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Nedkov, Ivan
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article
Microwave Properties of Polymer Composites Containing Combinations of Micro- and Nano-Sized Magnetic Fillers
Abstract
<jats:p>We investigated the microwave absorbing properties of composite bulk samples with nanostructured and micron-sized fillers. As magnetic fillers we used magnetite powder (Fe<jats:sub>3</jats:sub>O<jats:sub>4</jats:sub> with low magnetocrystalline anisotropy) and strontium hexaferrite (SrFe<jats:sub>12</jats:sub>O<jats:sub>19</jats:sub>with high magnetocrystalline anisotropy). The dielectric matrix consisted of silicone rubber. The average particle size was 30 nm for the magnetite powder and 6 <jats:italic>μ</jats:italic>m for the strontium hexaferrite powder. The micron-sized SrFe<jats:sub>12</jats:sub>O<jats:sub>19</jats:sub> powder was prepared using a solid-statereaction. We investigated the influence of the filler concentration and the filler ratio (Fe<jats:sub>3</jats:sub>O<jats:sub>4</jats:sub>/SrFe<jats:sub>12</jats:sub>O<jats:sub>19</jats:sub>) in the polymer matrix on the microwave absorption in a large frequency range (1 ÷ 18 GHz). The results obtained showed that the highlyanisotropic particles become centers of clusterification and the small magnetite particles form magnetic balls with different diameter depending on the concentration. The effect of adding micron-sized SrFe<jats:sub>12</jats:sub>O<jats:sub>19</jats:sub> to the nanosized Fe<jats:sub>3</jats:sub>O<jats:sub>4</jats:sub> filler in compositesabsorbing structures has to do with the ferromagnetic resonance (FMR) shifting to the higher frequencies due to the changes in the ferrite filler's properties induced by the presence of a magnetic material with high magnetocrystalline anisotropy. The two-component filler possesses new valuesof the saturation magnetization and of the anisotropy constant, differing from those of both SrFe<jats:sub>12</jats:sub>O<jats:sub>19</jats:sub> and Fe<jats:sub>3</jats:sub>O<jats:sub>4</jats:sub>, which leads to a rise in the effective anisotropy field. The results demonstrate the possibility to vary the composite's absorptioncharacteristics in a controlled manner by way of introducing a second magnetic material.</jats:p>