Preparation and microwave absorption properties of carbon-coated iron nitride composites

WANG Wenjing; DONG Haoqi; LU Jie; LI Wei; ZHU Lei; GUO Lisheng; ZHANG Chenhua; WEI Yuxue; SUN Song

doi:10.13732/j.issn.1008-5548.2024.03.004

Abstract：Objective The energy attenuation of wave-absorbing materials primarily occurs through two mechanisms:dielectric loss and magnetic loss.Conventional wave-absorbing materials are less effective because they cannot simultaneously use both electrical and magnetic losses to attenuate microwave interference.Iron nitride,characterized by magnetic properties,such as high satura-tion magnetization,low density,large surface area,and environmental friendliness,has applications in various high-tech fields.However,its widespread use is limited due to its poor dielectric loss characteristics.Carbon materials,known for their exceptional conductivity and dielectric loss properties,can be combined with iron nitride to form composite materials that exhibit both magnetic and high dielectric losses.To achieve this,metal-organic frameworks(MOFs)are used as precursors for the syn-thesis of Fe2N@C,Fe3N@C and Fe4N@C through a process involving calcination and nitriding.These core-shell wave-absorbing materials exhibit excellent stability.The incorporation of carbon increases the dielectric loss of iron nitride,generating compos-ites that exhibit high dielectric and magnetic losses,thereby improving the microwave absorption of Fe2N@C,Fe3N@C,and Fe4N@C.Further investigations will explore the microwave absorption variations between different compositions of Fe2N@C,Fe3N@C,and Fe4N@C. Methods The physical composition of Fe2N@C,Fe3N@C,and Fe4N@C was analyzed using X-Ray diffraction(XRD).Their micro-morphology was analyzed using ultra-high-resolution scanning electron microscopy(SEM)and high-resolution transmis-sion electron microscopy(TEM).The micro-morphology of Fe2N@C,Fe3N@C,and Fe4N@C was determined through the suc-cessful synthesis of carbon-encapsulated iron nitride.Raman spectroscopy and X-ray photoelectron spectroscopy(XPS)were also employed for this purpose.XPS techniques were used to characterize and investigate their conformational relationships.Microwave absorption properties,as well as the imaginary parts of the complex dielectric constant and complex permeability,were analyzed using a vector network analyzer(VNA).Their magnetic loss properties were quantified using a Vibrating Sample Magnetometer(VSM). Results and Discussion As shown in Fig.1,Fe2N,Fe3N,and Fe4N were synthesized using MOFs as precursors.Additionally,Fig.2 showed that highly dispersed Fe nanoparticles were successfully encapsulated in the carbon layer,confirming the synthe-sis of Fe2N@C,Fe3N@C,and Fe4N@C.Fig.4 showed that Fe3N@C had a relatively low degree of graphitization,resulting in a low permittivity.In contrast,Fig.6 demonstrated that Fe2N@C and Fe4N@C exhibited poor wave absorption properties,while Fe3N@C displayed good microwave absorption.Therefore,it could be concluded that Fe3N@C was a better candidate for micro-wave absorption compared to Fe2N@C and Fe4N@C.Fig.7 showed that Fe2N@C and Fe4N@C had significantly greater dielectric loss than magnetic loss due to their high imaginary dielectric constant,resulting in an imbalancd impedance matching.On the other hand,Fe3N@C had a lower imaginary dielectric constant,providing a better impedance matching due to its balanced dielectric constant and magnetic permeability. Conclusion Fe2N@C,Fe3N@C,and Fe4N@C were successfully prepared by nitriding MOFs as precursors.Fe2N@C and Fe4N@C exhibited an imbalanced impedance matching due to their dielectric constants being much higher than their magnetic permeability.In contrast,Fe3N@C had balanced values for both parameters,resulting in better impedance matching.Samples with a coating thickness of 2 mm had an effective absorbing bandwidth of less than-10 dB,with a reflection loss of-10 dB up to 2.4 GHz.The minimum reflection loss of-14.1 dB at 9.1 GHz indicated better absorbing performance.The differing electrical conductivity of the iron nitride cores and the carbon shells resulted from to the varying phase structures of the three iron nitrides and the degree of defects in the carbon layers.This difference led to charge aggregation between interfaces,causing interfacial polarization.As a result,the dielectric constants of Fe2N@C and Fe4N@C increased,exceeding their magnetic permeability and leading to an imbalanced impedance matching.