Tunable shell thickness of Fe-N@SiO2 nanoparticles for strong and stable microwave absorption properties with enhanced oxidation resistance

Abstract

Core-shell composites Fe-N @ SiO 2 with 500 nm average sizes of cores have been successfully prepared by a combination of hydrothermal, gaseous nitriding and Stöber methods. The Fe-N cores are composed of Fe 4 N as major phase and FeN as minority. The thickness of SiO 2 shell increases continuously with increase of the tetraethyl orthosilicate weight. Relative to the natures of Fe-N, Fe-N @ SiO 2 composites show slightly weaker ferromagnetic capacity, but much higher oxidation resistance. The Fe-N sample exhibits extremely high relative permittivity. With filling loading of 50 wt%, it has a moderate microwave absorption performance with a maximum reflection loss about −10 dB. Owing to low conductivity, non-magnetic property and superior anti-oxidation ability of the SiO 2 shell, the core-shell structures have considerably degraded permittivity, and nearly unchanged permeability. These changes bring a much better impedance matching between the permittivity and permeability, and further bring a greatly enhanced microwave absorption performance. All of the Fe-N @ SiO 2 composites have strong absorption abilities. They are quite stable with variation of shell thickness. The composite of 52 nm shell thickness has maximum reflection loss of −23.1 dB at 17.2 GHz, and effective absorption bandwidth (<−10 dB) of 2.4 GHz with absorber thickness 5.5 mm. The present study gives an insight on the roles of silica material influencing the iron-based absorber’s absorption ability. • Fe-N @ SiO 2 nanoparticles with tunable shell thickness were synthesized. • Ferromagnetic Fe-N @ SiO 2 composites have good oxidation resistance. • Core-shell absorbers exhibit strong and stable absorption performance. • Mechanism for SiO 2 modification of absorption performance was revealed.