Thin Magnetic Coating For Low-frequency Broadband Microwave Absorption

In this thesis, I fabricated the absorbers for low-frequency microwave application. I oriented the planar anisotropy carbonyl-iron(PACI) in magnetic field, and both the real part and the imaginary part of the permeability were improved greatly.Planar anisotropy carbonyl-iron(PACI) particles were prepared from 3μm diame-ter sphere-shaped carbonyl-iron materials by high-energy ball milling technique. And the oriented planar anisotropy carbonyl-iron(OPACI) composite was fabricated using rotational orientation method.The OPACI composite possessed much higher real and imaginary permeability and relatively lower real and imaginary permittivity than other absorbers in 1-18 GHz frequency range. The μ’ of OPACI composite is 10.2 at 1 GHz which is much higher than that of PACI composite(6.8). And the μ" of PACI composite increases from 2.8 to 3.4 in the range of 1-3.4 GHz, and then decreases in the higher frequency range. For the OPACI composite,μ"≈6 is almost the same from 1 GHz to 3.0 GHz, and then decreases in the range of 3.0-18 GHz.When the thickness of PACI coating is 2.9 mm, the RL can reach a minimum of-40 dB at 1.9 GHz, and the bandwidth(RL<-10 dB) is 1.1 GHz from 1.4 to 2.5 GHz. For OPACI composite, the minimum RL can be as low as -44 dB at 1.8 GHz with the thickness of 2.3 mm,-7.3 dB at 1 GHz,-20 dB at 2 GHz,-5.4 dB at 8 GHz, and the bandwidth can reach 1.9 GHz for the good performance (RL<-10 dB) in the frequency ranges of 1.2-3.1 GHz. Obviously, the composites can be applied for low-frequency broadband microwave absorption.For some special low-frequency applications, absorbers should be thinner than 1 mm. The OPACI composite also has excellent absorbing properties:-2.5 dB at 1 GHz, -5.8 dB at 2 GHz,-6.5 dB at 8 GHz and minimum reflection loss value -11.1 dB at 3.8 GHz for only 1 mm. For comparison, when the thickness is also 1 mm, the PACI composite has the absorbing properties: -1.0 dB at 1 GHz,-2.7 dB at 2 GHz,-7.9 dB at 8 GHz and minimum reflection loss value -9.3 dB at 5.6 GHz.The susceptibility versus frequency curves have been fitted quite well using Kittel equation and Debye equation.The OPACI composite can possess a remarkable reflection loss, indicating a promis-ing absorber for the low-frequency broadband application.