Polarization enhanced multi-grain-boundary dendritic micro–nano structure α-Fe for electromagnetic absorption applications: synthesis and characterization

Abstract

In this paper, multi-grain-boundary hierarchical dendritic micro–nano structure α-Fe was successfully synthesized in electrolyte containing rare earth (RE) ions by the electric field-induced and electrochemical reduction method. The results show that the size of the dendritic morphology samples decreases from about 10 μm to about 6 μm with increasing RE ion concentration, and plentiful nano-sized particles are generated on their surface due to the adsorption suppression effect of the RE ions, which facilitates a great increase in the BET surface area and the formation of abundant grain boundaries. All of these RE-added dendritic α-Fe exhibit excellently enhanced electromagnetic absorption performance. The minimum reflection loss (RL) value (around −60 dB) of the RE-added samples is twice that of the samples without added RE ions, and the absorption peak moves to higher frequency range as the RE ion concentration is increased. The widest absorption band (in which RL < −20 dB) of the La-added samples grows to 2 GHz. We mainly ascribe the great enhancement of the EMA performance to the unique structural characteristics, such as the great surface area and abundant grain boundaries, which could affect the dielectric loss and magnetic loss of the absorbers. Not only has this work directly confirmed the influence of the interface and grain boundaries on the electromagnetic absorption performance, but it has also provided a new way to control the morphology and microstructure using RE ions.

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Introduction

Due to the great impact of electromagnetic interference on both humans and the environment,^1,2 electromagnetic shielding has been one of the matters of most concern in daily life. In general, electromagnetic shielding is mainly realized by means of the reflection and absorption functions of surface materials or coatings. However, only electromagnetic absorption is a truly effective method to consume the electromagnetic energy. The mechanisms of electromagnetic absorption are different and complicated for different materials. The causes of electromagnetic absorption include dielectric loss and magnetic loss.^3,4 Nowadays, more and more attention is paid by scientists to dielectric loss, especially to polarization, which plays the pivotal role in dielectric loss of electromagnetic absorption.⁵ In order to increase the dielectric loss, many scientists are turning their attention to composites composed of two or more constituents, in which not only could the complementarity between the dielectric loss and the magnetic loss be tuned, but also the polarization is enhanced dramatically. The insulator or dielectric/magnetic composite materials (such as Fe@SiO₂,⁶ Ni/SiO₂,⁷ Fe@ZnO,⁸ Fe₃O₄/TiO₂,⁹ Fe₃O₄/SnO₂,¹⁰ and others^11–13) could enhance the interfacial polarization, and affect the eddy current loss and the associated relaxation.^8,12 Carbon/magnetic materials (such as graphene/Fe,¹⁴ porous carbon/Co,¹⁵ carbon nanotubes/CoFe₂O₄,¹⁶ and others¹⁷) also could enhance interfacial polarization^18,19 and improve electrical conductivity.²⁰ Polymer/magnetic or dielectric materials, especially the polyvinylidene fluoride composite materials,^21,22 are novel materials that have recently appeared and could improve the relaxation and interfacial polarization²³ more significantly by the formation of large dipoles, and further result in the Debye dipolar relaxation, Maxwell–Wagner relaxation and synergic effects.^24,25 All of these composites consisting of different forms, such as coated, doped and hybrid composites, provide multiple interfaces and heteronanostructures which further change their dielectric properties, and contribute to a great enhancement of the electromagnetic absorption properties. Therefore, the great significance of polarization is obvious.

Micro–nano structure iron metal materials are the electromagnetic absorbers with most potential due to their low price and excellent performance in the high frequency range.^26–28 In previous work, we have synthesized a hierarchical dendritic micro–nano structure α-Fe electromagnetic absorber which exhibits excellent electromagnetic absorption performance.²⁹ Based on the huge positive effect of grain boundaries for the electromagnetic absorption performance discussed above, the motivation of this work is to further enhance the polarization for electromagnetic waves by increasing the number of interfaces and grain boundaries of dendritic micro–nano structure α-Fe. According to the reports in the literature, the strong adsorption effect stemming from the special 4f atomic orbitals of rare earth (RE) ions in the electrolyte could increase the nucleation rate, and refine the grains.^30,31 Therefore, in this work, RE ions (La³⁺ and Ce³⁺) were added as additional agents in the electrolyte during the electrodeposition process, by which the smaller sized multi-grain-boundary hierarchical dendritic micro–nano structure α-Fe absorbers were successfully synthesized. The morphology, structure and properties of the samples were also systematically studied. Additionally, the relation between the multi-grain-boundary structure and the electromagnetic absorption performance was analyzed as well. This work not only provides an excellent idea to control the crystal growth under the electric field force in the electrolyte, but also extends the application of the RE elements dramatically.

Experimental section

Multi-grain-boundary dendritic micro–nano structure α-Fe absorbers were fabricated through an electric field-induced and electrochemical reduction method under constant current mode, which is reported in our previous work.^29,32 The ambient anodic area contained 250 mL 0.1 mol L⁻¹ sulfuric acid solution. Meanwhile, the center cathode area contained 40 mL 1.0 mol L⁻¹ iron(II) sulfate heptahydrate and a certain amount (0.25, 0.50, 1.00, 2.00 g L⁻¹) of lanthanum nitrate hexahydrate or cerium(IV) sulfate tetrahydrate mixed solution (actually, the cerium ions in the electrolyte are Ce³⁺, because the Ce⁴⁺ ions were reduced to Ce³⁺ by Fe²⁺ in the electrolyte, Ce⁴⁺ + Fe²⁺ = Ce³⁺ + Fe³⁺). Herein, we use Fe–1La, Fe–2La, Fe–3La and Fe–4La (or Fe–1Ce, Fe–2Ce, Fe–3Ce and Fe–4Ce) as the symbols to stand for the RE-added dendritic α-Fe synthesized under different RE ion concentrations. The current density and reaction time were 30 A cm⁻¹ and 20 seconds, respectively. The black flocculent samples generated on the copper electrode while the reaction was in progress were transferred into a centrifugal tube filled with oxygen-free deionized water and separated by ultrasonication. The products were washed and centrifuged three times with oxygen-free deionized water and ethanol. Then, the final product was dried at 60 °C under vacuum.

The crystalline structure and composition of the samples were characterized using X-ray diffraction (D/max-rB, RICOH, Japan) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Optima 8300, PerkinElmer, America). Their morphology and microstructure were obtained using Scanning Electron Microscopy (FEI Quanta 200F, America), Transmission Electron Microscopy and High-Resolution Transmission Electron Microscopy (FEI Tecnai G2 F30, America). Nitrogen adsorption isotherm measurements were carried out at 77.3 K with a high-performance Surface Area Analyzer and Pore Size Analyzer (Quadrasorb SI, Quantachrome, America). The magnetic properties of dendritic α-Fe were examined on a Vibrating Sample Magnetometer (Lake Shore 7404, America). The complex permittivity and permeability of dendritic α-Fe were tested on a vector network analyzer (Agilent N5230A, America). The samples for electromagnetic absorption property testing were dispersed in paraffin homogeneously with a sample-to-paraffin mass ratio of 7 : 3, and then the mixture was molded into an annular-shaped sample with a 7.00 mm external diameter, 3.01 mm internal diameter and 3.0 mm thickness.

Results and discussion

Fig. 1 shows scanning electron microscopy (SEM) images of La-added and Ce-added dendritic micro–nano structured α-Fe samples. The Fe–1RE sample (as shown in Fig. 1a and c) possesses almost the same size and morphology as the pure dendritic α-Fe (Fig. S1†).²⁹ Moreover, SEM images of Fe–4RE and other RE-added samples (Fig. S2†) show that the leaf-like dendritic micro–nano structure morphologies become a little untidy and disordered, but they still keep the leaf-like shape. However, with increasing RE concentration in the electrolyte, their lengths change significantly, decreasing from about 10 μm for Fe–1La to about 6 μm for Fe–4La (Fig. 1a and b). The Ce-added dendritic micro–nano structure α-Fe samples in Fig. 1c and d show a similar tendency to that of the La-added samples.

Fig. 1 SEM images of RE-added dendritic micro–nano α-Fe fractals: (a) Fe–1La, (b) Fe–4La, (c) Fe–1Ce and (d) Fe–4Ce.

The transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images in Fig. 2 clearly show the refined structure and morphologies of Fe–4La and Fe–4Ce. Both Fe–4La and Fe–4Ce are about 6 μm long and about 1.5 μm wide. Their branches and trunks become bent and asymmetrical compared to those of dendritic α-Fe samples without added RE ions or with less added RE ions. In addition, HRTEM micrographs show that there are a lot of nano-sized particles on the surface of the branches and tips (Fig. 2b, e and f) which could provide plentiful grain boundaries (the boundaries among nanosized particles). We propose that these particles are the result of the suppression effect of the RE ions.³³ Also, these generated nano-sized particles could suppress the growth of the dendritic α-Fe and promote the formation of the unique multi-nanoparticle structure, which could further decrease their size and provide plentiful grain boundaries (Fig. S3†). However, because the main segments of the trunks grow along the [110] direction, as shown in Fig. 2c, the samples still keep a similar dendritic shape. Selected area electron diffraction (SAED) patterns (Fig. S4†) indicate that the crystals are polycrystalline in structure for those dendritic samples as shown in Fig. 2a and d.

Fig. 2 TEM and HRTEM images of (a, b and c) Fe–4La and (d, e and f) Fe–4Ce dendritic micro–nano structure α-Fe fractals; (a and d) low-magnification TEM images; (b and e) HRTEM images taken from the branches labeled as (b) and (e) in (a) and (d), respectively; (c and f) HRTEM images taken from the areas labeled as (c and f) in (b) and (e), respectively.

The XRD patterns (Fig. 3a) indicate that all these samples are indexed to the body-centered cubic phase of α-Fe. With increasing amounts of added RE ions, the diffraction peaks become broader and their relative intensities decrease. It is well known that peak broadening can be caused by a reduction in crystallite size and/or an increase in lattice strain.³⁴ Therefore, this change may be further evidence for the formation of nano-sized particles. According to the Brunauer–Emmett–Teller (BET) theory, the BET surface areas of La-added and Ce-added samples were calculated from their N₂ adsorption–desorption isotherms (Fig. S5†). The values rose from about 10 m² g⁻¹ for the samples without added RE ions to 40.5 m² g⁻¹ for Fe–4La and 45.2 m² g⁻¹ for Fe–4Ce (Fig. 3d). The increase in the surface area results from the smaller size and the nano-sized particles after adding more RE ion agents in the electrolyte.

Fig. 3 (a) XRD patterns of the La-added and Ce-added dendritic micro–nano structure α-Fe; (b) BET surface areas of the dendritic micro–nano structure α-Fe synthesized with different RE ion concentrations.

Based on the adsorption suppression effect of the RE elements, we propose a possible formation mechanism for the nano-sized particles. During the reaction process, the Fe²⁺ ions are reduced and deposited on the electrode surface as illustrated in our previous work. However in this case, the RE³⁺ ions are also enriched on the cathode surface under the electric field force and further adsorbed on the surface of the cathode and the generated dendritic α-Fe due to their strong adsorption effect stemming from the RE ions’ special 4f atomic orbitals,^35,36 which further suppresses the dendritic structure growth and distorts the growth direction. The cathodic polarization curves (Fig. S6†) indicate that at the high polarization current density, RE-added samples show higher over-potentials than samples without added RE ions, which also confirms that RE ions play a role in suppressing the deposition of Fe²⁺. Due to the low standard reduction potential and the very low ion concentration of RE³⁺, they cannot be reduced from the electrolyte and enter the crystal lattice of iron. The amount of RE elements in the sample is lower than 0.78 wt% (Table S1†), which is from the adsorbed RE elements on the samples’ surface. This novel crystal structure is quite important to the chemical and physical properties of the dendritic micro–nano materials.

In order to elucidate the magnetic properties of the multi-grain-boundary dendritic micro–nano structure α-Fe, their static magnetic properties were measured using a Vibrating Sample Magnetometer at room temperature, and their magnetic hysteresis (M–H) loops are given in Fig. S7.† The results (in Fig. 4) indicate that the La-added and Ce-added dendritic α-Fe display a similar variation tendency for saturation magnetization (M_s) and coercivity (H_c) values. With increasing RE ion concentration in the electrolyte, the M_s value decreases from 215.46 emu g⁻¹ for Fe–1La to 109.53 emu g⁻¹ for Fe–4La; for the Ce-added samples, the M_s value also reaches a minimum of 117.77 emu g⁻¹ for Fe–4Ce. This decrease possibly results from the increased volume of the grain boundaries of the RE-added samples.^37,38

Fig. 4 (a) Saturation magnetization (M_s) and (b) coercivity (H_c) plots of the La-added and Ce-added dendritic micro–nano structure α-Fe synthesized in electrolyte with different RE concentrations.

However, the H_c value has the inverse tendency compared with M_s. It increases from about 330 Oe for Fe–1La and Fe–1Ce to maximums of 571.8 Oe (Fe–4La) and 624.56 Oe (Fe–4Ce). The larger H_c value is attributed to the electron structure, shape anisotropy and hierarchical structure of the RE-added dendritic α-Fe synthesized under a nonequilibrium state.^39–41 A magnetic material with a large coercive field (H_c) is expected to show a high-frequency resonance.⁴² So we predict that the sample containing more added RE would have a stronger absorption in the high-frequency region.

Fig. 5 shows the complex relative permittivity (ε_r = ε′ − jε′′, ε′ and ε′′ are the real and imaginary parts of the complex permittivity) and permeability (μ_r = μ′ − jμ′′, μ′ and μ′′ are the real and imaginary parts of the complex permeability) of the La-added and Ce-added dendritic micro–nano structure α-Fe. From Fig. 5 we know that the complex relative permittivity and permeability of all the samples have similar shapes due to their pristine electromagnetic properties. However, the addition of both La and Ce could obviously alter the values of the complex relative permittivity and permeability simultaneously at some specific frequency. For the La-added and Ce-added samples, the ε′ and ε′′ values of the dendritic micro–nano structure α-Fe are larger than those of the samples without added RE²⁹ in the whole frequency range. Meanwhile, the μ′ values keep a similar tendency to that of the pure sample in almost the whole frequency range. The μ′′ values decrease clearly with increasing RE ion concentration, especially between 2 to 9 GHz. The plentiful grain boundaries which act as the electric charge centers⁴³ in the samples could enhance the polarization and decrease the electrical conductivity. At the same time, the exchange energy and anisotropy energy are also changed by the plentiful grain boundaries.⁴⁴ These changes result in the variations in the complex relative permittivity and permeability after adding RE elements.

Fig. 5 Frequency dependence of the real and imaginary parts of (a and c) complex relative permittivity and (b and d) complex relative permeability of the La-added, Ce-added, and RE-free dendritic micro–nano structure α-Fe.

Especially in the La-added dendritic α-Fe samples (Fig. 5a and b), with increasing La³⁺ concentration, the response peak of ε′′ at about 16.5 GHz weakens, and the response peak of μ′′ at about 17.6 GHz strengthens. For Ce-added samples, the concentration has a more significant influence on the complex relative permeability. Two response peaks for ε′′ and ε′ (shown in Fig. 5c) at 12–14 GHz and 16–18 GHz appear and strengthen with increasing Ce³⁺ content. Most importantly, an interesting response peak in the μ′ and μ′′ spectra (Fig. 5d) appears between about 13 GHz and 17 GHz for both Fe–3Ce and Fe–4Ce. This indicates that the exchange resonances of Fe–3Ce and Fe–4Ce are enhanced dramatically. These changes are attributed to the enhancement of the energy anisotropy of the sample,⁴⁵ which affects their response frequency and resonance behaviours. In addition, we propose that all of these differences in the complex relative permittivity and permeability between the La-added and Ce-added samples may result from the different properties and orbital structures of the La and Ce atoms adsorbed on the samples’ surface.

To evaluate the electromagnetic absorption performance of multi-grain-boundary dendritic micro–nano structure α-Fe, the reflection loss (RL) values were calculated through eqn (1) and (2):^15,46

where ε_r and μ_r are the complex relative permittivity and permeability, f is the applied frequency, c is the velocity of light, and d is the thickness of composites. The calculated results are shown in Fig. 6 and 7, respectively.

Fig. 6 EMA performance of (a) Fe–1La, (b) Fe–2La, (c) Fe–3La, and (d) Fe–4La samples which were mixed with paraffin at different thicknesses in the 2–18 GHz frequency range.

Fig. 7 EMA performance of (a) Fe–1Ce, (b) Fe–2Ce, (c) Fe–3Ce, and (d) Fe–4Ce samples which were mixed with paraffin at different thickness in the 2–18 GHz frequency range.

In Fig. 6, the minimum RL value of Fe–1La is −12.5 dB at 2 GHz with a 4.8 mm thickness. When the La³⁺ concentration in the electrolyte is 1.00 g L⁻¹, the RL value decreases to a minimum (−62.1 dB) at about 3.5 GHz with the thickness of 3.3 mm. As the La³⁺ concentration continues to increase, the minimum RL value stays around −60 dB, and the absorption peaks shift towards higher frequency (6–10 GHz) with a thinner thickness of about 2.0 mm. The widest absorption band (in which RL < −20 dB) jumps from 0 for Fe–1La to about 0.5 GHz for Fe–2La with a thickness of 2.5 mm, and then reaches the maximum of more than 2 GHz for Fe–4La with a thickness of 1.2 mm. In Fig. 7, the minimum RL value drops from −34 dB at about 2.3 GHz to −66.5 dB at about 7.5 GHz when the Ce³⁺ concentration in the electrolyte increases from 0.25 g L⁻¹ to 2.00 g L⁻¹. Meanwhile, the corresponding thickness decreases from about 5.0 mm to 2.0 mm. However, the width of the absorption band is becoming narrower and narrower with increasing Ce³⁺ content, from 1 GHz for Fe–1Ce to about 0.5 GHz for other three Ce-added samples. In addition, another absorption peak appears in the range of 16–18 GHz when the RE ion concentration in electrolyte reaches 2.00 g L⁻¹, as shown in Fig. 6d and 7d. In short, both the La-added and Ce-added samples’ minimum RL values are superior to those of the dendritic α-Fe samples without added RE ions (RL_min = −32.4 dB, absorption band width = 1.5 GHz, thickness = 1.5 mm).²⁹ The absorption band width of Fe–4La is also obviously wider than that of the sample without added RE ions. Furthermore, the electromagnetic absorption performance of the RE-added dendritic α-Fe samples is superior to many other modified metal based absorbers (such as Ag encapsulated Fe@SiO₂,⁴⁷ Ni@TiO₂ and Ni@SiO₂ (ref. 48)), of which RL_min is −40 dB and the maximum width of the absorption band (in which RL < −10 dB) reaches 3.5 GHz.

From the above results, we propose that the excellent EMA performance may be attributed to the influence of the RE ions by adjusting the refined dendritic structure and size. The adsorption suppression effect of the RE ions in the electrolyte could lead to the generation of plentiful nanosized particles and decrease the size of the synthesized samples. The large BET surface area could increase the interfacial polarization between the conductive absorber and the insulating paraffin.^49,50 More importantly, the abundant nanosized particles on the RE-added dendritic α-Fe would provide more grain boundaries, which could also act as polarized centers and further enhance the polarization ratio.^19,51,52 Therefore, we propose that the abundant grain boundaries could enhance the dielectric loss of the samples dramatically.

What’s more, the formation of plentiful nanosized particles also affects the magnetic properties, and further affects the magnetic loss.⁴⁴ The plentiful nanosized grains on the hierarchical dendritic morphology α-Fe contribute to enhancing the shape anisotropy field (H_a) which is a key factor related to the resonance frequency.⁵³ According to the natural resonance equation, 2πf_r = νH_a, where ν is the gyromagnetic ratio,⁴² the added RE elements make the ferromagnetic response peaks shift to the high-frequency band.¹⁸ We propose that the response peak located between 8 GHz and 12 GHz may be ascribed to natural resonance (Fig. S8b and d†). Meanwhile, the response peaks between 13 GHz and 17 GHz in the magnetic loss tangent curves of Fe–3Ce and Fe–4Ce are strengthened dramatically because of the enhancement of exchange resonance.⁵⁴

Conclusions

In this work, multi-grain-boundary hierarchical dendritic micro–nano structure α-Fe was successfully synthesized. The addition of RE obviously affects the dendritic structure and size due to its suppression effect on crystal growth. With increasing RE ion content, the dendritic α-Fe becomes smaller and meanwhile plentiful nanosized particles are generated on its surface, which lead to the distinct increase of the BET surface area and the generation of abundant grain boundaries. The abundant grain boundaries and large BET surface area contribute to improving the polarization centers and increasing the interfacial polarization. Moreover, these nanosized particles could also increase the magnetic shape anisotropy, which changes the resonance behavior and makes the response frequency move towards the high-frequency range. The results show that the minimum RL could reach −66.5 dB for Fe–4Ce, and the widest absorption band, in which RL < −20 dB, is 2 GHz for Fe–4La. Obviously, the EMA performances are dramatically enhanced in absorption efficiency and absorption band width, compared with the dendritic α-Fe without added RE ions, and many other electromagnetic absorbers.

This work offers a novel idea to enhance the performance of electromagnetic absorbers by changing their microstructure and morphology. Meanwhile, it provides a meaningful way to control the morphology and structure of the products by adding RE ions to the electrolyte. It will extend the application of the RE elements dramatically in the electromagnetic absorption area and crystal structure control area.

Acknowledgements

This work is supported by State Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (no. 2015DX07).

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Footnote

† Electronic supplementary information (ESI) available: More SEM, HRTEM and SAED images of RE-added dendritic micro–nano α-Fe, specific ICP-OES data, the N₂ adsorption–desorption isotherms, cathodic polarization curves and magnetic hysteresis (M–H) loops of all the RE added dendritic micro–nano α-Fe, and the calculated dielectric loss and magnetic loss are given. See DOI: 10.1039/c5ra01665g

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