Electromagnetic characteristics and microwave absorption properties of carbon-encapsulated cobalt nanoparticles in 2–18-GHz frequency range

Carbon-encapsulated cobalt nanoparticles (Co(C)) with a diameter of 10–50 nm were synthesized by an arc discharge method. The inner crystal cobalt core was completely encapsulated by an outer carbon shell, which consisted of both amorphous carbon and 15–20 layers of graphite-like carbon. The electromagnetic characteristics of Co(C) embedded in paraffin at 10, 40, 50, 70, and 80 wt.% (2, 10, 15, 29, and 41 vol.%, respectively) at 2–18 GHz were investigated. The distinctive core–shell microstructure of Co(C) and the carbon shells were mainly responsible for the high complex permittivity. The real part of complex permeability (μ) decreased and the imaginary part of μ remained small with increasing frequency, which revealed that good insulation between the metallic cobalt cores was achieved by carbon encapsulation. The major electromagnetic absorption mechanism was dielectric loss. The minimum calculated reflection loss (RL) was −52 dB at 7.54 GHz with 50 wt.% Co(C) for a 3-mm Co(C)/paraffin composite. The measured RL of a Co(C)/epoxy coating with 40 wt.% loading at 3 mm showed good agreement with that of calculated RL. The electromagnetic property of Co(C) can be tailored through both control Co(C) concentrations and coating thickness.