Our group has made important progress in the field of aerogel phase stealth composites.
According to Stefan Boltzmann law (P = εσT^4), the radiation energy per unit area is proportional to the fourth power of emissivity and thermodynamic temperature. Therefore, infrared stealth can be achieved by adjusting infrared emissivity or temperature. For example, the thermal emission of targets can be regulated by manufacturing the surface with micro/nanostructures such, however, static micro/nanostructures do not endow targets with the ability of tunable thermal emissivity. Quantum wells, electrochromic dyes, phase change materials can dynamically control infrared radiation, however, the research around these have encountered multiple problems, such as sustained electric consumption, narrow spectral window, slow response speed, low tunability, and rigid substrates. On the other hand, modulating temperature has been proposed to hide infrared radiation and defeat thermal IR cameras. However, thermal insulators such as blankets are generally thick and heavy, which can cause heat buildup. Thermal cloaks which can manipulate heat diffusion are unable to realize large-scale manufacture.
The aerogel team led by Zhang Xuetong, a researcher at the Suzhou Institute of Nano-Tech and Nano-Bionic, China Academy of Sciences has prepared a flexible aerogel film with high porosity (98%) and high specific surface area (365.99 m2/g) by dissolving the Kevlar of DuPont TM, scraping coating, sol-gel and subsequent freeze-drying process. This KNA/PCM demonstrated (1) excellent energy storage properties (phase-change enthalpy of 179.1 J/g, (2) an infrared emissivity which was comparable with that of most backgrounds and (3) an ultralow average transmittance of a wide waveband of 3−15 μm. The KNA/PCM composite films demonstrated high performance in IR stealth in outdoor environments with solar illumination variation, where the target covered with a KNA/PCM film can blend their thermal appearance into the background. For continuous heating objects (such as engines), a combination structure of aerogel insulation layer and phase change composite film is proposed: According to the temperature difference between the target and the environment, selecting the aerogel layer with the appropriate number of layers or thickness can reduce the temperature to match the ambient temperature. The phase change composite film has low infrared transmittance, and the infrared light emitted by the high-temperature target cannot penetrate. Therefore, the high temperature target covered with this composite structure can also achieve infrared stealth in infrared photos.
On the basis of the use scenarios, the infrared stealth can be achieved by selecting the matching aerogel / phase change composite film or combined structure, as is shown in Figure 1. The related research achievements entitled " Nanofibrous Kevlar Aerogel Films and Their Phase-Change Composites for Highly Efficient Infrared Stealth" have been published online in the international journal ACS Nano (ACS Nano, 2019,13,2236-2245) and selected into ACS editors' choice article.
Figure 1. Schematic description of the preparation of Kevlar nanofiber aerogel (KNA) film and its phase-change composite film (KNA/ PCM) and schematic representation of the infrared stealth of KNA/PCM composite films