CREDIT: Giebink Lab / Penn State
Antimeflection (AR) coatings on plastic have many practical applications, including eye glasses, computer monitors and smartphone display outdoors. Now Penn State researchers have developed an AR coating that improves existing coatings to such an extent that it can make translucent plastics, such as Plexiglas, virtually invisible.
"This discovery came when we tried to make solar panels more efficient," said Chris Giebink, Associate Professor of Electrical Engineering at Penn State. "Our approach included focusing light on small, high-efficiency solar cells using plastic lenses, and we needed to reduce their loss of thought."
They needed an antireflection coating that worked well across the sun and at multiple angles when the sun crossed the sky. They also needed a coating that could withstand the weather for a long time.
"We would have liked to find a flawless solution, but there was no one that would meet our performance requirements," he said. "So we started looking for our solution."
It was a high order. Although it is relatively easy to create an overlay that will prevent reflection at a particular wavelength or a certain direction, there was no one that could meet all of their criteria. For example, glasses for AR coatings are directed to a narrow visible part of the spectrum. However, the solar spectrum is about five times wider than the visible spectrum, so such a coating would not be well suited to a solar system.
Reflected when light travels from one environment, such as air, to another environment, in this case plastic. If the difference in their refractive index, which determines how fast light goes into a particular material, is large – the air has a refractive index of 1 and plastic 1.5, then there will be a lot of reflection. The lowest rate for a natural coating material such as magnesium fluoride or Teflon is about 1.3. The refractive index can be classified – slowly varying from 1.3 to 1.5, mixing different materials, but the difference between 1.3 and 1 remains.
Recently published document online before printing in magazine Nano letters, Giebink and co-authors describe a new process to bridge the gap between Teflon and air. They used the victim's molecule to create nanos layer pores in evaporated Teflon, thus creating a graded index of Teflon-air film that fools the light to see a smooth transition from 1 to 1.5, essentially eliminating all reflections.
"Interesting about Teflon, which is a polymer, is when you heat it in a crucible, the large polymer chains are broken into smaller fragments that are small enough to evaporate and send steam. Repolymerize and make Teflon," said Giebinks.
When sacrificial molecules are added to the stream, Teflon will be transformed around the molecules. Getting rid of the victim's molecules leaves a nanoporous film that can be classified by adding more pores.
"We have collaborated with a number of companies looking for improved backflow coatings for plastics, and some applications have been surprising," he said. "They range from anti-glare to plastic thinking that protects security cameras to prevent false reflections on virtual / augmented headphones."
One unexpected application is high altitude UAV or unmanned aircraft. These are planes with giant wings covered with solar cells. These planes are mainly used for intelligence, so the sunlight remains in the nearest flight, so much of the light they receive is at an angle of reflection, with the highest reflections. One of the companies that make these solar cells explores AR coating to see if it can improve the amount of light acquired by UAV.
As technology is compatible with current production methods, Giebink believes that coating technology is adaptable and widely applicable. At this point, his test samples had been in Pennsylvania for two years, and the changes in properties were minor. In addition, coverage is also a contraceptive.
"The coating adheres well to various types of plastic, but not glass," he said. "So it's not going to be useful for your typical roof solar panel with safety glass. But if the concentration of the photovoltaic cells is a revenge, the critical part of them is the plastic Fresnel lens, and we could change it."
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