To cool a building below the ambient temperature in bright sunlight with no energy input seems to defy the laws of physics, which state that heat naturally flows from hot to cold. However, the sun, at about 6000\u00b0<\/sup>C, radiates largely at visible wavelengths and near infrared, whereas a building emits much longer infrared wavelengths. Conveniently, the atmosphere has an infrared \u2018transparency window\u2019 at which radiation can pass into outer space. Covering a building with a material that is reflective at visible wavelengths but emissive in this transparency window could, therefore, allow a building to radiate away its own heat into space, and thereby cool down in bright sunlight.<\/p>\n <\/cite>The experimental setup used to investigate the cooling performance of the white wood<\/p>\n<\/div>\n<\/div>\n That\u2019s the theory. In practice, the first such material, developed by Stanford researchers in 2014,\u00a0comprised seven layers fabricated by electron beam evaporation. Other teams have gone on to create simpler, more practical surfaces such as\u00a0paintsand\u00a0films\u00a0that achieve the same effect. However, nobody has previously developed a structural material that inherently cools down in bright sunlight.<\/p>\n The new work is a collaboration between the University of Colorado, Boulder team that developed a cooling film and the University of Maryland, College Park team that developed densified wood. Before it can be completely densified, wood must be partially stripped of lignin to reduce its brittleness \u2013 this can be achieved by boiling with hydrogen peroxide. The researchers found that, if they stripped out all the lignin, the resulting densified wood scattered visible light very well, while being highly emissive in the infrared transparency window. Nevertheless, the material remained around nine times as strong and 10 times as tough as natural wood. This is counter-intuitive as lignin provides the structural support to natural wood. \u2018If you strip out the lignin then a tree can\u2019t stand anymore,\u2019 explains the University of Maryland\u2019s\u00a0Liangbing Hu. \u2018But when you strip out the lignin and press the cellulose fibres so they form a lot of hydrogen bonds in the dry state, you can maximise their interaction.\u2019<\/p>\n![\"An](\"https:\/\/d1w9csuen3k837.cloudfront.net\/Pictures\/480xany\/0\/2\/6\/143026_1g-Cooling_wood_Science-1.jpg\")
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