Characterization of the interface reaction zone between iron and NiZn ferrite in a composite material - Study of a silica layer as a diffusion barrier

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 understand 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 SiO2 layer was deposited by PVD onto the NiZn-ferrite substrate, followed by iron deposition by PVD to study the role of SiO2 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 FexNi1-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.