[en] The results of work in which tungsten wafers (25x4x0.3mm), made by electrolytic polishing and ionic bombardment, were carburised by being surrounded by graphite pellets at 2000₀-2100₀C for several minutes (the amount of carbon so introduced varying from 1 per mille to several atom percent) are reported. The reported observations confirm previous results on the precipitation of the hexagonal phase of Me²C obtained by transmission electron microscopic observation of a variety of metals. The appearance of an incoherent precipitate, surrounded by helical dislocations and the presence of the W²C phase together with its orientation in the matrix are illustrated, consistent with the relations found by Burgers. Successive stages of the precipitation were observed qualitatively in different specimens, all suggesting that the precipitation of the W²C in the tungsten operates in the same way as in other systems of transition metal and carbon. Further work is required to clear up some finer points concerning deviation of axes. (C.J.O.G.)

[en] Nanostructured thin films were prepared from mixed Co-Ag clusters preformed in the gas phase, in order to improve the magnetic properties previously obtained for pure superparamagnetic Co clusters embedded in miscible matrices. In this paper, we focus on the morphology of such mixed nanoparticles to confirm a core-shell model. Electron diffraction has shown that a Co-Ag segregation occurs but preliminary local structure and magnetic results indicate that the Ag-shell around the Co-core is not perfect, some Co atoms being in direct contact with matrix atoms

[en] The crystallographic structure of 3Al₂O₃-2SiO₂ mullite has been studied by means of high-resolution electron microscopy. The best conditions for atomic imaging of this compound are discussed relative to the atomic resolution microscope at Berkeley. [001] multibeam images have been produced at a resolution better than 0.19 nm, allowing the cation sublattice to be directly imaged, with a firm transfer of information on the oxygen sublattice. Under optimal conditions of defocus setting and thickness, typical contrasts have been detected and consistently interpreted in terms of the presence of oxygen vacancies along the defective atomic columns. These observations are discussed in relation to the currently accepted model for the average structure of mullite. (orig.)

[en] A precise thermodynamic analysis of the mechanical test results carried out between 800 and 2200⁰C on polycrystalline W/sub 2/C allows us to determine the terms of the constitutive law. Particularly, the stress dependence of the preexponential factor is determined following the analysis of Surek and Co-workers. The preceding results are compared with those obtained by the measurement of carbon diffusion coefficient, electron microscope observations and crystallographic determinations by neutron diffraction. All the results are consistent with the proposed deformation mechanism

[en] The aim of the present paper is to discuss the advantages of High Resolution Electron Microscopy (HREM) studies of metal-ceramic interfaces. A review of methods available for the production of interfaces suitable for observation in a transmission electron microscopy, together with a literature survey of recent relevant studies will be presented first. A selection of examples will serve to illustrate the main points that HREM allows to be tackled: detection of nanometer-size intermediate compounds (Ni/MgO, NiO/Pt, MgO/Nb systems), accomodation of steps at the ceramic surface (Fe/Al₂O₃, Au/MgO, Cu/MnO systems), characterization of misfit dislocations (e.g., Cu/MgO), effect of the image force on the exact position of the dislocation cores, lattice expansion due to elastic mismatch (e.g., Pd/MgO), determination of the chemical nature of the terminating atomic planes (e.g., Pd/Al₂O₃). An introductive discussion of specific problems in the interpretation of HREM images (i.e., numerical image analysis and computer simulations) will be conducted and illustrated with the case of incoherent Fe/Al₂O₃ interfaces. (orig.)

[en] Full text.Since the discovery of carbon nanotubes (CNTs) in 1991 by S. Iijima, numerous studies have been devoted to the synthesis and characterisation of one dimensional materials. Actually, specific chemical and physical properties are expected for thin diameters nanotubes (NTs) and nanowires (NWs), due to quantum confinement effects. For instance, these nanomaterials offer exciting opportunities for thermostructural applications or for uses in electronic (field-emission properties). Various nanotubes and naowires of different chemical composition have been synthesised, including WS₂, BN, GaN, etc. Among this materials, cubic silicon carbide (β-SiC) in the form of nanomaterials is particularly interesting due to its intrinsic chemical and electronic properties. For instance, SiC NWs could be more eficient than carbon NTs in nanoelectronic for high temperature, high power and high frequency applications. Moreover, coaxial multielement nanostructures (nanocables, NCs) could offer good opportunities particularly as reinforcement agents for mechanical applications, the outer sheet acting as an interface between the enanofiber and the matrix. But only few methods have been developed to elaborate such nanocables and they usually involve heavy technical equipment. Recently, we reported the preparation of βSiC NWs by direct reaction between silicon and carbon at 1200 celsius degre under nitrogen. By changing the experimental conditions (atmosphere, temperature) this method has been extended to the preparation of various nanowires (like amorphous SiO₂-NWs or hexagonal Si₃N₄-NWs). Following a similar process, we prepared SiC and BN NCs composed of a SiC core coated with several layers of hexagonal boron nitride. Those nanocables are several micrometers long with 50 nm diameter. the influence of the experimental conditions on the thickness and structure of the BN coating will be discussed. This general method has been extended to the elaboration of others nanocables (like SiC and SiO₂ NCs). The structure of these nanomaterials has been investigated by the means of HRTEM, EELS and EDX spectroscopy

[en] The robust development of exotic devices strongly relies on physical characterization to better understand the stacked layers architecture and the resulting morphology. In such a context, a silicon nanowire, 200nm long and of around 15nm of diameter, encapsulated by a high-k metal gate stack (TiN/HfSiON) of few nanometers per layer, is characterized by 2D and 3D X-ray energy dispersive spectroscopy in a STEM to evaluate the process flow methodology

[en] With the development, over the past ten years, of a new generation of electron microscopes with advanced performance, incorporating aberration correctors, monochromators, more sensitive detectors, and innovative specimen environments, quantitative measurements at the sub-nanometer and, in certain cases, at the unique atom level, are now accessible. However, an optimized use of these possibilities requires access to costly instruments and support by specialized trained experts. For these reasons, a national network (METSA) has been created in France with the support of CNRS and CEA in order to offer, in centres with complementary equipment and expertise, an open access to an enlarged and multidisciplinary community of academic and industrial users. The eight METSA platforms offer an access to 3 atom probes and 17 electron microscopes, mainly aberration-corrected TEMs, but also one dedicated environmental SEM and 3 double-column SEM-FIB

[en] The first stages of NiSi phase formation at the expense of θ-Ni_2Si and a Si substrate are investigated by transmission electron microscopy (TEM). These measurements show the presence of a low density of NiSi particles at the θ-Ni_2Si/Si(1 0 0) interface and allow their complete shape to be determined. This stage corresponds to the lateral growth of NiSi at the epitaxial θ-Ni_2Si/Si(1 0 0) interface. The shape of these particles is in agreement with the predicted models, and the shapes were fitted by an analytical expression derived from the model developed by Klinger et al

[en] We present a method using high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) to determine the chemical composition of bi-metallic nanoparticles. This method, which can be applied in a semi-automated way, allows large scale analysis with a statistical number of particles (several hundreds) in a short time. Once a calibration curve has been obtained, e.g., using energy-dispersive X-ray spectroscopy (EDX) measurements on a few particles, the HAADF integrated intensity of each particle can indeed be directly related to its chemical composition. After a theoretical description, this approach is applied to the case of iron–palladium nanoparticles (expected to be nearly stoichiometric) with a mean size of 8.3 nm. It will be shown that an accurate chemical composition histogram is obtained, i.e., the Fe content has been determined to be 49.0 at.% with a dispersion of 10.4 %. HAADF-STEM analysis represents a powerful alternative to fastidious single particle EDX measurements, for the compositional dispersion in alloy nanoparticles.