Microwave Absorption Properties Of Ni@C Nanocapsules Based Composites With Core/Shell Structure

Recently,a novel multivariate nanocomposites which has strong absorption,wide frequency band,thin thickness and high temperature microwave absorber,has been arousing considerable attention.In this paper,Ni@C nanocapsules were prepared by arc discharge method.The different composites were prepared via changingexperimentparameters,rawmaterials,reactionmethod.Core/shell-structured nickel/carbon（Ni@C）nanocapsules with Ag₃PO₄ nanoparticle decoration（Ag₃PO₄@Ni@C）were prepared by an arc-discharge process and ion-exchange process.The novel structures of SrFe₁₂O₁₉nanoflakes decorated with Ni@C nanocapsules were successfully prepared by an arc-discharge process and hydrothermal method.The Ni@C/BiOI NCs nanocomposites,which Ni@C nanocapsules were conjugated on the flower-like BiOI microspheres,were prepared by an arc-discharge process and oil method.The composition and phase purity of the as-prepared samples were analyzed by X-ray diffraction（XRD）.The size distributions and microstructure of samples were characterized by scanning electron microscope（SEM）and transmission electron microscopy（TEM）in details.The static magnetic properties were measured by a vibrating sample magnetometer（VSM）at room temperature.The electromagnetic parameters were measured for the appropriate proportion paraffin matrix compositions nanocapsules/nanocomposites by a network vector analyzer（VNA）at the frequency range of 2¹8 GHz.The electromagnetic wave absorption properties were calculated from S-parameters tested by VNA,using the simulation program of Reflection/Transmission line theory.All the measurements were performed at room temperature.Core/shell-structured nickel/carbon（Ni@C）nanocapsules with Ag₃PO₄nanoparticle decoration（Ag₃PO₄@Ni@C）are prepared by an arc-discharge process and an ion-exchange process.The Ag₃PO₄@Ni@C nanocapsules show a clear decoration of Ag₃PO₄ naoparticles of 4–20 nm diameters on the C shell of the Ni@C nanocapsules of⁶0 nm diameter.The amount of Ag₃PO₄ nanoparticles that can be decorated on the Ni@C nanocapsules depends on the volume of Na₂HPO₄ reactant used in the ion-exchange process.The Ag₃PO₄@Ni@C nanocapsules demonstrate interestingly high and tunable electromagnetic absorption properties with different amounts of Ag₃PO₄ nanoparticle decoration in the paraffin-bonded composites over the 2–18 GHz microwave range.The nanocapsules prepared with 100 ml Na₂HPO₄exhibit much enhanced dielectric and magnetic losses for an improved electromagnetic impedance match.These result in a large reflection loss（RL）of-31.4dB at 12.3 GHz for a small absorber thickness of 2.6 mm in conjunction with a very wide effective absorption bandwidth（for RL<-10 dB）of 14 GHz（4–18 GHz）at a wide absorber thickness range of 1.4–5.0 mm.The novel structures of SrFe₁₂O₁₉nanoflakes decorated with Ni@C nanocapsules were successfully prepared by an arc-discharge process and hydrothermal method,in which Ni@C nanocapsules with the diameter of about 5 nm adhered to the surface of SrFe₁₂O₁₉hexagonal nanoflakes with the length of a side from 0.5 to 1.0μm and the thickness of about 80 nm.The electromagnetic properties of SrFe₁₂O₁₉/Ni@C-paraffin composite were investigated in the 2-18 GHz frequency range.The effects of Ni@C nanocapsules on the electromagnetic properties of SrFe₁₂O₁₉/Ni@C were studied.The reflection loss exceeding-10 dB is obtained in 14.4-18 GHz for an absorber thickness of 2.0 mm.Moreover,the strong absorption peak reaching-55.72 d B can be observed in 8.48 GHz when the thickness is 3.6 mm.Compared with the SrFe₁₂O₁₉nanoflakes,the nanocomposites exhibit the thinner thickness,the broader absorption bandwidth and the stronger absorption intensity,which is ascribed to the introduction of Ni@C nanocapsules.Ni@C nanocapsules can induce double dielectric relaxation loss and enhance dielectric loss and generate the cooperative effects of dielectric-magnetic properties.These excellent absorbing performances show that the SrFe₁₂O₁₉/Ni@C nanocomposites have great potential for application in a microwave-absorption field for their strong absorption and broad bandwidth.At present,the microwave absorbers are prepared by dispersing the absorbing nanomaterials in the binder,which can lead to the aggregation of nanomaterials in the binder and affect the optimization of the absorption performances.The micro/nano-hybrided particles are beneficial for the buffer of agglomeration phenomena and the construction of multiple interfaces.Ni@C nanocapsules are conjugated on the flower-like BiOI microspheres,forming the micro/nano-hybrided nanocomposites.The multiple interfaces between BiOI microspheres and Ni@C nanocapsules can bring enhanced dielectric loss and increased attenuation constant,resulting in the enhancement of absorption capacity（the optimal reflection loss reaches-61.35 dB）,the width of effective absorption band(the maximum of effective bandwidth,f_Emax,is 5.86 GHz)and the reduction of absorption thickness(the thickness corresponding to t_Emaxis 1.7 mm).This study lights on a simple idea for the optimization of electromagnetic absorbing performance,which is of great significance to the development of microwave absorbers.