Helium desorption in 3He implanted tungsten at low fluence and low energy

Nuclear Instruments and Methods in Physics Research B 268 (2010) 223–226

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Short Communication

Helium desorption in 3He implanted tungsten at low fluence and low energy A. Debelle *, P.-E. Lhuillier, M.-F. Barthe, T. Sauvage, P. Desgardin CNRS-CEMHTI, 3A Rue de la Férollerie, 45071 ORLEANS Cedex 2, France

a r t i c l e

i n f o

Article history: Received 5 May 2009 Received in revised form 8 October 2009 Available online 10 November 2009 Keywords: Implantation Helium Trapping Vacancy-like defects

a b s t r a c t The behaviour of helium in polycrystalline 3He implanted tungsten at low energy (60 keV) and low fluence (2  1013 cm 2) has been studied as a function of post-implantation annealing temperature until 1873 K by means of Nuclear Reaction Analysis. Helium desorption has been observed only from 1500 K, suggesting a helium trapping at mono-vacancies. Only 75% of the implanted helium has been released after the annealing during 1 h at high temperature (1873 K); besides, the desorption rate decreased from 1673 K. The presence of a second type of helium trapping site is likely to explain this strong helium retention. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The future International Thermonuclear Experimental Reactor (ITER), planned to operate in about ten years, must prove the scientific and technical feasibility of producing energy by atomic fusion. The components inside this fusion reactor will be submitted to a very severe environment, e.g. very high heat flux deposition and particles bombardment, that was not encountered in present-day machines. It is therefore crucial to define the materials that will be capable to resist to these extreme conditions. Tungsten, due to its intrinsic physical properties such as low sputtering yield for light elements and good thermo-mechanical behaviour, is a candidate material to cover parts of the divertor of controlled fusion experiments like ITER and DEMO. However, at present, studies must be carried out on its potential use in this type of environment. A major concern is the possible accumulation of helium in W. Indeed, the interaction of the high energetic 14 MeV neutrons generated during the fusion reaction with the Plasma-FacingMaterials (PFM), and in particular the divertor, will cause, among other phenomena, a continuous production of He by (n, a) reactions. It is well-known that the solubility of inert gases in metals is very low, and gas atoms tend to agglomerate and form bubbles, leading generally to blistering or cracking and finally the exfoliation of the material occurs [1]. In the field of materials for fusion, many studies dealing with He behaviour in W have already been carried out [2–4]. The general trend that emerges from these research works is that helium is easily and strongly trapped in the tungsten lattice. Two recent studies

well reflect this property: Iwakiri et al. [5] showed that when helium is introduced (1015 cm 2) in the tungsten by implantation at very low energy, 0.25 keV, i.e. below the threshold displacement energy, hence without creating any defect, it forms clusters by auto-agglomeration, even when the implantation temperature is increased up to 1073 K; for implantations at high energy (1.3 MeV) and high temperature (1123 K) and at a 1015 cm 2 fluence, Gilliam et al. [4,6] did not measure any helium desorption after a flash annealing at 2273 K, due to a strong helium trapping at vacancy-like defects. Recently, in order to better understand the He migration processes in W, and more precisely the role of the vacancy defects in implanted polycrystalline W, the helium behaviour in 500 keV 3 He implanted tungsten at 1015 cm 2 was studied by the authors [7]. No helium desorption was observed after post-implantation annealing up to 1773 K, helium being strongly trapped at large vacancy-like defects. In the present study, the objective is to examine the effect of a change in the implantation conditions on the behaviour of helium during implantation and during subsequent thermal treatments. In particular, a simultaneous decrease of the energy (still higher than the required one to produce defects) and of the implantation fluence will be tested, in order to modify both the absolute and the relative helium and radiation-induced defect concentrations.

2. Experimental details 2.1. Samples preparation

* Corresponding author. Address: Université Paris Sud 11 – CSNSM, Bâtiment 108, 91405 Orsay Cedex, France. E-mail address: [email protected] (A. Debelle). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.176

Polycrystalline one-side polished 150 lm thick W samples cut out of commercial laminated foils were used for this study. The

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purity of the samples was 99.95%. Many different impurities have been detected in the specimens, but in a very small amount (20– 30 weight ppm in average). Samples were then submitted to a thermal treatment to reduce the large concentration of laminating/polishing-induced defects [7,8]. This treatment also led to an increase of the mean grain size which is 20 lm. Annealings were performed in the induction furnace of the IPN-Lyon, 1 h at 1873 K under vacuum, i.e. a typical base pressure of 10 8 mbar that may reach 10 6 mbar at the maximum temperature. A positron annihilation study revealed that such type of treatment leads to a quasi-complete defect recovery since the corresponding positron annihilation characteristics are very close to the tungsten lattice ones [8].

Table 1 Helium content determined by NRA in 60 keV 3He implanted W at a 2  1013 cm fluence and annealed ex situ under different conditions. Temperature (K)

Atmosphere

Helium content (%)

RT 1473 1573 1673 1773 1873

/ Vacuum ArH2 ArH2 ArH2 Vacuum

100 94.5 ± 5.5 90.5 ± 5.5 56.5 ± 5.5 36.5 ± 5.5 24.5 ± 5.5

2

2.2. 3He implantations 3 He implantations were carried out with the 400 kV electrostatic accelerator of the IPN-Lyon. A 60 keV energy and a 2  1013 cm 2 fluence were chosen. During the experiment, the mean temperature of the sample holder was maintained below 333 K. SRIM calculations [9] were performed to determine the helium and the damage profiles (see Fig. 1). The maximum relative 3He on tungsten atomic concentration is situated at 132 nm and is 2  10 5; the average damage over the whole cascade region, considering a threshold displacement energy of 90 eV [10,11], is estimated to 2  10 4 displacements per atom (dpa), this parameter being the relative defects on tungsten atomic concentration.

2.3. 3He behaviour studied by means of NRA In the present case, the implementation of the Nuclear Reaction Analysis (NRA) technique relies on the 3He(d, a)1H nuclear reaction. To determine the He content in the W specimens, protons emitted during the nuclear reaction and transmitted through the samples are detected in the direction of the incident deuteron beam. More details of the technique can be found in [12]. For the present study, a 550 keV deuteron beam was used to probe the helium implanted samples. Experiments were carried out on the Van de Graaff accelerator of the CEMHTI-Orléans.

Fig. 2. Helium desorption curve obtained by NRA from 60 keV 3He implanted W at a 2  1013 cm 2 fluence and annealed ex situ at different temperatures.

2.4. Post-implantation thermal treatment After implantation, one sample was kept as a reference and the others were submitted to a thermal treatment. Annealings were realised either under vacuum (in an induction furnace or in an electronic bombardment furnace), or under an ArH2 atmosphere (in a classical tubular furnace). It will be shown that the annealing atmosphere did not play any role on the helium behaviour. In all cases, treatments lasted 1 h. Five temperatures were tested, 1473, 1573, 1673, 1773 and 1873 K.

3. Results and discussion

Fig. 1. SRIM calculations of the implantation profile (full symbols) of 60 keV 3He ions implanted at a 2  1013 cm 2 fluence at RT in tungsten and of the corresponding dpa damage profile (open symbols), taking a displacement threshold energy of 90 eV. Values on the Y-axis correspond to the relative helium or defect on tungsten atomic concentrations.

First of all, the total helium content in the reference sample was determined after implantation. A value of (1.89 ± 0.3)  1013 cm 2 was found, which is equal to the introduced amount (the nominal fluence was 2  1013 cm 2). This result shows the accuracy of the cross section determination and of the charge measurements. It also indicates that the whole implanted helium was trapped during implantation. Indeed, before reaching a thermal equilibrium with the lattice, the implanted helium could have migrated interstitially to the surface, since its migration energy in W is very low, 0.25 eV [13–15] and even lower at low He concentration [16,17]. Ex situ thermal treatments were then performed up to 1873 K. The helium content in the W specimens as a function of annealing conditions is presented in Table 1. A helium release is clearly observed when increasing temperature, irrespective of the annealing atmosphere. Thus, Fig. 2 displays the helium desorption as a function of the temperature only. In order to check any influence of the

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Fig. 3. Scanning electron microscopy pictures of the surface of (a) an as-implanted 60 keV 3He implanted W sample and (b) the corresponding specimen after an annealing at 1773 K under an ArH2 atmosphere.

analysing ion beam on the helium desorption measurements, the evolution of the protons signal during 8 h has been followed: it remained constant, which means that the observed helium release is not due to the probe beam. The surface state of the annealed specimens was also examined by scanning electron microscopy. Fig. 3a represents the surface of an as-implanted W sample and Fig. 3b shows the surface of a specimen annealed at 1773 K after implantation. No difference is observed, i.e. the surface state remained smooth after annealing, which means that the desorption did not proceed from a blistering phenomenon. Desorption starts at a temperature of about 1500–1600 K. Experimental results obtained by Picraux [19], calculations with empirical potentials carried out by Wilson et al. [13] and Kornelsen [20], and very recent ab initio calculations [21] indicate that this temperature corresponds to the helium release from mono-vacancies (V1). The radiation damage in the present implantation conditions is very low. A 2  10 4 dpa is found by SRIM calculations, which are known to over-estimate this value, and T1/2, which is the median energy of the primary knock-on atoms [22], determined in the present case taking into account the energy loss along the ion track, is 300 eV for an 3He incident energy of 60 keV. Similar radiation damage conditions were involved in our previous study [7], and it was found that the predominant radiation-induced defects were most likely mono-vacancies. Moreover, the estimated radiation-induced defects concentration is well higher than the helium one (see Section 2.2 and Fig. 1), which signifies that each introduced helium atom can easily meet one vacancy. Therefore, the hypothesis of the formation of simple He–V1 complexes during implantation could be envisaged. It must be mentioned that, since our samples contain impurities, the formation of He-impurity complexes could be envisaged. However, (i) the binding energy of these complexes is smaller (61 eV [23,24]) than the He-vacancy ones, and (ii) the vacancy defect concentration is well larger than the impurity amount. Hence, the trapping of helium by impurities can be disregarded. To finish, a maximum helium desorption of only 75% is obtained after a thermal treatment at high temperature (1873 K); moreover, the desorption rate decreases from 1673 K. These results indicate the presence of a second type of helium trapping sites, deeper than V1. Such high stability in temperature is characteristic of large Hen–Vp complexes. The question of the influence of the microstructure is then asked, since it is well-known that grain boundaries act as preferential bubbles nucleation sites [25]. Very recently, a similar study in 500 keV He implanted SiC clearly demonstrated the influence of grain boundaries on He retention. Actually, NRA measurements show a total He release from single crystals while 95% was retained in polycrystals implanted and annealed in the same conditions [26]. The low He release in this case as compared to our results is likely due the difference in both He concentration and He depth location (He is located deeper at 500 keV). To confirm the influence of the microstructure on the He behaviour in tungsten, experiments in single crystals are presently in progress.

4. Summary Polycrystalline tungsten samples were implanted with 60 keV He+ ions at a 2  1013 cm 2 fluence. The helium behaviour under thermal annealing up to 1873 K was studied by NRA. A helium desorption is observed from 1500 K, which is the temperature at which helium is released from mono-vacancies. The formation of He–V1 complexes during implantation is thus envisaged, since the radiation conditions imply (i) a weak damage compatible with the formation of this type of defects and (ii) a defect concentration well higher than the helium one. The helium release reaches only 75% after annealing at 1873 K, and the desorption rate decreases from 1673 K, which indicate that part of the implanted helium is strongly trapped in deep defects, presumably large Hen–Vp complexes. An effect of the grain boundaries on this strong helium trapping is suggested.

3

Acknowledgements This study is financially supported by the department for the controlled fusion (DRFC) of CEA in the framework of the Sixth European EURATOM Framework Programme. Authors would like to thank Ch. Viaud (CEA-Cadarache) and Ch. Peaucelle (IPN-Lyon) for thermal treatments and 3He implantations. References [1] S.E. Donnelly, J.H. Evans (Eds.), Fundamental Aspects of Inert Gases in Solids, vol. 279, Plenum, New York, 1991. [2] K. Tokunaga, R.P. Doerner, R. Seraydarian, N. Noda, Y. Kubota, N. Yoshida, T. Sogabe, T. Kato, B. Schedler, J. Nucl. Mater. 313–316 (2003) 92–96. [3] D. Nishijima, M.Y. Ye, N. Ohno, S. Takamura, J. Nucl. Mater. 313–316 (2003) 97–101. [4] S.B. Gilliam, S.M. Gidcumb, N.R. Parikh, D.G. Forsythe, B.K. Patnaik, J.D. Hunn, L.L. Snead, G.P. Lamaze, J. Nucl. Mater. 347 (2005) 289–297. [5] H. Iwakiri, K. Yasunaga, K. Morishita, N. Yoshida, J. Nucl. Mater. 283–287 (2000) 1134–1138. [6] N. Hashimoto, J.D. Hunn, N.R. Parikh, S.B. Gilliam, S.M. Gidcumb, B.K. Patnaik, L.L. Snead, J. Nucl. Mater. 347 (2005) 307–313. [7] A. Debelle, M.-F. Barthe, T. Sauvage, R. Belamhawal, A. Chelgoum, P. Desgardin, H. Labrim, J. Nucl. Mater. 362 (2007) 181–188. [8] A. Debelle, M.-F. Barthe, T. Sauvage, P. Desgardin, R. Belamhawal, A. Chelgoum, A.M. Brass, J. Chêne, Etude des défauts d’irradiation et du comportement de l’hélium, du deuterium et du tritium dans le tungstène, Rapport de fin de contrat de recherches V3466.001, www-drfc.cea.fr. [9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985, www.srim.org. [10] Standard Practice for Neutron Radiation Damage Simulation by ChargeParticle Irradiation, E521-96, Annual Book of ASTM Standards, vol. 12.02, American Society for Testing and Materials, Philadelphia, 1996. [11] P. Lucasson, in: Proceedings of the Conference on ‘‘Fundamental Aspects of Radiation Damage in Metals”, CONF 75-1006-P1, Gatlinburg, 1975, pp. 42–64. [12] F. Pâszti, Nucl. Instrum. Meth. B 66 (1992) 83. [13] W.D. Wilson, C.L. Bisson, M.I. Baskes, Phys. Rev. B 24 (1981) 5616–5624. [14] J. Amano, D.N. Seidman, J. Appl. Phys. 56 (1984) 983–992. [15] A. Wagner, D.N. Seidman, Phys. Rev. Lett. 42 (1979) 515–518. [16] C.S. Becquart, C. Domain, Phys. Rev. Lett. 97 (2006) 196402. [17] A.S. Soltan, R. Vassenet, P. Jung, J. Appl. Phys. 70 (1991) 793–797.

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