Electromagnetic properties of a composite made of metal particles dispersed in resin

The frequency dependences of the relative complex permeability μ_r ^* and relative complex permittivity ε_r ^* for the composite made of metal particles (aluminum particles) dispersed in polystyrene resin were measured in the frequency range from 1 MHz to 40 GHz. The volume mixture ratio and particle size of aluminum were varied and the dependences of the volume mixture ratio and particle size on the frequency dependences of μ_r ^* and ε_r ^* were investigated. In addition, theoretical values of the real and imaginary part of μ_r ^*, μ_r' and μ_r", for the composite made of metal particles dispersed in polystyrene resin were calculated using Maxwell's equations. The measured value of the real part of ε_r ^* was independent of frequency and both real and imaginary part of ε_r ^* increased with increasing the volume mixture ratio of aluminum particle. The measured value of μ_r' was found to decrease with increasing frequency in the low frequency range and became constant in the high frequency range. Meanwhile, the measured value of μ_r" increased with frequency, had a maximum and decreased with increasing frequency. Moreover, at high frequencies where the skin depth is much smaller than the radius of aluminum, μ_r' was found to depend only on the volume mixture ratio of aluminum, whereas μ_r" was determined by both the volume mixture ratio and the aluminum particle size. These results almost agreed with the calculated values. Thus, the frequency dependences of μ_r' and μ_r" were found to be predicted by the theoretical calculation and to be controlled by the volume mixture ratio and particle size of aluminum. For the application of this composite to an electromagnetic wave absorber, the return loss of this composite was calculated from the measured values of μ_r ^* and ε_r ^*. The return loss of the composite made of aluminum particles dispersed in polystyrene resin was less than -20 dB (absorption of 99% of an electromagnetic wave power) in the frequency range from 1 to 40 GHz when a suitable volume mixture ratio, particle size, and sample thickness were selected. Therefore, this composite can be used as an electromagnetic wave absorber in the gigahertz range. Furthermore, the absorption characteristics of such composite can be tailored based on the ability to control the values of μ_r' and μ_r" independently by adjusting the volume mixture ratio and aluminum particle size.