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The powder metallurgy processing (the compaction and the sintering) of aluminium nanocrystalline powder (with particle size of 100 nm) fabricated using a condensation process, has been studied. The compaction data were analysed using the Konopicky-Shapiro approach. This reveals that the particles are slightly harder than their coarse grained counterpart and that the powder densification is achieved by plastic flow of the nanoparticles. The sintering has been studied in vacuum and under hydrogen atmosphere and compared to the behaviour of a coarse particle powder (size 20 μm). The analysis, using the kinetic relations of sintering indicates that for nanoparticles, an initial stage of densification following a viscous flow mechanism occurs in the alumina hydroxide phase, located at the surface of the particles. Subsequently, the hydroxide is transformed into γ-alumina nanoparticles located at the aluminium grain boundaries. The sintering is then controlled by grain boundary diffusion coupled to aluminium plastic flow. The interpretations are supported by measurements of the activation energy and structural investigations using X-ray diffraction and transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Nanostructured ferrites have been prepared by milling starting from commercial ferrite powders. Subsequent compaction and sintering were applied to obtain cylindrical pieces. The magnetization as a function of the grain size follows the non-magnetic grain boundary model. The boundary thickness is estimated as about 1.5 nm, which is consistent with that calculated from the Mo¨ssbauer absorption areas. Due to the high lattice distortion, the small exchange length impedes the averaging of the anisotropy. [Copyright &y& Elsevier]

Abstract: The synthesis of Cu100−x Ag x (0.05at.%< x <0.30at.%) nanoparticles using an evaporation–condensation process has been studied. Particular attention has been paid to the composition of the as-prepared powders. Compared to the master alloy, systematic enrichment in Ag of the particles is observed at the beginning of the synthesis but a continuous decrease in the Ag content of the powders is observed during the process. Even though, after de-agglomeration, the chemical composition of the powders is, in average, homogeneous. We have shown that optimized synthesis conditions give spherical particles with a mean diameter of 50nm and a reasonable yield rate. [Copyright &y& Elsevier]

Nanocrystalline Fe-based spinels with composition Fe2.5Ti0.5O4 can be synthesized using two different routes: soft chemistry and high-energy ball milling. This paper is focussed on the fact that each type of synthesis process can lead to powders with a crystallite size of about 15 nm but with significant differences in the grain size distribution and the agglomeration state. Whereas in the case of mechanosynthesis, the ball-milled powders consist of aggregates, those obtained by soft chemistry are very well dispersed. Moreover the chosen investigated nanopowders present a blocked/superparamagnetic transition depending on the grain size. The grain size morphologies obtained by the two techniques of synthesis can then be fully characterized by complementary experiments: in addition to high-resolution image processing, specific measurements adapted to the study of magnetic relaxation can be used for weighting differently their small and large size tails: namely, magnetization measurements and Mo¨ssbauer spectrometry. [Copyright &y& Elsevier]

Abstract: In therecent years, magnetic nanoparticles have been extensively studied for their superparamagnetic properties providing useful labels in biology or for fundamental aspects including the size dependence of magnetic atomic moment and the effect of surface anisotropy. In most cases, the particles were smaller than 10nm and interestingly, the sizes ranging between 10 and 100nm have been poorly investigated until now. This is mainly due to the fact that usual chemical routes produce 5–10nm oxide or metallic particles or eventually 20nm at most. On the over side, atomization techniques yield particles in the micrometer range. Metallic particles are particularly interesting for better magnetic properties compared to oxides, but they have two big drawbacks: they are not biocompatible and they are conducting electricity. Consequently, it’s necessary to produce core–shell particles, for which the shell is biocompatible and insulating and with a perfect control of thickness and uniformity of that shell. In this work, we are studying metallic particles synthesized by an original evaporation–condensation technique that produces particles of several tens of nanometers. We prepared hard magnetic cobalt particles and soft FeNi ones coated with a silica shell using a modified sol–gel method. Morphological and magnetic properties are presented, showing the efficiency of ultrasonic sol–gel process for that purpose. [Copyright &y& Elsevier]

Abstract: We use an approximate micromagnetic model, based on geometrical simplifications of the problem, to describe the vortex core structure observed in spherical Permalloy nanoparticles using off-axis electron holography. The magnetisation distribution inside the vortex core is directly calculated by minimising the micromagnetic energy functional and is compared with the experimental results. The symmetry constraints underlying the model are discussed envisaging possible generalisation to the case of vortex cores with structure strongly dependent on the coordinate directed along the axis of the vortex. Moreover the many-body effect associated with the presence of two small satellite particles is described by rescaling the size of the particle. [Copyright &y& Elsevier]