Be/W and W/Be bilayers deposited on Si substrates with hydrogenated Fe-Cr and Fe-Cr-Al interlayers for plasma facing components
Introduction
Plasma facing components (PFCs) [1], [2], [3] are part of the test blanket modules in tokamak-type fusion reactors, such as ITER [4], which is expected to start plasma experiments in 2020, and future DEMO applications [5], [6]. These complex materials are envisioned for operation in extreme temperature and irradiation conditions, with minimal damages on their lifetime and device performances. However, the droplet spraying from melted material from the PFCs caused by plasma-wall interactions may induce contamination of the plasma core and negatively influence the plasma discharge [7], [8].
Tungsten-based materials, presenting low sputtering yields under ion bombardment, high melting point, good thermal conductivity, low tritium retention, are good candidates for plasma facing components materials [9], [10]. However, the impact on the plasma core is significant. On the other hand, light elements such as Be [11], present low influence on the plasma core but have high sputtering yields. Therefore, a viable approach for the top structure of the PFCs would be a heterostructure composed of these two materials with very distinct behavior. In order to improve the mechanical properties of the top layers, which are greatly affected by high temperatures, special reduced-activation ferritic-martensitic (RAFM) steels (e.g., Eurofer) [12], [13], [14], are used as underlayers [15].
During fusion experiments, an important effect induced by temperature and irradiation is the redeposition of intrinsic elements on the PFCs (some elements contained in the PFCs, such as Be or W or in the bottom structures, like Fe and other elements in the RAFM steels). The new structures may have different properties related to the designed PFCs. It is expected that such processes (induced by annealing and irradiation) will have a negative influence on the mechanical properties of the entire structures and a good knowledge of their effects is of great importance. Also, the generation of displacements per atom, inducing dislocation loops and transmutation reactions (which can generate He and H) may affect the brittleness of the steels.
In order to give more insight into redeposition processes possible to occur in PFCs, investigations of atomic intermixing in specifically designed thin films and multilayers represent a suitable approach. In some of our previous works [16], [17], effects of diffusion and atomic intermixing in as deposited and thermally annealed Be/W and Be/C bilayers as grown on Si(0 0 1) substrates with Fe thin interlayers, have been studied. The influence of special hydrogenation treatments of alloy films approaching the composition of Eurofer (Fe-Cr based layers) has also been previously investigated in Refs. [18], [19], emphasizing a higher degree of crystallization and an increased diffusion of different atoms in the alloy structure.
The purpose of this paper is to study the effects induced by atomic intermixing and diffusion in some thin multilayers which might be considered as case studies for components in nuclear fusion technology, such as complex structures of Be/W and W/Be bilayers deposited on Si substrates with Fe-Cr and Fe-Cr-Al interlayers. The characterization of the local phenomena and interfacial intermixing in the Be/W and W/Be layers deposited on Si substrates was made by means of grazing incidence X-ray diffraction (GIXRD), X-ray reflectometry (XRR), X-ray photoemission spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS).
Section snippets
Experimental details
Fe, Fe-Cr and Fe-Cr-Al thin films (4 nm thick), partially enriched in the 57Fe Mössbauer isotope, were deposited for 3 minutes on Si(0 0 1) substrates by radio-frequency (RF) sputtering. The substrate was held at room temperature, the discharge power was 100 W and the working Ar pressure was 5 × 10−2 mbar. The native Si oxide layer was not chemically removed from the substrate, but an etching process of 30 min was applied before deposition. The films were subsequently reduced in hydrogen
Results and discussions
The GIXRD patterns of the samples Fe0.89Cr0.11/Be/W, Fe0.89Cr0.11/W/Be, Fe0.84Cr0.11Al0.05/Be/W and Fe/W/Be are shown in Fig. 1. All patterns reveal relatively broad peaks for both bcc Fe (a = 0.287 nm) and bcc Cr (a = 0.288 nm), as well as for W and Be. The broadness of the peaks indicates a partial amorphous like structure.
The mass density profiles of samples Fe0.89Cr0.11/Be/W and Fe0.89Cr0.11/W/Be, shown in Fig. 2, have been obtained from the fitting of the XRR spectra (shown in the inset).
Conclusions
Bilayers of Be/W and W/Be were deposited on Si(0 0 1), with Fe, Fe-Cr and Fe-Cr-Al interlayers, in order to provide more insight into the effects of atomic intermixing and diffusional processes taking place in components with nuclear fusion applications. Some of the interlayers were annealed in hydrogen atmosphere, inducing oxygen reduction and crystallinity improvement in the films. The GIXRD results show amorphous like structures for all samples. Most of the Mössbauer spectra show clearly the
Acknowledgements
This work was supported by the Romanian National Programs (Core program PN16-480102/3, IDEI COMPLEXE C2-006 and PN-II-IDPCE-2011-3-0522) and the European Community (under the EUROFUSION contract 1 EU-1 WPPFC-RO).
References (29)
- et al.
J. Nucl. Mater
(2011) - et al.
J. Nucl. Mater.
(2007) J. Nucl. Mater.
(2007)- et al.
Fusion Eng. Des.
(2015) - et al.
Vacuum
(2002) J. Nucl. Mater
(2011)- et al.
J. Nucl. Mater.
(2004) - et al.
Beryllium as a plasma-facing material for near-term fusion devices
- et al.
J. Nucl. Mater
(2011) - et al.
J. Nucl. Mater
(2009)