Ordered Heterostructured Aerogel with Broadband Electromagnetic Wave Absorption Based on Mesoscopic Magnetic Superposition Enhancement

Abstract Demand for lightweight and efficient electromagnetic wave (EW) absorbers continues to increase with technological advances in highly integrated electronics and military applications. Although MXene‐based EW absorbers have been extensively developed, more efficient electromagnetic coupling and thinner thickness are still essential. Recently, ordered heterogeneous materials have emerged as a novel design concept to address the bottleneck faced by current material development. Herein, an ordered heterostructured engineering to assemble Ti3CNTx MXenes/Aramid nanofibers/FeCo@SiO2 nanobundles (FS) aerogel (AMFS‐O) is proposed, where the commonly disordered magnetic composition is transformed to ordered FS arrays that provide more powerful magnetic loss capacity. Experiments and simulations reveal that the anisotropy magnetic networks enhance the response to the magnetic field vector of EW, which effectively improves the impedance matching and makes the reflection loss (RL) peaks shift to lower frequencies, leading to the thinner matching thickness. Furthermore, the temperature stability and excellent compressibility of AMFS‐O expand functionalized applications. The synthesized AMFS‐O achieves full‐wave absorption in X and Ku‐band (8.2–18.0 GHz) at 3.0 mm with a RLmin of −41 dB and a low density of 0.008 g cm−3. These results suggest that ordered heterostructured engineering is an effective strategy for designing high‐performance multifunctional EW absorbers.

Snoek limit and its modified formulas: For most isotropic magnets, the microwave magnetic properties of the randomly distributed particles can be expressed as Equation S5, which is named by the Snoek limit [1]   i saturation magnetization, respectively. For this kind of material, the magnetic permeability drops rapidly to a very small value when the frequency reaches GHz frequency. However, when an easy magnetization plane exists in the material, its high-frequency property is expressed by Equation S6 [1a, 2] : H θ and H φ are out-of-plane anisotropy fields and in-plane anisotropy fields. Normally, H θ is always much larger than H φ for the soft magnetic materials with planar anisotropy.
Considering the influence of shape factors on the high-frequency permeability of singledomain particles, literature proposed the modified polar limit formula as Equation N k is the demagnetization factor along the easy magnetization axis of particles. H k is an anisotropic field. It can be proved by Equation S7 that particles with strong anisotropy and small demagnetization factor in the direction of easy magnetization can obtain better permeability in the GHz frequency band.

CST simulation:
Electromagnetic Energy Density: ·Electromagnetic waves fluctuate in a sinusoidal manner. When the electric field has a maximum strength, so does the magnetic field. Both are zero at the same instant.
·The energy carried by the wave fluctuates in the same manner.
·A way to quantify the energy of an electromagnetic wave is to measure the total energy density carried by an electromagnetic wave (energy per unit volume).
The following information is from the internal help file of CST software: Energy within magnetic materials is a consequence of complex microscopic interactions, and which can't be modeled by FEM. The numerical computation of the magnetic energy relies on the behavior law of the considered material. If no persistent magnetic field is considered, for linear or nonlinear materials, the magnetic energy density Ψ is defined as: The magnetic energy and coenergy densities can be computed using the B(H) curve defined within the numerical model. They represent the following areas: The sum of these two energies is defined as the product between B and H: BH   (10) The total magnetic energy and coenergy are computed integrating Φ and Ψ throughout the whole domain: The simulation was carried out by using CST microwave studio suite. Metamaterial-full structure workflow was chosen for simulation. Frequency domain solver was used in this simulation process. The overall properties of this simulation are shown in Figure S1.

Radar equation and radar cross section (RCS) simulation theory:
Thus, RCS can be intuitively understood as the characteristic of the target object itself, which has nothing to do with distance R. Besides, the dimension of RCS is area (m 2 ).
Here, E s and E i are the intensities of the scattered electric field and incident electric field, respectively. H s and H i stand for the intensities of the scattered magnetic field and incident magnetic field, and Ss and Si represent the power density of the scattered field and incident field. In addition, as explained in the Supporting Information, the decibel square meter unit (dB m 2 ) is typically used to measure radar cross section, where (dB m 2 ) = 10 log (m 2 ).
Due to the dynamic range of the radar cross section of the target is very large, so it is often expressed by decibel square meters (dB m 2 ): Other supporting materials：    we have analyzed the elemental composition of FeCo nanowires by EDS, as shown in Figure S5. The EDS images exhibit the uniform existence of Fe and Co in Figure S5b, c. On the other hand, it can be found that the diameter of the prepared FeCo wire is below 100 nm by TEM in Figure S5d, which can be recognized as nanowire. Furthermore, the XRD pattern of FeCo nanowires in Figure S5e reveal the similar structure with FeCo@SiO 2 nanobundles.

This work
Densities of pure wax, PDMS, and epoxy were 0.9, and 0.097 g/cm 3 , respectively.