Low Dimensional Carbon-based Composites Synthesized By Arc Discharge Method And Their Microwave Absorption Properties

Electromagnetic(EM)interference and radiation,originated from the development of electronic components constantly tending to be integrated,miniaturized and high frequency,have resulted in some intractable issues during the process of information transmission,such as leakage of private information,false images,transferring latency and instability.In order to eliminate the increasingly serious EM pollution,it is urgent to search for micro wave absorption materials with thin thickness,light weight,wide absorption band,strong anti-oxidation and corrosion resistance.As a type of potential candidates for microwave attenuation,low dimensional carbon-based nanocomposites have attracted much attention due to their relatively light weight,excellent weather resistance,high dielectric constant,special size effect and interface polarization mechanism.However,the factors of impedance mismatching,weak absorption intensity,narrow absorbption frequency band,complex synthesis process and low yield restrict the practical large-scale application of low dimensional carbon-based nanocomposites.Concerning about the above bottlenecks,a DC arc plasma method with high yield and easy operation was adopt in this work to control the electromagnetic properties of carbon-based nanocomposites by compositing hetero components and doping hetero atoms.Firstly,(Co/TiC)@C nanoparticles were prepared through combining metallic Co with high saturation magnetization,oxidation-resistant ceramic TiC and lightweight carbon.The product's special double-core@shell structure and multiple components are conducive to enhance the magnetic and dielectric loss,and we mainly studied the influence of the content of dielectric-loss components and the regulation of microstructure on the absorption capability.Secondly,by comparing the electromagnetic response characteristics of(TiC/Ni)@C with TiC@C nanoparticles,the relation between the magnetic-loss intensity and the absorption capability of double-shell@shell nanoparticles was investigated.Thirdly,nitrogen-doped carbon coated ceramic TiCN nanocubes was constructed.Compared with the magnetic metals with relatively high density,lightweight nitrogen atoms doping into carbon layer can increase the lattice defects and weaken the overall conductive loss,which also leads to the improvement of impedance matching of dielectric materials.On this basis,the defect-rich graphene nanoflakes doped by nitrogen atoms were synthesized to achieve lighter and more efficient carbon-based nanomaterials for microwave absorption.Compared with zero-dimensional nanoparticles,two-dimensional nanoflakes possess larger specific surface area and more electromagnetic response sites.The main research contents and related results are as follows:(1)(Co/TiC)@C double core@shell nanoparticles with magnetic metal Co and carbide TiC as the core and highly defective graphite C as the shell were prepared by DC arc-discharge plasma method.With elevating the gas ratio of CH4/Ar in the reaction chamber,the overall dielectric loss of(Co/TiC)@C can be enhanced due to the increase of the thickness of outer C shell and TiC content,further achieving the goal of the balance and synergy effect between magnetic and dielectric loss.The nanoparticles'microstructures of double core@shell were characterized by XRD and TEM.The results show that the metal Co and carbide TiC are both face-centered cubic structure,and no hexagonal close-packed Co,Co2Ti and CoTi2 alloys can be found.Raman spectra show that the ordered domain sizes of outer C shell gradually increase from 3.05 nm,3.52 nm to 4.70 nm with the increase of CH4 concentration.On the condition of CH4/Ar=l:4,the prepared(Co/TiC)@C nanoparticles display a minimum reflection loss(RL)of-66.59 dB at 8.76 GHz with a thickness of 2.56 mm.Such an excellent absorption capability can be ascribed to electrical dipole polarization,interface polarization and natural resonance and high-frequency exchange resonance.Due to the restricted effect of thicker carbon coatings on the formation of surface eddy current,the contribution of eddyt loss can be ignored.(2)Two kinds of nanocomposite particles,i.e.TiC@C and(TiC/Ni)@C,were prepared by utlizing different anode target materials,but keeping the same preparation atmosphere and arc output power.We used TEM technology to analyze the microstructure of the two kinds of nanoparticles.It was found that both of them exhibit irregular elliptical-shaped morphology.Their particles' diameter mainly distributes in the range of 20?70 nm.The result of Raman spectra show that the(ID/IG)A value of TiC@C nanocomposite is 1.15,while that of(TiC/Ni)@C nanocomposite increases to 1.32,further illustrating the addition of Ni into TiC/C matrix can elevate the lattice defective degree of outer C shell.The EM parameters of the products measured at room temperature shows that,compared with TiC@C nanoparticles,the real part and imaginary part of the complex permittivity of(TiC/Ni)@C nanoparticles decrease obviously.This finding is mainly attributed to the introduction of metallic Ni reduces the homogeneity of TiC/C matrix and enhances the scattering ability of electrons,which inevitably weakens the overall electric conductivity loss.On the other hand,combinating magnetic component into dielectric matrix can efficiently improve the overall magnetic loss capability.The minimum RL of(TiC/Ni)?C nanocomposite-43.4 dB,which is 2.38 times of that of TiC@C nanocomposite.(3)By utilizing CH4-N2-Ar tri-mixed gas as the preparation atmosphere,N-doped carbon coated TiCN nanocubes were self-assembled by high-temperature plasma evaporation of high-purity Ti ingot.The results of high-resolution TEM and selected electron diffraction(SAED)show that the core is TiCN monocrystal,which is six-fold rotationally symmetric along the[0-11]crystal direction and consists of six {100} crystal faces.The nanocube's shell is graphitic carbon with masses of lattice defects.Raman spectra show that the defect density on the outer C shells of the prepared products would increase from 1.80×1011 cn-2,2.79×1011 cm-2 to 3.10×1011 cm-2,when N2 concentration in the preparation atposphere improved from 16.7 vol.%,33.3 vol.%to 41.7 vol.%.As the center of electron scattering,the increased lattice defects weaken the electrical conductivity of the product.However,the corresponding saturation magnetization would increase from 0.1 7,0.31 to 0.54 emu/g,which can be attributed to a higher pyrrolic-N level doped into the outer carbon shells.The effective absorption bandwidth(reflection loss?-10 dB)of the optimized product can achieve 5.44 GHz with a thickness of merely 1.88 mm.Such an excellent absorption capability mainly stems from the improved impedance matching degree by reducing conductive loss and elevating magnetic resonance intensity together,meanwhile,the hetero interfaces between TiCN monocrystals and C shells are conducive to the generation of interfacial polarization.(4)By adjusting the gas ratio of N2/CH4 in the reactive chamber,graphene nanoflakes with different nitrogen content were prepared through DC arc plasma evaporating high-purity carbon.High-resolution TEM images show that pure graphene is about 100?300 nm in the lateral size and its thickness is?1.3 nm.In comparision,the lateral sizes of N-doped samples N1-GN and N2-GN are mainly distributed in the range of 50?150 nm,but the thicknesses of them present an increase tendency,measured as 3.5 and 7.2 nm,respectively.The results of SAED and Raman spectra both confirm that,a higher level N-doped into C lattices would generate more crystal defects in the prepared nanoflakes.The absoption capabilities of the products shows that,the minimum RL of pure graphene nanoflakes is only-16.6 dB,while that of sample N1-GN can achieve as high as-43.2 dB at 7.3 GHz with a thickness of 2.9 mm.Such a superior microwave absorbency is mainly due to the synergism between the optimized impedence matching and the appropriate attenuation ability towards EM waves.