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FEATURE ARTICLE
Superhydrophobic Glass Coating Created
September 2015

A moth’s eye and lotus leaf served as inspiration for an antireflective water-repelling, or superhydrophobic, glass coating developed by researchers at the Department of Energy’s Oak Ridge National Laboratory (ORNL) that holds significant potential for such diverse applications as solar panels, lenses, detectors, windows, as well as weapons systems.


Schematic representation of the coated product and applications. Image courtesy of ORNL.

Detailed in a paper, “Monolithic Graded-Refractive-Index Glass-based Antireflective Coatings: Broadband/Omnidirectional Light Harvesting and Self-Cleaning Characteristics,” published in the Journal of Materials Chemistry C, the discovery is based on a mechanically robust nanostructured layer of porous glass film that can be customized to be superhydrophobic, fog-resistant and antireflective.

“While lotus leaves repel water and self-clean when it rains, a moth’s eyes are antireflective because of naturally covered tapered nanostructures where the refractive index gradually increases as light travels to the moth’s cornea,” said Tolga Aytug, lead author of the paper and a member of ORNL’s Materials Chemistry Group. “Combined, these features provide truly game-changing ability to design coatings for specific properties and performance.”

To be superhydrophobic, a surface must achieve a water droplet contact angle exceeding 150 degrees. ORNL’s coating has a contact angle of between 155 and 165 degrees, so water literally bounces off, carrying away dust and dirt.

The base material—a special type of glass coating—is also highly durable, which sets it apart from competing technologies, according to Aytug.

“We developed a method that starts with depositing a thin layer of glass material on a glass surface followed by thermal processing and selective material removal by etching,” he said. “This produces a surface consisting of a porous three-dimensional network of high-silica content glass that resembles microscopic coral.”

The fact the coating can be fabricated through industry standard techniques makes it easy and inexpensive to scale up and apply to a wide variety of glass platforms.

“The unique three-dimensionality interconnected nanoporous nature of our coatings significantly suppresses Fresnel light reflections from glass surfaces, providing enhanced transmission over a wide range of wavelengths and angles,” Aytug said. The Fresnel effect describes the amount of light reflected versus the amount transmitted.

For solar panels, the coating is highly effective at blocking ultraviolet light and coupled with the superhydrophobic self-cleaning ability, could substantially reduce maintenance and operating costs. The suppression of reflected light alone equals a 3-6 percent relative increase in light-to-electricity conversion efficiency and power output.

Aytug emphasized that the impact abrasion resistance of the coating completes the package, making it suitable for untold applications.

“This quality differentiates it from traditional polymeric and powder-based counterparts, which are generally mechanically fragile,” Aytug explained. “We have shown that our nanostructure glass coatings exhibit superior mechanical resistance to impact abrasion – like sand storms – and are thermally stable to temperatures approaching 500 degrees Celsius.”

The work was supported by the Laboratory Directed Technology Innovation Program. STEM research was supported by the DOE Office of Science Basic Energy Sciences. A portion of the research was conducted at the Center for Nanophase Materials Sciences. Photovoltaic device measurements were done at the University of Utah.

Article reprinted from materials provided by Oak Ridge National Laboratory. ##


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