With the rapid development of infrared sensing and detection technology, research on flexible thermal camouflage materials for the human body is becoming increasingly important. Normally, the human skin temperature is between 32 and 35°C, and the thermal radiation characteristics are obvious. According to Wien's displacement law, the electromagnetic waves radiated by the human body are mainly distributed in the 7 to 14 μm band; the emissivity of human skin is close to that of a black body, and the peak infrared radiation is located near 9.5 μm. In view of the absorption and scattering effects caused by H2O, CO2 and suspended particles in the atmosphere, short-wavelength 3~5 μm and long-wavelength 8~14 μm are commonly used infrared detection bands, while 5~8 μm is a non-detection window band. As mentioned above, the thermal radiation distribution of the human body overlaps with the long-wave infrared detection window. On the other hand, referring to the AM 1.5G solar radiation spectral distribution characteristics, the solar spectrum covers the ultraviolet-visible-near-infrared (UV-VIS-NIR) band, with high energy density (1000W/m2), and the average reflectance of human skin in the solar band Usually less than 60%, that is, it will absorb the incident solar energy and heat up when exposed to sunlight. In summary, the human body is exposed to infrared radiation characteristics.
According to Stefan Boltzmann's law, lowering the surface temperature or changing the emissivity are the two main ways to achieve personal thermal camouflage. Although there has been a lot of research work on flexible camouflage devices, there are very few related studies on personal thermal camouflage materials.
The team of Liu Dongqing and Cheng Haifeng of the National University of Defense Technology published an article titled "A personal thermal camouflage material based on nanofibrous polyamide membrane" to address this issue. They studied the intrinsic infrared absorption characteristics of PA66 and the high reflection characteristics of metallic Al, using magnetic control. A metal-polymer composite multilayer film PA66/Al/nanoPE was prepared by a two-step method of sputtering and electrospinning. In the process of preparing the outer PA66 nanofiber membrane by electrospinning, the fibers are randomly stacked to form a variety of nano- and micron-scale irregular pore structures. When the size of the scatterers (fibers and pores) is close to or much smaller than the incident electromagnetic wavelength, Mie and Rayleigh resonance scattering occurs, reflecting sunlight and giving a white appearance. Since the size of the scatterers in the PA66 fiber membrane is mostly distributed in the hundreds of nanometers, it can produce strong Mie scattering of short-wave sunlight, providing the possibility of being compatible with UV camouflage. In addition, the staggered pore structure can provide channels for water vapor to pass through, giving the structure moisture permeability. The metal reflective layer can make up for the lack of reflectivity of the PA66 nanofiber film in the near-infrared band. Combined with FDTD simulation, the rationality of the experimental results is further verified.
The PA66/Al/nanoPE composite multilayer film achieves low emissivity in the detection band of 3~5 μm and 8~14 μm to solve the thermal camouflage needs, while achieving high emissivity in the non-detection band of 5~8 μm to achieve infrared selectivity. Radiation heat dissipation and cooling. Under the same heating conditions, the equilibrium temperature is lower than that of the non-selective irradiation sample. The nanopores of the electrospun PA66 fiber membrane, the Mie scattering effect brought about by the fiber structure, and the bottom metal reflective layer work together to achieve an average solar reflectance of >90%. Under peak solar radiation intensity, the solar reflectivity is not optimal. Compared with the low emissivity camouflage film, a cooling effect of about 20 °C can be achieved. In addition, the multi-layer film has high reflectivity in the ultraviolet band of 300~400 nm, which meets the requirements of ultraviolet camouflage. It can achieve thermal camouflage compatible with snow backgrounds, is moisture-permeable and windproof, and can be worn as personal thermal camouflage against snow backgrounds in winter.
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