Iron ferrite composites fabricated by powder metallurgy processes were studied for electromagnetic and large band microwave absorption applications. The sintering behavior of such composites is not well understood, and these materials have been found to lose their magnetic properties or mechanical properties. Different systems of Fe/NiZn ferrite composites were investigated in order to better under stand the chemical reactions that occur between oxide spinel and iron particles during the fabrication process. Three different systems, two models and one reference material were studied to analyze the chemical reactions in the aforementioned fabrication process. The first model consisted of iron films deposited by Physical Vapor Deposition (PVD) onto an NiZn ferrite substrate. The reference material was made of a mixture of hot pressed iron and ferrite powders. In the second model, a SiO 2 layer was deposited by PVD onto the NiZn ferrite substrate, followed by iron deposition by PVD to study the role of SiO 2 as a diffusion barrier. The materials were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), dilatometry, electron probe microscopy analysis (EPMA) and X ray diffraction (XRD). For the Fe/NiZn ferrite systems, the experimental results showed (i) an oxido reduction reaction above 600 C that produced FeO and Fe x Ni 1-x phases and (ii) a diffusion process of Ni and Zn. The combination of diffusion and oxido reduction reaction induced the total consumption of the initial phase and a considerable decrease in magnetic properties. By adding a silica layer between the iron and ferrite layers, the redox interfacial reaction and iron diffusion were prevented at temperatures up to 800 C.

International audienceWith the growth of miniaturization requirement for application such as transformers, computers, and inductors or motors, soft magnetic composites (SMC) with high flux density, high permeability and low magnetic loss are under development [1-5]. These powders are composed of an iron (or its alloys) core coated with a thin electrically insulating layer. This insulating layer is generally made of a polymer [6,7] or oxides [8-10]. In these cases, as the insulating layer is a nonmagnetic material, it leads to a decrease of the magnetic permeability or of the magnetic induction. One way to avoid or to reduce these properties decrease is to replace the non-magnetic layer by a magnetic electrically insulating layer. NiZn ferrite can appear as a good candidate. Abe have developed a process to deposit NiZn ferrite onto particles by an aqueous route using metal salt [11]. They reported that 1) the ferrite coating occurs thanks to the presence of hydroxyl groups (-OH) on the surface of the substrate and 2) the increase of the –OH group concentration on the surface of particles, after acid treatment, leads to an increase of the substrate reactivity. In this work, Abe et al. apparatus and process have been simplified and optimized and the effect of chemical surface treatments, of Fe particles, followed using X-ray photoelectron spectroscopy (XPS) analysis. This study demonstrates that surface reactivity of Fe particles on NiZn ferrite coating, using electroless process, is strongly link with OH groups chemically linked on Fe particles. Five different acid treatments have been tested onto Fe particles and the evolution of the XPS C1s and O1s followed. Finally the morphology and the adhesion strength of the NiZn ferrite coating, electroless deposited onto acid treated Fe particles, have been analyzed and correlated with the experimental procedures

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Silicon, with a theoretical capacity of 3579 mAh/g, is one of the most promising anode materials as a substitute to graphite for Li-ion batteries. However, the lithiation process involves a volume expansion of 270% causing electrode fracture and the creation of an unstable SEI and fast capacity fading. Si/C composites made using petroleum pitch as carbon precursor are a good alternative to minimize these problems. An original way of synthesis has been developed by dispersing Si nanoparticles (40 nm in diameter, prepared by laser pyrolysis) in a pitch (either molten, or dissolved in a solvent) in order to obtain Si/C composites after thermal treatment. Even though pitch is a disordered carbon, after pyrolysis at 900°C, it exhibits a reasonable electronic conduction, has a reversible capacity of 280 mAh/g and acts as a buffer for the Si volume expansion. Si/C composites prepared using this method with 12 wt.% of Si present a reversible capacity of 600 mAh/g at the rate of C/5 and a coulombic efficiency at the first cycle up to 85%. We will discuss about the influence of the synthesis conditions on the texture of the composites and the resulting electrochemical performances. In order to study deeper the capacity retention, the Si/C composites were tested in full cells using NMC as the cathode. We will show that the control of the Si/C interface allows to obtain composites materials with high energy density up to 290 Wh/kg with a nice capacity retention.

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