Microwave absorbing characteristics of Fe3O4@SiO2 core–shell polyaniline-based composites

In this study, Fe3O4@SiO2 core–shell/polyaniline composites were successfully prepared by mechanical and chemical methods. The crystalline structure, morphology, magnetic and microwave absorption property of the Fe3O4@SiO2 core–shell polyaniline-based composites were investigated with X-ray diffraction, scanning electron microscope, permagraph and vector network analyzer, respectively. The results indicate the composites of 4 wt% Fe3O4@SiO2 core–shell filleras a potential candidate for X-band electromagnetic absorbing material. The frequency bands for reflection loss below –10 dB (90% microwave absorption) areobtained from 8.0 to 12.2 GHz at the thickness of 2 to 5 mm. This enhancement could be attributed to the addition of Fe3O4@SiO2 core–shell as a filler.


Introduction
In recent years, rapid development in telecommunication devices such as military electronic systems, computers, and mobile phones has already attracted extensive concerns due to the electromagnetic interference (EMI) issues [1,2]. Now people are aware that the EMI environment and pollution are harmful to their health and degrade the quality of our lives [3][4][5][6][7]. Thus, the development of a variety of materials that can eliminate or absorb electromagnetic (EM) waves effectively becomes urgent and reduces the harmful EMI. To overcome this problem, the designing and fabrication of microwave absorbers are a very important subject in modern society. As a kind of typical microwave absorbers, magnetic and dielectric materials are required to fulfill the characteristics of the abundant resource, low cost, easy preparation, lightweight, relatively low density, high efficiency, and frequency range response [8][9][10][11][12][13][14][15].
Some results of studies on Fe 3 O 4 , SiO 2 , and polyaniline materials as microwave absorbers have been published in many papers [16][17][18]. Xiang Liu et al investigated the Fe 3 O 4 /C composites as a excellent microwave absorptionability with optimum reflection loss (RL) of −18.73 dB at 15.37 GHz [19]. Pouria Sardarian et al prepared Fe 3 O 4 /BaTiO 3 @MWCNT nanocomposite system by a hydrothermal sol-gel method which resulted inmaximum absorption for the high-frequency band  with 80%-93% absorption [20]. Biao Zhao et al reported Ni-SiO 2 composite microspheres, which show a minimum RL as low as −40.0 dB (99.99% absorption) at 12.6 GHz with a matching thickness of 1.5 mm [21]. Bin  Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. than shell materials. Several researchers have achieved some results in making a core-shell based on Fe 3 O 4 as a core for microwave absorbing application. Almost all of them use chemical methods to make Fe 3 O 4 nanometer structure [26][27][28]. So that the Fe 3 O 4 nanometer structure is produced in small amounts.
In this work, to make light-weight microwave absorbers, Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites were prepared by mechanical and chemical methods. Fe 3 O 4 @SiO 2 core-shell material is a filler and polyaniline as the matrix. Fe 3 O 4 as a core material is produced from iron sand which is milled to obtain a smaller particle size. SiO 2 as shell material is obtained from large amounts of rice husks. The complex permittivity and permeability of Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites with 0, 2, 4, and 6 wt% of Fe 3 O 4 @SiO 2 core-shell as a filler are investigated and the microwave absorbing characteristics are evaluated. The minimum RL and corresponding frequency at different thicknesses have been resulted in detail.

Materials
The iron sand, rice husks, hydrochloric acid (HCl, 70%), sodium hydroxide (NaOH, 96%), ammonium peroxodisulfateare all commercially local available market. Distilled water is also used for the preparation of the solutions. the synthesized polyaniline powder is ready to use.

Synthesis of Fe 3 O 4 fine particles from iron sand
Synthesis of Fe 3 O 4 fine particles using mechanical milling method. The iron sand is milled by SPEX M8000 highenergy ball milling (HEBM) to obtain smaller size and then separated magnetically. The magnetic material in the form of Fe 3 O 4 was milled in stages 0, 5, 20, and 560 min to obtain Fe 3 O 4 fine particles. The Fe 3 O 4 fine particles were dispersed in distilled water by ultrasonic for the coating process.

Synthesis of SiO 2 from rice husk.
Isolation of silica from rice husks begins with ashes of rice husks, then the ash of the rice husks is added with 3% HCl with a ratio of 10 mL of 3% HCl for 1 gram of rice husk. While heated and stirred with a hot plate stirrer for 2 h then filtered using Whatman filter paper no. 42. The filtered residue is then washed with demineralized water to neutral pH. The residue with a neutral pH was then dried at 105°C for 4 h to get white silica.

Preparation of sodium silicate solution
The obtained silica is then mashed and weighed as much as 10 grams. 82.5 ml of 4 M NaOH was added to 10 grams of fine silica in a moon flask then refluxed for 5 h. The solution is then filtered and the filtrate is taken. The resulting filtrate is a sodium silicate solution which is then used to make the core-shell.
2.5. Synthesis of core-shell Fe 3 O 4 @SiO 2 core-shell Fe 3 O 4 @SiO 2 was synthesized by reflux method. 2 grams of Fe 3 O 4 nanoparticles were dissolved in 400 ml of demineralized water. A total of 40 ml of synthesized sodium silicate solution was added dropwise to the mixture. The pH of the solution is lowered with concentrated HCl until it reaches pH 6. After the pH reaches 6 then the mixture is refluxed for 6 h. The mixture was then cooled to room temperature and filtered using Whatman paper No. 42. The resulting residue is washed using demineralized water repeatedly to a neutral pH. The resulting Fe 3 O 4 @ SiO 2 core-shell was dried at room temperature for 12 h.

Synthesis of polyaniline
Aniline monomer 5 ml was added to 50 ml of 1 M HCl under stirring until homogeneous. In a separate beaker, 6 grams of ammonium peroxodisulfate are dissolved in 50 ml of 1 M HCl until homogeneous. Into the aniline solution, ammonium peroxodisulfate solution is added dropwise under stirring conditions until the solution turns green. The polymerization process was continued for 2 h until a dark green solution was formed. The resulting solution was then stored at about 10°C for 24 h. The solution was then filtered with Whatman paper no. 42 and washed with demineralized water. The filtered residue is dried at room temperature until completely dry. The green solid formed is polyaniline.
2.7. Synthesis of Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites were synthesized by the oxidation method. core-shell Fe 3 O 4 @SiO 2 core-shell was added to 100 HCl 1 M then aniline monomer was added dropwise into the solution under stirring conditions. Stirring was continued for 10 min. Then ammonium peroxodisulfate as an oxidizing agent was dissolved in 50 ml 1 M HCl in a separate beaker (mole ratio with aniline 1: 1). Into the aniline solution, ammonium peroxodisulfate solution was added dropwise. The polymerization process is carried out for 3 h until the solution turns green. Filter the solution using Whatman paper No. 42. The resulting residue is then washed with demineralized water until the filtrate is colourless. The resulting composite was dried in the air for 12 h. The comparisons between aniline monomer and core-shell Fe 3 O 4 @SiO 2 were varied.

Characterization
The crystalline structures of the fine particles Fe 3 O 4 and SiO 2 were characterized by x-ray powder diffraction of the powder samples using a PanAnalitical x-ray diffractometer (XRD) using Cu-K α radiation (wavelength λ = 1.54 Å). The morphologies were observed using a scanning electron microscope SEC SNE4500M operating at an accelerating voltage of 15 kV. Magnetic measurements were performed using a Permagraph Electromagnet EP3 with an external field of 1 tesla. The reflected signal (S 11 or S 22 ) and transmitted signal (S 21 or S 12 ) of the samples were measured by A Rohde-Schwarz ZVA 67 vector network analyser (VNA) at a frequency of 8.0-12.2 GHz. The s-parameters (S 11 and S 21 ) and Nicolson-Ross-Weir (NRW) theoretical calculations are being used for resulting the complex permeability (μ=μ′-jμ″) and permittivity (ε=ε′-jε″) [29][30][31]. The reflection loss (RL) of samples were calculated by using the values of μ and ε based on transmit line theory through the formula using the following equations [32,33]: where Z in , μ, ε, f, c, t are the characteristic impedance, complex permeability, complex permittivity, X-band frequency, light velocity, and thickness of the samples, respectively.

Phase identification analysis
The crystalline phase of the milled Fe 3 O 4 powder and SiO 2 was analyzed by XRD, and the pattern of the sample is shown in figure 1. According to the reference patterns of international crystal diffraction data (ICDD) #98-065-4110 (Fe 3 O 4 ), the crystal structure of the sample is polycrystalline that consists of the Fe 3 O 4 phase ( figure 1(a)). This represents that the milled sample has been successfully prepared for the core material. Meanwhile, the pattern of SiO 2 shows that the sample is in amorphous form (figure1(b)). We observed five crystal orientations of the  there has been a change in particle size. This indicates that the preparation of Fe 3 O 4 nanoparticles has been successful in this method. The results of XRD measurements can be confirmed that Fe 3 O 4 after milling has a crystal size of less than 40 nanometers. In the Fe 3 O 4 @SiO 2 core-shell synthesis, it can be shown in figures 2 (e)-(f) that the colour changes to be whiter. This SiO 2 coating process has occurred and Fe 3 O 4 @SiO 2 core-shell particles tend to agglomerate.

Electromagnetic characteristics
It is well known that the complex permeability (μ=μ′ −jμ″) and permittivity (ε=ε′ −jε″) can be used to characterize the excellent microwave absorbing properties, which should have high the real (μ′) and imaginary (μ″) parts of permeability, large imaginary (ε″) and small real (ε′) parts of permittivity at microwave frequency [21]. As shown in figure 4 for polyaniline sample without filler of Fe 3 O 4 @SiO 2 core-shell. The μ′, μ″, ε′, and ε″ values of allsamples show fluctuation in the 8.2−12.2 GHz range. The μ′ values of 2 wt%, 4 wt%, and 6 wt% Fe 3 O 4 @SiO 2 core-shell fillershowed the tendency to decrease in the 9.0−12.2 GHz range. Generally, μ″ represents magnetic energy dissipation [24,36]. The μ″ value of 2 wt%, 4 wt%, and 6 wt% Fe 3 O 4 @SiO 2 coreshell fillers also exhibit decreasing in the 8.2−12.2 GHz range, which indicates a weak magnetic loss ability of  microwave energy. As shown in figures 4(c) and (d), the ε′ value of 2 wt%, 4 wt%, and 6 wt% Fe 3 O 4 @SiO 2 coreshell fillersare significant and tend to decrease in the 8.0−12.2 GHz range. The imaginary part (ε″) of 2 wt%, 4 wt%, and 6 wt% Fe 3 O 4 @SiO 2 core-shell filler showed constant at about 8.0-12.2 GHz, while electromagnetic fields are applied in the X-band frequency. The dielectric loss can be explained by the imaginary part of permittivity (ε″). Figure 4 shows the imaginary parts (ε″) of all samples. It can be noticed that the ε″ values of 4 wt% and 6 wt% Fe 3 O 4 @SiO 2 core-shell fillers increase with the increasing frequency and show higher than those of others.

Microwave absorption properties
To evaluate the microwave absorption characteristic of the Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites, their reflection loss (RL) values were calculated according to the complex permeability (μ = μ′-jμ″) and permittivity (ε = ε′ − jε″) values with the given frequency ( f ) and sample thickness (t) using equations (1) and (2). Now, to closely investigate and obtain impedance matching conditions, the effect of various sample thickness on absorption properties for composite materials in X band frequency range, the RL values have been shownin figure 5(a)-(d). The RL values are calculated using various sample thickness ranging from 1.0 to 5.0 mm. Based on the RL values of the Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites with 4 wt% (figure 5(c)) and 6 wt% fillers ( figure 5(d)) for each thickness, there is optimum result for a given absorbing ability. The optimum sample shows the minimum RL of 4 wt% Fe 3 O 4 @SiO 2 core-shell filler with a thickness of 1.0 − 5.0 mm reached∼−14.42 dB (over 95% absorption) at 8.74 GHz, and the bandwidth of the reflection loss less than −10 dB (over 90% absorption) was 4.2 GHz. Now, to closely investigate and obtain impedance matching conditions, the effect of various sample thickness on absorption properties for composite materials in the 8.0−12.2 GHz frequency range, the 2D map colour filling patterns of RL has been shown in figure 6(a). The RL values are calculated using various sample thickness ranging from 1.0 to 5.0 mm. In this study, to find impedance matching conditions of the samples, the graphical map method is commonly applied [37]. Based on the RL values of the Fe 3 O 4 @SiO 2 core-shell polyaniline-based composite with 4 wt% filler for each thickness, there is a relative bandwidth of W=f up /f low , where f up and f low are the upper-and lower frequency limits of the bandwidth for a given absorbing ability, in this case, −10 dB, respectively. Therefore, based on the curve of W versus t, the maximum relative bandwidth (W max ) and the corresponding optimum thickness (t m ) can be obtained, as shown in figure 6(b). Thus, the W max values of the sample are1.52 with an optimum thickness (t m ) of 2.0 mm (a bandwidth of 4.2 GHz) and 1.95 mm (a bandwidth of 3.69 GHz) with the absorption bandwidth below −10 dB. The magnetic loss ability is highly correlated with hysteresis loss, the eddy current effect, domain-wall resonance, exchange resonance and natural resonance. In our case, the exchange, natural, and domain wall resonance which are only existed in the MHz frequency range, could be excluded. Thus, the contribution of the magnetic loss of Fe 3 O 4 @SiO 2 as a filler is hysteresis loss and the eddy current effect that correlated with the irreversible magnetization because of a strong magnetic field and the penetrating electromagnetic wave into the magnetic materials. The eddy current effect can be expressed by μ″≈2πμ o (μ′) 2 σd 2 f/3, where μ o , σ and d are  the vacuum permeability, the diameter and the electric conductivity, respectively. Based on this equation, the eddy current loss is C o ≈μ″(μ′) −2 f −1 ≈2πμ o σd 2 /3 and nearly constant value with varying frequency. In Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites with 0 wt%, 2 wt% and 6 wt% fillers, the value of C o increases gradually and present some fluctuations with the increasing frequency range of 8.0-12.2 GHz. As shown in figure 6(d), the C o value of composite material with 4 wt% filler is constant and correlated with the increasing of the electric conductivity (σ). It is expectable that one may obtain high permeability in the present samples by adding partially magnetic filler, such as Fe 3 O 4 . The additional filler indicated that the attenuation of electromagnetic waves occurred at resonance frequencies below the frequency range of 8.0-12.2 GHz in Fe 3 O 4 @SiO 2 core-shell polyaniline-based composite with 4 wt% filler.
In table 2, the microwaves absorption performances of the typical Fe 3 O 4 @SiO 2 core-shell fillers in recent literature, which are very promising to be used as the main filler for the composite due to give the optimum RL of microwave absorbers. In this work, the frequency bandwidth of 4 wt% Fe 3 O 4 @SiO 2 core-shell filleris larger enough while the thickness is much larger, indicating that these composite materials are very promising microwave absorption candidates.

Conclusions
In summary, the Fe 3 O 4 @SiO 2 core-shell polyaniline-based composites consist of polyaniline matrix and fillers of Fe 3 O 4 @SiO 2 core-shell powders with various weight content have been synthesized. The potential candidate for electromagnetic absorbing material indicates that the optimum sample shows the minimum RL of 4 wt% Fe 3 O 4 @SiO 2 core-shell filler reached∼−14.42 dB (over 90% absorption) at 8.74 GHz with a thickness of 1.0 − 5.0 mm. The bandwidth of the reflection loss of less than −10 dB (over 90% absorption) was 4.2 GHz.