Iron-based soft magnetic composites with Al2O3 insulation coating produced using sol–gel method

The alumina sol was obtained using Al(C 3 H 7 O) 3 as a precursor. Pure Fe particles were coated with Al 2 O 3 using a sol–gel method and subsequently calcinated at 1100   °C. The resulting soft magnetic composites were investigated while varying the treating temperature. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses revealed that the surface of the Fe powder particles comprised a thin α-Al 2 O 3 insulating layer uniformly distributed after calcination at 1100   °C for 1   h. The permeability increased with the treating temperature, reaching a stable value of 93 from 675   °C to 725   °C. The hysteresis losses decreased, whereas the eddy losses increased with the treating temperature in the range of 650–725   °C. The core losses ( P cv ) decreased (lowest value: 10.2   kW/m 3 ) up to 675   °C and increased thereafter. The permeability of the material annealed at 675   °C remained stable with changes in the frequency, whereas P cv remained low over the 5–100   kHz range. A noticeable insulation layer separating the different Fe particles was observed by SEM for the samples annealed below 700   °C.