A simple method to obtain high magnetic permeability (mu(r)), high dielectric permittivity (epsilon(r)), and low loss polymer composites with core/shell iron/metal oxide particles is developed. Magnetic iron (Fe-0) particles with protective SiO2 shells are synthesized in aqueous environment at room temperature. The shells provide electrical insulting layers which decrease energy loss and also prevent the possibility of mu(r) decrease due to Fe-0 oxidation. Furthermore, Fe-0 particles could also be coated with TiO2 shells which allow epsilon(r) to be tuned. The epsilon(r) and mu(r) of the resulting polydimethylsiloxane composites are optimized for core-shell particle doping. The value of epsilon(r) is up to 9.8 and most importantly, the mu is close to 2 at 1 GHz with 43 wt% doping of core-shell particles. The dielectric loss of the composites is less than 0.02.
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