MATERIALS


Researchers at Colorado University, led by Ronggui Yang and Xiaobo Yin (front), have devised a metamaterial that allows passive radiative cooling of surfaces even in direct sunlight


or ultraviolet radiation, he said. The researchers also believe the material could be adapted to work with visible light.


in applications such as temperature regulation. He cites the example of an orbiting satellite, which goes through huge temperature swings depending on whether it is in sunlight or shadow. ‘By coating a satellite in this


material, we could tune it to either radiate extra heat, or hold onto it. By increasing emissivity in sunlight, the satellite would more readily radiate excess heat into space’, he says. Similarly, it could be tuned to hold onto heat when it is in shadow. The next stage, he says, is to


perform this passively – so that a material’s emissivity is linked directly to its temperature. ‘This way, there is no need to apply a voltage in order to change the emissivity,’ says Padilla.


Gold standard In 2016, researchers from the Australian National University (ANU) demonstrated a material based on nano-structured gold and magnesium fluoride, which could form the basis of highly efficient TPV cells. ‘Thermophotovoltaic cells could be much more efficient than solar cells – and our metamaterial might help to unlock their potential,’ says Sergey Kruk, from the ANU Research School of Physics and Engineering. TPV cells do not need direct


sunlight to generate electricity, and instead can harvest heat from their surroundings. Standard materials emit heat in all directions – across a broad spectrum of infrared frequencies – but the geometry of the metamaterial ensures that it emits in a specific spectral range, and in a specific direction. This makes it ideal for use as an emitter paired with a TPV cell. The key to this metamaterial’s


behaviour is that it has a far stronger interaction with the magnetic component of light than a conventional material, say the researchers. TPV cells based on the


metamaterial could be made even more efficient if the emitter and the receiver have a ‘nanoscopic’ gap between them. In such a configuration, radiative heat transfer between them might be 10 times more efficient than between conventional materials, said Kruk. The metamaterial was fabricated by a team at the University of California Berkeley, US (Sergey Kruk et al, Nature Communications; doi: 10.1038/ ncomms11329). Researchers at the same university


have applied metamaterials to optics – another promising area – in an application that could protect astronauts against harmful radiation (Advanced Functional Materials; doi: 10.1002/adfm.201700580). The team fabricated materials out of silicon nanometre-sized disks arranged in a two-dimensional lattice. Changing the temperature of the surface helped the team to take advantage of the thermo-optical effect – which affects the nanodisks’ refractive index. Heating the metasurface from


room temperature to 300°C caused a ‘spectral response’ of tens of nanometres, allowing the researchers to control whether wavelengths were scattered backwards (reflection) or forwards (transmission). This effect was reversible, the researchers found. The material is thin and could be


applied to any surface, said co-lead researcher Mohsen Rahmani. It could be used as an alternative to the thick filters that are traditionally used to absorb dangerous levels of infrared


Solar absorber At Arizona State University (ASU), Hao Wang and colleagues have developed yet another metamaterial for a highly efficient solar absorber. Solar absorbers harvest radiation from sunlight and convert it into usable thermal energy. For maximum conversion efficiency, they must absorb as much light as possible from across the electromagnetic spectrum, and have low emissions in the mid-infrared range – in order to cut losses from spontaneous thermal radiation.


>9%


Increased power conversion efficiency of solar cells boosted by a metamaterial cloak


Micro-electromechan- ical system technology has been deployed to make a reconfigur- able device that could convert waste heat into usable energy


> 90%


A typical commercial material used on solar absorbers, such as TiNOX – a coating developed by the company Almeco – absorbs 95% of light and emits only 4% of thermal radiation, but its performance is optimal at about 100°C. ‘Efficient solar absorbers – with both spectral selectivity and high-temperature compatibility – are still lacking,’ said the researchers. The metamaterial absorber


Amount of UV, visible and near-infrared light captured by a new metamaterial solar absorber. Solar absorb- ers harvest and convert radiation from sunlight into usable thermal energy. For maximum conversion efficiency, they must also have low emissions in the mid- infrared range


developed at ASU catches more than 90% of light in the UV, visible and near-infrared (NIR) regions. At the same time, it has an emittance of around 20% mid-IR radiation – thanks to the highly reflective nature of its metallic elements. The device operates at up to 350°C. The material is made of nanostructured titanium gratings deposited on a thin magnesium fluoride spacer and tungsten ground film. Tungsten was chosen because of its high- temperature stability, but titanium gratings are easier to pattern by standard processes. Overall conversion efficiency was


predicted to be 78% at 100°C, the researchers found. Metamaterials may have found


fame as invisibility cloaks. But as alternative energy sources continue to gain importance, they are likely to play an increasingly prominent role in boosting efficiencies – thus hastening their commercialisation.


09 | 2017 21


UNIVERSITY OF COLORADO


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