Dielectric Properties Of Discontinuous Metal Films

Electromagnetic wave absorbing materials, namely, microwave absorber is an important material in the weapon equipment development, The reasonable use of this kind of absorber can greatly reduce the radar cross section (RCS) of ships and planes, so as to improve the hidden and survival rate during the war. At present, our country’s research about electromagnetic wave absorbing materials is mainly focused on the magnetic powder absorber. After years of research, the performance of the magnetic powder absorber has already reached the theoretical limit, it is difficult to obtain a breakthrough. Also, the micron magnetic powder absorber has some shortcomings. First, the large surface density of the micron magnetic powder absorber. Second, because of the influence of demagnetizing effect, the real part and imaginary part of the microwave permeability are not so large. So the absorption rate of coating based on these materials is not high enough. Therefore, a new type of light, thin layer of absorbing material is necessary.The magnetic permeability of magnetic metal thin films can be very high, and it is expected to realize the light thin layer absorption. But the continuous metal film has high reflection on the incident electromagnetic wave, result in the difficulty for the electromagnetic wave to enter the interior of the material. In this paper, some surface chemical processes are carried out, which make the metal film form a periodic interval between metal and air.In this paper, three kinds of methods are used to calculate the permittivity of discontinuous metal films.1, calculating the permittivity of discontinuous metal by the Bruggeman effective medium theory.2, using finite element simulation software HFSS to simulate the free space method and calculate the permittivity of discontinuous metal films by Floquet ports and periodic boundary condition.3, using finite element simulation software HFSS to calculate the permittivity of discontinuous metal films by resonant cavity perturbation method.The calculation results indicate that the effective permittivity of discontinuous films can be analytically calculated using Bruggeman effective medium theory when the condition of long wave approximation is met, which gives a convenient routine to get the permittivity of discontinuous films. The metal permittivity based on Drude equation has been renormalized to be used in Bruggeman theory, and the renormalized metal permittivity effectively reflects the influence of eddy effect and enhanced effect of local electric field, so that the metal unit can be considered as a homogenously polarized medium in the discontinuous films. Due to eddy effect, the size of metal unit would remarkably influence the effective permittivity of discontinuous films, which means that the larger unit size will results in the higher renormalized metal permittivity. The effective permittivity could be tuned by controlling the metal unit size in the discontinuous metal films. Additionally, another key result is that the Bruggeman theory is effective only when the metal unit concentration is below the percolation threshold, because the effective medium theory is based on the assumption that the fields are the same in all metal units, which is the base to renormalize metal permittivity. This assumption does not hold when a metal concentration approaches the percolation threshold because the multipole scattering will take effect. All the results concluded in the paper give a rule to use EMT to handle the discontinuous metal films, and are very valuable in studying their dielectric properties.