‌Ultralight microwave absorber with an enhanced absorption performance based on chitosan aerogel

Abstract

In order to meet practical application, the fabrication and design of lightweight and multifunctional microwave absorbers is challenging. The strategy of this work, is the fabrication of polymer/carbon/mineral MoSe₂/RGO/MMTi-Chitosan as a hybrid aerogel via freeze-drying method by introducing the adjusted weight ratio of MoSe₂/RGO/MMTi (i = 0, 35, 50, and 75 wt% were named as: m1, m2, m3 and m4, respectively) nanocomposites as filler of chitosan polymer. FESEM confirmed the porous morphology of aerogels at different scales. The microwave absorption measurements showed that, raw chitosan polymer has a weak absorption efficiency. A significant improvement in microwave absorption efficiency was observed by introducing MoSe₂/RGO/MMTi filler. MoSe₂/RGO/MMT(50%wt)-Chitosan (m3) showed the RL_min of -72 dB with a bandwidth of 3.8 GHz at 2.7 mm. M4 showed the broadest bandwidth at 2.3 mm which covered the entire frequency X and Ku band. The energy conversion and absorption in MoSe₂/RGO/MMTi-Chitosan originated from synergetic effect of these factors: high electrical conductivity of MoSe₂/RGO, extensive surface of MMT which reduces MoSe₂ agglomeration and microporous structure of chitosan. Finite element simulation showed that by covering a PEC (perfect electrical conductor) sphere with 2.3 mm of each composite, the radar cross-section (RCS) reduced to 30–60 dB across the entire frequency band and the scattered far filed reduced 30-35dB.

Introduction

The advancement of information societies is contingent upon the use of electronic equipment, which is propelled by swift progressions in wireless technology, radar systems, and medical applications. These devices facilitate the transmission of information while simultaneously were exposed to electromagnetic (EM) radiation, which carries substantial implications for defense, security, and public health¹. Consequently, the development and application of microwave-absorbing materials (MAMs) have been recognized as a viable approach to mitigate the associated risks. MAMs absorb EM radiation and convert it into thermal energy, thereby potentially contributing positively to public health and enhancing the safety of information systems and electronic equipment^2,3. There are various types of MAMs, depending on the type of loss mechanism, which generally were categorized into carbon materials⁴, magnetic⁵, dielectric⁶, semiconductor, vernacular⁷, and more recently, porous materials⁸. Magnetic materials have garnered relatively less attention due to their low dielectric constants and susceptibility as well as rapid oxidation. In contrast, semiconductors such as molybdenum diselenide (MoSe₂), which belongs to the category of two-dimensional transition metal dichalcogenides (TMDs) within V-III group, have attracted significant interest owing to their distinctive layered structure. This structure comprises successive layers of molybdenum and selenium atoms, characterized by strong covalent bonds within MoSe₂ each layer and weak van der Waals forces between layers. These features render MoSe₂ physically analogous to graphene. The mobility of electrons within MoSe₂ is readily facilitated through electronic interactions, resulting in unique electrical and optical properties. Additional notable characteristics of MoSe₂ include a high specific surface area, an indirect energy band, and a substantial number of active sites. Such features render MoSe₂ a suitable candidate for applications that include photocatalysis, solar cells, supercapacitors, and microwave absorption. Furthermore, due to its lower bandgap and higher intrinsic conductivity compared to MoS₂, MoSe₂ generates a more dispersed current and exhibits enhanced efficiency in attenuation of microwaves. However, a significant challenge lies in reducing the agglomeration of MoSe₂ at the nanoscale to improve scattering and multiple reflections^9–11.

Integrating of molybdenum diselenide (MoSe₂) with carbon materials holds significant promise for addressing the challenges associated with agglomeration. Reduced graphene oxide (RGO), a carbon material introduced by its exceptional properties, including high electrical conductivity, substantial specific surface area, structural defects, and various functional groups, plays a crucial role in the field of microwave absorption. Unique features of carbon materials in combination with MoSe₂ leads to mitigate sheet aggregation and facilitate surface polarization. Furthermore, the enhanced surface area of the layered structure aids in microwave trapping, due to multiple reflections and scattering, which ultimately contributes to enhance energy absorption¹². Recent investigations have focused on the microwave absorption capabilities of MoSe₂/RGO composites. The findings indicate that the tuning of MoSe₂ phase, in conjunction with GOx, can enhance microwave absorption properties. The 1T′-MoSe₂ phase demonstrates low reflection loss and a broad absorption bandwidth, thereby surpassing the performance of other phases of MoSe₂ and underscoring the importance of phase tuning in optimizing the microwave absorption of MoSe₂¹³. Additionally, surface engineering and impedance matching characteristics are a crucial factor in microwave absorption. One study revealed that the incorporation of GO enhanced the impedance matching properties, conduction losses, and polarization losses of RGO/MoSe₂ heterojunctions, achieving a minimum reflection loss of -65.34 dB at a frequency of 6.40 GHz and an adequate absorption bandwidth of 4.20 GHz¹⁴.

Montmorillonite (MMT), as a two-dimensional layered material of the smectite family, has attracted many people due to its low cost, high specific surface area, suitable dielectric properties, and very natural. The layered structures and chemical properties of MMT enhance the scattering and multiple reflection of incident light in nanocomposites by increasing the surface area and preventing the aggregation of nanoparticles¹⁵. Research indicates that incorporating nano clay into polymers significantly improves their mechanical and thermal properties. In one study, the shielding efficiency of polypropylene/montmorillonite/polypyrrole ternary nanocomposites was assessed within the telecommunication frequency band. The results demonstrated that these nanocomposites exhibit twice the shielding efficiency of the polypropylene/polypyrrole binary composite, attributed to higher dielectric losses. Additionally, anisotropic montmorillonite layers coated with a conductive polypyrrole layer significantly enhanced the complex permittivity of the ternary composite¹⁶. Other materials of interest in microwave absorption include biopolymers. Chitosan, a polysaccharide primarily extracted from crustacean shells, presents notable biocompatibility and biodegradability, making it an ideal candidate for medical and biomedical applications. Chitosan (CTS), a biocompatible polysaccharide, is derived through the deacetylation of chitin. The presence of amino and hydroxyl groups in its structure endows it with the capacity to chemically or physically adsorb and trap various metal ions. Applying chitosan biopolymer in developing environmentally friendly and economical adsorbents can significantly contribute to the recycling of marine food waste while simultaneously aiding in the reduction of electromagnetic pollution^17,18. Recently, C/SnO2 honeycomb-like porous materials were synthesized using freeze-drying and carbonization techniques with varying tin ratios. Chitosan forms a honeycomb-like conductive network, significantly contributing to conduction losses. The presence of amine groups facilitates dipolar polarization which enhanced the wave absorption performance, as in the optimized sample, a minimum reflection loss of -48.80 dB and a bandwidth of 5.20 GHz were achieved at a thickness of 3.46 mm¹⁹. In another study, reduced graphene oxide (RGO) was combined with chitosan nanofibers through electrostatic attraction. Following carbonization at various temperatures, porous carbon nanofiber absorbers were fabricated. The porous structure and functional groups present in chitosan carbon enhances EM absorption as well as reducing density and optimizing impedance. With a filler content of 10 wt% at a carbonization temperature of 800 °C, a minimum reflection loss of -51.04 dB was achieved at a thickness of 0.9 mm, along with an absorption bandwidth of 12.08 GHz and a reflection loss of less than − 20 dB³. A cobalt-decorated, chitosan-derived carbon-based aerogel was synthesized using a uniform solidification casting method, followed by the lyophilization of the chitosan-based precursor. EM absorption property of dielectric-magnet composite revealed that this aerogel demonstrated a minimum reflection loss of -44.30 dB with a bandwidth of 6.4 GHz at a thickness of 2.5 mm, achieved after carbonization of the precursor at 800 °C²⁰. Aerogels is one of the innovative and compelling interest for researchers which is characterized by porous architecture that can exhibit various applications. The diversity in the design of pore structures, encompassing features such as shape, dimensions, and anisotropy in orientation, creates new opportunities to enhance the synergistic effects between material components and their structural properties^21,22. In previous works, a MoSe₂/RGO binary composite was synthesized, demonstrating a significant reflection loss of − 99 dB with a bandwidth of 10 GHz at a thickness of 3.4 mm¹². Then, this composite was subsequently deposited onto montmorillonite (MMT). The inherent negative charges and high specific surface area of MMT enhanced the polarizability of MoSe₂/RGO, resulting in a reflection loss of − 76 dB at a lower thickness of 1.5 mm. Thus, MMT facilitated a decrease in thickness and weight, while maintaining absorption performance, but with a reduction in the absorption bandwidth (5.83 GHz)²³.

In this study, a novel MoSe₂/RGO/MMT–Chitosan aerogel was fabricated by adjusting the filler ratio to chitosan matrix using glutaraldehyde as a crosslinking agent and developing the structure through freeze-drying. Chitosan, as a lightweight biochemical matrix with –OH and –NH functional groups, after protonation in acidic medium, makes major electrostatic interactions with MoSe₂, RGO and MMT layers. The integration of biomaterials and inorganic materials (Na-MMT) leads to the formation of a three-dimensional porous structure with high surface area and multiple dipoles, which enhances microwave scattering and absorption. FESEM images show the formation of multiple pathways and uniform generation of components in the lightweight matrix, which, together with different polarizations, increases the dielectric loss. This aerogel showed complete coverage of the X and Ku bands at a thickness of 2.3 mm, and a minimum significant reflectance loss of − 72 dB was achieved at a thickness of 2.7 mm. The combination of low thickness, low weight, and wide absorption bandwidth demonstrates the superior performance of this aerogel over previous composites. Also, finite element simulations on a PEC sphere with a 2.3 mm coating of aerogel showed a radar level reduction of 30 to 60 dB over the entire French range, accompanied by a reduction of more than 35 dB in the far-scattered intensity field compared to the uncoated sphere.

Experimental

Materials

Chitosan (C), characterized by a deacetylation degree of 75% and a high molecular weight, was used in conjunction with sodium montmorillonite (Na-MMT), sodium molybdate dihydrate (Na₂MoO₄.2H₂O), selenium powder (Se), sodium borohydride (NaBH₄), ethanol (C₂H₅OH), acetic acid (CH3COOH), glutaraldehyde, and reduced graphene oxide (RGO), which possessed a particle size ranging from 3.4 to 7 nm and a minimum purity of 95%.

Synthesis of MoSe₂

Sodium molybdate dihydrate was mixed with selenium powder in a molar ratio of 2:1, along with 0.007 g of sodium borohydride as a reducing agent, in 50 ml of DI water was stirred using a magnetic stirrer until a black solution was observed. Subsequently, the mixture was sonicated for 45 min. The resultant solution with pH = 9 was then transferred to an autoclave and heated at 200 °C for 24 h. Finally, the solution was washed and allowed to dry at room temperature.

Synthesis of MoSe₂/RGO/Na-MMT

Initially, 0.5 gr of MoSe₂ was mixed with 0.3 gr of MMT nanosheets and stirred in 50 ml of deionized water using a magnetic stirrer for 2 h. Subsequently, 0.3 gr of RGO was separately mixed in 50 ml of water/ethanol at a 1:1 ratio and subjected to ultrasonic bath for 30 min. This mixture was then added dropwise to the initial solution, which was stirred for 24 h at room temperature. Finally, the solution was dried in an oven at 50 °C.

Synthesis of MoSe₂/RGO/Na-MMT-Chitosan

Initially, 2 g of chitosan was added to 50 mL of 2% (v/v) acetic acid and stirred for 2 h to form a uniform hydrogel through the sol-gel process. Subsequently, different proportions of MoSe₂/RGO/Na-MMT nanocomposite (35, 50, and 75 wt%) were separately dissolved in 20 mL of deionized water and added dropwise to the initial hydrogel. At this stage, the amino and hydroxyl groups of chitosan interacted with the oxygen groups on the RGO and Na-MMT surface through hydrogen bonds and electrostatic interactions. After 1 h, 20 µL of glutaraldehyde was added as a crosslinking agent. The mixture was stirred for 24 h at room temperature with a magnetic stirrer and then solidified into a uniform casting. Subsequently, it was placed in a freeze-dryer at -40 °C and 0 bar pressure for 48 h. The synthesis process of MoSe₂/RGO/Na-MMT-chitosan is shown in Fig. 1, The weight% of aerogel’s component was listed in Table 1.

Fig. 1.

Steps for the Preparation of MoSe₂/RGO/MMT-Chitosan Aerogel.

Table 1.

Weight% of aerogel’s component.

Aerogel’s component	(wt%)
MoSe2	16.1
rGO	9.6
Na-MMT	9.6
Chitosan	64.7

Results and discussion

XRD analysis

Figure 2 (a) shows the X-ray diffraction pattern of MoSe₂, Na-MMT and MoSe₂/RGO/MMT-Chitosan Aerogel. In this pattern, the angles 2Ɵ=13.45°,32.11°,41.13°and 53.80°, corresponding to the (002), (102), (006) and (102) planes of the hexagonal 2 H-MoSe₂, respectively, according to the card number (JCPDS: 29–0914) no additional peak is observed, indicating the high purity of the MoSe₂^24,25. Furthermore, the diffraction peaks of montmorillonite at angles of 19.75°, 20.7, 23.28°, 28.32°, 34.75°, 36.17°, and 54.13° correspond to the PDF card number 13–0135^26,27. Additionally, the interlayer spacing for montmorillonite sheets was measured at 9.6 nm with an angle of 2θ = 9.50° using Bragg’s law. In the MoSe₂/RGO/Na-MMT polymer/carbon/mineral aerogel nanocomposite, all the peaks of MoSe₂, Na-MMT, and RGO are present in conjunction with the chitosan polymer, exhibiting an angular shift of 7.20° from montmorillonite and a reduction in the intensity of the MoSe₂ peaks. This shift and decrease in peak intensity can be attributed to alterations in the interlayer spacing, suggesting that the chitosan polymer has not introduced in crystalline structure. Two additional peaks at angles of 44.45° and 24.64° are also associated with the (003) and (001) planes of RGO nanosheets²⁵. Furthermore, a pronounced diffraction peak at 21.18° corresponding to chitosan is observed, attributable to its macromolecular crystalline structure, which arises from various intramolecular and intermolecular hydrogen bonds between functional groups such as hydroxyl and amino groups²⁸.

Fig. 2.

(a) XRD patterns, (b) FTIR spectra, (c) TGA of MoSe₂/RGO/MMT-Chitosan and macroscopic images of (Chitosan (d)- MoSe₂/RGO/MMT 35wt%-Chitosan (e)).

FTIR analysis

In Fig. 2 (b), the functional groups of the samples are analyzed using Fourier Transform Infrared (FTIR) spectroscopy. The broad absorption band observed at 3431 cm⁻¹ is attributed to the O-H stretching vibrations of water in montmorillonite and the N-H stretching vibrations of amino groups in chitosan. The absorption bands at 2945 cm⁻¹ and 2848 cm⁻¹ in chitosan correspond to the asymmetric C-H bonds stretching, relate to the CH₃ and CH₂ groups, respectively. The band at 1690 cm⁻¹ is associated with the N-H bending vibrations of chitosan and the Se-O vibrations of MoSe₂. The band at 1563 cm⁻¹ pertains to the C = C bond in graphene oxide, while the band at 1405 cm⁻¹ is similarly linked to the C-H bond. 1038 cm⁻¹ band indicates the Si-O stretching vibrations of montmorillonite and the C-O vibrations of the polysaccharide backbone, thereby confirming the presence of the chitosan biopolymer. The absorption band at 455 cm⁻¹ signifies the Se-O bond in molybdenum diselenide^29–32.

TGA analysis

The results of thermogravimetric analysis (TGA) of the aerogels are presented in detail in Fig. 2(c). TGA analysis is an effective method to investigate the thermal stability of materials and their chemical composition. According to the graph, at temperatures below 200 °C, the weight loss is mainly due to the evaporation of solvents adsorbed on the aerogel surface and moisture contained in the chitosan and Na-MMT nanoclay structure. In the temperature range of 300–500 °C, a rapid and significant weight loss is observed, which may be attributed to two decompositions.1) As the temperature increases, the polymer chains of chitosan release the gases under the RGO decomposition temperature, and a concomitant reduction in mass. 2) Separation of functional groups: the dissociation of functional groups, such as amines and hydroxyls, can also contribute to mass loss. These functional groups are prone to decomposition at elevated temperatures, leading to the emission of various gases. Within the temperature range of 150–650 °C, the weight loss of MoSe₂/RGO/Na-MMT-Chitosan aerogel with filler ratios of 0.35%, 0.50%, and 0.75% was approximately 50.62%, 55.14%, and 56.71%, respectively. An increase in the proportion of MoSe₂/RGO/Na-MMT-Chitosan fillers in the chitosan matrix enhances polymer stability, attributed to the layered structure of MoSe₂/Na-MMT, which acts as a thermal barrier to impede heat transfer. Additionally, RGO facilitates the formation of robust bonds with the chitosan polymer chains^33,34. Figures 2(d&e) shows macroscopic images of chitosan and MoSe₂/RGO/MMT35wt%-Chitosan.

Figure 3 illustrates the FESEM images of the MoSe₂/RGO/MMT-Chitosan at varying weight ratios (35, 50, and 75 wt%) of the MoSe₂/RGO/MMT filler. As shown in Fig. 3(a), the multifunctional aerogel exhibits a porous structure at the microscale, characterized by relatively uniform porosities. Upon the incorporation of 35 wt% of the MoSe₂/RGO/MMT nanocomposite filler, as shown in Fig. 3 (b), the aerogel retains its porous structure while demonstrating significantly more regular and interconnected porosities. This enhancement can be attributed to the intermolecular interactions between the positive charges of chitosan and the negative charges on the surface of the montmorillonite, which possesses a layered structure, as well as the negative charges from molybdenum diselenide and reduced graphene oxide. Furthermore, with the increasing weight ratios of 50 wt% (Fig. 3(c)) and 75 wt% (Fig. 3(d)), the porous structure of the multifunctional aerogel is further refined. However, at a 75 wt% concentration of the MoSe₂/RGO/MMT nanocomposite, the pore size diminishes and the structural organization becomes more disordered, resulting in the formation of heterogeneous interfaces among the aerogel components. It can be concluded that variation in the mass of the MoSe₂/RGO/MMT nanocomposite within the chitosan polymer significantly influences the morphology of the composite. Chitosan macromolecules with a 50 wt% ratio of the MoSe₂/RGO/MMT nanocomposite are identified as the optimal sample due to their ability to maintain the porous structure as well as enhance microwave absorption efficiency. EDS analysis confirms the presence of carbon, oxygen, and nitrogen elements in the chitosan polymer, as depicted in Fig. 3(aʹ), while Figs. 3(bʹ), 3(cʹ), and 3(dʹ) reveal the percentages of molybdenum, selenium from molybdenum diselenide, magnesium, silicon, aluminum, and calcium from sodium montmorillonite, as well as carbon and oxygen from reduced graphene oxide. Additionally, mapping analysis in Figs. 3(d1-d4) corroborates the presence of these elements³⁵.

Fig. 3.

FE-SEM (a₁-a₃,b₁-b₃, c₁ –c₃ and d₁-d₃), macroscopic image (a-d) and EDS (aʹ,bʹ,cʹ and dʹ) images of MoSe₂/RGO/MMT(i) (i = 0, 35,50 and75wt%) -Chitosan and element mapping images of (MoSe₂/GO/MMT 35wt% (d₁ʹʹ-d₄ʹʹ)).

Microwave absorption

Electromagnetic parameters play a key role in determining the microwave absorption properties. The microwave absorption in these aerogels is mainly determined by the complex permittivity (ε = εʹ + iεʹʹ) and to a lesser extent by the complex magnetic permeability (µ = µʹ + iµʹʹ), since the components of the system are non-magnetic and in this case, µʹ = 1 and µʹʹ = 0. The real components (εʹ and µʹ) represent the energy storage and the imaginary components (εʹʹ and µʹʹ) represent the electromagnetic energy dissipation. The dominant attenuation mechanisms include dielectric losses, electrical conductivity, and interfacial and dipole polarization. The samples were embedded in 30% paraffin wax uniformly dispersed in X- and Ku-band waveguides and the EM parameters were measured with the Agilent WR-90 kit^36–38.

As illustrated in Fig. 4(a), the polarizability (ɛʹ) of chitosan (m1) ranges from 1.14 to 1.26, exhibiting minimal variation with changing frequency. Upon the incorporation of 0.35 wt% (m2) of MoSe₂/RGO/MMT into the chitosan polymer, the ε′ or polarizability value increases to 1.8. This enhancement is attributed to the superior conductivity of reduced graphene oxide (RGO) and the resultant surface polarization between the MoSe₂ and MMT sheets. Additionally, the presence of interconnected pores within the chitosan matrix facilitates charge accumulation in the interfacial regions, thereby contributing to significantly elevated polarizability and the migration of free electrons across the surface. Furthermore, by increasing the MoSe₂/RGO/MMT filler concentration to 0.50 wt% (m3), increasing the number of MMT sheets further increases the interfacial polarization, which increases the contact area between particles. The role of MMT in increasing the interfacial contact area and preventing aggregation is due to a well-known mechanism in layered materials. The structure of the MMT sheets, together with its wettability and high surface area, causes the MoSe₂ and RGO sheets to be uniformly distributed on the MMT layers rather than adhering to the composite. The negative surface charge and ion exchangeability of MMT also create an electrostatic repulsion force that prevents the sheets from approaching and reattaching to each other. These interactions create a stable and dispersed substrate in which the nanosheets are completely separated, resulting in a high interface between the components. This larger interface helps to create stronger interfacial polarities and higher activity, ultimately increasing the absorption of electromagnetic waves. These modifications improve the charge distribution and minimize energy losses compared to M2. The polarizability value reaches two and shows a positive trend with increasing frequency. Finally, for m4 with 0.75 wt% of MoSe₂/RGO/MMT, oscillations in the X band remain consistent, with increasing frequency (Ku band), the polarizability reached to 2.3^22,39. The values in Fig. 4 (b) for m3 is the largest one. This suggests that conductivity predominates over polarization. In the X band, the value is an oscillating and increasing trend, reaching 0.8. Subsequently, in the Ku band, the becomes constant at 0.08. Samples m1, m2, and m4 exhibited a similar trend, the value decreasing from 0.15, 0.30 and 0.13 to 0.06, 0.07, and 0.05, respectively. The values of in these samples decline as frequency increases. This decrease is attributed to the gradual mismatch between the time required to generate internal polarization within the material and the alterations in the electromagnetic field resulting from the increased frequency of electromagnetic waves. In other words, this phenomenon reflects the impact of frequency divergence on the electrical properties of materials²⁰. Concurrently, at high frequencies, the effects associated with dipole and interfacial polarization are considerably diminished due to the inability of charge carriers to respond rapidly to changes in the electric field⁴⁰.

Fig. 4.

Electromagnetic parameters, real and imaginary parts of permittivity (a,b), dielectric (tanẟ_ε (c)), Attenuation constant (d) and dielectric conductivity of chitosan and MoSe₂/RGO/MMT(i) (i = 0, 35, 50 and 75wt%)-Chitosan respectively.

To conduct a more comprehensive evaluation of dielectric losses, the dielectric loss tangent of the samples within the frequency range of 8–18 GHz was computed and illustrated in Fig. 4(c). Generally, an increase in the value of tanδ_ε indicates greater energy loss of input microwave, manifesting as dielectric losses. The dielectric loss tangent curve reveals that m3 exhibits the highest dielectric loss, demonstrating a decreasing trend within the range of 0.19 to 0.03. The dielectric loss tangent follows the order: m3 > m4 > m2 > m1. For all samples, an oscillatory and unstable trend was observed in the X-band range; conversely, in the Ku-band, this trend remained relatively constant, with oscillations diminishing to a lower value. This behavior may be attributed to a reduction in dielectric relaxation associated with dipole and surface polarization, which arises from the unbalanced charge distribution in the three-dimensional structures of porous conductors⁴¹.

After the electromagnetic wave penetrates the absorber, the rate of energy decay is represented by the expression 1 − e^−2dα. In this equation, the energy decay constant (α) is introduced as the damping constant, a crucial parameter in assessing the absorption capacity of the absorber materials. This constant is determined based on the permittivity and permeability characteristics of the materials, which are accurately articulated in Eq. (1).

1

In this study, Fig. 4 (d) is defined as the length of the electromagnetic wave transmission path. All samples, m1, m2, m3, and m4, are non-magnetic; therefore, they do not possess the ability to induce magnetic losses, resulting in the values of µʹ and µʹʹ remaining almost constant and equal to 1 and 0, respectively. Additionally, the value of α in the examined frequency spectrum (Fig. 3d) exhibits an increasing trend with the augmentation of the amount of MoSe₂/RGO/MMT filler within the chitosan polymer. This leads to an enhanced energy dissipation capabilities at elevated frequencies, indicating an increase in electromagnetic energy consumption due to significant conductive and dielectric losses, which correlates with the value of ɛʹ in Fig. 4 (a). As a standalone absorber with suboptimal intrinsic electromagnetic properties, such as low electrical conductivity or low dielectric constant, chitosan aerogel frequently encounters challenges in achieving acceptable loss levels for practical applications. Furthermore, the requirement for lightness may impose restrict the quantity of additives permissible in this absorber, potentially resulting in diminished attenuation capability. However, in this study, the MoSe₂/RGO/MMT-Chitosan aerogel, characterized by the excellent conductivity of RGO and the dielectric enhancement from MoSe₂, along with a multilayer structure that increases the contact area of MMT and creates a porous framework, results in an elevation of the damping constant. These attributes allow absorbers to incorporate across a broad frequency range, demonstrating optimal performance. To further elucidate the conductivity characteristics of EM absorbers, the conductivity versus frequency plot is illustrated in Fig. 4 (e). In this plot, AC conductivity is directly associated with the imaginary component of the dielectric permittivity.

2

where σ is the conductivity with unit (S/m), ε₀ is the permittivity of free space (8.854 × 10⁻¹² F⁄m) and is the microwave angular frequency.

The integration of conductive and semiconducting components in nanocomposites can significantly enhance conductivity. The conductivity value of aerogels was calculated according to Eq. (2). At a frequency of 8–12 GHz, m3 exhibited the highest conductivity value, which corresponds to a substantial decrease in electrical permeability. The presence of reduced graphene oxide, attributed to functional groups such as hydroxyl and carbonyl, induces electric dipoles, while the incorporation of MoSe₂ further enhances surface polarization within the layered structure. Additionally, sodium montmorillonite, characterized by its layered structure, contributes to an increased surface area and facilitates interconnected pores that accelerate the electron transfer. Consequently, conductivity has been found to increase with the addition of MoSe₂/RGO/MMT fillers to chitosan^42–45.

The Cole-Cole curves (ɛʹʹ-ɛʹ) in Fig. 5 contain semicircles that represent the Debye dipole relaxation process in m1, m2, m3 and m4. These semicircles were calculated using the Debye formula as follows:

3

Fig. 5.

Electromagnetic parameter, Cole-Cole semicircle of samples (a,b,c and d).

In this formula, denotes the permittivity at infinite frequency, while signifies the static permittivity.

m1 (a) and m2 (b) show polarization relaxation processes characterized by more distorted semicircles compared to the m3 (c), m4 (d). This phenomenon may be attributed to defects and imperfections, as well as surface polarizations, since such defects and imperfections can contribute to the formation of dipole polarizations and their subsequent relaxation. The distortion evident in the diagram indicates the activity of dipoles and their capacity to respond to an electric field. The presence of multiple semicircles in the m3 and m4 curves suggests that several relaxation mechanisms are active within these materials, each relaxation potentially enhances the EM absorption property sample. Conduction loss and polarization loss occur concurrently^46,47.

To conduct a more precise evaluation of the electromagnetic wave (EMW) absorption performance of the absorber, the reflection loss (RL) and effective absorption bandwidth (EAB) are calculated in accordance with the transmission line theory, as described in Eqs. (1) and (2). Generally, an RL<-10 dB indicates that the absorption rate of incoming electromagnetic waves can achieve 90%. The frequency range in which this condition is satisfied at a specified thickness is referred to adequate absorption bandwidth (EAB).

EM absorber materials are required to possess certain characteristics, including lightweight, thinness, and a broad EAB. To this end, the reflection loss (RL) of the prepared samples was measured across a frequency range of 8 to 18 GHz, with thicknesses varying from 2.3 mm to 3.3 mm.

These calculations were conducted in accordance with the transmission line theory and derived from the relative complex permeability (µ_r) and relative complex permittivity (ε_r), as extracted from the following relations.

4

5

In these relations, Z₀ refers to the impedance of the vacuum environment, while Z_in represents the input impedance of the absorbing material. The variable (f) denotes frequency, (c) signifies the propagation speed of the electromagnetic wave in a vacuum, and (d) indicates the thickness of the absorbing material. Furthermore, (j) is considered an imaginary unit in these equations.

A high absorption capacity can significantly enhance the performance of various systems that rely on microwave absorption. Moreover, a comprehensive analysis and understanding of these parameters can facilitate the development of new materials and the optimization of absorber structures. Such advancements can ultimately lead to increased efficiency and improved performance of electronic and telecommunications systems, which is of particular importance across various fields of technology and industry. Consequently, research in this area not only aids in enhancing the technical properties of materials but also fosters significant innovations in the design and application of microwave-based systems^48,49.

Figure 6 (a, b, c and d) shows the two-dimensional (2D) plots of reflectance loss (RL) versus frequency, while Figs. 6 (aʹ, bʹ, cʹ and dʹ) show the three-dimensional (3D) plots of reflectance loss and Fig. 6 (aʹʹ, bʹʹ, cʹʹ and dʹʹ) show the two-dimensional thickness plot of the aerogel polymer composite versus frequency.

Fig. 6.

From left to right the dependence of reflection loss on frequency in 2D, 3D projection (a-d and aʹ-dʹ) images and Thickness versus frequency contour plot (aʹʹ-dʹʹ) of samples.

Sample (m1) consisting of chitosan (a, aʹ and aʹʹ), demonstrates a minimum reflection loss of -18 dB at 10.9 GHz, with a bandwidth of 1.2 GHz at a thickness of 3.3 mm. This dielectric polymer, owing to its porous structure, facilitates multiple reflections and scattering of electromagnetic waves, thereby reducing the overall scattering effect. MoSe₂/RGO/MMT (35% wt)-Chitosan(m2) (b, bʹ and bʹʹ), exhibited a minimum reflection loss of -20 dB at 10.21 GHz, maintaining the same bandwidth of 1.2 GHz at a thickness of 3.3 mm. The cross-links formed between the conductive reduced graphene oxide nanosheets, semiconducting molybdenum diselenide, and dielectric montmorillonite create a three-dimensional conductive network that enhances electron transfer, converts electric field energy into heat, and increases electromagnetic wave losses. In sample m2, the reflection loss value exhibited a slight increase. Subsequently, sample MoSe₂/RGO/MMT (50% wt)-Chitosan (m3) (c, cʹ and cʹʹ ) achieved a reflection loss of -72 dB at 9.80 GHz, with a bandwidth of 3.8 GHz at a thickness of 2.7 mm. The addition of conductive, semiconductor, and dielectric fillers to the chitosan polymer, while retaining the porous structure, contributes to reductions in multiple reflections, scattering, conduction losses, and polar losses induced by the presence of numerous defects and polar functional groups within the composites. This results in the formation of polar centers, leading to bipolar polarization and presenting an optimal reflection loss performance. The minimum reflection loss for m4, which consists of MoSe₂/RGO/MMT (75% wt)-Chitosan (d, dʹ and dʹʹ), is -22 dB at 10.75 GHz, with a wide bandwidth of 2.3 mm thickness, encompassing the entire X and Ku bands. Despite, the reflection loss of m4 decreased significantly due to the elevated filler ratio within the polymer, which may have resulted in alterations to the porous structure and changes in electrical and structural properties. Nonetheless, it maintains a broad bandwidth with a reduced thickness within the X and Ku bands compared to the other samples. In summary, polymer aerogels, focusing on a porous structure and incorporating conductive and semiconductor fillers, contribute to (1) multiple reflections and scattering (2) conduction losses, and (3) polarization losses^50–55. A summary from the microwave absorption properties of the aerogels is given in Table 2.

Table 2.

Microwave absorption properties of the composites which investigated in this study.

Composite	RLmin (dB)	Effective Bandwidth (GHz) (RL<-10 dB)	Optimum thickness (mm)	Density of aerogels (g/cm3)
Chitosan (m1)	-18	2.10	3.3
MoSe2/RGO/MMT(35%wt)-Chitosan (m2)	-20	2.10	3.3
MoSe2/RGO/MMT(50%wt)-Chitosan (m3)	-72	3.80	2.7
MoSe2/RGO/MMT(75%wt)-Chitosan (m4)	-22	10	2.3

The high efficiency of electromagnetic absorption in MoSe2/RGO/MMT–Chitosan composite aerogels with different filler weight percentages (35, 50 and 75%) is the result of the synergy of polarization, multiple scattering and conductivity loss mechanisms (Fig. 7). The presence of RGO with medical functional groups and MoSe₂ semiconductor with a band gap and structural weaknesses enhances the dipole polarization and defect polarization. Also, MMT nanosheets prevent the aggregation of MoSe₂, increasing the phase dispersion, specific surface area and interfacial polarization.

Fig. 7.

Schematic diagram from microwave absorption mechanisms of MoSe₂/RGO/MMT–Chitosan Aerogel.

As a polymer, chitosan creates a porous and stable three-dimensional structure that is stabilized by hydrogen bonds and interfacial interactions with RGO, MoSe₂ and MMT. These porous structures increase the time of EM waves by creating zigzag propagation paths, spurious reflections and multiple scattering, leading to attenuation of electromagnetic energy.

In addition, the MoSe₂/RGO network provides electron transfer, hopping and transfer paths, and the structure of the MMT layers facilitates charge transfer. The formation of a three-dimensional conductive network through cross-linking between the nanocomposites and chitosan significantly increases the electrical conductivity loss. Based on structural evidence and reflection loss results, multiple scattering in the porous structure is the dominant mechanism of EM attenuation in these aerogels. These features make the studied aerogels lightweight and efficient absorbers for reducing electromagnetic interference in advanced telecommunication systems and 5G/6G technologies.^56–63. Figure 7 illustrates the microwave absorption mechanisms of MoSe₂/RGO/MMT-Chitosan. The MA characteristics of analogous composites are detailed in Table 3.

Table 3.

Brief reports on the microwave absorption properties of composites similar to those investigated in this study.

Composite	RLmin (dB)	Effective Bandwidth (GHz) (RL<-10 dB)	Optimum thickness (mm)	Ref.
rGO–MoSe2	-99	10	3.4	11
MoSe2/rGO/Na-MMT	-76	5.83	1.5	22
FeNi-NiO/SiO2/GO	-49.12	8.2	2.3	56
CoNC HCS@MoSe2	-44.30 -56.19	2.5 7.43	5.0 3.75	19 36
CCA	-68.8	4	5.1	20
MoSe2/RGO/MMT(50%wt)-Chitosan	− 72	3.8	2.7	this work
MoSe2/RGO/MMT(75%wt)-Chitosan	− 22	10	2.3	this work

Radar cross section and scattered Far field

The scattering characteristics and radar cross-section (RCS) in Fig. 8(a0-a6) were examined using a numerical approach based on the Finite Element Method (FEM)^60,61. The simulation framework consists of a spherical perfect electric conductor (PEC) with a 2.0 cm radius, positioned at the coordinate origin. This core structure is enveloped by nanocomposite coatings of thicknesses (2.3 mm) tuned to the minimum reflection loss (RL_min) of each sample.

Fig. 8.

RCS versus frequency (a0-a6), the scattered far field norm versus azimuthal coordinate and the scattered far field norm for: metal sphere and m1, m2, m3 and m4 (b1-b6) covered metal sphere with 2.3 mm thickness.

The surrounding region of the scattering sphere is modeled as two concentric domains: the inner domain represents free space (air), at the same time, the outer layer serves as a perfectly matched layer (PML) to absorb outgoing waves and approximate an infinite domain without reflections.

The incident wave propagates along the positive x-axis with its electric field oriented in the z-direction. By applying the time-harmonic wave equation within the FEM framework⁴⁶, the scattered far-field electric field distribution can be determined. The RCS is subsequently obtained using the relation:

Where r denotes the simulation domain radius, while and represent the scattered and incident electric fields, respectively.

According to Fig. 8, the RCS exhibits variations from 1 dB to -1 dB for the metallic sphere and significantly lower values (between − 30 dB and − 60 dB) for the nanocomposite-covered sphere. Figure 8 (b1-b6) also illustrates the azimuthal distribution of the scattered far-field norm at various frequencies. Also, if the scattering is restricted to two dimensions (the x–y plane with z = 0), the detector can be positioned on a circular trajectory of radius r and scanned over the azimuthal angle φ, while the polar elevation angle is kept at zero. As φ increases, the detector’s position shifts relative to the x-axis. The azimuthal reference φ = 0 corresponds to the forward propagation direction of the incident plane wave, where the detector is located at a distance 2r from the radiating source. At φ = 180°, the detector is positioned at the source location.

As expected, the maximum detected field occurs at φ = 0, where most of the incident plane wave bypasses the sphere and continues to propagate around it. At φ = 180°, the detector captures the backscattered field at the source position. The analysis further on firms a reduction in scattered field intensity for the coated sphere compared to the bare PEC sphere across all frequency ranges. Figure 8 presents the 3D far-field distribution at RL_min frequencies for each sample. The simulation results highlight the strongest received electric field norms, measured at approximately 30 dB for the uncoated metallic sphere and 60 dB for the absorber-coated metallic sphere^64–69.

Conclusions

In this study, a lightweight polymer/carbon/mineral aerogel was synthesized by ice casting and freeze-drying. XRD results confirmed the presence of 2 H-MoSe₂ phase and other components, and FESEM images showed a hierarchical 3D porous structure. This porosity, by creating mutually conductive networks and increasing the wave propagation path length, along with functional groups and structural structures as polarization centers, enhances the surface and interlayer polarities. Multiple scattering, as the dominant mechanism, traps the microwave and directly dissipates the electromagnetic energy. Microwave absorption results in X and Ku bands showed that adjusting the weight ratio of MoSe₂/RGO/MMT in the chitosan matrix, simultaneously improves the reflection loss and bandwidth. The optimized MoSe₂/RGO/MMT (50%wt)–Chitosan (m3) sample exhibits the lowest reflection loss of − 72 dB with a bandwidth of 3.8 GHz at a thickness of 2.7 mm and an ultra-low density of 4.63 × 10⁻ m¹. The thickness of 2.3 mm provided complete coverage of the X and Ku bands. This performance was much better than that of the pure MoSe₂/RGO composite. Also, finite element simulations investigated the significant radar cross-section and scattered far-field intensity for the PEC sphere coated with these aerogels. Overall, the MoSe2/RGO/MMT–Chitosan aerogels become promising candidates for electromagnetic absorption applications in the X and Ku bands as an ultra-light, thin, and high-performance microwave absorber.

Author contributions

Mahdieh Dehghani-Dashtabi: experimental work/analysis of data/writer the draft of manuscript. Hoda Hekmatara: Supervisor/writer . Masoud Mohebbi: simulation/theoretical section.

Funding

Authors received NO FUNDING for this work.

Data availability

The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.