Effects of microstructure and filling ratio on electromagnetic properties of Co microspheres

• An EABW up to 6.1 GHz was obtained in a thin coating using Co particle as fillers.
• The electromagnetic properties can be tailored via annealing and filling ratio.
• High filling ratio is favorable to excellent EMA performance.

Cobalt microspheres with diameters of 1.5–3.5 µm were synthesized by a liquid phase reduction method. The effects of hydrogen annealing on microstructure evolution and electromagnetic properties of Co microspheres were investigated. The influence of filling ratio on the electromagnetic properties of specimens containing Co microspheres as fillers was also examined. The results indicated that the annealing leads to increase in Co microspheres' permittivity as the improved conductivity that developed during annealing contributes to enhanced dielectric relaxation. High filling ratio is found to be favorable for achieving high electromagnetic properties and thus higher electromagnetic absorbing performances, which is of technical significant for application in low frequency band. Coatings containing 30, 45 and 50 vol% Co particles as fillers present excellent EMA performance, even very thin thickness is applied. High electromagnetic wave absorbing efficiency of −10 dB was observed at thickness of 1.5 mm in C band and the electromagnetic wave absorption bandwidth reaches up to 6.3 GHz (6.7–13 GHz) when the filling volume is 45 vol%.