Namkyu Lee1
Yonsei University1
Camouflage is an innate behavior observed in creatures that allows them to blend in with their surroundings, thereby increasing their chances of survival by evading predators [1]. The artificial camouflage materials is emerging from the inspiration from this natural phenomenon. These materials can be designed to exhibit specific electromagnetic properties across various domains such as acoustics, microwave, infrared (IR) waves, visible light, and ultraviolet light, depending on their intended applications. Among these domains, IR wave manipulation has gained significant attention due to its relevance in energy and military sectors [2-5].<br/><br/>Micro-nano structures show great promise in overcoming the limitations of conventional materials and realizing the designed properties for camouflage materials. However, these structures are susceptible to external stress such as mechanical or chemical, which can lead to a sudden degradation in material performance, making them challenging to use in practical applications. For this reason, various strategies have been explored, including chemical treatment, the implementation of hierarchical structures, and thin-film coatings for enhancing durability.<br/><br/>However, directly applying conventional methods to improve durability in the context of manipulating electromagnetic waves poses challenges. This is because electromagnetic properties are highly sensitive with residues on the surface, which can generate additional resonances and opaqueness in the target wave, resulting in different material performance. To meet requirements of IR camouflage materials, a polyimide nanofilm is used as a dielectric layer firstly. In the previous work, the polyimide nanofilm exhibits flexibility, anomalous dispersions, and, in particular, satisfies the IR camouflage requirements in the absorbed band based on atmospheric transmittance. Moreover, polyimide is chosen for its excellent mechanical properties and thermal stability. Based on these characteristics and optical properties, we utilize the nanofilm coating of polyimide on the structures, which can enhance mechanical durability.<br/><br/>In this study, we present durable camouflage materials specifically designed for IR radiative energy [6]. We coated ordinary metal-dielectric-metal (MDM) structures with a polyimide nanofilm to improve their mechanical durability. The electromagnetic performance of these durable materials was evaluated through measurements using Fourier transform infrared spectroscopy (FT-IR) and simulations using COMSOL Multiphysics. To check the mechanical durability, two methods, namely manual brushing and the root cause analysis (RCA) test, were employed for comparing conventional camouflage materials. Finally, the IR radiative energy was assessed by IR camera and calculation. We demonstrate that the mechanical stress doesn’t change the IR camouflage performance.<br/><br/><br/>References<br/>[1] Barnett, James B., et al. "Imperfect transparency and camouflage in glass frogs." <i>Proceedings of the National Academy of Sciences</i> 117.23 (2020): 12885-12890.<br/>[2] Kim, Taehwan, et al. "Hierarchical metamaterials for multispectral camouflage of infrared and microwaves." <i>Advanced Functional Materials</i> 29.10 (2019): 1807319.<br/>[3] Lee, Namkyu, et al. "Multiple resonance metamaterial emitter for deception of infrared emission with enhanced energy dissipation." <i>ACS applied materials & interfaces</i> 12.7 (2020): 8862-8869.<br/>[4] Lee, Namkyu, et al. "Transparent metamaterials for multispectral camouflage with thermal management." <i>International Journal of Heat and Mass Transfer</i> 173 (2021): 121173.<br/>[5] Lee, Namkyu, et al. "Flexible Thermocamouflage Materials in Supersonic Flowfields with Selective Energy Dissipation." <i>ACS Applied Materials & Interfaces</i> 13.36 (2021): 43524-43532.<br/>[6] Lee, Namkyu, et al. "Durable camouflage materials by polyimide nanofilm with thermal management." <i>Applied Surface Science</i> 608 (2023): 155107.