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Nature Inspires Anti-Reflective Coating

Wednesday, January 13, 2016

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Using millions of tiny hollow spheres of carbonized sugar, arranged in a tightly packed hexagonal monolayer, a team of researchers has developed a lightweight anti-reflective coating that could be used to hide aircraft from detection by radar.

Scientists at the Research Institute for Nuclear Problems of Belarusian State University (INP BSU) in Belarus and Institut Jean Lamour-Université de Lorraine in France looked to the structure of moth eyes when creating this low-cost, ultra-lightweight material, the American Institute of Physics (AIP) announced Jan. 5.

D. Bychanok Research Institute for Nuclear Problems BSU
D. Bychanok / Research Institute for Nuclear Problems BSU

A lightweight anti-reflective coating, created by arranging millions of tiny hollow spheres of carbonized sugar in a tightly packed hexagonal monolayer, could be used to hide aircraft from radar, researchers say.

The research team published its findings in the journal Applied Physics Letters, published by AIP, in a paper titled “Hollow carbon spheres in microwaves: Bio inspired absorbing coating.”

“Based on the experimental and modeling results, we found that using hollow carbon spheres with larger spherical diameters and optimal shell thickness it is possible to achieve almost perfect microwave absorption,” said Dzmitry Bychanok, the primary author and a researcher at the INP BSU.

Microwaves and Natural Inspirations

While antireflective coatings are used to reduce surface glare on items ranging from camera lenses to solar cells, this research team chose to focus on applications that cut down on reflections from microwaves, not visible light.

A coating that blocks microwave reflection—invisible energy from a different part of the energy spectrum—may be used as a radar-absorbing material in stealth technology, the team suggests, like making an airplane invisible to radar or a vehicle invisible to police traffic radar. 

Like engineers at the University of Illinois at Urbana-Champaign and the University of Massachusetts at Lowell who referenced moth-eye structures when developing a coating to increase efficiency in devices such as LEDs, solar cells and sensors, the team from Belarus and France saw promise in the physiology of a moth’s eye.

istock/Estellez
© iStock.com / Estellez

The pattern of hexagonal bumps on a moth's eye both enables the insect to see at night and acts as one of the most effective antireflective coatings found in nature.

The eyes of moths are covered with a periodic, hexagonal pattern of bumps smaller than the wavelength of the incident light, the team said, which act as a continuous refractive index gradient. This enables the insect to see at night, and makes the moth’s eye one of the most effective antireflective coatings in nature.

To replicate the structure of moth eyes, the researchers packed hollow carbon spheres in two dimensions to form a hexagonal-patterned monolayer, which is a strong, electrically conductive coating material as well, they said.

Making the Material

Hollow carbon spheres with uniform diameter can be used for producing ordered periodic structures, Bynachok said, explaining “filling a flat surface with identical balls will lead to a spontaneous hexagonal self-ordering.”

While the spatial distribution of the hollow sphere monolayer is ideally hexagonal, Bynachok said it actually straddles the line between cubic and hexagonal. The thickness of the monolayer measures around one to two millimeters, he added.

In the experiment, the team fabricated the carbon hollow spheres by a template method based on fish eggs or sugar-based polymer beads. The researchers coated the bio-based template spheres with sugar, then "pyrolysed" them—a process that thermally decomposes the spheres in an inert atmosphere. This converts the sugar coating into char at the surface of the spheres, while the inner template sphere was largely destroyed and decomposed into gas, leaving a hollow carbon sphere.

D. Bychanok Research Institute for Nuclear Problems BSU
D. Bychanok / Research Institute for Nuclear Problems BSU

Decorative cake beads demonstrate an example of how spheres spontaneously "self-order."

Using theoretical modeling based on long-wave approximation and experimental measurements, the team studied the electromagnetic properties of monolayers based on different-parameter hollow spheres in Ka-band (microwave) frequency.

For electromagnetic applications requiring high absorption, the most preferable hollow spheres are those with larger radii or diameters, the scientists found. Additionally, each hollow sphere radius has an optimum shell thickness to achieve the highest absorption coefficient.

“Our study showed that the monolayer formed by spheres with a radius of six millimeters and a shell thickness of about five micrometers enables the highest microwave absorption coefficient, which is more than 95 percent at 30 gigahertz,” said Bychanok.

A New Coatings Technology

The new technology has been demonstrated to achieve almost perfect microwave absorption, according to the paper—near 100 percent absorption of microwaves in the Ka-band (26-37 gigahertz) frequency range, the first antireflective material to achieve this.

The new coating material can be completely derived from biological resources, Bychanok said, which may make it greener, lower-cost, easier to fabricate and very lightweight compared to conventional antireflective coatings.

The team’s next step is to investigate and develop three-dimensional periodic structures for an effective manipulation of microwave radiations.

   

Tagged categories: Asia Pacific; Coating Materials; Coatings; Coatings Technology; EMEA (Europe, Middle East and Africa); Latin America; North America; Reflectance; Research and development; Specialty Coatings

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