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Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001) α-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 ...nm thick films were investigated between 973 K and 1423 K.
The fcc-phase persists while the original texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 µm wide grains upon annealing. Only the metallic M grains having two specific orientation relationships (ORs) to the c-sapphire grow. These ORs are OR1 (M(111)11¯0//α-Al2O3(0001)11¯00) and OR2 (M(111)11¯0//α-Al2O3(0001)112¯0)and their twin-related variants (OR1t and OR2t). They are identical to those reported for several pure fcc metal (M) films. Thus, the ORs in these fcc/c-sapphire systems appear not to be controlled by the fcc phase chemistry or its lattice parameter as usually assumed in literature.
Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. Below 1000 K, the film microstructure stabilizes into 10 µm wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures.
The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are ...similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO2 and c-sapphire substrates by magnetron co-sputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.
•Equiatomic CoCrFeNi alloy thin films reported to be multiphase at room temperature•Presence of Cr rich σ phase in as deposited films in addition to the expected fcc phase•Stability of the σ phase on annealing up to 673 K, disappearance on the formation of Cr2O3•Fcc phase stability up to a temperature of 1223 K for films on Si/SiO2 and up to 1373 K for films on c-sapphire
The infrared gas-phase absorption spectrum of methane was used to determine its Clapeyron solid–gas equilibrium curve in the 40–77 K temperature range. For comparative purposes and to obtain more ...reliable results, two different optical experimental setups were used. At higher temperatures (53–77 K), a single pass cryogenically cooled cell was coupled to a standard low-resolution Fourier transform infrared spectrometer. The second system was a state-of-the-art vertical-external-cavity surface-emitting laser tunable source operating at around 2.3 μm, combined with a 7 m path Herriott cell, to record methane absorption features down to 40 K. From the measurements, the vapor pressure curve ln(p/Pa) = −(1191.92 ± 8.92)/(T/K) + (22.49 ± 0.16) was derived in the range 40–77 K. This corresponds to a value of 9910 ± 75 J mol–1 for the sublimation enthalpy. The relation was validated down to 40 K, increasing our knowledge of the saturation pressure by 2 orders of magnitude. Data were compared with available pressure measurements from the literature, obtained by manometric or mass spectrometry techniques, and the sublimation enthalpy was compared with a thermodynamic approach based on heat capacity measurements in the solid and gas phases.
The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in literature. Under experimental conditions, it shows a single-phase fcc structure and its physical and mechanical properties are ...similar to those of the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have focused on the mechanical properties of bulk nanocrystalline high entropy alloys or compositionally complex alloys, and their microstructure evolution upon annealing. The thin film processing route offers an excellent alternative to form nanocrystalline alloys. Due to the high nucleation rate and high density of defects in thin films synthesized by sputtering, the kinetics of microstructure evolution is often accelerated compared to those taking place in the bulk. Here, thin films are used to study the phase evolution in nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by magnetron cosputtering from elemental sources. The phases and microstructure of the films are discussed in comparison to the bulk alloy. The main conclusion is that second phases can form even at room temperature provided there are sufficient nucleation sites.
Homogeneous face-centered cubic (fcc) polycrystalline CoCrFeNi films were deposited at room temperature on (0001)alpha-Al2O3 (c-sapphire). Phase and morphological stability of 200 to 670 nm thick ...films were investigated between 973 K and 1423 K. The fcc-phase persists while the original texture of 30-100 nm wide columnar grains evolves into ~10 or ~1000 micron wide grains. Only the grains having specific orientation relationships to the sapphire grow; as in the case of pure fcc metal (M) films 4 orientation relationships (OR) are found: OR1 (M(111)1-10//alpha-Al2O3 (0001)1-100) and OR2 (M(111)1-10//alpha-Al2O3 (0001)11-20) and their twin-related variants (OR1t and OR2t). Below 1000 K, the film microstructure stabilizes into 10 micron wide OR1 and OR1t twin grains independent of film thickness. Above 1000 K, the OR2 and OR2t grains expand to sizes exceeding more than a 1000 times the film thickness. Upon annealing, the films either retain their integrity or break-up depending on the competing kinetics of grain growth and grain boundary grooving. Triple junctions of the grain boundaries, the major actors in film stability, were tracked. Thinner films and higher temperatures favor film break-up by dewetting from the holes grooved at the triple junctions down to the substrate. The grain boundaries of the OR2 and OR2t grains migrate fast enough to overcome the nucleation of holes from which break-up could initiate. The growth of the OR2 and OR2t grains in this complex alloy is faster than in pure fcc metals at equivalent homologous annealing temperatures.
•New data on dust produced by plasma-wall interaction.•Dust size larger than in any other tokamaks.•Dust produced in large quantities by material overheating and melting.•Production of Mo, B, W and ...Mo/B droplets-rounded dust.
Post mortem analyses of dust collected in Alcator C-Mod have highlighted a production of large size dust particles. The quantities of such large particles are higher than in any other tokamak. They are divided in two classes as a function of their shape and consequently, their origin. Rounded dust particles such as spheres and splashes constitute the first class. These particles are the result of high heat loads on various leading edges of plasma facing components and possibly, their melting during plasma operation. The heated or already molten material can be destabilized during disruptions and droplets are emitted across the vacuum chamber. After solidification, the resulting rounded particles are either in pure elements or in alloys. Flake-like dust particles, which are mainly due to light material coating delamination, constitute the second class of dust particles.
Fusion fuel retention (trapping) and release (desorption) from plasma-facing components are critical issues for ITER and for any future industrial demonstration reactors such as DEMO. Therefore, ...understanding the fundamental mechanisms behind the retention of hydrogen isotopes in first wall and divertor materials is necessary. We developed an approach that couples dedicated experimental studies with modelling at all relevant scales, from microscopic elementary steps to macroscopic observables, in order to build a reliable and predictive fusion reactor wall model. This integrated approach is applied to the ITER divertor material (tungsten), and advances in the development of the wall model are presented. An experimental dataset, including focused ion beam scanning electron microscopy, isothermal desorption, temperature programmed desorption, nuclear reaction analysis and Auger electron spectroscopy, is exploited to initialize a macroscopic rate equation wall model. This model includes all elementary steps of modelled experiments: implantation of fusion fuel, fuel diffusion in the bulk or towards the surface, fuel trapping on defects and release of trapped fuel during a thermal excursion of materials. We were able to show that a single-trap-type single-detrapping-energy model is not able to reproduce an extended parameter space study of a polycrystalline sample exhibiting a single desorption peak. It is therefore justified to use density functional theory to guide the initialization of a more complex model. This new model still contains a single type of trap, but includes the density functional theory findings that the detrapping energy varies as a function of the number of hydrogen isotopes bound to the trap. A better agreement of the model with experimental results is obtained when grain boundary defects are included, as is consistent with the polycrystalline nature of the studied sample. Refinement of this grain boundary model is discussed as well as the inclusion in the model of a thin defective oxide layer following the experimental observation of the presence of an oxygen layer on the surface even after annealing to 1300 K.
Raman microscopy is one of the methods that could be used for future post-mortem analyses of samples extracted from ITER plasma facing components. This study shows that this technique is useful for ...studying tungsten-based materials containing impurities including oxides and nitrides. Here, we apply pulsed laser deposition and DC argon glow discharges to produce tungsten-containing synthetic films (compact, and porous), and nanoparticles, and investigate the influence of their morphology on the measured Raman spectra. The amounts of oxygen and/or nitrogen in the films are also investigated. Comparative data are obtained via x-ray Photoelectron Spectroscopy, Atomic Force Microscopy, Electron Microscopies (Scanning and Transmission), Energy Dispersive x-ray spectroscopy, and Time-of-Flight Elastic Recoil Detection Analysis. The power density of the laser beam used to perform the Raman microscopy is varied by up to 4 orders of magnitude (0.01-20 mW μm−2) so as to investigate the thermal stability of films and nanoparticles. As a first result, we give evidence that Raman microscopy is sensitive enough to detect surface native oxides. Secondly, more tungsten oxides are detected in porous materials and nanoparticles than in compact films, and the intensities of the Raman band correlate to their oxygen content. Thirdly, the thermal stability of these films (i.e. structural and chemical modification under laser heating) is poor when compact films contain a sufficiently large amount of nitrogen. This finding suggests that nitrogen can be substituted by oxygen during Raman laser induced heating occurring in ambient air. Finally, our methodology can be used to rapidly characterize the morphology and chemistry of the samples analyzed, and also to create oxides at the micrometer scale.
Dans ce travail, nous étudions la stabilité thermique et les effets des irradiations par un plasma d'hélium ou de deutérium de films minces de WO3 d’intérêt pour la fusion magnétique (projet ITER). ...L’objectif est de comprendre comment une oxydation du divertor modifie les interactions plasma paroi. Pour cela, nous avons synthétisé des films de WO3 par oxydation thermique de substrats de W à 400°C et caractérisé les effets du type de substrat, de la pression d’oxygène et du temps d’oxydation sur la structure et sur l’épaisseur des oxydes formés. La structure (monoclinique nanocristalline), la morphologie et les défauts des échantillons ont été analysés avant et après traitement, à différentes échelles, en utilisant la microscopie électronique, la microscopie Raman, la diffraction de rayons X, et la microscopie à force atomique.Le chauffage sous vide (400 - 800°C) a conduit à la formation de WO2. Le bombardement aux ions D+ (11 eV) a mené à une diffusion profonde du deutérium à travers le film d’oxyde, engendrant un effet électrochimique, observé ici pour la première fois sous irradiation plasma. Cet effet, réversible, est associé à la formation de bronzes de tungstène (DxWO3) et à une transition de phase vers une structure hexagonale. Des bombardements aux ions He+ (20 eV) ont été réalisés afin de dissocier les effets physiques et chimiques. A température ambiante, le bombardement a causé peu de changements morphologiques et structuraux. Par contre, le autre bombardement à 400°C a causé une érosion du film d’oxyde accompagnée d’un changement de couleur, une amorphisation en surface et la formation de bulles à l’interface W / WO3.
As part of laboratory studies devoted to magnetic fusion we have investigated the thermal stability and the effects of helium and deuterium plasma irradiation on tungsten oxide thin films. The objective is to predict the consequences of the oxidation of the W plasma facing component (divertor) for plasma wall interactions.To this aim, we have synthesized WO3 films by thermal oxidation of W substrates at 400°C and we have characterized the effects of the W substrate, the oxygen pressure and the oxidation duration on the structure and the thickness of the oxide films. The sample crystalline structure (monoclinic nanocrystalline), defects and morphologies were characterized before and after treatment using scanning and transmission electron microscopies, Raman microscopy, X-Ray diffraction and atomic force microscopy. Heating under vacuum up to 800°C leads to changes in the film structure and composition which results in the formation of WO2. D+ bombardment (11 eV) leads to D+ diffusion throughout the oxide film and to an electrochromic effect, here observed for the first time under plasma irradiation. This effect - which turned out to be reversible - is related to the formation of W bronzes (DxWO3) and to a phase transition of the oxide toward a hexagonal structure. Helium bombardments (20 eV) have then been performed to unravel physical and chemical processes at play. He+ bombardment at room temperature causes slight structural and morphological changes. On the contrary, He+ bombardment at 400°C leads to a significant erosion of the oxide film, accompanied by a colour change, the surface amorphisation and the formation of bubbles at the W / WO3 interface.
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