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Technology : Coated windows will let the sun shine in

作者:谯甸碜    发布时间:2019-02-27 10:11:08    

By Jonathan Beard ANYONE who has tried to look into a house or car on a sunny day can testify that glass is not completely transparent. But new antireflective coatings developed in Germany should make it possible to treat windows and solar collectors so that they let in more light than even the finest of today’s optical instruments. Ordinary glass reflects about 8 per cent of sunlight in the visible spectrum because of the large difference in the refractive indices of air (1.00) and glass (1.51). Optical glasses and camera lenses are coated with an alternating series of films of high and low refractive index. These set up an interference pattern that prevents incoming light from being reflected. “But they are quite expensive, both literally and optically,” says Dieter Sporn of the Fraunhofer Institute for Silicate Research in Würzburg. “They are too costly for large surfaces, and they prevent some of the light from getting through.” Instead, Sporn and his colleagues at the Fraunhofer Institute for Solar Energy Systems in Freiburg have developed a coating made of silicon dioxide, which creates a porous covering on the glass. The researchers dip the glass in a coating solution of silicon dioxide and then bake it at 500 °C. “The coating looks like a sponge consisting of a silicon dioxide framework riddled with pores about 15 nanometres in diameter,” says Sporn. Because these microstructures are much smaller than the wavelength of visible light, the light enters the coating as if it were simply air. “The silicon layer has an effective refractive index of 1.22,” says Sporn. “This means that for visible wavelengths, 99.5 per cent of light is transmitted, while about 97 per cent of the solar spectrum passes through.” Sporn expects the first applications to be in solar energy. “Solar collectors today use ordinary glass that reflects over 9 per cent of the incoming energy,

 

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