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

• The Fe particle surface adhered well to a α-Al2O3 forming a dense and uniform layer upon calcination of the gel at 1100 °C.
• Optimal µ and Pcv values were obtained for SMC treated at 675 °C with the Fe particles being separated from each other.
• Very stable permeability was obtained in the frequency range of 5–100 kHz.

The alumina sol was obtained using Al(C3H7O)3 as a precursor. Pure Fe particles were coated with Al2O3 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-Al2O3 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 (Pcv) decreased (lowest value: 10.2 kW/m3) up to 675 °C and increased thereafter. The permeability of the material annealed at 675 °C remained stable with changes in the frequency, whereas Pcv 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.

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