Synthesis and characterization of MoS2/Fe@Fe3O4 nanocomposites exhibiting enhanced microwave absorption performance at normal and oblique incidences

1. Introduction

The growing extent of electromagnetic (EM) pollution, which can cause the failure of sensitive electronics and seriously harm human health, has led to a surge of interest in microwave-absorbing materials (MAMs). These materials have a broad range of military applications (e.g., aircraft stealth) and should exhibit strong absorption at low thickness, wide absorption frequency range, low density, high chemical/thermal stability, good mechanical properties, and low cost. To achieve these goals, the microwave absorption performances of different materials such as magnetic metals and alloys, functional ceramics, conductive polymers, ferrites, and carbon materials and their composites have been intensively investigated [[1], [2], [3], [4], [5], [6], [7], [8]]. Among these materials, composites are viewed as particularly promising ones, as they can be engineered to exhibit the desired physicochemical properties. For example, graphene-based composites feature the advantages of excellent microwave absorption ability and low density [9,10], but are very expensive and hence, of limited practical applicability.

MoS₂, a typical transition metal sulfide with a 2D graphene-like layered structure, has been well characterized and widely applied in lithium batteries, field-effect transistors, sensors, and numerous other optoelectronic conversion devices because of its excellent electrical properties [[11], [12], [13]]. Compared to graphene, which does not have a band gap, bulk MoS₂ exhibits an indirect band gap of 1.2 eV, whereas exfoliated MoS₂ features a direct band gap if the number of layers is sufficiently small, e.g., single-layer MoS₂ exhibits a direct band gap of 1.8 eV. Thus, MoS₂ exhibits a tunable band gap and hence, adjustable semiconductor properties [14], as electrons in the valence band of MoS₂ can be easily excited to the conduction band by thermal or electric-field activation. Activated electrons can propagate like free electrons and cause great conduction loss, and MoS₂ can therefore be classified as a dielectric loss-type MAM. In particular, sheet-like MoS₂ is considered to be a promising microwave absorption material because its large specific surface area provides numerous active sites capable of interacting with EM radiation.

Recent investigations have demonstrated that the microwave absorption performance of MoS₂ nanosheets (NSs) [15,16] can be enhanced by hybridization with magnetic materials, which not only improves the impedance match but also increases the magnetic loss. Among the broad variety of magnetic materials, Fe₃O₄ nanoparticles (NPs) have been widely used as MAMs because of their low cost and good chemical stability [17,18]; however, the microwave permeability of these NPs is very low. Conversely, magnetic metal (e.g., Fe) NPs exhibit higher permeability [19], but are very easily oxidized in wet air or at relatively high temperature. This oxidation can be prevented by coating Fe NPs by an oxide layer, as exemplified by core-shell Fe@Fe₃O₄ NPs, which are a good choice to enhance the microwave absorption performance of MoS₂ NSs.

Herein, we prepare MoS₂ NS/Fe@Fe₃O₄ NP composites with different compositions, showing that at an optimal Fe@Fe₃O₄ NP to MoS₂ NS mass ratio and normal incidence, these materials exhibit a high absorption ability and a wide absorption frequency range even at a small thickness and can therefore be used under far-field conditions. Moreover, to probe whether these composites maintain the above properties under near-field conditions, we examine their absorption peak intensities and bandwidths at oblique incidence.

2. Materials and methods

2.1. Materials

MoS₂ NSs were synthesized from (NH₄)₆Mo₇O₂₄·4H₂O (Tianjin Kemiou Chemical Reagent Co., Ltd.) and SC(NH₂)₂ (thiourea; Tianjin Guangfu Fine Chemical Research Institute), while Fe@Fe₃O₄ NPs were synthesized from Fe(NO₃)₃·9H₂O (Chongqing Xiyi Technology Co., Ltd.) and KBH₄ (Tianjin Guangfu Fine Chemical Research Institute). All chemicals were of analytical grade and were used without further purification.

2.2. Synthesis of MoS₂ NSs

Typically, a solution of (NH₄)₆Mo₇O₂₄·4H₂O (0.5 mmol) and SC(NH₂)₂ (7 mmol) in deionized water (25 mL) was magnetically stirred for 10 min and then transferred to a 50 m L Teflon-lined autoclave. The autoclave was maintained at 180 °C for 24 h and then naturally cooled to room temperature. The black precipitates were collected by centrifugation, sequentially washed with deionized water and ethyl alcohol, and vacuum-dried at 200 °C for 1 h.

2.3. Synthesis of Fe@Fe₃O₄ NPs

Typically, a solution of Fe(NO₃)₃·9H₂O (5.0 mmol) in deionized water (50 m L) was treated with KBH₄ (1.0 g) upon ultrasonication, and the obtained black suspensions of Fe NPs were held for 15 min at room temperature to induce the oxidation of Fe on the NP surface to Fe₃O₄ [20]. The precipitates were collected by centrifugation, sequentially washed with deionized water and ethyl alcohol, and vacuum-dried at room temperature. For crystallinity improvement, Fe@Fe₃O₄ NPs were further annealed at 400 °C for 3.0 h in vacuum and then naturally cooled to room temperature.

2.4. Preparation of MoS₂ NS/Fe@Fe₃O₄ NP composites

MoS₂ NS/Fe@Fe₃O₄ NP composites were prepared by ball-milling and co-firing in vacuum using Fe@Fe₃O₄ NP to MoS₂ NS mass ratios of 30:70, 40:60, 50:50, and 60:40. MoS₂ NSs, Fe@Fe₃O₄ NPs, ZrO₂ balls (20 g, diameter = 5.0 mm), and ethyl alcohol were put in a stainless steel tank that was purged with Ar to remove oxygen and attached to a planetary ball-milling machine. Mixtures obtained by ball milling (150 rpm, 1.0 h) were dried in vacuum, co-fired at 200 °C for 5.0 h in vacuum (to make Fe@Fe₃O₄ NPs tightly adhere to the surface of MoS₂ NSs), and naturally cooled to room temperature to afford Fe-n composites, where n stands for the content of Fe@Fe₃O₄ NPs (wt%).

2.5. Characterization

Sample crystal structure was identified by X-ray diffraction (XRD; Philips, Cu K_α radiation), and morphology was observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM). Selected area electron diffraction (SAED) patterns were recorded and the crystal lattice structure was probed by high-resolution TEM (HRTEM). Surface composition was characterized by X-ray photoelectron spectroscopy (XPS). EM parameters in the frequency range of 2-18 GHz were measured by a vector network analyzer (Agilent E8363B) using a coaxial method. Specifically, MoS₂ NSs/Fe@Fe₃O₄ NPs (65 wt%) and paraffin (35 wt%) were uniformly dispersed in n-hexane by ultrasonication, and mixtures formed after the solidification of paraffin and evaporation of n-hexane were pressed into toroids with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm.

2.6. Calculation of reflection loss (RL)

When EM waves are incident normally to the surface of an absorber backed with a metal plate, RL can be calculated as

$RL=20log|\frac{Z_{in}-Z_{0}}{Z_{in}+Z_{0}}|$ (1)

$Z_{in}=Z_{0}\sqrt{\frac{\mu_{r}}{\varepsilon_{r}}}tanh(j\frac{2\pi ft}{c}\sqrt{\varepsilon_{r}\mu_{r}})$ (2)

where Z_in is the input impedance of the absorber, Z₀ is the impedance of free space, ε_r is complex permittivity, μ_r is complex permeability, j is an imaginary unit, f is the EM wave frequency, t is the absorber thickness, and c is the velocity of light in vacuum. The calculation of RL in the case of oblique incidence is more complicated, with details found elsewhere [21].

3. Results and discussion

3.1. Crystal structure

The XRD pattern of MoS₂ exhibited three diffraction peaks at 14.398, 33.455, and 58.235° (reflections from the (002), (101) and (110) planes of MoS₂ (PDF#06-0097), respectively) and thus indicated that as-synthesized MoS₂ was phase-pure (Fig. 1(a)). The XRD pattern of Fe@Fe₃O₄ featured three strong peaks at 44.642, 65.030, and 82.243° (indexed to the (110), (200) and (211) planes of cubic Fe (PDF#06-0696), respectively) and two weak peaks at 35.525 and 62.602° (indexed to the (311) and (440) planes of Fe₃O₄ (PDF#89-0691), respectively), indicating the successful synthesis of Fe@Fe₃O₄ (Fig. 1(b)). The XRD pattern of Fe-30 revealed the co-presence of MoS₂, Fe, and Fe₃O₄, and thus demonstrated that the MoS₂/Fe@Fe₃O₄ composite was successfully prepared (Fig. 1(c)).

3.2. Surface composition

The surface compositions of MoS₂ and Fe@Fe₃O₄ were probed by XPS. Fig. 2(a) displays the Mo 3d spectrum of MoS₂, demonstrating the presence of two peaks at 228.91 eV (Mo 3d_5/2) and 231.99 eV (Mo 3d_3/2) that could be deconvoluted into the contributions of 2H-MoS₂ (229.60 and 232.80 eV), 1T-MoS₂ (228.95 and 232.09 eV), and MoO₃ (233.60 and 236.70 eV) [22,23]. Thus, the surface of MoS₂ was concluded to comprise 2H-MoS₂, 1T-MoS₂, and a small amount of MoO₃. The Fe 2p spectrum of Fe@Fe₃O₄ NPs featured two peaks at 710.8 eV (Fe 2p_3/2) and 724.5 eV (Fe 2p_1/2) that were assigned to Fe₃O₄ [24] (Fig. 2(b)).

3.3. Morphology and microstructure

SEM image revealed that MoS₂ comprised frizzy nanosheets (width = 100-200 nm), some of which formed aggregates (Fig. 3(a)). TEM image revealed that Fe@Fe₃O₄ contained nearly spherical NPs with a mean size of 10 nm and a core-shell structure (Fig. 3(b) and (c)). The SAED pattern of these NPs featured typical polycrystalline diffraction rings corresponding to Fe (211), Fe₃O₄ (440), Fe (110), and Fe₃O₄ (311) crystal planes, which indicated the co-existence of Fe and Fe₃O₄ (Fig. 3(d)). Fig. 3(e) displays an HRTEM image of the core-shell structure. The lattice fringe of 0.2050 nm was in accordance with the interplanar spacing of Fe (110), while the fringe of 0.4844 nm was in accordance with the interplanar spacing of Fe₃O₄ (111), i.e., the core and shell contained Fe and Fe₃O₄, respectively. To further investigate the core, the NP surface was treated with dilute acetum solution to remove the Fe₃O₄ shell, and the obtained powders were annealed at 400 °C for 12 h in high vacuum to increase crystallinity. Fig. 3(f) displays a representative HRTEM image of the thus treated NPs, revealing the presence of exposed Fe cores and obvious Fe (110) lattice fringes, which indicated that the core comprised pure Fe. Fig. 3(g-k) displays TEM and elemental mapping images of Fe-30, respectively, clearly showing that Fe@Fe₃O₄ NPs uniformly adhered to the surface of MoS₂ NSs.

3.4. Complex permittivity

Fig. 4(a) and (b) shows the frequency-dependent real (ε') and (b) imaginary (ε”) parts of the complex permittivity of Fe-n, respectively, revealing that both ε' and ε” were very high at low frequencies and rapidly decreased with increasing frequency. Usually, permittivity depends on many factors, with the most important ones corresponding to free electron concentration and mobility. For our samples, the free electron concentration was very high, as MoS₂ and Fe have excellent semiconductor and metallic properties, respectively, while the electron mobility was also high because of electron transfer between Fe³⁺ and Fe²⁺ ions in Fe₃O₄. Hence, upon the application of an alternating electric field, numerous free electrons could be shuttled along the MoS₂-Fe₃O₄-Fe path, which resulted in high dielectric polarization (ε') and large energy loss (ε”) at low frequencies. The rapid permittivity decrease with increasing frequency was ascribed to electronic relaxation polarization. The maximum electronic relaxation time is estimated as 10^-9 s, i.e., when the frequency of the alternating electric field surpasses 10⁹ Hz, this kind of polarization cannot be established, and permittivity decreases. In addition, permittivity decreased with increasing content of Fe@Fe₃O₄ NPs, which may reflect the fact that the permittivity of Fe@Fe₃O₄ NPs is lower than that of MoS₂ NSs.

3.5. Complex permeability

Fig. 5(a) and (b) displays the frequency-dependent real (μ') and imaginary (μ”) permeabilities of Fe-n, respectively, revealing that (i) both of these parameters increased with n and (ii) μ' quickly decreased with increasing frequency because of hysteresis, eddy, and resonance losses, with resonance loss being particularly obvious at high frequency. The above μ”-f spectra featured two resonance loss peaks. The first peak at 2.66 GHz was ascribed to the natural resonance of Fe NPs. In this case, the contribution of Fe₃O₄ could be neglected, as the content of Fe₃O₄ was very small. For nearly spherical Fe NPs with a cubic crystal structure, the natural resonance frequency f_γ can be expressed as [25,26]

f_γ = γ₀(H_k + H_d) (3)

where γ₀ = γ/2π = 2.8 GHz/kOe is the gyromagnetic ratio, H_k = 2K₁/μ₀M_s is the magnetocrystalline anisotropy field, K₁ = 4.2 × 10⁵ erg/cm³ is the magnetocrystalline anisotropy constant of Fe, μ₀ = 1 is the permeability of vacuum, M_s =220 emu/g is the saturation magnetization of Fe, and H_d = M_s/3 is the demagnetizing field. The theoretical value of f_γ (estimated as 2.97 GHz) was close to the observed value of 2.66 GHz, which demonstrated that the first peak originated from natural resonance.

The second resonance peak, which was ascribed to exchange resonance between magnetic NPs [27], was only observed for Fe-40 and Fe-50. In Fe-30, the mass fraction of Fe NPs was too low for this peak to be well visible, while in Fe-60, the overly high mass fraction of Fe NPs resulted in agglomeration and thus prevented exchange resonance. The exchange resonance frequency f_ex can be expressed as

f_ex = γ₀(Cμ²_kn/R²M_s + H₀ - 4πM_s/3 + 2K₁/M_s) (4)

where C = 2A = 4 × 10^-6 erg/cm is the exchange constant of Fe, μ_kn is the eigenvalue of the derivative of the spherical Bessel function j_n(μ), which is optional (reference 27 provides a series of μ_kn values, here, we chose μ_kn = μ₁₁ = 2.08), R is the radius of spherical Fe NPs, and H₀ is the strength of the applied DC magnetic field. At R = 9.84 and 9.58 nm, Eq. (4) yields f_ex values of 10.8 and 12.4 GHz, respectively, which are nearly equal to the observed resonance frequency. Thus, the second peak was confirmed to originate from the exchange resonance.

3.6. Microwave absorption performance

Usually, absorber thickness strongly influences microwave absorption parameters such as absorption peak frequency, intensity, and bandwidth (RL < - 10 dB). Fig. 6(a) displays the effects of Fe-30 thickness on RL-f curves for the case of normal incidence, revealing that absorption peak frequency decreased with increasing thickness. Previous studies demonstrated that the absorption peak frequency (f_m) and absorber thickness (t_m) satisfy the quarter-wave formula: t_m = c/4f_m|ε_rμ_r|^0.5 [28]. This formula shows that t_mf_m|ε_rμ_r|^0.5 is a constant, i.e., f_m can only decrease if t_m concomitantly increases. Fig. 6(a) also shows that the absorption peak intensity of Fe-30 decreased with decreasing f_m, as the permittivity of Fe-30 was much larger than its permeability at low frequencies, which resulted in a poor impedance match. The minimum reflection loss (RL_min) of Fe-30 reached - 18.67 dB at a thickness of 1.4 mm. As RL = - 10 dB corresponds to 90% energy loss, and RL = - 20 dB corresponds to 99% energy loss, we concluded that Fe-30 was unable to fully absorb the incident EM radiation. Besides, Fig. 6(a) also shows that the bandwidth of RL < - 10 dB decreased with decreasing f_m, as has been explained in our previous research [29]. Fig. 6(b) displays the t-f curve of Fe-30 described by the quarter-wave formula. Notably, the thickness given in Fig. 6(a) is approximately equal to the thickness calculated by the quarter-wave formula in Fig. 6(b) at identical f_m, i.e., the quarter-wave formula can well describe the relationship between t_m and f_m. In fact, the quarter-wave formula is based on the interface cancellation model [30], which assumes that the EM waves reflected by the air-absorber (AA) and absorber-metal (AM) interfaces can cancel each other by destructive interference to form the RL peak. Thus, a strong RL peak can be realized only when the two waves exhibit opposite phases and identical or nearly identical amplitudes. Fig. 7 provides a schematic diagram of this model and demonstrates the derivation of the quarter-wave formula and the destructive interference condition. In numerous reports, the quarter-wave formula is only used to calculate f_m at a fixed thickness, while the interface cancellation model is used to explain more microwave absorption characteristics such as RL_min and bandwidth [29,30].

Fig. 6(a) shows that Fe-30 exhibits a weak absorption peak and is therefore unable to fully attenuate incident EM waves. To achieve a better impedance match and stronger absorption, the EM parameters of composites need to be further optimized. As the proportion of composite components usually has a strong effect on EM parameters, we characterized Fe-40, Fe-50, and Fe-60 (Fig. 4, Fig. 5). The thickness-dependent RL-f curves of these samples (Fig. 8) demonstrate that the RL_min of Fe-40 still cannot exceed - 20 dB, while that of Fe-50 can exceed - 20 dB, and that of Fe-60 can exceed - 25 dB. In particular, the RL_min of Fe-60 reaches - 37.95 dB at 10.78 GHz and a thickness of 2.0 mm. The RL peak enhancement observed for Fe-50 and Fe-60 was attributed to the improved impedance match, which allowed more EM waves to enter the absorber instead of being largely reflected by the AA interface. In this case, the energy of EM waves reflected by AA and AM interfaces becomes very close, and hence, the effect of destructive interference is very strong. On the contrary, for Fe-30 and Fe-40, the incident EM waves were largely reflected by the AA interface because of the poor impedance match, and the energy of EM waves reflected by the above interface was therefore much larger than that of waves reflected by the AM interface. As a result, the RL_min of Fe-30 and Fe-40 did not exceed - 20 dB.

As the RL peak of Fe-60 was the strongest among the samples, we decided to further investigate the microwave absorption performance of this composite. Fig. 9(a) displays a three-dimensional plot of the RL of Fe-60 as a function of thickness and frequency, revealing that RL can exceed - 10 dB in the range of 1.2-5.0 mm and 3.3-18.0 GHz, whereas RL_min can reach - 39.46 dB (Fig. 9(b)) at a thickness of 1.96 mm and a frequency of 11.10 GHz. At this thickness, the bandwidths of RL < - 10 dB and RL < - 20 dB equal 3.9 GHz (9.49-13.39 GHz) and 1.15 GHz (10.58-11.73 GHz), respectively. Fig. 9(c) displays the projection of RL in the t-f plane, where one can obtain bandwidth at random thickness. Notably, the bandwidth of RL < - 10 dB first increased and then decreased with increasing thickness, with maximum bandwidth observed at 4.8 GHz (13.2-18.0 GHz) at a thickness of 1.52 mm (Fig. 9(d)). At this thickness, the corresponding peak intensity and frequency equaled - 31.8 dB and 15.3 GHz, respectively, and the bandwidth of RL < - 20 dB reached 1.45 GHz (14.64-16.09 GHz). Table 1 lists representative microwave absorption data of some absorbers reported elsewhere and reveals that the absorption peak intensity of Fe-60 is stronger and the bandwidth (RL < - 10 dB) is wider than those of previously reported absorbers at close-to-identical absorber thickness, indicating that Fe-60 exhibits superior microwave absorption performance. At this point, it is worth mentioning that the above comparison is meaningful only when the corresponding thicknesses are approximately equal.

For an absorber with a fixed thickness, the incidence angle has an important effect on RL_min and bandwidth (RL < - 10 dB). Here, it is worth mentioning that in the case of oblique incidence, one needs to consider two kinds of EM waves, namely transverse electric (TE) and transverse magnetic (TM) ones, which exhibit different microwave absorption behaviors. Fig. 10 displays the effects of incidence angle on the RL_min of Fe-60 with a thickness of 1.52 mm, demonstrating that although the RL_min values of TE and TM waves were same (around -31.8 dB) at an incidence angle is 0°, they progressively diverged with increasing incident angle. For example, the RL_min of the TE wave first decreased and then increased with increasing incidence angle, with the lowest RL_min of -43.1 dB observed at an angle of 17.3° and a value of -20 dB observed at 37.8°. For the TM wave, RL_min monotonically increased with increasing incidence angle, reaching -20 dB at an angle of 31.7°. These results showed that strong absorption can be achieved for Fe-60 in the angle range of 0-30° at oblique incidence, which is very significant for microwave absorption applications under near-field conditions.

Fig. 11 displays the effects of incidence angle on the bandwidth of Fe-60 with a thickness of 1.52 mm, demonstrating that the bandwidth decreases with increasing incidence angle and remains relatively wide in a large angle range. For example, the bandwidth of RL < - 10 dB can reach 4.2 GHz (13.5-17.7 GHz) for the TE wave at an incidence angle of 40.0° and can reach 4.0 GHz (14.0-18.0 GHz) for the TM wave at an incidence angle of 50.4°. Moreover, the bandwidth of RL < - 20 dB can reach 1.2 GHz (14.9-16.1 GHz) for the TE wave at an incidence angle of 30.0° and reaches 1.0 GHz (15.2-16.2 GHz) for the TM wave at an incidence angle of 29.0°. These results show that Fe-60 features a wide absorption frequency band in angle ranges of 0-40° (for the TE wave) and 0-50° (for the TM wave), which is also very significant for microwave absorption applications under near-field conditions.

3.7. Microwave absorption mechanism

Dielectric and magnetic losses are considered to be the two major mechanisms of microwave absorption [37,38] and are characterized by the loss angle tangents (tanδ_e and tanδ_m, respectively). Fig. 12 displays the frequency-dependent tanδ_e and tanδ_m of the four samples, revealing that the former decreases and the latter increases with increasing Fe@Fe₃O₄ content and thus indicating that the dielectric loss ability concomitantly weakens and the magnetic loss ability concomitantly increases. However, for all cases, tanδ_e exceeded tanδ_m, which implies the dominance of dielectric loss.

Relaxation polarization usually significantly contributes to dielectric loss and, in the case of our samples, was believed to involve electron and dipole relaxation polarization. This kind of relaxation polarization can be investigated using the Debye relaxation model, in which relative permittivity (ε_r) is expressed as [39]

$\varepsilon_{r}=\varepsilon_{\infty}+\frac{\varepsilon_{s}-\varepsilon_{\infty}}{1+j2\pi ft}=\varepsilon'(f)+i\varepsilon''(f)$ (5)

where f is EM wave frequency, τ is relaxation time, and ε_s and ε_∞ are the static and optical permittivities, respectively. From Eq. (5), one can obtain Eq. (6):

$(\varepsilon'-\frac{\varepsilon_{s}+\varepsilon_{\infty}}{2})^{2}+(\varepsilon'')^{2}=(\frac{\varepsilon_{s}-\varepsilon_{\infty}}{2})^{2}$ (6)

which shows that plots of ε' vs. ε” should feature semicircles (the so-called Cole-Cole circles) representing individual types of relaxation polarization. Fig. 13 displays ε' vs. ε” plots obtained for Fe-n, demonstrating that no Cole-Cole circles (and hence, no relaxation polarization) were observed for Fe-30. Consequently, the dielectric loss of Fe-30 was ascribed to the occurrence of conduction loss. However, two semicircles were observed for the other three samples, which indicated the occurrence of two kinds of relaxation polarization processes. As electronic relaxation time is usually shorter than dipole relaxation time, the electronic relaxation frequency is generally higher than the dipole relaxation frequency. As permittivity decreases with increasing relaxation frequency, the small semicircle corresponding to low permittivity represented electronic relaxation polarization, while the larger semicircle represented dipole relaxation polarization.

During eddy loss, which is significant in bulk metals because of their high conductivity, magnetic energy is dissipated via conversion of the eddy into thermal energy. In the case of nanocomposites, which comprised a semiconductor and a metal, the effects of eddy loss had to be carefully investigated. The imaginary permeability μ” can be expressed as μ” = 2πμ₀(μ')²d²fσ, where μ₀ is the permeability of vacuum, μ' is the real permeability, d is absorber thickness, f is EM wave frequency, and σ is conductivity [40]. From this equation, one obtains μ”(μ')^-2f^-1 = 2πμ₀d²σ/3, which shows that σ is constant if μ”(μ')^-2f^-1 is constant over the whole frequency band. If this condition is met, the magnetic loss mainly originates from eddy loss, and largely originates from magnetic resonance otherwise. Fig. 14 displays plots of C = μ”(μ')^-2f^-1vs. f for the four Fe-n samples. Notably, C decreased with increasing f, especially in the case of Fe-60, which indicated that eddy loss was relatively weak and that magnetic loss mainly originated from magnetic resonance.

4. Conclusion

Herein, we successfully prepared and characterized MoS₂/Fe@Fe₃O₄ nanocomposites with different compositions. At normal incidence, a strong absorption peak of -39.46 dB@11.10 GHz was observed for 1.96-mm-thick Fe-60, and a wide absorption frequency band of 4.8 GHz (13.2-18.0 GHz) was achieved for 1.52-mm-thick Fe-60. At oblique incidence, the absorption peaks of TE and TM waves were still below - 20 dB at incidence angles of up to 30°, and the bandwidth still reached 4.2 GHz (for TE waves) and 4.0 GHz (for TM waves) at incidence angles of 40.0 and 50.4°, respectively. Therefore, we conclude that the Fe-60 composite can be employed as a strong, broadband, and thin microwave absorber at both normal and oblique incidences.

Acknowledgment

This work was financially supported by the National Natural Science Foundations of China (Nos. 11574122 and 51731001) and the Fundamental Research Funds for the Central Universities (No. lzujbky-2017-k20).

The authors have declared that no competing interests exist.

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