with an activation energy of 61.5 kJ/mol (0.64 eV) [28]. The decrease of D atom content in a BeO layer with time [24,25] (Fig. 4) is thought to be attributable to dehydration of beryllium hydroxide which is formed during simultaneous action of D atoms and oxygen-containing molecules on the Be sample surface. An Arrhenius plot of the dehydration rate constant for beryllium hydroxide is shown in Fig. 5.
1.5 2.0 2.5 3.0 3.5
1000/T (K-1)
1E-5 1E-4 1E-3 1E-2 0.1 1
Dehydration rate constant (min-1 )
Livey and Williams (1958) Alimov and Sharapov (1998)
Figure 5. Arrhenius plot of the dehydration rate constant for beryllium hydroxide. The experimental data are taken from Livey and Williams [28] and Alimov and Sharapov [24,25].
The appearance of molecular deuterium in the BeO layer growing during D atom exposure (Fig. 4) is related to accumulation of deuterium atoms at structural defects of oxide layer where they form molecules.
Due to the presence of oxygen-containing molecules as trace impurities, the beryllium oxide layer is formed on the surface of the Be sample exposed to D atoms at elevated temperatures. It has been found that the deuterium atom exposure leads to the deuterium retention in this layer. The majority of deuterium (> 90%) retains as D atoms, the other part is accumulated in the form of D2 molecules. After termination of D atom exposure the D concentration in BeO layer decreases with time at room temperature. It is supposed that the formation of beryllium hydroxide Be(OD)2 under D atom exposure of a growing BeO layer takes place. The decrease of deuterium concentration with time is explained by dehydration of the beryllium hydroxide.
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DEUTERIUM RETENTION IN GRAPHITE, TUNGSTEN AND TUNGSTEN-CARBON MIXED MATERIALS
V.Kh. ALIMOV, D.A. KOMAROV, R.Kh. ZALAVUTDINOV Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russian Federation
Abstract
Deuterium (D) retention in graphite, tungsten single crystal and tungsten-carbon mixed materials implanted with keV D ions has been investigated by means of secondary ion mass spectrometry (SIMS) and residual gas analysis (RGA) measurements. The mixed materials were prepared by chemical vapor deposition (CVD) and reactive magnetron sputtering of carbon-tungsten cathode. In the implantation zone, the maximum D concentration in tungsten and tungsten-carbon mixed materials is lower than that in graphite by one-two orders of magnitude depending on implantation temperature. Deuterium incorporated into the tungsten-carbon mixed materials prepared by magnetron sputtering in a D2 atmosphere is accumulated up to concentrations which are typical for pure tungsten or tungsten carbides but not for graphite inclusions.
1. INTRODUCTION
According to the requirements and selection criteria in the ITER design, the plasma facing material will be chosen from beryllium, carbon fibre composite and tungsten. Physical and chemical sputtering causes erosion of plasma-facing materials and impurity release into the plasma. These elements and hydrogen from the plasma will subsequently be co-deposited back onto the wall and divertor surface forming mixed layers.
In view of the above mentioned fact, not only pure materials such as carbon and tungsten should be studied, but also carbides and mixed carbon-containing materials.
The retention of hydrogen isotopes in the carbon layer has been reviewed in the past by Wilson and Hsu [1] and by Möller [2] and is discussed in numerous other publications. When a hydrogen atom is embedded into graphite (or carbon composite) at temperatures below ∼800 K, it is effectively immobile [3,4]. It remains there until the surrounding conditions (temperature or crystalline structure) are altered. Additional atoms are retained in the same manner until a saturation is reached [5]. The hydrogen to carbon ratio in the saturated layer is controlled by the temperature [6] and equals to 0.40-0.65 at 300 K and to about 0.05 at 1150 K [5,7-10]. Direct information on the deuterium-graphite bonding in the implanted layer has been acquired from X ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) measurements [11-13]. In XPS experiments a chemical shift as well as broadening of the C 1s peak was observed together with the appearance of CD- and C2D- lines in the SIMS spectra. It was also shown that CD- and C2D- lines arise due to the formation of chemical bonds between implanted deuterium and carbon. Considering the trapping of H and D ions one has to bear in mind the evidence that hydrogen molecules are formed in the bulk during implantation and these molecules diffuse to the surface through a highly damage matrix via a network of open paths of subnanometer scale [14,15].
When carbon atoms are sputtered from a graphite or carbon composite surface, they are re-deposited on the surrounding surfaces. As the sputtered atoms arrive at their new location, they are co-deposited with energetic hydrogen isotope ions and neutrals. The characteristics of
the co-deposited layers and other similar coatings are reviewed in Refs. [16-24]. The hydrogen content in the layers can reach value of ∼1 H/C [23]. As to chemical composition of the co-deposited layer, summarising the results reported in Refs. [19,25-28] it can be said that the co-deposited carbon-hydrogen layer is primarily amorphous, contains a large amount of hydrogen, has hydrogen formed monohydride with sp3, sp2, and sp1 hybridized carbon atoms in decreasing probability, and has hydrogen that is chemisorbed on the carbon as well.
Few data obtained by different experimental methods have been reported on deuterium inventories in tungsten after implantation of hydrogen isotope ions at energies in the range 0.1 to 8 keV [29-36]. It has been shown that the hydrogen isotope inventory in tungsten materials depends strongly on the material structure at temperatures in the range from 300 to 600 K [32]. More than 70% of the implanted deuterium diffuses into the bulk even at room temperature and is captured by lattice imperfections [32,34,35].
There are only a few works on deuterium trapping in and thermal release from tungsten containing carbon, prepared by chemical vapour deposition (CVD) [37] or by annealing tungsten films kept in contact with carbon films at 1500-1673 K [38,39]. It has been found that the amount of deuterium retained in W0.85C0.15 and W0.60C0.40 produced by CVD is more than double the value in CVD tungsten free from carbon atoms. In the tungsten layer containing carbon the concentration of retained D atoms is higher than that in pure tungsten by about 20 percent [38] and reaches the value of ∼1.5×1028 D/m3 at room temperature [38,39]. The deuterium inventory in tungsten containing carbon steeply decreases with increasing target temperature from 300 to 550 K and reaches the value of pure tungsten at higher temperatures [37]. No data on hydrogen solubility and diffusivity in tungsten carbides are available.
Retention of deuterium implanted at room temperature in pure W single crystal and that pre-irradiated with 40 keV C ions has been studied in Ref [40]. D ion implantation was performed at energies of 10 keV with the range confined in the carbon modified layer and 100 keV with the range exceeding the carbon modified layer. Carbon pre-implantation influences the deuterium retention only if the range of the D ions is confined within the carbon modified surface layer. In this case, deuterium diffusion beyond the ion range distribution does not occur and the retained amount of deuterium is smaller than in the pure W crystal. At D ion energy where the deuterium range exceeds the carbon modified layer the retention occurs in the dislocation zone extending up to 1 µm and the total retained amount is the same for carbon implanted and pure W samples.
The present work was done to study the influence of carbon atoms deposited together with tungsten and deuterium atoms on the retention of implanted deuterium. In order to understand D behaviour in the mixed W-C material, the additional experiments on the study of the deuterium retention in graphite, pure tungsten and tungsten carbide irradiated with D ions were also performed.