Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance

Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance

Accepted Manuscript Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance Yu z...

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Accepted Manuscript Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance Yu zhang, Xujian Lyu, Zhihong Yang, Meng Li, Lieji Yang, Juncen Liu, Renbing Wu PII:

S0022-3697(18)32616-7

DOI:

https://doi.org/10.1016/j.jpcs.2018.11.015

Reference:

PCS 8801

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 21 September 2018 Revised Date:

16 November 2018

Accepted Date: 23 November 2018

Please cite this article as: Y. zhang, X. Lyu, Z. Yang, M. Li, L. Yang, J. Liu, R. Wu, Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance, Journal of Physics and Chemistry of Solids (2018), doi: https://doi.org/10.1016/ j.jpcs.2018.11.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile chemical synthesis of amorphous FeB alloy nanoparticles and their superior electromagnetic wave absorption performance Yu zhanga, Xujian Lyub, Zhihong Yanga*, Meng Lia, Lieji Yanga, Juncen Liua, Renbing Wuc* College of Material Science and Technology, Nanjing University of Aeronautics and Astronautics,

b

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Nanjing 210016, P. R. China

School of Energy and Power Engineering,Nanjing University of Science and Technology, Nanjing

210094, P. R. China c

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China

*E-mails: [email protected]; [email protected]

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Abstract

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a

Currently, the magnetic metal based electromagnetic wave absorber have attracted extensively interest. However, how to control the high conductivity of magnetic metals

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and achieve good impedance matching of electromagnetic absorbing materials made by them have become a big problem at present. Herein, amorphous FeB nanoparticles were rationally synthesized by a wet method, which involves a simple three-step

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solution reaction. The obtained products were characterized by X-ray diffraction

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(XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM). Benefiting from their amorphous structure, the resulting FeB nanoparticles show good magnetic properties, dielectric losses and proper impedance matching, thus good microwave attenuation ability can be achieved in the low frequency range. The maximum reflection loss is -15 dB at 3.5 GHz with a thickness of 8 mm which can be mainly attributed to its excellent impedance matching in the frequency range of 2.5-4.5 GHz. These amorphous FeB alloy

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ACCEPTED MANUSCRIPT nanoparticles are expected to be promising candidates for efficient microwave absorbers in the S band. Keywords:

FeB

nanoparticles;

amorphous

structure;

impedance

matching;

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electromagnetic property

1. Introduction

With the rapid popularization and application of electromagnetic (EM) wave circuit

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devices, the electromagnetic interference (EMI) issues caused by various advanced

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electronic devices have brought great concerns in both civil and military fields. To solve the EMI problems, an effective solution is to apply the EM wave absorbing materials to absorb the undesirable EM wave

[1-3]

. The EM absorbing materials should

possess the features of light weight, strong absorption ability, thin thickness, broad

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absorption bandwidth and high thermal stability simultaneously [4-5]. Recently, the soft magnetic materials, including Fe, Co, Ni and its alloys have attracted extensive

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attention from many researchers because they exhibit the excellent magnetic, catalytic, optical properties which allow them to be widely used in the information storage, [6-9]

. Furthermore, these soft magnetic metal

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catalysis, biomedical applications

materials have also been used in the field of EM attenuation purposes benefiting from their high saturation magnetization and high Snoek’s limit as compared to other kinds of EM wave absorption materials which make them be utilized in the range of high frequency [10-11]. As is known to all, the EM wave absorbing properties are mainly depended on the complex permeability (µ r = µ̍ - jµ̎) and the dielectric constant (εr = ε̍ - ε̎) of the material 2

ACCEPTED MANUSCRIPT [12-13]

. The high magnetic or dielectric loss can give rise to the strong electromagnetic

wave absorption performance. Furthermore, another key thing which need to be concerned is the balance between the permeability and permittivity, that is, EM

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impedance matching, which is essential to improve the absorption of EM wave [14]. The soft magnetic metal materials such as Fe, Co, Ni and its alloys commonly shows the more suitable permeability and the higher magnetic loss values than other soft [10]

. However, conventional magnetic

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magnetic oxide materials such as the ferrites

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metals usually have high conductivity values which result in a large dielectric constant and mismatched impedance condition. In that way, the EM absorption capacity will be decayed, which will greatly limit its application in the EM wave absorption purpose. On the other hand, for magnetic metal materials, the eddy currents caused by high

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frequency electromagnetic waves will reduce its permeability and thus also depress their EM attenuation performance [6]. A reasonable way to overcome all these problems is to embed magnetic metal particles into an insulating layer, such as silica. In that way,

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the eddy current effect can be restricted and the magnetic metal particles can also be

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separated from each other which can effectively lower their conductivity value and improve the impedance matching condition [15]. However, the non-magnetic SiO2 shell causes an undesired decrease in permeability. Therefore, if the essential conductivity of magnetic metals can be effectively reduced, it will be a more effective way to solve these problems. Ferromagnetic amorphous materials with good soft magnetic properties of high saturation magnetization, low coercivity and low electric resistivity have been 3

ACCEPTED MANUSCRIPT commercially produced and widely applied as magnetic cores of distribution transformers in power grid [16-17]. Therefore, if soft magnetic metal or alloy can be made into amorphous state, its high saturation magnetization can be maintained, its high

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conductivity can be greatly reduced, and its high frequency electromagnetic properties can be greatly improved. Hamayun et al. reported the magnetic and magnetothermal properties of iron boride (FeB) nanoparticles prepared by surfactant-assisted ball [18]

. Oprea et al. reported a study of magnetic properties of FeB [19]

. However, to the best of our

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nanoparticles grown using an ion cluster source

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milling process

knowledge, there are few reports on the high frequency and electromagnetic wave absorption properties of soft magnetic amorphous materials. In this study, we studied a simple chemical method to synthesize the soft magnetic FeB alloy nanoparticles with

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pure amorphous phase. The resultant nanoparticles show the uniform spherical morphology with well-dispersed characteristic. It shows that making the soft magnetic metal into the amorphous state can effectively decrease the dielectric constant and

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result in the better impedance matching condition and enhance the microwave

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attenuation performance. It is expected that these as-prepared soft magnetic amorphous FeB nanoparticles can be an ideal candidate of EM wave absorption materials in the high frequency range.

2. Experimental details 2.1. Synthesis of FeB alloy nanoparticles All reagents including ferrous sulfate (FeSO4), tetrahydrofuran (THF), sodium 4

ACCEPTED MANUSCRIPT borohydride (NaBH4) are analytical reagent grade without further purification. FeB alloy nanoparticles were prepared by a solution method under a nitrogen atmosphere in a dry three-neck flask. First, 0.01mol (2.7802g) of FeSO4 was dissolved in 20ml of

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THF and stirred at room temperature for 30 minutes. When the solution was mixed homogeneously, transferred the mixture into a three-necked flask. In order to control the temperature of the reaction solution to be 0 °C, the three-neck flask was immersed

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in an ice bath. Then 0.01mol NaBH4 was added to the prepared solution, the

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precipitates with black colour were appeared after that addition of NaBH4. After reaction for 2h, the precipitates were collected by centrifugation and washed with distilled water and ethyl alcohol for several times followed by drying in a vacuum oven at 30 °C for 5h.

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2.2. Characterization

The samples were analyzed by a Rigaku D/MAXRC X-ray diffractometer with a Cu Kα radiation source (45.0kV, 50.0mA). The amounts of element Fe and B were

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determined through an inductively coupled plasma-atomic emission spectrometer

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(ICP-AES, USA VARIAN). Scanning electron microscopy (SEM) images of three samples were obtained on the in-situ emission scanning electron microscope (FESEM) at an accelerating voltage of 20kV. High-resolution transmission electron microscopy (HRTEM) images of the samples were observed using a TEM (JOEL JEM 2010 at an accelerating voltage of 150kV). The magnetic hysteresis loops of the FeB alloy nanoparticles at room temperature were measured by using a vibrating sample magnetometer (VSM, ADE Magnetics EV-9). For the microwave measurements, 30 5

ACCEPTED MANUSCRIPT wt%, 50wt% and 70wt% amorphous FeB nanoparticles were uniformly mixed with paraffin wax to form toroidal-shaped composite samples with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Complex permeability and permittivity of the

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composites were measured by Agilent Vector Network Analyzer (VNA, Agilent N5232A) with a reflection-through-line calibration, over 0.5–10 GHz, using a set of 7 mm coaxial air-line with length of 49.96 mm. The corresponding electromagnetic

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properties was calculated based on the American Standard (ASTM 893-1997,

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American Society Testing and Materials). Based on the obtained values of complex permittivity (ɛr = ε'r – jε"r) and permeability (µ r = µ'r – jµ"r), the frequency dependence of reflection losses (RL) of the composites were estimated according to the following equations:

µr  2πft  µrε r  tanh j εr  c 

RL(dB ) = 20 lg

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Z in Z 0 =

Z in − Z 0 Z in + Z 0

(1)

(2)

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where Zin is the impedance of the composite material supported by the ground, Z0 is the

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inherent impedance of free space, c is the speed of light in free space, t is thickness of the microwave absorbing material, ƒ is the frequency of the EM wave incident to the material.

3. Results and discussion 3.1. XRD of FeB nanoparticles The crystalline structure of the FeB alloy nanoparticles was preliminarily studied by 6

ACCEPTED MANUSCRIPT wide-angle X-ray diffraction which is shown in Fig. 1. There are no obvious diffraction peaks have been detected for the samples whether low angles or high angles, indicating that the obtained FeB alloy nanoparticles are amorphous nature. In order to

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find out the actual elements and contents in the sample, the sample was tested by ICP-AES and the result is shown in Table 1. The ICP-AES analysis revealed that the

3.2.SEM micrographs of FeB nanoparticles

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atomic ratio of Fe and B was 1:1, suggesting a successful preparation of Fe-B.

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The morphology of the FeB alloy nanoparticles was characterized by scanning electron microscope (FESEM) and transmission electron microscopy (TEM). It can be observed from Fig. 2(a) that there are homogeneous particles in the typical FESEM image of the sample. The high-magnification FESEM image (Fig. 2(b)) reveals that

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the as-synthesized magnetic alloy nanoparticles are nanospheres with an average diameter of 80 nm and the surface of the nanosphere is quite smooth. The microstructures of magnetic alloy nanoparticles were further studied by HRTEM. The

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representative TEM and HRTEM images of the magnetic alloy nanoparticles are shown

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in Fig. 2(c) and Fig. 2(d). It can be observed that these nanospheres are solid spheres. The HRTEM image obtained from the core of a single FeB nanoparticle is shown in figure 2(d). The absence of obvious diffraction stripes proves that these nanoparticles are amorphous which is consistent with the results of XRD. 3.3. Magnetic hysteresis loops of FeB nanoparticles The room temperature magnetization hysteresis loops of the prepared FeB alloy nanoparticles were measured by VSM and it is shown in Fig. 3. The results demonstrate 7

ACCEPTED MANUSCRIPT the ferromagnetic behavior of the prepared samples, and the related saturation magnetization ( M s ) is around 133.5 emu·g-1. The sample is ferromagnetic and becomes saturated in a 10 kOe magnetic field. It is found that the Ms value is [20]

, that is mainly due to the

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significantly lower than that of carbonyl iron particles

nonmagnetic of nonmetallic element boron in the composition, which will reduce the magnetic response of the sample in an external magnetic field. The inset in Fig. 6

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reveals that the coercivity value of the FeB nanoparticles is around 150 Oe.

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3.4. Complex permittivity and permeability of FeB nanoparticles

The microwave absorption characteristics of the absorbers are closely related to their real permittivity (ɛʹ), the imaginary permittivity (ɛʺ), the real permeability (µʹ) and the imaginary permeability (µʺ). The real parts (ɛʹ and µʹ) represent the storage capability of

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electric and magnetic energy, and imaginary parts (ɛʺ and µʺ) represent the loss capability of electromagnetic energy [21-23]. The complex permittivity and permeability of the paraffin wax composites loaded with 30w%, 50w% and 70w% of the FeB alloy

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nanoparticles are studied in the frequency range of 0.5-10 GHz. As shown in Fig. 4, the

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values of complex permittivity increase with the increase of the filling ratio of the composite samples. With increasing the filling ratio from 30 wt% to 70 wt%, the values of ɛʹ and ɛʺ of the composite samples increase from 4 to 8.3 and 0.3 to 1.2, respectively. It is known that the ɛʹ and ɛʺ values have an important relation with their electrical conductivity [24,25] and according to the free electron theory, the higher the conductivity, the higher the dielectric constant

[26]

. Thus, it is clear that the enhancement in the

complex permittivity values are attributable to the fact that the electrical conductivity 8

ACCEPTED MANUSCRIPT will be increased for composite samples with the high filling ratio. When the filling ratio increases, the high filling ratio makes it easier for clusters to occur between the metal nanoparticles, which leads to the enhancement of the electrical polarization

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between metal nanoparticles and the higher conductivity value of the sample. However, it is found that when the filling ratio is 70 wt%, the real part of dielectric constant is 8.3, which is still much lower than the dielectric constant of the composites filled with other [27]

. This is mainly due to the amorphous state of FeB metal

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metal particles

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nanoparticles, which reduces its metal conductivity by several orders of magnitude. The composite permeability of paraffin wax composites filled with 30 wt%, 50 wt% and 70 wt% of FeB nanoparticles varies with frequency as shown in Fig. 4(b). The real part of the complex permeability µʹ of the samples with different filling ratios

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decreases as the frequency increases and eventually tends to be stable. The imaginary part of the complex permeability µʺ increases first and then decreases with the increase of frequency, and finally approaches to zero. It is clearly shown that, for the samples

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with filling ratio from 30 wt% to 70wt%, the real part of the complex permeability (µʹ)

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increases from 1.25 to 1.5, and the maximum imaginary part of complex permeability (µʺmax) increase from 0.15 to 0.28, respectively. It is generally believed that the magnetic loss mainly comes from hysteresis, natural resonance, eddy current effect and skin effect [28]. In the weak field, the hysteresis loss is negligible due to the existence of irreversible magnetization and the domain wall resonance loss can also been ignored because it usually occurs at much lower frequency (MHz) [29]. And the size of these magnetic alloy nanoparticles is in the nanometer range which is much smaller than the 9

ACCEPTED MANUSCRIPT skin depth of bulky iron (1µm), so the eddy current effect can also be excluded

[30]

.

Thus, the main cause of magnetic loss is natural resonance, which can be confirmed by the multiple peaks in the curves, especially the obvious peak at the frequency of 2 to 4

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GHz as shown in Fig. 4(b). It is well known that dielectric loss and magnetic loss are two possible contributions to the EM wave absorption. In order to study which is the main loss of amorphous

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FeB nanoparticles, the dielectric loss tangent (tanδɛ = ɛʺ/ɛʹ) and magnetic loss tangent

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(tanδµ = µʺ/µʹ) of amorphous FeB nanoparticles are calculated, as shown in Fig. 4 (c) and (d). The values of tanδɛ for the samples filled with 30 wt%, 50 wt% and 70 wt% of FeB nanoparticles display the similar variation within the whole frequency range. It is clear to see that the samples filled with 70 wt% of FeB nanoparticles possesses much

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higher tanδɛ than others, implying its strong dielectric loss capability. As shown in Fig. 4 (d), a generally increasing and declining tendency can be observed for the magnetic loss tangents of all three samples with the increase of frequency. The samples filled

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with 70 wt% of FeB nanoparticles exhibit the higher tanδµ value and their resonance

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peak located at 2-4 GHz should be mainly induced by the natural resonance which is in accordance with the peak in the complex permeability curve. In the whole frequency range, the value of tanδµ is much larger than that of tanδɛ, indicating that the magnetic loss plays a major role in the intrinsic electromagnetic wave absorption of the three samples. 3.5. Reflection loss (RL) of FeB nanoparticles Based on the experimentally determined complex permittivity and complex 10

ACCEPTED MANUSCRIPT permeability, the reflection loss (RL) properties of composite samples filled with 30 wt%, 50 wt% and 70wt% of amorphous FeB nanoparticles can be calculated using the formulae depending on the transmission line theory. The calculated results for the

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reflection loss of the three samples with different filling ratios at different thicknesses are shown in Fig. 5(a)-(c). The thickness d of absorbing material is one of the key parameters affecting the response frequency of reflection loss, and in order to eliminate

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the influence of the thickness of absorbing material, the reflection losses of different

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thicknesses (3.0 4.0 5.0 6.0 7.0 and 8.0 mm) are further studied. It is clear that the values of the reflection loss increase with increase in the filling ratio and the thickness of the absorber. Thus, the maximum reflection loss for these three samples is -15 dB at 3.5 GHz in the filling ratio of 70w% and the absorber thickness is 8 mm. Furthermore,

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with the increase in the thickness, the reflection loss peaks for these three samples shift from the higher frequency to the lower frequency, which is related to the attenuation of 1 4 wavelength

[31]

. In order to reveal more details on the influence of

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thickness to the electromagnetic wave absorption properties, the RL 3D-contours of

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these three composites in the frequency range of 0.5-10 GHz with the absorber thickness of 3-8 mm have been calculated and drawn in Fig. 5 (d)-(f). As shown in Fig. 5 (d) and (e), the RL value of composite samples filled with 30 wt% and 50 wt% of FeB nanoparticles can hardly reach -10 dB which means 90% EM wave absorption in the range of 3-8 mm. For the composite sample filled with 70 wt% of FeB nanoparticles, the absorption band for RL values below -10 dB almost covers the whole S band with the of thickness of 8mm. As we all know, in order to maximize the absorption of 11

ACCEPTED MANUSCRIPT incoming electromagnetic waves, two basic conditions must be satisfied: 1) the incoming electromagnetic waves are transmitted to the material and the reflection wave are quite small; 2) the substance effectively attenuates electromagnetic waves

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transmitted to them. In order to transmit incident electromagnetic waves into the material rather than reflecting them into space, the impedance of the material should match the free space, so the dielectric constant and permeability of the material should [32-33]

. When the complex permittivity is much higher than the complex

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be close

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permeability, due to the low surface resistance, most of the incident electromagnetic waves will be reflected to the surface, rather than transmitted to the absorber [34]. In our case, due to the amorphous phase of FeB nanoparticles, the conductivity of FeB nanoparticles decreases significantly, and the real part of the dielectric constant also

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decreases significantly compared with other metals. Thus, the impedance matching in space is realized better, and the absorption ability of electromagnetic wave is improved. 3.6. Impedance matching and attenuation constant of FeB alloy nanoparticles

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To further understand the electromagnetic wave absorption behavior for these three

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samples, some critical parameters such as the impedance matching and the attenuation constant have been calculated. The impedance matching property of the absorbing material determines the ability of the material to let the electromagnetic wave into its interior and the coefficient of electromagnetic matching (Z) can be calculated by the following equation [15]:

Z=

Z in = Z0

 2πfd µ r ε r µr tanh j  εr c 

   

(3)

where Zin is the input impedance, Z0 is the impedance of the free space, and Z is the 12

ACCEPTED MANUSCRIPT normalized input impedance. Generally, perfect impedance matching is desired with the values of Z are close to one

[35]

. Fig. 6 (a)-(c) shows the resultant curves of

impedance matching at the different thickness. The results show that the composite

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samples with 70 wt% filling ratio have the better impedance matching than 30 wt% and 50 wt% samples, especially in the frequency range of 2.5-4.5 GHz. In fact, the strongest RL peak is also in this band.

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Furthermore, the EM wave attenuation in the interior of the absorber is another key

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factor for an excellent absorber. The attenuation constant α is commonly being used to determine the attenuation properties of the materials and it can be expressed by the following equation [36]:

α=

2πf × c

(µ ′′ε ′′ − µ ′ε ′) + (µ ′′ε ′′ − µ ′ε ′)2 + (µ ′ε ′′ − µ ′′ε ′)2

(4)

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where c represents the velocity of light and the f represent frequency. The Fig. 6 (d) shows the attenuation constant α for the composite materials filled with 30 wt%, 50

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wt% and 70 wt% of amorphous FeB nanoparticles in the frequency range of 0.5-10 GHz. The α values of all samples increase in the frequency range of 0.5-10 GHz and the

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samples filled with 70 wt% of FeB nanoparticles show the maximum α value of 550 among the three samples. Therefore, it can be concluded that the reason why the samples with 70 wt% filling ratio have higher electromagnetic wave absorption ability than other samples is mainly due to its better impedance matching characteristics and strong electromagnetic wave attenuation ability. These results show that the as-prepared amorphous FeB nanoparticles are very promising to be used as highly efficient EM wave absorption materials in the S band. 13

ACCEPTED MANUSCRIPT 4. Conclusions In summary, the amorphous magnetic FeB nanoparticles with high-performance EM wave absorption ability in the low frequency range have been successfully prepared

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through a solution transformation method at special temperature under the nitrogen atmosphere. The XRD and TEM images show that the magnetic FeB nanoparticles are amorphous nature, and the electrical conductivity is lower than that of traditional metal.

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The SEM images show that the magnetic FeB nanoparticles are spherical with a

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diameter of ~80 nm. Owing to the contributions from their lower electrical conductivity, corresponding lower dielectric constant and the proper dielectric and magnetic loss, the better impedance matching condition can be achieved for the composites made of the FeB nanoparticles and thus the good electromagnetic wave absorption can be obtained.

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The composite samples filled with 70 wt% of FeB nanoparticles can achieve the minimum reflection loss of -15 dB at 3 GHz with a thickness of 8 mm and the effective absorption bandwidth can cover the S band. It is believed that these amorphous

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magnetic FeB nanoparticles prepared by the facile chemical process may open up a new

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avenue for the design and preparation of EM wave absorption materials and it also has a broad application prospect in other areas.

Acknowledgements

This work is financially supported by the Fundamental Research Funds for the National Natural Science Foundation of China (Grant Nos: 51602154), and Fundamental Research Funds for the Center Universities (Grant Nos: NE2018103), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education 14

ACCEPTED MANUSCRIPT Institutions (PAPD) and the Recruitment Program of Global Youth Experts (National Thousand Young Talents Program).

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Figure captions: Fig. 1. XRD patterns of the as-prepared FeB alloy nanoparticles.

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Fig. 2. (a) and (b) SEM images of the sample. (c) TEM image of sample. (d) HRTEM image of the FeB alloy nanoparticle.

Fig. 3. Magnetic hysteresis loops of FeB alloy nanoparticles at room temperature.

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Fig. 4. Complex permittivity of composite samples filled with 30wt%, 50wt%, 70wt

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of FeB nanoparticles: (a) real part and imaginary part, (b) dielectric loss tangents; complex permeability of composite samples filled with 30wt%, 50wt%, 70wt of FeB nanoparticles: (c) real part and imaginary part, (d) magnetic loss tangents.

Fig. 5. Electromagnetic wave reflection losses of composite samples filled with 30w%

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Fig. 6. Impedance matching |Zin/Z0| values of composites at variable absorber

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Table 1 The results of ICP-AES analysis for the prepared FeB nanoparticles.

ICP

FeB

Fe( (wt%) )

B( (wt%) )

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0.50

FeB

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FeɑBβ

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Catalysts

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Research Highlights Amorphous FeB nanoparticles have been rationally synthesized.

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Amorphous structure can effectively decrease the conductivity and permittivity values. Benefiting from the improved the permittivity property, good impedance matching is achieved.

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Good microwave attenuation ability is obtained in the S band.