The influence of interfaces on the dielectric properties of MnZn-based hybrid polymer composites

In the present paper we report on the specific features of the dielectric properties of a MnZn ferrite/Al based composite. Previously, it has been shown that high-frequency magnetic losses in such hybrid composites (HCs) can be enhanced due to the formation of a core-shell-like structure of a composite, in which ferrite particles are immersed into a conducting medium formed by a continuous network of conducting particles that spans throughout the polymer matrix. Simultaneously, one can vary the dielectric properties of HCs by changing the type and the concentration of conducting particles. Dielectric constant and ac conductivity measurements of MnZn-based composites have been made over the frequency range of 10 Hz-100 kHz in temperature interval from -30 to 100 degrees C and at ambient temperature up to 3 GHz. The results obtained show that addition of aluminum into the MnZn ferrite/polyurethane composite leads to a decrease in the dc conductivity due to the insulating barrier of Al-Al2O3. On the other hand, ac conductivity of MnZn ferrite and aluminum/polyurethane composite is greater than that of a two-component system due to the occurrence of Maxwell-Wagner-Sillars relaxation processes with a rather low value of activation energy and significantly higher relaxation time. Thus, on one side, aluminum provides sufficient conductivity of the shell and thus leads to the enhancement of effective permeability, but, on other side, it does not significantly contribute to the total conductivity (effective permittivity) of HCs. The analysis of the efficiency of HCs with different types of conducting filler as electromagnetic wave absorbers (EWAs) has shown that the matching frequency of EWAs can be effectively controlled in the radio-frequency range through an appropriate choice of the type of conducting filler.