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A Review based on dependence of microwave absorbing materials upon number of layers and thickness of the material

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A Review based on dependence of microwave absorbing materials upon number of layers and thickness of the material Shreya Kaushal, Ramanjyot Kaur Electronics and Communication Department IET Bhaddal Technical University Shreya121089@gmail.com Abstract-This review is essentially based on the results of microwave absorbing materials which are increasing in demand due to their unique absorbing microwave energy and promising applications in the stealth technology of aircraft, television image interference of high-rise buildings, and microwave dark-room and protection. Extensive study has been carried out to develop new microwave absorbing materials with a high magnetic and electric loss. Electromagnetic absorbers are specifically chosen or designed materials that can inhibit the reflection or transmission of electromagnetic radiation. This can be accomplished with materials such as dielectrics combined with metal plates spaced at prescribed intervals or wavelengths. Various parameters such as particular absorption frequencies, thickness, component arrangement and configuration of the materials determine the capabilities and uses of these materials. In this review the effect of single and multilayer types of materials as well as effect of variation in thickness of materials on microwave absorption properties is examined and concluded. I INTRODUCTION Absorbers in the RF/microwave realm are the materials that attenuate the energy in an electromagnetic wave. Absorbers are used in a wide range of applications to eliminate stray or unwanted radiation that could interfere with a system’s operation. Absorbers can be used externally to reduce the reflection from or transmission to particular objects. They can also be used to recreate a free space environment by eliminating reflection in an anechoic chamber. It is possible to describe the interaction of an electromagnetic wave with microwave absorbing materials (RAM’s) as a phenomenon where the electromagnetic energy is transformed into thermal energy. According to the principle of energy conservation, the electromagnetic wave impinging on a material can be reflected, attenuated or transmitted through the material. The response of the material to the wave depends on its intrinsic characteristics [6]. Absorbing material can be of different types, shapes and forms such as: Microwave Absorbing Materials (Foam) • Microwave Absorbing Materials (Rubber) • Microwave Absorbing Materials (Coating) • Ferrite Absorbing Block • Absorbing Putty • Single and multilayer materials Here the last category is taken into account , evaluated and concluded. RAM’S are broadly divided into two types [8]: a) Narrowband or “tuned” RAM is one in which its peak performance is focused to a specific, narrow frequency or frequency band. This type of RAM is more likely used for attenuating the undesired output from radar or other type of system that transmits a narrow frequency spectrum. b) Broadband RAM has its performance spread out over a wide frequency range. This type of RAM is more likely used for simultaneously attenuating the undesired emissions from

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an EW system or several narrowband systems, where attenuation of signal energy across a wide band of frequencies is required. In general, greater attenuation performance (25–30 dB) can be achieved with a tuned RAM, but the performance is available only over a narrow frequency range. A broadband RAM will provide somewhat less attenuation performance (15–20 dB) but will do so over a much wider frequency range[13]. permittivity are farads/meter and the permeability units are henrys/meter. The actual values for most materials can be cumbersome in calculation. For this reason they are usually compared to the permittivity and permeability of a vacuum. These values are Absorber theory: Absorbers generally consist of a filler material inside a material matrix. Absorbers are characterized by their electric permittivity and magnetic permeability. The permittivity is a measure of the material’s effect on the electric field in the electromagnetic wave and the permeability is a measure of the material’s effect on the magnetic component of the wave. The permittivity is complex and is generally written as: 𝜺 ∗ = 𝜺′ – 𝒋 𝜺" The values ε* and μ* then become dimensionless. Since ε is dependent on the dielectric polarization which always opposes the electric field, ε for all materials is greater than that of free space and hence is always greater than 1. Eq 1 [6] The permittivity arises from the dielectric polarization of the material. The quantity ε’ is sometimes called the dielectric constant which is something of a misnomer when applied to absorbers as ε’ can vary significantly with frequency. The quantity ε” is a measure of attenuation of the electric field cause by the material. The electric loss tangent of a material is defined as: 𝑡𝑎𝑛 𝛿𝑒 = 𝜀"/ 𝜀′ Eq 2 [6] The greater the loss tangent of the material, the greater the attenuation as the wave travels through the material. Analogous to the electric permittivity is the magnetic permeability which is written as: 𝜇 ∗ = 𝜇 ′ − 𝑗𝜇" 𝜀𝑜 =8.854x10 12 farads /meter and 𝜇𝑜 = 4 πx10-7henrys /meter [6] L. C. Folgueras et al [1] produced RAM’s in the form of single and multi-layer lightweight thin flexible sheets containing the conducting polymer polyaniline dispersed in a polyurethane matrix. The sheets consisted of 1 to 4 layers of absorbing materials, each having the same electromagnetic properties. Bulk electromagnetic properties of these RAMs were analyzed using the waveguide technique in the frequency range of 8 to 12 GHz (X-Band). Conducting polyaniline was prepared and obtained as a powder. The polyaniline powder was mixed with polyurethane at the proportion of 15% w/w. The attenuation of the incident radiation by the RAMs was obtained from the difference between the reflectivity of an aluminium plate (reference material) and the same aluminium plate coated with the RAMs. The prepared mixture of polyaniline and polyurethane was applied to a substrate as layers. Measurements were carried out considering only the bulk properties of the layered material. Therefore the measured values of permittivity and permeability refer to 1, 2, 3and 4layer materials; the thickness of these layers were 2.2, 2.6, 2.8, and 3.2 mm, respectively. Eq 3 [6] With magnetic loss tangent defined as: 𝑡𝑎𝑛 𝛿𝑚 = 𝜇"/ 𝜇′ Eq 4 [6] The permeability is a measure of the material’s effect on the magnetic field. Both components contribute to wavelength compression inside the material. Additionally, due to the coupled EM wave, loss in either the magnetic or electric field will attenuate the energy in the wave. In most absorbers, both permittivity and permeability are functions of frequency and can vary significantly over even a small frequency range. If the complex permittivity and permeability are known over a frequency range then the material’s effect on the wave is completely known. The units of Fig 1 Relative permittivity of layered [1] materials From Fig 1, it can be observed that although only one type of material (polyaniline dispersed in

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polyurethane) was used, the effect of using layers is to change the bulk property of the RAM, with an average change of about 100% in ε' and ε ".The bulk real permittivity increased with the number of layers, whereas the imaginary permittivity showed significant changes when the number of layer used increased from 2 to 3. Fig 3 Curves of coefficients (in percentages) absorbed (Ea), transmitted (Et) and reflected (Er) energies: RAM thickness, 2.80 mm, silicone matrix L9000 [2] Fig 2 Attenuation of the electromagnetic radiation by the layered RAMs [1] Fig 2 shows, that the material consisting of 3 layers, presented good absorption characteristics about 10.6 GHz. The resonance peaks of the other RAMs consisting of 1 and 2 layers are outside the range of frequencies studied. The 4-layer RAM has a weaker resonance peak at about 8.2 GHz. The 4 RAMs behaved as narrowband absorbers[11]. Luiza de Castro Folgueras et al [2] produced sheets of microwave absorbing materials using conductive polyaniline dispersed in a silicone rubber matrix and to characterize the electromagnetic properties like absorption, transmission and reflection of electromagnetic energy; and electric permittivity and magnetic permeability of these sheets in the X-band (8 – 12 GHz). Variation of the thickness and number of layers leads to alteration of electromagnetic properties of RAM’S. Small samples (about 2 cm) were cut from the sheets and inserted into the waveguide. The complex electromagnetic parameters, (permittivity and permeability) were obtained from the measured values of the S-parameters using commercial software specifically designed for this task. The attenuation of the incident radiation by the RAM’s was obtained from the difference between the reflectivity of an aluminium plate and that of the same aluminium plate covered with the RAM. Fig 4 Curves of coefficients (in percentages) absorbed (Ea), transmitted (Et) and reflected (Er) energies: RAM thickness 4.39 mm, silicone matrix RTV630 [2] S. no Substrate Thickness (mm) 1 Metal Plate Metal Plate Aluminium Plate Aluminium Plate 2.80 Max Energy Absorption (%) 18.5 4.39 16.2 2.80 88 4.39 71 2 3 4 Table 1 Observation from Error! Reference source not found. Fig 3 and Fig 4 [2]

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Fig 7 Simulations of two-layer RAM’s attenuation of electromagnetic radiation as a function of frequency, thickness and stacking order of the absorbing materials. Z1, RAM produce with L9000 silicone rubber; Z2, RAM produce with RTC6300 silicone rubber: a) layers 2 and 8 mm thick [2] Fig 5 Energy absorption of single-layer RAMs as a function of frequency and different layer thicknesses: RAM produced with silicone rubber L9000 [2] Fig 8 Simulations of two-layer RAM’s attenuation of electromagnetic radiation as a function of frequency, thickness and stacking order of the absorbing materials. Z1, RAM produce with L9000 silicone rubber; Z2, RAM produce with RTC6300 silicone rubber: both layers are 5mm thick [2] Fig 6 Energy absorption of single-layer RAMs as a function of frequency and different layer thicknesses: RAM produced with silicone rubber RTV6300 [2] From Fig 5 and Fig 6, it can be seen that there is displacement of resonance peak (maximum absorption) as a function of the thickness. Also, the RAM produced with the silicone rubber L9000 absorbs electromagnetic energy more efficiently. Fig 7 Fig 7 and Fig 8 shows that the order in which these materials are stacked plays an important role in determining the final properties of the RAM, affecting the amplitude and the position of the resonance peak. [9] [15] It can be concluded that the attenuation pattern of electromagnetic energy by the absorbing materials suggests that the electrical conductivity of these materials is related to the quantity of absorbing centres (conducting polyaniline) and type of polymer matrix (silicone rubber), which modify the impedance of absorbing materials. In absorbing materials, a large fraction of the incident energy must be attenuated, which is a consequence of the equilibrium between electric conductivity and electric losses[14]. Mingxia Chen et al [3] Multilayered carbon nanotubes/silicon dioxide (CNTs/SiO2) electromagnetic wave absorbing ceramic matrix composite material was fabricated by hot-pressed sintering. The gradient layer structure was designed to improve its absorbing properties. The multilayered CNTs/SiO2 composite was shaped by adding five kinds of mixed powders with different CNTs content to the graphite die in turn, and its average CNTs content was 5 wt%. Meanwhile, a single layered composite with the same content of

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