Radar absorbing structures using periodic pattern fabric implemented through photolithography

Abstract

In this study, various types of radar absorbing structures (RAS) were implemented by utilizing the photolithography process. The conventional RAS has been implemented through a method of dispersing a lossy material in a matrix material of a composite material, a method of using a lossy fiber, and a method of printing a conductive pattern on a polymer substrate. However, conventional methods have limitations such as increase in viscosity of matrix material, difficulty in controlling electromagnetic properties, and deterioration of mechanical properties. Therefore, in this study, in order to overcome these limitations, a method for implementing a RAS with improved mechanical properties and easy control of electromagnetic properties was studied. In this study, a periodic pattern fabric (PPF) having a desired shape was realized by applying a photolithography process to a electrolessly nickel plated glass fiber fabric (Ni-glass). And using this PPF, an CA-absorber and an pixelated PPF were implemented. First, in Chapter 2, a photolithography process was applied to the Ni-glass fabric. Based on the conventional photolithography process, photoresist (PR) film adhesion, exposure, etching, and peeling processes were performed in this order. And a parameter study was performed for each process parameter required for the application of photolithography on Ni-glass fabric. In particular, in the process of attaching the PR film, a process of attaching the PR film using an autoclave has been proposed. This process allowed the effective adhesion of the PR to textile materials with curved surfaces. Therefore, photolithography process parameters have been established, and RAS can be implemented using Ni-glass fabric through these process parameters. Next, in Chapter 3, a CA-broadband RAS with improved mechanical properties was implemented. Conventionally, in order to implement a CA-absorber, a method in which a conductive pattern is printed on a polymer-based printed circuit board material has been used. However, these polymer-based substrate materials cause deterioration of mechanical properties when manufacturing composite materials. Therefore, using the process established in the previous chapter, a PPF in which a loop-type conductive pattern is repeated was manufactured. And a RAS with -10 dB absorption band in part of C-band and X-band was designed through genetic algorithm technique. Three-point bending shear and tensile tests were performed to measure the mechanical properties. As a result, the proposed RAS showed -10 dB bandwidth of the 6.3 GHz (6.7 ~ 13 GHz), and showed superior mechanical properties without degradation of interlaminar shear strength and tensile stiffness compared to previous studies. Finally, in Chapter 4, a sandwich RAS with light and broadband absorption performance was implemented with a pixelated pattern implemented through a foam core and a genetic algorithm. In the conventional CA-absorber, simple patterns such as patches, loops, and crosses were used. However, these patterns have been confirmed to have limitations in their electromagnetic wave absorption performance through many studies. Laminate materials, such as glass fiber reinforced composites, also increase weight due to their high thickness when used as spacers for broadband absorbers. Therefore, in this study, thin and light broadband radar RAS with -10 dB bandwidth in C-band and X-band using pixelated patterns were proposed and implemented. The pixelated pattern and the structure of the RAS were designed using a design tool of the CST software. And the designed pattern was fabricated to a pixelated PPF through a photolithography etching process with the advantage of sophisticated patterning. Sandwich pixelated PPF-RAS was fabricated using pixelated PPF, GFRP, foam core, and Ni-glass fabric for PEC. The fabricated sandwich pixelated PPF-RAS had a -10 dB bandwidth at 5.8–12.4 GHz. The simulation result is expected to have -10 dB bandwidth in the 5–13 GHz band. This absorption performance has superior broadband absorption performance compared to previous studies in terms of thickness and weight. In addition, mechanical properties were measured through a four-point bending test. As a result of the measurement, there was no deterioration in structural properties compared to the conventional composite sandwich structure. Therefore, it was confirmed that it has superior structural reliability as well.