CoFe2O4 NANOSTRUCTURED MAGNETIC MATERIALS: INFLUENCE OF NOT-CONVENTIONAL METHODS OF COMPACTION (SPS, HIP) ON THE MICROSTRUCTURE AND PHYSICAL CHARACTERISTICS.

This work presents a Bottom-Up strategy implementing methods of soft chemistry followed by processes of not conventional compaction for making of CoFe2O4 spinel nanostructured materials. The optimization of the process routes to obtain bulk and dense materials will be described in details along with the material nanostructures and magnetic properties. Initially, the nanopowders (10 nm) are obtained in solution by hydrolysis forced in polyol medium. CoFe2O4 nanoparticles have been obtained starting from ionic salts (cobalt acetate and iron chlorate) dissolved in a polyol and heated at a boiling point. The chemical reactions are made possible by several properties of polyols, including high boiling point, dissolving, complexing, hydrolysing and protective properties. This route enables to synthesize particles of size and morphology controlled. These powders prove to have interesting magnetic properties in comparison with those worked out by other ways of soft chemistry [1]: high saturation magnetization near to the bulk materials and temperature of blocking higher than the ambient one

Engineered Materials: Micro-Nanostructure, Properties and Applications

The special issue presents a series of 12 contributions related to recent developments on the link between functionalities and structure in micro and nanostructured materials. The main singularity of our collection is that we proposed to bring together the know-how of several disciplines (physics, mechanics and chemistry) for a transdisciplinary discussion on the multi-scaled properties on nanostructured systems. We put together different communities for a better understanding of the relations between elaboration-structure-and-properties in bulk nanostructured materials. Particular attention has been paid to the parameters which seem to govern the macroscopic physical properties of these structures: the size of the grains, the interaction between these grains but also the influence and the nature of the size of the crystallites constituting these grains as well as their confinement by different designed nanostructuration. The main idea of this collection is to open the debate on the possible reliable strategies able to characterize the microstructure and link them to the performances of the nanostructured materials. As reported in the contributions presented in this collection, this latter point is essential for improving the physical functionalities of new nanostructures

Microstructure and mechanical properties of ultrafine-grained fcc/hcp cobalt processed by a bottom-up approach

Bulk Co samples having a mean grain size of ∼300nm were processed by hot isostatic pressing of a high purity Co nanopowder synthesized by chimie douce. The grain interior exhibited a highly faulted nanoscale lamellar microstructure comprising an intricate mixture of face-centered cubic, hexagonal close-packed phases and nanotwins. Room temperature compression tests carried out at a strain rate of ∼2×10−4 s−1 revealed a yield stress of ∼1 GPa, a strain to rupture of ∼5%. During straining it was found that the hexagonal close-packed phase content increased from 55% to 65% suggesting a deformation mechanism based on stress-assisted face-centered cubic to hexagonal close-packed phase transformation. In addition, an apparent activation volume of ∼3b3 was computed which indicates that the deformation mechanism was controlled by dislocation nucleation from the numerous boundaries. Nonetheless, in such an intricate microstructure, the overall mechanical properties are discussed in term of a complex interplay between lattice dislocation plasticity, transformation-induced plasticity and possibly twin-induced plasticity

Effect of SPS process sintering on the microstructure and mechanical properties of nanocrystalline TiC for tools application

International audienc

Promising high temperature thermoelectric properties of dense Ba 2 Co 9 O 14 ceramics

International audienc

Microstructure effects on thermal and electrical conductivities in the intermetallic compound Ag

Sintered intermetallic $$\hbox {Ag}_{3} \hbox {Sn}$$ powders by spark plasma sintering (SPS) have been studied in order to analyse the effect of the microstructure on the transport properties at room temperature. Firstly, we synthesized $$\hbox {Ag}_{3}\text {Sn}$$ submicronic particles by polyol process. Then, we sintered them by SPS at different temperature (473, 573 and 663 K) and pressure (20 and 100 MPa) using fixed dwelling time of 5 min. X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) show that all sintered samples remained single-phased according to the $$\hbox {Ag}_{3}\text {Sn}$$ orthorhombic crystal structure. Their corresponding grain size ($$\approx$$ 0.65 to 1.75 \upmu m) and relative density ($$\approx$$ 88–98%) were calculated and show similar evolution respect to the sintering parameters. As a matter of fact, sintering temperature remains the key factor to affect the microstructure of the sintered material s. Finally, the thermal conductivity and the electrical resistivity were investigated at room temperature by using the Flash Laser technique and the Physical Property Measurement System (PPMS) respectively. The obtained values were highly dependent on densitiy and grain showing the key role of the microstructure and thereby confirming the fact that $$\text {Ag}_{3}\text {Sn}$$ compound is a good candidate for a lead-free die-attach material in power module devices

Role of Morphology on the Large Coercive Behavior in Co 80 Ni 20 Nanowires

ABSTRACT Ferromagnetic metal CoNi-based nano-objects have been synthesized in a polyol media within different elaboration conditions in order to drive their morphology (i.e. enhancing their length-to-diameter ratio ﴾ d / L ﴿, and changing the diameter d ratio over edge T width ﴾ d / L ﴿). Transmission Electron Microscopy (TEM) studies revealed unexpected effects on the Co 80 Ni 20 nano-objects arising from the magnetic field assisted synthesis. This gave us the opportunity to compare this latter to coming from the variation of Ruthenium (III) chloride hydrate nucleating agent concentration. A Co 80 Ni 20 anisotropic particles elaboration was successfully achieved under zero magnetic field assisted synthesis, while an important percentage of isotropic nanoparticles appeared immediately under the application of a small magnetic field (i.e. H > 500 Oe). In the first case we were able to sharply drive both the aspect ratio and head morphology of nanowires ( T and ﴾ d / T ﴿). The good crystallinity and structures symmetry of all our samples have been proved by X-Ray Diffraction (XRD) pattern analysis. Magnetic static properties showed a ferromagnetic standard behavior with a coercive field efficiency which was strongly dependent on shape parameters. The magnetic static behavior was studied within a standard Stoner-Wohlfart model as a function of the observed morphologies. Our observations are fully consistent with a shape anisotropy origin behavior of the enhanced coercivity measured as function of the decreasing ﴾ d / L ﴿ ratio. However, they revealed the presence of contributions to the global effective anisotropy coming from other complex terms then the shape one (i.e. conic head impressiveness, dipolar interactions and magnetocrystalline anisotropy)

Etude du frittage de particules d'argent pour la connexion dans un système électronique de puissance

International audienceCe travail propose l'explication et la mise en place d'une technique innovante pour l'obtention de matériaux en fonction des conditions d'élaboration et de frittage d'un matériau conducteur thermique, possédant des propriétés élastiques et élaboré à basse température et à basse pression (avec une maîtrise de la densité et de la taille de grain). Ce matériau fritté doit être ensuite utilisé pour la connexion par diffusion dans un assemblage électronique de puissance. Le choix du matériau s'est porté sur une poudre d'argent. Trois techniques de frittage ont été explorées pour connecter une poudre d'argent entre deux substrats métallique : une presse chauffante, SPS, et CIC. L'objectif est ici de comparer les propriétés structurales en fonction des paramètres de frittage (température, pression et temps) pour chaque technique de mise en forme explorée. Des observations par MEB ainsi que des mesures par DRX ont permis d'obtenir des informations sur les propriétés microstructurale et morphologique du matériau fritté telles que la densité et la taille des cristallites