Preparation And Properties Of N-doped C Coated Ferromagnetic Metal Nanocapsules

In this paper,an arc-discharge method together with in-situ self-polymerization method was adopted to synthesize Ni@C/NC nanocapsules.Then the Ni@NC nanocapsules were synthesized by an arc-discharge method in a mixture atmosphere of methane and nitrogen.The phase,morphology and surface compositions of as-prepared samples were analyzed by X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,Raman spectrometer and transmission electron microscopy（TEM）.The microwave absorption properties of as-prepared nanocapsules/paraffin were measured by a vector network analyzer.The results show that the Ni@C/NC nanocapsules synthesized by an arc-discharge method together with in-situ self-polymerization method have a distinct core/shell microstructure with a size range of 15-50nm,and its shell thickness is about 4-8nm.The results of microwave absorption properties show that an optimal reflection loss（RL）value-38.3dB is observed at 11.7GHz on Ni@C/NC nanocapsules-paraffin composities with an absorber thickness of 2.4mm.A reflection loss（RL）below-20dB is obtained in the wide frequency range of 4.4-18GHz of Ni@C/NC nanocapsules,when an appropriate absorber thickness from 1.5 to 5.2mm is chosen.Moreover,the amplitudes of RL values of the Ni@C/NC nanocapsules is about 2-3 times as many as that of Ni@C nanocapsules within absorber thickness ranging from 1.5 to 5.2mm.The Ni@C/NC nanocapsules have different absorption peak and frequency widths due to their thickness,and the attenuation peak shift to a lower frequency when absorbing layer thickness increases.The Ni@NC nanocapsules were prepared by arc-discharging a Ni metal target in a mixture atmosphere of argon,methane and nitrogen.The effect of different nitrogen doping amount on the microwave absorption properties of the prepared nanocapsules was studied by changing the nitrogen pressure.The results show that the Ni@NC nanocapsules have a distinct core/shell microstructure with a size range of 20-70nm.The results demonstrate that the Ni@NC nanocapsules have enhanced microwave absorption characteristic,compared to the Ni@C nanocapsules.As the amount of nitrogen doping increases,the microwave absorption performance of the Ni@NC nanocapsules increases first and then decreased.The Ni@NC nanocapsules have optimal performance with N₂ gas of 1.5×10⁴Pa.The strongest RL of the Ni@NC-parrafin composite reaches-50.7dB at 14.43GHz when the absorbent thickness is1.8mm.That RL values below-10dB in the 2.8-18GHz range are obtained while layer thickness ranges from 1.5 to 6mm.The enhanced microwave absorption performance of as-prepared samples is ascribed to N-doped C shell.Nitrogen-doped graphite C elevates lattice defects of carbon-based material,inducing more polarized centers.For microwave absorbing materials,Nitrogen-doped graphite C can effectively improve impedance matching of the absorbing materials and enhance their microwave absorption performances.The increase of lattice defects reduces the transmission capacity of electrons in the surface of the graphite layer,resulting in a decrease in dielectric loss capability.Therefore,the microwave absorption performance of as-prepared samples will also decrease as the doping amount of nitrogen further increases.