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What is the future of metamaterials? Time:2017/01/19 14:53:47 Hit:776

The word ¡°metamaterial¡± is coming up more and more often in the news, often referring to some pretty esoteric characteristics that have been changed by scientists.


For most of us, the subject is mysterious. All we know is that changing the characteristics of some materials imparts a whole new set of behaviours in that material.


The basic definition of metamaterials is simple: They are materials engineered to have properties that have not yet been found in nature. The key is in the engineering, and to do that, scientists have to use knowledge gleaned from many disciplines.  That's why members of the research team can be electrical engineers, or material sciences, optics, microwave and antennae engineering, and nanoscience, plus a handful of others.


The concept of metamaterials originated late in the 19th Century, but work on them remained under the radar until just a couple of decades ago. The growth of the nanotechnology industry then opened the gate for progress in metamaterials.


So far, metamaterials haven't made much of an inroad into the construction industry. There are sensors used mainly for infrastructure monitoring, and not much else.


That could be about to change because of work on two fronts ¡ª roofing and photovoltaic cells.


Materials scientists at the University of Technology in Sydney, Australia, believe they have made a breakthrough with a surface they've developed that will stay cooler than the air around it, even under the heat of a mid-Australian summer. If their work results in a commercially available roofing product, it would reduce the heat load in urban areas ¡ª often called simply the "urban heat island effect." That would reduce energy use for cooling, and, as a result, a reduction of emissions of greenhouse gases.


Angus Gentle and Geoff Smith, have published a paper outlining their work in the latest edition of the journal Advanced Science


At the heart of their work is a "coated polymer stack." Think of it as simply a combination of specially chosen polyester films and mounted on a layer of metallic silver.


Dark roofs can get really hot in an Australian summer. Even white roofs, says Smith, can absorb enough sunlight to warm up by 9 to 12 C. The new surface devised by the researchers stayed 11 C or cooler than a nearby state-of-the-art white roof during nine days of testing under the Aussie sun. In fact, it stayed cooler than the air around it, earning it the nickname "super-cool roofing."


That, says Smith, is because the new surface absorbs only three per cent of incident sunlight, while at the same time radiating infrared heat that is not absorbed by the atmosphere.


The "stack" they used in their product was built up of 300 layers of specially selected polymer films underlain by a layer of silver and topped by a layer of polyethylene terephthalate, or PET, a tough polymer. The thickness of the layers averaged 110 nanometres (nm). That's thin. For reference think of an average human hair, which is about 60 nm in diameter.


The thickness of the stack, then, was somewhere around 33,000 nm, or 33 microns, including the tough top layer of PET, that's still thin.


Successful though the Sydney test has been, there is still work to do, the researchers say. For example, refinement is needed to diffuse the reflectance in order to avoid glare without affecting the material's performance.


On the plus side, the materials needed are not only available, but are suited to large-scale production, the scientists say.


If further testing and tweaking the materials are successful, we may be closer than we think in seeing a metamaterial in widespread application by the construction industry.


It could also be coming to another corner of the construction industry, although still up on the roof ¡ª solar photovoltaic panels. I'll return to that next week.