Electromagnetic wave absorbing behavior of nano-Fe₁Co_0.8Ni₁@C single-layer core-shell and nano-Fe₁Co_0.8Ni₁@SiO₂ @C double-layer core-shell composite particles

To effectively explore the influence of nano core-shell structure on the absorption properties of Fe₁Co_0.8Ni₁@X composite particles, three groups of Fe₁Co_0.8Ni₁@C single-layer core-shell structures with varying C coating layer thicknesses and Fe₁Co_0.8Ni₁@SiO₂@C double-layer core-shell structure magnetic composite particles were prepared via chemical liquid phase deposition and high-temperature carbonization, and corresponding electromagnetic absorption properties were investigated. The study revealed that the prepared nano core-shell Fe₁Co_0.8Ni₁@X composite particles exhibited a near-spherical shape with particle size of approximately 100 nanometers. The core of the FeCoNi alloy particles was FCC polycrystalline with C coating layer thicknesses ranging from 2 to 3 nm, 10–20 nm, to 30–50 nm, while Fe₁Co_0.8Ni₁@SiO₂@C composite particles with SiO₂ coating layer thicknesses of 5–10 nm and C coating layer thicknesses of 2–3 nm. As the C coating layer thickness increased, the specific saturation magnetization intensity showed a decreasing trend while the coercivity exhibited an increasing trend, all composite particles presented soft magnetic properties. With the increase of the C coating layer thickness, the real and imaginary parts of the dielectric constant of composite particles continued to rise, while real part of the permeability showed minimal change and imaginary part of permeability exhibited significant variation. Polarization relaxation and electric conduction were observed as C layer thickness increased and magnetic loss primarily including eddy current loss and resonance loss. Additionally, the peak of the dielectric loss of the composite particles demonstrated a shift towards higher frequency with increasing C coating layer thickness, and the dielectric loss of the Fe₁Co_0.8Ni₁@SiO₂@C double-layer coating structure sample was lower than that of the Fe₁Co_0.8Ni₁@C. The attenuation constant of Fe₁Co_0.8Ni₁@C composite particles increased with C coating layer thickness and rising continuously with the electromagnetic wave frequency. The double-layer coating structure exhibited a lower attenuation constant than single-layer coating structure and FeCoNi alloy particles, which presented the best impedance matching. Compared with the Fe₁Co_0.8Ni₁ alloy particles, the electromagnetic wave absorption performance of the Fe₁Co_0.8Ni₁@C composite particles was enhanced in both high and low frequency bands, and with the increase of the C coating layer thickness the main absorption frequency band of the samples with the same thickness shifted towards lower frequency. Furthermore, the thickness of sample decreased from 1.5mm to 2.0 mm to 1–1.5 mm as the C coating layer thickness increased. Maximum reflection loss |RLmax| of 61.76 dB and effectively absorbed bandwidth of 4.64 GHz was obtained for Fe₁Co_0.8Ni₁@C composite particle.