Facile synthesis of porous coin-like iron and its excellent electromagnetic absorption performance

Abstract

In this paper a novel electromagnetic absorbent, porous coin-like iron with a diameter of ∼10 μm and a thickness of 2 μm, was fabricated using a hydrogen gas reduction process. This special porous coin-like structure was attributed to a decrease in density and exceeded the Snoek limitation. It was observed that these coin-like iron structures exhibit excellent microwave absorption properties. An optimal reflection loss value of −53.2 dB was obtained at 16 GHz, moreover, the effective frequency bandwidth could be up to 6.3 GHz (11.7–18 GHz) at a thickness of 1.4 mm. The microwave absorption mechanism may have originated from the following factors: firstly, these coin-like irons were favorable for obtaining a lower real part of permittivity value and thus gained the improvement of impedance matching behavior, as compared with other reported irons. Secondly, the coin-like morphology exhibited a strong magnetic loss ability. Further analysis revealed that the magnetic loss mechanism may rely mainly on the resonance. In addition, the porous feature of the coin-like iron offered a rough surface on the large size of the coin-like structure, which was beneficial for electromagnetic wave scattering and further enhanced their microwave absorption properties.

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1. Introduction

Since the rapid development of wireless communications, high frequency circuit devices and radar systems, the growth of electromagnetic interference (EMI) has become a negative influence in our daily lives.^1,2 In order to eliminate the severe electromagnetic pollution, electromagnetic absorbents have attracted a lot of interest in recent years. As we know, the microwave attenuation process can be divided into two steps. In the first stage, the ideal absorbent should let more and more electromagnetic wave incidence into the coating layer and reduce the reflection from the surface. Subsequently, the absorbent needs to attenuate the incident electromagnetic wave and convert it into thermal energy via its intrinsic magnetic or dielectric properties.^3–5 It is well known that microwave absorbents can generally be classified into two types depending on their way of attenuating electromagnetic waves. An ideal absorbent has to exhibit not only a low reflection loss value but also a broad frequency width.^6–8 Among all the microwave absorption materials, magnetic metals (e.g.: Fe, Co and Ni) have been a hot spot in previous studies because of their high permeability, large saturation magnetization and so on.^9,10 As a representative of magnetic absorbents, iron has superior advantages over other magnetic metals or oxides. It is well known that the saturation magnetization value of bulk iron can be as high as 220 emu g⁻¹, larger than 180 emu g⁻¹ of cobalt and 55 emu g⁻¹ of nickel. Such a high M_s value is related to a high real part and imaginary value of permeability (μ′, μ′′), which is favorable for microwave absorption. However, being used as microwave absorbent materials, metal iron still exhibits two serious shortcomings. On one hand, the μ′ value decreases with increasing frequency in the high frequency range according to the Snoek limitation and 1/4 wavelength equation:^11–13

(μ₀ − 1) f_o = (1/3π)M_s (1)

t_m = nc/4f_m(ε_rμ_r)^1/2 (2)

where t_m and f_m are the matching thickness and frequency of a reflection loss (RL) peak, μ_r (μ′ − jμ′′) and ε_r (ε′ − jε′′) are the complex permeability and permittivity at matching frequency, respectively. From eqn (1), we can see that the μ′ value decreases as the frequency increases. Therefore, the μ′ value at high frequencies is small, which may lead to the poor impedance matching behavior. Furthermore, it is difficult to obtain an ideal absorption within a low coating thickness (t_m), as indicated by eqn (2). In other words, the absorbent cannot meet the requirement of being lightweight, which restricts its application. However, some studies proved that a flake-shaped absorbent may exceed the Snoek limitation due to its larger shape anisotropy,¹⁴ and therefore great efforts have been focused on the synthesis of a flake-like absorbent. For instance, Yan et al. used a ball-milling technique to prepare Fe nanoflakes with a thickness of 20 nm.¹⁵ Furthermore, Ma et al. synthesized Co nanosheets with a diameter of several micrometers and a thickness of 80 nm via a simple low temperature hydrothermal process.¹⁶ On the other hand, the density of metal iron can be as heavy as 7.87 g cm⁻³, which makes it unsuitable for aircraft application.¹⁷ In order to solve the above problem, porous materials have aroused great attention due to their large surface area.^18,19 More importantly, material with a porous structure present low-density characterization. To date, many attempts have been made to fabricate a porous absorbent for microwave applications. For instance, the minimum RL value of nanoporous Fe₃O₄ microspheres can reach −16.3 dB at 2 mm thickness.²⁰ Meanwhile, porous Co spheres with a diameter of 2–4 μm have an optimal value of −45 dB at a coating thickness of 1.5 mm.²¹ To the best of our knowledge, producing magnetic metal iron with a porous structure still remains a big challenge.

In this paper, we offer a strategy to acquire an excellent microwave absorbent by effectively solving those two big limitations, as discussed above. Firstly, a porous coin-like Fe₂O₃ was obtained, and subsequently heated under hydrogen gas at 500 °C.

2. Experimental section

2.1 Materials

Barium nitrate (Ba(NO₃)₂), sodium hydroxide (NaOH) and iron nitrate (Fe(NO)₃) were purchased from Nanjing Chemical Reagent Co. Poly ethylene (PEG-200) was purchased from the Sinopharm Chemical Reagent Co. All of the chemical regents were used as received and were analytically pure, not requiring any further purification.

2.2 Synthesis of porous coin-like iron

3 g Fe(NO₃)₃ and 0.25 g Ba(NO₃)₂ were first dissolved in 40 mL PEG(200)/distilled water mixed solution with mechanical stirring to form a clear solution. 1.946 g NaOH was then added into the mixed solution with a further 10 minutes of stirring. After that, the mixed solution was transferred into a Teflon-lined stainless steel autoclave (100 mL) and subsequently sealed and heated at 220 °C for 24 hours. Finally, the resulting samples were collected by centrifuge, washed with absolute ethanol and distilled water several times, and then dried in a vacuum at 60 °C for 6 hours. The as-prepared coin-like Fe₂O₃ was deducted in a hydrogen atmosphere at 500 °C for 2 hours, with an increase rate of 1 °C min⁻¹.

2.3 Characterization

Powder X-ray diffraction (XRD) pattern measurements were carried out on a Bruker D8 ADVANCE X-ray diffractometer using Cu Kα radiation (λ = 0.154178 nm) with 40 kV scanning voltage, 40 mA scanning current and a scanning range from 20° to 60°. A Hitachi S4800 type scanning electron microscope (operating at an acceleration voltage of 3.0 kV and equipped with an energy dispersive X-ray spectroscope) were used to observe the morphological features and size of these samples. Magnetic properties including coercive force and saturation magnetization were investigated using a vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) at room temperature. The S parameter, including S₁₁, S₁₂, S₂₁ and S₂₂, were tested using an Agilent PNA N5224A vector network analyzer with a coaxial-line method, and the ring was prepared by using the homogeneously mixed sample and paraffin wax (mass ratio of 7 : 3) and then pressing into toroidal-shaped samples (Φ_out: 7.0 mm, Φ_in: 3.04 mm). After that, the ε′, ε′′, μ′, μ′′ values could be obtained using a software installed in the Agilent PNA. Finally, the RL value with different thicknesses could be calculated based on the following formulas:²²where Z_in is the input impedance of the absorber, f is the frequency of the electromagnetic wave, d is the coating thickness of the absorber, and c is the velocity of the electromagnetic wave in free space. ε_r (ε_r = ε′ − jε′′) and μ_r (μ_r = μ′ − jμ′′) are the complex permittivity and permeability of the absorber, respectively.

3. Results and discussion

From Fig. 1, it can be observed that all the diffraction peaks match well with the hematite of α-Fe₂O₃ (JCPDS no. 89-0597). Meanwhile, the sharp diffraction peaks indicate that the as-prepared coin-like α-Fe₂O₃ is well-crystallized.

Fig. 1 XRD patterns of the starting material α-Fe₂O₃.

Fig. 2 displays the FE-SEM images and element mappings of α-Fe₂O₃. One can clearly see that there are large quantities of mono-dispersed coin-shaped Fe₂O₃, as exhibited in Fig. 2a. Furthermore, it can be noted that the surface of these coin-like structures is smooth, with a diameter of 9–11 μm, as shown in the inset of Fig. 2b. The element mappings of Fe and O are shown in Fig. 2c and d. Clearly, both Fe and O are uniformly distributed in each coin-like structure.

Fig. 2 FE-SEM images and element mapping of the coin-like α-Fe₂O₃.

In order to investigate the possible formation mechanism of the coin-like structure, a series of time-dependent experiments were performed to reveal the morphological evolution. Fig. 3a shows the sample collected within 6 hours, a large number of irregular flakes can be observed with about 1 μm in length and 200–300 nm in thickness. When the reaction time reached 12 hours, as shown in Fig. 3b and c, these flakes had accumulated with a gradual increase in size including both length and thickness. However, the edges of the flakes are not flat, because when more than two flakes are accumulated the length of each flake is unequal and results in a groove among the flakes (inset of Fig. 3c). When the time was increased to 18 hours, the flakes became more round, while the outline is still a little irregular, however, their size and thickness are close to the final coin-like structure (Fig. 3d–f). Based on the experimental results above, we may speculate on the growth procedure of the coin-like shape. During the early stages, irregular flake-like Fe₂O₃ is obtained, and the high surface energy and edge of each flake make it unsteady. Therefore, accumulation may be the best way to decrease the surface energy. Consequently, two types of accumulation are occupied: one is face-to-face accumulation which is used to increase the thickness of the flake, and the other is edge-to-edge accumulation which is able to enlarge the surface area of the flake-structure. During the accumulation process, it is apt to form a new interface via re-assembly of the Fe and O bands. Such a re-assembly procedure is also efficient for reducing the surface energy. After several flakes assemble into a one big flake, the outline of the flake is similar to the coin-like structure, but with rough edges. The samples then further decrease the surface energy by making the prominent edge smoother and finish forming the stable coin-like structure.

Fig. 3 SEM images of the coin-like Fe₂O₃ at different times: (a) 6 hours; (b and c) 12 hours and (d–f) 18 hours. (g) Schematic diagram of the possible formation mechanism of the coin-like structure.

The coin-like iron is obtained via hydrogen reduction at 500 °C with a slow increase rate of 1 °C min⁻¹. Fig. 4 indicates that the main diffraction peaks are due to the (110) and (200) crystal planes of the metal iron (JCPDS card no.: 06-0696). Nevertheless, the presence of weak diffraction peaks are mainly assigned to non-stoichiometric Fe oxide, corresponding to Fe_0.902O (JCPDS card no.: 86-2316) and Fe_2.897O₄ (JCPDS card no.: 86-1343). These non-stoichiometric Fe oxides may result from incomplete reduction, or an oxide formed due to long time exposure in air.

Fig. 4 XRD patterns of the porous coin-like iron.

SEM images of the coin-like iron are displayed in Fig. 5. It is interesting that after the hydrogen gas reduction, all the samples still maintain the original coin-like structure (Fig. 5a). From Fig. 5b and c, one can see that this coin-like structure contains numerous pores with sizes of about 10–50 nm. The reason for this may be that the reduction of iron oxide results in the removal of oxygen atoms, and the empty space remain as pores inside the coin-like structure. In Fig. 5d, we may conclude that the size of both Fe₂O₃ and Fe are almost unchanged and mainly range between 9–11 μm.

Fig. 5 SEM images of the porous coin-like iron; (a) an over view of the coin-like iron; (b) the surface of the porous coin-like iron; (c) the edge of the porous coin-like iron; (d) the size distribution of Fe₂O₃ and Fe.

The magnetization properties of these coin-like iron structures were measured using a VSM with an applied field −10 kOe < H_c < 10 kOe at room temperature. It can be observed from Fig. 6 that the saturation magnetization value can reach 149 emu g⁻¹, smaller than the 220 emu g⁻¹ of bulk iron. Such a decrease in saturation magnetization value can be ascribed to the following factors. Firstly, the surface of the coin-like structure consists of numerous Fe nanoparticles which are easily oxidized to nonmagnetic or low magnetization Fe oxides, as shown in Fig. 4. Secondly, the samples presenting a power form may not be as beneficial to the anisotropic value as the large value of the bulk structure. The coercivity of the sample is up to 296.08 Oe. This high coercivity value of the coin-like iron can make the resonance peak shift to a high frequency range according to the following formulas:^23,24

K = μ₀M_sH_c/2 (5)

H_a = 4|K|/3μ₀M_s (6)

2πf_r = rH_a (7)

where μ₀ is the universal value of permeability in free space (4π × 10⁻⁷ H m⁻¹), r is the gyromagnetic ratio, and H_a is the anisotropy energy. From eqn (5) and (6) we know that coin-like Fe with a larger H_c value and a moderate M_s value relate to a big anisotropy (H_a). Such a large H_a value can make f_r transfer to a high frequency region which is good for obtaining an excellent microwave absorption at a thin thickness, based on eqn (2).

Fig. 6 The M–H loops of the as-prepared porous coin-like iron.

Fig. 7 shows the relationship between RL value and frequency of the coin-like iron. It is worth mentioning that the RL_min value < −10 dB is regarded as an ideal value, which corresponds to the absorbent and attenuates 90% of the electromagnetic wave. Meanwhile, the frequency bandwidth of the RL_min exceeding −10 dB should be as wide as possible. From Fig. 7, we discover that with increasing coating thickness, the RL_min values are shifted to a low frequency range. Obviously, the sample has an optimal reflection loss value of −53.2 dB at 16 GHz while the coating thickness is just 1.4 mm, which is much higher than that of the reported iron. Furthermore, at this thin thickness, the effective frequency bandwidth can reach 6.3 GHz (11.7–18 GHz).

Fig. 7 The reflection loss of the porous coin-like iron.

Electromagnet parameters can be applied to analyze the microwave absorption properties as shown in Fig. 8. It is worth pointing out that the as-prepared coin-like iron exhibits a relatively lower ε′ value of 9–10 as compared to other types of iron or iron based alloy, such as hexagonal flake Fe (25–15), dendrite nanostructure Fe (12–10) and hexagonal-cone like FeCo alloy (14–10).^25–27 Such a low ε′ value greatly improves the impedance matching properties, allowing more and more electromagnetic wave incident into the absorbent. For the ε′′ value, a peak can be observed at 12.3 GHz. As for the real part of permeability (μ′), the value decreases from 1.9 to 1 with increasing frequency. The μ′′ value is within a narrow region of 0.4–0.6. The magnetic loss factor (tan μ = μ′′/μ′) and dielectric loss factor (tan δ = ε′′/ε′) possess a key parameter to decide material attenuation ability. Fig. 8b clearly shows that the magnetic loss ability is higher than the dielectric loss, which means that the attenuation loss ability may mainly depend on magnetic loss. To the best of our knowledge, the high magnetic loss may derive from natural resonance, eddy current loss and exchange resonance. In general, exchange resonance can be ruled out because this attenuation only happens at a low frequency region. However, if the loss comes from the eddy current loss, the eddy current loss (C_o) will be a constant based on the equation:²⁸

C_o = μ′′(μ′)f⁻¹ = 2πμ₀d2σ (8)

where σ is the electric conductive. We find that C_o presents a decreased tendency as the frequency increases, meaning that the eddy current loss can be precluded (Fig. 8c). Thus, the natural resonance may be the main attenuation way.

Fig. 8 The electromagnetic parameters (a), dielectric loss (b), and C_o curves (c) of the porous coin-like iron.

As a result, the excellent microwave absorption can be explained based on Fig. 9; where L₀ is the emission of the electromagnetic wave, L₁ represents the reflection of the electromagnetic wave from the interface of the coating layer, L₂ is the penetration electromagnetic (diffraction of microwave) wave from the coating layer, L₃ is the attenuated electromagnetic wave, and L₄ is the scattering of the electromagnetic wave among the coin-like structures. In general, L₀ = L₁ + L₂ + L₃ + L₄. An excellent electromagnetic absorption material should increase L₃ and L₄, and decrease L₁ and L₂. It is well known that L₁ is related to the impedance matching properties. A high impedance matching property is attributed to a lower L₁. In this study, the novel microwave absorption of the as-prepared porous coin-like Fe mainly results from the following aspects: firstly, the relatively lower ε′ value may lead to the high impedance matching properties which can effectively decrease the electromagnetic reflection (L₁); secondly, the coin-like Fe exhibits a strong magnetic loss ability allowing most of the electromagnetic to be attenuated and transferred into thermal energy (L₂); finally, the porous coin-like structure with a rough surface may further increase the scattering of electromagnetic wave (L₃) and indirectly increase the electromagnetic absorption properties.

Fig. 9 Schematic illustration of the absorption mechanism of the porous coin-like iron.

4. Conclusion

Porous coin-like iron has been synthesized using a simple solvothermal process. During the first stage, the coin-like Fe₂O₃ was produced and then reduced by hydrogen gas. The reduction of Fe₂O₃ resulted in a loss of oxygen atoms and the formation of a porous structure. The porous coin-like iron showed an excellent electromagnetic wave absorption performance with a minimum reflection loss of −53.2 dB, with a thickness of 1.4 mm. Furthermore, the frequency bandwidth was less than −10 dB and ranged from 11.7–18 GHz. The superb microwave properties may contribute to the high impedance matching, magnetic resonance and special coin-like structure. The novel coin-like Fe presented in this paper provides a promising absorbent for intensity absorption, is lightweight, and has a broad frequency.

Acknowledgements

The financial support by the Aeronautics Science Foundation of China (2014ZF52072), the National Natural Science Foundation of China (51172109 and 11475086) and the Fundamental Research Funds for the Central University (no.: 3082014NS2014057) are gratefully acknowledged.

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