Fabrication and enhanced microwave absorption properties of Al₂O₃ nanoflake-coated Ni core–shell composite microspheres

Abstract

Core–shell composite microspheres with Ni cores and Al₂O₃ nanoflake shells have been successfully fabricated by the hydrothermal deposition method. The electromagnetic parameters of the Ni microspheres and Ni/Al₂O₃ composites are measured by a coaxial line method. The tangent losses for the Ni/Al₂O₃ composite are larger than those of the Ni. A significant enhancement of electromagnetic absorption (EMA) performance of Ni microspheres coated by the alumina shells was achieved over the 1–18 GHz. The reflection loss (RL) less than −10 dB of the composite was obtained over 7.5–18.0 GHz by tuning an appropriate sample thickness between 1.3 and 2.2 mm, and an optimal RL of −33.03 dB was obtained at 9.2 GHz with a thin absorber thickness of 2.0 mm. The coating of the dielectric alumina shell significantly enhanced the microwave absorption performance due to the enhancement of interface polarization between the metals and dielectric interfaces, the synergetic effect between the dielectric loss and magnetic loss and unique flake-like dielectric alumina.

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1. Introduction

Serious electromagnetic (EM) interference pollution arising from the rapidly expanding business of communication devices, such as mobile telephones, local area network systems, and radar systems, has become a concern. Therefore, microwave absorption materials have attracted a great deal of interest to deal with this problem.^1–6 Among the microwave absorption materials, core–shell structured absorbers have received much attention for their improved microwave absorption properties, such as ZnO-coated iron,⁷ graphite-coated FeNi₃,⁸ Fe/SiO₂,⁹ Fe₃O₄/TiO₂ nanorods,¹⁰ carbon-coated cementite,¹¹ Ni@Ni₂O₃ particles,¹² carbon-coated iron.¹³ In these kinds of core–shell structures, the magnetic core acting as a magnet, which increases the permeability of the composites leading to the increase of the magnetic loss, was coated by a dielectric shell, which acts not only as a centre of electric polarization but also as an insulating matrix improving impedance match. The good microwave absorption performance was obtained due to the improved magnetic loss, dielectric loss, reduced eddy current loss and impedance match.¹⁴ Thus, preparing the traditional microwave absorption materials with a core–shell structure might be a promising way to enhance their microwave absorption capability.

Nickel as a magnetic absorbent has received much attention from many researchers, due to high permeability at GHz frequency ranges, high saturation magnetization, easy preparation, as well as low cost.^1,15–17 However, because of its metallic nature, the eddy current generation severely limits their applications at high frequency.¹⁸ Therefore, in order to optimize microwave absorption performance, a strategy to solve the problem is to cover the magnetic Ni metallic particles by an inorganic and nonmagnetic coating to create a core–shell microstructure.

Al₂O₃ is a dielectric material, which can improve the dielectric properties of Al₂O₃ based composite.¹⁹ The microwave absorption performances of Al₂O₃ based core–shell absorbers were also reported in the literature.^20,21 However, to best of our knowledge, flake-like Al₂O₃ coated Ni composite microspheres have not been reported. Herein, the core–shell Ni/Al₂O₃ composite microspheres with Ni cores and Al₂O₃ nanoflake shells were synthesized through a simple hydrothermal deposition method. Compared with the pure Ni microspheres, the microwave absorption performance of Ni microspheres can be remarkably enhanced after coating with Al₂O₃ nanoflakes.

2. Experimental section

All chemicals used in this paper were of analytical grade without further purification. Al₂O₃ nanoflake-coated Ni composites were prepared through a two-step method. The Ni microspheres were synthesized by a solvothermal route, which was described in our pervious literature.²² Ni/Al₂O₃ core–shell microspheres were prepared through a templating method. Firstly, the as-prepared Ni microspheres (0.05 g) were added into 60 mL distilled water. After mechanical stirring for 15 min, NaOH (0.17 M) and AlCl₃·6H₂O (0.017 M) were introduced to the solution subsequently. Secondly, the mixture was transferred into a Teflon-lined stainless steel autoclave, and heated at 200 °C for 15 h. When cooled to room temperature, the obtained precipitates were washed with absolute ethanol and distilled water.

Phase analysis was carried out by X-ray diffraction (XRD, XD-3, Beijing Purkinje General Instrument Co. Ltd. Cu Kα radiation source, λ = 0.15406 nm). The morphology, size and chemical composition of the synthesized samples were characterized by field-emission scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (FESEM/EDS; JSM-7001F). The surface compositions of the core–shell composites were determined by X-ray photoelectron spectroscopy (XPS), which was carried out on an AXIS ULTRA (Manchester, U.K.) apparatus. For measurement of the microwave properties, the samples were dispersed in paraffin homogeneously with a sample-to-paraffin weight ratio of 2 : 3, and then the mixture was pressed into a toroidal shape with an inner diameter of 3.04 mm and an outer diameter of 7.00 mm. The complex permittivity and permeability of the composites were measured between 1–18 GHz on a vector network analyzer (Agilent N5244A).

3. Results and discussion

Fig. 1 shows XRD patterns of the Ni microsphere and Ni/Al₂O₃ composite. It can be seen that the XRD patterns of the two samples match well with face-centred cubic (fcc) nickel (JCPDS card no. 04-0850). There are no detectable peaks of nickel oxides. It should be noted that there is no evidence for the presence of crystalline Al₂O₃ in the XRD pattern. As a result, we deduce that the alumina in the composites is in an amorphous state. In order to obtain more information of the alumina, the core–shell composites were investigated by XPS. The XPS pattern of Al 2p_3/2 on the surface of Ni/Al₂O₃ is shown in detail in Fig. S1.† The shell (alumina) of core–shell structured composites can be determined as Al₂O₃ based on the binding energy of 74.5 eV, which are also confirmed by other reports.^23,24

Fig. 1 XRD spectra of (a) Ni microspheres and (b) Ni/Al₂O₃ composites.

Fig. 2a shows the representative FESEM image of the Ni particles, which possess uniform sphere-like shape and the diameter of 0.8–1.0 μm. The citrate groups were grounded on the particle surface during the solvothermal reaction, which facilitate the subsequent coating with the Al₂O₃ layer.²⁵ Fig. 2b and c show the FESEM images of the obtained Ni/Al₂O₃ microspheres with the diameter of 0.9–1.1 μm. One significant difference is clearly discerned between the Ni/Al₂O₃ microspheres and the naked Ni microspheres. The difference is that flake-like Al₂O₃ nanoparticles were deposited on the surfaces of Ni microspheres, which indicates Ni microspheres were coated by Al₂O₃ layer to form core–shell structure. From the high magnified SEM image (the inset of Fig. 2c), the diameter of nanoflakes are mostly in the range of 100–200 nm and the thickness is about 30 nm. The EDS spectrum shown in the Fig. 2d indicates the presence of three elements of Ni, Al and O in the Ni/Al₂O₃ product. The C element signal originates from the carbon conductive tape to support the samples during the test.

Fig. 2 (a) SEM image of the Ni microspheres, (b and c) different magnified SEM image of Ni/Al₂O₃ particles and (d) EDS spectra of the Ni/Al₂O₃ composite. High magnified SEM image of Ni/Al₂O₃ composite is shown in the inset of (c).

To further confirm the core–shell structure of Ni/Al₂O₃, the enlarged magnification FESEM image and the elemental mappings of Ni/Al₂O₃ were performed in Fig. 3. The Ni element can be clearly detected in the core region, while the Al element and O element can be detected in the shell regions. This further validates the unique core–shell structures with Ni cores and Al₂O₃ shells.

Fig. 3 (a) The enlarged magnification FESEM image of Ni/Al₂O₃, (b–d) elemental mappings of Ni, Al and O.

Fig. 4a and b shows the real part (ε′) and the imaginary pat (ε′′) of complex permittivity for the paraffin matrix composites containing Ni microspheres or Al₂O₃@Ni core–shell structures in the 1–18 GHz range. It can be seen that ε′ of the Ni sample is almost constant (∼5) in the 1–18 GHz range. The real part ε′ of the Al₂O₃@Ni sample shows some variations (11.3–20.2) in the 1–18 GHz range, and has a maximum value (20.2) at 7.4 GHz. The imaginary part ε′′ of the Ni sample is in the range of 0.001–0.71. The ε′′ of the Al₂O₃@Ni sample shows larger values and the maximum value of 7.00 is obtained at 8.0 GHz. According to the free electron theory,^7,18 ε′′ ≈ 1/πε₀ρf, where ρ is the resistivity. The low imaginary component of the complex permittivity indicates a high resistivity of the materials. The electric resistivity of the Ni samples is higher than that of the Al₂O₃@Ni samples. From previous reports,^23,26 the higher electric resistivity of the Ni samples was attributed to the low filler ration (40 wt%) and high dispersion in the paraffin composites. Metal Ni particles can not connect with each other, which leads to high electric resistivity. However, for the Al₂O₃ nanoflake-coated Ni composites, the unique Al₂O₃ nanoflakes were deposited on the surfaces of Ni microspheres, which introduce additional interfaces and more polarization charges on the surface of the particles. This makes interfacial polarization important and the associated relaxation will give rise to low electric resistivity.²¹ The Ni/Al₂O₃ sample has higher real and imaginary parts of the relative permittivity than that of the Ni sample since the shell of dielectric alumina enhances the complex permittivity.^20,21 It is known that the real part of permittivity is an expression of the polarization ability of a material which arises mainly from the dipolar and interfacial polarization at microwave frequency.^27,28 Interfacial polarization always presents in materials comprised of more than one phase composites. This kind of polarization arising at the interfaces is due to the migration of charge carriers through different phases of the composite material, which results in differential charge accumulation at the interfaces. When these charges are made to move by the application of an external electric field, the motion will be hampered at various points of the composite material differently, causing space charge to appear. In this case, the coating of alumina on Ni microspheres introduces metal–dielectric interfaces. The interface polarization between metal and alumina results in a higher real permittivity. The imaginary part ε′′ of the Ni/Al₂O₃ sample presents a significant peak at 7.4 GHz, which is related to the interface relaxation.²⁹ During the activation of an electromagnetic wave, a redistribution process of charges occurs periodically between the Ni cores and Al₂O₃ shells. As a result, besides the enhanced dielectric relaxation of the Al₂O₃ shell, an additional interfacial relaxation between core–shell interfaces is constructed. Apart from the main resonance peak at 7.4 GHz, several other peaks are observed, which can be due to different kinds of relaxation process. The complex permittivity of the Ni/Al₂O₃ sample presents larger fluctuations in high frequencies, which is ascribed to displacement current lag at the ‘core–shell’ interface as the frequency varied.³⁰ The higher values and larger fluctuation of complex permittivity of the Ni/Al₂O₃ sample indicate higher dielectric loss. It is beneficial for obtaining the enhanced microwave absorption performance at high frequencies.

Fig. 4 Frequency dependence of (a) real part (ε′) and (b) imaginary part (ε′′) of the complex permittivity (ε_r), (c) real part (μ′) and (d) imaginary part (μ′′) of complex permeability (μ_r) of Ni– and Al₂O₃@Ni–paraffin composites.

Fig. 4c and d shows the real part (μ′) and imaginary part (μ′′) of complex permeability (μ_r) of Ni and Al₂O₃@Ni as a function of frequency. The μ′ of Ni and Ni/Al₂O₃ decrease from 1.15 and 1.45 to about 0.93 and 0.67 with increasing the frequency from 1 to 18 GHz, respectively. The μ′′ curves of both Ni and Al₂O₃@Ni present a broad peak at around 6 GHz with maxima of 0.11 and 0.13, respectively. Generally, the permeability spectra is explained by hysteresis loss, domain-wall resonance, eddy current effect and natural resonance. The magnetic hysteresis can be excluded at high frequency and weak applied field.³¹ The domain-wall resonance usually appears in the 1–100 MHz range in the multidomain materials and certainly is absent in the present samples.¹⁸ Otherwise, the eddy current loss in these core–shell-type samples can be effectively suppressed by the outer insulator.³² Therefore, we can conclude that natural resonance and non-uniform exchange resonance^17,33 are the main magnetic loss mechanisms for Ni and Al₂O₃@Ni. Besides, the μ′′ value of Ni/Al₂O₃ is larger than those of Ni, which would be significant to its use as an electromagnetic wave absorption material in gigahertz frequency region.

The microwave absorption property, denoted by the reflection loss (RL), can be calculated based on the relative permeability and permittivity for a given frequency and absorber thickness, by means of the following equations:^21,34

RL = 20 log₁₀|(Z_in − Z₀)/(Z_in + Z₀)| (1)

where Z₀ is the impedance of free space, Z_in is the input characteristic impedance, ε_r is the complex permittivity, μ_r is the complex permeability, f is the frequency, c is the velocity of light, and d is the thickness of the composites. Fig. 5 shows the relationship between RL and frequency for the paraffin wax composites with 40 wt% Ni and Ni/Al₂O₃. One can see that the microwave absorption properties of Ni particles were weak and the minimum reflection loss value is only −4.55 dB at 13.6 GHz with the absorber thickness of 2.2 mm. In comparison with bared Ni microspheres, after coating with Al₂O₃ nanoflakes, the Ni/Al₂O₃ shows excellent microwave absorption properties. The optimal RL of −33.03 dB was obtained at 9.2 GHz and with the thickness of 2.0 mm. The RL values less than −10 dB were recorded in the 7.5–13.3 GHz range with the absorber thicknesses of 2.0 mm. The effective absorption (below −10 dB) bandwidth can be tuned between 7.5 GHz and 18.0 GHz by adjusting thickness in 1.3–2.2 mm. It can be found that the peak values of RL shift to a lower frequency with increasing absorber thickness in Fig. 5b. A quarter-wavelength theory can explain this phenomenon.³⁵ If an absorber thickness (d) equals 1/4λ_in, the incident and reflected waves in the absorber are out of phase 180° and giving rise to the reflected waves in the air-absorber interface are totally cancelled. For quarter-wavelength cancellation, the relationship between d_m and f_m is given by the following equation:where μ_r is the complex permeability, ε_r is the complex permittivity, c is the velocity of light in the free space. It can be seen that f_m will shift to lower frequency with increasing absorber thickness. Table 1 exhibits the microwave absorption properties of typical Ni-based composites and some reported core–shell samples in recent literatures.^{7,10,22,36–41} According to the comparison, the core–shell structured Ni/Al₂O₃ composite microspheres are very promising to be used as thin-thickness, and high EM wave absorptive materials in a wide frequency range.

Fig. 5 Frequency dependence of the reflection loss for the paraffin matrix composites containing 40 wt% (a) Ni and (b) Ni/Al₂O₃ with different thicknesses.

Table 1 Electromagnetic absorption properties of some reported samples and Al₂O₃ nanoflake-coated Ni composites

Samples	Minimum RL value (dB)	Minimum RL fm (GHz)	Bandwidth (RL <−10 dB)	Optimum thickness (mm)	Reference
Ni nanowires	−8.5	10	—	3.0	36
Flowers-like Ni	−17	13	11.5–14	3.0	37
Fe3O4/graphene	−8.75	8.11	—	1.5	38
Ni chains	−25.29	9.6	8.3–10.4	2.0	39
Ni/graphene	−13	11	9.6–12.2	2.0	40
Ni/polypyrrole	−15.2	13.0	11–15.4	2.0	41
ZnO-coated iron	−14.2	7.8	7.4–8.3	3.0	7
Ni/SnO2 microspheres	−18.6	14.7	13.8–15.3	7.0	22
Fe3O4/TiO2	−20.6	17.28	16–18	5.0	10
Ni/Al2O3	−33.03	9.2	7.5–13.3	2.0	This work

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The dielectric loss factor (tan δ_ε = ε′′/ε′) and the magnetic loss factor (tan δ_μ = μ′′/μ′) may well explain why the Ni/Al₂O₃ composites have excellent microwave absorption properties in so wide frequency range, as shown in Fig. 6. Ni/Al₂O₃ sample exhibits the higher dielectric loss and magnetic loss between the two samples, which indicates the excellent capability of microwave absorption. On the other hand, the Ni/Al₂O₃ core–shell microspheres exhibit strong magnetic loss at the low-frequency range and significant dielectric loss at the high-frequency range. Such complementarities between the dielectric loss and the magnetic loss imply that the core–shell microspheres have excellent EM absorption properties. It is evident that the improved microwave absorption properties for the Ni/Al₂O₃ composites are a consequence of better electromagnetic matching due to the existence of dielectric Al₂O₃ shells, as well as its particular ‘core–shell’ microstructure. Moreover, the unique flake-like Al₂O₃ structure is also beneficial for the enhancement of EM wave absorption.⁴² Flake-like morphology can lead to multiple scattering and further enhance the attenuation of the electromagnetic wave.

Fig. 6 The dielectric loss factor (a) and magnetic loss factor (b) of Ni and (b) Ni/Al₂O₃ microspheres–paraffin wax sample versus frequency.

From the RL versus frequency patterns, it is obvious that the coating of the dielectric flake-like Al₂O₃ shell can obviously enhance the electromagnetic wave absorption performance. According to transmission line theory, effective absorbers are dominated by two key factors. One is the impedance match, which requires the complex permittivity and permeability to be equal, and the other is the EM attenuation in the interior absorber. The EM attenuation was determined by the attenuation constant α, which can be described as:^13,29

where f is the frequency of the EM-wave and c is the velocity of light. Fig. 7 shows the frequency dependence of the attenuation constant. The Ni/Al₂O₃ possesses biggest α in all frequency ranges, indicating the excellent attenuation or EM wave absorption. From the above equation, it is noted that high values of ε′′ and μ′′ would result in high α. The higher ε′′ and μ′′ values seem important for obtaining highest α in Ni/Al₂O₃ core–shell structures, which is related to interface polarization and relaxation. Consequently, the enhancement of the EMA performance for the dielectric coating comes from the increase in dielectric loss and magnetic loss. Otherwise, the dielectric coating introduces the metal–dielectric interfaces, at which the interface polarization increases the microwave attenuation.

Fig. 7 Attenuation constant of Ni and Ni/Al₂O₃–paraffin composites versus frequency.

4. Conclusion

In summary, the Ni microspheres were successfully coated by amorphous Al₂O₃ nanoflakes via a two-step process. Compared with naked Ni microspheres, the Ni/Al₂O₃ composites possess excellent microwave absorption properties with wide frequency band, strong absorption and thin absorber matching thickness. The excellent microwave absorption properties are attributed to high attenuation constant, interface polarization of core–shell structure and synergetic effect between the dielectric loss and magnetic loss. Consequently, Ni/Al₂O₃ core–shell structured microspheres may prove to be an attractive candidate for microwave absorption material to applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10638e

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