Preparation and microwave absorbing properties of polyaniline/NiFe₂O₄/graphite nanosheet composites via sol–gel reaction and in situ polymerization

Abstract

Excellent microwave absorption of polyaniline composites with NiFe₂O₄/graphite nanosheet was prepared via sol–gel method and in situ polymerization reaction. The obtained composites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, thermogravimetric analyzer, vibrating sample magnetometer and a vector network analyzer. The results indicated that the graphite nanosheet was completely covered by NiFe₂O₄ particles and NiFe₂O₄/ graphite nanosheet is wrapped by polyaniline. Polyaniline /NiFe₂O₄/ graphite nanosheet has better thermal stability than polyaniline, and its conductivity and Ms are 0.2 S/cm and 10 emu/g, respectively. Measurement of the vector network analyzer indicates that the microwave absorbing properties of polyaniline /NiFe₂O₄/ graphite nanosheet are better than those of graphite nanosheet, NiFe₂O₄, polyaniline and polyaniline / graphite nanosheet. Its maximum reflection loss value is −21 dB at 11.5 GHz when the thickness is 2.0 mm, and polyaniline /NiFe₂O₄/ graphite nanosheet’s best match thickness is 2.5 mm, its reflection loss value can reach −30 dB at 9.1 GHz in the X band.

Graphical Abstract

1 Introduction

With the advancement of modern science and technology, microwave absorption materials have played an important role in our society for its special functions. For instance, in the application of high power radar, communication equipment, microwave heating device, absorbing material can prevent leakage of electromagnetic radiation and protect the health of the operator. Meanwhile, with a growing importance in stealth technology and electromagnetic compatibility (EMC), electromagnetic wave absorbing material also has an outstanding role. So absorbing materials with light weight, broad frequency and strong absorption peak become the focus of current research. Generally, microwave absorption materials are divided into two types on their different mechanism: electronic materials such as graphite nanosheet (NanoG) and magnetic materials such as ferrite [1–3]. When the conductive materials were combined with magnetic materials, perfect microwave absorbing materials with electrical conductivity and magnetic properties can be prepared [4–6].

Ferrite is a traditional microwave absorbing material with high saturation magnetization and magnetic loss, and has been widely applied in many fields such as military area and industrial. However, ferrite is a non-conductive material with very high density, which restricts its potential in applications requiring light mass and electrical conductivity. In order to prepare light-weight composites with efficiency microwave absorbing performance, carbon materials such as carbon nano-tube (CNT), carbon fiber (CF) and graphite nanosheet (NanoG) are always combined with ferrite [7–9]. Out of these materials, NanoG was choosen for its excellent electrical conductivity, cheap price, low density and other strengths [10].

Conductive polymers such as polyaniline (PANI), polypyrrole (PPY), poly thiophene (PHT) and polyacetylene (PAC) are popular research subjects for their special structures and properties. Conductive polymers are always used in the preparation of microwave absorbing composite materials [11–13]. As a conducting polymer, PANI has useful properties such as its cheap price, low weight and high conductivity, but its microwave absorbing property is low and frequency band is narrow [14]. To obtain perfect absorbing property, magnetic materials such as metal cobalt, metal nickel, ferrites and conductive materials such as carbon nano-tube, carbon fiber, graphene are always added to improve PANI’s microwave absorbing property [15–17].

In this study, the expanded graphite was firstly stripped into nanometer size by ultrasonic treatment in ethanol solution to prepare the NanoG. Then, nickel ferrite (NiFe₂O₄)/NanoG composites were synthesized via sol–gel reaction, and the graphite nanosheet was coated by NiFe₂O₄. Then, homogeneous PANI/NiFe₂O₄/NanoG composites with low density and excellent absorption properties were prepared. The morphology structures of NanoG, NiFe₂O₄/NanoG, PANI and PANI/NiFe₂O₄/NanoG were examined by TEM, SEM and XRD. Other instruments such as four probes resistance tester, vibrating sample magnetometer, thermogravimetric analyzer and vector network analyzer were also used to characterize the reaction products.

2 Experimental

2.1 Preparation of NanoG

Graphite nanosheet was obtained from oxidized graphite (OG) via an uncomplicated two-step reaction. First, OG was subjected to a thermal expansion at 850–950 °C for about 15 s in a tube furnace to form expanded graphite (EG). Then EG was immersed in ethanol solution and stripped into nanometer size by ultrasonic treatment. Then, the product was filtered, rinsed with ethanol solution for several times and dried in a vacuum at 70 °C for about 12 h, finally the NanoG with nanometer size was successfully prepared.

2.2 Preparation of NiFe₂O₄/NanoG

NiFe₂O₄/NanoG was obtained by sol–gel reaction. Before the sol–gel reaction, the surface of NanoG was pretreated with KH550. Ni(NO₃)₂, Fe(NO₃)₃ (Fe/Ni ratio of 2) and the pretreated NanoG were mixed with the appropriate amount of C₆H₈O₇. The mixed components were ground until the fine mixture powder was obtained. Then, the mixture powder was put at 90 °C water bath and a small amount of (CH₂OH)₂ was dropwise into, the PH value was adjusted to 7–8 by adding NH₃·H₂O solution, keeping the temperature for more than 2 h, until the solution became viscous and a gel was formed. The gel was then washed with deionized water several times to remove possible residues and dried at 65 °C for 10 h, then calcined at 700 °C (argon atmosphere) for 4 h. The reaction is shown in Fig. 1.

Fig. 1

Preparation of NiFe₂O₄/NanoG and PANI/NiFe₂O₄/NanoG

2.3 Synthesis of PANI/NiFe₂O₄/NanoG

The PANI/NiFe₂O₄/NanoG composite was prepared via in situ polymerization reaction, the process is shown in Fig. 1. 1.0 g NiFe₂O₄/NanoG was added into 30 ml HCl solution (0.5 mol l⁻¹), the mixture was then placed in an ice water bath and stirred. Then, 0.6 ml aniline was added into the mixture. When the temperature of the mixture was dropped to about 2–4 °C, 40 ml another solution was added into the mixture, containing (NH₄)₂S₂O₈ (APS, 0.5 mol l⁻¹) and HCl (0.5 mol l⁻¹). The reaction was kept for 8 h. After the reaction, the reaction liquid was filtered, and washed several times with HCl solution, then the solid products were placed in a 90 °C vacuum for 6 h to obtain the PANI/NiFe₂O₄/NanoG.

2.4 Characterization

The structure of NanoG, NiFe₂O₄/NanoG, PANI and PANI/NiFe₂O₄/NanoG was characterized by scanning electron microscopy (SEM; JSM-6390, HITACHI, Japan) and X-ray diffraction (XRD; PANalytical, Holland). In addition, the NiFe₂O₄/NanoG and PANI/NiFe₂O₄/NanoG samples were observed by transmission electron microscopy (TEM, H-7650, HITACHI, Japan). The thermal stabilities were analyzed using a thermogravimetric analyzer (TGA, SDT-2960, USA) with a heating rate of 10 °C/min in the range from 40 to 800 °C under argon atmosphere. The electrical conductivities of compounds were measured by a SZ-82 digital four probes resistance tester (Suzhou Electronic Equipment Factory, China). For measurement of electrical properties, circle samples with a diameter of 15 mm and a thickness of 2 mm were prepared by casting them into stainless forms and cold-pressing them. The magnetic properties were measured using vibrating sample magnetometer (VSM, Lake Shore7307). The absorption properties of PANI, PANI/NanoG and PANI/ NiFe₂O₄/NanoG were analyzed using a vector network analyzer(Agilent technologies E8362B) in X-band, the samples were prepared using the mix of the products (20 wt%) and paraffin (80 wt%) in a mould with the size of 22.86 × 10.16 × 2 mm.

3 Results and discussion

3.1 SEM/TEM analysis

The SEM and TEM images of the products are shown in Fig. 2. Figure 2a is the SEM image of NanoG, NanoG was lustrous, smooth and with uniform lamellar structure, its thickness is less than 100 nm, about 40–80 nm. Figure 2b is the SEM image of NiFe₂O₄/NanoG, it can clearly observed that the NiFe₂O₄ layer is well-distributed on the surface of NanoG. Figure 2c and d show the microcosmic appearance of PANI and PANI/NiFe₂O₄/NanoG, respectively. After the in situ polymerization, the surface of NiFe₂O₄/NanoG was completely covered by PANI. Figure 2e and f are the TEM images of NiFe₂O₄/NanoG and PANI/NiFe₂O₄/NanoG, which can further confirm the conclusions obtained by SEM images. These results indicate that the NiFe₂O₄/NanoG and PANI/NiFe₂O₄/NanoG are all successfully prepared.

Fig. 2

SEM/TEM images of NanoG (a), NiFe₂O₄/NanoG (b, e), PANI (c) and PANI/NiFe₂O₄/NanoG (d, f)

3.2 XRD analysis

Figure 3 illustrates the XRD patterns of NanoG (a), NiFe₂O₄/NanoG (b), PANI (c) and PANI/NiFe₂O₄/NanoG from 2θ = 10° to 80°. The two peaks locate at 2θ = 26.46° and 55.64°of curve (a), (b) and (d) are corresponding to (0 0 2) and (1 1 0) planes respectively and are the typical diffraction peaks of graphite (JCPDS. File No. 75-1621). Compare with curve (a), other six peaks can be observed at curve (b), which indicate the (220), (311), (400), (422), (511) and (440) planes of NiFe₂O₄’s cubic face-centered structure(JCPDS, File No. 10-0325). It suggests that the NiFe₂O₄/NanoG contains both NiFe₂O₄ and NanoG. The three peaks before 30° can easily be found at curve (c), which are the characteristic peaks of PANI. Figure 3d is the curve of PANI/NiFe₂O₄/NanoG, containing the characteristic peaks of PANI, NiFe₂O₄ and NanoG. These results confirm that NiFe₂O₄/NanoG and PANI/NiFe₂O₄/NanoG were both obtained.

Fig. 3

XRD curves of NanoG (a) NiFe₂O₄/NanoG (b) PANI (c) and PANI/NiFe₂O₄/NanoG (d)

3.3 TGA analysis

The thermal stability of material is an important property, which determines the temperature of the application of the material, so it is necessary to carry out thermal analysis. The thermal stabilities of PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG were characterized by TG analysis, and the curves are demonstrated in Fig. 4. The degradation temperature (T _d) of PANI is 511 °C for the pyrolysis of its main chain. Residual ratio (wt%) of PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG at different temperatures including 200, 400, 600 and 800 °C are shown in Table 1. Figure 4 and the table indicate that the PANI/NanoG’s residual ratio and PANI/NiFe₂O₄/NanoG’s residual ratio are relatively close at the four temperatures, while all the corresponding residual ratio values of the PANI are smaller than those of PANI/NanoG and PANI/NiFe₂O₄/NanoG. Meanwhile, the PANI’s curve is lower than PANI/NanoG’s curve and PANI/NiFe₂O₄/NanoG’s curve. It can be attributed to the role of NanoG in imposing restriction on the pyrolysis of PANI chains and avoiding heat concentration [18].

Fig. 4

TG curves of PANI (a) PANI/NanoG (b) and PANI/NiFe₂O₄/NanoG (c)

Table 1 Residual ratio (wt%) of PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG at different temperatures

3.4 Electrical conductivity

The conductivities of NanoG, NiFe₂O₄, NiFe₂O₄/NanoG, PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG are summarized in Tables 2 and 3. NanoG is an excellent conductive material with the high conductivity of 5.8 S/cm. The low conductivity of NiFe₂O₄ at 8.1 × 10⁻⁸ S/cm indicates that it is an insulating material. However, after the sol–gel reaction and combined with NanoG, the conductivity of NiFe₂O₄/NanoG can reach 0.24 S/cm, which can mainly attribute to the carbon material NanoG. As a conducting polymer, proton acid PANI has excellent conductivity, the four probes suggest that its conductivity can reach 0.19 S/cm. However, after adding the NiFe₂O₄/NanoG, its conductivity increased to 0.2 S/cm, which is situated between the PANI’s 0.19 S/cm and NiFe₂O₄/NanoG’s 0.24 S/cm.

Table 2 Electrical conductivity values of NanoG, NiFe₂O₄, NiFe₂O₄/NanoG (S/cm)

Table 3 Electrical conductivity values of PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG (S/cm)

3.5 Magnetic performance

To verify the magnetic properties of the samples, the hysteresis loops were measured with VSM at room temperature, and the result was exhibited in Fig. 5. The hysteresis loops of the samples exhibit typical soft magnetic behavior. The magnetization saturation (Ms) of NiFe₂O₄ can reach 49.7 emu/g. After the sol–gel reaction and in situ polymerization, and being combined with nonmagnetic materials NanoG and PANI, its magnetism drops to 33 and 10 emu/g, respectively. It’s mainly attributing to the contribution of the volumes of the non-magnetic NanoG and PANI to the total sample volume.

Fig. 5

Magnetization curves of NiFe₂O₄ (a) NiFe₂O₄/NanoG (b) and PANI/NiFe₂O₄/NanoG (c)

3.6 Microwave absorbing performance analysis

The mechanism of microwave energy loss in a material is the result of its magnetic and electronic properties, which is related to the complex permittivity and mainly characterized by reflection loss (RL). According to the transmit-line theory, when the electromagnetic wave vertically incidents upon the samples, the reflection loss can be calculated using the following equations [19–21]:

$$R = 20\log \left| {\frac{{{Z_{{\rm{in}}}} - {Z_0}}}{{{Z_{{\rm{in}}}} + {Z_0}}}} \right|$$

(1)

$${Z_{{\rm{in}}}}\, = \,{Z_0}\left( {\sqrt {\frac{{{\mu _r}}}{{{\varepsilon _r}}}} } \right)\tanh \left[ {j\left( {\frac{{2\pi fd}}{c}} \right)\left( {\sqrt {{\mu _r}{\varepsilon _r}} } \right)} \right]$$

(2)

where Z ₀ is the characteristic impedance of vacuum, Z _in is the normalized input impedance of the absorber, c is the velocity of light in free space, ε _r = ε′ − jε″ , μ _r = μ′ − jμ″ is the relative complex permeability and permittivity of the material, f is the frequency, a is the velocity of the light, and d is the thickness of the absorber, respectively. It indicates that the absorption capacity of the material at the same frequency mainly depends on the thickness of the material [22, 23].

The electromagnetic parameters of NanoG (a), NiFe₂O₄ (b), PANI (c), PANI/NanoG (d) and PANI/NiFe₂O₄/NanoG (e) were exhibited in Fig. 6, and their dielectric loss (tanδ _e = ε″/ε′) and magnetic loss (tanδ _m = μ″/μ′) were calculated. The real permittivity (ε′) and real permeability (μ′) represent the storage ability of electromagnetic energy, and the imaginary permittivity (ε″) and imaginary permeability (μ″) are connected closely with the energy dissipation and magnetic loss, respectively. In the X band, the permittivities’ real part of PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG are very close, all higher than NiFe₂O₄ and lower than NanoG. However, the permittivitie imaginary part of PANI/NiFe₂O₄/NanoG is higher than PANI/NanoG. Therefore, the PANI/NiFe₂O₄/NanoG’s curve of tanδ _e is above those of PANI/Nano’s and PANI’s. In terms of magnetic parameters, due to the existence of the ferrite, all the parameters include μ′, μ″ and tanδ _m of NiFe₂O₄ and PANI/NiFe₂O₄/NanoG are higher than PANI, NanoG and PANI/NanoG sample.

Fig. 6

Electromagnetic parameters of NanoG (a) NiFe₂O₄ (b) PANI (c) PANI/NanoG (d) and PANI/NiFe₂O₄/NanoG (e)

The calculated reflection loss curves of NanoG, NiFe₂O₄, PANI, PANI/NanoG and PANI/NiFe₂O₄/NanoG composite in X band are displayed in Fig. 7. The RL values of NiFe₂O₄ and PANI are all less than −10 dB (90 % absorption), which exhibit poor microwave absorbing property. The RL value of NanoG is ranging from −6 to −13 dB. After the sol–gel reaction and was combined with NanoG, the absorbing performance of PANI/NanoG was improved, and the maximum value of the reflection loss can reach −17.2 dB. However, the PANI/NiFe₂O₄/NanoG’s curve is below the PANI/NanoG’s curve and PANI’s curve, and it can reach −21 dB (more than 99.9 % absorption) when the frequency is 11.5 GHz. In addition, when the frequency is ranging from 10 to 12.4 GHz, its RL curve is below −10 dB. These results indicate that the microwave absorbing properties of PANI/NiFe₂O₄/NanoG were greatly improved compared with the other four materials. Meanwhile, the frequency band of PANI/NiFe₂O₄/NanoG is wider. This is due mainly to the addition of magnetic property by NiFe₂O₄, making the ternary composite has better electromagnetic matching than these materials.

Fig. 7

RL curves of NanoG (a) NiFe₂O₄ (b) PANI (c) PANI/NanoG (d) and PANI/NiFe₂O₄/NanoG (e)

As the matching thickness of the absorber is one of the crucial parameters which could determine the incident impedance, it is necessary to explore the influence of thickness on the absorption properties in detail. The simulated RL curves of PANI/NiFe₂O₄/NanoG at different thicknesses are shown in Fig. 8. It is obvious that the peak of RL moves toward the lower frequency region with the thickness increasing, and a maximum peak value can be observed at a certain thickness, which may be explained by the quarter-wavelength model. When the thickness is 1.5 mm, the RL value is less than −5 dB. However, when the thickness increased to 2.5 mm, its RL curve is ranging from −6.3 to −30 dB, and the maximum value appeared at F = 9.1 GHz. When the frequency ranges from 8.2 to 10.8 GHz, its RL values are all more than −10 dB. Its microwave absorbability is better than the 2.0 mm sample, and the frequency band became wider. When the thickness reached 3.0 and 3.5 mm, its RL value is decreased with the increase of the thickness, which is less than the 2.5 mm sample. So we can draw the conclusion that the best match thickness of PANI/NiFe₂O₄/NanoG is 2.5 mm in the X band, and the RL is lower both in low and high frequency.

Fig. 8

RL curves of PANI/NiFe₂O₄/NanoG at different thicknesses

4 Conclusions

In summary, the PANI/NiFe₂O₄/NanoG ternary composite with perfect microwave absorption properties were prepared by a facile two-step method. According to measurements from SEM, TEM and XRD, the NanoG is coated by NiFe₂O₄ layer after sol-gel reaction, and NiFe₂O₄/NanoG is uniformly wrapped by PANI after in situ polymerization. Measurement of TGA shows that the thermal stability of PANI is improved by the addition of NanoG. The four-point-probe and VSM test suggest that the PANI/NiFe₂O₄/NanoG’s conductivity and Ms value are 0.2 S/cm and 10 emu/g, respectively. Above all, the PANI/NiFe₂O₄/NanoG has excellent microwave absorbing properties, its RL value can reach up to −21 dB when its thickness is 2.0 mm and −30 dB when its thickness is 2.5 mm. Based from the above results, the PANI/NiFe₂O₄/NanoG composite is therefore an excellent candidate for application as a microwave absorber.