Using γ-Fe₂O₃ to tune the electromagnetic properties of three-dimensional (3D) polypyrrole (PPy) and its broadband electromagnetic absorber

Abstract

Electromagnetism such as gravity can be considered an omnipresent force, which plays an important role in the functional activities of organisms. However, electromagnetic waves with different frequencies have aroused various harmful factors in human beings and wildlife. With the tunable dielectric property, the material can get a controllable electromagnetic absorption range against different types of electromagnetic pollution. In this study, we reported that nano γ-Fe₂O₃ can regularly tune the dielectric properties of the three dimensional (3D) polypyrrole (PPy) aerogel. On the one hand, it has advantages such as a rapid synthesis strategy (only several minutes), facile purification process (with only water), and on the other hand, this aerogel based absorber can reach effective EA bandwidths of 6.76 GHz (≤−10 dB, 90% electromagnetic energy was attenuated) and 1.88 GHz (≤−20 dB, 99% electromagnetic energy was attenuated) simultaneously at a thickness of 2.5 mm.

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Introduction

Electromagnetic pollution has been considered a worldwide hazard. In recent years, human beings and wildlife have been exposed to electromagnetic waves, with the potential risks to the organisms being mentioned by researchers.^1–3 Reflection and absorption are two main methods to design a shield to avoid electromagnetic pollution. In practical applications, reflection based shields lose their effectiveness when the inner electric equipment generates strong electromagnetic fields by itself, because not only the electromagnetic waves are totally reflected, but also self-interference will lead to serious potential hazards arising. On the other hand, electromagnetic absorbers can weaken the electromagnetic energy sharply and make sure that the transmitted electromagnetic waves do not have a harmful effect on organisms and equipment. Thus, electromagnetic absorption (EA) is much more important than reflection during the design of shielding materials.

Recently, and in the future, an ideal EA material should be light weight and possess a low filler loading ratio, high and broad EA performance and a tunable absorption frequency range.⁴ With the development of nanotechnology, nano-sized dielectric or magnetic materials have been fabricated to improve the EA performance such as BaTiO₃,⁵ TiO₂,⁶ ZnO,^7,8 CuS,^9–11 and CoFe₂O₄.¹² However, these nanoparticles or nanostructures often need a high filler loading ratio within the composites. A one dimensional (1D) dielectric nanostructure, with a large length–diameter ratio, can build a conductive network in the absorber and possess a fascinating EA performance.¹³ Further studies focus on tuning the dielectric properties of SiC nanowires and CNTs by decorating them with nanoparticles.^14–20 It is found that the intensity and the bandwidth of their EA performance were enhanced simultaneously.

Polypyrrole (PPy), as one of the intrinsically conducting polymers (ICPs), has many advantages in the manufacturing of absorbers, such as gentle synthesis conditions, controlled dielectric properties and is much cheaper, when compared with SiC and CNTs. Nowadays, PPy can be fabricated into a 3D gel through 1D PPy chains, rather than irregular particles, benefiting from a fast oxidation procedure.²¹ The 3D PPy is made up of many 1D PPy chains.^22,23 Thus, similar to the abovementioned SiC nanowires and CNTs, this aerogel also possesses a continuous, conductive network inside of composites to make sure that there is an effective attenuation of electromagnetic waves.²²

In this study, we reported that the dielectric properties of 3D PPy can be regularly tuned by γ-Fe₂O₃. Results show that at a 10 wt% of this tuned PPy aerogel in a composite, the effective EA bandwidths can reach up to 6.76 GHz (≤−10 dB, 90% electromagnetic energy was attenuated) and 1.88 GHz (≤−20 dB, 99% electromagnetic energy was attenuated) simultaneously at a thickness of 2.5 mm. This absorber, with a rapid synthesis strategy (only several minutes), facile purification process (with only water), wide and strong absorption, and low filler loading ratio, makes a promising candidate against electromagnetic pollution.

Results and discussion

The XRD pattern of the as-prepared γ-Fe₂O₃ tuned 3D-PPy is shown in Fig. 1. The diffraction data of the γ-Fe₂O₃ reported in PDF 80-2186 is also presented for comparison. The peaks observed at 2θ = 30.26°, 35.64°, 43.32°, 43.48°, 57.30°, 62.93° and 63.05° are attributed to the (220), (313), (400), (308), (513), (440) and (508) planes of γ-Fe₂O₃. The high consistency between them confirms the existence of γ-Fe₂O₃ in the 3D-PPy. Signals of PPy can hardly be detected from the powder pattern, due to the low crystallinity. Nevertheless, 3D-PPy can be readily identified by its special morphology in SEM and TEM images (Fig. 2). γ-Fe₂O₃ tuned 3D-PPy was formed by countless micro and nano platelets (Fig. 2a and b), and these platelets grew into the irregular chain-like structure, which resulted from the polymerization of pyrrole (Fig. 2c). The detailed morphology of the γ-Fe₂O₃ can hardly be found, even in high-resolution TEM images (Fig. 2d). However, the result from the EDX mapping analysis shows that this aerogel contained the elements C, N, Fe and O, uniformly. The elements C and N were obviously attributed to PPy, while Fe and O can only come from the γ-Fe₂O₃ nanoparticles, without doubt (Fig. 3). With the abovementioned discussions, we can presume that the most probable structure is a γ-Fe₂O₃ tuned 3D-PPy aerogel (Fig. 4). In brief, the surface of the γ-Fe₂O₃ nanoparticles was initially attached by means of a layer of pyrrole monomers. When the solution of FeCl₃ was added in, the polymerization process forced the γ-Fe₂O₃ nanoparticles to lock into the PPy platelets, and finally formed a 3D gel.

Fig. 1 XRD pattern of γ-Fe₂O₃ tuned 3D-PPy aerogel (upper) and the calculated data for γ-Fe₂O₃ reported in PDF 80-2186 (bottom).

Fig. 2 SEM (a and b) and TEM (c and d) images of γ-Fe₂O₃ tuned 3D-PPy aerogel.

Fig. 3 EDX mapping of γ-Fe₂O₃ tuned 3D-PPy aerogel.

Fig. 4 The most probable structure of γ-Fe₂O₃ tuned 3D-PPy aerogel.

The EA performance of a material is decided by its complex permittivity (ε = ε′ − iε′′) and permeability (μ = μ′ − iμ′′), whose real and imaginary parts represent the storage and loss capacity of the electromagnetic energy. The complex permittivity of the four samples are shown in Fig. 5a and b. It is obvious that both the real (ε′) and imaginary (ε′′) parts of the permittivity decrease with increasing frequency and γ-Fe₂O₃ mass ratio. The ε′ values of the four samples are in the range of 15.7–8.1, 15.6–8.0, 10.1–4.9 and 6.2–3.8. The ε′ represents the polarization of the composites, and it may be related to the number of interfaces and orientation dipoles. Because the dielectric property of γ-Fe₂O₃ is poor, the electric storage ability of the composites was weakened with increasing γ-Fe₂O₃ mass ratio. The ε′′ values of the four samples are in the range of 10.7–5.6, 10.3–5.2, 6.1–3.5 and 4.2–2.0. The ε′′ should be ascribed to the relaxation and conductivity of the material. When the conductivity of PPy aggregations is high, the relaxation peaks were not obvious. However, the fluctuation of ε′′ indicates the existence of a relaxation mechanism. The conductivity plays a key role in the electric loss ability of the composites, and it can also be ascribed to the conductive network constructed by PPy platelet aggregation.^22,23 With an increasing γ-Fe₂O₃ mass ratio, the conductivity of the composites should be decreased.

Fig. 5 Complex permittivity (a and b) and complex permeability (c and d) of the wax based composites in the frequency range of 2–18 GHz.

According to the Debye theory, ε′ and ε′′ can be written as:

where ε_s is the static permittivity, ε_∞ is the relative dielectric permittivity at the high-frequency limit, ω is the angular frequency, τ is the polarization relaxation time, σ is the electrical conductivity and ε₀ is the dielectric constant in a vacuum (ε₀ = 8.854 × 10⁻¹² F m⁻¹). By eliminating ωτ in eqn (1) and (2), the relationship of ε′ and ε′′ can be expressed in the following form:

The conductivities of the four samples are calculated by fitting eqn (3), whose values and contributions to ε′′ are shown in the Fig. 6a and b. The conductivity is inversely proportional to the mass ratio of γ-Fe₂O₃, which agrees with the discussion above. As is shown in Fig. 6c, compared with the conductivity, the relaxation procedure makes lesser contributions to ε′′. An important phenomenon is delivered by eqn (3), wherein the plot of ε′ versus ε′′ − σ/ωε₀ should be a semicircle. The cole–cole plots of the samples are shown in Fig. 6d, where one semicircle appears for each sample, which corresponds to the relaxation procedure of the interface capacitor-like structure.^17,23,24

Fig. 6 (a) The conductivity (σ); (b) the contribution of σ to ε′′; (c) the contribution of relaxation to ε′′ and (d) the cole–cole plots of the composites.

The complex permeability is shown in the Fig. 5c and d. The real part (μ′) of the permeability increases with increasing frequency, and its values are in the range of 0.97–1.05, 0.98–1.03, 0.95–1.03 and 0.96–1.05. Owing to the additive amount of γ-Fe₂O₃ being limited, the μ′ changes slightly with respect to the dielectric material. The imaginary part (μ′′) fluctuates in the investigated frequency range; however, there is a common feature shared by the μ′′ of all four samples, the resonance peak that appears at ∼8.1 GHz, which should be derived from the natural resonance. On the other hand, another resonance peak was observed at ∼16.0 GHz for S1 and S2, which benefits from the magnetic loss property in the nearby frequency range.

It has been widely accepted that the magnetic loss ability of composites is ascribed to hysteretic lag, domain wall resonance, natural ferromagnetic resonance and the eddy current effect.²⁵ In general, the intensity of the electromagnetic wave is not strong enough to motivate a hysteretic lag.²⁶ Second, the radius of the γ-Fe₂O₃ nanoparticles is around 20 nm, which is so small that a domain wall can't exist in them. In other words, only the natural ferromagnetic and eddy current effects may make contributions to the magnetic loss of the composites. When the aerogel is mixed with wax, the natural ferromagnetic resonance will be attenuated. The magnetic loss originating from the eddy effect can be written in the following form:^27,28

By changing the form of eqn (4), it can be found that the values of μ′′(μ′)⁻²f⁻¹ should be constant in the investigated frequency range. Fig. 7a shows the μ′′(μ′)⁻²f⁻¹ values of the four samples. The μ′′(μ′)⁻²f⁻¹ almost keeps unchangeable when the frequency exceeds 10 GHz, which indicates the existence of the eddy current effect. Fig. 7b illustrates the effect of the eddy current, which appears at the surface of coaxial ring samples when electromagnetic waves pass through. At last, the energy is depleted in the form of joule heat.

Fig. 7 (a) Frequency dependence of the μ′′(μ′)⁻²f⁻¹ of the composites; (b) the schematic of the current effect.

An effective way to estimate the EA ability of a material is to calculate its value of reflection loss (RL). Based on measured data of the permittivity and permeability of materials, the RL of a metal-backed single absorber layer can be calculated by the transmission line theory:²⁹

where the Z_in is the normalized input impedance of the absorber. c, f and d represent the speed of light, frequency of incident electromagnetic waves and the thickness of the absorber. Fig. 8 shows the 3D and contour plots of RL versus frequency and thickness for each sample. It is clear that both the thickness and the γ-Fe₂O₃ mass ratio exerts an impact on the RL of the composites. When the frequency is lower than 8.1 GHz, the RL peaks appear at a thickness of ∼2.2 m. Once the frequency exceeds 8.1 GHz, the thickness needs to be increased continuously to get higher loss effects. As discussed above, the dielectric constant of the composites decreases with the increasing γ-Fe₂O₃ mass ratio. However, the EA ability of the composites reaches a peak when the mass ratio of γ-Fe₂O₃ is equal to 7 wt%, and then decreases forthwith. For the purpose of assessing the EA ability of the four samples specifically, a detailed comparison of the minimum and bandwidth of the RL values are shown in Table 1. S3 possesses the best EA performance, its RL peak reaching −50 dB and its bandwidth of RL ≤ −10 dB reaching 6.76 GHz. Furthermore, S3 has a bandwidth of RL ≤ −20 dB, unlike the other four samples. There are several points that we should pay attention to: first, for higher mass ratio γ-Fe₂O₃ samples, the thickness of the absorber needs to be increased to enable the RL to exceed −10 dB or −20 dB, which is consistent with the discussion above; second, both the RL peak and RL bandwidth of S3 are better than other samples. This means that the EA performance of the composites will not always be improved by adding more magnetic material.

Fig. 8 3D plots of RL with the filler of S1 (a), S2 (b), S3 (c) and S4 (d) at thicknesses ranging from 1.5 to 8.0 mm in the frequency range of 2–18 GHz.

Table 1 The details of the minimum values of RL/dB and bandwidth/GHz of the as-prepared composites

γ-Fe2O3 mass ratio	Min RL/dB				Band width (≤−10 dB)/GHz				Band width (≤−20 dB)/GHz
	1.5 mm	2 mm	2.5 mm	3 mm	1.5 mm	2 mm	2.5 mm	3 mm	1.5 mm	2 mm	2.5 mm	3 mm
3%	−12.75	−15.27	−13.55	−11.58	1.36	5.92	3.96	1.84	—	—	—	—
5%	−13.03	−15.99	−14.78	−13.11	1.32	6.12	4.08	3.04	—	—	—	—
7%	−6.32	−50.26	−24.22	−20.75	—	3.6	6.76	5.72	—	1.20	1.88	0.72
10%	−3.36	−9.80	−16.47	−16.20	—	—	3.48	5.28	—	—	—	—

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The EA performance of composites is determined by the impedance matching degree and loss tangent of electricity (tan δ_e) and the magnetism (tan δ_m). The better the impedance matching, the more the electromagnetic waves can get into the absorber. The modulus of impedance matching coefficient (ρ) represents the matching degree of the absorber against a vacuum.³⁰ When its value is equals to zero, all the incident electromagnetic waves can get into the absorber. Fig. 9a and b show the value of |ρ| and the sum of two loss tangents. Both the |ρ| and (tan δ_e + tan δ_m) of the composites decrease with an increase in the γ-Fe₂O₃ mass ratio, which means more and more incident electromagnetic waves get in, but less and less transmitted electromagnetic waves are exhausted. As a result, the absorbing ability of the composites reaches a peak with a certain γ-Fe₂O₃ mass ratio, and then decrease with it. This is the reason why S3 acquires the best EA performance experimentally.

Fig. 9 The mismatch coefficient modulus (a) and the sum of tan δ_e and tan δ_m (b) of the composites.

Conclusions

In summary, the dielectric properties of a 3D PPy aerogel can be tuned by γ-Fe₂O₃ nanoparticles regularly. Effective EA bandwidths of 6.76 GHz (≤−10 dB, 90% electromagnetic energy was attenuated) and 1.88 GHz (≤−20 dB, 99% electromagnetic energy was attenuated) can be obtained with only 10 wt% in the composite. Furthermore, it has advantages in the synthesis strategy and purification process.

Experimental section

Materials

γ-Fe₂O₃ nanoparticles of 20 nm in diameter were purchased from Shanghai Macklin Biochemical Co., Ltd, China. The pyrrole monomer, FeCl₃·6H₂O and ethanol were purchased by Shanghai GENERAL-REAGENT Titan Scientific Co., Ltd, China. Deionized water was obtained from Direct-Q3 UV, Millipore.

Fabrication of γ-Fe₂O₃ tuned 3D-PPy aerogel

Pyrrole monomer (10 mmol, 0.67 g) and γ-Fe₂O₃ were dispersed in 6 mL of ethanol. Then, FeCl₃·6H₂O (6.2 g) in 20 mL of water was added into the abovementioned solution under quick stirring, and a wet gel was formed immediately. The as-prepared gel was left undisturbed for 24 hours to form hydrogel networks, which was further purified via a solvent exchange process with large amounts of distilled water to remove unreacted monomer and other impurities. After drying in a vacuum oven at 50 °C, a γ-Fe₂O₃ tuned 3D-PPy aerogel was obtained. The samples, containing with different amounts of γ-Fe₂O₃ (3 wt%, 5 wt%, 7 wt% and 10 wt% of pyrrole), were denoted as S1, S2, S3 and S4.

Characterization and measurements

The crystal structures were identified using an X-ray diffractometer (XRD, D8 Advance, Bruker AXS), using Cu Kα (λ = 1.54 Å) radiation from 10° to 80°. The detailed morphologies were observed with a field emission scanning electron microscope (FE-SEM, S4800, Hitachi) and a field emission high resolution transmission electron microscope (FE-HRTEM, Tecnai G2 F30 S, FEI). Energy dispersive X-ray (EDX) analysis was also carried out on the S4800. The relative complex permittivity (ε_r) and permeability (μ_r) were measured by a vector network analyzer (VNA, N5242A PNA-X, Agilent) in the frequency range of 2–18 GHz. The measured samples (10 wt%) were prepared by uniformly mixing with wax (90 wt%) at 85 °C. The mixture was then pressed into toroidal shaped samples with an outer diameter of 7.00 mm and inner diameter of 3.04 mm. In a coaxial wire analysis, ε_r of the dielectric material was calculated from the experimental scattering parameters S11 (or S22) and S21 (or S12) using the standard Nicolson–Ross–Weir (NRW) algorithm.^31,32

Conflict of interest

All authors have given their approval for the submission of the final version of the manuscript. The authors declare no competing financial interest.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (51403236) and the China Postdoctoral Science Foundation Funded Project (2015M570452).

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