Preparation Of Fe_xCo_1-x/C And C@C Microwave Absorbers And The Performance

Carbon materials are promising microwave absorbers due to their light weight,stable properties,and low cost.Recently,the fabrication of carbon-based materials includes the compounding-reduction process and several unconventional techniques,such as chemical vapor deposition and arc discharge.Unfortunately,the complex preparation methods,tedious preparation process and uneven distribution of various components limit their application.In this aspect,it is urgent to seek the simple method to obtain the magnetic metal/carbon composites with the uniform distribution.In this study,metal organic frameworks-derived（MOFs）method is utilized for preparing the Fe_xCo_1-x/C composites.In addition,a series of pure carbon absorbers with special microstructures are prepared through rational design,and the effects of microstructures in electromagnetic（EM）wave attenuation are studied.Fe/C nanotubes and Co/C polyhedrons are prepared through MOFs-derived method by using the Prussian blue and ZIF-67 as precursor.SEM and TEM images show that the Fe/C nanocubes or Co/C polyhedrons exhibit an ordered morphology with the amorphous carbon framework decorated with well-dispersed core-shell Fe@graphitic carbon or Co@graphitic carbon nanoparticles.Raman spectra reveal that the degree of graphitization of carbon component in the Fe/C and Co/C composites increase at the elevated temperature.The reflection loss performance shows that the Fe/C composites exhibit a super broad response bandwidth（7.2 GHz,10.8-18.0 GHz）,and the Co/C composites display excellent microwave absorption properties.The minimum reflection loss can reach up to-32.4 d B and the effective response bandwidth achieves up to 3.8 GHz（10.7-14.5 GHz）at a thickness of 2.0 mm.The microwave absorption performances of these Fe_xCo_1-x/C composites relate to their efficient attenuation properties and proper impedance matching characteristics.In addition,the well-dispersed magnetic metal components are suitable for obtaining the stable and effective microwave absorbers,where the blind absorbing area reduces due to the good chemical homogeneity and special microstructures.To utilize the MOFs-derived methods in magnetic alloy/carbon composites,PB analogues are used as the precursors in preparing Fe Co/C composites.XRD results show that the introduction of Co atoms suppresses carbide formation in these composites,which results in the significant enhancement of magnetic properties.SEM and TEM images reveal that lower Fe/Co ratio has caused the reduced alloy particle sizes because of more nucleation sites under relatively mild pyrolysis temperature.EDS images show that the constituent of alloy in Fe Co/C composites varies with the content of melt salts.Raman spectra reveal that low Fe/Co ratio is favorable for improving the graphitization degree of carbon frameworks,which can influence the corresponding dielectric loss performance.As a result,the superior performance is achieved at the Fe/Co ratio of 1:1,where the minimum reflection loss can reach up to-57.4 d B（17.7 GHz,1.26 mm）.The above results reveal that manipulating the alloy compositions is a good way to regulate the EM parameters of composites,and thus influence the reflection loss properties.In order to obtain the carbon materials with lightweight,amorphous carbon/onionlike carbon composites are prepared through the MOFs-derived method.N2 adsorption-desorption isotherms show that the composites have vari ous pore structures,the specific surface area of AC/OLC-600,AC/OLC-700,and AC/OLC-800 composites are 273.3,408.6,and 527.3 m2/g,respectively.The appropriate pore structures can promote the repeated oscillation and absorption of EM waves and contribute to the attenuation behavior.The reflection loss performance is that the effective response bandwidth achieves up to 3.8 GHz（10.7-14.5 GHz）at a thickness of 2.0 mm,and the valid bandwidth of composites can achieve 3.3 GHz（12.1-15.4 GHz）with the thickness ranges of 1.0-5.0 mm.In this aspect,carbon absorbers with good performance can be obtained through the rational design on microstructure.Carbon materials exhibit ideal microwave absorption performance based on the rational design of microstructure.In this regard,we demonstrate the successful fabrication of yolk-shell C@C microspheres through a “coating-coating-etching” route with enhanced microwave absorption.SEM and TEM images reveal that the yolk-shell carbon microsphers can be divided into three parts,a core with diameter at about 300 nm,a shell with thickness at about 20 nm,and a distinguishable void between core and shell,leading to a standard yolk-shell structure.Unfortunately,the carbon microsphers only have the spherical structures.XRD and Raman results show that yolk-shell carbon microspheres and the carbon microsphers have the similar composition and the degree of graphitization.N2 adsorption-desorption isotherms show that the yolk-shell carbon microspheres have the larger specific surface area and pore volume compared with the carbon microspheres.The as-prepared yolk-shell carbon microspheres exhibit superior microwave absorption properties compared with solid carbon microspheres,where the minimum reflection loss can even reach up to-34.8 d B and the effective response bandwidth achieves up to 5.4 GHz（12.6-18.0 GHz）at a thickness of 2.0 mm.The superior microwave absorption performance can be attributed to its yolk-shell structure,which facilitate multiple reflection and absorption behavior,enhance the interactions between the incident EM wave and absorber,and thus promote the EM wave attenuation process.Moreover,the yolkshell carbon microspheres with gradient graphitization degree are prepared,where the gradient structures fulfil the double-layered（matching and absorbing layers）design concept for the practical application of microwave absorbers.The as-prepared materials show excellent microwave absorption performance with 30% filler loading amount,where the minimum reflection loss can reach-45.1 d B and the effective response bandwidth can achieve 3.8 GHz（9.8-13.6 GHz）at a thickness of 2.5 mm.The good EM absorption performance is achieved because the gradient graphitization microstructures can create the double-layered structures for the attenuation of EM waves,where the double-layered structures stimulate the multiple reflection and scattering behaviors and provide the further microwave attenuation.The microwave absorption properties are realted to two factors,namely,the minimum reflection loss and effective response bandwidth,where the effective bandwidth is important in terms of practical use.Among the three magnetic metal/carbon composites,Fe/C composites possess the best microwave absorption performance due to their superbroad response bandwidth（7.2 GHz）.For the three pure carbon materials,yolk-shell C@C microspheres show the best performance,while the yolk-shell C@C-IM microspheres demonstrate lower loading amount and moderate microwave absorption performance.These findings suggest that C@C-IM microspheres can be competitive in practical applications.In this regard,yolk-shell C@C and C@C-IM microspheres are superior EM wave absorbers due to their good microwave absorption performance and low density.Furthermore,the yolk-shell structure can not only improve the impedance matching and reflection loss properties,but also manipulate the transmission behaviour of EM wave and facilitate the rapid transformation of EM energy,which provides a new concept for the designed synthesis of carbon-based microwave absorbers.