Controllable Preparation of Fe₃O₄@RF and Its Evolution to Yolk–Shell-Structured Fe@C Composite Microspheres with High Microwave Absorbing Performance

Abstract

Fe₃O₄@RF microspheres with different phenolic (RF) layer thicknesses are prepared by adjusting the polymerization time. With the prepared Fe₃O₄@RF as the precursor, Fe@C composite microspheres with rattle-like morphology are obtained through one-step controlled carbonization. This method simplifies the preparation of rattle-shaped microspheres from sandwich microspheres. Fe@C microspheres exhibit excellent microwave absorbing properties. The morphology and composition of the product are investigated depending on the effects of carbonization temperature, time and thickness of the RF layer. When the carbonization temperature is 700 °C, the carbonization time is 12 h and the polymer shell thickness is 62 nm, the inner hollow Fe₃O₄ is completely reduced to Fe. The absorption properties of the materials are compared before and after the reduction of Fe₃O₄. Both Fe@C-12 and Fe₃O₄@C-700 show excellent absorbing properties. When the filler content is 50%, the maximum reflection loss (RL_max) of the rattle-shaped Fe@C microspheres is −50.15 dB, and the corresponding matching thickness is 3.5 mm. At a thickness of 1.7 mm, the RL_max of Fe₃O₄@C-700 is −44.42 dB, which is slightly worse than that of Fe@C-12. Both dielectric loss and magnetic loss play a vital role in electromagnetic wave absorption. This work prepares rattle-shaped absorbing materials in a simple way, which has significance for guiding the construction of rattle-shaped materials.

1. Introduction

As an effective means to prevent electromagnetic radiation and realize electromagnetic stealth, microwave absorbers have attracted more and more attention in civil and military fields [1,2]. With the diversification of radiation sources and radiation forms, and the upgrading of monitoring devices in electromagnetic countermeasures, new and high requirements for the performance of microwave absorbers are constantly being put forward. The wave absorber is required to have a high absorption capacity and a wide absorption band, as well as lightness, thin thickness, high thermal stability, oxidation resistance and corrosion resistance. In terms of improving the performance of the wave absorber, the choice of suitable raw materials and a reasonable microstructure are crucial [3,4,5].

Studies have shown that porous or hollow nanostructures exhibit strong interface polarization and multiple reflection capabilities, which can help improve the electromagnetic wave (EMW) absorption capability of materials [6]. Carbon material is an ideal choice for practical application because of its inherent hydrophobicity, good thermal stability and electrical conductivity, low original density and excellent chemical corrosion resistance [7]. Meanwhile, the low density caused by the above structure will give the wave-absorbing coating lightweight performance. The precursors of hollow carbon materials are prepared using the template method and then by etching and carbonization. Xu et al. [8] prepared SiO₂ nanoparticles by Stöber hydrolysis and used them as seeds to react with resorcinol and formaldehyde to prepare SiO₂@RF microspheres; they were then carbonized in an argon atmosphere and etched with hydrofluoric acid to obtain mesoporous carbon hollow microspheres (PCHMs). When the filler content was 20 wt% and the thickness was 3.9 mm, the maximum reflection loss (RL_max) of PCHMs at 8.2 GHz reached −84 dB, and the effective absorption bandwidth (EAB) was 4.8 GHz. Wang et al. [9] used super-crosslinked tubular polymer nanofibers as a precursor to carbonization to obtain tubular carbon nanofibers (TCNFs) with secondary pores. When the filler content was 10%, the RL_max was −61.5 dB, and the EAB was 4.25 GHz.

Magnetic materials are considered to be one of the most promising materials for broadband microwave absorption, but, generally, the RL_max of pure magnetic nanoparticles are relatively small, mainly due to their poor electrical conductivity. Therefore, the preparation of magnetic composite materials by compounding magnetic nanoparticles with carbon with good conductivity has become a research hotspot. Researchers have carried out a lot of work in this field, designing and preparing magnetic composite microspheres with core–shell [10], hollow [11], sandwich [12], flower-like [13,14,15] and bell-shaped [16] structures, as well as one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) composites with similar structures to wave absorbers [17]. On the basis of TCNFs, Zhang et al. prepared magnetic wave absorbers with multi-shell heterostructures, including Fe@C [18], Fe/Fe₃O₄@TCNFs@TiO₂ [19] and TCF@Fe₃O₄@NCLs [20], and all of them showed excellent reflection loss and absorption bandwidth. Li et al. [21] prepared hollow Fe@C (HFC) microspheres through emulsion polymerization, electrostatic assembly, polydopamine (PDA) coating and carbonization. HFC had both magnetic loss and dielectric loss, and RL_max was as high as −54.4 dB at 3.0 mm thickness and 8.8 GHz.

The rattle-shaped structure is a kind of special multi-layer core–shell structure, which is characterized by a unique core–void–shell configuration, low density, large surface area and many gaps. This structure is beneficial to reduce the density of the absorbent, adjust the absorption rate and effective dielectric constant and improve impedance matching [22]. Dai et al. [23] prepared CIP@void@NC (carbonyl iron powder@void@nitrogen doped carbon) microspheres with a bell structure by using carbonyl iron powder (CIP) as raw material through multi-layer coating and etching. Compared with CIP alone, the microspheres showed enhanced characteristics in EAB and absorption strength. The magnetic particles are encapsulated inside the carbon shell with movable space, which maintains the electrical conductivity of the material, increases the magnetic loss and multiple reflection at the interface and also protects the magnetic particles from external corrosion. Therefore, the construction of magnetic metal-based particles@air@carbon with a rattle-shaped structure provides a new strategy for the synthesis of a high-efficiency absorber. However, the construction of rattle-shaped microspheres is usually based on sandwich microspheres, and the intermediate layer of the sandwich microspheres is etched away by a chemical or physical means to achieve its preparation. The process requires two steps of cladding and one step of etching, and the process is relatively cumbersome.

In this work, we simply prepare a rattle-shaped magnetic composite microsphere Fe@C through a one-step coating and carbonization process. Hollow Fe₃O₄ particles are prepared using the hydrothermal method, and a series of Fe₃O₄@RF microspheres with different phenolic (RF) layer thicknesses are obtained by adjusting the coating time. The carbonization time and temperature are controlled, and the reductive effect of the phenolic aldehyde conversion carbon layer is used to reduce the hollow Fe₃O₄ to Fe nanoparticles. The coating and etching steps of the interlayer are omitted, and the preparation method is simplified. The microwave absorption properties of the rattle-shaped Fe@C microspheres are studied, and the absorption mechanism of the material is revealed.

2. Experimental Section

2.1. Materials

Urea, FeCl₃·6H₂O, formaldehyde, trisodium citrate dihydrate and ethanol absolute were obtained from Guangzhou Chemical Reagent Factory (Guangzhou, China). Hydroquinone and sodium polyacrylate were bought from Adamas Reagent Co., Ltd. (Shanghai, China). The chemicals were used as received without any purification. The water used in the work was deionized water.

2.2. Preparation of Fe₃O₄@RF Microspheres

The Fe₃O₄ particles with a hollow structure were synthesized using the hydrothermal method [24]. A total of 1.35 g of FeCl₃·6H₂O, 0.60 g of sodium polyacrylate, 1.00 g of urea and 3.20 g of trisodium citrate dihydrate were dissolved in 75 mL of deionized water. Then, the mixture was transferred to a 100 mL high-pressure hydrothermal reactor and reacted at 200 °C for 12 h. After the reaction, it was cooled to room temperature, washed with deionized water several times and magnetically separated to obtain hollow Fe₃O₄ particles.

We weighed 0.30 g of Fe₃O₄ particles and 0.60 g of hydroquinone in 120 mL of ethanol absolute, dissolved and dispersed them by ultrasound and transferred them to a 250 mL three-necked flask. Then, 550 μL of formaldehyde was added and, after mechanical stirring for 5 mins, 8 mL of ammonia water was added to it dropwise and reacted for a certain time at room temperature. The product was magnetically separated and washed with ethanol to obtain Fe₃O₄@RF composite microspheres. In order to control the thickness of the phenolic layer, the reaction time was controlled to 4 h, 6 h, 10 h and 12 h, and the products obtained were named Fe₃O₄@RF-4, Fe₃O₄@RF-6, Fe₃O₄@RF-10 and Fe₃O₄@RF-12.

2.3. Fabrication of Fe@C Composite Microspheres

We weighed 0.2 g of Fe₃O₄@RF composite microspheres and placed them in a quartz boat. We carried out high-temperature carbonization in a vacuum tube furnace at a heating rate of 5 °C/min. After the carbonization was completed, it was cooled to room temperature in the furnace to obtain magnetic carbon microspheres. The carbonization temperature was 550 °C, 650 °C and 700 °C, and the carbonization time was 5 h. The products obtained were Fe₃O₄@C-550, Fe₃O₄@C-650 and Fe₃O₄@C-700. Fe₃O₄@RF-6, Fe₃O₄@RF-10 and Fe₃O₄@RF-12 were carbonized for 12 h, and the products with a carbonization temperature of 700 °C were Fe₃O₄@C-6, Fe@C-10 and Fe@C-12, respectively.

2.4. Analysis and Characterization

The morphology of the material was observed using AMERy-1000B scanning electron microscope (SEM) (FEI, Hillsboro, FL, USA) and JEM-2100F transmission electron microscope (TEM) (FEI, Hillsboro, FL, USA). The magnetic properties were measured by LakeShore7307 vibrating sample magnetometer (VSM) (CFMS-14T Cryogenic, GEKO, Dusseldorf, Germany). The crystal structure of the material was confirmed by XRD-7000 X-ray diffractometer (D8 DISCOVER A25, Bruker, Karlsruhe, Germany), with Cu target Kα radiation, tube pressure 40 KV and tube flow 40 mA. The thermal stability of the material and the proportion of each component were measured by the Q50 thermogravimetric analyzer (TGA) (Mettler tolido Instrument Company, Zurich, Switzerland). The temperature range was 35–650 °C, the heating rate was 10 °C/min, and the oxygen flow rate was 50 mL/min. Functional groups were determined by Tensor 27 Fourier transform infrared spectrometer (Bruck GMBH, Herne, Germany) manufactured by Bruck GMBH in Germany, and KBr powder tablets were pressed. The sample was mixed with solid paraffin in a certain proportion and pressed into ring samples. The electromagnetic parameters were measured by N5227 vector network analyzer manufactured by Agilent Technology Co., Ltd. (Agilent Technology Co., Ltd., Beijing, China). The pore properties of the microspheres were measured by tristAR-3020 N₂ adsorption–desorption apparatus manufactured by Micromeritics Co., Ltd. (Micromeritics Co., Ltd., Beijing, China).

3. Results and Discussion

3.1. Shell Control of Fe₃O₄@RF Microspheres

The morphology of the Fe₃O₄ particles prepared using the hydrothermal method is shown in Figure 1A,B. The particle size distribution was uniform, the surface was rough, and the average particle size was about 230 nm (Figure 1A inset). The TEM images showed that the grain had a non-single crystal structure, which showed that the grain was stacked with a morphology of deep surrounding contrast and shallow middle contrast, indicating that it had an obvious hollow structure (Figure 1B). A series of hollow Fe₃O₄@RF composite microspheres with different shell thicknesses were prepared by controlling the polymerization reaction time of hydroquinone and formaldehyde. TEM images of the products are shown in Figure 1C–F. Compared with Figure 1B, it can be clearly seen that Fe₃O₄ had a phenolic shell layer on its surface, which means that the precipitation polymerization method successfully prepared Fe₃O₄@RF microspheres with a core–shell structure. The coated products still had good dispersion. With the increase in polymerization time, it was obvious that the thickness of the phenolic shell increased. When the polymerization time was 4 h, the surface change in Fe₃O₄ was not obvious, and only a few nanometers of the non-fully coated polymer layer could be seen in magnification (Figure 1C). When the reaction time exceeded 6 h, the phenolic shell became obvious and had been completely coated. The phenolic layer thickness of Fe₃O₄@RF-6, Fe₃O₄@RF-10 and Fe₃O₄@RF-12 was 18 nm, 62 nm and 67 nm, respectively.

3.2. Characterization of Fe₃O₄@C Composite Microspheres

Phenolic is a polymer material with a high carbon content, which is why it was chosen for the shell. Fe₃O₄@RF microspheres were carbonized under vacuum conditions, and the phenolic layer was converted into a carbon layer. Magnetic microspheres composed of different components were prepared. Using Fe₃O₄@RF-12 as a precursor, the effect of calcination temperature on the properties of the product was investigated. TEM and SEM images of Fe₃O₄@C microspheres prepared at different carbonization temperatures are shown in Figure 2. It can be seen that the morphology of the microspheres was not significantly damaged by the high-temperature carbonization, and the obtained Fe₃O₄@C still maintained a good spherical morphology and a narrow particle size distribution (Figure 2B,D,F). The shell thickness of the carbonated products at 550 °C, 650 °C and 700 °C was 35 nm, 32 nm and 31 nm, respectively. With the increase in the carbonization temperature, the decomposition degree of the phenolic layer on the surface of the microspheres deepened slightly. When the organic matter was fully lost, part of the carbon was also lost, which led to the gradual thinning of the carbon layer remaining in the product (Figure 2A,C,E). A careful observation of Figure 2C,E showed that the aggregation degree of Fe₃O₄ in the microsphere increased, the particle size became larger, the hollow structure was more obvious, and the lattice stripes were vaguely visible. The lattice fringes were measured, and there was a 0.2 nm lattice spacing corresponding to the (110) crystal plane of Fe (Figure 2E inset), indicating that part of Fe₃O₄ may be reduced to Fe by high-temperature carbon.

In order to confirm the crystal information of the inorganic components in the prepared Fe₃O₄@C microspheres, XRD detection was carried out as can be seen in Figure 3A. The diffraction peaks of Fe₃O₄@C-550 and pure Fe₃O₄ (JCPDS card no. 19-0629) were exactly the same. The main diffraction peaks of Fe₃O₄@C-650, Fe₃O₄@C-700 and Fe₃O₄@C-550 were basically the same, indicating that the magnetic components in the Fe₃O₄@C microspheres were mainly Fe₃O₄. Figure 3B shows the FTIR spectra of hollow Fe₃O₄, Fe₃O₄@RF and Fe₃O₄@C. After coating with phenolic aldehyde, the elastic shock absorption peak of the C-H bond on the aromatic hydrocarbon appeared near 3100 cm⁻¹. Moreover, 1612 cm⁻¹, 1448 cm⁻¹ and 1406 cm⁻¹ were the stretching shock absorption peaks of the benzene ring C=C skeleton, indicating the presence of a benzene ring (phenolic). After carbonization, the absorption peak of the sample was significantly reduced, and the absorption peak of the organic matter basically disappeared, indicating that the carbonization was relatively complete. The absorption peak of 1614 cm⁻¹ was attributed to the stretching vibration of the unsaturated C=C bond. The TGA curves of the Fe₃O₄@C microspheres prepared at different carbonization temperatures are shown in Figure 3C. The mass remaining after high-temperature calcination was used as the content of the magnetic component, and the mass increase caused by the conversion of Fe₃O₄ to Fe₂O₃ during this process was ignored. Fe₃O₄@C began to lose weight when the calcination temperature reached about 350 °C. With the increase in the carbonization temperature during the preparation process, the temperature of the initial weight loss increased, indicating that the carbonization temperature can improve the thermal stability of Fe₃O₄@C microspheres. The magnetic content of Fe₃O₄@C-550, Fe₃O₄@C-650 and Fe₃O₄@C-700 was 19.58%, 21.56% and 25.22%, respectively [25,26].

3.3. Morphology and Structure of Rattle-Shaped Fe@C Composite Microspheres

It can be seen from Figure 2 and Figure 3A that increasing the carbonization temperature may cause the carbon formed by the carbonization of the phenolic shell to reduce the internal Fe₃O₄. Based on this discovery, by extending the carbonization reaction time to deepen the degree of reduction, rattle-shaped Fe@C composite microspheres were prepared. Figure 4 shows the TEM and SEM images of Fe₃O₄@RF microspheres with different shell thicknesses carbonized at 700 °C for 12 h. Compared with the situation prior to carbonization (Figure 1D), the morphology of Fe₃O₄@RF-6 after carbonization has no significant change (Figure 4A,B). This may be due to the thin and incomplete coating of the polymer shell, resulting in less carbon converted, which had no significant effect on the reduction of Fe₃O₄ to Fe. When the thickness of the Fe₃O₄@RF phenolic shell increased, it was found that the Fe₃O₄ with high-quality and thick contrast in the microspheres changed significantly. The inorganic crystals clustered together and separated from the surrounding low-contrast carbon shell, presenting a rattle-like structure (Figure 4C,E). When the carbonization time was prolonged, the thickness of the carbon shell decreased, but the overall microsphere morphology was well maintained, as shown in Figure 4C–F. The above results show that the thickness of the polymer shell had a significant effect on the morphology of subsequent carbonized products, and the influence mechanism was mainly the amount of the carbon layer determining the reduction degree of Fe₃O₄.

The crystal form of the carbonized products of Fe₃O₄@RF microspheres with different shell thicknesses was determined, and the XRD pattern is shown in Figure 5A. The XRD peaks of the Fe₃O₄@RF-6 carbonization products at 18.3°, 30.1°, 35.5°, 43.1°, 53.4°, 57.1° and 62.8° corresponded to the inverse spinel structure Fe₃O₄ (JCPDS75-0033) crystal faces of (111), (220), (311), (400), (422), (511) and (440) in the face-centered cubic lattice [27,28]. This showed that Fe₃O₄ had not been reduced at this time. The XRD patterns of the products with the carbonization time of 10 h and 12 h had the same peak positions. The diffraction peaks at 44.7°, 65.2°, and 82.5° in the spectrum corresponded to the (110), (200), and (211) crystal planes of elemental Fe. This was highly consistent with the standard XRD pattern of Fe (JCPDS 87-0722) in the peak position and intensity. There were no other impurity peaks, indicating that Fe₃O₄ was completely reduced to Fe crystals with higher crystallinity. The magnetic properties of the prepared products were analyzed using the vibrating sample magnetometer. The VSM curve is shown in Figure 5B. The maximum specific saturation magnetization of Fe₃O₄@C-6, Fe@C-10 and Fe@C-12 was 77 emu/g, 123 emu/g and 177 emu/g, respectively. Before Fe₃O₄ was reduced to Fe, the magnetic properties of the microspheres were relatively low, but after the reduction, the magnetic properties were significantly enhanced. Meanwhile, the coercivity of the three samples, Fe₃O₄@C-6, Fe@C-10 and Fe@C-12, was 25 Oe, 265 Oe and 14 Oe, respectively, and the three were quite different. The coercive force represented the magnetic loss performance of the sample to a certain extent, but the subsequent analysis showed that the dielectric loss still dominated the EMW absorption.

The nitrogen adsorption–desorption isotherms and pore size distribution curves of Fe₃O₄@RF microspheres before and after carbonization are shown in Figure 6. The nitrogen adsorption–desorption isotherm of Fe₃O₄@RF-12 belonged to type III (Figure 6A), indicating that there were almost no pores in the material. The pore information was caused by the stacking between particles, and its BET specific surface area was 7.60 m²/g (

Table 1. Pore performance data of samples before and after calcination.

Sample	BET (m2/g)	Pore Volume (cm3/g)	Average Pore Size (nm)
Fe3O4@RF-12	7.60	0.0168	14.71
Fe3O4@C-700	114.25	0.0970	5.13
Fe@C-12	116.95	0.0986	5.71

). The small specific surface area and pore volume proved this again (Figure 6B). This was because phenolic aldehyde effectively coated Fe₃O₄ with a hollow and stacked structure, so its surface area was only the outer surface of the microspheres and the surface of the stacked holes. As shown in Figure 6C,E, the nitrogen adsorption–desorption isotherm of Fe₃O₄@C-700 and Fe@C-12 microspheres belonged to type IV, which was a typical mesoporous material with an H2-type hysteresis ring, indicating that the material had ink bottle-shaped pores [29]. The high surface area of Fe@C-12 comes from the pores created by the loss of organic matter from RF. The pore size distribution curve of Fe@C-12 (Figure 6F) basically showed a single peak, indicating that the pore size was narrow and concentrated.

3.4. Evaluation of Microwave Absorption Performance

In order to systematically evaluate the absorbing performance of the composite microspheres, the electromagnetic parameters of Fe@C-12 and Fe₃O₄@C-700 microspheres were measured at 50 wt% filler content with paraffin as the matrix. Based on the transmission line theory, the reflection loss of the samples with different matching thicknesses was calculated according to the following formula [30,31,32]:

$$\mathrm{Z}=\left|\frac{{\mathrm{Z}}_{\mathrm{in}}}{{\mathrm{Z}}_{\mathrm{O}}}\right|=\sqrt{\left|{\mathsf{\mu }}_{\mathrm{r}}∕{\mathsf{\epsilon }}_{\mathrm{r}}\right|} \tan \mathrm{h}\left[\mathrm{j}\left(2\mathsf{\pi }\mathrm{fd}∕\mathrm{c}\right)\sqrt{{\mathsf{\mu }}_{\mathrm{r}}{\mathsf{\epsilon }}_{\mathrm{r}}}\right]$$

(1)

$$\mathrm{RL}\left(\mathrm{dB}\right)=20 {\log }_{10}\left|\frac{{\mathrm{Z}}_{\mathrm{in}}-{\mathrm{Z}}_0}{{\mathrm{Z}}_{\mathrm{in}}+{\mathrm{Z}}_0}\right|$$

(2)

where Z_in is the input impedance of the wave absorber, Z₀ is the impedance of free space, f is the frequency of EMW, c is the speed of light, and d is the thickness of the wave absorber. As can be seen from Figure 7A,B, as the thickness of the absorber layer continued to increase, the reflection loss curve of Fe@C-12 moved to the high-frequency direction. When the thickness was 3.5 mm, the peak value of the curve was the largest; that is, the ability to lose EMW was the strongest, and RL_max reached −50.15 dB. At this time, the effective absorption frequency range was 7.8 GHz–11.9 GHz (reflection loss was less than −10 dB), covering almost the entire X-band (Figure 7B), which was the working band of military radars, such as THAAD. Figure 7C,D showed 3D and 2D reflection loss curves of Fe₃O₄@C-700. At a thickness of 1.7 mm, RL_max was −44.42 dB, which was slightly worse than that of Fe@C-12. However, at a thickness of 1.8 mm, the EAB was 6.0 GHz (12 GHz–18 GHz), covering the entire Ku band; at this time, RL_max was −35.12 dB. The above results indicate that the magnetic composite microspheres prepared in this paper had excellent EMW absorption effects and can meet the requirements of related fields. Meanwhile, the effective frequency range can also be expanded to other wavebands by changing the thickness of the absorbing layer to enrich application scenarios.

Table 2. The EMW absorbing properties of similar materials.

Materials	Thickness (mm)	Effective Bandwidth (GHz)	RLmin (dB)	Reference
FeCPNFs	2.0	3.0	−26.1	[18]
Fe/Fe3O4@TCNFs@TiO2	1.6	3.7	−44.8	[19]
TCF@Fe3O4@NCLs	3.3	4.6	−43.6	[20]
Fe@C (HFC)	4.5	8.1	−54.4	[21]
Fe73.5Si13.5Nb3Cu1B9	0.14	4.4	−45.97	[33]
Nickel Slag	5.0	1.7	−34.00	[34]
Fe3O4@C-700	1.8	6.0	−35.12	This work
Fe@C-12	3.5	4.1	−50.15	This work

compares the absorbing properties with Fe-based materials and other magnetic materials [18,19,20,21,33,34]. Obviously, both Fe@C-12 and Fe₃O₄@C-700 showed absolute advantages in performance. The reason is the great relationship with the design of the material. That is, the composite of the dielectric component and the magnetic component and the design of the rattle-like structure had an important influence on its absorbing performance.

In order to reveal the EMW loss mechanism of the magnetic composite microspheres, the electromagnetic parameters of the composite microspheres were analyzed. The real parts (ε′ and μ′) of the complex dielectric constant (ε_r = ε′ − jε″) and the complex permeability (μ_r = μ′ − jμ″) represented the capacity of the material to store electric and magnetic energy. The imaginary parts (ε″ and μ″) represented the capacity of the material to lose electric and magnetic energy [35]. As can be seen from Figure 8, the ε′ and ε″ of Fe@C-12 and Fe₃O₄@C-700 gradually decreased with the increase in electromagnetic frequency (Figure 8A,C), while μ′ and μ″ approached 1 and 0, respectively (Figure 8B,D). In addition, in Figure 8B,D, μ″ was almost unchanged, and negative values appeared in some regions, indicating the existence of eddy current losses. This was because the vortex created an opposing magnetic field that canceled out the natural field, causing the permeability to become negative [12,13,14,15,16]. Meanwhile, it can be found by comparison that the material permeability parameter was much smaller than the dielectric constant parameter, indicating that the dielectric loss dominated the EMW loss process of the two materials.

Dielectric loss and magnetic loss can be expressed by the tangent value of the dielectric loss (tanδ_e = ε″/ε′) and magnetic loss (tanδ_m = μ″/μ′). Figure 9A,C show the relationship curves of the dielectric loss tangent value and the magnetic loss tangent value of Fe@C-12 and Fe₃O₄@C-700 with frequency. It can be seen from the curve that the dielectric loss tangent value was obviously greater than the magnetic loss tangent value, which further proved the dominance of dielectric loss. The dielectric loss tangent value of Fe@C-12 first decreased and then increased as the frequency increased, while the magnetic loss tangent value presented the opposite trend, and the difference value between the two appeared to be the minimum at about 12 GHz (Figure 9B). The smaller the value, the smaller the difference between the dielectric loss and the magnetic loss, which meant better impedance matching. The dielectric loss tangent value and the magnetic loss tangent value of Fe₃O₄@C-700 fluctuated with the increase in frequency, and the minimum value of the difference appeared at 13 GHz. The EMW loss ability of materials was mainly controlled by the dielectric loss, and the dielectric loss was greater in the low-frequency region. Under the combined action of the EMW loss ability and impedance-matching factors, the RL_max appeared in the X and Ku bands, respectively.

Dielectric loss mainly included four ways of interface polarization, dipole polarization, conduction loss and Debye relaxation. Among them, the interface polarization occurred on the interface with different electromagnetic parameters on both sides. For the above two materials, there were many heterogeneous interfaces between the carbon shell, cavities, Fe₃O₄/Fe and matrix paraffin. In addition, a large amount of unevenly distributed space charges accumulated between the interfaces, resulting in the continuous occurrence of polarization and dipolar polarization. Dipolar polarization was also related to defects formed inside the material, including crystal defects and atomic doping. The Debye relaxation phenomenon occurred in various polarization processes, and the Debye relaxation process can be easily judged from ε″-ε′ curves. If Debye relaxation occurred, then ε″ and ε′ would have satisfied the following equation [36,37,38]:

$${\left({\mathsf{\epsilon }}^{\prime }-\frac{{\mathsf{\epsilon }}_{\mathrm{s}}+{\mathsf{\epsilon }}_{\infty }}{2}\right)}^2+{\left({\mathsf{\epsilon }}^{″}\right)}^2={\left(\frac{{\mathsf{\epsilon }}_{\mathrm{s}}-{\mathsf{\epsilon }}_{\infty }}{2}\right)}^2$$

(3)

where ε_s is the static permittivity, and ε_∞ is the relative permittivity at the high-frequency limit. In this case, the ε″-ε′ curve should be semicircle, namely, a Cole–Cole semicircle, and each semicircle corresponded to a Debye relaxation process. Figure 10 shows the ε″-ε′ curves of the two magnetic composite microspheres. From the enlarged illustration, multiple obvious semicircles can be observed, confirming the existence of multiple Debye relaxation processes. A long linear part appeared after the semicircle. The existence of the linear part was a typical feature of strong conductive loss, which indicated that the materials formed an effective 3D conductive network in the matrix paraffin.

The magnetic loss came from the hysteresis effect and the eddy current effect. The hysteresis effect can be judged by the area surrounded by the hysteresis loop in the VSM curve of the absorber, and its value was the loss energy of the hysteresis effect. The eddy current loss effect can be described by the following equation [39]:

C₀ = μ″(μ′)⁻²f⁻¹ = 2πμ₀d²σ

(4)

where μ₀ is the transmittance in the vacuum, and σ is the electrical conductivity. If the eddy current loss existed in the EMW absorption process, C₀ was always constant with the change of frequency. Figure 11 displays the curves of the magnetic composite microsphere C₀ and frequency. In Figure 11A, the curve fluctuated slightly in the high-frequency band and was close to a straight line, indicating that the eddy current loss existed in the high-frequency region. The fluctuation in the low-frequency region was mainly related to natural resonance. For Fe₃O₄@C-700 (Figure 11B), there were fluctuations in the high and low-frequency curves, indicating that ferromagnetic resonance had played an important role. The fluctuations at the low frequency were mainly related to natural resonance, while those at high frequency were related to exchange resonance [40].

In summary, Figure 12 shows the absorbing mechanism of rattle-shaped magnetic composite microspheres. Excellent impedance matching performance made EMW penetrate the material and be absorbed. The loss of EMW was mainly provided by dielectric loss and magnetic loss. Magnetic loss mainly included hysteresis loss and eddy current loss, as well as natural resonance and exchange resonance. Dielectric loss mainly included interface polarization, dipole polarization, conduction loss and Debye relaxation. The existence of heterogeneous interfaces and the accumulation of a large number of unevenly distributed space charges between the interfaces enhanced the occurrence of interface polarization. Abundant lattice defects and atomic doping were conducive to the occurrence of dipole polarization. In addition, the design of the rattle-like structure was conducive to the multiple reflections of EMW in the cavity structure, allowing the EMW to be converted into heat and dissipated. Meanwhile, the effective construction of the 3D conductive network provided a strong conductive loss, making the material exhibit excellent wave-absorbing properties.

4. Conclusions

To summarize, we prepared a sandwich structure Fe₃O₄@RF precursor, combined with a high-temperature annealing process, to obtain Fe₃O₄@C-700 and a rattle-shaped Fe@C-12 composite wave absorber. Comparing the absorbing performance of the two, the results show that the construction of the yolk–shell structure is conducive to the occurrence of multiple interface reflections and effectively dissipates EMW. The influence of electromagnetic parameters on the absorbing performance was analyzed in detail, and the absorbing mechanism was revealed. The synergistic effect of multiple components makes the material possess excellent impedance matching, strong interfacial polarization, dipole polarization and enhanced conduction loss, which, together, enhance the dielectric loss performance of the material. Coupled with excellent magnetic loss, the material exhibits excellent absorbing properties. We believe that Fe@C-12 is expected to become one of the candidate materials for the next generation of new wave-absorbing materials. It has potential application values in civil applications, such as anti-electromagnetic pollution, building wave absorption, electromagnetic protection and electronic equipment.