Formation of Fayalite for Insulating Layer of Powder Magnetic Core

Abstract

For powder magnetic cores, a spinel ferrite insulating layer offers the advantages of high magnetic flux density and low iron loss. However, FeO is generated in the insulting layer after annealing at 873 K, which causes a decrease in electrical resistivity. To overcome this problem, we focused on fayalite (Fe₂SiO₄), which exhibits a high electrical resistivity and is stable even at temperatures above 1273 K. Fe₂SiO₄ can be formed by a reaction between FeO and SiO₂. Therefore, if Fe powder particles mixed with both spinel ferrite and SiO₂ are heated at a temperature that is sufficiently high to form FeO, Fe₂SiO₄ is expected to be produced, and the resulting particles are likely to exhibit high electrical and thermal resistance. To examine this possibility, a mixture of Fe₃O₄ of spinel-type ferrite, SiO₂, and Fe powder was heated at 1023 K, and the structure and composition of the resulting material were investigated. Analyses using X-ray diffraction and Fourier-transform infrared spectroscopy confirmed the formation of Fe₂SiO₄, which showed a high electrical resistivity. These results indicate that Fe₂SiO₄ can be used as an insulating layer for powder magnetic cores.