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The design of the dissimilar metal weld investigated here is aimed at applications in the steam generator of a sodium-cooled nuclear reactor, with a multi-decade lifespan in demanding operational conditions. It consists in a narrow-gap joint between 2.25Cr-1Mo low-alloy steel and an austenitic alloy using a nickel-based alloy 82 as filler material. This study focuses on understanding the microstructural and micromechanical evolution in the near fusion boundary region between the low-alloy steel and the nickel alloy filler metal during post-weld heat treatment, using notably electron probe micro-analysis and nano-indentation. The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical potential for carbon, which is mobile at the post-weld heat treatment temperature. A number of fine-scale characterization techniques were used to assess the gradient of composition, hardness, and microstructures across the fusion boundary, both as-welded and after post-weld heat treatment. This complete analysis permits to highlight and understand the main microstructural and micromechanical changes occurring during post-weld heat treatment and opens the way to their long term study in service conditions.

Core–shell nanoparticles are synthesized by nanosecond-pulsed discharges in liquid nitrogen using a two-step process. In a first step, copper nanoparticles (resp. zinc nanosheets) are synthesized by eroding pure copper (resp. zinc) electrodes. In a second step, copper (resp. zinc) electrodes are removed and replaced by zinc (resp. copper) electrodes in the liquid loaded with copper nanoparticles (resp. zinc nanosheets). After erosion and air oxidation, once nitrogen has evaporated, Cu@ZnO core–shell nanoparticles are obtained in both configurations. The shell is always ZnO, because of the unusual formation of zinc nanosheets instead of zinc nanoparticles. When Cu electrodes are used first, copper nanoparticles are wrapped in ZnO nanosheets. When Zn electrodes are used first, copper nanoparticles hit zinc nanosheets and get coated to form also Cu@ZnO. In this latter case, Cu2O@ZnO are sometimes encountered too but to a much lesser extent. Copper oxidation is then attributed to a failure in the zinc protective shell. Time-resolved optical emission spectroscopy measurements reveal that Cu lines and Zn lines are never emitted simultaneously, the element in the liquid remaining outside the discharge area. If zinc nanosheets are synthesized first, N II lines are exclusively observed during the first 200 ns of discharges with copper electrodes, revealing for this process the possibility of highly energetic processes.

The origin of isosahedral short range order (ISRO)-mediated nucleation in additively manufactured Inconel 718 is investigated in this work. 3D EBSD-EDS characterization of the equiaxed grain zone revealed a 5-fold symmetry axis emerging from the surface of a TiC particle. Using high resolution electron microscopy, the Nb2Ni and Ni6Nb7phases have been identified around a TiC particle for the first time in this alloy. Such phases are associated with local Nb enrichment around the TiC particles. The Nb-rich liquid and the solidification conditions of the additive manufacturing process induce the formation of phases containing a high concentration of icosahedral clusters and are thought to be the origin of ISRO-mediated nucleation in this alloy.

In additive manufacturing of Ni-based alloy IN718, what causes Icosahedral Short-Range Order (ISRO) or iQC mediated nucleation of equiaxed grains is still not understood. In order to have a better comprehension of the microstructure formation, a more comprehensive characterization both on grain structure and chemical heterogeneities must be achieved. In this work, a scanning electron microscope equipped with a focused ion beam (FIB) allowed to perform a 3D EDS chemical analysis and EBSD orientation mapping of the volume in an equiaxed grain zone. After the 3D reconstruction of the equiaxed grains has been performed, grain-to-grain orientation analysis allowed to analyze multiple twin boundaries orientation relationships in a volume containing hundreds of grains. Chemical mapping using EDS allowed to identify TiC inclusions from which a five-fold symmetry axis seem to emerge, indicating a potential nucleation site. Such areas were analyzed in transmission electron microscopy (TEM) to investigate the different present phases. The nucleation sequence and potential origin of mediated nucleation are further discussed, opening new prospects for grain refinement in additively manufactured nickel-based alloys.

The possibility to create homogeneous nanoparticles containing iron and gold by nanosecond-pulsed discharges in liquids, without any laser assistance, is demonstrated. Thanks to a new kind of electrode where elements are coated by PVD on a silicon plate, AuFe nanoparticles, ranging from 10 to 50 nm, are synthesized. Depending on their residence time in the discharge, the structure of the homogeneous nanoparticles evolves to become a core–shell structure, either nested or not. Larger submicrometric particles (from 100 to 500 nm) are specifically formed with the present process by the ejection of liquid droplets from the molten pool created by the discharge on electrode surfaces. These larger particles are produced by solidification of a AuFe shell on a silicon core. This new mechanism can be used to create core–shell particles with a shell containing several, possibly immiscible, elements. Thermal post-treatment performed in situin a TEM microscope shows the AuFe shell, where Au and Fe separate, turning either into a Janus structure or into a SiFe core facetted by gold pyramids.

The present paper focuses on the direct synthesis of NiGa layered double hydroxides (LDHs) by concomitant addition of Ni and Ga chloride in solution with NaOH solution, at a constant pH value of 6.5. Various Ni/Ga ratios (Ni/Ga = 2, Ni/Ga = 4, or Ni/Ga = 6) were investigated, and samples were collected after successive additions. Each sample was then analyzed by small-angle X-ray scattering (SAXS) to derive information about the shape of the objects formed upon synthesis. In parallel Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) analysis were carried out on relevant samples to obtain information on the local structure of the species in suspension. SAXS curves reveal significant changes. At the beginning of the synthesis, for low added volumes (Vs), the suspension appears to contain both monodimensional and bidimensional objects. For higher Vs, the monodimensional objects disappear and only platelike particles are detected in the suspension. EXAFS results show that all the added gallium atoms are involved in a solid Ni−Ga LDH phase throughout the whole synthesis. In contrast, some nickel cations do not precipitate in the Ni−Ga LDH. At the beginning of the synthesis, for low added volumes they are also involved in the formation of fibrous polycations. For higher added volumes, the fibers are not observed anymore and monomeric nickel species are then present in the suspension.

Takovites are nickel-based layered double hydroxides (LDH) with a general formula that can be written as Ni1-xAlx(OH)2, A^z-x/z, yH2O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses on the hydrolysis of Ni²⁺ and Ga³⁺ ions, together in solution, carried out by base addition. The samples will be defined by their hydrolysis ratio R = [OH^-]/([Ni²⁺] + [Ga³⁺]). Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) were used to obtain information on the colloidal species size and shape on a large scale. Each hydrolyzed sample was also studied by Ni K-edge and Ga K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the species in suspension. SAXS curves reveal the presence of bidimensional objects whatever the R values. The platelets formed for R values ≥1.1 are slightly thicker and smaller in size, which may be linked to their different natures. Complementary information is provided by TEM analysis:  the first colloids formed have a structure very close to that of α-GaOOH, as shown by electronic diffraction. Those structures are progressively replaced by Ni−Ga LDH platelets with increasing hydrolysis ratio, which are the only species in suspension for R = 2.0, as shown by XRD. EXAFS results confirm the complete hydrolysis of gallium before the formation of Ni−Ga LDH phases.

Takovites are nickel-based layered double hydroxides (LDHs) with a general formula that can be written as Ni1-xAlx(OH)2, A^z-x/z, yH2O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses only on the hydrolysis of Ni²⁺ ions carried out by base addition. The samples will be defined by their hydrolysis ratio R = [OH^-]/[Ni²⁺]. For all R values, colloids are observed in the final suspensions. Each hydrolyzed sample was studied by Ni K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the colloids. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) are also used to obtain concomitant information at larger scale. EXAFS results reveal that upon Ni²⁺ hydrolysis by NaOH, the number of nickel neighbors increases with R, atomic distances corresponding only to edge-sharing nickel octahedra. These results could then be interpreted as revealing the precipitation of increasing amounts of Ni(OH)2, with increasing R values. However, SAXS curves display a significant evolution with R. Modeling shows the presence of rod-shaped colloids for low R values. These rodlike particles progressively disappear in favor of disklike platelets, that are the only species present in suspension for R = 2.0. Such an evolution is confirmed by TEM analysis.

A novel chemical reduction method using an activated alkaline hydride (LiH or NaH-t-BuONa) in tetrahydrofuran solvent has been applied to antimony salt reduction. X-ray diffraction and transmission electron microscopy studies have been carried out to characterize the morphology and structure of the materials. Alkali hydride nature influence has been proved. In both cases the process allows to prepare antimony particles in nanometer range from few nanometers to about 20nm which could be used as anodic materials for lithium–ion batteries. With lithium hydride well-crystallized particles inclined to agglomeration were observed whereas finely dispersed amorphous particles were pointing out after activated sodium hydride reduction.

To reach a better understanding of the behavior of suspensions of charged colloidal platelets, takovite samples with a general formula Ni1-x, Alx(OH)2, An^z-x/z, yH2O, where An is an interlayer anion, were synthesized. These materials belong to the family of layered double hydroxides (also referred to as anionic clays). They can be easily prepared in the laboratory by coprecipitating mixed nickel and aluminum salts by NaOH. Their charge can be adjusted by changing the synthesis conditions, while the hydration properties can be modified by using various interlayer anions. On the basis of oscillatory measurements, rheological phase diagrams were obtained by plotting the sol−gel transition as a function of solid content and ionic strength for three monovalent anions:  F^-, Cl^-, and NO3^-. In all cases, the phase diagrams exhibit a negative slope, i.e., the sol−gel transition shifts toward lower solid contents for increasing ionic strength. The nature of the interlayer anion has a strong influence, the sol−gel line moving toward higher solid content in the order takovite F^- < takovite Cl^- < takovite NO3^-. These differences cannot be assigned to changes in the surface charge of the samples as zeta potential measurements reveal a constant charge for the three anions. Postsynthesis hydrothermal treatments were then performed in order to modify the size and anisotropy of takovite NO3^-. The aspect ratio of the particles appears as a key parameter controlling gel formation as it has a very pronounced effect on the rheological phase diagrams. Particles with higher aspect ratio exhibit a sol/gel transition at lower solid contents and with a reduced dependence on ionic strength.

Electroreduction of C60 was carried out in all-solid-cells operating with polymer electrolytes composed of polyethylene oxide (PEO), and salts like LiClO4, NaCF3SO3 or KCF3SO3. Some selected reduced phases of C60 were isolated and characterized by electron diffraction microscopy.

AbstractPolyparaphenylene powders are electrochemically reduced in LiCIO4-ethylene carbonate electrolyte. Intercalation of Li+ions occurs and leads to the formation of Li0.5(C6H4) compounds. Side reactions due to the electrolyte decomposition are responsible for the important irreversible electrochemical capacity observed during the first charge-discharge cycles. The degradation products were analyzed by TEM and EELS between 2.85 and 0.75 V.

The evolution of the electrical charge injected into Ni-doped phyllosilicate nanoscrolls composites deposited on a conductive substrate was studied by Kelvin probe force microscopy. The nanoscrolls were synthesized by hydrothermal method and then annealed in H2/Ar flow at 400-900degC in order to reduce Ni up to metal. A typical agglomerate of the Ni3Si2O5(OH)4 nanoscrolls accumulated the charge on one nanoscroll followed by a subsequent partial destruction of the agglomerate. For the Ni2MgSi2O5(OH)4 nanoscrolls, the whole agglomerate could be charged, and the injected charge was retained for hours. The shortest charge relaxation time was revealed for the nanoscrolls annealed at the highest temperature, when Ni was completely reduced.

The production of single-walled carbon nanotubes (SWNTs) requires the use of catalysts, so the as-obtained compounds can contain traces of iron and nickel. The characterization by XRD and TEM of the nanotubes-based compounds was carried out at different steps of their thermal and chemical purification. Prolongated electron beam exposure results in structural modifications of the nanotubes which were mainly studied by electron energy loss spectroscopy (EELS).

The evolution of the electron spin resonance parameters (ΔHpp, A/B and g) of vapour phase sodium intercalated polyparaphenylene was examined as a function of the intercalation duration and the temperature T. The linear relationship between ΔHpp and T observed for compounds intercalated to saturation is characteristic of a metallic behaviour.