The flaky porous Fe₃O₄ with tunable dimensions for enhanced microwave absorption performance in X and C bands

Ferromagnetic media occupy a crucial position in microwave absorption because of their unique magnetic property and optimal impedance matching [1–4]. However, the decline of permeability in the GHz region caused by Snoek's limit harms not only their magnetic attenuation competency, but also good impedance match. The evident drawbacks have impeded its practical exploitation. As we known, Snoek's limit can be written as follows [5]:

$$\begin{eqnarray}&&({\mu }_{i}-1){f}_{r}=(1/3\pi )\gamma {M}_{s},\end{eqnarray} \tag{ 1 }$$

where μ_i is the initial permeability, f_r is the resonance frequency, γ is the gyromagnetic ratio, and M_s is the saturation magnetization. Fortunately, the ferromagnetic flakes may break through the Snoek limitation and offer high permeability values in the GHz range [6]. Combining the Landau–Lifshitz–Gilbert equation with Snoek's law, the following formula can be speculated [7]:

$$\begin{eqnarray}&&({\mu }_{i}-1){f}_{{\rm{r}}}=\displaystyle \frac{2}{3}\gamma {M}_{{\rm{s}}}\sqrt{\displaystyle \frac{{H}_{ha}}{{H}_{ea}}},\end{eqnarray} \tag{ 2 }$$

where H_ha and H_ea stand for the out-plane anisotropy field and in-plane anisotropy field, respectively. With respect to the anisotropy materials, the value of H_ha is far greater than that of H_ea, and thus, μ_i and f_r should be much bigger than those of isotropy media [8]. As a result, compared with isotropy media, flaky media are much more likely to be a superior absorber because of the large shape anisotropy. Apart from holding the μ_i value at a high frequency, the flaky absorbers also possess other two attractions. On one hand, the flake framework can restrain the eddy current effect when the thickness is smaller than the skin depth, which would result in less of the microwave being reflected [9]. On the other hand, the individual microstructure is beneficial for improving the dielectric property. Because the flake configuration usually has a large size and surface area, it has better space charge polarization than granular powder [10].

Fe₃O₄, as a significant member of ferromagnetic materials, has representative magnetism and strong spin polarization grounded to the division of electrons between Fe³⁺ and Fe²⁺ in the octahedral sites [11]. For its hypotoxicity, favorable biocompatibility, and low cost, Fe₃O₄ magnetite has attracted considerable attention in the electromagnetic (EM) wave absorption domain [12]. Nevertheless, single Fe₃O₄ often suffers from unavoidable intrinsic shortcomings such as inferior dielectric and electric properties, and therefore the desired reflection loss (RL) value in a broad frequency range at a low thickness can hardly be achieved [13, 14]. For overcoming this weakness, integrating Fe₃O₄ with other typical dielectric dissipation materials, e.g. carbon nanotubes (CNTs), [15] rGO, [16] MnO₂, [17] TiO₂ [18], has been a popular approach. Although these methodologies have obtained remarkable microwave absorption performances, the relatively complex synthetic procedures, as well as raised costs, inhibit their diffuse applications. Alternatively, it is more advisable to modify the morphology of Fe₃O₄ to perfect its absorbing capability. As aforementioned, the flaky structure is different for enhanced dielectric capability and attenuating the incident microwave. Therefore, designing flaky Fe₃O₄ is another valid way to fulfill high-performance EM wave absorbency. For example, Yang et al [19] fabricated Fe₃O₄ nanodiscs and got a maximum RL value of −37.0 dB.

Recently, porous configurations with a large specific surface area, have been discovered to be useful for accelerating EM wave absorption intensity, owing to their interfacial polarization and multi-reflection [20, 21]. Therefore, the combination of a flaky structure and porous architecture is expected to further improve the EM wave absorbency of Fe₃O₄. Herein, flaky Fe₃O₄ with different dimensions are prepared by the reduction of different sizes of α-Fe₂O₃ flakes using sodium borohydride (NaBH₄). These Fe₃O₄ have an uneven porous surface, and their thicknesses and diameters are about 0.6–0.7 μm and 4.7–6.0 μm, respectively. When the coating thickness is 2.05 mm, the maximum RL value is up to −49.0 dB. The widest effective absorption bandwidth (EAB) of 4.32 (7.52–11.84) GHz at 2.25 mm nearly amounts to whole the X band. Moreover, when the matching thickness is between 2.05 and 3.05 mm, the EAB can even cover the integrated C and X bands.

2.1. Materials

2.2. Synthesis of flaky α-Fe₂O₃

2.3. Preparation of porous flake-like Fe₃O₄

2.4. Characterization

The synthetic process for flaky Fe₃O₄ with a rough porous surface is presented in figure 1. In the first step, by regulating the amount of NaOH, different aspect ratios of α-Fe₂O₃ flakes were obtained. As shown in figures 2(a)–(c), these α-Fe₂O₃ samples have an obvious flaky structure with a smooth surface. The diameter and thickness are about 0.14 μm and 80 nm for α-Fe₂O₃-2, 4.6 μm and 0.6 μm for α-Fe₂O₃-10, and 5.98 μm and 0.72 μm for α-Fe₂O₃-20, respectively. Hence, their aspect ratios (the ratio of diameter to thickness) are around 1.75, 7.6, and 8.6 for α-Fe₂O₃-2, α-Fe₂O₃-10, and α-Fe₂O₃-20, respectively. Clearly, with the content of NaOH increasing, the aspect ratios of α-Fe₂O₃ flakes raises as well. As we know, α-Fe₂O₃ is a kind of semiconductor with a wide band gap and no magnetism, and thus cannot dissipate microwaves effectively [22]. Innovatively, in the second step, we reduced α-Fe₂O₃ to Fe₃O₄ with the help of NaBH₄ based on its predominate microstructure. As seen in figure 2(d), none of the flaky configurations are found in the Fe₃O₄-2 sample, which comprises of many irregular particles and shows some agglomerations. The Fe₃O₄-10 and Fe₃O₄-20 inherit the flaky structures of the precursor α-Fe₂O₃, and the diameters and thicknesses are almost equal to the corresponding α-Fe₂O₃ (figures 2(e), (f)). Meanwhile, the surface becomes rough and porous in comparison with the precursor α-Fe₂O₃, which is due to the lost part of the oxygen during the phase transformation process. In terms of equation (3), when 12 mol of α-Fe₂O₃ are converted to 8 mol of Fe₃O₄, the 4 mol of O atoms are lost in the system.

$$\begin{eqnarray}&&12\alpha -{{\rm{Fe}}}_{{\rm{2}}}{{\rm{O}}}_{{\rm{3}}}+{{\rm{NaBH}}}_{4}\to 8{{\rm{Fe}}}_{3}{{\rm{O}}}_{4}\\ &&\,+{{\rm{NaBO}}}_{2}+2{{\rm{H}}}_{2}{\rm{O}}\end{eqnarray} \tag{ 3 }$$

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By further magnifying their FESEM images, it is clear that these pores, with corresponding sizes of ca. 10–110 nm, are randomly dispersed on the flake surface (figure S1 is available online at stacks.iop.org/NANO/29/295603/mmedia). The apparent porous structures can extend the transmission path of the microwave, being advantageous for the microwave attenuation.

The phase information of all samples was determined by XRD analysis. All diffraction signals in figure 3(a) are well matched with the rhombohedral hematite Fe₂O₃ standard peaks (PDF#33-0664). In figure 3(b), the symbols located at 18.27°, 30.06°, 37.12°, 43.03°, 57.16°, and 62.72° are ascribed to (111), (200), (311), (400), (511), and (440) planes of cubic magnetite Fe₃O₄ (JCPDS No.: 19-0629), respectively. No other impurity peaks could be observed, demonstrating the α-Fe₂O₃ were successfully reduced to the Fe₃O₄. Furthermore, based on Debye–Scherrer's formula [23], the estimated average crystalline size was around 41 nm, 44.3 nm, and 38.1 nm for Fe₃O₄-2, Fe₃O₄-10, and Fe₃O₄-20, respectively.

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In order to further identify the chemical valence of iron ions, the XPS profiles are displayed in figure 3(c). The peaks centered at about 709 and 723.1 eV correspond to the Fe 2p_3/2 and Fe 2p_1/2, respectively [1]. The existence of the satellite peak at around 719.2 eV is characteristic of Fe₂O₃ [24]. However, no satellite is detected in the curve of Fe₃O₄, thus ruling out the presence of α-Fe₂O₃ or γ-Fe₂O₃ in the Fe₃O₄ specimens. These conclusions are in accordance with the XRD results, and further prove that thorough phase transformation could be realized by the reduction of NaBH₄.

The magnetic properties of specimens were measured by VSM analysis. As displayed in figure 3(d), all three samples present the typical magnetic behaviors. In detail, the saturation magnetization (M_s) values of Fe₃O₄-2, Fe₃O₄-10, and Fe₃O₄-20 are about 85.7, 86.4, and 87.9 emu g⁻¹, respectively, which are quite close to the bulk value of Fe₃O₄ (92 emu g⁻¹). Meanwhile, the H_c values are 111.4 Oe for Fe₃O₄-2, 114 Oe for Fe₃O₄-10, and 116 Oe for Fe₃O₄-20. The strong magnetism would result in magnetic dissipation, which benefits the EM wave absorption behavior.

The EM wave absorption performances were assessed based on the tested EM parameters. According to the transmission line theory, the RL can be written as follows [25, 26]:

$$\begin{eqnarray}&&{Z}_{in}={Z}_{0}\sqrt{{\mu }_{r}/{\varepsilon }_{r}}\,\tanh [j(2\pi \pi f/c)]\sqrt{{\mu }_{r}{\varepsilon }_{r}},\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&RL=20\,\mathrm{log}| ({Z}_{in}-{Z}_{0})/{Z}_{in}+{Z}_{0}| ,\end{eqnarray} \tag{ 5 }$$

where Z_in is the input impedance of the absorber, Z₀ represents impedance of free space, f is the frequency, c is the velocity of light, d is the thickness, and ε_r and μ_r correspond to the complex permittivity and permeability, respectively. For fulfilling the demand of practical application, the RL value is required to be below −10 dB, indicating that 90% of the incoming EM wave could be absorbed [27]. Figure 4 displays the EM wave absorbency of the three Fe₃O₄ samples from 1 to 5 mm over the range of 2–18 GHz. For the Fe₃O₄-2 sample, a maximum RL value of −44.3 dB is obtained at 4.5 mm (figure 4(a)). The large thickness obviously is not suitable for commercial application. Delightedly, the maximum RL value of the Fe₃O₄-10 sample could reach −49.0 dB at 2.05 mm. Furthermore, the broadest EAB of 4.32 GHz with a frequency from 7.52 to 11.84 GHz almost covers all the X band. Even more interestingly, when adjusting the thickness between 2.05 and 3.05 mm, the EAB could cover nearly all the C and X bands (figure 4(d)). At the same time, it is obvious that the EAB shifts from high frequency to low frequency with increasing thickness. This phenomenon can be interpreted by the quarter-wavelength laws, which illustrate the inverse proportional relationship between matching frequency and matching thickness [20]. Figure 4(c) shows the RL results of Fe₃O₄-20. Although the RL value of −33.5 dB at 1.9 mm and EAB of 3.88 GHz at 2.15 mm could be achieved, these values are still undesirable compared with those obtained with Fe₃O₄-10. Ultimately, the Fe₃O₄-10 sample presents the best EM wave absorption properties among the three Fe₃O₄ samples, and has great potential to be an outstanding absorber with strong absorbency, wide absorption bandwidth, and low thicknesses.

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For comprehending the conceivable EM wave absorption mechanism, the EM parameters of three Fe₃O₄ samples are presented in figure 5. In general, the real parts of complex dielectric (ε') and permeability (μ') relate to the storage competences for electric and magnetic energy; meanwhile, the imaginary parts (ε'' and μ'') represent the attenuation capacities for electric and magnetic energy [28]. From figure 5(a), one can observe that the ε' values are in the ranges of 5.4–6.6, 12.3–13.0, and 13.87–15.2 (with slight fluctuations) for Fe₃O₄-2, Fe₃O₄-10, and Fe₃O₄-20, respectively. The FESEM results show that the Fe₃O₄-2 sample consists of many anomalous particles, while the other samples show micron-sized flaky architectures. It is obvious that the formation of a flake-shaped structure is conductive to the increase of the dielectric coefficient. Unexpectedly, the Fe₃O₄-10 and Fe₃O₄-20 samples have similar flaky configurations, but the ε' values of Fe₃O₄-20 are higher than those of Fe₃O₄-10. This can be interpreted by their diverse aspect ratios (Fe₃O₄-20, 8.6 versus Fe₃O₄-10, 7.6), which are demonstrated by the FESEM images. The larger aspect ratio and porous characteristics increase the number of dipoles and the area of interfaces, which enhances the dipoles polarization and interfacial polarization at the air–sample interface. Hence, the ε' values rise with the increase in aspect ratio. The phenomenon where ε' rises with increasing aspect ratio in flake-shaped samples has been previously reported [23]. Besides, conductivity may also contribute to ε' values. When the three Fe₃O₄ samples are dispersed in an insulating paraffin matrix, the larger unique flake structure may be conductive to the formation of partial connectivity, thus leading to an improvement in electrical conductivity, which results in higher ε' values. Additionally, multiple resonance peaks can be distinctly seen in the ε'' curves of Fe₃O₄-10 and Fe₃O₄-20, which may be due to the interface polarization caused by the porous structures [29]. Figure 5(b) displays the μ' and μ'' values of the three Fe₃O₄ samples. The μ' values decline rapidly from 1.65, 1.82, and 1.75 to around 0.8 with increasing frequency for Fe₃O₄-2, Fe₃O₄-10, and Fe₃O₄-20, respectively. Meanwhile, parallel variation tendencies can be found in the μ'' plots, where the values sharply decrease from 0.9, 0.97, and 0.98 to about 0 for Fe₃O₄-2, Fe₃O₄-10, and Fe₃O₄-20, respectively. This phenomenon may result from the relaxation of magnetic moments instead of the magnetic hysteresis; therefore, the testing frequency is higher than the relaxation frequency of the mediums [30]. Furthermore, these original high μ' and μ'' values imply promising impedance matching and strong magnetic loss [31].

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The dielectric loss tangent (tan δ_ε = ε''/ε') and magnetic loss tangent (tan δ_μ = μ''/μ') are calculated to clarify the contribution of different attenuation mechanisms [32]. From figure 6(a), it is observed that the magnetic loss factors of the three Fe₃O₄ samples are all higher than the dielectric loss during the measured frequency range. Accordingly, the magnetic loss plays a dominant role in dissipating the incident microwave energy. In the microwave region, the magnetic loss mainly derives from the natural resonance, exchange resonance, and eddy current loss [33]. Among them, the eddy current loss may prevent the incoming EM wave from entering into the interior of the absorber for latter attenuation, and should be inhibitive. The eddy current loss coefficient (C₀) is connected to the thickness (d) and conductivity (σ) of the absorber, and can be expressed as follows: C₀ = μ'' (μ')⁻²f⁻¹ = 2πμ₀σd [34]. According to this formula, if the eddy current loss is the main cause for magnetic loss, the C₀ values should remain constant with the variation of frequency. From figure 6(b), it is clear that the C₀ values of all specimens decrease as the frequency increases from 4–15 GHz, and then remains constant from 15–18 GHz. Hence, the eddy current loss mainly occurs at high frequency. At low frequency, the magnetic loss competence is mainly aroused from the nature resonance and exchange resonance.

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Based on the above comparison and discussion, it is concluded that the different microstructures and aspect ratios should be responsible for the distinct dielectric coefficients and EM wave absorption characteristics. It is worth noting that when the complex permeability of samples are at the proximal level, appropriate ε' and ε'' values can meet the demands of optimal impedance matching, and lead to a strong RL intensity and wide absorption bandwidth. Among the three specimens, the Fe₃O₄-10 medium possesses a moderate aspect ratio and individual micro-architecture, and exhibits prominent EM absorption characteristics. The effect of a filler ratio for Fe₃O₄-10 in a paraffin matrix on the final EM absorption performance was also investigated. As shown in figure S2, for filler ratios of 60%, the maximum RL value was only −22 dB at 3.8 mm because of the lower permittivity and permeability (figures S2(a), (d)). When the filler content increases to 65%, although the maximum RL peak could reach −56 dB at 3.8 mm (figures S2(b), (e)), the large coating thickness is not desirable. The microwave performance becomes poor upon further increasing the mass percentage to 75% (figures S2(c), (f)), owing to the imbalance between the complex permittivity and complex permeability. The optimal filler proportion was 70%. Under this condition, the maximum RL of −49.0 dB at 2.05 and effective frequency bandwidth of 4.32 GHz at 2.25 mm are achieved. Table 1 lists the EM absorption performances of different morphologies of Fe₃O₄ samples in previous studies. Compared with these materials, the Fe₃O₄ sample in this work has a strong RL of −49 dB at a relatively low thickness of 2.05 mm. Furthermore, the EAB of 4.32 GHz can nearly cover the whole X band at 2.25 mm. These outstanding performances suggest that the Fe₃O₄ sample can be regarded as a candidate microwave absorber with high absorption intensity and broad frequency bandwidth.

In summary, by adjusting the amount of NaOH, micro-sized α-Fe₂O₃ flakes with different aspect ratios were fabricated through the solvothermal method. Then, complete phase transformation from α-Fe₂O₃ to Fe₃O₄ was fulfilled by a reduction with NaHB₄. Apart from the Fe₃O₄-2 sample, the obtained Fe₃O₄-10 and Fe₃O₄-20 samples preserved the flake morphologies, and the surface became rough and porous compared with the corresponding precursor α-Fe₂O₃. It was found that the dielectric values of Fe₃O₄ were vitally influenced by their micro-configurations and dimensions. The construction of a flake-shaped structure favored an increasing dielectric coefficient. At the same time, the dielectric values increased with increasing aspect ratios for the flaky framework. The Fe₃O₄-10 sample exhibited outstanding microwave performance through the strong cooperation between dielectric and magnetic properties. Specifically, the maximum RL of −49.0 dB was obtained at only 2.05 mm. The widest EAB of 4.32 GHz at 2.25 mm could cover almost the whole X band. More interestingly, when the thickness was between 2.05 and 3.05 mm, the frequency range with an RL lower than −10 dB width could spread over both C and X bands.

Financial support from the National Nature Science Foundation of China (No. 11575085), the Qing Lan Project, Six Talent Peaks Project in Jiangsu Province (No.XCL-035), Jiangsu 333 Talent Project, the Funding for Outstanding Doctoral Dissertation in NUAA (No. BCXJ17-07), Postgraduate Research & Practice Innovation of Jiangsu Province (KYCX17_0252), and the Priority Academic Program Development of Jiangsu Higher Education Institutions are gratefully acknowledged.