Synthesis, characterization, and optimization of a quaternary nanocomposite for efficient electromagnetic absorption coating in the X-band

2.1 Materials

Ferrous sulfate heptahydrate (FeSO₄·7H₂O), aniline (C₆H₅NH₂) purity ≥99.5 %, cobalt chloride hexahydrate (CoCl₂·6H₂O) ≥99 %, sodium hydroxide (NaOH) ≥99 %, potassium nitrate (KNO₃) ≥99 %, ammonium persulfate (NH₄)₂S₂O₈ ≥98 %, hydrochloric acid (HCl) ≥35 %, tetrahydrofuran (THF) ≥99 % and graphite powder ≥97 % were purchased from Merck company and used without purification. TPU was purchased from the Epaflex Company (with Shore 85A).

2.2 Characterization

In this research, the phase structure of samples was analyzed by X-ray diffraction (XRD) (PHILIPS-PW 1730). Field emission scanning electron microscopy (FESEM) was used to investigate the surface morphology of the samples (MIRA3 TESCAN-XMU). The hysteresis loops of powders and films were determined at room temperature using a vibrating-sample magnetometer (VSM) (LBKFB). An ultrasonic probe (UP400S-Hielscher) was employed during composite creation to disperse particles. For thermal analysis of the samples and measurement of both heat flow and weight changes in material, Thermogravimetric analysis (TGA) (Q600) was used. A four-probe electrical conductivity instrument (Huanyu-model) was used at room temperature. The ASTM D570-98 Standard Test Method was also performed to decide the water absorption percentage. The microwave wavelength absorption was examined in the X-band (8–12 GHz) by a Vector Network Analyzer (VNA) (HP-8410C). The samples were exposed in the X-band range and S₁₁ and S₁₂ (S-parameters are the electrical characteristics of a signal in a complex network) were established. Design-Expert software applied design optimization for broadband electromagnetic shielding of the composite film.

2.3 Experimental methods

3.1 X-ray diffraction

XRD patterns for CF, CF/PANI, CF/PANI/G, and CF/PANI/G/TPU are shown in Figure 1a. The CF nanoparticles were assigned by the standard (JCPDS no. 00-022-1086). The observed peaks at 2θ = 21.24, 35.10, 41.4, 50, 62.2, 67.5, and 74.8° were assigned (111), (220), (311), (400), (422), (511), and (440) respectively, with an inverse cubic spinel structure with space group [26]. The XRD pattern of PANI had broad peaks at 15–30° and peaks at 2θ = 15.45, 20.45, 25.45, and 25.9°, which is similar to PANI. Two peaks were observed in the XRD pattern of CF/PANI/G at 26.45° (large) and 54.7° (small), which confirms the presence of G in the nanocomposite [27, 28]. Based on the results, the broad peak in the 15 < 2θ < 30 was increased because TPU and PANI had a higher percentage by weight than other materials. Besides the peaks above, there was a peak at 20.55°, which matched TPU [29]. By using X’pert Highscore software and the Williamson–Hall method the following qualitative and quantitative results were obtained. The output parameters were; Crystal system (Rhombohedral) – Space group (R 3 ̄ m) Space group number (166) – a (Å): 5.9366 – b (Å): 5.9366 – c (Å): 14.5430 – Alpha (°): 90.0000 – Beta (°): 90.0000 – Gamma (°): 120.0000 – Crystallite size (nm) = 131.

Figure 1:

XRD patterns of cobalt ferrite-cobalt ferrite/polyaniline-cobalt ferrite/polyaniline/graphite and cobalt ferrite/polyaniline/graphite/thermoplastic polyurethane.

3.2 Morphological characterization

The FESEM technique was used to determine the particle morphology. Based on the results, the morphology of the CF nanoparticles was octahedral with 50–150 nm (Figure 3a). In Figure 2b, PANI surrounds the CF nanoparticles. Figure 2c shows CF/PANI/G. The morphology of the composites shows clustering and layering. In Figures 2d and e FESEM images for TPU/PANI/CF/G are shown on 2 µm and 200 nm scales. The filler particles have a distribution of 50–100 nm in the TPU matrix, despite the magnetic property of CF, which leads to agglomeration.

Figure 2:

FESEM images of (a) cobalt ferrite in 500 nm, (b) cobalt ferrite/polyaniline in 200 nm, (c) cobalt ferrite /polyaniline/graphite in 1 µm, (d) cobalt ferrite /polyaniline/graphite/thermoplastic polyurethane composites in 2 µm, and (e) cobalt ferrite /polyaniline/graphite/thermoplastic polyurethane composites in 200 nm.

3.3 Magnetic properties

The magnetic measurements of CF, PANI/CF, PANI/CF/G, and TPU/PANI/CF/G composites were performed using a vibrating sample magnetometer (VSM) hysteresis loop recorded at room temperature, as shown in Figure 3. CF’s saturation magnetization (M_s), 57.89 emu g, almost matches a previous report [30]. CF’s coercivity (H_c) was recorded at 493 Oe, and remanent magnetization (M_r) was 13.06 emu g⁻¹. “M_s” and “M_r” of PANI/CF were 18.31 and 10.78 emu g⁻¹, respectively, and coercivity was 1910 Oe. The interaction between CF and PANI led to a significant decrease in the magnetic properties of the nanocomposite, which matches a previous report [31]. For PANI/CF/G, M_s and M_r were 4.92 and 1.11 emu g⁻¹, respectively. Also, coercivity was 490 Oe. Magnetic saturation was further decresed by G because it is a diamagnetic material [32]. The CF had an intense ferromagnetic phase, so the hysteresis diagram for PANI/CF/G showed the same state.

Figure 3:

The magnetic hysteresis loops of (a) cobalt ferrite, (b) cobalt ferrite /polyaniline, (c) cobalt ferrite /polyaniline/graphite, and (d) cobalt ferrite /polyaniline/graphite/thermoplastic polyurethane composites.

The quaternary composite had an M_s of 1.03 emu g⁻¹ and remanent magnetization of about 0.3 emu g⁻¹. Also, coercivity was 737 Oe. TPU also decresed magnetic saturation. TPU is a non-magnetic material, so it further decreases magnetic saturation.

3.4 Water absorption – environmental stability against humidity

Water absorption in composites causes damage and disorder communication between the matrix and fillers. It thus decrases mechanical properties (tensile strength, impact resistance). It also changes the form of composites (swelling caused by water absorption). Also, water in a material affects its electrical properties, thermal performance, and dielectric properties.

ASTM D570 is one of the most commonly used standard methods for measuring water absorption in polymers. This method was performed on composites at room temperature and in humid conditions. First, the samples were placed in an oven for 24 h at 50 °C and then in a dryer to cool. Immediately after cooling, the samples were weighed. Then, the samples were immersed in water at 23 °C for 24 h. After that, the samples were dried with a lint-free cloth and weighed again. Finally, the percentage of water absorption was calculated by Equation (1) [33]:

Here, w₁ is the weight of the sample after drying in the oven, and w₂ is the weight after immersion in water.

Table 1 shows the percentage of water absorption for pure TPU (matrix), pure PANI, and composites.

Table 1 shows the water absorption percentage of two pure thermoplastic materials, TPU and PANI. The results show that TPU was hydrophobic, and PANI was hydrophobic with less intensity. For various composites, an inverse relationship was shown between the water absorption percentage and the weight percentage of TPU. Results reveal that the G in composites shows a hydrophilic property, confirming a previous report [34]. The effect of G on water absorption in composites causes layered structure and porosity.

On the other hand, for similar concentrations (wt.%) of G and TPU, the weight changes of PANI due to water absorption percentage were noticeable (sample no. 12 with 8). However, samples 2 and 24 showed an increased PANI wt.% (20–25) with a decreasing percentage of water absorption. That can be attributed to water molecules absorbed through the hydrogen bonds of PANI. Additionally, groups such as –OH and –COOH in the composite are important for water absorption, increasing the weight percent PANI by up to 20 % increased water absorption. However, a further increase in PANI from 20 % led to accumulation and restraint of water uptake.

3.5 Thermogravimetric analysis – thermal stabilities of the composite

Thermal gravimetric analysis (TGA) is a method used to evaluate a material’s thermal stability. All samples were heated in air to 800 °C at 10 K min⁻¹. The weight loss percentages of the samples with temperature are shown in Figure 4. All the samples had the same thickness (2 mm) and constituents such as CF, TPU, PANI, and G.

Figure 4:

TGA curve of 6 composites at a thickness of 2 mm and cobalt ferrite, graphite, polyaniline and thermoplastic polyurethane.

The curves of CF show weight loss in two steps. The first weight loss occurred at 50–125 °C, which was related to the evaporation of water absorbed on the surface of the CF. In stage 2, the weight loss was steady, ranging from 150 to 400 °C. From 500 to 800 °C, the slope of the curve remains constant. CF has thermal stability of up to 800 °C. The TGA curve of G showed negligible weight loss up to 800 °C. Graphite was thermally stable at temperatures less than 800 °C. The TGA curve of PANI shows that weight loss occurs in three stages. The first zone, up to 130 °C, showed a modest drop in weight, which might be attributed to the water molecule and dopant (HCl) loss from the PANI chains. The second zone, which ranged from 130 to 230 °C, was where functional groups (–COOH) were lost. In this area, the polymer had thermal stability. The third area related to continuous weight loss from 230 to 550 °C was the breakdown of the polymer skeleton at 600–800 °C, a slope of zero related to the residual PANI. The maximum thermal stability of PANI was recorded at 230 °C.

Thermal decomposition of TPU occurred in three steps. In the first stage, moisture was evaporated at up to 100 °C. The second stage took place at around 300 °C, and the third stage was at 410 °C.

Destruction in steps 2 and 3 was due to urethane decomposition in the hard part and polyol decomposition in the soft part of TPU. Step 2 involves separating urethane from the primary polyol and isocyanate. In step 3, the polyol was destroyed [35, 36]. At 500 °C, the slope of the curve decreased to near zero, indicating that the polymer had been destroyed, leaving only the remaining carbon. Between 600 and 800 °C, TPU recorded the remaining percentage (3.5 %). Pure TPU remained stable up to 290 °C. Almost all composites decomposed at 260–450 °C, and the remaining components were G and CF. Sample no. 15 had the highest thermal stability at 310 °C and the highest residual weight of 35.27 %, followed by samples no. 21, 18, 22, 20, and 26, in that order. The results showed that adding fillers, G, and CF to the matrix increased the composites’ thermal stability. Compared to the curves of samples no. 18 and 22, increasing the percentage of PANI decreses thermal stability. The form of the curves was similar to that of pure TPU for composites with a more significant TPU weight percentage. They were also thermally stable between 260 and 310 °C.

3.6 D.C. Electrical conductivity

The electrical conductivity test was performed on composite films (1.5 mm in thickness) at room temperature. The results are shown in Table 2.

The weight percent of G in films is nearly directly linked with the increase in electrical conductivity. The electrical conductivities of pure TPU, CF, PANI, and G were 10⁻¹³, 8 × 10⁻⁹, 4.1, and 352. TPU was almost an electrical insulator, and CF was almost non-conductive. The conductivity of the quaternary composites varied from 18.9 to 78.9 S cm⁻¹.

PANI placement in the space between the G layers has a synergistic effect. PANI acts as an electron donor and G as an electron acceptor [37]. Therefore, when exposed to a different potential, the separated electrons move freely, leading to a significant increase in electrical conductivity. With 15 wt.% G and 15–20 wt.% of PANI, about 22 % of the conduction of pure G was achieved (despite the electrical resistance of TPU in the composite matrix). When CF was combined with G, an interaction was created; CF was converted from an insulator to a semiconductor. According to Table 2, when the wt.% of CF increased composites no. 12 and 16, the electrical conductivity increased.

3.7 Principles of electromagnetic interference (EMI) shielding theory

The EMI SE theory is defined as the shielding restraint of propagating EM waves. The shielding effectiveness of a material is determined by the decrease of the magnetic or electric field caused by shielding. Each shielding material decreses EM radiation through three mechanisms. The sum of the three, reflection, absorption, and multiple internal reflections, defines SE_T and is given by Equation (2):

When SE_T is greater than 15 dB, SE_M can be ignored, and Equation (3) results [38]:

Shielding efficacy is the ratio of power received with and without material for the same incoming power. It is represented in decibels and is computed as follows [39]:

Here, P₁ is the electromagnetic power before installing the shield, P₂ is the same quantity after installing the shield, and both are measured at the same point. A shielding effectiveness of 10 dB equals 90 % of reflected or absorbed electromagnetic energy. To achieve 99.9 % EM energy reflection or absorption by a medium, a minimum of 20 dB SE is required [40]. According to another assessment, 30 dB for SE is suitable for 50 % of applications in the automotive and computer industries [41]. Another study [42] suggests an SE of 20 for general use. According to previous research [43], an EMI SE of 20 dB is suitable for commercial use, matching a transmission of less than 1 % of the EM wave. Another study [38] found that SE with 30 dB matching and 99.9 % wavelength weakening is deemed high for SE applications. Term SE_T is described as a reflection, absorption, and multiple reflections [44].

In the military field, SE_T values of EMI shielding materials of at least 30 dB are needed [45]. A SE_T equivalent to 20 dB is appropriate for various electronic applications [46], [47], [48]. According to the relations and values of the S parameters of the network analyzer, transmission (T), reflection (R), and absorption (A) are calculated using Equations (5)–(7) [49, 50]:

Therefore, the shielding mechanisms by reflection (SE_R) and absorption (SE_A) are defined by Equations (8)–(10) [51]:

S₁₁, S₂₂, S₁₂, and S₂₁ are the input reflection coefficient, output reflection coefficient, reverse gain coefficient, and forward gain coefficient.

The effectiveness of the EMI shielding (dB) with the following Equation converted to the EMI shielding efficiency (%) [52]:

3.8 EMI shielding properties and mechanism

According to previous research, SE_T = 30 dB was deemed ideal. We constructed twenty-seven film composites with 1, 1.5, and 2 mm thicknesses. We calculated the SE_T using the S parameters of the VNA by Equation (10); the results are given in Figure 6a–c.

As shown in Figure 5a, the ideal sample is composite film no. 4 with SE_T = 30.7. Because Sample no. 1 comprised the lowest weight percentages of PANI and SE_T, composites containing less than 20 % PANI are not acceptable for shielding at a 1 mm thickness. When samples no. 27 and no. 4 are compared, the effect of G weight gain and CF weight loss on increasing SE_T from 26.8 to 30.7 is evident. Increasing PANI’s weight to greater than 20 % did not substantially raise SE_T (samples no. 23 and 4). Also, the change in the SE_T was the opposite of the change in the weight percent of the matrix TPU.

Figure 5:

The relationship between to weight percentage of materials and SE_r at thickness: (a) 1 mm, (b) 1.5 mm, and (c) 2 mm.

In Figure 5b, at a thickness of 2 mm, sample no. 14 with SE_T = 30.1 was selected as the optimal sample. Also, the highest SE_T was related to no. 6. Because of the more significant interaction of materials, the electrical conductivity of sample no. 6 was higher than that of sample no. 13. When samples no. 6 and 9 are compared, the binding effect of CF as a magnetic material in increasing SE_T is observed.

In Figure 5c, sample no. 18 with SE_T = 30.1 was selected as the optimal sample. Sample no. 15, with SE_T = 47.9, received the highest SE_T and best performance. When comparing samples 26 and 15, raising the G of weight percent from 5 to 10 and decreasing the weight percent TPU increased SE_T from 25.5 to 47.9.

Numerous studies have revealed that absorption loss on SE_T is significantly greater than reflection loss and plays a critical impact. In several investigations [53, 54] absorption loss ranged between 75 and 85 % of SE_T. Composites with high electrical conductivity and high SE_A are used in stealth technology.

Therefore, the following shows the change of absorption loss with frequency in the X-band; at a thickness of 1 mm, none of the samples had an absorption loss of more than 30 dB, so they were not chosen for specialized and advanced uses.

The maximum absorption loss at 1.5 mm thickness was 35.6 dB (samples no. 6), adequate for stealth technology. The sample with the lowest absorption loss (sample no. 10) had a loss of 75 %, while the sample with the highest loss had a loss of 92 % (sample no. 13).

At a thickness of 2 mm, samples no. 21, 22, and 15 exhibited absorption losses of 31.4, 37.5, and 45 dB, respectively, adequate for advanced applications. The sample with the lowest absorption loss (82 %) was sample no. 20, whereas the sample with the highest absorption loss (96 %) was sample no. 22.

The changes in absorption loss at different thicknesses in different samples were such that at a thickness of 1 mm, the maximum absorption loss was 81 %. The minimum absorption loss was 72 %, and at a thickness of 1.5 mm, the maximum absorption loss was 92 %. The minimum absorption loss was 75 %, and at a thickness of 2 mm, the maximum absorption loss was 96 %, and the minimum absorption loss was 89 %, which indicates the effect of extreme thickness on the increase in the amount of absorption pressure on the reflection loss. It is important to remember that the reflection loss is determined according to Equation (3) by the values left at 100 %. The relationship between SE_T and SE_A in the X-band is given in Figure S2.

Because CF had magnetic characteristics, the change in the weight percent of CF was substantial for two samples of the same thickness.

At 10 GHz (composites 13 and 21), the percentage absorption loss rose, as did the percentage absorption loss above 10.5 GHz (composites no. 15 and 22). SE_T was more than 30 dB across the whole frequency range X, showing that four composites are better for use in industrial applications.

3.9 Response surface methodology

The response surface methodology (RSM) collects statistical techniques used to optimize, affecting several variables’ responses. This strategy minimizes the number of experiments and allows for the estimation of all second-order regression model coefficients and interacting components. This study examined the relationship between the earned responses and variables. We have also optimized variables by the surface response method with Box Behnken Design (BBD) of Design-Expert software. The effects of independent variables such as percentage by weight of CoFe₂O₄ (levels 2, 6, 10), PANI (levels 15, 20, 25), G (levels 5, 10, 15), and the thickness of three levels (1, 1.5, 2 mm) on SE_T (as response) were investigated. In total, 27 samples were tested.

Finally, data were entered into the Design-Expert software to determine and achieve the best thickness, composition, and weight percent to achieve an ideal composite. Based on how the software set up the tests, the highest value was SE_T = 47.9 dB.

According to the ANOVA table, the R-squared and adj R-squared values were equal to 0.9086 and 0.8020, respectively, and showed the model’s accuracy. First, the wt.% of G, then the thickness of the composite, then the wt.% of CF, and finally the wt.% of PANI affected the SE_T.

A second-order multivariate Equation was obtained according to the obtained results, which showed the experimental relationship between the tested variables and SE_T. The results of the variance analysis for the quadratic response level model on the experimental data are shown in Table 3.

Shielding Efficiency (Total) = 29.70 + (2.75 × A) + (2.70 × B) + (2.58 × C) + (4.34 × D) + (2.02 × A × B) + (0.35 × A × C) + (5.13 × A × D) − (0.22 × B × C) + (0.80 × B × D) − (4.55 × C × D) + (0.94 × A²) − (3.34 × B²) + (1.10 × C²) + 1.52 × D²

Figure 6a shows the effect of wt.% CF on total shielding. SE_T increased with CF in the 2 to 7 wt.% range and then declined. This effect is evident in the SE_T change from 25.4 to 33.1 dB based on data from the ANOVA (Table 3), which shows that thickness and then wt.% of the CF effectively increased SE_T.

Figure 6:

Response surface plot of interacting effects of cobalt ferrite, polyaniline, and graphite on SE_T.

Figure 6b shows the effect of wt.% PANI on total shielding. SE_T increased with wt.% of PANI, especially in the 20 to 25 wt.% range. This effect is evident in the SE_T change from 28.5 to 35.6 dB. From a comparison of the slopes of Figures 6a and b, the difference between the effects of CF and PANI was noticeable.

Figure 6c shows the effect of wt.% G on total shielding. SE_T increased with G in the 7.5 to 15 wt.% range. The effect of graphite on SE_T was outstanding, from 28.6 to 41.2 dB.

Software output shows the different values of the optimal film composite FM scroll file placed in the below path . According to the thickness of 1.26 mm, the weight percentages of CF, PANI, and G were 6.95 %, 20.04 %, and 12.13 %, respectively, to achieve the minimum value of SE_T = 30. Also, the weighted TPU was 60.88 %.