# Preparation and Conductive and Electromagnetic Properties of Fe3O4/PANI Nanocomposite via Reverse In Situ Polymerization

Authors: Di Zhang; Huaiyin Chen; Ruoyu Hong
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

In this paper, the magnetite/polyaniline (PANI) nanocomposite was prepared by the novel reverse in situ polymerization method. Fe3O4 magnetic nanoparticles were synthesized in situ in PANI chloroform solution to form a suspension containing the Fe3O4/PANI nanocomposite. It overcame the disadvantage of oxidation of the Fe3O4 by the oxidant in conventional method. The Fe3O4/PANI chloroform suspension and the Fe3O4/PANI powder were characterized by FT-IR, TEM, XRD, vibrating sample magnetometer, Gouy magnetic balance, conductivity meter, and vector network analyzer. It is demonstrated that the Fe3O4/PANI suspension has a good electrical conductivity that is up to 2.135 μS/cm at the optimal ratio of reactants. The Fe3O4 nanoparticles are well dispersed in the PANI network with a particle size of about 10 nm. Fe3O4/PANI powder has high saturation magnetization and magnetic susceptibility, as well as a broad application prospect in the field of electromagnetic devices. The Fe3O4/PANI powder exhibits an excellent microwave absorption behavior, which can be an outstanding candidate for the rapid development of broadband shielding materials.


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## Body

## 1. Introduction

In recent years, with the development of science and technology, more and more wireless devices have appeared in human society. These wireless systems bring convenience to people’s lives, but in the meantime show many negative effects. Firstly, the human body will suffer a series of diseases under the electromagnetic radiation with high intensity for a long time. Secondly, the surrounding electromagnetic waves will cause Electromagnetic Interference (EMI). It is of great significance to develop efficient absorbing materials to solve the electromagnetic pollution [1–4].While searching for more effective absorbing agents, people are paying increasing attention to the nanocomposite of magnetic particle and conductive polymer. Some previously reported studies have shown that the magnetic conductive polymers have a high spin density and exhibit ferromagnetic and antiferromagnetic properties, which have broad application prospects in sensing technology, nonlinear optical materials, molecular electrical devices, medical research, electromagnetic shielding, and microwave absorption [5–9].The magnetic particle-conductive polymer nanocomposite generally possesses the core-shell structures. Most of them are composed of metal oxides (such as iron, cobalt, nickel, and other oxides) as the core and polymer materials as the shell, while a small part is made of a polymeric material as a core and an inorganic magnetic material as a shell layer. In addition, there exists a sandwich structure, with the outer and inner interlayers of polymeric materials and the middle interlayer of inorganic particles [10–12]. In general, the core-shell materials have special conductivity, magnetic properties, and microwave absorption properties due to interfacial charge polarization and multilevel structure interfacial scattering. Also, in common magnetic particles, nickel and cobalt are toxic, and their applications are severely restricted in the fields of life science and medicines [13–15]. Iron oxide (Fe3O4, γ-Fe2O3) is usually used as a magnetic component, because of its low toxicity (LD50 about 2000 mg/kg, much higher than the current clinical application dose), availability, and so on [16–18].The conductive polymers can be classified into the doping-type or intrinsic electroactive polymers. At present, poly-p-phenylene (PPP), polypyrrole (PPY), polythiophene (PTH), polyphenylacetylene (PPV), and polyaniline (PANI), with enormous monomers, have mostly been studied [19–22]. Compared with other conductive polymers, PANI attracts much more attentions of researchers due to its simple preparation method, excellent conductivity, and stability [23, 24]. In order to increase the multifunctional properties and get improved performances, many scientists have conducted extensive research on conductive polymer/magnetic particle composites, which possess advantages of both the magnetic and the conductive materials [25, 26].Although previous researchers have prepared Fe3O4/PANI nanocomposite, i.e., direct dispersion of magnetite nanoparticles in PANI solution or in situ preparation by polymerization of aniline in magnetite suspension [27–30], it is difficult to uniformly disperse the Fe3O4 nanoparticles in the PANI solution or prevent the oxidization of Fe3O4 nanoparticles by an oxidant (e.g., APS). In addition, the magnetic properties of Fe3O4/PANI nanocomposite are closely related to the size of the Fe3O4 particles. In order to synthesize uniformly dispersed Fe3O4 nanoparticles with smaller particle size and solve the drawbacks of the traditional in situ preparation method [31–35], the reverse in situ polymerization method was proposed, which alleviated the agglomeration of magnetic nanoparticles and inhibited the oxidation of magnetite. The magnetic properties and electrical conductivity of composite materials were investigated [36, 37].


## 2. Experimental

### 2.1. Materials

Aniline (AN), ferric chloride (FeCl3·6H2O), ferrous sulfate (FeSO4·7H2O), ammonium persulfate ((NH4)2S2O8), ammonia water (25%), ammonium persulfate, and chloroform were of analytical grade and used as received. 4-Dodecylbenzenesulfonic acid (DBSA, 96%) and 1 M hydrochloric acid were used directly. All the aqueous solutions were deionized water used as a solvent.


### 2.2. Preparation of PANI Chloroform Solution

100 mL of deionized water was added to a three-necked flask; 1.40 g of AN and 1.46 g of DBSA were added in turn with vigorous agitation. Then, 0.56 g of additional AN was added to the flask, and the solution became milky white. After the suspension was kept at a constant temperature of 5°C by the ice water bath, 50 mL of ammonium persulfate aqueous solution (68.4 g/L) was added dropwise. The reaction mixture was stirred continuously, and the color change was observed. A complete reaction has taken place until the color changed from milky white to yellowish green, blue, blue-green, green, and dark green. Finally, extraction with 300 mL of chloroform was performed to obtain the PANI chloroform solution.


### 2.3. Preparation of PANI/Fe3O4 Nanocomposite

30 mL of PANI chloroform solution was poured into a three-necked flask. 70 mL of FeCl3·6H2O aqueous solution (0.1 mol/L) and 40 mL of FeSO4·7H2O aqueous solution (0.1 mol/L) were added to the chloroform solution in sequence and stirred continuously to form an emulsion in nitrogen atmosphere. 10 mL ammonia water was then quickly added to the mixture when the temperature rose to 60°C. After stirring for 1 hour at 80°C, the flask was cooled down to room temperature. At last, extraction with 300 mL of chloroform was carried out to obtain the suspension containing PANI/Fe3O4 nanocomposite. The suspension would be washed using the deionized water for five times.


### 2.4. Characterization

The analysis of the surface functional groups of magnetic nanoparticles was done using the Acotar 360 type Fourier transform infrared (FT-IR) spectrometer. The particle size and crystalline structure of the nanocomposite were observed using a Hitachi H-600 transmission electron microscope (TEM) and an X-ray powder diffractometer (XRD, Japanese Society of Science and Technology D/Max-IIIC). The magnetic properties of nanocomposite were studied by a Gouy magnetic balance and a vibrating sample magnetometer. The electrical conductivity of Fe3O4/PANI chloroform solution was measured using a conductivity meter (DDS-11C, Leici Instruments Co. Ltd., Shanghai, China). Dielectric constant and magnetic permeability of nanocomposite at 2~18 GHz were measured using an Agilent 8720E vector network analyzer. The pure Fe3O4 nanoparticles were made similarly using coprecipitation. All the tests performed in this work were repeated for at least three times.


## 2.1. Materials

Aniline (AN), ferric chloride (FeCl3·6H2O), ferrous sulfate (FeSO4·7H2O), ammonium persulfate ((NH4)2S2O8), ammonia water (25%), ammonium persulfate, and chloroform were of analytical grade and used as received. 4-Dodecylbenzenesulfonic acid (DBSA, 96%) and 1 M hydrochloric acid were used directly. All the aqueous solutions were deionized water used as a solvent.


## 2.2. Preparation of PANI Chloroform Solution

100 mL of deionized water was added to a three-necked flask; 1.40 g of AN and 1.46 g of DBSA were added in turn with vigorous agitation. Then, 0.56 g of additional AN was added to the flask, and the solution became milky white. After the suspension was kept at a constant temperature of 5°C by the ice water bath, 50 mL of ammonium persulfate aqueous solution (68.4 g/L) was added dropwise. The reaction mixture was stirred continuously, and the color change was observed. A complete reaction has taken place until the color changed from milky white to yellowish green, blue, blue-green, green, and dark green. Finally, extraction with 300 mL of chloroform was performed to obtain the PANI chloroform solution.


## 2.3. Preparation of PANI/Fe3O4 Nanocomposite

30 mL of PANI chloroform solution was poured into a three-necked flask. 70 mL of FeCl3·6H2O aqueous solution (0.1 mol/L) and 40 mL of FeSO4·7H2O aqueous solution (0.1 mol/L) were added to the chloroform solution in sequence and stirred continuously to form an emulsion in nitrogen atmosphere. 10 mL ammonia water was then quickly added to the mixture when the temperature rose to 60°C. After stirring for 1 hour at 80°C, the flask was cooled down to room temperature. At last, extraction with 300 mL of chloroform was carried out to obtain the suspension containing PANI/Fe3O4 nanocomposite. The suspension would be washed using the deionized water for five times.


## 2.4. Characterization

The analysis of the surface functional groups of magnetic nanoparticles was done using the Acotar 360 type Fourier transform infrared (FT-IR) spectrometer. The particle size and crystalline structure of the nanocomposite were observed using a Hitachi H-600 transmission electron microscope (TEM) and an X-ray powder diffractometer (XRD, Japanese Society of Science and Technology D/Max-IIIC). The magnetic properties of nanocomposite were studied by a Gouy magnetic balance and a vibrating sample magnetometer. The electrical conductivity of Fe3O4/PANI chloroform solution was measured using a conductivity meter (DDS-11C, Leici Instruments Co. Ltd., Shanghai, China). Dielectric constant and magnetic permeability of nanocomposite at 2~18 GHz were measured using an Agilent 8720E vector network analyzer. The pure Fe3O4 nanoparticles were made similarly using coprecipitation. All the tests performed in this work were repeated for at least three times.


## 3. Results and Discussion

### 3.1. Composition Analysis

Figure1 shows the FI-IR spectra of pure Fe3O4 nanoparticles and Fe3O4/PANI nanocomposite. The absorption peaks at 560.34 cm-1 and 577.30 cm-1 in both curves are the characteristic absorption peaks of Fe-O bonds. The peak at 3419.6 cm-1 in Figure 1(a) is a strong absorption peak of hydroxyl groups [38, 39]. This indicates that the surface of Fe3O4 magnetic nanoparticles is rich in hydroxyl groups, which is the basis for the surface modification of nanoparticles. The absorption peaks at 1491.56 cm-1 and 1299.61 cm-1 in Figure 1(b) correspond to the C=C stretching vibration of the quinone ring and C-N stretching vibration, respectively. 1124.62 cm-1 is the vibration absorption peak of quinone nitrogen atom, which indicates that PANI has been successfully coated onto the surface of Fe3O4 particles [40–43].Figure 1
FT-IR spectra of (a) pure Fe3O4 nanoparticles and (b) Fe3O4/PANI nanocomposite.


### 3.2. Structural Characterization

Figure2(a) is the XRD pattern of Fe3O4 nanoparticles. As it can be seen, the characteristic peaks were at 30.03o (220), 35.41o (311), 43.0o (400), 53.0o (422), 57.0o (511), 62.6o (440), and 74.6o (533) as compared with the JCPDS standard card (No. 75-1609). This indicates that the obtained Fe3O4 particles are inverse-spinel type. According to the Scherrer formula D=Kλ/βcosθ (K is the Scherrer constant, λ is the X-ray wavelength, θ is the Bragg diffraction angle, and β is the half-peak width of the diffraction peak), the average particle size of the particles is calculated to be 10 nm.Figure 2
XRD spectra of (a) pure Fe3O4 nanoparticles and (b) Fe3O4/PANI nanocomposite.Figure2(b) is the XRD pattern of Fe3O4/PANI nanocomposite. It can be seen that new diffraction peak appears at 20°~30o due to the parallel vertical periodicity of the PANI chain, indicating that PANI has been successfully integrated with Fe3O4 [44, 45]. In addition, after integration with PANI, the characteristic peak of Fe3O4 remains, and just the intensity and width of the peak are changed, indicating that the particle is still the inverse-spinel type. It is worthwhile to note that the coating does not change the crystalline structure of the nanoparticles.


### 3.3. Morphology Characterization

Figure3(a) is a TEM image of PANI, from which it can be seen that PANI is a fiber-interwoven network structure. Figure 3(b) shows that the Fe3O4 nanoparticles with a particle size of about 10 nm are uniformly embedded in the network structure of PANI. The Fe3O4 nanoparticles prepared through reverse in situ polymerization are smaller than those in the in situ polymerization. This may be related to the PANI formation [37, 40]. The scattered PANI acts as a coating agent and a surfactant, which inhibits the growth of Fe3O4 nanoparticles to preferentially avoid the agglomeration of Fe3O4 nanoparticles.Figure 3
TEM images of (a) PANI and (b) Fe3O4/PANI nanocomposite.
(a)
(b)


### 3.4. Magnetic Property Analysis

Figure4 shows the hysteresis loops of pure Fe3O4 nanoparticles and Fe3O4/PANI nanocomposite. It can be perceived that both materials are superparamagnetic. The saturation magnetization of pure Fe3O4 nanoparticles was 84.83 emu/g. After the combination of Fe3O4 with PANI, the saturation magnetization of the nanocomposite decreased significantly, which was about 70.12 emu/g. This was mainly because the nonmagnetic PANI coated on Fe3O4 nanoparticles, resulting in a decrease in the relative content of Fe3O4 in the Fe3O4/PANI nanocomposite, so the saturation magnetization per unit mass was inevitably reduced [38, 42, 44, 46].Figure 4
Magnetization curves of (a) pure Fe3O4 and (b) Fe3O4/PANI nanocomposite.Figure5 shows the relationship between the magnetic weight of the Fe3O4/PANI chloroform suspension and the applied magnetic field intensity. The magnetic weight of the Fe3O4/PANI chloroform suspension has a good linear relationship with the applied magnetic field intensity under a relatively low intensity of magnetic field (less than 30 mT). According to the slope of the magnetic weight and the applied magnetic field intensity curve, the magnetic susceptibility of the Fe3O4/PANI chloroform suspension was calculated as about 7.51×10-5.Figure 5
Relationship of magnetic weight and magnetic field intensity.


### 3.5. Conductivity Analysis of Chloroform Suspension

In the present work, the conductivity of chloroform suspension containing PANI and magnetic nanoparticles was determined. As listed in Table1, the chloroform and chloroform-AN monomer solutions are not conductive. With the addition of surfactant DBSA, the solution begins to be electrically conductive with a conductivity of about 0.145 μS/cm, which can be attributed to the free movement of the anions and cations of the surfactant. After the polymerization of the AN monomer, the conductivity of the PANI chloroform solution was significantly improved and increased to 1.172 μS/cm. This is due to the presence of large conjugated groups in the PANI structure [41, 47–50], whereby the free movement of electrons is improved.Table 1
Electrical conductivity of the samples.
Materials
Electrical conductivity (μS/cm)
CHCl3
0
AN (1.96 g)+CHCl3
0
AN (1.96 g)+DBSA (1.46 g)+CHCl3
0.145
PANI (1.96 g AN)+DBSA (1.46 g)+CHCl3
1.172
PANI (1.96 g AN)+DBSA (1.46 g)+Fe3O4+CHCl3
1.835
PANI (2.66 g AN)+DBSA (1.46 g)+Fe3O4+CHCl3
1.923
PANI (2.66 g AN)+DBSA (1.56 g)+Fe3O4+CHCl3
2.135When Fe3O4 nanoparticles were formed in situ in PANI chloroform solution, the conductivity of the suspension was further increased to 1.835 μS/cm. This may be due to the fact that the Fe3O4 is an ionic crystal with more free electrons and hence increasing the density of the electron cloud. A similar experimental phenomenon was reported by Husain et al. [51]. When the amount of AN monomer is increased, the conductivity of the PANI chloroform solution containing magnetic nanoparticles continues to increase to a climax of 1.923 μS/cm, which is clearly attributed to the increase in PANI concentration in the suspension. At the same time, it can be seen that the solution conductivity increases as the amount of DBSA is increased. The PANI molecule exists in the conformation of the extended chain because of more DBSA, which is beneficial to the delocalization and transition of the charge, resulting in higher conductivity.From the abovementioned, we can conclude that the main factors affecting the conductivity of PANI chloroform suspension are the concentration of PANI, the amount of DBSA, and magnetic nanoparticles.


### 3.6. Electromagnetic Parameter Analysis

The suspension containing PANI/Fe3O4 nanocomposite was washed with a large amount of deionized water to remove the DBSA and (NH4)2SO4, then washed with acetone several times and followed by vacuum drying to obtain the Fe3O4/PANI nanocomposite powder sample. The nanocomposite and paraffin were mixed at a mass ratio of 1 : 1 to make a coaxial sample with an inner diameter of 3 mm and an outer diameter of 7 mm; then electromagnetic parameters were tested using an Agilent 8720E vector network analyzer.The complex permittivity of Fe3O4 and Fe3O4/PANI is presented in Figure 6. The size of the real part of the dielectric constant is related to the amount of energy storage while the imaginary part is related to the electric field loss of the material [36, 52–56].Figure 6
Complex permittivity curves of pure Fe3O4 (e1) and Fe3O4/PANI nanocomposite (e2).It is noticed that the real part (e2′) and imaginary part (e2″) of Fe3O4/PANI are higher than the respective part of Fe3O4 (e1′, e1″) in the range of 2~18 GHz. That is because PANI as a conductive polymer will produce many electric dipoles in the electric field, which will bind most of the energy of the electric field and eventually converted into heat. Meanwhile, the Fe3O4 is embedded in the PANI network structure which also increases the dielectric loss due to interfacial synergistic effects between the two materials [57–59].The complex permeability of Fe3O4 and Fe3O4/PANI is presented in Figure 7. The real part (μ′) is related to the amount of magnetic field energy storage while the imaginary part (μ″) is related to the magnetic field loss of the material.Figure 7
Complex permeability curves of pure Fe3O4 (μ1) and Fe3O4/PANI nanocomposite (μ2).The real part of the Fe3O4/PANI magnetic particles gradually decreases in the S-band (2~4 GHz). When the frequency is greater than 4 GHz, the real part slowly rises. This is because the Fe3O4 magnetic nanoparticles have a natural resonance in the S-band, and as the frequency increases, the “exchange mode resonance” between the magnetic nanoparticles becomes more and more obvious [60–62]. Also, the magnetic permeability of the S-band of Fe3O4 is relatively high, but as the frequency increases, the conjugated main chain structure of the conductive polymer inhibits the agglomeration of the magnetic nanoparticles, which reduce the reflection of electromagnetic waves and enhance the energy storage capacity of the composite material. Therefore, the real part of the Fe3O4/PANI nanocomposite is slowly increased at 4~18 GHZ. The imaginary part of the Fe3O4/PANI nanocomposite is smaller than that of the pure Fe3O4 in the range of 2~18 GHz. This may be because the PANI encapsulates the magnetic nanoparticles to cause a decrease in saturation magnetization and a decrease in hysteresis loss [63, 64].From Figure8, the perception is made about the Fe3O4/PANI nanocomposite that the dielectric loss tangent is much larger than the magnetic loss tangent, which indicates that the effect of dielectric loss in 2~18 GHz is much larger than hysteresis loss. For pure Fe3O4 particles, dielectric loss is dominant with increasing frequency over 7.2 GHz. Figure 8 also displays that the magnetic loss tangent of the Fe3O4 particles is larger than that of the Fe3O4/PANI nanocomposite at 2~18 GHz [11, 65].Figure 8
Tangent loss curves of pure Fe3O4 (e1; μ1) and Fe3O4/PANI nanocomposite (e2; μ2).


### 3.7. Refection Loss Curve Analysis

The electromagnetic wave properties of materials can be characterized by the RL, according to the transmission line theory. The RL can be expressed by the following equations:(1)RL=−20logZ−Z0Z+Z0,Z=μrμ0ere0tanhj2πfdcμrer,where Z0 and Z are the impedance of free space and absorbing materials; μr and er are the complex permeability and complex dielectric constant of wave absorbing material; f is the frequency of the incident electromagnetic wave; d is the thickness of the absorber; and c is the velocity of light in free space [66–68].The microwave reflectivity of Fe3O4/PANI nanocomposite is represented in Figure 9. PANI/Fe3O4 nanocomposite owns an excellent microwave absorbing property which includes a wide reflection (reflection loss<‐10 dB) band (from 6.00 to 8.08 GHz) and has a minimum reflection value of -17.08 dB at 6.22 GHz at the thickness of 5.0 mm. Meanwhile, it is noticed that the thinner the coating, the more the absorbing band shifts towards low frequencies.Figure 9
The calculated reflection losses curve of PANI/Fe3O4.


## 3.1. Composition Analysis

Figure1 shows the FI-IR spectra of pure Fe3O4 nanoparticles and Fe3O4/PANI nanocomposite. The absorption peaks at 560.34 cm-1 and 577.30 cm-1 in both curves are the characteristic absorption peaks of Fe-O bonds. The peak at 3419.6 cm-1 in Figure 1(a) is a strong absorption peak of hydroxyl groups [38, 39]. This indicates that the surface of Fe3O4 magnetic nanoparticles is rich in hydroxyl groups, which is the basis for the surface modification of nanoparticles. The absorption peaks at 1491.56 cm-1 and 1299.61 cm-1 in Figure 1(b) correspond to the C=C stretching vibration of the quinone ring and C-N stretching vibration, respectively. 1124.62 cm-1 is the vibration absorption peak of quinone nitrogen atom, which indicates that PANI has been successfully coated onto the surface of Fe3O4 particles [40–43].Figure 1
FT-IR spectra of (a) pure Fe3O4 nanoparticles and (b) Fe3O4/PANI nanocomposite.


## 3.2. Structural Characterization

Figure2(a) is the XRD pattern of Fe3O4 nanoparticles. As it can be seen, the characteristic peaks were at 30.03o (220), 35.41o (311), 43.0o (400), 53.0o (422), 57.0o (511), 62.6o (440), and 74.6o (533) as compared with the JCPDS standard card (No. 75-1609). This indicates that the obtained Fe3O4 particles are inverse-spinel type. According to the Scherrer formula D=Kλ/βcosθ (K is the Scherrer constant, λ is the X-ray wavelength, θ is the Bragg diffraction angle, and β is the half-peak width of the diffraction peak), the average particle size of the particles is calculated to be 10 nm.Figure 2
XRD spectra of (a) pure Fe3O4 nanoparticles and (b) Fe3O4/PANI nanocomposite.Figure2(b) is the XRD pattern of Fe3O4/PANI nanocomposite. It can be seen that new diffraction peak appears at 20°~30o due to the parallel vertical periodicity of the PANI chain, indicating that PANI has been successfully integrated with Fe3O4 [44, 45]. In addition, after integration with PANI, the characteristic peak of Fe3O4 remains, and just the intensity and width of the peak are changed, indicating that the particle is still the inverse-spinel type. It is worthwhile to note that the coating does not change the crystalline structure of the nanoparticles.


## 3.3. Morphology Characterization

Figure3(a) is a TEM image of PANI, from which it can be seen that PANI is a fiber-interwoven network structure. Figure 3(b) shows that the Fe3O4 nanoparticles with a particle size of about 10 nm are uniformly embedded in the network structure of PANI. The Fe3O4 nanoparticles prepared through reverse in situ polymerization are smaller than those in the in situ polymerization. This may be related to the PANI formation [37, 40]. The scattered PANI acts as a coating agent and a surfactant, which inhibits the growth of Fe3O4 nanoparticles to preferentially avoid the agglomeration of Fe3O4 nanoparticles.Figure 3
TEM images of (a) PANI and (b) Fe3O4/PANI nanocomposite.
(a)
(b)


## 3.4. Magnetic Property Analysis

Figure4 shows the hysteresis loops of pure Fe3O4 nanoparticles and Fe3O4/PANI nanocomposite. It can be perceived that both materials are superparamagnetic. The saturation magnetization of pure Fe3O4 nanoparticles was 84.83 emu/g. After the combination of Fe3O4 with PANI, the saturation magnetization of the nanocomposite decreased significantly, which was about 70.12 emu/g. This was mainly because the nonmagnetic PANI coated on Fe3O4 nanoparticles, resulting in a decrease in the relative content of Fe3O4 in the Fe3O4/PANI nanocomposite, so the saturation magnetization per unit mass was inevitably reduced [38, 42, 44, 46].Figure 4
Magnetization curves of (a) pure Fe3O4 and (b) Fe3O4/PANI nanocomposite.Figure5 shows the relationship between the magnetic weight of the Fe3O4/PANI chloroform suspension and the applied magnetic field intensity. The magnetic weight of the Fe3O4/PANI chloroform suspension has a good linear relationship with the applied magnetic field intensity under a relatively low intensity of magnetic field (less than 30 mT). According to the slope of the magnetic weight and the applied magnetic field intensity curve, the magnetic susceptibility of the Fe3O4/PANI chloroform suspension was calculated as about 7.51×10-5.Figure 5
Relationship of magnetic weight and magnetic field intensity.


## 3.5. Conductivity Analysis of Chloroform Suspension

In the present work, the conductivity of chloroform suspension containing PANI and magnetic nanoparticles was determined. As listed in Table1, the chloroform and chloroform-AN monomer solutions are not conductive. With the addition of surfactant DBSA, the solution begins to be electrically conductive with a conductivity of about 0.145 μS/cm, which can be attributed to the free movement of the anions and cations of the surfactant. After the polymerization of the AN monomer, the conductivity of the PANI chloroform solution was significantly improved and increased to 1.172 μS/cm. This is due to the presence of large conjugated groups in the PANI structure [41, 47–50], whereby the free movement of electrons is improved.Table 1
Electrical conductivity of the samples.
Materials
Electrical conductivity (μS/cm)
CHCl3
0
AN (1.96 g)+CHCl3
0
AN (1.96 g)+DBSA (1.46 g)+CHCl3
0.145
PANI (1.96 g AN)+DBSA (1.46 g)+CHCl3
1.172
PANI (1.96 g AN)+DBSA (1.46 g)+Fe3O4+CHCl3
1.835
PANI (2.66 g AN)+DBSA (1.46 g)+Fe3O4+CHCl3
1.923
PANI (2.66 g AN)+DBSA (1.56 g)+Fe3O4+CHCl3
2.135When Fe3O4 nanoparticles were formed in situ in PANI chloroform solution, the conductivity of the suspension was further increased to 1.835 μS/cm. This may be due to the fact that the Fe3O4 is an ionic crystal with more free electrons and hence increasing the density of the electron cloud. A similar experimental phenomenon was reported by Husain et al. [51]. When the amount of AN monomer is increased, the conductivity of the PANI chloroform solution containing magnetic nanoparticles continues to increase to a climax of 1.923 μS/cm, which is clearly attributed to the increase in PANI concentration in the suspension. At the same time, it can be seen that the solution conductivity increases as the amount of DBSA is increased. The PANI molecule exists in the conformation of the extended chain because of more DBSA, which is beneficial to the delocalization and transition of the charge, resulting in higher conductivity.From the abovementioned, we can conclude that the main factors affecting the conductivity of PANI chloroform suspension are the concentration of PANI, the amount of DBSA, and magnetic nanoparticles.


## 3.6. Electromagnetic Parameter Analysis

The suspension containing PANI/Fe3O4 nanocomposite was washed with a large amount of deionized water to remove the DBSA and (NH4)2SO4, then washed with acetone several times and followed by vacuum drying to obtain the Fe3O4/PANI nanocomposite powder sample. The nanocomposite and paraffin were mixed at a mass ratio of 1 : 1 to make a coaxial sample with an inner diameter of 3 mm and an outer diameter of 7 mm; then electromagnetic parameters were tested using an Agilent 8720E vector network analyzer.The complex permittivity of Fe3O4 and Fe3O4/PANI is presented in Figure 6. The size of the real part of the dielectric constant is related to the amount of energy storage while the imaginary part is related to the electric field loss of the material [36, 52–56].Figure 6
Complex permittivity curves of pure Fe3O4 (e1) and Fe3O4/PANI nanocomposite (e2).It is noticed that the real part (e2′) and imaginary part (e2″) of Fe3O4/PANI are higher than the respective part of Fe3O4 (e1′, e1″) in the range of 2~18 GHz. That is because PANI as a conductive polymer will produce many electric dipoles in the electric field, which will bind most of the energy of the electric field and eventually converted into heat. Meanwhile, the Fe3O4 is embedded in the PANI network structure which also increases the dielectric loss due to interfacial synergistic effects between the two materials [57–59].The complex permeability of Fe3O4 and Fe3O4/PANI is presented in Figure 7. The real part (μ′) is related to the amount of magnetic field energy storage while the imaginary part (μ″) is related to the magnetic field loss of the material.Figure 7
Complex permeability curves of pure Fe3O4 (μ1) and Fe3O4/PANI nanocomposite (μ2).The real part of the Fe3O4/PANI magnetic particles gradually decreases in the S-band (2~4 GHz). When the frequency is greater than 4 GHz, the real part slowly rises. This is because the Fe3O4 magnetic nanoparticles have a natural resonance in the S-band, and as the frequency increases, the “exchange mode resonance” between the magnetic nanoparticles becomes more and more obvious [60–62]. Also, the magnetic permeability of the S-band of Fe3O4 is relatively high, but as the frequency increases, the conjugated main chain structure of the conductive polymer inhibits the agglomeration of the magnetic nanoparticles, which reduce the reflection of electromagnetic waves and enhance the energy storage capacity of the composite material. Therefore, the real part of the Fe3O4/PANI nanocomposite is slowly increased at 4~18 GHZ. The imaginary part of the Fe3O4/PANI nanocomposite is smaller than that of the pure Fe3O4 in the range of 2~18 GHz. This may be because the PANI encapsulates the magnetic nanoparticles to cause a decrease in saturation magnetization and a decrease in hysteresis loss [63, 64].From Figure8, the perception is made about the Fe3O4/PANI nanocomposite that the dielectric loss tangent is much larger than the magnetic loss tangent, which indicates that the effect of dielectric loss in 2~18 GHz is much larger than hysteresis loss. For pure Fe3O4 particles, dielectric loss is dominant with increasing frequency over 7.2 GHz. Figure 8 also displays that the magnetic loss tangent of the Fe3O4 particles is larger than that of the Fe3O4/PANI nanocomposite at 2~18 GHz [11, 65].Figure 8
Tangent loss curves of pure Fe3O4 (e1; μ1) and Fe3O4/PANI nanocomposite (e2; μ2).


## 3.7. Refection Loss Curve Analysis

The electromagnetic wave properties of materials can be characterized by the RL, according to the transmission line theory. The RL can be expressed by the following equations:(1)RL=−20logZ−Z0Z+Z0,Z=μrμ0ere0tanhj2πfdcμrer,where Z0 and Z are the impedance of free space and absorbing materials; μr and er are the complex permeability and complex dielectric constant of wave absorbing material; f is the frequency of the incident electromagnetic wave; d is the thickness of the absorber; and c is the velocity of light in free space [66–68].The microwave reflectivity of Fe3O4/PANI nanocomposite is represented in Figure 9. PANI/Fe3O4 nanocomposite owns an excellent microwave absorbing property which includes a wide reflection (reflection loss<‐10 dB) band (from 6.00 to 8.08 GHz) and has a minimum reflection value of -17.08 dB at 6.22 GHz at the thickness of 5.0 mm. Meanwhile, it is noticed that the thinner the coating, the more the absorbing band shifts towards low frequencies.Figure 9
The calculated reflection losses curve of PANI/Fe3O4.


## 4. Conclusions

In this paper, the Fe3O4/PANI composite was successfully prepared by the reverse in situ polymerization method. The conductivity, magnetic properties, and microwave absorption properties of the composite were elaborated in detail. The conductivity of the suspension of PANI/Fe3O4 nanocomposite is higher than that of the pure PANI solution and increases with the increase of PANI concentration and DBSA. The dried PANI/Fe3O4 powder has a high saturation magnetization of 70.12 emu/g and owns an excellent microwave absorbing property, which includes a wide reflection (reflection loss<‐10 dB) band (from 6.00 to 8.08 GHz) and has a minimum reflection value of -17.08 dB at 6.22 GHz at the thickness of 5.0 mm. The present work provides an evident basis for the preparation and potential applications of the electrical sensing and shielding coatings.



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*Source: 7962754-2019-12-05.xml*