Effects of particle size on electromagnetic properties of spherical carbonyl iron

Classification experiments using the spherical carbonyl iron (SCI) with particle size ranged from 1 to 7 µm were conducted by an air classifier. The electromagnetic properties of the SCI with different particle size were investigated in the frequency range of 2–18 GHz. The effects of particle size on the microwave absorption properties of the SCI were discussed on the results of scanning electronic microscopy, vibrating sample magnetometer, and frequency dependence of complex permittivity and permeability. The results show that the larger particle size leads to higher permittivity and lower permeability. The dielectric loss is attributed to the Debye dipolar polarization, while the magnetic loss arises from natural resonance and blocking resonance between superparamagnetism and ferromagnetism. The SCI with average particle size of 1.49 µm exhibits the optimal microwave absorption performance. A minimum reflection loss (RL) of − 50.8 dB is achieved at 15.6 GHz with a thickness of 1.27 mm, along with the corresponding bandwidth of RL less than − 10 dB (90% absorption) covering 7.04 GHz. Notably, the SCI with smaller particle size (about 1–3 µm) show superior microwave absorption properties at higher frequency range and thinner thickness than the corresponding larger ones (about 5 µm), which is mainly attributed to the improved impedance matching.