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[en] Metallic uranium is an important material for many applications, especially nuclear energy for low enriched metallic fuel forms and new reactor concepts. Due to the its high electropositivity, production pathways to the metallic form are limited. One of the historically most common synthesis routes involves the heating of a uranium (IV) halide, in this study UF4, with a highly electropositive metal like calcium. This synthesis is referred to as “bomb reduction” due to the temperatures and pressures released by the resulting exothermic reaction. This synthesis route is important for production, but it has also been shown to separate a decay product, thorium, from the parent uranium. This fractionation could be important for purification, but also presents the opportunity for radiochronometric dating of the reduction date. Unfortunately, little characterization has been performed on the products of this reaction. The present study performed bomb reduction on ~10 g of thorium-doped (2000 ppm) UF4 followed by scanning electron microscopy and energy dispersive spectroscopy. This characterization revealed morphological features of the metal product and slag, the latter of which displayed a wide range of features that indicates a complex reaction in which many variables are involved. Initial characterizations also identified thorium-rich particles, which were extracted and analyzed via transmission electron microscopy and atom probe tomography. These characterizations identified a new thorium-bearing phase, Al9-xFe7-xTh2Si<1, and also indicated thorium fractionation from uranium via at least four mechanistic pathways.
[en] Fe-6.5wt%Si steel surpasses the current extensively used Fe-3.2wt%Si steel in lower iron loss, higher permeability, and near zero magnetostriction. As a cost effective soft magnetic material, Fe-6.5wt%Si may find applications in motors, transformers, and electronic components. However, the brittleness of the alloy poses processing challenges. The brittleness in Fe-6.5wt%Si is attributed to the formation of ordered phases. Evaluation of the amount of ordered phases is important for the research and development of Fe-6.5wt%Si. This paper aims to find effective ways to evaluate the ordering degree through a comparison of various characterization techniques. In order to tune the ordering degree, various speeds were used to prepare Fe-6.5wt%Si samples via melt spinning. The varying wheel speed changes the cooling rate, which was confirmed by thermal imaging. In addition to the widely used TEM and normal theta-2theta X-ray diffraction methods, two quantitative methods were adopted for this Fe-6.5wt%Si system to study the ordering degree. One method is based on rotating crystal XRD technique, and the other is magnetic thermal analysis technique. Finally, these two methods effectively quantified the varying degree of ordering presented in the samples and were deemed more suitable than the TEM, normal theta-2theta XRD methods for Fe-Si due to their ease of sample preparation and short turn-around time.
[en] A ceramic sample of uranium dioxide is probed by Raman imaging followed by a combined Lorentzian fitting - Principal Component Analysis process. This allows to evidence structural or chemical inhomogeneities of the material, which affect Raman line intensities but also line positions, evidencing local symmetry lowering. The inhomogeneities were observed not only between grain cores and boundaries, but also inside grain cores themselves. Only a part of these intensity inhomogeneities is as expected due to different orientations of the ceramic grains. Besides, a zone noticeably differing from others has been distinguished, presumably due to local strains or to chemical nature (oxygen stoichiometry), showing the sensitivity of the analysis. This underlines the importance to perform Raman analysis in such ceramic materials at least on several points and better, in imaging mode. (authors)
[en] A surface state analysis of an UO2 ceramic is performed by combining Raman imaging and electron back-scattering diffraction. Special attention is paid to the behavior of the T2g band intensity versus the crystalline orientation. In order to clarify the origin of the T2g intensity variation in the appropriate Raman image, electron back-scattering diffraction (EBSD) measurements are carried out. EBSD data allow a theoretical estimation of the Raman T2g band intensities. Both maps have been compared in order to correlate them, and to distinguish regions where they are analogous, from those where some physical or chemical effects (stoichiometry, or strains) induce different behavior. More generally, this highlights the interest and the complementarity of combining Raman spectroscopy and EBSD for a better microstructure knowledge. (authors)
[en] 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. (authors)
[en] We report monodisperse, chain-like particles (nanochains) consisted of silica-coated maghemite (γ-Fe2O3) nanoparticle clusters prepared by colloidal chemistry and magnetic field-induced self-assembly of nanoparticle clusters. In order to quantify the shapes of chain-like particles, we have used the measure for shape convexity which is also called solidity. We functionalize the surface of the nanochains with amino (–NH2) and carboxyl groups (–COOH) in order to modify surface charge. These surfaces of nanochains provide better colloidal stability and their potential for practical applications in biomedicine. The enhanced colloidal stability of the surface modified nanochains is confirmed by Zeta potential (ζ-potential) analysis. Magnetic properties of the nanochains show superparamagnetic state at room temperature since the nanochains are composed of tiny nanoparticles as their building blocks. The measured M(H) data at room temperature have been successfully fitted by the Langevin function and magnetic moment μp = 20,526 μB for sphere-like nanoparticle clusters and μp = 20,767 μB for nanochains are determined. The determined magnetic parameters have revealed that the nanochains show a magnetic moment of the nanoparticles higher than the one of individual nanoparticle clusters. These differences can be attributed to the collective magnetic properties of superparamagnetic iron oxide nanoparticles (SPION) assembled in different morphologies (isotropic and anisotropic morphology). © 2018
[en] An accurate method for the characterization of the topographic evolution of materials during dissolution at the submicrometric scale is reported. This method is based on the recording of Environmental SEM tilted images of a selected zone at the surface of a sample for various dissolution times. After each observation, a 3D surface reconstruction was made leading to a series of height maps of the selected zone of interest. One 3D reconstruction was compared to an AFM image of the same zone in order to estimate the accuracy of the heights determined using stereoscopic images. The maximum achievable resolution along the z-axis on the 3D reconstructions was found to be 38 nm. Several micro-structural parameters were extracted from the 3D-image series and their evolutions were related to the dissolution process. This method offers new insights for the monitoring of the changes occurring at the surface of a sample at the micrometer scale during dissolution or corrosion of the material. (authors)
[en] In-situ time-resolved Synchrotron X-ray diffraction analyses were performed on zirconium alloy (Zircaloy-4) sheet samples, during their heating, isothermal oxidation at 700, 800 and 900 degrees C under a flowing mixture of He and O2 and cooling. The oxide growth and the evolution of the oxide structure as a function of time and temperature were studied with suitable time resolution. Oxide layer thicknesses of approximately 10 mu m were formed during the experiments. The incident X-rays penetrated the whole oxide thickness. The samples were examined after the experiments by field emission gun scanning electron microscopy, electron backscatter diffraction and electron-probe microanalysis. The results showed that the oxide contains a mixture of monoclinic and tetragonal zirconia evolving during heating, oxidation and cooling. The average volume fraction of tetragonal zirconia decreases during oxidation. This fraction is larger at 900 degrees C than at 700 and 800 degrees C. For oxide layers thinner than approximately 5 mu m, this fraction is larger at 800 degrees C than at 700 degrees C, but it is rather equivalent for both temperatures when the oxide thickness ranges between 5 and 8 mu m. Some of the tetragonal zirconia crystals transforms into the monoclinic phase during cooling after oxidation. This fraction of transformed tetragonal zirconia is larger after oxidation at 900 degrees C than after oxidation at 700 and 800 degrees C. It is suggested that these evolutions of the oxide crystallographic structure are related to micro-stresses and to temperature dependences of the critical size of zirconia crystals below which tetragonal zirconia is stabilized. (authors)
[en] Anomalous small-angle X-ray scattering (ASAXS) is a technique developed in the 1980's giving access to chemical information of nano-objects besides characteristic features like size and volume fraction given by classical SAXS. ASAXS is an element-selective technique based on the anomalous variation of the scattering factor near the absorption edge of one chosen element. A simple approach is proposed to extract chemical information from anomalous SAXS data. To illustrate the procedure, data treatment is applied to discriminate between different possible phases that may form nano-oxides in oxide-dispersion-strengthened (ODS) steels. (authors)