High Strength of Powder Magnetic Cores Made from Powders Coated with Low-Melting-Point Glass

Powder magnetic cores have higher resistivity and lower iron loss in the high-frequency range than electrical steel sheets. However, there is a trade-off between increasing the resistivity and the strength of powder magnetic cores. In this study, we aimed to achieve high strength and resistivity in powder magnetic cores by using low-melting-point glass to the insulation films. We selected glasses with good wettability and insulating properties through a simple wettability test and fabricated powder magnetic cores using Fe-Si spherical powder. The annealing temperature ranged from 400 to 800°C, and we examined the relationship between strength and resistivity. As the annealing temperature increased, strength increased, but resistivity decreased. Electrical resistance measurements using a microprobe in the SEM showed that the glass films with high resistance (over MΩ) formed on the Fe-Si powder, but it was observed that short circuits occurred between the Fe-Si powders. It was presumed that the Fe-Si powders contacted each other before the glass melted and formed the insulation films, resulting in decreased resistivity. The glass films effectively improved the strength of powder magnetic cores. However, it was found that the glass film must be multilayered with another insulating layer to prevent resistivity degradation.