Microwave Absorption Mechanism Of Magnetic Metal Powder/Binder Composite

Magnetic metal particles are widely used in the field of microwave absorption because of their high Curie temperature, high saturation magnetization, low coercivity and high permeability in the GHz frequency range. Fe, Co, Ni and their alloys are the main metal as the microwave absorption materials. At present, for the microwave absorption mechanism of magnetic metal powder, the mainstream view is the electromagnetic loss in the metal materials. But, the theory of electromagnetic loss can not explain the reflection loss curves reasonably. In this work, the magnetic metal powder/binder composite with good microwave absorption properties are prepared using carbonyl iron and nickel carbonyl particles and the microwave absorption mechanism is discussed. The main results are shown as follows:(1) The surface of carbonyl iron particles were modified by using titanate coupling agent NXT-312. The dispersion property of the particles is improved. The dielectric constant of the composite decreases and the impedance matching is improved, so the microwave absorption properties of carbonyl iron/epoxy composite are enhanced.(2) The surface of nickel carbonyl particles were oxidized by hydrogen peroxide, and a nano-shell of NiO was coated on the surface of nickel particles. There is no electrical contact between the particles because of the NiO nano-shell, so the dielectric constant of the composite was greatly reduced. Reflection losses (RL) exceeding -20 dB in the frequency range of 4.0-12.6 GHz were obtained for the absorber thicknesses of 1.8-4.3 mm, while an optimal RL of -53.5 dB was achieved at 7.9 GHz for the thickness of 2.5 mm.(3) For the magnetic metal powder/binder composite, the peak position, the number of the peak and the intensity of the peak of reflection loss were explained by using the interface reflection model or the quarter-wavelength (λ/4) matching model. It is found that this model can be successfully applied to understand and predict not only the peak frequency but also the number of the peaks. The energy reflected from the interface and the energy of the electromagnetic loss at the peak frequency of reflection loss were calculated by the S parameters. And the change of the intensity of reflection loss can be explained by the interface reflection model.(4) The microwave absorption mechanism of magnetic metal powder/binder composite is the destructive interference of the reflection wave from the interface and the electromagnetic loss in the composite.