Passive radiative cooling materials absorb energy and emit infrared radiation in the so-called atmospheric transparency window. This means the deep sky is used as a heatsink, and the materials can give off more heat than they take up from the sun’s rays, leading to a net cooling effect. Such materials can save energy when they replace active cooling methods such as air conditioning. Good passive daytime radiative cooling (PDRC) materials need to combine a high solar reflectance to reduce solar energy gain and a high infrared emittance to increase radiative heat loss. If they are used as fabrics, the materials also need additional properties such as flexibility and breathability. Achieving all of this at the same time can be challenging.
Fuqiang Wang, Harbin Institute of Technology, China, and colleagues have used the skin as an inspiration for the design of a PDRC “metafabric” that could be useful, e.g., for personal cooling, tents, car covers, etc. The team’s design includes two layers that mimic the two topmost layers of the skin (epidermis and dermis). They used a cotton polyester fiber as a base fabric, which was soaked with a precursor solution and then dried. This wetting and drying process was repeated to create the desired layers. The precursor solutions were made by grinding BaSO4, SiO2, or TiO2 in a ball mill and mixing the resulting optically selective microparticles with polyvinylidene fluoride (PVDF) and N-methyl pyrrolidone (NMP) using ultrasound.
In the resulting metafabric, the cotton-polyester fibers are arranged similarly to collagen in the skin and encapsulate the microparticles and PVDF while leaving microcavities. The thickness of the lower layer is 150 μm and that of the upper layer is 600 μm. In the lower layer, the volume fractions of TiO2, BaSO4, SiO2, and PVDF are 40, 40, 6, and 14 %, respectively; in the upper layer, the volume fractions of BaSO4, SiO2, and PVDF are 80, 6, and 14 %, respectively.
The microcavities improve sunlight backward scattering and breathability. According to the researchers, both the solar reflectance and atmospheric window emittance of the metafabric can reach 97 %. In outdoor tests during the daytime, the metafabric reached temperature drops below the ambient temperature of up to 12.6 °C. The team tested the material, e.g., in hats. Using infrared imaging, they found that a hat made of the PDRC metafabric can be 16.6 °C cooler than a black cotton hat and 5.0 °C cooler than a white cotton hat, for example.
- Scalable Bio-Skin-Inspired Radiative Cooling Metafabric for Breaking Trade-Off between Optical Properties and Application Requirements,
Xinping Zhang, Ziming Cheng, Dongling Yang, Yan Dong, Xuhang Shi, Huaxu Liang, Fuqiang Wang, Han Han, Weifeng Meng, Yong Shuai, Yuying Yan,
ACS Photonics 2023.