Theoretical aspects

MAM have to achieve general requirements which are difficult to accomplish simultaneously. While designing a MA structure, the aim is to accomplish two targets: (i) reducing reflection and (ii) increasing absorption over broadband frequency. Reducing the reflection can be met by wave impedance matching and increasing the absorption can be met by controlling the dielectric properties of the materials.

When an EM wave penetrates and propagates through the material, the experience is divided into three mechanisms which are a reflection of the incoming wave, absorption of the wave as it passes through the thickness of the material, and transmission of the wave. The absorption characteristics of the material rely on its dielectric properties such as complex permittivity, loss tangent, and conductivity [Reference Vinoy and Jha28]. The complex permittivity is a dominating factor which determines how the EM field interacts with the molecular structure of the material. When an EM field is applied across a dielectric material, translational motion of electrons or ions is induced by the internal field, thus it rotates the dipole and dipoles are aligned accordingly and the material is said to be polarized and the polarization mechanism is known dipolar polarization. Inertial, elastic, and frictional forces impede this induced motion as well as the realignment of dipoles and cause energy loss which leads to heat dissipation. Another polarization mechanism which also plays a role in absorption mechanism is conduction mechanism. In this mechanism, any mobile charge carriers (electrons, ions, etc.) move back and forth through the material under the influence of the electric field, creating an electric current. These induced currents will cause heat generation in the materials due to any electrical resistance caused by the collisions of charged species with neighboring molecules or atoms. The complex permittivity is a frequency-dependent parameter which can be expressed with the following expression [Reference Nath29]:

(1)$$\;\varepsilon ^\ast{ = } {\varepsilon }^{\prime}-j\varepsilon ^{\prime\prime}\comma \;$$

where ɛ ^′ is known as the dielectric constant which characterizes the material's ability to store and release EM energy, ɛ ^′′ is the loss factor or dissipative factor which corresponds to the material's ability to absorb or attenuate EM energy to create thermal energy.

There is another related factor to microwave absorption namely loss tangent, commonly used to indicate the efficiency of conversion of microwave energy into thermal energy within the dielectrics and is given in (2) [Reference Nath29]:

(2)$$\tan \delta = \displaystyle{{{\varepsilon }^{\prime \prime}} \over {{\varepsilon }^{\prime}}}.$$

It is defined as the ratio of the energy dissipated to the energy stored in the dielectric material. The more energy that is dissipated into the material, the less energy will penetrate into the dielectric material. This dissipated energy is converted into heat. The main principle of absorption mechanism is heat dissipation. Absorption is a necessary parameter in determining the optimum condition when designing an MA. An optimal absorber should provide 100% absorption of the entering signal. The power loss in terms of heat generation due to the electrical component of the EM wave can be further explained by the following equation [Reference Tsubaki, Azuma, Fujii, Singh, Thallada, Wada, Bhaskar, Pandey, Venkata Mohan, Lee and Khanal30, Reference Rosa, Veronesi and Leonelli31]:

(3)$$P = \displaystyle{1 \over 2}\sigma \vert E \vert ^2 + \omega \varepsilon _0{\varepsilon }^{\prime \prime}\vert E \vert ^2\comma \;$$

where P represents the power density in the material (W/m³), σ is the electrical conductivity (S/m), ω = 2πf(Hz); f is the frequency of the incident microwaves, ɛ′′ is the dielectric loss factor, ɛ ₀ is the permittivity of free space, E represents the electric field strength (V/m).

Conductivity is another factor which contributes to the conductive loss and it is the main parameter in determining the loss factor. The conductivity of the material is given in (4) [Reference Green and Chen32] Fig. 1:

(4)$$\sigma = 2\pi f\varepsilon _{0} \varepsilon _{r}^{\prime\prime} \comma \;$$

where ɛ ₀ = 8.854  ×  10⁻¹² F/m.

Fig. 1.

Possible paths of the incident electromagnetic wave in a material.

The components of the incident power distribution inside the material where P_i, P_{r ,} P_a, and P_t are incident, reflected, absorbed, and transmitted power, respectively, are expresses as:

(5)$$P_i = P_r + P_a + P_t.$$

Equation (6) shows the expression of return loss:

(6)$$RL\;\lpar {dB} \rpar = 20\log \displaystyle{{P_r} \over {P_i}}.$$

Absorption profile AR(ω) of the material can be determined as [Reference Nath29]:

(7)$$AR\lpar \omega \rpar = 1-\vert {S_{11}} \vert ^2-\vert {S_{21}} \vert ^2\comma \;$$

where S ₁₁ is the reflected signal and S ₂₁ is the transmitted signal.

In the context of the measurement setup, we measured the S ₁₁ parameter and the ground plane (PEC) allows minimal transmission. From both the point of view, transmission loss (S ₂₁ ~ 0) can be neglected. Therefore, (8) is the reduced form of (7) and it validates the measurement setup:

(8)$$AR\lpar \omega \rpar = 1-\vert {S_{11}} \vert ^2.$$

Within the MAM, the microwave energy decays exponentially with distance x by a factor e ^−αxwhere attenuation constant (α) is given by [Reference Rubrice, Castel, Himdi and Parneix33]:

(9)$$\alpha = A \times \displaystyle{\omega \over C} \times \sqrt {\displaystyle{{{\mu }^{\prime}{\varepsilon }^{\prime}} \over 2}} \times \sqrt {\left[{\sqrt {1 + {\lpar {\tan \delta } \rpar }^2} -1} \right]} \comma \;$$

where ω = 2πf and f is the frequency of microwave, C is the speed of light in vacuum, μ ^′ is the relative permeability of the material (μ ^′ = 1), ɛ^′ and tan δ are the EM parameters of the material. A is the adjustment factor (A = 8.68/100 dB/Np) to convert α in dB/cm. The above expression indicates that the attenuation of microwave power for a dielectric absorber depends on ɛ ^′ and tan δ. From (2), it can be concluded that the higher the imaginary part of the complex permittivity, the bigger the loss tangent and the better the wave absorption effect.

When an EM wave penetrates into the material and propagates through the material, the power of the wave drops to 37% of its initial value at a certain distance from the surface. The distance is known as skin depth and it is given by [Reference Yah, Rahim, Lee, Wee and Zainal34]:

(10)$$D = \displaystyle{C \over {2\pi f\sqrt {2{\varepsilon }^{\prime}\left[{\sqrt {1 + {\lpar {{\varepsilon }^{\prime \prime}/{\varepsilon }^{\prime}} \rpar }^2} -1} \right]} }}.$$

Sample fabrication method

Figure 2 illustrates the flow of work for developing the flat-shaped particle board. The structure of the work plan consists of five phases, i.e. collection, grinding, measurement of dielectric properties, the addition of resin and hardener agent, and simulative analysis. Shape plays a significant role in the absorption performance of a good MA. The frequency range considered in this investigation is 1–20 GHz. In this research work, three particle boards of various thicknesses are fabricated and the dimension of the boards is given in Table 1. Banana leaves are taken and dried for a week under the sun. They are blended using a grinder to make a fine microscopic powder. Figure 3 shows the dried banana leaves in microscopic powder form.

Fig. 2.

Steps of developing banana leaves-based rectangular-shaped microwave absorber.

Fig. 3.

Dried banana leaves in powder form.

Table 1.

Sample preparation for microwave property measurement and mould sample dimensions

To fabricate a particle board, different types of resin and hardener can be used. Urea formaldehyde, phenol formaldehyde, and polyurethane were used in some studies [Reference Nornikman, Soh, Azremi, Wee and Malek13, Reference Liyana, Malek, Nornikman, Mohd Affendi, Mohamed, Saudin and Ali21]. But these resins are not eco-friendly and are human health hazards. In this work, polyester resin, methyl ethyl ketone peroxide, and cobalt have been used as a binding agent, hardening agent, and an accelerator for the fabrication of the particle board. These compounds are also USDA approved for food container production [35]. Hence, they are not malign agents for either human health or the environment. Resin with a hardening agent binds the material, and the layers of the two constituents impart strength to it and protect it from abrasion. Figure 4 shows the particle board fabricated by banana leaves powder.

Fig. 4.

Banana leaves particle board.

Measurement method

Particle size measurement

Figure 5 shows the microscopic view of the banana powder particles with 20× optical zoom. The size of the particles is not uniform as it is crushed using a grinder. The size of the particles is in the micrometer range, i.e. from 100 to 500 μm. The size of the grain plays an important role in microwave absorption performance. Small size grain means a higher surface area, more surface atoms, multiple reflections, and thus larger dielectric constant or/and dielectric loss. These lead to the interfacial polarization and multiple scatter which is useful to enhance the microwave absorption performance [Reference Idris, Hashim, Abbas, Ismail, Nazlan and Ibrahim36].

Fig. 5.

Microscopic view of banana leaves powder.

Thickness measurement

The thickness of the sample has an important role in microwave absorption performance. The microwave absorption performance is analyzed by varying the thickness of the rectangular sample. Hence, it becomes very much necessary to measure the sample's thickness. In this study, the digital caliper is used to measure the thickness of the samples. Figure 6 shows the thickness measurement setup using a digital caliper and the measured value is given in Table 1.

Fig. 6.

Thickness measurement using a digital caliper: (a) samples and (b) PEC.

Hardware measurement

Different measurement methods such as OCP method, transmission line technique, free space measurement technique, and resonant cavity technique can be used to determine dielectric properties [37]. In this investigation, the OCP method has been used to measure the dielectric properties of the banana leave powder and the particle board. The sample is placed in close contact with the probe or the probe is pressed against a material under test or immersed into the liquid sample. Therefore, the dielectric properties are calculated from the phase and amplitude of the reflected signal using software embedded in the vector network analyzer (VNA). Before measurement, the dielectric probe is first calibrated with the help of air, a metallic shorting block, and water [Reference Pattanayak, Bachhar and Sahoo38].

In this investigation, R&S ZNB-20 VNA and DAK (Dielectric Assessment Kit) are used to measure the dielectric properties of the banana leaves powder as shown in Fig. 7. The measurement set up is shown in Fig. 8.

Fig. 7.

Dielectric parameter measurement of dried banana leaves to powder using a dielectric probe technique.

Fig. 8.

Measurement setup.

Simulation strategy

The single-layer absorber has been designed in CST Microwave Studio (CST MWS) 2019 3D EM simulation software for high-frequency components. The dimension taken for the simulation is given in Table 1. The material is dispersive and that is why the absorber has been designed and simulated in CST MWS using the 1000 value of dielectric constant and loss tangent at 1000 frequency point in the range from 1 to 20 GHz obtained from the measurement. The CST simulations are carried out under plane wave incidence along with open-add space boundary conditions.