroscopy showed that a mixed carbon layer had been formed during the implantation, resulting from the carbon deposition and accumulation near the surface of tungsten during the implantation. The carbon layer would enhance the chemical sputtering with deuterium and reduce the deuterium retention, as the implantation temperature increases. During discussion, a simple retention mechanism has been proposed, which shows the importance of implantation temperature. Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction Tungsten and graphite have been selected as the divertor materials in ITER, which will be exposed to a high flux plasma [1,2]. Under these conditions, graphite could be sputtered as C+ and hydrocarbons, and implanted into tungsten along with hydrogen isotopes, including tritium. Therefore, mixed layers are thought to be formed on the surface of tungsten dynamically during discharges, which complicates hydrogen retention and recovery processes of damage [3–9]. In our previous studies [9], sequential and simultaneous C+ and Dþ 2 implantations into tungsten were carried out and it was found that the chemical sputtering of deuterium with carbon could reduce deuterium retention. In addition, the temperature dependences of the tungsten sputtering and the accumulation of implanted carbon in tungsten under only C+ and simultaneous C+ + and Dþ 2 irradiations were reported [8]. For only C irradiation, the accumulation of carbon layer was occurred, indicative of the C deposition regime, which would protect the underlying tungsten from further sputtering. The C deposition regime decreased and transition to a tungsten sputtering regime occurred with increasing irradiation ⇑ Corresponding author at: Radioscience Research Laboratory, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529, Japan. Tel.: +81 54 238 4752; fax: +81 54 238 3989. E-mail address: [email protected] (W. Wang).
temperature. For simultaneous C+ and Dþ 2 irradiation, the transition from the C deposition regime to a tungsten sputtering regime was influenced by the implantation temperature and C fraction in the total implantation flux contributed to chemical erosion in the tungsten sputtering regime. However, deuterium retention during the sputtering and the accumulation of carbon has not been discussed. This motivated us to investigate further the retention mechanism of energetic deuterium and carbon in tungsten. This study was focused on the deuterium trapping and desorption processes during the implantation as a function of implantation temperature and investigate the role of carbon on the deuterium retention. In this work, the simultaneous implantation of C+ and Dþ 2 were carried out at temperatures ranging from room temperature to 673 K. The thermal desorption processes of implanted deuterium and the chemical states of implanted carbon were studied by means of thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS), respectively. The glow discharge-optical emission spectroscopy (GD-OES) was also performed to estimate depth profile of deuterium and carbon in the mixed layer.
2. Experimental Disk-shaped samples, 10 mm diameter and 0.5 mm thickness, were cut from a rod of stress-relieved tungsten supplied by Allied
0022-3115/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2010.12.111
W. Wang et al. / Journal of Nuclear Materials 417 (2011) 555–558
Materials Co. Ltd. The samples were mechanically polished and heated to 1173 K for 10 min in vacuum to remove surface impurities and damages introduced by the polishing process. CO2 was used as a C+ source gas to minimize hydrogen impurities. An E B mass separator was located at the head of the C+ ion gun to remove all impurity ion species. To evaluate the interaction mechanism of energetic deuterium and carbon in tungsten, it is important to maintain the same implantation depth for C+ and Dþ 2 . Therefore the ion energies of C+ and Dþ 2 were fixed to be 10 keV and 3 keV, respectively, which correspond to implantation depths of 11 nm [10]. In order to evaluate the temperature dependence, the simultaneous implantations of C+ and Dþ 2 were performed at selected temperatures ranging from room temperature to 673 K. The ion flux and fluence of C+, was 0.2 1018 C+ m2 s1 and 0.2 1022 C+ m2, 18 + respectively, and those of Dþ D m2 s1 and 1.0 2 , 1.0 10 1022 D+ m2, for a flux ratio C+/D+ of 0.2. After implantation, the chemical states of tungsten and carbon were evaluated by XPS (ESCA1600 system, ULVAC-PHI Inc.) using an Al Ka X-ray source (1486.6 eV) and a hemispherical electron analyzer [9]. The desorption and retention behavior of deuterium was evaluated by TDS with a heating rate of 0.5 K s1 up to 1173 K. Measurements of GD-OES were also carried out at University of Toyama using Ar+ plasma to obtain the depth profiles of deuterium and carbon in tungsten. 3. Results and discussion
7.5
D2 TDS Spectrum 6.0
Peak 1 Peak 2
17
Desorption rate /10 D2m s
(a)
-2 -1
Fig. 1a shows the typical D2 TDS spectrum for the simultaneous C+ and Dþ 2 implanted tungsten at room temperature. The TDS spectrum was consisted of two desorption stages, the peaks of which were located around 410 and 580 K, named Peaks 1 and 2,
4.5 3.0 1.5 0.0 300
500
700
900
1100
respectively. According to the previous study [11,12], Peak 1 was assigned to the desorption of deuterium trapped in intrinsic defects in tungsten, such as grain boundaries and dislocations, and Peak 2 was corresponded to the deuterium retained in some other ion-induced defects. The D2 TDS spectra for various implantation temperatures between R.T. and 673 K was shown in Fig. 1b. It was clear that the total deuterium retention decreased as implantation temperature increased. For the sample irradiated at 373 K, Peak 1 in the D2 TDS spectrum decreased and shifted to higher temperature side. Then all thermal desorption peaks have disappeared for 673 K implantation. These results indicated that as the implantation temperature was elevated, the trapped deuterium would be detrapped and desorbed. In Fig. 2, the decrease of deuterium retention as a function of implantation temperature, including the fraction of Peaks 1 and 2, has been shown clearly. In Ref. [11], for WC sample a deuterium desorption stage above 700 K has been observed, indicating that C–D bonds have formed. However, here we have not found these desorption stage. In the following, we will discuss the deposition of carbon on the tungsten and its influence on the deuterium retention. Fig. 3a shows the XPS spectra for the binding energy of 1s shell electron in carbon atom (C-1s) on the surface of C+ and Dþ 2 implanted tungsten [13]. The C-1s XPS spectra were divided into two peaks: the lower peak located at 282.7 eV was attributed to C–W bonds and the higher one at 284.6 eV was attributed to C–C bonds [9,13]. The areas of these peaks as a function of implantation temperature are summarized in Fig. 3b. This figure shows that the ratio of C–W bond slightly decreased as the implantation temperature increased. However, the amount of C–C bond was almost constant among these implantation temperatures, indicating that the carbon with forming C–C bond would be accumulated near the surface of the tungsten during the implantation. The XPS spectra just provided the chemical state of carbon on the surface. In order to obtain more information on the accumulation of carbon and deuterium near the tungsten surface, GD-OES measurements were carried out. Fig. 4 shows the GD-OES spectra for deuterium and carbon after the simultaneous implantation in tungsten at various temperatures. The GD-OES spectra for deuterium (Fig. 4a) show that the deuterium retention decreased quickly as the implantation temperature increased. This behavior is almost consistent with the TDS result as shown in Fig. 2. The GD-OES spectra for carbon (Fig. 4b) showed that the carbon is mainly aggregated near the implanted tungsten surface, within 10 nm of the surface, which is the same as the expected implantation depth calculated by SRIM [10]. The GD-OES spectra of carbon also
6
6.0 4.5 3.0 1.5
0.55
-2
Total(TDS) Peak 1(TDS) Peak 2(TDS) GD-OES
4
0.50
20
C+D imp. at RT C+D imp. at 373K C+D imp. at 473K C+D imp. at 673K
D retention / 10 D2 m
7.5
17
Desorption rate /10 D2m s
(b)
-2 -1
Temperature /K
0.45
2 0.40
0 0.0
D2 peak area in GD-OES / -
556
0.35
300
500
700
900
1100
Temperature /K Fig. 1. (a) Peak analysis of D2 TDS spectra for the simultaneous C+ and D+ implanted tungsten at room temperatures. (b) D2 TDS spectra for the simultaneous C+ and D+ implanted tungsten at various implantation temperatures.
RT
473K
673K
Implantation temperatures Fig. 2. Decrease of deuterium retention as a function of implantation temperature (retention in TDS and peak area in GD-OES). The fraction of each peak has also been shown.
W. Wang et al. / Journal of Nuclear Materials 417 (2011) 555–558
C-C bond
(a) 1.0
C-W bond
Intensity / -
0.8 673K
0.6
473K
0.4 R.T.
0.2 0.0 290
285
280
275
Binding energy /eV
Peak area of C-1s / -
(b) 1.0 0.8 0.6 0.4 0.2
C-C bond C-W bond
0.0 RT
473K
673K
Implantation temperature Fig. 3. (a) XPS spectra of C-1s at various implantation temperatures. (b) Peak area for C–C and C–W bonds.
3.0
R.T. 473 K 673 K
(a) Deuterium
Count / -
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
50
Depth /nm 3.0
Count / -
2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
40
showed that the retention of carbon decreased as the implantation temperature increased, especially large reduction of carbon aggregation near the surface has observed, indicating that the dynamic desorption during the implantation has occurred at higher implantation temperature, which can be explained by the increase in C sputtering with increasing temperature [8]. Based on the above experimental results and previous studies, we can summarize the deuterium retention mechanism simply as follows: During the simultaneous implantation of C+ and Dþ 2, most of the energetic ions are reflected [14,15] and the other part of them is implanted into the tungsten. The implanted carbon and deuterium would diffuse and be trapped by grain boundaries, interstitial sites and vacancies in tungsten, if the sample temperature is not so high that not all of traps are inactive. In addition, during the ion implantation, some ion-induced defects will be produced by the energetic C+ and Dþ 2 and these defects will also trap the implanted deuterium [12]. Various types of trapping sites have different activation energies [12]. When the sample temperature is elevated, the deuterium trapped in sites with low activation energy can be detrapped. In addition, some implanted deuterium will diffuse back to the surface and can be sputtered by energetic ions implanted sequentially [8]. The sample temperature also influences the sputtering process [14]. When the sample temperature is high, the trapped deuterium would diffuse to surface and be sputtered more easily [8]. The signal of QMS during simultaneous implantation at various temperature showed the enhancement of CDx desorption with increasing implantation temperature, meaning that chemical sputtering has occurred during implantation and this chemical sputtering would increase as temperature increases [8]. In all, on one hand the energetic ions, such as C+ and Dþ 2 , would like to implant, diffuse and be trapped near the surface of tungsten during implantation; on the other hand they also are like to be detrapped, desorbed and sputtered. In this processes, the temperature plays a dependent role. In summary, the implanted carbon would create many defects in the bulk tungsten [10], which can trap the implanted deuterium. Therefore, more deuterium would be retained in the simultaneous implantation of deuterium and carbon. However, the mixed layer formed near the surface of tungsten will react with the implanted deuterium and the chemical sputtering will occur. As a result, the deuterium retention will decrease and the concentration of carbon close to the surface would decrease. As the implantation temperature increases, the implanted deuterium would be detrapped and diffuse back to surface where the deuterium atoms recombine into molecules for the desorption, accompanied with the sputtering including chemical sputtering. So only a few of deuterium has been trapped in the tungsten. As shown in Figs. 2 and 4a, most of the implanted deuterium was thermal desorbed when the implantation were performed above 473 K. 4. Conclusions
R.T. 473K 673K
(b) Carbon
557
50
Depth /nm Fig. 4. GD-OES spectra for (a) deuterium and (b) carbon at various implantation temperatures.
The temperature dependence of retention of energetic deuterium and carbon implanted in tungsten was investigated by TDS and XPS. The TDS and GD-OES spectra shown that the deuterium retention decreased obviously with increasing implantation temperature. The experimental results also showed that a mixed tungsten–carbon layer has formed near the surface of implanted tungsten, resulting from the trapping and deposition during implantation. The mixed layer would react and chemical sputter with deuterium and then reduce the deuterium retention. During the discussion of the effects of implantation temperature and carbon layer, a simple deuterium retention mechanism has been proposed. However, no C–D bond has been observed in this experiment, which is different from the deuterium retention in tungsten carbide (WC) implanted by deuterium [11]. Therefore, further studies are required to determine
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the role of the mixed layer on the deuterium trapping and detrapping. Acknowledgements This study has been supported by NIFS collaboration Program No. NIFS07KOBA020, JSPS Kakenhi Nos. 19686055 and 19055002 from MEXT, Japan, and the Center for Instrumental Analysis at Shizuoka University. This work was supported partially by the National Natural Science Foundation of China under Contract No. 10905070. We also would like to acknowledge the Hydrogen Isotope Research Center, University of Toyama, for the measurements of GD-OES. References [1] R. Tivey, T. Andoa, A. Antipenkova, V. Barabasha, S. Chiocchioa, G. Federicia, C. Ibbotta, R. Jakemana, G. aneschitza, R. Raffraya, M. Akibab, I. Mazulc, H. Pacherd, M. Ulricksone, G. Vieder, Fusion Eng. Des. 46 (1999) 207. [2] G. Federici, P. Andrew, P. Barabaschi, J. Brooks, R. Doerner, A. Geier, A. Herrmann, G. Janeschitz, K. Krieger, A. Kukushkin, A. Loarte, R. Neu, G. Saibene, M. Shimada, G. Strohmayer, M. Sugihara, J. Nucl. Mater. 313–316 (2003) 11.
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