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In recent years, the electromagnetic radiation with the frequency in range
of 1-100 GHz has great application in telecommunication, medical
treatment, and military. In company with that electromagnetic radiation also
brings problems such as: electromagnetic interference, health diseases.
Therefore, developing absorbing materials, which has able to absorb
electromagnetic radiation, have paid much attention in GHz frequency.
Microwave absorption materials (MAM) helps to prevent electromagnetic
interference issue, reduce the cross-section reflectivity, and ensure the
security of electronic systems. Radar absorption materials (RAM) worked in
frequency range of 8-12 GHz is widely used in military systems for stealth
technology. Generally, the study on electromagnetic absorption material
mainly focuses on three ways: (1) preventing reflectivity signal, (2)
enhancing the absorbability of material, and (3) extending frequency range.
The increase of loss tangent and absorption efficiency can be obtained if
absorbing material can observe both electric and magnetic energy.
Moreover, nanotechnology provides the other ways to fabricate absorption
material in nanoscale for shielding. MAM with nano-size displays the
improvement of absorption ability in comparison with micro-size.
Nanotechnology also helps to make the light weight and thin absorbed layer.
The microwave absorption ability of material can be determined by relative
permeability (r), permittivity (r), and impedance matching between
environment and material. The reflection loss (RL) is used to determine the
quality of MAM via the formula: RL = 20log|(Z - Z0)/(Z + Z0)|, where Z =
Z0(r/r)1/2 is the impedance of material, Z0 is the impedance of air. The
maximum reflection loss can be obtained via two mechanisms: (i) the
impedance of material equals to impedance of air, |Z| = Z0, which is so called
Z matching; (ii) the thickness of absorbing layer satisfies the phase matching
or quarter-wavelength condition (d = (2n+1)c/[4f(|r||r|)1/2], n = 0, 1, 2, ).
Z matching normally achieves by balancing the permeability and
permittivity values, r = r. It can be obtained by fabricating a composite of
dielectric and ferrite materials. Recently, there are a lot of publications on
MAM based on the nanocomposite of magnetic and dielectric materials in
which the RL can be obtained below -50 dB. The RL of nanocomposite is
much higher than that of traditional materials such as carbon black-C and
carbonyl-Fe.
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