Microstructure Regulation And Microwave Absorption Properties Of Ni/C Nanocapsules

Core/shell-structured carbon-coated magnetic metal nanocapsules show highly efficient microwave absorption properties due to their multi-microwave loss mechanisms.In this paper,Ni/C nanocapsules were prepared by an arc discharge method.The structural parameters and compositions were adjusted via changing experimental conditions and reaction materials.The phases,size distributions,and microstructure were characterized by means of X-ray diffraction(XRD),transmission electron microscopy(TEM)in details.The static magnetic properties were measured by a vibrating sample magnetometer(VSM)at room temperature.The electromagnetic properties of the paraffin matrix compositions containing nanocapsules/nanocomposites as fillers were measured by a network vector analyzer(VNA)at the frequency range of 2~18 GHz.The electromagnetic wave absorption properties were valued from electromagnetic parameters by transmission line theory.Ni/C nanocapsules were prepared by evaporating nickel in ethanol atmosphere using an arc discharge method,where their particles sizes were adjusted by varying arc current values from 40 to 100 A.The average particle sizes of the Ni/C nanocapsules increase from 25 to 53 nm,while the thicknesses of the carbon shell keep independence with the arc current value.The saturation magnetization and coercivity increase with the particle size,due to the small size effect.The complex permittivity,dielectric loss and attenuation constant of paraffin-Ni/C composites are in inverse proportion to the particle size of Ni/C nanocapsules,ascribed to enhance interfacial polarization relaxation from larger interface area in the smaller nanocapsules.With the reduction of particle size of Ni/C nanocapsules,the peak of reflection loss(RL)at fixed absorber shifts to lower frequency and the bandwidth becomes wider,while the maximum RL of the composites is achieved at thinner absorber layer.This results from the bigger dielectric loss,larger attenuation constant and better impedance matching degree.Core/shell-structured Ni/(C,N)nanocapsules with the nitrogen-doped onion-like carbon(C,N)as the dielectric shell were prepared by the modified arc discharge method using nitrogen gas as the N atom source.Core/shell-structured Ni/C nanocapsules were also prepared for comparison.Raman spectra reveals more disorderly carbon states in the(C,N)shell because of the introduction of more lattice defects/imperfections into the carbon via doping of N atoms.The complex permittivity spectrums of the Ni/(C,N)nanocapsules measured in 2~18 GHz show the increase in dielectric loss tangent in comparison with the Ni/C nanocapsules because of the additional dielectric contribution from the doped N atoms in carbon shells acting as the polarization centers.The RL curves show the microwave absorption properties of Ni/(C,N)nanocapsules are improved in the 2~8 GHz.The Ni/C-C-NbC nanocomposites,comprising of core-shell structured Ni/C nanocapsules,defective onion-like carbon nanoparticles and tiny NbC nanoparticles,were prepared by evaporating NiNb alloys in methane atmosphere using the arc discharge method.Compared with Ni/C nanocapsules,the Ni/C-C-NbC nanocomposites exhibit large permittivity,enhanced dielectric loss and three Debye dipolar relaxations.An anomalous electromagnetic resonance in the Ni/C-C-NbC nanocomposites appears at 9~14 GHz,revealing the energy transform from the permeability to permittivity,which results from the radiation of magnetic energy.The reflection loss value of the Ni/C-C-NbC nanocomposites can reach-40.4 dB at 8.48 GHz with the absorbent thickness of 3.2 mm,which is better than that of Ni/C nanocapsules.