WANG Wenjing1，DONG Haoqi2，LU Jie1，LI Wei，ZHU Lei，GUO Lisheng1，
ZHANG Chenhua1，WEI Yuxue1，SUN Song1
（1a. School of Chemistry and Chemical Engineering，1b. School of Materials Science and Engineering， Anhui University，
Hefei 230601， China；2. Anhui Tanxin Technology Co. ， Ltd， Huaibei，235141， China）
Abstract
Objective The energy attenuation of wave-absorbing materials occurs primarily through two mechanisms： dielectric loss and magnetic loss. Conventional wave-absorbing materials are often limited in their effectiveness because they cannot use both electrical and magnetic losses simultaneously to attenuate microwave interference. Iron nitride， characterised by magnetic properties such as high saturation magnetisation, low density， large surface area and environmental friendliness，has found applications in various high-tech fields. However，its widespread use has been hampered by 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 synthesis of Fe2N@C， Fe3N@C and Fe4N@C through a process involving calcination and nitriding. The resulting core-shell wave-absorbing materials exhibit excellent stability. The incorporation of carbon materials increases the dielectric loss of iron nitride， resulting in composites that exhibit high levels of both dielectric and magnetic losses， thereby improving the microwave absorptivity of Fe2N@C， Fe3N@C and Fe4N@C. Subsequent investigations will further explore the microwave absorptivity 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 analysed using X-Ray Diffraction （XRD）. The micro-morphology of Fe2N@C， Fe3N@C and Fe4N@C was analysed using Ultra-high resolution Scanning Electron Microscope （SEM） and High resolution Transmission Electron Microscope （TEM）. The micro morphology of Fe2N@C， Fe3N@C and Fe4N@C was determined through the successful synthesis of carbon-encapsulated iron nitride. The Transmission Electron Microscope （TEM） was used to analyze the microscopic morphology of Fe2N@C， Fe3N@C， and Fe4N@C to determine the successful synthesis of carbon-coated iron nitride. Raman spectra and X-ray photoelectron spectroscopy （XPS） were also employed for this purpose. Photoelectron spectroscopy （XPS） techniques were used to characterize and investigate the conformational relation⁃ships of Fe2N@C， Fe3N@C and Fe4N@C. The microwave absorption properties， as well as the complex dielectric constant imaginary part and complex permeability imaginary part of Fe2N@C， Fe3N@C and Fe4N@C， were analyzed using a Vector Network Analyzer （VNA）. The magnetic loss properties of Fe2N@C， Fe3N@C and Fe4N@C were quantitatively analyzed using a Vibrating Sample Magnetometer （VSM）.
Results and Discussion Based on Fig. 1， Fe2N， Fe3N， and Fe4N were synthesized using MOFs as precursors. Additionally，Fig. 2 shows that highly dispersed Fe nanoparticles were successfully encapsulated in the carbon layer，confirming the synthesis of Fe2N@C，Fe3N@C，and Fe4N@C. Fig. 4 shows that the degree of graphitization of Fe3N@C is relatively low，resulting in a low permittivity. In contrast， Fig. 6 demonstrates that Fe2N@C and Fe4N@C exhibit poor wave absorption properties，while Fe3N@C has good microwave absorption ability. Therefore，it can be concluded that Fe3N@C is a better candidate for microwave absorption compared to Fe2N@C and Fe4N@C. Fig. 7 shows that Fe2N@C and Fe4N@C have a significantly larger dielectric loss than magnetic loss due to their high dielectric constant imaginary part， resulting in an imbalance in impedance matching. On the other hand， Fe3N@C has a lower dielectric constant imaginary part， leading to a better impedance matching as the dielectric constant and magnetic permeability are similar.
Conclusion Fe2N@C， Fe3N@C， and Fe4N@C were successfully prepared by nitriding metal-organic frameworks （MOFs） as precursors. Fe2N@C and Fe4N@C exhibit an imbalance in impedance matching due to their dielectric constants being much larger than their magnetic permeability. In contrast， Fe3N@C has similar values for both parameters， resulting in better impedance matching. Samples with a coating thickness of 2 mm have 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 indicates better absorbing performance. The reflection loss at 9. 1 GHz is a minimum of -14. 1 dB， indicating good wave-absorbing performance. The electrical conductivity of the iron nitride cores and the carbon shells differs due to the varying phase structures of the three iron nitrides and the degree of defects in the carbon layers. This difference leads to charge aggregation between the interfaces， causing interfacial polarization. As a result， the dielectric constants of Fe2N@C and Fe4N@C increase， making them much larger than the magnetic perme-ability. This ultimately leads to an imbalance of the impedance matching.
Keywords：iron nitride；composites；impedance matching；wave-absorbing properties
Get Citation: WANG W J， DONG H Q， LU J， et al. Preparation and microwave absorption properties of carbon-coated iron nitride composites ［J］. China Powder Science and Technology，2024，30（3）：39−50.
Received:2024-02-03.Revised:2024-03-26,Online:2024-04-26。
Funding Project:国家自然科学基金项目，编号：21902001，22179001，22102001；安徽省高校杰出青年科研项目，编号：2022AH020007；安
徽高校协同创新项目，编号： GXXT-2023-009；安徽省高等学校自然科学基金项目，编号：2023AH050114
First Author:王文敬（1998—），女，硕士生，研究方向为化工新材料。E-mail：wwjfp1015@163. com。
Corresponding Author:魏宇学（1991—），女，副教授，博士，硕士生导师，研究方向为化工新材料。E-mail：weiyuxue@ahu. edu. cn。
DOI：10.13732/j.issn.1008-5548.2024.03.004
CLC No:TM25； TB4 Type Code:A
Serial No:1008-5548（2024）03-0039-12
