Enhanced magnetic properties of iron-based soft magnetic composites with phosphate-polyimide insulating layer

Abstract

Iron-based soft magnetic composites with phosphate-polyimide (PI) insulating layer were successfully fabricated by phosphating and imidization. The effect of phosphoric acid content on microstructure and compressibility of iron powder, and magnetic properties of the cores were investigated. The results demonstrate that the inner inorganic coated layer mainly consists of iron phosphate and ferrous phosphate. With increasing phosphoric acid content, the thickness of phosphate increases gradually, resulting in the reduction of density, maximum permeability and magnetic induction intensity. When the content of phosphoric acid reaches above 0.3 wt%, the bonding strength between the PI coating and iron particles is reduced, leading to the breakage of insulating layer and the augment of core loss. Owing to the excellent thermal stability of PI, magnetic powder cores can be cured at the temperature up to 400 °C. The sample coated with 0.1 wt% phosphoric acid and 1.5 wt% PI exhibits excellent magnetic properties. The maximum permeability reaches 201.4, and the core loss (186.6 mW/g) is less than 15% of monolayer phosphate coated sample (measured at 50 mT and 100 kHz). Additionally, the corrosion resistance of the sample is significantly improved.