In the volcanic areas of Indonesia and Thailand, the ground temperature is as high as 70 degrees Celsius, and in such a harsh environment, an insect, the long-horned beetle, is active. This beetle has a tiny triangular structure on its wings, which not only reflects sunlight, but also helps it dissipate heat from the body.
Inspired by it, a research team from Shanghai Jiaotong University, the University of Texas at Austin, and the Royal Swedish Institute of Technology, based on a new method of etching spin coating vacuum thermal curing, bionic prepared a flexible film with a long-horn beetle-like structure At the same time as passive radiation cooling, it realizes the macro preparation of radiation cooling film.
Related research was published in the Proceedings of the National Academy of Sciences recently.
Longhorn beetle brings inspiration
In nature, some organisms have a special surface structure, and through passive radiation, show amazing thermal regulation ability. The long-horned beetle is one of them.
"We stumbled upon the appearance of this beetle in the Insect Museum with a golden luster, which attracted our attention." Zhou Han, one of the authors of the paper and a professor at the School of Materials Science and Engineering of Shanghai Jiaotong University, told the China Science News Most of them live near the crater with very high temperature, which also aroused their interest, and conducted a series of studies on their micro-morphology, optical and thermal characteristics, and finally found some novel characteristics.
The researchers first observed the microstructure of the long wing beetle's front wing, and found that the surface of the front wing was covered with villi, with a density of more than 25,500 per square centimeter. The color of the front wing can also effectively resist the fading treatment, showing the structural color characteristics of the photonic crystal.
Through further observation, they found that each fluff is a triangular structure composed of two smooth surfaces and a rough surface. The rough surface is a corrugated structure with a width of 1 μm and a height of 0.18 μm. Grade rough structure.
After grasping the microstructure of the front wing surface, Zhou Han et al. studied its optical properties and temperature adjustment ability. By observing the reflection of the front wings with or without fluff, it is found that the fluff can increase the light reflectance by more than 35%. Through the immersion experiment of ethanol solution, it was further determined that the high reflectivity is due to the multi-level microstructure existing on the surface of the villi.
The time-temperature curve also shows that the surface fluff has a significant cooling effect, reaching 3.2 degrees Celsius and 1.5 degrees Celsius temperature drop in vacuum and air, respectively. This excellent temperature control capability is beneficial for insects to carry out daily foraging activities in high-temperature, sun-drenched environments.
"Longhorn beetles have visible near-infrared high reflection characteristics and mid-infrared band high emission characteristics, which can effectively cool the surface and make it heat-resistant." Zhou Han added.
Preparation of Bionic Film
Based on research on the surface structure and temperature control ability of the long-horn beetle, the researchers tried to prepare a bionic radiation cooling film with a similar structure. Zhou Han told reporters that the difficulty of bionic-like beetle-like structures lies in the precise preparation of microstructures and the preparation of large areas.
In the preparation process, the researchers first prepared a silicon template with a triangular structure by photolithography, and then spin-coated the precursor solution containing silicone and alumina microspheres on the surface of the template. After thermal polymerization, the surface was separated into triangles. Microstructured film. This method can realize large-scale and macro-preparation of thin films, and has certain versatility. It can achieve the doping of various ceramic particles such as zinc oxide, zirconium oxide, magnesium oxide, and titanium dioxide.
After obtaining the biomimetic film, the researchers tested its performance, and the results showed that the average reflectivity in the solar spectrum range was about 95%. The evaluation results of the actual cooling capacity of the film showed that the average temperature of the bionic film dropped to 5.1 degrees Celsius and the maximum temperature dropped to 7 degrees Celsius under the conditions of an average solar intensity of about 862 watts per square meter and a humidity of 22.7%.
"Before preparation, we carried out simulation analysis on the microstructure of the beetle and modified the structure to make the mid-infrared emissivity higher." Zhou Han said that they refined the optimized bionic structure model through theoretical research and revealed that Its mechanism. On this basis, by imitating the bionic mechanism of the optimized structural model, the bionic film has similar functions. The researchers also optimized the optical and thermal properties of the biomimetic film through structural optimization and functional material compounding.
The prospect of passive radiant cooling is expected
According to statistics, my country's building energy consumption accounts for about 35% of the country's total energy consumption, while the energy consumption of refrigeration and air conditioning systems accounts for about 50% to 60% of building energy consumption. Most existing refrigeration technologies are based on vapor compression refrigeration, which requires a lot of power. Therefore, refrigeration technology has become a major energy consumer, and the technology needs to be updated.
Learning nature, preparing special surface structures, and achieving passive radiant cooling are undoubtedly a promising technology. The biomimetic film can not only cool itself, but also significantly reduce the temperature of the surrounding environment and the equipment or heating body covered by the film.
Zhou Han said that in the future, the bionic film can be used in the field of passive radiant cooling without the need for external power sources, such as building cooling, water system cooling, desert management, solar cell thermal management, etc. In addition, in addition to radiation cooling, the flexibility and hydrophobicity of this biomimetic film also lay the foundation for its application in various wearable devices, personal electronic devices, and vehicles.
"For future large-scale application, we need to expand production scale and product specifications, and further reduce costs." Zhou Han said that this passive radiation cooling heat regulation technology is undoubtedly more energy-saving and environmentally friendly. This work also paved the way for the subsequent mass production of radiation cooling technology based on high-performance photon radiators. (â– Reporter Li Huiyu)
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