Formation of Fayalite for Insulating Layer of Powder Magnetic Core

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.