A novel three-layer woven fabric composite reinforced by (polyaniline + carbonyl iron) core-shell/carbonyl iron/polyaniline fillers for high-performance broadband radar absorbing structure

Abstract

One of the most important applications of electromagnetic wave absorption is in stealth aircrafts and electromagnetic protection of avionic systems. The main limitations in the design of these structures are aerodynamics, thickness or weight, mechanical strength, manufacturing process, and reasonable cost. In this study, a novel three-layer woven fabric composite laminate (with a total thickness of about 3 mm) is proposed which each layer is reinforced by individual polyaniline, carbonyl iron, or (PANI + CI) core-shell fillers. The developed Non-dominated Sorting Genetic Algorithm II optimization algorithm suggests the stacking sequence of layers, the appropriate thickness of each layer, and the filler weight fraction in each layer to achieve a broadband absorption. Due to using both dielectric and magnetic absorbing fillers, this structure shows well-impedance matching and approximately absorbs 80% of the X-band (8-12 GHz) electromagnetic waves. The maximum reflection loss is about −14dB. Finally, the effect of the addition of absorbent particles on the mechanical properties has been investigated. Experimental results showed that the tensile modulus and strength decrease by about 21.5% and 20.6%, respectively, and the flexural modulus and strength reduce by 21.7% and 19.7%, respectively. However, the (PANI + CI) core-shell filler can be introduced as a high performance absorber filler because it suggests maximum reflection loss with low weight fraction compared to other fillers and consequently the minimum reduction in mechanical properties.

Keywords

Absorbent fillers electromagnetic waves composites radar-absorbing multi-objective optimization

Introduction

Today, the use of electromagnetic waves (EM) in many applications such as wireless communication systems, satellite communications, and radar systems has been significantly developed. This causes problems such as environmental pollution, and disruption of the functioning of electronic systems.^1,2 In aerial military affairs, the need to camouflage equipment and devices from enemy radars is important, as governments spend large sums of money each year to develop materials capable of absorbing EM within the radar frequency range. Radars are important devices that use modulated EM and directional antennas to send EM to a specific area of space to detect, intercept and destroy enemy targets.^3,4 Targets in the search range, reflect some of these waves toward the radar, and the radar receiver processes these return waves to extract target information such as distance, speed, and other characteristics that determine the target. The detection rate of a target by radar is determined by the radar cross section (RCS). RCS means the ratio of reflective power to land power, depending on various factors such as shape, size, and target material. Primarily, in order to reduce the RCS, two techniques of shape design and the use of radar wave-absorbing materials are employed.⁵ In conventional designs, shape design techniques are considered the first step in controlling the RCS. In these techniques, the shape surface angles of a device are designed in such a way that the radar radiation wave is scattered in different directions after colliding with the target device and does not return to the radar.^5,6 However, Haoqin et al. stated that aircraft modification is limited to the basic principles of aerodynamics and the optimization process is necessary.⁷ Most of the shape design techniques have been used in the design of the F-117 fighter. Lee et al. proposed a robust evolutionary algorithm with extensive calculations to optimize aerodynamic and RCS parameters simultaneously when designing unmanned aerial vehicles (UAVs).⁸ Li et al. use a gradient-based optimization design method for a special type of UAV, flying wing aircraft, to optimize ideal stealth aircraft layout.^9,10 Xu et al. used a trajectory design method to optimize RCS parameters for aircraft.¹¹ With the advancement of helicopter stealth technology, Zhou et al. investigated special cases, dynamic RCS of rotor or multi-rotors.¹² A review of the literature shows that achieving optimal shapes in terms of aerodynamics and stealth requires complex calculations and is limited to specific shapes. So, new design methods are generally optimized to strike a balance between shape design and other reduction techniques. Therefore, the engineering of radar absorbers has become an important field of research. Electromagnetic absorbers material, also known as radar absorbers in the military industry, prevent the sight of the target device by absorbing the emitted waves and preventing their reflection back to the radar. These materials are based on the proper arrangement of dielectric and magnetic materials to create a suitable impedance at the surface of the radar absorber coating that provides a good degree of adaptation and absorption. Magisetty et al. categorized these materials as metalic alloys, conductive polymers, ceramics, and carbons.¹³ Przybył et al. used carbonyl iron for the manufacture of radar-absorbing paints reducing the radar signature of the objects that they cover.^14,15 Bahri-Laleh et al. used carbonyl iron (CI) as a magnetic filler and polyaniline (PANI) as a polymeric filler via in-situ polymerization to produce a core-shell composite material with multidisciplinary properties.¹⁶ Wang et al.¹⁷ and Ruiz-Perez et al.¹⁸ studied various types of carbon fillers according to the electrical features of the carbon materials to reach suitable EM wave shielding. Polymers play a key role in radar absorbers, because polymers are easier to process than metals and can be used to prepare a solution containing the desired absorbent and coat them on different surfaces or use them inside composite structures.¹⁶ Vasconcelos da Silva et al.¹⁹ and Wang et al.²⁰ stated that due to the low mechanical properties and brittleness of absorbent particles, it is necessary to reinforce these materials in the form of composite structures with fibers to show good mechanical and EM properties. Therefore, the electromagnetic properties of the reinforcing fibers were investigated. Liang et al.²¹ and Pang et al.²² investigated carbon fabric and glass fabric for better mechanical and electromagnetic performance. Also, a group of researchers studied the interaction of absorbent particles and fibers to gain better EM and mechanical properties. Salimkhani et al. investigated CI with carbon fabric for absorbent goals.²³ Lyu et al. used PANI with aramid fabric for high strength structure.²⁴ Guo et al. used FeNi/C film on SiC fabric for high efitency microwave absorption in the low-frequency band.²⁵ An ideal EM absorber should have special properties such as low weight and thickness, desirable mechanical properties, high absorption of EM in a broadband, and adjustable absorption frequency.²⁶ Diwiny et al. reviewed different stealth methods, and special use of EM3500 absorber material to present stealth UAVs; however, this material has production difficulties.²⁷ Nam et al. present a wing airfoil model with the nickel-coated glass fabric radar-absorbing structure.²⁸ In a different application, Turczyn et al. claimed the structure based on electrically conducting epoxy resins filled with PANI and polypyrrole (PPy) for electromagnetic interference shielding of remotely piloted aircraft systems.²⁹

Due to the significance of aerial structures' performance, these structures face limitations such as aerodynamic parameters, weight (or thickness), mechanical strength, operational manufacturing process, and appropriate production cost. A comprehensive design is needed if appropriate electromagnetic properties and radar absorption are also added to these limitations. Most investigations to date have been based on the spraying or coating of absorbent particles on an aerial structure surface, which has led to aerodynamic problems such as deceleration, drag, and increases in thickness and consequently weight, which practically limit flight and complicate the execution process as well. In this study, using the optimized design algorithm based on Non-dominated Sorting Genetic Algorithm (NSGA) II, a novel three-layer woven fabric laminated composite (with a thickness of about 3 mm) based on polyaniline, carbonyl iron, and (PANI + CI) core-shell fillers is presented instead of spraying and coating the absorbent material on the aerial structure surface. It is manufactured based on the combination of these three absorbent materials with resin and woven fabrics during the layering process. Thickness and weight are drastically reduced and aerodynamic problems are minimized. Due to using both dielectric and magnetic absorbing fillers, this structure shows well-impedance matching and approximately absorbs 80% of the X-band (8-12 GHz) electromagnetic waves. That is the maximum reflection loss is about −14dB. Finally, the effects of the addition of absorbent particles on the mechanical tensile and flexural properties of the designed structure are also investigated. It shows that reduction in the tensile modulus and strength by about 21.5% and 20.6%, respectively, and the flexural modulus and strength by 21.7% and 19.7%, respectively.

Theoretical background

Aerospace structures are usually composed of fiber-reinforced composites with lower weight and higher mechanical properties. In this study, a multi-layer absorber model was used to study the electromagnetic properties of these structures. The basic variables in this model are frequency band, thickness, and material electromagnetic properties including complex electric permittivity and magnetic permeability.³⁰ The relative (frequently dropped) complex permittivity and permeability are presented by equations (1) and (2).

{ε_{r} = ε}_{r}^{'} - {i ε}_{r}^{″}

(1)

{μ_{r} = μ}_{r}^{'} - {i μ}_{r}^{″}

(2)

where

ε_{r}^{'}

and

ε_{r}^{″}

are alluded to relative permittivity and the loss factor.

μ_{r}^{'}

and

μ_{r}^{″}

are often alluded to relative permeability and the magnetic loss factor. In this model, in order to fully measure the return waves, it is assumed that the waves enter the multi-layer composite structure perpendicular to the surface and a perfect electric conductor is placed under this structure as shown in Figure 1. By means of the transmission line theory, the input impedance (Z) of every layer with the thickness (d_i) and the relative complex permeability and permittivity (

ε_{i}^{'}

μ_{i}^{'}

) for each layer are (i = 1, 2, . . ., n)^31,32:

Z_{1} = η_{1} \tanh (k_{1} d_{1})

(3)

Z_{i} = η_{i} \frac{Z_{i - 1} + η_{i} \tanh (k_{i} d_{i})}{η_{i} + Z_{i - 1} \tanh (k_{i} d_{i})}

(4)

where η_i is the ith layer characteristic impedance and defined as follows:

η_{i} = \sqrt{\frac{μ_{i}^{'} - {i μ}_{i}^{″}}{ε_{i}^{'} - {i ε}_{i}^{″}}} i = \sqrt{- 1}

(5)

Also, k_i is the ith wave vector. The incident of the EM wave is assumed to be vertical, so it is constant and given as follows:

k_{i} = \frac{2 i π f}{c} \sqrt{μ_{i} ε_{i}}

(6)

The total reflection loss of Radar Absorbing Structure (RAS) is calculated by:

R = - 20 \log | \frac{z_{i} - 1}{z_{i} + 1} |

(7)

According to the wide range of effective variables in the wave absorption mechanism including the number of layers, the reinforcing material’s permittivity and permeability, and layer thickness range, it is necessary to perform the optimization process on the model to achieve a RAS with appropriate electromagnetic and mechanical performance. In this research, a code is developed in Matlab to predicte the reflection loss by using the transmission line theory and the optimization process of NSGA II. The decision variables are limiting and adjusting in such a way as to satisfy the intended functions including, the RAS total average reflection loss and RAS total thickness directly. Also mechanical properties, feasible manufacturing controlling, weight and materials cost are satisfied indirectly by adjusting the constraints.³³ Flowchart of the explained design algorithm is clarified in Figure 2. As an iterative optimization process, NSGA II is a search-based algorithm that starts with a population of randomly chosen potential solutions and attends to gradually move towards better solutions by applying genetic operators.³⁴

Experimental procedure

Materials and characterizations

Epoxy resin EC130 and hardener W340 were produced by Elantas Inc. The carbonyl iron powder, with 99.9% purity and the size of 4 microns, was sourced from Merck Inc. Aniline, HCl, ammonium persulfate, were supplied by Merck Co. The 200 g/m² E-glass woven fabrics were each produced by Taishan Fiberglass Inc. Polyaniline was synthesized by the chemical reaction of aniline with the ammonium peroxydisulfate with the specific instructions. The Core-shell form of polyaniline containing carbonyl iron (PANI + CI) was prepared via in-situ polymerization. The molar ratio of carbonyl iron to aniline was chosen to be 1:1 for the synthesis of this composite. Also, for the polymerization of aniline, ammonium persulfate was used as an initiator and hydrochloric acid as a dopant. First of all, 2 grams of carbonyl iron (CI) powder was added to 100 mL of 1.5 M hydrochloric acid solution and placed in an ultrasonic bath for 60 minutes. Then 3.39 mL of distilled aniline was added to it and the above mixture was placed in an ultrasonic bath for another 15 minutes. After that, the reaction container is placed in the water and ice bath while being stirred by a magnetic stirrer, and the temperature of the reaction mixture is allowed to reach 0-5 degrees Celsius. Then, 42.5 mL of pre-made ammonium persulfate solution was added drop by drop to the mixture of aniline and acid. By adding ammonium persulfate, the color of the solution changed to light green, dark green, dark blue and finally black. The mixture was gently stirred in an ice water bath for 2-3 hours and then vigorously stirred for one hour at room temperature. The resulting sediment is separated using a Buchner funnel and filter paper and is thoroughly washed several times with distilled water. At the end, the black sediment was placed in an oven at 60 degrees Celsius for 24 hours to dry completely and be ready for use.^16,35–37 The polymerization is schematically shown in Figure 3.

For achiving better characterizations of synthesized PANI and (PANI + CI), the FTIR, XRD and SEM image is provided in Figure 4, 5 and 6. According to Figure 4 (a) the peaks above 3000 cm⁻¹ are related to the stretching vibrations of the N-H bond. Also, the peaks in the regions of 1100-1600 cm⁻¹ are related to N-H bond bending vibrations, quinoid vibrations, benzoid vibrations and N = Q = N bond vibrations. These results indicate that the pure PANI was highly doped and exists in synthesized poweder. In Figure 4 (b) besides characteristic absorption bands of PANI, a characteristic peak at 1683 cm⁻¹ for the C = O of CI was detected implying successful formation of the (PANI + CI) composite. According to Figure 5 (a), the X-ray diffraction peaks in the regions of 2θ = 21° and 2θ = 26° represent the crystal structure for chemically synthesized polyaniline. The broad peak appearing in the region of 2θ = 20° is characteristic of the polyaniline salt. In the (PANI + CI) composite, broad diffraction peaks of the PANI as well as narrow diffraction peaks of CI are both seen in Figure 5 (b), implying successful formation of the (PANI + CI) composite.

Specimen preparation

Specimens are fabricated through two different methods. Silicone mold is employed for casting of samples without fabric reinforcement and vacuum assistant hand-layup method is employed for making glass woven fabric-reinforced samples. Fabrication of EM composites structure was first performed by mixing the EC130 resin and specific absorbing filler with a mechanical stirrer or hot plate for 90 min at 800 rpm at 50°C, depending on the filler type magnetic property. Curing time was slowed down after decreasing temperature by adding the hardener to the resin at a ratio of 30% and stirring it by a mechanical mixer for 5 min at 100 rpm. Finaly the glass fabrics with size of 300 × 300 mm² were laminated with the stirried absorbent filler/resin. The fabrication process is schematically depicted in Figure 7. Ultimately, jigsaw and sandpaper were used to cut the test specimens to the ultimate size as shown in Table 1. Figure 8 shows the complete casting and laminated specimens.

Table 1.

Test specimen specifications.

Type of test	Dimensions	Standard
Microwave	22.86 × 10.16 mm	ASTM D5568
Tension	250 × 25 × 2.5 mm	ASTM D3039
Bending	127 × 12.7 × 3.2 mm	ASTM D790

Experimental

Electromagnetic setup

For the electromagnetic study, the Agilent Technologies E8362 B network vector analyzer used a waveguide with a rectangular cross-section. This setting measures the material's S-parameters, transmission (S12/S21), and reflection coefficients (S11/S22). Commercial software is employed to calculate complex permittivity and permeability as a function of frequency.

Mechanical setup

The STM-150 made by Santam Co. was used for the tensile and bending tests according to ASTM D3039 and ASTM D790 standards, respectively. Five specimens are tested to determine the tensile and bending properties. The loading rate is 2 mm/min. Figure 9 shows the microwave and mechanical experimental test setup.

Results and discussion

Optimization of radar absorbing structure

In order to design an aerial structure with suitable mechanical properties and wave absorption, the optimized design algorithm is employed. The selected parameters, components and performance of multi-objective genetic algorithm are reported in Table 2.

Table 2.

The components and performance of multi-objective genetic algorithm.

Component	Introduction	Function	Details
Objective description function	Inputs receiving operator	• Importing initial population number • Importing generations total number • importing number of the objective functions • Importing number of random variables • Receiveing each random variable variation range	200-1000 100-900 2 6 ➢ Materials Permittivity and permeability library ➢ Layer thickness range
Evaluate objective	Objective functions optimizing operator	Obtaining random variables and calculating objective functions	o RAS total average reflection loss o RAS total thickness
Initialize variables (Chromosome)		Creating the initial population randomly and calling the evaluate objective function and calculating the objective functions in selected random values
Non domination sort mod		Using non-dominant sorting, the population is sorted
NSGA_II	The main operator receiving inputs and providing outputs	The main operator that provides the desired output by utilizing genetic sub-operators and the process of removing non-dominant chromosomes and performing the final optimization process.	Three-layer composite with the specific layer material, thickness and total average reflection loss

For providing an aerial structure with ideal properties a three-layer composite structure with optimization of material type, thickness, and stacking sequences is presented in Table 3 and schematicaly showed in Figure 10. In the design of this composite, glass woven fabrics have been used to provide the required mechanical properties of the aerial structure; furthermore. The low difference between the actual and predicted thickness is due to difficulties in control of thickness in vacuum assistant hand layup. Also an average reflection loss line in the total band is added for easier comparison. Figure 11 shows the good agreement between presented aerial structure and the experimental results. It shows suitable absorption on average of -8 dB in the X-band and approximately absorbs 80% of the electromagnetic waves. The maximum reflection loss peak is about −14dB.

Table 3.

Stacking sequences of a three-layer (PANI + CI)/CI/PANI/epoxy hybrid composite.

Layer number	Material	Predicted thickness (mm)	Actual Thickness (mm)	Difference, %
1	Metal sheet	—	—	—
2	30wt. % PANI/HCl + Glass fabric + Epoxy resin	1.11	1.15	3.6
3	70wt. % CI + Glass fabric + Epoxy resin	1.15	1.14	0.9
4	30wt. % (PANI + CI) + Glass fabric + Epoxy resin	0.69	0.74	7.2

In order to reveal the ability of the proposed algorithm, each of the absorbents used in the proposed structure is evaluated as a separate single-layer in the designed thickness presented in Table 3 as well as the whole structure thickness (3.03 mm) and their reflection loss are investigated in Figure 12. A comparison of Figures 11 and 12 reveal that none of these materials can absorb even as much as (PANI + CI)/CI/PANI/epoxy multi-layer composite based on the reported thicknesses. In addition, due to the fact that the absorbent particles are located inside the resin of each layer, the total thickness, weight, and cost are reduced compared to the coating absorbing fillers on the structure surface. Also, the aerodynamic parameters such as the drag of the aerial structure are not disturbed. Finally, the aerial structure with the optimized diffrent parameters is presented. In order to describe the comprehensive optimization performed in wave absorbing and reducing the structure weight, the specific reflection loss diagram which is the reflection loss over density of the composite based on the density of (PANI + CI)/CI/PANI/epoxy is shown in Figure 13. The specific reflection loss of the novel three layer nanocomposite is also compared with other absorbent materials and depocted in Table 4. It shows the presented material reports as the best specific absorption, which offers the structure with low weight an high absorbing feature.

Table 4.

Comparison of specific average reflection loss for different absorbing material.

Absorbing material	Thickness (mm)	Specific average reflection loss (dB*m³/kg)
Novel three layer nanocomposite	3.03	2.1
15wt. % Carbon black	3.4	1.3
70wt. % CI	3.3	0.7
30wt. % (PANI + CI)	3.1	1.8

Mechanical properties

After the electromagnetic investigation, mechanical properties are one of the most important parameters in the design process. Experimetal results in previous section showed that three-layer glass woven fabric/epoxy hybrid composite which each layer is reinforced by individual PANI, CI, (PANI + CI) core-shell filler gives an appropriate absorbtion. To investigate the effect of the addition of absorbent particles on the mechanical properties of composite material, quasi-static tensile and bending tests were performed. Five samples were tested for each case. Engineering tensile stress-strain curves of the glass fabric/epoxy composite and (PANI + CI)/CI/PANI/glass fabric/epoxy hybrid composite are compared in Figure 14. Table 5 reportes the average of tensile modulus and strength. It is found that the average of tensile modulus and strength have reduced by 21.5% and 20.6%, respectively.

Table 5.

Tensile properties of glass fabric/epoxy composites in comparison with and without (PANI + CI)/CI/PANI filler.

Composite type	Total thickness (mm)	Tensile modulus (GPa)	Tensile strength (MPa)
Glass fabric/epoxy	2.69 ± 0.05	21.4 ± 0.9	400.6 ± 7
(PANI + CI)/CI/PANI/glass fabric/epoxy	3.10 ± 0.1	16.8 ± 0.5	318 ± 5
Difference, %		21.5	20.6

The load-dispalcement of the glass fabric/epoxy composite and (PANI + CI)/CI/PANI/glass fabric/epoxy hybrid composites is compared under bending loading in Figure 15. Table 6 shows that the average of bending modulus and strength have reduced by 21.7% and 19.7%, respectively.

Table 6.

Bending properties of glass fabric/epoxy composites reinforced with and without (PANI + CI)/CI/PANI filler.

Composite type	Total thickness (mm)	Max. load (N)	Bending modulus (GPa)	Bending strength (MPa)
Glass fabric/epoxy	2.69 ± 0.05	168.9 ± 5	16.1 ± 0.4	270 ± 6
(PANI + CI)/CI/PANI/glass fabric/epoxy	3.10 ± 0.1	202.2 ± 7	12.6 ± 0.5	216.9 ± 5
Difference, %			21.7	19.7

Generally, the mechanical propertis reduction is due to the presence of a large amount of radar absorbing particles in structural resin and fabrics (i.e., 30wt. % (PANI + CI), 70wt. % CI, 30wt. % PANI/HCl) which causes some typical laminating difficulties and errors in compareing with pure resin and fabrics. By considering the significant reduction in overall weight, about by half, due to the addition of particles inside the structure instead of coating on the surface, this reduction in properties can be compensated for by considering a reliability factor in the design of aerial structures. However, the (PANI + CI) core-shell filler can be introduced as a high performance absorber filler because it suggests maximum reflection loss with low weight fraction compared to other fillers and consequently the minimum reduction in mechanical properties.

Conclusion

In this study, a novel three-layer woven fabric composite laminate, i.e., glass fabric/epoxy hybrid composite that each layer reinforced by individual (PANI + CI), CI, or PANI is proposed with total thickneses of about 3 mm. This composite material is optimized for the stacking sequence of layers, thickness of individual layer and volume fraction of filler. Adding absorbent particles inside the structure reduces the aerodynamics problems, thickness, weight, and cost to a great extent. The presented structure shows proper impedance matching and excelent absorbtion up to 80% in the X-band. The proposed composite structure has been evaluated by mechanical tensile and bending tests. Experimental results suggested that tensile modulus and strength have reduced by 21.5% and 20.6%, respectively, and the flexural modulus and strength have reduced by 21.7% and 19.7%, respectively. However, the (PANI + CI) core-shell filler can be introduced as a high performance absorber filler because it suggests maximum reflection loss with low weight fraction compared to other fillers and consequently the minimum reduction in mechanical properties.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Hamid Beheshti

Mohammad Heidari-Rarani

Data Availability Statement

The datasets generated and analyzed during the current study are not publicly available due to data also forms part of an ongoing study, but are available from the corresponding author on reasonable request.

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