Camouflage is an emerging application of metamaterials owing to their exotic electromagnetic radiative properties. Based on the use of a selective emitter and an absorber as the metamaterials, most reported articles have suggested the use of single‐band camouflage, however, multispectral camouflage is a challenging issue owing to a difference of several orders of magnitude in the unit cell structure. Herein, hierarchical metamaterials (HMMs) for multispectral signal control when dissipating the absorbed energy of microwaves through the selective emission of infrared (IR) waves from the unit cell structure of the HMM are demonstrated. Integrating an IR selective emitter (IRE) with a microwave selective absorber, multispectral signal control with the large‐sized unit cell structures of up to 10 cm are realized. With an IRE, the emissive power from the HMM toward 5–8 µm is 1570% higher than the Au surface, which is preventing the occurrence of thermal instability. Furthermore, we determine that the signature levels of targeted IR waves (8–12 µm) and microwaves (2.5–3.8 cm) are reduced by up to 95% and 99%, respectively, when applying the HMM.
Camouflage is a method evading predators in nature by assimilating into the environment. To realize an artificial camouflage surface for displays and sensors, many researchers have introduced several concepts including a metamaterial-selective absorber/emitter (MSAE). When an MSAE is adopted for camouflage at infrared (IR) wave, the energy dissipation of reduced emitting energy, as well as the reduction of emitting energy to deceive the IR signature from the surface, must be considered from the viewpoint of energy balance due to thermal instability. The integrated investigation of radiative heat-transfer characteristics and IR signature control of MSAE remains, however, poorly understood. Here, we investigate MSAE for IR camouflage by considering the energy balance in terms of reduction of emitting energy and dissipation of reduced emitting energy. On the basis of the atmospheric transmittance at an IR band, we designate the detected band as having wavelengths of 3–5 and 8–14 μm and the undetected band as having a wavelength of 5–8 μm. We investigate, via experiments and simulations, the optical characteristics required for IR camouflage and extract the factor that controls the emissive power. Furthermore, we suggest an integrated factor for evaluating the camouflage performance based on the concept of energy balance and propose a design guideline for MSAE with the aim of maximizing the camouflage performance at the IR band. This study will help to expand the range of applications (such as energy harvester and sensors) and others that are based on selective absorption/emission.
In article number https://doi.org/10.1002/adfm.201807319, Hyung Hee Cho and co‐workers successfully demonstrate a hierarchical metamaterial (HMM) for multi‐spectral control. The intermediate dielectric layer enables the integration of the microwave absorber and infrared selective emitter. The HMM shows not only the signature control against the microwave and infrared spectra, but also the dissipation of the reduced emitting energy through the undetected band of infrared waves.
Enhancing the critical heat flux (CHF), which is the capacity of heat dissipation, is important to secure high stability in two-phase cooling systems. Coolant supply to a dry hot spot is a major mechanism to prevent surface burn-out for enhancing the CHF. Here, we demonstrate a more ready supply of coolant using aligned silicon nanowires (A-SiNWs), with a high aspect ratio (>10) compared to that of conventional random silicon nanowires (R-SiNWs), which have a disordered arrangement, for additional CHF improvement. We propose the volumetric wicking rate, which represents the coolant supply properties by considering both the liquid supply velocity and the amount of coolant (i.e., wicking coefficient and wetted volume, respectively). Through experimental approaches, we confirm that the CHF is enhanced as the volumetric wicking rate is increased. In good agreement with the fabrication hypothesis, A-SiNWs demonstrate higher coolant supply abilities than those of R-SiNWs. The longest (7 μm) A-SiNWs have the highest volumetric wicking rate (25.11 × 10 mm/s) and increase the CHF to 245.6 W/cm, which is the highest value obtained using nanowires among reported data (178 and 26% enhanced vs unmodulated plain surface and R-SiNWs, respectively). These well-aligned SiNWs can increase the CHF significantly with efficient coolant supply, and it can ensure high stability in extremely high thermal load systems. Moreover, our study provides nanoscale interfacial design strategies for further improvement of heat dissipation.
Artificial camouflage surfaces for assimilating with the environment have been utilized for controlling optical properties. Especially, the optical properties of infrared (IR) camouflage materials should be satisfied with two requirements: deception of IR signature in a detected band through reduced emissive energy and dissipation of reduced emissive energy for preventing thermal instability through an undetected band. Most reported articles suggest the reduction of emissive energy in the detected band; however, broadband emission for enough energy dissipation through the undetected band simultaneously is still a challenging issue. Here, we demonstrate the multiresonance emitter for broadband emission with IR camouflage utilizing the electromagnetic properties of dielectric material. We reveal that the interaction between the magnetic resonance and dielectric layer’s property in a metal–dielectric–metal structure induces the multiple resonance at the specific band. We present an IR camouflage behavior of multiresonance emitter on a curved surface through the IR camera (8–14 μm). We evaluate the energy dissipation in the undetected band, which is 1613% higher than metal and 26% higher than conventional selective emitters. This study paves the way to develop broadband emitters for radiative cooling and thermophotovoltaic applications.
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