Large scale synthesis of superparamagnetic face-centered Cubic Co/C nanocapsules by a facile hydrothermal method and their microwave absorbing properties

A large scale direct hydrothermal method has been used to prepare face-centered cubic (FCC) Co/C nanocapsules with a core of FCC-Co nanoparticles and a shell of amorphous C. The size distribution is 5-20 nm. The TB of FCC-Co/C nanocapsules is determined as 85 K, indicating that FCC-Co/C nanocapsules are superparamagnetic at 300 K. For FCC-Co/C nanocapsules-paraffin composite, an optimal reflection losbs (RL) of-33.4 dB is observed at 9.6 GHz for the 2.6 mm thick layer. RL values exceeding-20 dB in the 7.4-15 GHz range are obtained by choosing an appropriate absorption-layer thickness between 1.7 and 3.3 mm. The good microwave absorbing properties of FCC-Co/C nanocapsules is considered to result from the excellent synergetic effect of the multi-dielectric relaxation loss and the multi-magnetic resonance loss. Quantitative calculation demonstrated that not only RL peak position but also the number of the peaks are determined by the quarter-wavelength cancellation model.