Analyzing the Impact of thermal variations on agricultural waste-based Microwave absorbers

AnjuTripathi1✉Emailanju.tripathi89@gmail.com

KarmjitSinghSandha1Emailkssandha@thapar.edu

1Department of Electronics and Communication EngineeringThapar Institute of Engineering and Technology147004PatialaPunjabIndia

“Corresponding author: Anju Tripathi^1”,

¹ Research Scholar, Department of Electronics and Communication Engineering, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004, India

¹anju.tripathi89@gmail.com

Karmjit Singh Sandha²

² Associate Professor, Department of Electronics and Communication Engineering, Thapar Institute of Engineering and Technology, Patiala, Punjab 147004, India

²kssandha@thapar.edu

Abstract

This paper investigates the microwave absorption characteristics of dried banana leaves (DB) and their composite with multi-walled carbon nanotubes (DB-CNTs) in the C-band frequency range (3.7–7.4 GHz) across a temperature range of 20°C to 120°C. Using a one-port vector network analyzer (VNA) and WG13 waveguide. The reflection loss (S₁₁ parameter) was measured to evaluate absorption efficiency. The results indicate that DB alone exhibits good microwave absorption at room temperature, achieving a minimum reflection loss (RL) of -26.44 dB at 5.02 GHz. However, as the temperature increases, its absorption capability weakens, reaching − 16.96 dB at 120°C, primarily due to moisture loss, reduced dipolar polarization, and structural degradation. But the incorporation of CNTs significantly enhances absorption performance with DB-CNT achieving a minimum RL of -40.43 dB at 20°C and maintaining − 24.18 dB at 120°C, demonstrating superior thermal stability. The improved absorption is attributed to enhanced conduction loss, multiple scattering effects, and better impedance matching, which remain effective even at elevated temperatures. The findings suggest that DB-CNT composites are promising candidates for thermally stable microwave absorber applications, outperforming DB alone in high-temperature environments. This research paper highlights the potential of biowaste-derived materials for sustainable electromagnetic interference (EMI) shielding and radar absorption applications. The DB-CNTs composite presents an eco-friendly alternative to conventional microwave absorbers with strong absorption efficiency and stability across varying temperatures, making it suitable for aerospace, defense, and industrial applications.

Keywords:

Bio-composite

Carbon nanotubes

Temperature

Reflection loss

Absorption power

Microwave absorber

Statement of Novelty

The novelty of this work lies in the use of dried banana leaves (DB), an underutilized agricultural byproduct, and their composites with multi-walled carbon nanotubes (DB-CNTs) as sustainable microwave absorber materials. To date, thermal stability has primarily been explored in chemically synthesized absorbers, whereas the thermal behavior of biowaste-based absorbers has received very limited attention. This study addresses that gap by systematically investigating the microwave absorption characteristics of DB and DB-CNT composites across a temperature range of 20°C to 120°C in the C-band (3.7–7.4 GHz). Unlike conventional chemical absorbers that are costly, toxic, and non-biodegradable, the DB-CNT composite offers an eco-friendly, low-cost, and biodegradable alternative with competitive absorption efficiency and superior thermal stability. The integration of agricultural waste with CNTs for thermally stable absorber development represents a new direction toward sustainable EMI shielding solutions for aerospace, defense, and industrial applications.

1 INTRODUCTION

Microwave absorbing materials (MAMs) have gained significant attention due to their critical role in electromagnetic interference (EMI) shielding, stealth technology, and radar absorption. These materials efficiently absorb and dissipate microwave radiation, minimizing signal reflection and interference, which is essential for applications in wireless communication systems, aerospace, defense, and advanced electronic devices [1]. An ideal MAM should possess low density, superior mechanical properties, and high dielectric or magnetic loss to ensure strong microwave attenuation across a broad frequency spectrum [2]. With the growing demand for high-performance microwave absorbers, researchers are increasingly focusing on materials that can withstand elevated temperatures. High-temperature MAMs are crucial for civil, commercial, military, and aerospace vehicles operating in extreme thermal conditions [3]. These materials must maintain their structural integrity and electromagnetic properties under high thermal loads to ensure effective EMI shielding and stealth functionality.

Traditionally, chemical-based MAMs such as carbon fibers, silicon carbide (S_iC), Ferrite, M_nZ_n, nickel, cermic, metal sulfide and titanium carbide (T_iC) have been widely used due to their high electrical conductivity, excellent dielectric properties, and superior absorption efficiency [4–8]. But nowadays, several studies have demonstrated the strong absorption capabilities of these chemical-based MAMs at high temperatures. For example, T_iC-based composites exhibit strong absorption with reflection loss (RL) values of − 55.87 dB at room temperature, -48.49 dB at 50°C, and − 40.36 dB at 100°C[4]. Similarly, CNT/silica nanocomposites show enhanced permittivity and absorption in the X-band frequency range (8.2–12.4 GHz) at temperatures up to 600°C. The Z_nO@CNT/S_iO₂composites display efficient and thermally stable microwave absorption achieving an RL ≤ -10 dB across the full X-band (8.2–12.4 GHz) in the 373–673 K temperature range [5]. Moreover, Fe₃O₄-CNT composites act as tunable microwave absorbers with high efficiency and thermal stability, achieving reflection loss ≤ − 20 dB at 323 K[6]. Studies on Al₂O₃–MoS_i2/C_u composite coatings indicate that at 700°C a 1.4 mm-thick coating achieves a reflection loss of -19.09 dB and an effective absorption bandwidth of 2.83 GHz in the X-band [7]. Likewise, S_iC fiber-reinforced ceramic matrix composites using Al₂O₃–SiO₂ hybrid matrices achieve a minimum reflection loss of -37 dB at 200°C (8.6 GHz) and an effective absorption bandwidth of 4.2 GHz below 400°C making them suitable for aerospace and military applications [8, 9, 10]. Although these chemical-based MAMs provide excellent microwave absorption performance at the high temperature they pose significant environmental and health risks [12]. The production of synthetic materials such as ferrite, copper, T_iC, and S_iC involves complex chemical processing leading to the release of hazardous byproducts. Additionally, these materials are often non-biodegradable, raising concerns regarding their long-term environmental impact [13, 14]. Some chemical-based absorbers degrade at high temperatures (above 573 K), releasing toxic emissions or undergoing structural instability, making them less reliable for long-term applications.[15] To overcome these challenges, researchers are exploring eco-friendly and cost-effective alternatives particularly biowaste-derived microwave absorbers. These materials, obtained from agricultural waste, are abundant, renewable, and naturally rich in carbon, making them highly suitable for microwave absorption [16, 17]. Various biowaste materials such as dried banana leaves (DB), rice husk, corn husk, sugarcane bagasse, and coconut shell powder have been investigated due to their high carbon content and porous structure, which enhance their dielectric and microwave absorption characteristics [18, 19, 20]. For example, DB contains approximately 42.34% carbon [21], while corn husk and coconut shell powder contain 79.11% and 46.7% carbon, respectively [22, 23]. Research has shown that DB-based absorbers achieve a reflection loss of -25.663 dB with an effective absorption bandwidth of 1.133 GHz [24], while Bael leaf-based composites exhibit an RL of -28.64 dB with a bandwidth of 3.20 GHz in the X-band range [25]. To further enhance the microwave absorption capabilities of biowaste-derived materials, researchers have incorporated carbon-based additives such as charcoal and CNTs into the composites [26, 27, 28]. These additives increase dielectric loss, improve impedance matching, and enhance overall absorption efficiency. Studies have demonstrated that charcoal-enriched corn husk composites achieve a return loss of less than − 10 dB in the 2–20 GHz frequency range, while charcoal-coconut fiber coir composites record a reflection loss exceeding − 25 dB in the X-band range [29]. Also, studies show that adding CNTs with rice, sugarcane bagasse, and dried banana leaves increases their microwave absorption characteristics [26, 30] because CNTs have high carbon purity (95–99%) and excellent electromagnetic properties, which boost the absorption efficiency of biowaste-based composites [31].

Despite significant advancements in high-temperature chemical-based absorbers, limited research has been conducted on the thermal stability and microwave absorption performance of biowaste-derived materials. Given the increasing need for sustainable, environmentally friendly MAMs for high-temperature applications, so it is essential to investigate their performance under elevated temperatures. This study specifically focuses on investigating the microwave absorption behavior of dried banana leaves (DB) and DB-multiwalled carbon nanotube (DB-CNT) composites under different high-temperature conditions, ranging from 20°C (room temperature) to 120°C. The research aims to analyze their microwave absorption efficiency at these elevated temperatures. By examining their microwave absorption efficiency, this research will provide crucial insights into the potential of biowaste-derived MAMs as sustainable, high-temperature microwave absorbers for aerospace, military, and EMI shielding applications.

2 MATERIALS AND METHODS

The preparation of the dried banana leaves (DB) and DB-CNTs (dried banana leaves with multi-walled carbon nanotube) composite samples involved a systematic process to ensure uniformity. After the samples were successfully prepared their carbon composition was analyzed to determine the elemental content and distribution. The microstructural characteristics of the sample surfaces were examined using a scanning electron microscope (SEM) which provided detailed imaging of their morphology and surface features. Furthermore, the high-temperature electromagnetic and microwave absorption characteristics of the samples were evaluated using a Rohde & Schwarz ZVH8 vector network analyzer (VNA). The VNA measurements were conducted to assess the reflection and absorption capabilities of the samples across the C-band frequency range ensuring their effectiveness as potential microwave absorbers at high temperature.

2.1 SAMPLE FABRICATION METHOD

In this study, we used banana leaves and polyester resin along with CNTs with 97% purity, surface area ranging from 90 to 220 m^2/g, 10µm length, and 5 to 20 nm diameter to develop a microwave absorber material. The fabrication of the DB & DB-CNTs composite based on microwave absorber begins with taking green banana leaves, which are shown in Fig. 1. In the first step, the leaves need to be cleaned thoroughly by washing them under running water multiple times. In next step, the biomass needs to be sun-dried for 3–4 weeks. After this, a home base grinder is used to grind the banana leaves. In next step, crude powder is sieved for finer powders. The banana powders can be characterized by non-uniform distribution of particles, which range from 140 µm to 520 µm. The size of the particles has major impact on material properties such as microwave absorbing capacity. When the particles are reduces to the micron range. The material displays higher degree of porosity, greater surface area, increased light scattering on the surface, and higher surface atom concentration. With reference to [32]. When the dimension of an object decreases, its specific surface area increases. Once the powder of banana leaves integrated with CNTs is obtained, the following step is to develop the microwave absorber composite material. First mix the powder of banana leaves with CNTs and subsequently add it as a loading filler. Thereafter methyl ethyl ketone peroxide and cobalt are added to the mixture of polyester resin and stirred until a homogeneous mixture is achieved. The material is sonicated in the ultrasonic processor for 35 minutes to ensure optimal dispersion of the material.

Fig. 1

Steps for fabrication DB and DB-CNTs composite-based microwave absorber

Polyurethane and phenol-formaldehyde were used previously by many of studies towards the development of microwave absorbers [33–34]. However, these types of resin are not eco-friendly and can be hazardous to one’s health. Thus, in this research, a binder of polyester resin was used and the hardener agent of methyl ethyl ketone peroxide along with cobalt as an accelerator was used for making samples of microwave absorbers. The USDA approval for methyl ethyl ketone peroxide and polyester resin as constituents of food containers testifies to lower cost and eco-friendliness [32]. Also, the use of polyester resin and methyl ethyl ketone peroxide enhances erosion resistance and helps in binding materials, thus preventing air gaps between particles. This weight ratio must sustain 80:20 of polyester resin and methyl ethyl ketone peroxide for the production of a microwave absorber sample. Then, composites of DB-CNTs are positioned in a rectangular mold of 40.386mm×20.193mm × 3mm to set at room temperature for 1 day. Thus, after 1 day, we have created the DB and CNTs composite samples. I will then proceed to measure all the samples with a vector network analyzer to determine the microwave absorption characteristics for the C-band frequency.

2.2 CHARACTERIZATIONS

2.2.1 Scanning Electron Microscopy (SEM)

Fig. 2

SEM image of DB-CNTs with magnification of (a) 1µm (b) 4µm (c) 10µm

Figure 2( a- c) shows an SEM image of the DB-CNTs composite material taken at different magnifications, revealing the details about its microstructure. At lower magnification, the surface exhibits an undulating sheet-like structure that looks broken, dense, and tough. This indicates a strong and stable material structure that is likely due to the successful incorporation of CNTs into the dried banana leaves (DB) matrix. Furthermore, the image shows the presence of numerous pores within the composite. These pores or voids are important for improved microwave wave absorption capabilities. Indeed, porous structures are beneficial for the absorption and dissipation of microwave waves. Therefore, the SEM image does more than demonstrate the DB-CNTs composite’s surface morphology and porosity; it illustrates the DB-CNTs composite’s structure characteristics in relation to its EM wave absorption effectiveness.

2.3 Elemental Analysis of DB and DB-CNT Composite

Table 1 presents the elemental analysis of dried banana leaves (DB) using CHNS analysis. The carbon content in DB is 42.34%, with additional elements like hydrogen and nitrogen.

Table 1
CHNS Elemental Analysis of Dried Banana Leaves
Elements	Carbon (C)	Hydrogen (H)	Nitrogen (N)	Sulphur (S)
DB (Dried banana leaves)	42.34%	5.47%	0.56%	-

Table 2
ELEMENT Analysis of DB-CNT composite (EDS Analysis)
Points	Carbon (C)	Oxygen(O)	Chlorine (Ci)	Gold (Au)
B.1	77.53%	20.07%	1.37%	1.03%
B.2	78.199%	19.33%	1.34%	1.14%
B.3	83.46%	14.01%	1.12%	1.41%
B.4	87.17%	10.54%	1.225	1.08%

Fig. 3

Scanning Electron Microscope analysis with an Energy-Dispersive X-ray Spectroscopy detector analysis of elemental composition of DB-CNTs composite.

After incorporating 5 wt% of CNTs into DB, the carbon content increased significantly, as shown in Table 2. Scanning Electron Microscope (SEM) analysis with an Energy-Dispersive X-ray Spectroscopy (EDS) detector was conducted to determine the elemental composition of the dried banana leaves (DB) combined with multi-walled carbon nanotubes (CNTs). The EDS analysis was performed at four specific points (B.1, B.2, B.3, and B.4) on the composite sample, and the results confirm a notable rise in carbon content are shown in Fig. 3. The SEM image taken at 2500x magnification (scale bar: 20 µm) shows the surface morphology of the DB-CNT composite. The image reveals the presence of DB powder particles and CNT fibers with the EDS analysis performed at four selected points. Point B.1 shows the Carbon content is 77.53%, with 20.07% oxygen. Point B.2 shows Carbon content is 78.199%, with 19.33% oxygen. Point B.3 is located on the CNT fibers, showing a higher carbon content of 83.46% and a lower oxygen content (14.01%). Point B.4 is another CNT fiber point, with 87.17% carbon and 10.54% oxygen. These results clearly indicate a significant increase in carbon content compared to pure dried banana leaves (42.34% carbon). The addition of CNTs enhances the carbon composition and strengthens its potential as an efficient microwave-absorbing material with enhanced thermal stability.

2.4 EXPERIMENTAL SET UP

The microwave absorption characteristics of the prepared samples were tested within the C-band frequency range using a one-port vector network analyzer (VNA, ROHDE & SCHWARZ ZVH8) for data collection. A WG13 rectangular waveguide was employed in the experiment and the samples were precisely cut to match its dimensions (40.386 mm × 20.193 mm × 3 mm). To prevent microwave transmission through the sample a metal back plate was placed behind it ensuring that the measurements focused solely on the reflected microwaves. Figure 4(a) illustrates the schematic representation of the experimental setup, where a single port of the VNA is connected to a coaxial cable. The cable is linked to the waveguide where the material under test (MUT) is positioned. A metal back plate is placed behind the sample to block microwave transmission allowing for accurate reflection measurements. Figure 4(b) depicts the actual experimental setup where the VNA is connected to the waveguide via a coaxial cable. The sample is placed inside the waveguide, followed by the metal back plate, and the reflection loss (S₁₁ parameter) is displayed on the VNA screen. The S₁₁ parameter, also known as reflection loss, represents the amount of microwave energy reflected by the material, thereby indicating its absorption efficiency. By analyzing the reflection loss across different frequencies within the C-band, the microwave absorption performance of the samples can be effectively assessed.

Fig. 4

(a) Schematic representation of the experimental setup. (b) Actual experimental setup used for measuring the S₁₁ scattering parameter with a one-port VNA. The MUT was positioned between the WG 13 waveguide and a metal back plate for accurate reflection loss measurements.

3 RESULTS

3.1 Reflection loss

The microwave absorption characteristics of the synthesized samples, DB and DB-CNTs, were evaluated within the C-band frequency range of 3.7–7.4 GHz using a one-port vector network analyzer (VNA) at temperatures ranging from 20°C to 120°C. The experiment utilized a WG 13 waveguide, and the dimensions of the samples (40.386 mm x 20.193 mm ×3mm) were compatible with those of the WG 13 waveguide, as illustrated in Fig. 5. As shown in Fig. 5, the samples were heated on a heating plate to achieve the target temperature. An infrared thermometer was employed to accurately measure the temperature of the samples prior to testing, ensuring precise thermal readings. Following the heating process, the sample under examination was positioned between the WG 13 waveguide and a metal back plate for testing.

Fig. 5

Schematic representation of reflection loss measurement of DB and DB-CNT at different temperature.

The metal back plate is used to obstruct the transmission of a microwave. A coaxial cable was then connected from the waveguide to Port A of a Vector Network Analyzer (VNA), which enabled the measurement of the S₁₁ parameter representing the amount of microwave signal reflected from the sample. The VNA displayed the S₁₁ data, which provided insights regarding the materials’ capability for microwave absorption efficiency. The S₁₁ parameter assesses how well a material can reflect or absorb the microwaves. Reflection loss is another term used to describe the S₁₁ parameter. This experimental outcome was instrumental in evaluating the influence of temperature on the microwave absorption behavior of the DB and DB-CNT composite, especially within the C-band frequency range, as depicted in Fig. 6.

Fig. 6

VNA display showing the reflection loss of the DB-CNT composite at 20°C with a metal back plate in the C-band frequency range

In this study, we investigated the microwave absorption characteristics of the prepared samples within the C-band frequency range using a one-port vector network analyzer (VNA). A metal backing plate was positioned behind the samples to eliminate electromagnetic (EM) wave transmission ensuring that only reflection and absorption phenomena were considered. When an EM wave interacts with a material’s surface, it can either be reflected, absorbed, or transmitted. However, due to the metallic backing, transmission is effectively blocked and interaction is limited to reflection and absorption. The one-port VNA measurement provides only the S₁₁ scattering parameter, which represents the ratio of reflected power to incident power. A lower S₁₁ value indicates reduced reflection and consequently enhanced microwave absorption by the material. This measurement setup was used to evaluate and compare the EM wave absorption performance of both dried banana leaves(DB) and DB-CNTs composite samples. The S₁₁ parameter is also known as reflection loss (RL), which is defined according to the transmission line theory as

$\:RL=20\text{log}\left|\frac{{Z}_{in}-{Z}_{0}}{{Z}_{in}+{Z}_{0}}\right|$

$\:{Z}_{in}={Z}_{0}\sqrt{\raisebox{1ex}{${\mu\:}_{r}$}\!\left/\:\!\raisebox{-1ex}{${\epsilon\:}_{r}$}\right.}tanh\left[j\frac{2\pi\:fd}{c}\sqrt{\left({\mu\:}_{r}{\epsilon\:}_{r}\right)}\right]$

Using Eq. 1, we found reflection loss (RL) parameters. This equation considers metal-backed materials’ complex relative permittivity

$\:\left({\epsilon\:}_{r}\right)$

, complex relative permeability

$\:\left({\mu\:}_{r}\right)$

, C band frequency

$\:\:\left(f\right)$

, absorber thickness

$\:\left(t\right)$

, speed of electromagnetic waves in empty space

$\:\left(c\right)$

, and input impedance

$\:\left({z}_{in}\right)\:,\:$

$\:({Z}_{0}$

, impedance of free space shown in Eq. 2. The material employed in this study to create microwave absorber material is bio waste material and has a complex relative permeability

$\:\left({\mu\:}_{r}\right)$

of 1 indicating that the material which used in this study to fabricate microwave absorber material is bio waste material and it does not possess magnetic properties. A candidate microwave absorber material must achieve below − 10 dB minimum RL which means that more than 90% microwave energy is absorbed by a material. According to transmission line theory as defined in Eqs. (1) & (2), Two major factors play important role in RL: (1) the impedance matching (the match between input impedance

$\:\left({z}_{in}\right)\:$

and free space impedance

$\:({Z}_{0}$

) and (2) the dielectric and magnetic properties of the absorber material. An ideal microwave absorber material will therefore possess multiple loss mechanisms with strong microwave attenuation combined with perfect impedance matching.

The microwave absorption characteristics of the dried banana leaves (DB) were analyzed within the C-band frequency range (3.7–7.4 GHz). The Fig. 7 illustrates the variation of reflection loss (RL) with increasing temperature. The temperature-dependent microwave absorption performance of dried banana leaves (DB) was investigated across the temperature range of 20°C to 120°C at a fixed sample thickness of 3 mm within the C-band frequency range. The reflection loss (RL) curves at each temperature reveal a consistent peak absorption centered around 5.05 GHz. However, the intensity of absorption exhibits noticeable decline as the temperature increases.

Fig. 7

Temperature-dependent reflection loss (RL) performance of dried banana leaves (DB) microwave absorber at 3 mm thickness in the C-band (3.7–7.4 GHz). Each subplot (a)–(f) represents the RL value at a specific temperature (20°C to 120°C)

At room temperature (20°C) the DB sample demonstrates the highest microwave absorption capability with a maximum RL value of -26.44 dB. As the temperature increases to 40°C and 60°C the peak RL reduces to -23.84 dB and − 21.92 dB respectively indicating a progressive degradation in absorption performance. Further heating to 80°C, 100°C, and 120°C results in RL values of -20.96 dB, -19.24 dB, and − 16.96 dB, respectively. This decreasing trend can be attributed to temperature-induced changes in the physicochemical and dielectric properties of the material. At elevated temperatures the internal structure of the banana leaves fibers undergoes partial degradation including the breakdown of hemicellulose and lignin components which are critical to maintaining effective dielectric polarization. Additionally, the evaporation of inherent moisture content and the collapse of natural pores and interfaces likely reduce the capacity for interfacial and dipolar polarization both of which are essential mechanisms in microwave absorption. These structural and dielectric deteriorations ultimately weaken the materials ability to convert electromagnetic energy into thermal energy resulting in reduced reflection loss values. Thus, the results suggest that the microwave absorption efficiency of DB is optimal at lower temperatures where the material retains its natural dielectric characteristics and internal morphology.

Figure 8 illustrates the temperature-dependent reflection loss (RL) characteristics of the dried banana leaves with Multi-Walled Carbon Nanotubes (DB-CNTs) composite measured within the C-band frequency range (3.7–7.4 GHz) at temperatures ranging from 20°C to 120°C. All measurements were conducted at a fixed material thickness of 3 mm. The RL spectra reveal that the composite consistently exhibits strong absorption around 5.05 GHz across the entire temperature range. However, the magnitude of absorption progressively decreases with increasing temperature. At 20°C, the composite demonstrates its highest microwave attenuation performance, achieving a maximum RL of -40.43 dB. As the temperature rises to 40°C and 60°C, the RL values slightly reduce to -32.23 dB and − 30.24 dB, respectively, indicating sustained but gradually diminishing absorption. Further increases in temperature to 80°C, 100°C, and 120°C result in RL values of -29.14 dB, -28.5 dB, and − 24.18 dB, respectively, confirming a clear trend of reduced microwave absorption efficiency as thermal conditions intensify.

Fig. 8

Temperature-dependent reflection loss (RL) performance of Dried banana leaves with multiwalled carbon nanotubes (DB-CNT) composite microwave absorber at 3 mm thickness in the C-band (3.7–7.4 GHz). Each subplot (a)–(f) represents the RL value at a specific temperature (20°C to 120°C).

This reduction in absorption capability at elevated temperatures can be attributed to several key factors. Firstly, thermal agitation at higher temperatures disrupts the structural and interfacial integrity of the composite material, particularly at the interface between the dried banana leaves matrix and the embedded multi-walled carbon nanotubes (MWCNTs). This disruption weakens the multiple dielectric relaxation processes and reduces the interfacial polarization effects that are essential for effective electromagnetic wave attenuation. Secondly, the increase in temperature may cause partial thermal degradation or softening of the DB matrix, resulting in reduced compatibility and contact between the DB and the conductive CNT network, thereby diminishing the composite’s ability to dissipate electromagnetic energy. Lastly, excessive temperature may also reduce the dielectric contrast between the filler and matrix, limiting the composite’s ability to trap and absorb microwave energy through multiple reflections and scattering mechanisms. These observations suggest that while the DB-CNTs composite retains good microwave absorption properties even at higher temperatures, its optimal performance is achieved at lower thermal conditions where structural integrity and interfacial interactions are better preserved.

The study of temperature-dependent microwave absorption characteristics reveals distinct behaviors for both pure DB and DB-CNT composite materials. For the DB sample, the reflection loss (RL) progressively decreased from − 26.44 dB at 20°C to -16.96 dB at 120°C, indicating a steady decline in absorption performance with rising temperature. This degradation is attributed to the breakdown of natural components such as hemicellulose and lignin, as well as the collapse of porous structures and loss of dipolar polarization, all of which reduce the material’s ability to absorb electromagnetic waves effectively.

In contrast, the DB-CNTs composite showed superior absorption capabilities across the entire temperature range, with the highest RL of -40.43 dB at 20°C and still maintaining a strong RL of -24.44 dB at 120°C. The presence of MWCNTs significantly enhances dielectric loss, interfacial polarization, and electrical conductivity, making the composite stronger to temperature-induced degradation. However, a decreasing trend with temperature was also observed in the composite, though the extent of decline was less severe compared to pure DB. Overall, the results confirm that the DB-CNTs composite not only outperforms the pure DB in terms of reflection loss values but also maintains better thermal stability. Therefore, incorporating carbon nanotubes into agricultural biowaste matrices like dried banana leaves is an effective strategy to develop high-performance, thermally stable, and eco-friendly microwave absorber materials for C-band applications.

3.2 Absorption Power

When electromagnetic waves hit the sample that’s supported by a metal plate, they either get absorbed or reflected but they don't pass through. The percentage of power that's absorbed calculated by Eq. (3). This equation quantifies the microwave absorption efficiency of the material by calculating the proportion of incident power absorbed by the sample.

$\:Absorption\:\%=100-Reflected\:power=\:\left[100-100*\left({10}^{{^}^{\left(-RL/10\right)}}\right)\right]$

The microwave absorption efficiency of the DB and DB–CNTs composites was further investigated by calculating the absorption power percentage at varying temperatures. As RL is a direct measure of how much incident microwave energy is reflected by a material a more negative RL value indicates stronger microwave attenuation. Once the RL values across different temperatures were obtained the corresponding absorption percentage was calculated. Table 3 presents the absorption power percentages of both DB and DB–CNTs samples across a temperature range from 20°C to 120°C at a fixed thickness of 3 mm. For the DB sample a high initial absorption of 99.7% was recorded at 20°C. However, as the temperature increased, a gradual reduction in absorption efficiency was observed with values decreasing to 99.5% at 40°C, 99.4% at 60°C, 99.2% at 80°C, 98.8% at 100°C, and finally 97.8% at 120°C. This trend can be attributed to the thermal degradation of organic constituents in the dried banana leaves, such as cellulose, hemicellulose, and lignin, which lead to a decline in dipolar polarization and interfacial interactions, both of which are critical for effective dielectric loss mechanisms. Despite this decline, the DB material still maintains RL values below − 10 dB across all temperature points, confirming that it continues to function effectively as a microwave absorber. According to standard electromagnetic absorption criteria, an RL below − 10 dB indicates that more than 90% of incident energy is being absorbed, thereby validating the pure dried banana leaves (DB) usability even under elevated thermal conditions.

Table 3
Absorption power percentage of DB and DB-CNTs materials at different temperatures
Material	Thickness (mm)	Temperature °C	Absorption percentage (%)
Dried banana leaves (DB)	3	20	99.7
		40	99.5
		60	99.4
		80	99.2
		100	98.8
		120	97.8
Dried banana leaves with multiwalled carbon nano tubes (DB-CNTs)	3	20	99.9
		40	99.9
		60	99.9
		80	99.8
		100	99.8
		120	99.6

In contrast, the DB–CNTs composite sample exhibited remarkably stable absorption behavior throughout the same temperature range. At 20°C, 40°C, and 60°C, the absorption power was measured consistently at 99.9%, with only a marginal decrease to 99.8% at 80°C and 100°C, and 99.6% at 120°C. This outstanding thermal stability can be credited to the synergistic effect between the banana leaves matrix and the embedded MWCNTs. The incorporation of MWCNTs introduces enhanced electrical conductivity, interfacial polarization, and multiple scattering effects which significantly boost dielectric loss mechanisms. Additionally, the highly conductive network formed by the MWCNTs ensures that the composite maintains effective impedance matching with free space even as the temperature increases thereby minimizing reflection and maximizing absorption. Unlike the pure DB sample which experiences a moderate decline in performance at higher temperatures, the DB–CNTs composite retains near-constant absorption levels showcasing its superior electromagnetic attenuation capabilities.

From a comparative perspective, both materials exhibit high absorption efficiency above 97% across the entire temperature range. However, the DB–CNTs composite clearly outperforms the pure DB, particularly under high-temperature conditions where its stability and effectiveness remain nearly unchanged. This indicates that while DB alone is a promising eco-friendly and cost-effective microwave absorber, the integration of MWCNTs substantially improves its performance especially in thermally stressed environments. From the result, it’s observed that both DB and DB–CNTs samples are viable candidates for use in high-temperature microwave absorption applications. The DB sample, despite minor performance drops at elevated temperatures, still meets the critical RL <- -10 dB requirement, validating its potential. Meanwhile, the DB–CNTs composite demonstrates excellent thermal stability and near-perfect absorption, making it exceptionally well-suited for demanding applications such as stealth technology, radar absorption, and thermal-resistant electromagnetic shielding, where consistent performance under varying thermal conditions is essential.

3.3 Comparative Analysis microwave Absorption Performance of DB, DB-CNTs, and Conventional Absorbers

Table 4 compares the microwave absorption performance of various materials, including chemically synthesized composites and the natural DB and DB-CNTs composites. The chemically engineered materials, such as Z_nO@CNTs, Fe₃O₄-CNTs/S_iO₂, MWNT/S_iO₂, Al₂O₃-M_OS_i2/C, and S_iC-based composites generally operate in the X-band (8.2–12.4 GHz) with high-temperature stability up to 600–700°C. These materials exhibit strong absorption with minimum reflection loss (RL) values ranging from − 17 dB to -37 dB, making them highly effective for microwave wave attenuation.

Table 4
Comparison of Microwave Absorption Performance of DB and DB-CNTs Composites with Chemically Synthesized Absorbers
Materials	frequency	Thickness (mm)	RL(dB)	T(°C)	References
ZnO@CNTs	(8.2–12.4 GHz)	2.5	-20.7dB at 50°C	50°C -400°C	[5]
Fe₃O₄-CNTs/S_iO₂	(8.2–12.4 GHz)	3.2	-24.8dB at 50°C	50°C-200°C	[6]
MWNT/S_iO₂	(8.2–12.4 GHz)	2.5	-20 dB at30°C	30°C − 600°C	[4]
Al₂O₃–MoS_i2/C	(8.2–12.4 GHz )	1.4	-17dB at 20°C	20°C -700°C	[7]
S_iC_f/Al₂O₃–S_iO₂	(8.2–12.4 GHz)	3	-37 dB at 200 C	200°C -600°C.	[8]
S_iC_f/BN/AlPO₄/CNTs	(8.2–12.4 GHz)	3.1	-33dB at 200°C	25°C -600°C	[9]
DB	(3.7–7.4 GHz)	3	-16.96 dB at 120°C	20°C-120°C	This work
DB-CNTs	(3.7–7.4 GHz)	3	-24.8 dB at 120°C	20°C-120°C	This work

In contrast, DB and DB-CNTs composites operate in the C-band (3.7–7.4 GHz) and show moderate absorption. The DB-CNTs composite exhibits enhanced performance compared to DB alone, achieving a minimum RL of -24.8 dB at 120°C, while DB alone reaches only − 16.96 dB. Although DB-based materials have lower absorption performance than advanced ceramic-based absorbers, their biodegradability, cost-effectiveness, and sustainability make them attractive alternatives for environmentally friendly microwave absorption applications.

While high-temperature chemical materials remain superior in extreme environments, DB-CNTs composites offer a promising balance of absorption efficiency, lightweight properties, and eco-friendliness, making them viable for EMI shielding and radar absorption in lower-frequency applications. Future work can focus on enhancing their thermal stability and absorption efficiency to compete with synthetic materials.

Conclusions

In this study, it is concluded that dried banana leaves (DB) exhibit moderate microwave absorption performance within the C-band frequency range (3.7–7.4 GHz) with reflection loss (RL) decreasing at elevated temperatures. This decline is attributed to moisture loss, reduced dipolar polarization, and structural changes that adversely affect impedance matching. However, the incorporation of multi-walled carbon nanotubes (MWCNTs) into DB significantly enhances both the microwave absorption efficiency and thermal stability of the composite. The DB-CNTs composite demonstrates superior microwave attenuation compared to DB alone, maintaining strong absorption even at higher temperatures. At 20°C, the DB-CNTs composite achieves a minimum RL of -40.43 dB in contrast to -26.44 dB for DB alone, highlighting the role of CNTs in promoting conductive losses, multiple scattering, and improved impedance matching. Although the RL slightly decreases with increasing temperature, the DB-CNT composite consistently maintains absorption levels exceeding 99%, confirming its reliability and effectiveness under thermal variation.

These results indicate that DB-CNT composites hold significant promise for practical applications such as electromagnetic interference (EMI) shielding, radar-absorbing materials, and high-frequency communication systems. Furthermore, the environmentally friendly and cost-effective nature of DB-based absorbers makes them attractive as sustainable alternatives to conventional synthetic microwave absorbing materials. Future research should focus on evaluating the long-term thermal stability, mechanical durability, and scalability of DB-CNTs composites to support their potential for industrial and aerospace applications.

Author Contributions

Anju Tripathi: Conceptualization, Investigation & Measurement, Methodology, Simulation, Writing- Original draft.

Karmjit Singh Sandha

Investigation, Reviewing and Editing Supervision.

Data Availability

The authors express their gratitude to Microwave Lab, TIET Patiala, for analyzing performance of microwave absorption characteristics, MCF@DPMS Lab, TIET, Patiala for conducting SEM and EDS analyses & SAI Lab of TIET, Patiala, for conducting CHNS analyses

Declarations

Conflict of Interest:

The author declare that they have no known competing financial interest or personal relationship that could appeared to influence the work reported in this paper. This article does not contain any studies connecting animals and human participant performed by any of the authors.

Funding

No funding is received for this study.

REFERENCES

Zeng, L., Cheng, H., Yu, R., Stucky, G.D.: Electromagnetic microwave absorption theory and recent achievements in microwave absorbers. Carbon. 168, 606–623 (2020). https://doi.org/10.1016/j.carbon.2020.07.028

Yusoff, A.N., Abdullah, M.H., Ahmad, S.H., Jusoh, S.F., Mansor, A.A., Hamid, S.A.A.: Electromagnetic and absorption properties of some microwave absorbers. J. Appl. Phys. 92, 876–882 (2002). https://doi.org/10.1063/1.1489092

Hou, Z., Yin, X., Xu, H., Wei, H., Li, M., Cheng, L., Zhang, L.: Reduced graphene oxide/silicon nitride composite for cooperative electromagnetic absorption in wide temperature spectrum with excellent thermal stability. ACS Appl. Mater. Interfaces. 11, 5364–5372 (2019). https://pubs.acs.org/doi/10.1021/acsami.8b20023

Wang, Y., Sun, X., Xiao, Z., Gu, J., Dong, Q., Yi, S., Jin, J.: Influence of absorber contents and temperatures on the dielectric properties and microwave absorbing performances of C@TiC/SiO2 composites. Nanomaterials. 14, 2033 (2024). https://doi.org/10.3390/nano14242033

Song, W.L., Cao, M.S., Hou, Z.L., Yuan, J., Fang, X.Y.: High-temperature microwave absorption and evolutionary behavior of multiwalled carbon nanotube nanocomposite. Scripta Mater. 61, 201–204 (2009). https://doi.org/10.1016/j.scriptamat.2009.03.048

Lu, M.M., Cao, W.Q., Shi, H.L., Fang, X.Y., Yang, J., Hou, Z.L., Jin, H.B., Wang, W.Z., Yuan, J., Cao, M.S.: Multi-wall carbon nanotubes decorated with ZnO nanocrystals: mild solution-process synthesis and highly efficient microwave absorption properties at elevated temperature. J. Mater. Chem. A. 2, 10540–10547 (2014). https://doi.org/10.1039/C4TA01715C

Lu, M.M., Cao, M.S., Chen, Y.H., Cao, W.Q., Liu, J., Shi, H.L., Zhang, D.Q., Wang, W.Z., Yuan, J.: Multiscale assembly of grape-like ferroferric oxide and carbon nanotubes: a smart absorber prototype varying temperature to tune intensities. ACS Appl. Mater. Interfaces. 7, 19408–19415 (2015). https://doi.org/10.1021/acsami.5b05595

Gao, C., Jiang, Y., Cai, D., Xu, J., Xiao, W.: Effect of temperature on the microwave-absorbing properties of an Al2O3–MoSi2 coating mixed with copper. Coatings. 11, 940 (2021). https://doi.org/10.3390/coatings11080940

Wan, F., Luo, F., Wang, H., Huang, Z., Zhou, W., Zhu, D.: Effects of carbon black (CB) and alumina oxide on the electromagnetic- and microwave-absorption properties of SiC fiber/aluminum phosphate matrix composites. Ceram. Int. 40, 15849–15857 (2014). https://doi.org/10.1016/j.ceramint.2014.07.113

10.

Gao, H., Luo, F., Wen, Q., Jia, H., Zhou, W., Zhu, D.: Enhanced high-temperature dielectric and microwave absorption properties of SiC fiber‐reinforced oxide matrix composites. J. Appl. Polym. Sci. 136, 47097 (2019). https://doi.org/10.1002/app.47097

11.

Wan, F., Yan, J., Xu, H.: Improved mechanical and high-temperature electromagnetic wave absorption properties of SiCf/BN/AlPO4 composites with absorber multiwalled carbon nanotubes. Compos. Interfaces. 28, 809–826 (2021). https://doi.org/10.1080/09276440.2020.1812336

12.

Ren, J., Mu, Z., Sellami, R., El-Bahy, S.M., Liang, G., Guo, J., El-Bahy, Z.M., Xie, P., Guo, Z., Hou, H.: Multifunctions of microwave-absorbing materials and their potential cross-disciplinary applications: a mini-review. Adv. Compos. Hybrid. Mater. 8, 1–25 (2025). https://doi.org/10.1007/s42114-025-01258-5

13.

Li, H., Sun, W., Qiu, B., et al.: Temperature-stable dielectric constant and low thermal expansion in linear low-density polyethylene/copper vanadate composites. Polym. Bull. (2025). https://doi.org/10.1007/s00289-025-05769-5

14.

Omidian, H., Akhzarmehr, A., Chowdhury, S.D.: Advancements in cellulose-based superabsorbent hydrogels: sustainable solutions across industries. Gels. 10, 174 (2024). https://doi.org/10.3390/gels10030174

15.

Zhang, F., Li, N., Shi, J.F., Xu, L., Jia, L.C., Wang, Y.Y., Yan, D.X.: Recent progress on carbon-based microwave absorption materials for multifunctional applications: a review. Compos. B Eng. 111646 (2024). https://doi.org/10.1016/j.compositesb.2024.111646

16.

Nornikman, H., Soh, P.J., Azremi, A.A.H., Wee, F.H., Malek, M.F.: Investigation of an agricultural waste as an alternative material for microwave absorbers. PIERS Online 5. (2009)

17.

Mezan, M.S., Malek, M.F.A., Jusoh, M.S., Abdullah, F.S., Affendi, N.A.M.: Reflection loss performance and performance assessment of pyramidal microwave absorber using agriculture waste. Prog. Electromagn. Res. Symp. Google scholar (2014)

18.

Verma, G., Ray, K.P.: Design, fabrication, and characteristics of eco-friendly microwave-absorbing materials: A review. IETE Tech. Rev. 39(6), 756–774 (2022). https://doi.org/10.1080/02564602.2021.1927865

19.

Appusamy, S., Krishnan, S., Gopikrishna, M., Raman, S.: Bio-based materials for microwave devices: a review. J. Electron. Mater. 50, 1893–1921 (2021). https://doi.org/10.1007/s11664-020-08672-z

20.

Pattanayak, S.S., Laskar, S.H., Sahoo, S.: A review on microwave absorber using agricultural residues. In: 7th Indian Young Geotechnical Engineers Conference, 4–8.Google scholar (2019)

21.

Tripathi, A., Sandha, K.S.: Broad microwave absorption bandwidth analysis of biocomposite microwave absorber material for X-band applications. Biomass Convers. Biorefinery. 1–19 (2024). https://doi.org/10.1007/s13399-024-06157-1

22.

Pattanayak, S.S., Laskar, S.H., Sahoo, S.: Microwave absorption performance enhancement of corn husk-based microwave absorber. J. Mater. Sci. : Mater. Electron. 32, 1150–1160 (2021). https://doi.org/10.1007/s10854-020-04888-

23.

Yah, N.F.N., Rahim, H.A., Lee, Y.S., Wee, F.H., Zainal, H.H.: Electromagnetic wave absorption properties of novel green composites coconut fiber coir and charcoal powder over X-band frequency for electromagnetic wave absorbing applications. Adv. Electromagn. 7, 13–18 (2018). https://doi.org/10.7716/aem.v7i1.598

24.

Pattanayak, S.S., Laskar, S.H., Sahoo, S.: Microwave absorption study of dried banana leaves-based single-layer microwave absorber. Int. J. Microw. Wirel. Technol. 13, 154–163 (2021). 10.1017/S1759078720000707

25.

Panda, S.S.S., Gandi, S., Panigrahi, T., Parne, S.R.: Structural, optical, and electromagnetic microwave absorption properties of bael leaves: a simple approach to investigate microwave absorption properties with 3D Printed PLA Tubes. J. Electron. Mater. 52, 6254–6268 (2023). https://doi.org/10.1007/s11664-023-10575-8

26.

Tripathi, A., Sandha, K.S.: Impact of thickness and wt% on the dielectric properties of Sugarcane Bagasse and MWCNT composite material as microwave absorber. Waste Biomass Valor. 14, 2009–2024 (2023). https://doi.org/10.1007/s12649-022-01977-6

27.

Lee, Y.S., Wee, F.H., You, K.Y., Liyana, Z., Lee, C.Y., Azol, M.E., Ramli, N., Gan, H., Shakhirul, M., Idris, N.: Study of single layer microwave absorber based on rice husk Ash/CNTs composites. Indones J. Electr. Eng. Comput. Sci. 14, 929–936 (2019). 10.11591/ijeecs.v14.i1.pp929-936

28.

Pattanayak, S.S., Laskar, S.H., Sahoo, S.: Design from waste: an eco-efficient microwave absorber using dried banana leaves and charcoal based composite. J. Mater. Sci. : Mater. Electron. 33, 13398–13407 (2022). https://doi.org/10.1007/s10854-022-08276-9

29.

30.

Seng, L.Y., Wee, F.H., Rahim, H.A., AbdulMalek, M., You, Y.K., Liyana, Z., Ezanuddin, A.A.M.: Design of multiple-layer microwave absorbing structure based on rice husk and carbon nanotubes. Appl. Phys. A. 123, 1–5 (2017). https://doi.org/10.1007/s00339-016-0618-2

31.

Ghasempour, R., Narei, H.: CNT basics and characteristics. In: Carbon Nanotube-Reinforced Polymers, pp. 1–24. Elsevier (2018). https://doi.org/10.1016/B978-0-323-48221-9.00001-7

32.

Pattanayak, S.S., Laskar, S.H., Sahoo, S.: Design and development of banana leaves-based double-layer microwave absorber. IETE J. Res. 69, 924–931 (2023). https://doi.org/10.1080/03772063.2020.1844073

33.

Yang, H., Jian, Z., Li, K., Chang, H., Yang, W., Liu, S.: Band gap, ferromagnetic resonance and strong microwave absorption of BaFe₁₂–₂ₓZnₓSnₓO₁₉ compounds and enhanced dielectric loss in BaFe₁₂–₂ₓZnₓSnₓO₁₉/carbon nanotube composites. J. Magn. Magn. Mater. 596, 171997 (2024). https://doi.org/10.1016/j.jmmm.2024.171997

34.

Menon, A.V., Madras, G., Bose, S.: Ultrafast self-healable interfaces in polyurethane nanocomposites designed using Diels–Alder click as an efficient microwave absorber. ACS Omega. 3, 1137–1146 (2018). https://doi.org/10.1021/acsomega.7b01845

FIGURES

Figure 1 Steps for fabrication DB and DB-CNTs composite-based microwave absorber

Figure 2 SEM image of DB-CNTs with magnification of (a) 1µm (b) 4µm (c) 10µm

Figure 3 Scanning Electron Microscope analysis with an Energy-Dispersive X-ray Spectroscopy detector analysis of elemental composition of DB-CNTs composite.

Figure 4 (a) Schematic representation of the experimental setup. (b) Actual experimental setup used for measuring the S₁₁ scattering parameter with a one-port VNA. The MUT was positioned between the WG 13 waveguide and a metal back plate for accurate reflection loss measurements.

Figure 5 Schematic representation of reflection loss measurement of DB and DB-CNT at different temperature.

Figure 6 VNA display showing the reflection loss of the DB-CNT composite at 20°C with a metal back plate in the C-band frequency range

Figure 7 Temperature-dependent reflection loss (RL) performance of dried banana leaves (DB) microwave absorber at 3 mm thickness in the C-band (3.7–7.4 GHz). Each subplot (a)–(f) represents the RL value at a specific temperature (20°C to 120°C)

Figure 8 Temperature-dependent reflection loss (RL) performance of Dried banana leaves with multiwalled carbon nanotubes (DB-CNT) composite microwave absorber at 3 mm thickness in the C-band (3.7–7.4 GHz). Each subplot (a)–(f) represents the RL value at a specific temperature (20°C to 120°C).

TABLES

Table 1. CHNS Elemental Analysis of Dried Banana Leaves

Elements	Carbon (C)	Hydrogen (H)	Nitrogen (N)	Sulphur (S)
DB (Dried banana leaves)	42.34%	5.47%	0.56%	-

Table 2. ELEMENT Analysis of DB-CNT composite (EDS Analysis)

Points	Carbon (C)	Oxygen(O)	Chlorine (Ci)	Gold (Au)
B.1	77.53%	20.07%	1.37%	1.03%
B.2	78.199%	19.33%	1.34%	1.14%
B.3	83.46%	14.01%	1.12%	1.41%
B.4	87.17%	10.54%	1.225	1.08%

Table 3 Absorption power percentage of DB and DB-CNTs materials at different temperatures

Material	Thickness (mm)	Temperature °C	Absorption percentage (%)
Dried banana leaves (DB)	3	20	99.7
		40	99.5
		60	99.4
		80	99.2
		100	98.8
		120	97.8
Dried banana leaves with multiwalled carbon nano tubes (DB-CNTs)	3	20	99.9
		40	99.9
		60	99.9
		80	99.8
		100	99.8
		120	99.6

Table 4 Comparison of Microwave Absorption Performance of DB and DB-CNTs Composites with Chemically Synthesized Absorbers

Materials	frequency	Thickness (mm)	RL(dB)	T(°C)	References
ZnO@CNTs	(8.2–12.4 GHz)	2.5	-20.7dB at 50°C	50°C -400°C	[5]
Fe₃O₄-CNTs/S_iO₂	(8.2–12.4 GHz)	3.2	-24.8dB at 50°C	50°C-200°C	[6]
MWNT/S_iO₂	(8.2–12.4 GHz)	2.5	-20 dB at30°C	30°C − 600°C	[4]
Al₂O₃–MoS_i2/C	(8.2–12.4 GHz )	1.4	-17dB at 20°C	20°C -700°C	[7]
S_iC_f/Al₂O₃–S_iO₂	(8.2–12.4 GHz)	3	-37 dB at 200 C	200°C -600°C.	[8]
S_iC_f/BN/AlPO₄/CNTs	(8.2–12.4 GHz)	3.1	-33dB at 200°C	25°C -600°C	[9]
DB	(3.7–7.4 GHz)	3	-16.96 dB at 120°C	20°C-120°C	This work
DB-CNTs	(3.7–7.4 GHz)	3	-24.8 dB at 120°C	20°C-120°C	This work