The Effect of Core-Shell Structure on Microwave Absorption Properties of Graphite-Coated Magnetic Nanocapsules

Interface polarization is one important factor in producing a good electromagnetic impedance match and enhancing the microwave-absorption properties of core/shell nanocomposites. However, the effect of the core-shell nanocapsules having insulating shells, and the polarization effect at interfaces of graphite-coated magnetic metallic nanoparticles, on the microwave absorption properties, are not clear. In this work, the microwave-absorption properties of magnetic nanocapsules with alpha-Fe/gamma-Fe(C)/Fe3C as cores and graphite as shells have been investigated in the frequency range 2-18GHz. Partial magnetic cores were removed by a similar to 19wt.% HCl solution from the as-prepared nanocapsules (A-nanocapsules), but the carbon shells were kept constant. Transmission electron microscopy confirms that hollow carbon nanocages are almost the same as the as-prepared shells, except for slight change in shape due to removing the cores. As a result, the complex permeability drops slightly, while the complex permittivity has an obvious increase, which can be ascribed to percolation effects due to the hollow carbon nanocages, rather than the interface polarization originating from the core/shell interfaces and interfaces between the core components of alpha-Fe, gamma-Fe(C) and Fe3C nanocrystals. The optimal reflection loss (RL) value of the A-nanocapsules reaches -27.6 dB at 16.2 GHz for a thickness of 2 mm. This study clarifies that the interface polarization effect in the A-nanocapsules is negligible in enhancing the complex permittivity and the synergetic effect of the magnetic loss of cores, and the dielectric loss of graphite shells is the dominant mechanism in attenuating microwaves.