Journal of the Japan Society of Powder and Powder Metallurgy

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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.

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A uniform carbon coating was applied to stainless steel (SUS316L) particles by liquid-phase carbon coating using soluble carbon material. The resulting carbon-coated stainless steel particles have improved powder flowability, especially angle of repose and spatula angle, comparing with uncoated stainless steel particles, because of the lubricating properties of the carbon layer. Carr’s flowability index was improved from 80.5 to 91. Furthermore, when sintered or melted objects were prepared from carbon-coated stainless steel particles with heating over 1250 K, the carbon coat layer reduced the oxide layer (especially chromium oxide) of the stainless steel. By heating over 1250 K, carbon coat layer removed oxygen from surface of stainless steel and also itself disappeared, therefore carbon coat layer improved purity of melted objects without changes of components and structures. The liquid-phase carbon coating improved the flowability of the stainless steel particles while not adversely affecting their sintering and melting properties.

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