Hydrogen accumulation in nanostructured as compared to the coarse-grained tungsten
Introduction
Due to its properties tungsten is considered nowadays the best candidate for plasma facing materials (PFM) applications, in both magnetic confinement (MC) and inertial (laser) confinement nuclear fusion reactors (IC) [1], [2], [3], [4], [5]. However, some difficulties have been identified in coarse grained W (from now on CGW) which have to be overcome in order to fulfill specifications [6], [7]. In particular, special attention should be paid to the high capacity for light species retention. Light species in CGW tend to nucleate in vacancy clusters forming overpressurized bubbles that lead, among other effects, to surface blistering and exfoliation [8], [9], [10]. In view of this, the light species behaviour in W has been experimentally studied [11], [12], [13], [14], [15] and modelled [16], [17] by many groups all over the world.
Most of these studies were devoted to analyse the W behaviour under irradiation conditions taking place in magnetic fusion reactors. In these reactors, PFM will be mainly exposed to H-isotopes, He and plasma impurity ions such as carbon, all of them impinging into the PFM with energies below the displacement damage threshold. On this frame, the data on H behaviour reported so far evidence that H retention (i) is quite large in W [18], especially in samples containing radiation induced defects that even delay the H transport [19]. (ii) H retention strongly depends on irradiation conditions (single beam versus multiple beams, sample temperature and ion flux) [20], [21] as well as on sample microstructure [22], [23].
In laser confinement fusion, light species are generated after the explosion of the (deuterium–tritium) pellets. In this fusion approach, the energy of the light species impinging into the PFM strongly depends on the target illumination mode: direct or indirect. In reactors like HiPER, [24], [25], [26], operating with direct drive targets, ions are expected to reach PFM with energies well above the displacement threshold. Thus, the radiation-induced damage configuration, in laser fusion reactors with direct targets will be quite different from that expected in MC reactors. Therefore, the hydrogen behaviour in PFM in laser fusion reactors with direct targets cannot be reliably extrapolated from the results obtained for the evaluation of W under MC conditions.
One alternative to delay the appearance of blistering and exfoliation effects is the use of materials with a grain size in the nanometer range, so called nanostructured materials [27], [28], [29]. The behaviour of these materials under irradiation is very much dominated by the large density of grain boundaries (GBs), which at room temperature (RT) act as (i) annihilation centres for Frenkel pairs (self-healing behaviour) [30], [31] and (ii) pinning centres for light species [32], [33]. These two facts may drive the delay (shift to higher irradiation fluences) for the formation of overpressurized bubbles. Moreover, if as reported by von Toussaint et al. [17], the trapping energy for H at the grain boundaries is smaller than that at the radiation-induced defects, GBs may favour H release, performing as effective diffusion channels. Nevertheless, the behaviour of light species and in particular of H in nanostructured materials has not been sufficiently addressed, especially from the experimental point of view. Indeed, most part of the present conclusions have been deduced from computer simulation data in which a small number of atoms and a low number of ideal grain boundaries have been considered [34], [35]. Because of the notable differences in microstructure and defect configuration between modelled and real nanostructured samples, the results of computer simulations must be carefully validated. Nowadays, to the best of our knowledge, only few works have been devoted to this topic. On the other hand the scarce experimental work devoted to the characterisation of nanostructured materials under irradiation has been performed on materials consisting of a metallic matrix (i.e. steel) with embedded oxide-based nanoinclusions (oxide dispersion strengthened steels, ODS) [36], [37], [38]. In these systems, the large amount of different phenomena, in particular phase segregation and chemical bonding, taking place at the GBs hamper a fundamental understanding of the role of GBs in the final radiation-induced defect configuration. To overcome these problems and to have a more fundamental picture of the influence of GBs on the radiation behaviour, especially designed samples consisting of immiscible multilayers of pure elements have been deposited and studied. The obtained results show that the interface play a role in defect reduction [39].
In this paper, we analyse the role of GBs on the H behaviour for pure nanostructured W samples. For this purpose, commercial coarse grained W and home-deposited nanostructured W samples were single- (with H), sequentially- (with C plus H) and simultaneously- (with C and H) implanted. The stability of the nanostructures under irradiation, the sample surface and the phase composition of the samples are characterised by comparing scanning electron microscopy images and X-ray diffraction patterns measured prior to and after implantation. The H depth profiles for samples with different microstructure implanted under diverse conditions is characterised by resonant nuclear reaction analysis (RNRA). RNRA results show that the H behaviour in NW samples is dominated by the presence of native defects.
Section snippets
Experimental
W coatings with a thickness of ∼1.2 μm were deposited by DC triode sputtering from a pure W commercial target (99.95%) at a normal incidence angle on single-side polished Si (1 0 0) substrate. The deposition setup consists of a high vacuum chamber with a base pressure in the 10−8 mbar range which is equipped with a 5 cm diameter magnetron designed and manufactured by Nano4Energy SL [40]. Deposition took place in the presence of a pure argon atmosphere (99.9999%) at room temperature. The Ar pressure
Morphology
As reported in literature, the behaviour of light species is highly influenced by the sample morphology [46], [47]. This is particularly relevant in the case of nanostructured materials in which the light species behaviour is highly affected by the density and architecture of grain boundaries (GBs). Because of this reason, the morphological characterisation of the samples is crucial in order to properly describe the hydrogen behaviour. In our particular case, special attention is paid to the
Discussion
The effect of microstructure on H behaviour can be assessed by comparing the H depth profiles measured for NW to those for CGW. From the quantitative point of view the H content is higher in the NW than in the CGW samples for each of the implantation conditions. These results show that H retention strongly depends on microstructure. The fact that the H content is higher for the NW than for the CGW samples points out that NW samples have a larger density of trapping sites than CGW. This occurs
Conclusions
The H behaviour in nanostructured W (NW) coatings as compared to coarse-grained W (CGW) samples has been studied as a function of (i) implantation conditions single- (H), sequential- (C plus H) and simultaneous- (C and H) implantations as well as, (ii) H implantation temperature.
Nanostructures are stable under the studied implantations conditions (energy of the beams, implantation fluence and temperature). No surface modification is deduced from top view FEG-SEM images for any sample. According
Acknowledgments
Research by N. Gordillo was supported by a PICATA postdoctoral fellowship of the Moncloa Campus of International Excellence (UCM-UPM). Research by E. Tejado was supported by a C.S.I.C JAE-Predoc fellowship co-financed by FSE. The work was financed by the M.I.N.E.C.O (Spain) under the projects (AIB2010DE-00358 and MAT2012-38541-C02-01 and 02).
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