Metafilms for visible and infrared compatible camouflage of high-temperature targets

Abstract

With the rapid development of multispectral detection, infrared and visible compatible camouflage becomes necessary. Metafilms with dielectric/metal/dielectric (D/M/D) structures can be highly transparent in visible band (380 ∼ 780 nm) and highly reflective in infrared atmospheric windows (3 ∼ 5 μm, 8 ∼ 14 μm). The metafilm can be deposited on the equipment surface, and the high visible transmittance can make the original camouflage coating continue to achieve visible camouflage, while the low infrared emissivity can inhibit the infrared signal to achieve infrared camouflage. Compatible camouflage is urgently needed by high-temperature targets such as exhaust pipes and engine cabins. Therefore, the thermal stability of multilayer structure is very important. In this study, a D/M/D-structured metafilm with improved thermal performance is proposed. Al-doped zinc oxide (AZO) is selected as the material of the dielectric layers due to good thermal stability, and high visible transmittance is realized through the mechanism of admittance matching. Ag is selected as the material of the metal layer to increase infrared reflectance. The metafilm with the structure of AZO/Ag/AZO is rigorously designed and fabricated. The results from Fourier transform infrared spectrometer and spectrophotometer show that the integrated visible transmittance and infrared emissivity at room temperature is higher than 0.87 and lower than 0.05, respectively. The camouflage performance of the metafilm is demonstrated on a flexible polyethylene terephthalate (PET) substrate. The camouflage performance of metafilm samples at 20 ∼ 140 °C is tested on a model cabin. The metafilm does not affect the original camouflage coating, so it can achieve visible camouflage. The radiation temperature of the metafilm is approximately 80 °C lower than that of the control surface, and the infrared signature is significantly attenuated. In order to further investigate the thermal stability and thermal fatigue resistance of the metafilm, metafilm deposited on quartz substrate is continuously heated and periodically heated at different temperatures. It is found that the sample can withstand continuous heating at 450 °C for 4 h or repeated heating for 20 cycles. SEM (scanning electron microscope) and EDS (energy dispersive spectrometer) scanning shows that if heated at higher temperature or for more cycles, the AZO layer becomes blocky, and the proportion of Ag and O changes significantly. This leads to the decrease of visible transmittance and the increase of infrared emissivity of samples.