Enhanced microwave absorption of Fe nanoflakes after coating with SiO2 nanoshell

Fe nanoflakes were prepared by the ball-milling technique, and then were coated with 20 nm-thick SiO2 to prepare Fe/SiO2 core-shell nanoflakes. Compared with the uncoated Fe nanoflakes, the permittivity of Fe/SiO2 nanoflakes decreases dramatically, while the permeability decreases slightly. Consequently, reflection losses exceeding -20 dB of Fe/SiO2 nanoflakes are obtained in the frequency range of 3.8-7.3 GHz for absorber thicknesses of 2.2-3.6 mm, while the reflection loss of uncoated Fe nanoflakes almost cannot reach -10 dB in the same thickness range. The enhanced microwave absorption of Fe/SiO2 nanoflakes can be attributed to the combination of the proper electromagnetic impedance match due to the decrease of permittivity and large magnetic loss due to strong and broadband natural resonance. The key to the combination is the coexistence of the nanoshell microstructure and the nanoflake morphology.