The evaporation of chromium at elevated temperatures in vacuum, commonly employed in the processing and heat treatment of Ni–Cr–Fe heat-resistant alloys, has been investigated. Two variants of these alloys, each containing distinct alloying elements, were exposed at 1100 °C for durations of 3 to 500 hours. These exposures were conducted within a vacuum environment, both with and without applied stress. During the experiments, the samples displayed the development of a carbide-free layer (M23C6/M7C3) with thicknesses ranging from 10 to 100 μm. Employing Energy Dispersive Spectroscopy (EDS), the composition of the alloys was analyzed as a function of depth beneath the surface. Notably, the samples exhibited a pronounced gradient of chromium content. This gradient corresponded to the dimensions of the carbide-free layer, suggesting that the dissolution of carbides is due to the chromium evaporation process. A noticeable difference in the behavior of the two alloys was identified and deduced to be related the effect of aluminum on the development of concentration gradients and the associated surface evaporation of chromium. These observations were interpreted within the framework of a diffusion-based evaporation model which shed light on the intricate interplay between alloy composition, carbide dissolution, and chromium evaporation during high-temperature exposure.

Atomic probe tomography (APT) is able to generate three-dimensional chemical maps in atomic resolution. The required instruments for APT have evolved over the last 20 years from an experimental to an established method of materials analysis. Here, we describe the realization of a new modular instrument concept that allows the direct attachment of APT to a dual-beam SEM microscope with the main achievement of fast and direct sample transfer and high flexibility in chamber and component configuration. New operational modes are enabled regarding sample geometry, alignment of tips, and the microelectrode. The instrument is optimized to handle cryo-samples at all stages of preparation and storage. It comes with its own software for evaluation and reconstruction. The performance in terms of mass resolution, aperture angle, and detection efficiency is demonstrated with a few application examples.

Atomic probe tomography (APT), based on the work of Erwin Mueller, is able to generate three-dimensional chemical maps in atomic resolution. The required instruments for APT have evolved over the last 20 years from an experimental to an established method of materials analysis. Here, we describe the realization of a new instrument concept that allows the direct attachment of APT to a dual beam SEM microscope with the main achievement of fast and direct sample transfer. New operational modes are enabled regarding sample geometry, alignment of tips and microelectrode. The instrument is optimized to handle cryo-samples at all stages of preparation and storage. The instrument comes with its own software for evaluation and reconstruction. The performance in terms of mass resolution, aperture angle, and detection efficiency is demonstrated with a few application examples.

The oxidation behavior of alumina-forming alloys was studied after oxidation treatments conducted at 900 °C for 24 h in air or in steam. Alloys with sulfur contents ranging from 1 to 82 ppm in wt% were used. The influence of trace sulfur as well as the presence of steam in the oxidizing atmosphere were investigated. Under oxidation conditions, higher sulfur contents led to higher mass gains and the trend was more pronounced in steam than in air. Oxides were identified by Raman spectroscopy: a thin and continuous α-Al2O3 layer was formed at the metal-oxide interface in all cases. The mass gain differences were caused by other oxides formed at the surface of the samples—mainly spinel and Cr2O3—and in the internal oxidation zone too— mainly α-Al2O3 and θ-Al2O3—indicating that the protectiveness of the alumina layer greatly depends on the sulfur content in the base material and the oxidizing atmosphere. In order to explain this phenomenon, oxide structures were analyzed at various scales using scanning electron microscopy, transmission electron microscopy and nanoscale secondary ion mass spectrometry. Sulfur was detected at metal-oxide interfaces and also in the alumina layer in regions enriched with chromium. In addition, we demonstrate that steam oxidation leads to finer alumina grains as compared to air oxidation. Finally, the relationship between oxidation conditions, nanoscaled structural features and oxidation kinetics is discussed.

This work focusses on the link between microstructure and creep properties for heat-resistant austenitic alloys with high aluminum content (3-5 wt %). An emphasis was put on the coupling of thermodynamic simulations, microstructural characterizations by scanning electron microscopy and transmission electron microscopy, and creep testing. The phase predictions performed by the calculation of phase diagrams method are in good agreement with the observed microstructure after creep at 1000 degrees C and 1050 degrees C. Correlation between creep properties and microstructure characterizations at 1000 degrees C and 1050 degrees C revealed that NiAl and alpha' (chromium-rich base centered cubic phase) phases are deleterious for the creep properties at service temperature. Several high Al-content alloys are selected in order to replace the chromia-forming alloys standardly used in cracking furnaces.

With their tendency to form low-dimensional materials with high surface area, cobalt hydroxide carbonates represent an important class of precursors to Co3O4-a heterogeneous catalyst with various applications including sustainable energy conversion. We here present a facile methodology for the room temperature precipitation of cobalt hydroxide carbonate under additive-free conditions. Upon aging in aqueous solution, the initially deposited amorphous bulk solid slowly transforms into macroscopic crystals with an unusual spherulitic morphology interfacially templated by glass surfaces. Most intriguingly, the individual branches of these hemispherical particles represent highly anisometric, elongated platelets reaching millimeter dimensions along the growth direction, while their lamellar architecture indicates the formation of a layered double hydroxide structure. Gradual substitution of Co(II) with Mn(II) results in the deposition of spheroidal stoichiometric carbonates with a calcite structure, in which the crystal lattice progressively expands with increasing Mn content. Replacement of Co(II) by Ni(II), in contrast, preserves the spherulite morphology at low Ni(II) content (Co/Ni > 1), but prevents precipitate maturation at higher proportions of Ni(II). Calcination converts the hydroxide carbonate precursor into hierarchical Co3O4 spherulites with superstructures composed of interconnected nanoparticles, where this nanoscale arrangement is shown to positively affect the electrocatalytic activity toward the oxygen evolution reaction.

alpha' precipitation in a Fe-19 at.%Cr alloy aged at 500 degrees C up to 2008 h has been characterized by both APT and SANS. This paper shows that when using an appropriate method for SANS data treatment, both APT and SANS yield consistent results regarding not only volume fraction and size but also alpha and alpha' composition. Good agreement is achieved when alpha' particles are considered as magnetic scattering features at the early stage of the kinetics.

Among the different nano-features at the origin of embrittlement of Fe-Cr alloys and steels, alpha' precipitates play a major role in alloys with Cr content higher than about 10at.%. If Atom Probe Tomography (APT) is recognized as an efficient technique for characterizing alpha' precipitates, discussions remain on its ability to measure the actual composition of small particles. Two APT limitations are at the origin of these discussions: its lateral resolution (and on a smaller scale the depth resolution) and local magnification effects due to the difference in field evaporations between the matrix and the particles. In this study, the impacts of these two limitations are quantified for the first time using numerical approaches and an analytical model. This will provide an overview of the interpretation for the alpha' particles chemical composition experimentally measured by APT. The results show that: (i) the major effect of local magnification is ion focussing with no mixing in the core of the particles for particle radius larger than 1 nm, (ii) lateral resolution is the main contributor to the composition bias. Depending on the lateral resolution, the core of the small particles may be diluted by matrix atoms but the dilution does not exceed 5.2 at.%. The extent of the decrease in measured Cr concentration of the particles depends on the Cr concentration difference between particles and the surrounding matrix. (C) 2019 Elsevier B.V. All rights reserved.

Materials consisting of silicon nanocrystals (Si-Ncs) embedded in silicon dioxide (SiO 2 ) are the subject of an intense research activity due to their potential applications for optoelectronic and photonic devices.Moreover, providing charged carriers by introducing n-or p-type dopants in these materials can drastically modify their electrical or optical properties.A plenty of studies focused on the capabilities to change the indirect bandgap of Si-Ncs to direct one and to improve the luminescence efficiency of Si-Ncs by single or co-doping.Recently a particular interest was focused on the localized surface plasmon resonance exhibited by highly doped Si-Ncs [1].In both cases, a perfect control of the doping level and size of Si-Ncs should allow to tune the material properties.However, efficiency of these doped materials is correlated to the dopant location, which should be located in a substitutional site of the Si-Ncs.Then, it requires an accurate control of this parameter to improve and control the properties of these systems.Numerous studies concern the characterization of P doping in Si-Ncs, here, we propose to compare undoped and p-doped (with P or As) Si-Ncs by the use of Atom Probe Tomography (APT) to perform a deep structural analysis at the atomic scale.Further investigations will be done on n-type (B) doping.Three silicon rich silicon oxide films, undoped and As or P doped, were elaborated by using ion beam synthesis process.Implantation was divided into two parts.First one consists in a 29 Si implantation in a 28 SiO 2 200 nm thick layer.Second one consists, for two samples, in another implantation of 75 As or 31 P chose to match with the 29 Si implantation range.Finally, samples were annealed at 1100°C during 4h in pure N 2 to form Si-Ncs. Structural characterization of these thin films was performed combining Timeof-Flight Secondary Ion Mass Spectrometry (Tof-SIMS) and Atom Probe Tomography.Measurements performed in Tof-SIMS allowed us to ensure the reliability of 3-D reconstruction of the tip samples.In undoped as in As or P doped samples, Tof-SIMS revealed a region of interest, corresponding to the implantation range of 29 Si in the 28 SiO 2 thin films, of almost 110 nm of thickness and centered at barely 55 nm from the film surface.Moreover, for As or P doping, it confirms that the implantation of 29 Si and both dopants species match well in the same region.We must note that considering APT composition profiles, the composition reached at the peak center of the implantation by 29 Si (~11 at.%) and As or P (~1.2 at.%) in each sample is almost the same.In each case, 3-D chemical maps obtained after the APT reconstruction show that the annealing treatment performed at 1100°C leads to the clustering of 29 Si and 28 Si without any distinction.The mapping of As and P atoms allowed us to highlight the aggregation of the impurities at the same position than the clustering of Si atoms.Thereby, from a first global view, both doping cases seems to be equivalent.The use of APT allowed us to perform deeper investigation on the Si-Ncs characteristics and on the dopant location.In all samples, the diameter of Si-Ncs has been measured in order to compare the size distributions of Si-Ncs.In undoped as in As doped samples, mean diameter and shape of size distribution are almost the same, but we observed strong difference of these parameters in P doped sample (figure 1.a, 1.b).In fact, for undoped and As doped samples, size 2540

Secondary precipitation of Cr-rich carbides in heat resistant austenitic stainless steels has been investigated both experimentally and using finite element simulations. The microstructural evolutions in two commercial grades were characterized using electron microscopy. A special emphasis was given on the peculiar spatial distribution of M23C6 secondary carbides exhibiting precipitate free zones surrounding primary carbides, and high density precipitate zones extending with longer aging times. Solidification induced chemical composition gradients were clearly exhibited in the as-cast alloys with significant chromium depletion in the vicinity of primary M7C3 carbides. It is then proposed that these gradients play a major role in the secondary precipitation mechanism. Classical nucleation and growth theories have been adapted to account for (i) the flux of solutes with large difference in diffusion coefficients, (ii) the initial composition gradients in the matrix and, (iii) the chemical driving force for nucleation and growth of M23C6 carbides. Within this framework, the whole kinetics has been reproduced. It clearly shows that the spatial distribution of secondary carbides that play a key role in the creep resistance of these alloys is the result of a complex interaction between initial composition gradients in as-cast alloys and solute flux resulting from phase transformation during aging.

Titanium is presently discussed as a catalyst to accelerate the hydrogenation kinetics of hydrogen storage materials. It is however known that H absorption in Ti decisively depends on the surface conditions (presence or absence of the natural surface oxide). In this work, we use Ti thin films of controlled thickness (50-800 nm) as a convenient tool for quantifying the atomic transport. XRD and TEM investigations allow us to follow the hydrogenation progress inside the film. Hydrogenation of TiO2/Ti bi-layers is studied at 300 degrees C, for different durations (10 s to 600 min) and at varying pressures of pure H-2 atmosphere. Under these conditions, the hydrogenation is found to be linear in time. By comparing films with and without TiO2, as well as by studying the pressure dependence of hydrogenation, it is demonstrated that hydrogen transport across the oxide represents the decisive kinetic barrier rather than the splitting of H-2 molecules at the surface. Hydrogenation appears by a layer-like reaction initiated by heterogeneous nucleation at the backside interface to the substrate. The linear growth constant and the H diffusion coefficient inside the oxide are quantified, as well as a reliable lower bound to the hydrogen diffusion coefficient in Ti is derived. The pressure dependence of hydrogen absorption is quantitatively modelled. (C) 2018 Elsevier B.V. All rights reserved.

In this study, a carbon supported oxynitrides catalyst with ultra-low Pt concentration (2wt%, Pt-ON/C) is synthesized from Co(NO3)2⋅6H2O, (NH4)6Mo7O24⋅4H2O and PtCl4 precursors by using NH3 as reducing agent and nitrogen source. It is investigated as oxygen reduction reaction (ORR) catalysts for proton exchange membrane (PEM) fuel cells and compared to conventional 2wt% Pt/C (ETEK) catalyst. Electrochemical and physical properties of both materials were characterized in detail. Pt-ON/C shows competitive ORR activity similar to Pt/C (ETEK) while demonstrating an improved stability. By using post mortem analysis with transmission electron microscope/scanning transmission electron microscope (TEM/STEM), the degradation mechanisms of both catalysts are investigated. Two different dominant mechanisms were suggested to explain the decreased activity of Pt/C (ETEK) under different operation protocols: for an accelerated stress test (AST) with low maximum potentials, a loss of Pt surface area associated with carbon oxidation leads to the decreased activity; while for AST with high maximum potentials, Pt particle growth, detachment, dissolution/re-deposition and severe carbon corrosion dominate the performance loss. In addition, in the catalyst of Pt-ON/C, Mo dissolution occurs under the entire potential window which, however, leads to the enhanced activity after lifetime stability test protocol.

Heterostructured nanowires are of prime interest in nowadays technology such as field-effect transistors, field emitters, batteries and solar cells. We consider their aging behavior and developed a model focusing on reactive diffusion in core-shell nanowires. A complete set of analytical equations is presented that takes into account thermodynamic driving forces, vacancy distribution, elastic stress and its plastic relaxation. This complete description of the reactive diffusion can be used in finite element simulations to investigate diffusion processes in various geometries. In order to show clearly the interplay between the cylindrical geometry, the reactive diffusion and the stresses developing in the nanowire, we investigate the formation of an intermetallic reaction product in various core-shell geometries. Emphasis is placed on showing how it is possible to control the kinetics of the reaction by applying an axial stress to the nanowires.

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH2 is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO2). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.

Pure polyurethane and nanocomposite carbon black (CB) polyurethane solutions were deposited by spin-coating on a silicon substrate using gold as the adhesion layer and electrode. Different test structures were achieved for electrical and mechanical characterizations. The incorporation of CB nanoparticles in the polyurethane matrix has a significant influence on the dielectric permittivity of the material with an increase of about one third of its value. The Young's modulus of PU and nanocomposite PU films was determined by different characterization methods. Nanoindentation experiments have pointed out a Young's modulus gradient through the film thickness. By performing mechanical tests (tensile, bulge, point deflection) on freestanding films, an average Young'smodulus value of about 30 MPa was found as well as a residual stress value of about 0.4 MPa. However, no influence of the presence of the nanoparticles was found. Finally, several MEMS actuators were realized and characterized. At their fundamental resonance frequency, the actuation of the nanocomposite membranes is more efficient than that of pure polyurethane. However, the time constant of the material seems to provide a major barrier for the development of high-frequency PU-based micro-actuators.

Phase separation in silicon-rich silica/silica multilayers was investigated using Atom Probe Tomography and Atomistic Kinetic Monte Carlo simulation. It is shown that the thickness of silicon-rich silicon oxide sublayers plays an important role during phase transformation. It determines the morphology of Si-rich phase formed after subsequent annealing, which is of prime interest for microelectronic and optoelectronic applications. Monte Carlo simulation reveals that the formation of isolated Si clusters can be achieved even in the case of spinodal decomposition and is directly related to the ratio between the spinodal wavelength and the sublayer thickness.

SiOX/SiO2 multilayers have been prepared using magnetron sputtering and annealed in order to induce the growth of Si nanoparticles in Si-rich sublayers. This sample has undergone several successive annealing treatments and has been analyzed using a laser-assisted tomographic atom probe. This allows the phase separation between Si and SiO2 and the growth process to be studied at the atomic scale as a function of annealing temperature. Si diffusion coefficient is estimated from the accurate measurement of matrix composition and Si particle size. We demonstrate that the diffusion coefficient in SiOX is supersaturation dependent, leading to a decrease in silicon particle growth kinetics during annealing. In addition, we use our measurements to predict the critical thickness for efficient SiO2 diffusion barriers.