Flexible Thermocamouflage Materials in Supersonic Flowfields with Selective Energy Dissipation

Abstract

Camouflage refers to a creature’s behavior to protect itself from predators by assimilating its signature with the environment. In particular, thermal camouflage materials in the infrared (IR) wave are attracting interest for energy, military, and space applications. To date, several types of camouflage materials such as photonic crystals and metal–dielectric–metal structures have been developed. However, flexible camouflage materials still face challenging issues because of the material’s brittleness and anomalous dispersion. Herein, we propose flexible thermocamouflage materials (FTCM) for IR camouflage on an arbitrary surface without mechanical failure. Without using a polymer as a dielectric layer, we realized FTCM by changing the unit cell structure discretely. By imaging methods, we verified their flexibility, machinability, and IR camouflage performance. We also measured and calculated the spectral emissivity of FTCM; they showed electromagnetic behavior similar to a conventional emitter. We quantified the IR camouflage performance of FTCM that the emissivity in the undetected band (5–8 μm) is 0.27 and the emissivity values in detected bands are 0.12 (3–5 μm) and 0.16 (8–14 μm) in the detected bands, respectively. Finally, we confirmed the IR camouflage performance on an arbitrary surface in a supersonic flowfield. FTCM are expected to help to improve our basic understanding of metamaterials and find widespread application as IR camouflage materials.

Flexible Assembled Metamaterials for Infrared and Microwave Camouflage

“…[ 37,45 ] Hence, the structure of IR camouflage materials should be changed, such as disconnecting the brittle part to reduce the mechanical stress. [ 42 ] As a dielectric layer, silicon nitride (Si 3 N 4 ) was adopted for FAM. The transparent material, such as ZnS [ 46 ] or Al 2 O 3 , [ 47 ] is generally used for IR camouflage materials because its lattice vibration in IR band induces additional resonances.…”

“…[ 37 ] It implies that the flexible IR camouflage material with the dielectric layer not to induce multi‐resonances should be assembled to the flexible microwave absorber (FMWA) separately. Disconnecting the brittle layer as the engineering solution to reduce the mechanical stress leads to the flexible structure for arbitrary surfaces [ 42 ] similar to the conventional IR camouflage materials. Based on these materials, we change the function of flexible IR camouflage materials in having IR camouflage and microwave transparency, then multispectral camouflage materials can be realized by assembling each IR and microwave camouflage material.…”

Multispectral Optical Confusion System: Visible to Infrared Coloration with Fractal Nanostructures

“…As a countermeasure to novel detection systems, many researchers have studied various camouflage materials whose electromagnetic (EM) behaviors can be manipulated in particular spectral ranges. Because visible colors are generated by the reflection of visible light and infrared signatures are obtained from surface emissions based on temperature and emissivity, color filters with subwavelength structures have been studied extensively for controlling visible colors ,− as well as photonic or plasmonic resonators for controlling infrared signatures. − …”

“…Additionally, to address the EM properties in a multispectral range, several artificial materials have been studied, such as thin-film stacks, , hierarchical materials combining two or more materials working in different spectral ranges, − and metal–dielectric-metal (MDM) structures having broad tunable ranges. , Based on these materials, few recent studies have reported on multispectral camouflaging (e.g., visible to microwave, visible to infrared, ,,, and infrared to microwave) with the aim of lowering signature levels by assuming a constant-intensity background. − , …”

“…22,23 Based on these materials, few recent studies have reported on multispectral camouflaging (e.g., visible to microwave, 18 visible to infrared, 9,22,24,25 and infrared to microwave 21 ) with the aim of lowering signature levels by assuming a constant-intensity background. [13][14][15][16]22 However, considering the characteristics of the aforementioned state-of-the-art imagery detectors, signature lowering is not always effective owing to the size contrast between the signature pattern of the target and surrounding background, independent of the low signature level. 26 Thus, optical confusion strategy is essential over entire spectral ranges like the conventional visible camouflage pattern, which assimilates both colors and patterns with the surroundings by creating pixelated patterns.…”