Hollow core-shell CoNi@C and CoNi@NC composites as high-performance microwave absorbers

Highlights: : • Hollow and porous structure is utilized to improve the impedance matching. • Soft magnetic material is chosen as the core to improve permeability. • Electromagnetic parameters are tuned by varying the carbonization temperature. • The effect of various carbon shells on the microwave absorption property is studied. -- Abstract: Magnetic nanoparticles are readily oxidized in air, resulting in a poor electromagnetic wave (EMW) absorption performance. In this study, hollow CoNi alloy was used as the core, and two kinds of carbon shells were explored to protect it from oxidation and improve its complex permittivity. The hollow core-shell shaped CoNi@C and CoNi@NC absorbers were synthesized by in-situ polymerization of phenolic resin (PR) and polydopamine (PDA) on the surface of CoNi alloy, respectively, which was followed by high-temperature pyrolysis. The EMW absorption performance of the absorbers was tuned by varying the calcination temperature. Compared to the pristine CoNi alloy, the CoNi@C and CoNi@NC composites displayed significantly enhanced microwave absorption (MA) performance. The CoNi@NC-700 microspheres exhibited a maximum reflection loss (RL) of −47.1 dB at a thickness of 2 mm. Further, CoNi@NC-700 and CoNi@C-700 displayed effective bandwidths (RL ≤ −10 dB) of 5.1 GHz at 1.7 mm-thickness and 4.97 GHz at 1.5 mm-thickness, respectively. Overall, this study establishes a facile and effective approach for the preparation of high-performance absorbers with tunable properties.