Microwave characterization and modelization of magnetic granular materials

Abstract

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 (ε, μ) 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 (μ′, μ″) 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.

Controlled synthesis and shape-dependent electromagnetic wave absorption characteristics of porous Fe3O4 sub-micro particles

“…The measured results are shown in Figure 10. [33,48]. The Fe 3 O 4 composites exhibit not only high dielectric loss, but also relatively large magnetic loss, suggesting that they have excellent EM absorption properties.…”

“…On the basis of the factors above, the Fe 3 O 4 particles exhibit high dielectric loss. [33,35,48]. For example, Olmedo et al [48] investigated the complex permittivity and complex permeability of ferromagnetic particles by the extending Bruggeman method, and found that nonspherical particles showed higher ″ and ″ compared with spherical magnetic particles.…”

High frequency complex permeability of flake-shaped Fe74Cr2Mo2Sn2P10Si4B4C2 particle composite material

“…(1)) or the extended Bruggeman effective medium theory [4,9], we can not calculate the effective permeability of particle oriented-distributed composites. …”

Preparation and microwave absorption properties of electroless Co–P-coated nickel hollow spheres

“…Natural resonance frequency of NHSs is different with that of Ni nano-particle (5.2 GHz) [20] because of the presence of phosphorus. The interaction of NHSs and Co-P coatings causes that natural resonance frequency of samples lies between that of Ni nanoparticles and Co (hcp) particles (about 6.5 GHz) [23][24][25]. For the Co-P coatings and NHSs, it can be estimated that naturalresonance frequency will be around several tens of megahertz.…”