Granular composites containing "micro-onions," permeability, and permittivity calculated for application to microwave absorbers

The formula for the effective permeability <mu> (or permittivity <epsilon>) is derived, based on the Maxwell Garnett approximation, for the granular composites having "micro-onions" (micron-meter-sized, multifold core-shell structured particles) dispersed in matrices. We calculated <mu> and <epsilon> up to 3 GHz for the composites in which micro-onions of threefold [i.e., air/(silica+air)/NiZn-ferrite or water/(silica+water)/NiZn-ferrite] structure are dispersed in a Fe/silicon-rubber matrix. Introducing the micro-onions into the Fe/silicon-rubber matrix improves the wave absorber characteristics, increasing absorption and decreasing reflection, at 0.5-3 GHz. Confining the magnetic substance to the outermost shell layers increases the natural ferromagnetic resonance frequency, thus enhancing permeability at high frequencies. It also facilitates fabrication of lightweight electromagnetic wave absorbers, and suppresses eddy current loss in case the magnetic substance is a metal. (C) 2002 American Institute of Physics.