Ni@C nanocapsules-decorated SrFe₁₂O₁₉ hexagonal nanoflakes for high-frequency microwave absorption

Highlights

• SrFe₁₂O₁₉ nanoflakes decorated with Ni@C nanocapsules were prepared.

• The RL exceeding −10 dB is obtained in 14.4–18 GHz for the thickness of 2.0 mm.

• The strong absorption peak reaches −55.72 dB at 8.48 GHz for the thickness of 3.6 mm.

Abstract

The novel structures of SrFe₁₂O₁₉ nanoflakes decorated with Ni@C nanocapsules were successfully prepared by the conjugating assemble process, in which Ni@C nanocapsules with the diameter of about 5 nm adhered to the surface of SrFe₁₂O₁₉ hexagonal nanoflakes with the length of a side from 0.5 to 1.0 μm and the thickness of about 80 nm. The electromagnetic properties of SrFe₁₂O₁₉/Ni@C-paraffin composite were investigated in the 2–18 GHz frequency range. The effects of Ni@C nanocapsules on the electromagnetic properties of SrFe₁₂O₁₉/Ni@C were studied. The reflection loss exceeding −10 dB is obtained in 14.4–18 GHz for an absorber thickness of 2.0 mm. Moreover, the strong absorption peak reaching −55.72 dB can be observed in 8.48 GHz when the thickness is 3.6 mm. Compared with the SrFe₁₂O₁₉ nanoflakes, the nanocomposites exhibit the thinner thickness, the broader absorption bandwidth and the stronger absorption intensity, which is ascribed to the introduction of Ni@C nanocapsules. Ni@C nanocapsules can induce double dielectric relaxation loss and enhance dielectric loss and generate the cooperative effects of dielectric-magnetic properties. These excellent absorbing performances show that the SrFe₁₂O₁₉/Ni@C nanocomposites have great potential for application in a microwave-absorption field for their strong absorption and broad bandwidth.

Introduction

Electromagnetic (EM) interference and radiation in microwave frequency band become more and more serious with the development of information and communication techniques [1], [2], [3], [4], [5]. Microwave absorption materials have attracted increasing attention on reduction of EM pollution/radiation and shielding interference in the GHz range [6], [7], [8], [9], [10]. The microwave absorption performance can be determined by the complex permeability/permittivity, EM impedance matching and the microstructure of the absorbents [8]. In order to obtain the good EM impedance matching, researchers have fabricated core-shell structured nanocapsules microwave absorbents, in which magnetic metal/alloy nanoparticles act as core, while dielectric materials, like onion-like carbon, graphene, ZnO, TiO₂, CuO, SnO, SiO₂ and polyaniline, are often selected as the shells of nanocapsules absorbents [1], [2], [6], [11], [12], [13], [14], [15]. Among the above core-shell structure, the permittivity is far larger than the permeability, which is not helpful for building the good EM match degree. Consequently, a magnetic nanoparticle with high permeability and high resonance frequency characteristics at microwave range need to be exploited to adapt to high-speed developing microwave absorber.

Snoek pointed that the product of the initial permeability and the resonance frequency was proportional to the saturation magnetization: (μ_S−1)f_r = 4γM_S/3 [5]. Because of the precession of magnetization under uniaxial anisotropy filed, both the extent of such precession and the loss of energy are small. In order to solve this problem, planar anisotropy picture has been proposed [5], [16], which bases on the uniaxial anisotropy. According to the planar anisotropy picture, the amplitude of precession is relatively bigger, which is ascribed to the correction of Snoek's constant with the increase of the ratio ($\sqrt{H_{\theta }/H_{\varphi }}$). Larger H_θ with smaller H_ϕ can get higher permeability and higher resonance frequency, where H_θ is the out-of-plane anisotropic field and H_ϕ is the in-plane anisotropy field [5].

The planar anisotropy ferromagnetic particles have been attracting considerable interest in experiment. The M-type hexaferrites are suitable as microwave absorbers due to their appropriate permeability values, high magnetization and planar anisotropic behavior in microwave frequencies [17]. Due to their high anisotropy field, hexafferrites can be used at much higher frequencies than spinel ferrites or garnets [17], [18]. M-type strontium hexafferrite SrFe₁₂O₁₉ (SFO) have attracted scientific interests as they offer promising technological applications in the field of telecommunications, microwave devices, magnetic recoding, etc [19]. SFO with planar hexagonal structure can be applied in GHz region because of its strong magnetocrystalline anisotropy field and high cut-off frequency. At present, a large amount of work has been done to investigate its EM properties by changing the method of preparation (such as solid-state reaction method, sol-gel method, and co-precipitation), chemical composition, additive, sintering temperature, time, etc [19], [20], [21], [22]. Several researchers have addressed the challenge of finding a radar-frequency absorbing material with wide frequency bandwidth. Since materials based only on SFO cannot fulfill these demands, composites have been explored in order to combine SFO with other materials. To mediate between the advantages and disadvantages of each kind of filler, it might be a strategy mixing the dielectric materials with SFO nanofakes. The thickness of nanofakes is generally less than their skin depth, so eddy current in high frequency is suppressed. Li et al. reported the preparation of polyaniline/SFO/multiwalled carbon nanotubes composites and the outstanding EM absorption properties [23]. SFO/NiFe₂O₄ nanostructure particles were synthesized by the co-precipitation of chloride salts using the sodium hydroxide solution and the maximum microwave absorption reached −35 dB (at resonance frequency) [22].

Ni@C nanocapsule, in which Ni nanoparticle is coated by onion-like C layer, is an attractive kind of absorption materials, due to its advantages of simple preparation process, cheap composition and good environmental stability and the complementarity between magnetic Ni core and dielectric C shell. Zhang et al. reported the synthesis of Ni@C nanocapsules through the arc discharge method and investigated their EM performances. The maximum reflection loss (RL) of Ni@C nanocapsules reached −32 dB at 13 GHz with 2 mm thickness layer [24]. In our previous work, the effects of particle size on the microwave absorption properties of Ni@C nanocapsules were analyzed [25]. In order to enhance the microwave absorption properties of Ni@C nanocapsules, Ag₃PO₄ nanoparticles were introduced to modify Ni@C nanocapsules, constructing the special microstructure in which Ag₃PO₄ nanoparticles anchored on the surface of Ni@C nanocapsules [26]. Compared with Ni@C nanocapsules, Ag₃PO₄-modifed Ni@C nanocapsules exhibited the enhanced microwave absorbing abilities with broad frequency bandwidth at thin absorber thickness. Ag₃PO₄ nanoparticles could increase permittivity values and induce the strong dielectric resonance [26].

To the best of our knowledge, there is no report on Ni@C nanocapsules-decorated SrFe₁₂O₁₉ hexagonal nanoflakes as high-frequency microwave absorber up to now. Therefore, the development of Ni@C nanocapsules-decorated SrFe₁₂O₁₉ hexagonal nanoflakes is imperative to new generation microwave absorbers with high performance. In this paper, the core/shell structured Ni@C nanocapsules are chemically modified and subsequently adhered to the surface of SFO nanoflakes. The effect of Ni@C nanocapsules on the microstructure, EM properties and the microwave absorption of the SFO are investigated in detail.