MATERIALS
TERIALS
M
aterials generally behave in predictable ways. We can make turbine blades from titanium, microchips from silicon and hot water piping from copper, knowing they have the correct properties for the job in hand.
However, an emerging breed of materials – dubbed metamaterials
– turn convention on its head by having ‘impossible’ characteristics: bending light in the ‘wrong’ direction, for instance, or allowing current to flow in reverse. Many metamaterials are still confined to the laboratory, but if
commercialised could find use in a range of areas – including the energy industry. Some materials have already been shown to improve the efficiency of solar arrays, while others could be used in areas such as radiative cooling and energy harvesting.
Invisible dream The most celebrated potential application of a metamaterial is that of a Harry Potter-style ‘invisibility cloak’, which has been the subject of many studies. For instance, in a 2016 paper published in Nature (doi:10.1038/srep29363), researchers from Queen Mary University in London, UK, described how a curved metal plate – coated with a nano- structured material – could be used as a ‘surface wave cloak’. Other research has described materials that can ‘bend’ radio waves around an object, making it potentially undetectable by radar, for instance. These unnatural properties of a metamaterial arise because of how the material is designed at a microscopic level. In effect, the material’s behaviour is governed by its geometric design, rather than by its inherent physical properties. Attributes such as conductivity,
transparency or refractive index are then at odds with those expected of the ‘pure’ material. The ‘light bending’ principle has been used to boost the efficiency of solar panels. Scientists at Karlsruhe Institute of Technology in Germany have succeeded in adding a cloaking material to the metal contact ‘fingers’ on a solar array. These strips of metal are a vital part of the array, helping to extract the generated current. However, they shade part of the cell’s active area, which can reduce overall power efficiency.
Adding the cloaking material bends the incident light away from the metal strips – and onto the active part of the cell instead. The metal strips occupy anything from 4% to 9% of the solar
array’s surface, while the cloaking material can boost a cell’s power conversion efficiency by more than 9%, according to a July 2017 paper in Advanced Optical Materials (doi: 10.1002/adom.201700164). ‘Our experiments have shown that the cloak layer makes the
contact fingers nearly completely invisible,’ says lead author Martin Schumann. The researchers covered the surface of the whole solar cell – which
amounted to only a few square centimetres – with a polymer, whose surface was etched with a series of ‘grooves’. These create the cloaking effect by refracting the light around the strip and onto the active surface of the solar cell. The grooves were etched using a 3D printing technique called direct laser writing (DLW). The effect could one day be incorporated into cells in a low-cost
way, the researchers believe. ‘A key advantage of our approach is the straightforward fabrication involving a low number of processes,’ they
09 | 2017 19
JOHAN SWANEPOEL / SCIENCE PHOTO LIBRARY
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
