STUDY AND DEVELOPMENT OF BROADBAND MICROWAVE ABSORBERS

Abstract

In recent years, there has been a growing global focus on Microwave Absorbing Materials (MAMs), drawing considerable attention for their crucial role in civil and defense areas. It is noteworthy that traditional radar systems were confined to the 8.2-12.4 GHz (X-band) frequency range. In contemporary times, a substantial transformation has occurred, with radar and other electronic systems now operating across a broad frequency range, covering 2 – 18 GHz. This shift underscores the need for innovative solutions. The ideal microwave absorber needs to have multiple key attributes: efficient absorption, thin coating, wide absorption range, and easy synthesis process. Achieving these properties simultaneously is a complex engineering challenge. To create thin and broadband MAMs, researchers have explored composites with various dielectric, magnetic, and conductive components. Notably, most materials developed so far excel in X-band (8.2-12.4 GHz) and Ku-band (12.4-18.0 GHz). However, evolving electronics technology landscape demands coverage in S-band (2-4 GHz) and C-band (4.0-8.2 GHz) also. While significant efforts have been made to enhance the structural and component design of microwave absorbers, the mechanism influencing microwave absorption still has potential for improvement. A key aspect in this endeavor involves gaining a fundamental mechanistic understanding of how materials with diverse geometries interact with microwaves. The influence of different material morphologies on absorption behavior underscores the need for exploration. A systematic examination of these intricacies is expected to yield internal insights into the complex interplay among the components of microwave absorbers and their subsequent structural design. This detailed understanding of the underlying mechanism of microwave absorption can serve as the foundation for developing broadband microwave absorbers. Thus, this thesis incorporates a comprehensive investigation into the understanding of the microwave absorption mechanism, with the objective of developing broadband microwave absorbers. Researchers face a significant challenge in developing MAMs with the right balance of permittivity, permeability, and conductivity, having low coating thickness and broad absorption bandwidth. Microwave absorption heavily depends on the magneto-dielectric properties of materials. The real parts of complex permittivity and permeability (ε' and μ') signify the efficiency of storing electric and magnetic energy, while the imaginary parts (ε" and μ") represent energy loss. By carefully considering various factors influencing absorption, along with incorporation of specialized morphological materials, this thesis endeavors to develop broadband microwave absorbers. In contrast to complex and time-consuming methods employed in previous research endeavors, this thesis leverages the simplest ball milling technique. This approach facilitates the facile modification of nanocomposite morphology. A common challenge faced in the realm of MAMs is the inherent trade-off between absorption bandwidth and coating thickness. The MAMs developed in this study overcome this limitation by achieving a balance between wide absorption bandwidth and thin coating thickness. Moreover, this research addresses the scarcity of MAMs excelling in the low-frequency range, as most existing materials for low-frequency absorption tend to have narrow bandwidths. The nanocomposite synthesized in this work exhibits broad bandwidth in the low-frequency range, covering the L, S, C, and X bands. To further improve the absorption performance of developed materials electromagnetic (EM) techniques like Frequency Selective Surfaces (FSS) and multi-layering were employed to enhance microwave absorption bandwidth. The primary objective of this thesis is to develop MAMs by comprehensively understanding the microwave absorption mechanism. By carefully considering various factors influencing absorption and incorporating specialized morphological materials, this thesis aims to create broadband microwave absorbers. Additionally, the utilization of EM techniques to achieve enhanced microwave absorption with less coating thickness in the 2 – 18 GHz frequency range has been explored. The main objective of this research work is the development of broadband microwave absorbers by understanding the underlying absorption mechanism and employing advanced EM techniques for various applications, which has been obtained by following tasks: Task 1: Study the effect of particle size and nanomaterials content on microwave absorption. Subtask 1: To study the effect of particle size and 1D MWCNTs ratio on microwave absorption of Co. Subtask 2: To study the effect of particle size and 0D BFO nanoparticles ratio on microwave absorption of CI/BFO Task 2: Effect of ball size heterogeneity on microwave absorption properties of cobalt. Task 3: Design of wideband microwave absorbers with the application of FSS and multilayering. Subtask 1: Development of an analytical approach to design FSS absorber as per user requirement. Subtask 2: Study of multilayered structures for microwave absorption bandwidth enhancement. This thesis consists of seven chapters which are briefly outlined as follows: Chapter 1 serves as the foundational introduction to the thesis. It outlines the core motivation driving the research, highlights the specific problem statement being addressed, and introduces the concept of MAMs. The chapter proceeds to delve into absorption mechanism, discussing the underlying physics of absorption. Furthermore, EM techniques for absorption bandwidth enhancement are introduced. It also explains the principles of FSS and multi-layering. Chapter 2 provides a literature review with a comprehensive current state of research in the field of MAMs. The chapter begins by presenting a historical perspective on the development of MAMs. It then delves into a thorough review of the current state of research in the field, identifying the research gaps. Furthermore, the literature review on the effect of various parameters on microwave absorption is explored, emphasizing the importance of particle size, morphology, and morphology in microwave absorption characteristics. Additionally, the literature review encompasses FSS based microwave absorbers and multilayered microwave absorbers, shedding light on the latest research and developments and research gaps in these areas. Chapter 3 is dedicated to the synthesis, characterization, and measurement techniques related to MAMs. The chapter begins with an introduction, setting the stage for a detailed exploration of the synthesis processes employed in developing MAMs. Various methods are discussed, including the solid-state reaction method, sol-gel method, hydrothermal method, and mechanical alloying (ball milling). Each method plays a significant role in tailoring the properties of MAMs. Additionally, Chapter 3 delves into the characterization and measurement techniques used to evaluate MAMs. These techniques include X-Ray Diffraction (XRD) for analyzing crystalline structures. Field-Emission Scanning Electron Microscopy (FESEM) and High-Resolution Transmission Electron Microscopy (HRTEM) for visualizing material morphology and microstructure of developed MAMs. The Brunauer-Emmett-Teller (BET) analysis helps determine the surface area and porosity of MAMs, which is crucial for understanding their surface properties. Vibrating Sample Magnetometer (VSM) is utilized to investigate the magnetic characteristics of MAMs. The chapter also highlights microwave absorption measurement techniques, which encompass the waveguide method, coaxial airline method, and free space measurement that are used to assess the microwave absorption properties of MAMs Chapter 4 is dedicated to the development of broadband MAMs by exploring the microwave absorption mechanism and investigating the effect of particle size and different dimensional nanomaterials content on microwave absorption. The first subtask of this chapter includes the study of particle size and one-dimensional multiwall carbon nanotubes (1D MWCNTs) content on microwave absorption properties of Cobalt (Co). The selection of Co is based on its distinctive characteristics including large saturation magnetization, high magnetic loss, and significant permeability in the GHz frequency range, making it a noteworthy candidate for microwave absorption. However, pure Co materials have inherent limitations, including weaker absorption capabilities due to single type of losses. Two effective strategies are being explored to overcome these challenges and enhance the microwave absorption performance of Co-based materials. The first approach involves designing Co absorbers with specific microstructural features, low density, and a large surface area, to regulate EM parameters and achieve impedance matching. This is achieved through a simple and cost-effective ball milling process, which tailor’s particle size and shape, introducing structural changes that enhance dielectric relaxation and induce multi-interfacial polarization. Moreover, this method shows promise in producing bulk materials suitable for practical applications, enabling convenient particle size and morphology adjustments to achieve desired microwave absorption properties. The second approach focuses on mixing Co particles with dielectric materials, like 1D MWCNTs. These 1D structures offer unique advantages for microwave absorption enhancement, their special properties, including a high aspect ratio, exceptional carrier mobility, and high current-carrying capacity, make them effective at absorbing microwaves and converting them into other forms of energy through conductance and electric polarization. The composite of Co and 1D MWCNTs leverages the advantages of both components, with enhanced dielectric and magnetic losses, improved impedance matching, and an expanded bandwidth. The effects of different ball milling times (5h, 10h, 15h, and 20h) and varying loading ratios of MWCNTs (1 wt%, 2 wt%, 3 wt%, 4 wt% and 5 wt%) on the microwave absorption properties of Co are meticulously studied. This study aims to gain a profound understanding of the microwave absorption mechanism, particularly as it relates to changes in particle size and shape caused by different milling times. Milling of raw Co powder is conducted using a high-energy mechanical ball mill for varying durations, and the microwave absorption performance is comprehensively evaluated across the 2-18 GHz frequency range. The results highlight significant improvements in the microwave absorption characteristics of Co after milling, with enhancements further observed as different wt% of MWCNTs are introduced. The interfacial polarization and multiple reflections induced by MWCNTs within milled Co contribute substantially to microwave attenuation. The Co/MWCNTs composite samples exhibit broad bandwidth while maintaining a low coating thickness. Notably, a highly effective bandwidth of 10 GHz (ranging from 3.73-13.73 GHz) is achieved for 20 h milled Co sample mixed with 2 wt% MWCNTs, with a coating thickness of 2.5 mm. The good microwave absorption in developed Co/MWCNTs nanocomposite samples can be attributed to the various possible mechanism like dielectric losses from surface defects, conductive networks, synergistic effects of dielectric and magnetic losses, morphological effects causing wave interference, and interfacial polarization, making the composite highly effective for microwave absorption. This study provides insights into the design, synthesis, and performance of Co-based MAMs with enhanced microwave absorption capabilities, driven by the special properties of MWCNTs' morphology. The microwave absorption obtained using Co/MWCNTs showed the absorption bandwidth in C and X bands. However, no coverage in the L and S bands, prompting the next subtask of Chapter 4: expanding absorption to include these lower frequencies. This strategic shift recognizes the need for comprehensive coverage, focusing on materials for broad absorption bandwidth in the L, S, C, and X bands. Thus, the second subtask of Chapter 4 is focused on the development of broadband MAMs covering the low-frequency range (L, S, C, and X-bands). This involves understanding the microwave absorption mechanisms and studying how different milling durations and the introduction of zero-dimensional barium ferrite (0D BFO) nanoparticles affect the microwave absorption capabilities of Carbonyl Iron (CI). Dielectric-based MAMs typically excel in X- and Ku-bands due to their polarization properties. In contrast, magnetic materials and ferrites prove to be efficient microwave absorbers in lower frequency ranges. Their distinctive magneto-crystalline anisotropy confers a significant advantage in achieving high absorption in lower frequency bands. Among these magnetic materials, CI garners substantial attention due to its unique attributes, including high thermal stability, saturation magnetization, and dielectric constant values. However, pure CI materials exhibit limitations stemming from high density and restricted absorption due to a single loss mechanism. This prompts the exploration of optimization strategies to increase CI powder's microwave absorption performance. One such avenue involves the incorporation of CI with other materials, each contributing distinct microwave loss mechanisms. CI particles with flake-like morphology show promise as good absorbing materials since they can overcome the Snoek limit and also due to increased shape anisotropy. However, flake-like CI particles present a challenge due to their significantly high permittivity values, primarily attributed to increased interface polarization. This, in turn, results in poor impedance matching and consequently narrow absorption bandwidth. To address these challenges, various additives, including metal oxides, carbon compounds, ferrites, and ceramics, are often employed to broaden the absorption bandwidth and enhance impedance matching in CI-based materials. Among these additives, hexaferrites, particularly barium hexaferrite (BFO) nanoparticles, emerge as promising candidates for microwave absorption within the GHz range. BFO particles possess high magnetic anisotropy, high permeability values, strong magnetization, chemical stability, and ease of processing. Additionally, research has demonstrated that reducing particle size enhances EM interaction, rendering BFO nanoparticles an adaptable and effective option for microwave absorption. In contrast to more complex and time-consuming methods utilized in prior studies for developing microwave absorbers with varying morphologies, simple high-energy ball milling technique was employed in this work. This method induces physical and chemical changes, including particle size reduction, defect generation, alloying, and interfacial polarization, all of which may collectively enhance microwave absorption characteristics. By altering CI particle morphology (through varied milling time – 5h, 10h, 15h and 20h in CI/10%BFO nanocomposite) and by varying the quantity of BFO nanoparticles (2%, 6%, and 10% in 15h milled CI/BFO nanocomposite), adjustments in EM parameters were made, resulting in improved impedance matching and significantly enhanced microwave absorption. The microwave absorption performance of the developed samples was investigated across a frequency range 0.5 to 18 GHz, with a specific focus on studying the effect of different BFO nanoparticle loading ratios on microwave absorption characteristics. The modified morphology of the CI/BFO nanocomposite resulted in improvement in absorption bandwidth. Specifically, for a 15h milled sample containing 10 wt% of BFO nanoparticles, broad bandwidth (RL < − 10 dB) covering 9.15 GHz within the 1.9-11.05 GHz frequency range was achieved, all while maintaining a thin coating thickness of 2.5 mm. The CI/BFO nanocomposite demonstrates good performance, offering a broad bandwidth in the low-frequency range, encompassing L, S, C, and X bands. The microwave absorption mechanism in CI/BFO nanocomposites involves a synergistic interaction of dielectric and magnetic losses. This synergy is driven by strong magnetic coupling interactions, breaking the Snoek limit, and enhancing magnetic loss in lower frequency bands. Chapter 5 explores the complex interaction of various factors influencing microwave absorption and highlights the crucial role played by altering material morphology in achieving the optimal results. Extensive research has explored the profound impact of size, morphology, and material structure on microwave absorption characteristics. Ball milling is a commonly employed method for synthesizing MAMs. The effect of its various parameters for their role in shaping material properties has been extensively analyzed by various research groups. Various ball milling parameters, including rotation per minute, ball-to-powder ratio, milling media, and milling duration, have been studied extensively by research groups worldwide for their influence on microwave absorption. However, one crucial milling parameter, ball size heterogeneity, has largely remained unexplored area in the realm of research. Chapter 5 covers the often-overlooked milling parameter, i.e., ball size heterogeneity and investigates its effect on microwave absorption performance. Researchers commonly utilize same-sized milling balls as ball milling media. However, using different-sized milling balls results in the maximum collision energy, which leads to the production of finer particles. Therefore, to randomize the effects of the balls, it is advantageous to utilize a mixture of smaller and bigger milling balls. In this Chapter, the effect of ball size heterogeneity on the microwave absorption performance of Cobalt (Co) has been investigated for the first time. An attempt has been made to explore how the use of different-sized milling balls changes the structural, magnetic, and dielectric properties of Co and how these modifications affect the material's microwave absorption capabilities. The ball milling of commercially available Co powder was performed at various times (5h, 10h, 15h, and 20h) using different-sized milling balls (3 mm and 5 mm), and the results obtained were compared to those using same-sized milling balls (3 mm). The results show that by using different-sized milling balls, the particle size distribution in 0.1-1 μm range has been significantly increased, clearly indicating that much finer particles are obtained using different-sized milling balls. The value of relative permittivity also drastically increases due to the changes in particle size and morphology by using different-sized milling balls. A significant change in the microwave absorption characteristics of Co milled using different-sized milling balls has been observed. For a 15h milled Co sample using different-sized milling balls, the effective bandwidth was increased to 9.65 GHz (4.75-14.4 GHz) compared to no absorption bandwidth when using same-sized milling balls, with the same coating thickness of 2.0 mm. Thus, this work provides new insights into the preparation of materials for microwave absorption applications employing the ball milling method. Chapter 6 is divided into two subtasks. The first subtask focuses on the development of an analytical approach to design FSS absorber as per user requirement. When integrated with composite materials, FSS absorbers enhance absorption characteristics, including reflection loss and bandwidth, due to their combined attributes of frequency selectivity, dielectric, and magnetic losses. Moreover, FSS structures are lightweight and thin, improving absorption properties while reducing thickness and enhancing physical strength. While numerous studies exist on the development of FSS absorbers that are created by imprinting FSS structures on composite materials. The right FSS design parameters for specific absorbing materials remains challenging. The existing simulation techniques, such as parametric analysis, are time-consuming and computationally complex, emphasizing the pressing need for an analytical approach to design FSS structures according to user-defined parameters and specifications. In light of these considerations, Chapter 6 presents a proof of concept demonstrating the design of FSS structure aligned with user requirements. The developed analytical approach-based methodology enables users to obtain desired results from user-defined parameters. This chapter showcases a cross-dipole as an example, analyzed parametrically, and details its EM response using an equivalent circuit model (ECM), where circuit parameters (L, C) depend on FSS geometry. The model takes user-defined substrate data and provides FSS structure geometrical parameters to achieve the user-defined absorption bandwidth. The microwave absorption results of FSS absorber obtained using the developed approach were then compared with full-wave simulations results, displaying alignment with the design model. An assessment of the developed design indicates a root mean square error of 0.47, primarily attributed to substantial discrepancies at the frequency band edges. Additionally, the comparative analysis of microwave absorption with frequency is presented for both FSS-imprinted absorbing material and without FSS imprinted-absorbing material, using user-defined design FSS parameters. The effective bandwidth, initially at 5.41 GHz (11.38-16.72 GHz) when only absorbing material was used was increased to 10 GHz (8-18 GHz) with the incorporation of FSS structure featuring user-defined design parameters on an absorbing material with same coating thickness of 2.5 mm. This innovative design model effectively addresses the computational complexities and costs that conventional methods typically encounter. In second subtask of Chapter 6, further advancements were made for enhancing the bandwidth of the microwave absorbers developed in Chapter 4. This was achieved by exploring the potential of a multi-layering technique, which involves arranging microwave-absorbing materials layer by layer. The aim was to improve the absorption capabilities of single-layer absorbers by achieving good absorption, reducing coating thickness, and increasing the bandwidth. The primary mechanism behind multi-layer absorbers lies in the front layer, known as the matching layer, which contributes to impedance matching and decreases coating thickness due to its limited absorbing capacity. Meanwhile, the absorbing layers (inner layers) are responsible for dissipating the maximum amount of incident microwave energy. In this work, a triple-layer microwave absorber was chosen as an example of a multi-layer absorber, aiming to explore its potential for increasing the bandwidth of developed samples. To optimize and design these multi-layered microwave absorbers effectively Genetic Algorithm (GA) can be used. GA is a powerful tool for navigating complex design spaces, enabling to identify optimal solutions among various design options for multi-layered absorbers, resulting in highly efficient absorber designs. In Chapter 6, the GA approach was used to develop triple-layer microwave absorber with low thickness and broad absorption bandwidth. The approach involved systematically selecting design parameters, including material composition, layer sequence, and thickness, to achieve the desired performance. To facilitate the design process, the database that included samples prepared in Chapter 4 as input for GA optimization was used. This database consisted of two sets: Set Ⅰ, which comprised 25 samples of Co/MWCNTs nanocomposites study, and Set Ⅱ, which included 7 samples of CI/BFO nanocomposite study. The results obtained through the multi-layering technique demonstrated reduced thickness, an ultra-wide bandwidth, and enhanced absorption compared to the single-layer absorbers discussed in Chapter 4. For Co/MWCNTs based sample, when the first layer is taken as 5W4, the second layer as 10W1, and the third layer as 20W2, with thickness 0.43 mm, 0.98 mm, and 0.90 mm, respectively, the effective bandwidth of 13.65 GHz is obtained in the frequency range 4.35-18 GHz. Similarly, for CI/BFO based sample, for layer combination CI+2%BFO:15h, CI+10%BFO:5h, and CI+10%BFO:15h broad absorption bandwidth of 15 GHz (3-18 GHz) was obtained when the thickness of each layer is 1.26 mm, 0.31 mm, and 0.20 mm (total thickness of 1.77 mm). The results obtained through the multi-layering approach exhibit significantly improved absorption, ultra-wide bandwidth, and reduced thickness compared to the single-layer absorbers. The absorption mechanism of the triple-layer microwave absorber shows that the improved microwave absorption in this type of absorber could be due to the synergistic influence of several factors. These factors include discontinuity, multiple internal reflections, and phase cancellation phenomena encountered by incoming microwaves within the multilayered structure. The computed RL results for the triple-layer microwave absorber using GA were validated using the HFSS software, demonstrating a strong agreement between the results obtained from both approaches. Chapter 7 presents the concluding remarks and a glimpse into future prospects of the present work. The 20h wet-milled Co and 2 wt% MWCNTs composite sample prepared in Chapter 4 shows maximum absorption bandwidth of 10 GHz within the frequency range of 3.73-13.73 GHz, with a low coating thickness of 2.5 mm. Additionally, the CI+10%BFO:15h sample, with a coating thickness of 2.5 mm, demonstrated an effective bandwidth of 9.15 GHz, covering the lower frequency bands within the range of 1.9-11.05 GHz. Also, an analytical approach for designing FSS absorbers tailored to user specifications was developed in Chapter 6. This approach eliminates the trial-and-error processes, thereby reducing computational complexities. Moreover, further bandwidth enhancement of developed samples was done by exploring multi-layering techniques. The GA optimized results for CI/BFO nanocomposites based triple layer absorbers showed an absorption bandwidth of 15.12 GHz (2.88-18 GHz). The optimized thickness of each layer was 0.75 mm, 0.56 mm, and 0.46 mm (total thickness of 1.77 mm). The potential for future research endeavors is also explored in this Chapter. Various possibilities are discussed for further advancing the field and addressing emerging challenges.