MICROWAVE CHARACTERIZATION AND MODELIZATION OF MAGNETIC GRANULAR-MATERIALS

The use of dispersion techniques for development of microwave absorbers is a convenient way of realization of coatings with good attenuation characteristics in the 1- to 20-GHz range. Particularly, elastomer mixing is a very good technique for dispersion and individualization of each grain up to 60% volume. A first generation of materials, developed with ferrite powders, has been extensively studied. It was found possible to predict properties of composites with classical mixture laws using intrinsic properties of filler and matrix. When using conductive ferromagnetic fillers, the inability to measure intrinsic magnetic properties does not permit prediction of behavior of composites. Therefore, a method of inverse calculation has been developed which, from (epsilon, mu) parameters measured on composites, allows inferrence of intrinsic properties of each filler (iron, cobalt, permalloy,...). It is shown that this method, employed for several amounts of filler, gives significant accuracy on (mu', mu'') with intermediate concentrations (typically 25% vol. to 45% vol. with spheres). For permittivity, this method is satisfying in the case of nonpercolating systems. Also studied was the effect of morphology on properties of granular materials, particularly in the case of sphericity and grain size of iron and cobalt particles. Permittivity level is directly correlated to sphericity, and magnetic relaxation frequency to grain size. It is also shown that the chemical nature, particularly for ex-carbonyl origin of iron, does not play a significant role in composite material properties.