Facile synthesis of hollow porous cobalt spheres and their enhanced electromagnetic properties

We report a template-free approach to prepare hollow porous cobalt spheres by combining a facile solvent-thermal route with a chemical reduction process. The synthesized cobalt microspheres with diameter of approximately 2-4 mu m have a shell thickness of about 150 nm. The hollow Co microsphere is built from particle-like ligaments with diameter of ca. 80-150 nm and there are a lot of pores with size of several tens of nm on the shell. The hollow porous cobalt spheres exhibited saturation magnetization (M-s) of 163.0 emu g(-1) and large coercivity (H-cj) of 307.9 Oe. The epoxy resin composites with 30 wt% and 60 wt% hollow porous cobalt sample showed efficient EM wave absorption characteristics (RL < -20 dB) in the ranges of 11.3-18.0 GHz and 4.0-12.0 GHz over absorber thicknesses of 1.4-2.0 mm and 1.4-4.0 mm, respectively. Our study demonstrates the potential application of hollow porous ferromagnetic materials as a more efficient electromagnetic wave absorber.