Electrospinning Device for Nanofibers| Tailored Multi-Band Microwave Absorption Performance viaEntropy Engineering in Spinel Ferrite/Carbon NanofiberComposites

As an effective means to address electromagnetic radiation pollution, electromagnetic protection materials, especially electromagnetic wave absorbing materials, have long attracted significant attention. However, increasingly complex electromagnetic environments not only lead to higher electromagnetic intensity but also create diversified frequency trends, posing major challenges for the research and application of non-magnetic carbon-based electromagnetic wave absorbing materials that primarily function in high-frequency bands (12–18 GHz). Although these materials, which mainly rely on conductive loss, can demonstrate excellent electromagnetic wave absorption performance after solving impedance mismatch issues, their constant permeability limits their absorption capability in medium-to-low frequency bands, hindering their further development as novel electromagnetic wave absorbing materials.

Recently, a research team led by Professor Fenglong Wang and Professor Jiurong Liu from Shandong University, in collaboration with Professor Ke Bi from Beijing University of Posts and Telecommunications, published their latest findings titled "Tailored Multi-Band Microwave Absorption Performance via Entropy Engineering in Spinel Ferrite/Carbon Nanofiber Composites" in the journal Small. The researchers employed an entropy engineering strategy to design the tetrahedral interstitial sites of spinel ferrites and prepared carbon nanofibers with in-situ grown spinel ferrites (MNZCFO/C) through electrospinning and high-temperature carbonization processes. The incorporation of multiple elements increased the entropy-driven structural disorder in the ferrite crystal lattice, opening new possibilities for enhancing its electrical and magnetic properties.

Among the samples, MNZCFO/C-2 nanofibers exhibited an ultra-wide effective absorption peak covering 14.16 GHz (3.84–18 GHz) at thicknesses ranging from 1–5 mm, including the entire C-band (4–8 GHz), X-band (8–12 GHz), and Ku-band (12–18 GHz). Additionally, the MNZCFO/C-3 sample demonstrated an effective absorption bandwidth covering 4.72–18 GHz at the same thickness range, with a maximum effective absorption bandwidth of 7.28 GHz and a minimum reflection loss of −54.62 dB. These results highlight the significant potential of entropy engineering in developing electromagnetic wave absorbing materials with multi-band absorption characteristics.

Figure 1 Preparation process and microscopic morphology of MNZCFO/C nanofibers

A series of composite nanofibers were prepared via electrospinning, followed by pre-oxidation and carbonization to obtain MNZCFO/C nanofiber samples (Figure 1a). SEM and TEM images confirmed the presence of uniform nanofibers embedded with nanoparticles. EDS elemental mapping provided preliminary evidence of successful entropy engineering, with the relative brightness of colors reflecting the varying concentrations of each element (Figures 1g, m, and s). The subtle structural differences in the fibers, influenced by the types and proportions of added atoms, were observable in their morphology.

Figure 2: Microstructure of MNZCFO/C nanofibers and schematic illustration of entropy engineering strategy

Calculations based on the configurational entropy formula revealed that MNZCFO/C-2 and MNZCFO/C-3 samples exhibited medium-entropy (1.38 R) and low-entropy (0.65 R) characteristics, respectively. Figures 2a–e demonstrate that the entropy engineering strategy increased structural disorder in the spinel ferrite crystal lattice, leading to variations in interplanar spacing and the emergence of defects and lattice distortions in HR-TEM images. These defects and distortions significantly enhanced polarization loss in MNZCFO/C nanofibers.

Figure 3: Electromagnetic wave absorption performance and electromagnetic parameters of MNZCFO/C nanofibers

By applying entropy engineering to the microscopic crystal structure through component and structural design, the researchers increased the structural disorder of spinel ferrites simply by adjusting the types and ratios of transition metal ions, thereby optimizing their electromagnetic properties. Ultimately, the balance between dielectric loss and magnetic loss endowed the material with excellent multi-band electromagnetic wave absorption performance. The MNZCFO/C-2 and MNZCFO/C-3 nanofibers achieved ultra-wide effective absorption peaks covering 3.84–18 GHz and 4.72–18 GHz, respectively, enabling compatibility from high-frequency to medium-low-frequency absorption.

Figure 4: Multi-band absorption characteristics and electromagnetic wave absorption mechanism of MNZCFO/C nanofibers

The entropy engineering strategy successfully achieved compatibility between synergistic electromagnetic loss and optimal impedance matching, as well as an appropriate balance between dielectric and magnetic losses. The carbon nanofibers with in-situ grown spinel ferrites integrated multiple electromagnetic wave dissipation mechanisms through macroscopic conductive networks, abundant heterogeneous interfaces, and microscopic defects and lattice distortions. The MNZCFO/C samples exhibited outstanding electromagnetic wave absorption performance, with their excellent multi-band absorption properties making them promising for efficient performance across a broader frequency range.

Paper link: https://onlinelibrary.wiley.com/doi/10.1002/smll.202502349