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The trend towards maximising natural light in buildings not only creates a pleasant atmosphere, it also saves energy costs. However, conventional glass roofs and walls also cause problems, such as glare in the surroundings, a lack of privacy and overheating. Alternative solutions such as coatings and light-diffusing materials do not yet offer a comprehensive remedy.

New material combines several functions

Researchers at the Institute of Microstructure Technology (IMT) and the Light Technology Institute (LTI) at KIT have therefore developed a new type of polymer-based metamaterial that combines various properties and could replace glass components in the construction sector in the future. The so-called Polymer-based Micro-Photonic Multi-Functional Metamaterial (PMMM) consists of microscopically small pyramids made of silicone. These micropyramids measure around ten micrometres, which corresponds to around a tenth of the diameter of a hair. This structure gives the PMMM film several functions: Light scattering, self-cleaning and radiative cooling while maintaining high transparency. "A key feature is the ability to efficiently radiate heat through the long-wave infrared transmission window of the Earth's atmosphere, thus releasing heat into the cold expanse of the universe. This enables passive radiative cooling without power consumption," explains Bryce S. Richards, professor at IMT and LTI.

Cooling, translucent and glare-free

The researchers tested the properties of the material in the laboratory and with experiments in the open air under real outdoor conditions, using modern spectrophotometry to measure light transmission, light scattering, reflective properties, self-cleaning ability and cooling performance. The result: in the tests, a cooling effect of six degrees Celsius compared to the ambient temperature was achieved. In addition, a high spectral transmittance, i.e. transparency of 95 per cent, was achieved. In comparison, glass usually has a transparency of 91 per cent. At the same time, 73 per cent of the incident sunlight is scattered by the micro-pyramid structure. This creates a blurred appearance. "When the material is used in roofs and walls, it enables bright and at the same time glare-free and protected interiors for working and living. In greenhouses, the high light transmission could increase yields because the efficiency of photosynthesis is estimated to be nine per cent higher than in greenhouses with glass roofs," says Dr Gan Huang, group leader at the IMT. The micropyramids also give the PMMM film superhydrophobic properties, similar to a lotus leaf: water rolls off in the form of droplets, removing dirt and dust from the surface. This self-cleaning function makes the material easy to clean and durable.

Potential for construction and urban development

"Our newly developed material has the potential to be used in various areas and makes an important contribution to sustainable and energy-efficient architecture," explains Richards. And Dr Huang predicts: "The material can simultaneously ensure optimal use of sunlight indoors, provide passive cooling and reduce dependence on air conditioning systems. The solution can be scaled up and seamlessly integrated into plans for environmentally friendly house building and urban development."

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