K. Zehani

High Magnetic Moment of FeCo Nanoparticles Produced in Polyol Medium

High magnetization Fe55Co45 alloy nanoparticles have been successfully synthesized by the polyol reduction process followed by annealing under argon. The diethylene glycol was used as solvent and reducing agent simultaneously in this process. All the synthesized samples of FeCo nanoparticles were annealed at 873 K for different times. The alloy formation processes, the evolution of the microstructure, and the magnetic properties have been investigated before and after samples annealing. The X-ray diffraction (XRD) of the synthesized product before annealing shows that a cobalt ferrite is spinel structure of crystallite size of about 10 nm. XRD analysis of the samples annealed at 873 K for different times also shows that of the FeCo alloy has been obtained by reducing the cobalt ferrite. It has been confirmed that the formation of a body-centered cubic (bcc) single-phase structure where the wt% increases with annealing times leading to a pure phase after annealing during 4 h. These results are confirmed by transmission electron microscopy study. The saturation magnetization of the Fe-Co alloys increases with annealing time, indicating an increasing homogeneity in composition and the single bcc FeCo phase formation. The highest saturation magnetization of 235 Am2 · kg-1 with a low coercivity of 7.5 mT was obtained for the Fe55Co45 nanoparticles annealed during 4 h. The anisotropy constant K has been extracted and we have shown that increases with the annealing time.

Spark Plasma Sintering Co-Sintered Monolithic Transformers for Power Electronics

Power integration is a key issue to reduce the volume and weight of electronic devices in power applications. However, transformers produced using classical planar assembly are limited in design. Spark plasma sintering (SPS) is a relatively new technology to produce multi-material compact systems with higher density using lower sintering temperature. The purpose of this paper is to manufacture cubic-centimeter-sized transformers for the high-frequency application using SPS. The prototype presented in this paper is composed of two spiral copper coils separated by an insulated layer and encapsulated in ferrite powder. The assembly is then co-sintered by SPS. The magnetic material used for the transformers is a nickel-zinc-based ferrite, with copper substitution to allow sintering at low temperatures. The low conductivity of this mixed ferrite ensures operation in the frequency range of 1-20 MHz of our final system. Computed tomography scanning has been used to optimize the design of the co-sintered structures. The effects of the composition of the ferrite and the sintering temperature on the transformation ratio are discussed. It is shown that a ferrite with very low conductivity is required to ensure galvanic insulation, as there is a direct contact between the copper spirals and the magnetic parts.

Grain refinement and Lattice Imperfections in Commercial Aluminum Alloy Processed by Severe Plastic Deformation

In the present work, investigations on the microstructure of an aluminum alloy that had been subjected to severe plastic deformation (SPD) by equal channel angular pressing (ECAP), filing and ball milling, were carried out using X-ray diffraction and scanning electron microscopy. SPD leads to lattice distortions, increased dislocation density and an intensive refinement of the microstructure. The refinement and lattice imperfections of the material are greatly affected by the deformation modes and loading performance occurring during SPD. During the milling, the dislocation annihilation increases at higher strains thereby resulting in a smaller crystallite size. After ECAP, the material manifests a strong shear texture and anisotropy of the deformation behavior. Strain anisotropy is less pronounced in filed and ball milled powder particles.