Electrospun Fe@Fe4N/amorphous-carbon (a-C) submicron fibers were reported by Zheng et al. (Journal of Materials Science 55, 16954, 2020) to strongly outperform otherwise identically processed Fe@g-C fibers as microwave absorbers, and the difference was attributed to the crystalline Fe4N interfacial phase. The two materials, however, also differ simultaneously in Fe particle size (about 25 nm vs about 70 nm), fiber diameter and morphology, and apparent mat density (about 0.25 vs about 0.93 g/cm3), and were compared as equal-mass paraffin composites, so that the two composites contain very different filler volume fractions, leaving the central attribution untested. The objective is to quantify, by controlled parametric variation on this fiber system, the separate contributions of the Fe particle-size distribution, the filler packing density (volume fraction), and the Fe4N interfacial content/crystallite size to the complex permittivity, complex permeability (2-18 GHz), and reflection loss RL, and decide whether the Fe4N interface remains a principal cause of the performance gap (i.e. the factor accounting for the largest share of that gap) once the confounds are removed.

Background

Radar and communication equipment require materials that convert incident microwave energy into heat rather than reflecting or transmitting it. The standard figure of merit is the reflection loss RL, in decibels, computed for a planar layer of thickness tt backed by a conducting plate: transmission-line theory gives the input impedance at the air surface as Zin=Z0μr/εrtanh⁡(j 2πftμrεr/c)Z_{\mathrm{in}}=Z_0\sqrt{\mu_r/\varepsilon_r}\tanh\left(j\,2\pi ft\sqrt{\mu_r\varepsilon_r}/c\right), where Z0Z_0 is the free-space impedance, ff the frequency, cc the speed of light, and εr=ε′−jε′′\varepsilon_r=\varepsilon'-j\varepsilon'' and μr=μ′−jμ′′\mu_r=\mu'-j\mu'' are the measured complex relative permittivity and permeability of the composite; RL=20log⁡10∣(Zin−Z0)/(Zin+Z0)∣\mathrm{RL}=20\log_{10}|(Z_{\mathrm{in}}-Z_0)/(Z_{\mathrm{in}}+Z_0)|. Deep absorption requires both strong internal loss (dielectric loss governed by ε′′\varepsilon'', magnetic loss by μ′′\mu'') and good impedance matching (ZinZ_{\mathrm{in}} close to Z0Z_0), since a mismatched surface reflects the wave before internal loss can act. The effective absorption bandwidth (EAB) is the frequency range with RL≤−10\mathrm{RL}\le -10 dB, i.e. at least 90% of incident power absorbed.

Magnetic metal particles (Fe, Co, Ni) provide large magnetic loss but are dense and prone to eddy-current losses, while carbons provide light dielectric loss but poor matching to free space; a leading strategy therefore embeds magnetic nanoparticles in electrically moderate carbon fibers. Electrospinning, extruding submicron fibers from a polymer solution containing an iron salt followed by carbonization, is the standard route to such fibers. During carbonization the precursor chemistry controls what forms at the nanoparticle-carbon interface: graphitic shells, carbides, or nitrides such as γ′\gamma'-Fe4_4N, the ferromagnetic iron nitride. Interface phases matter because interfacial (Maxwell-Wagner) polarization, the accumulation of space charge at conducting/insulating heterointerfaces, contributes to ε′′\varepsilon'', and because the filler volume fraction relative to the percolation threshold controls whether a conductive network forms, which raises ε′\varepsilon' and ε′′\varepsilon'' sharply.

Zheng et al. (2020) electrospun two Fe/amorphous-carbon (a-C) submicron-fiber batches, polyvinylpyrrolidone (PVP) with iron nitrate versus polyacrylonitrile (PAN) with iron acetylacetonate, carbonized identically at 730 C in nitrogen. Different thermal-decomposition chemistry produced graphitic-carbon shells around the Fe cores in one system (Fe@g-C) and crystalline Fe4_4N shells in the other (Fe@Fe4_4N). Measured on paraffin-matrix toroids at equal fiber/paraffin mass ratio (2-18 GHz, vector network analyzer with Nicolson-Ross extraction, the standard method for recovering εr\varepsilon_r and μr\mu_r from coaxial scattering parameters), the Fe@Fe4_4N composite reached RLmin⁡=−39.8\mathrm{RL}_{\min}=-39.8 dB at 5.4 GHz for t=5.0t=5.0 mm and an EAB of 5.0 GHz at t=2.5t=2.5 mm, while the Fe@g-C composite never fell below −10-10 dB. The authors attributed the gap to the Fe4_4N interface: it preserves magnetic loss (natural- and exchange-resonance features in μ′′\mu'', i.e. magnetic-loss peaks from spin precession at the whole-particle scale and from spin waves exchange-coupled over nanometer distances) while suppressing the conductive dielectric loss responsible for the Fe@g-C composite's severe impedance mismatch.

The comparison is, however, confounded: the two fibers differ simultaneously in average Fe particle diameter (about 25 nm versus about 70 nm), annealed fiber diameter, surface morphology, and apparent mat density (about 0.25 versus about 0.93 g/cm3). At equal mass fraction in paraffin these densities correspond to very different filler volume fractions, hence different percolation states and interfacial areas. The same group separately showed that changing the electrospinning solvent, which changes Fe particle size, fiber diameter, and graphitic-shell thickness together, measurably changes the absorption of Fe/C nanofibers, so these morphology variables are demonstrably potent. Whether the crystalline Fe4_4N interface is the principal cause of the performance gap, rather than the co-varying particle size, fiber geometry, and packing, has therefore never been established.

Problem Statement

For electrospun Fe-based/amorphous-carbon submicron-fiber microwave absorbers of the type studied by Zheng et al. (2020): determine, by controlled parametric variation, the separate contributions of (i) the Fe nanoparticle size distribution, (ii) the fiber packing density, equivalently the filler volume fraction of the composite test specimen, and (iii) the Fe4_4N interfacial-phase content (stoichiometry and crystallite size) to the complex permittivity (ε′\varepsilon', ε′′\varepsilon''), the complex permeability (μ′\mu', μ′′\mu'') over 2-18 GHz, and the reflection loss RL(f,t)\mathrm{RL}(f,t). The variables that co-vary in the original comparison must be decoupled, for example by comparing Fe@Fe4_4N and Fe@g-C fibers at matched particle-size distributions and matched filler volume fractions, or by a factorial design spanning the three variable groups; in either case the co-varying annealed fiber diameter and surface morphology are held fixed by matching. The answer must state, with quantified sensitivities or a per-factor decomposition of the performance gap with stated uncertainty, whether the crystalline Fe4_4N interface remains a principal cause of the superior absorption of Fe@Fe4_4N/a-C relative to Fe@g-C/a-C (i.e. the factor accounting for the largest share of the performance gap) once the confounding variables are controlled.

References

  1. Qi Zheng, Meijie Yu, Xueping Gao, Yanjun Wang, Ze Zhang, Haiping Zhou, YouYong Dai (2020). Microwave absorption performance of Fe@Fe4N/amorphous carbon submicron fibers: critical role of the interface. Journal of Materials Science 55:16954-16968. DOI: 10.1007/s10853-020-05189-y. https://doi.org/10.1007/s10853-020-05189-y
  2. Qi Zheng, Meijie Yu, Wen Wang, Yanjun Wang, Ze Zhang, Haiping Zhou, YouYong Dai, Chengguo Wang, Yong Xu (2020). Enhanced microwave absorption performance of Fe/C nanofibers by adjusting the magnetic particle size using different electrospinning solvents. Ceramics International 46:28603-28612. DOI: 10.1016/j.ceramint.2020.08.018. https://doi.org/10.1016/j.ceramint.2020.08.018
  3. Sijia Zhong, Meijie Yu, Xuechen Liang, Yuqing Dong, Jianjun Liu, Chengguo Wang (2022). Microwave absorption performance and multiple loss mechanisms of three-dimensional porous Fe4N@Fe3O4@Fe/carbon composite. Journal of Materials Science 57:16649-16664. DOI: 10.1007/s10853-022-07663-1. https://doi.org/10.1007/s10853-022-07663-1
  4. Xuechen Liang, Chengguo Wang, Meijie Yu, Zhiqiang Yao, Ye Zhang (2022). Fe-MOFs derived porous Fe4N@carbon composites with excellent broadband electromagnetic wave absorption properties. Journal of Alloys and Compounds 910:164844. DOI: 10.1016/j.jallcom.2022.164844. https://doi.org/10.1016/j.jallcom.2022.164844
  5. Xuechen Liang, Chengguo Wang, Meijie Yu, Siyu Liu, Ye Zhang, Sijia Zhong (2023). Enhanced microwave absorption properties of spherical Fe4N derived from spherical Fe. Journal of Materials Science: Materials in Electronics 34:31. DOI: 10.1007/s10854-022-09476-z. https://doi.org/10.1007/s10854-022-09476-z
  6. Yuqing Dong, Chengsen Li, Shuaiyu Gao, Meijie Yu, Genban Sun (2026). 3D porous Fe4N/C via nitridation engineering for high-performance microwave absorption. Journal of Alloys and Compounds 1067:188388. DOI: 10.1016/j.jallcom.2026.188388. https://doi.org/10.1016/j.jallcom.2026.188388
  7. Shixiang Dong, Jing Li, Shuai Zhang, Ning Li, Bo Li, Qianli Zhang, Liqin Ge (2022). Excellent microwave absorption performance of PAN-based Fe/C nanofibers with low loading fillers. Colloids and Surfaces A: Physicochemical and Engineering Aspects 655:130280. DOI: 10.1016/j.colsurfa.2022.130280. https://doi.org/10.1016/j.colsurfa.2022.130280
  8. Shuoyu Lian, Xiang Li, Hongbo Li, Xingwang Cheng (2025). Particle size and loading ratio dependent microwave absorption performance of FeSiAl alloy and the mechanism study. Materials Today Communications 45:112230. DOI: 10.1016/j.mtcomm.2025.112230. https://doi.org/10.1016/j.mtcomm.2025.112230
  9. Xuewei Su, Yanyan Liu, Zijian Liao, Yuxin Bi, Yan Chen, Yong Ma, Kwok L. Chung, Fei Wan, Mingliang Ma (2023). A review of 1D magnetic nanomaterials in microwave absorption. Journal of Materials Science 58:636-663. DOI: 10.1007/s10853-022-08054-2. https://doi.org/10.1007/s10853-022-08054-2
  10. Benqiang Wang, Zhuangzhuang Wu, Yang Xu, Jin Hou, Tingdong Zhou, Huying Yan, Tianxiu Song, Jialing Tang, Ling Zhong, Bingfei Zou (2025). The enhanced microwave absorption in S-band of flaky FeSiCr@FexN composites. Journal of Materials Science 60:23447-23462. DOI: 10.1007/s10853-025-11716-6. https://doi.org/10.1007/s10853-025-11716-6
  11. Shuting Zhang, Xiangwei Meng, Chengguo Wang, Meijie Yu (2025). Design and synthesis of Fe4N/MnO/carbon nanofibers for efficient electromagnetic wave absorption. Chemical Engineering Journal 524:169749. DOI: 10.1016/j.cej.2025.169749. https://doi.org/10.1016/j.cej.2025.169749
  12. Shuting Zhang, Chengguo Wang, Meijie Yu (2026). Design and synthesis of core-shell Fe4N@C nanowires for efficient electromagnetic wave absorption and corrosion resistance. Carbon 250:121306. DOI: 10.1016/j.carbon.2026.121306. https://doi.org/10.1016/j.carbon.2026.121306