Tuesday , March 9 2021

The anti-reflective coating causes the plastic to be invisible

plasticPlastic dome covered with a new anti-reflective coating (right) and an uncoated dome (left)CREDIT: Giebink Lab / Penn State

Anti-reflective coatings (AR) of plastics have a multitude of practical applications, including the reduction of bursting on glasses, computer monitors and the screen on your smartphone at # 39; outdoors Now, Penn State researchers have developed an AR coating that improves existing coatings to the point that it can make transparent plastics, such as Plexiglas, virtually invisible.

"This discovery occurred as we tried to make high efficiency solar panels," said Chris Giebink, associate professor of electrical engineering, Penn State. "Our focus was on concentrating light on small solar cells and high efficiency using plastic lenses, and we have to minimize their loss of reflection."

They needed an anti-reflective coating that would work well across the solar spectrum and in multiple angles as the sun crossed the sky. They also needed a coating that could withstand long periods of time in the open air.

"We would have liked to find an offline solution, but there was not one that fulfilled our performance requirements," he said. "Therefore, we started looking for our own solution."

That was a high order. Although it is comparatively easy to make a coating that eliminates reflection at a given length or in a particular direction, there was no one that could fit all its criteria. For example, coatings of AR glasses are directed to the visible narrow portion of the spectrum. But the solar spectrum is about five times larger than the visible spectrum, so that this coating would not work well for a concentrated solar cell system.

The reflections occur when the light travels from a medium, such as air, to a second medium, in this case the plastic. If the difference in its refractive index, which specifies the speed that the light travels in a given material, is large-air has a refractive index of 1 and 1.5 plastic, then there will be a lot of reflection. The lowest index for a natural coating material such as magnesium fluoride or Teflon is approximately 1.3. The refractive index can be classified (varying slowly) between 1,3 and 1,5 mixing different materials, but the difference between 1.3 and 1 remains.

In an article recently published online before publishing it to the magazine Nano LettersGiebink and the co-authors describe a new process to overcome the gap between teflon and air. They used a sacrifice molecule to create evaporated Teflon nanoscale pores, thus creating a classified Teflon-air film index that triggers light to see a smooth transition from 1 to 1.5, essentially eliminating all reflections.

"The Teflon's interesting, which is a polymer, is when it is heated in a crucible, the large polymer chains are closed in small fragments that are small enough to volatise and send a steam flow. It can repolimise and train Teflon, "Giebink said.

When the sacrifice molecules add to the flow, the Teflon will be reformed around the molecules. The dissolution of sacrifice molecules leaves a nanoporous film that can be classified by adding more pore.

"We have been interacting with several companies looking for improved anti-reflective coatings for plastic, and some of the applications have been surprising," he said. "They are going to eliminate the glare of the plastic domes that protect the security cameras to eliminate outside reflections within the virtual / augmented headphones."

An unexpected application is in high altitude UAVs, or unmanned aerial vehicles. They are planets with large giants that are covered with solar cells. Used mainly for recognition, these planes rely on sunlight to stay on an almost perpetual flight and, therefore, much of the light they receive is at an angle of observation where the reflections are higher. One of the companies that manufacture these solar cells is exploring the AR coating to see if it can improve the amount of light collected by a UAV.

Because technology is compatible with today's manufacturing techniques, Giebink considers that the coating technology is scalable and widely applicable. At this point, his trial samples have been in central Pennsylvania for two years, with little change in properties. In addition, the coating is also antifogging.

"The coating adheres well to different types of plastics, but not glass," he said. "Therefore, it will not be useful for the typical solar panel on the terrace with a glass protective cover. But if the concentration of photovoltaic energy is reappearing, a critical part of these are the Fresnel lens of plastic and here we could make a difference ".

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