Demonstration of self-ordering by Christmas cake decoration balls in a Petri dish. Photo: D. Bychanok/ Research Institute for Nuclear Problems BSU, Belarus.
Demonstration of self-ordering by Christmas cake decoration balls in a Petri dish. Photo: D. Bychanok/ Research Institute for Nuclear Problems BSU, Belarus.

Antireflective coatings are used to cut surface glare in everything from eyeglasses and camera lenses to solar cells, TV screens and LED devices. Now, inspired by the eyes of moths, researchers from the Research Institute for Nuclear Problems of Belarusian State University in Belarus and Institut Jean Lamour-Université de Lorraine in France have developed a novel, low-cost, ultra-lightweight material that can act as an effective anti-reflective surface for microwave radiation.

The eyes of moths are covered with a periodic, hexagonal pattern of tiny bumps smaller than the wavelength of the incident light. They act as a continuous refractive index gradient, allowing the moths to see at night and avoid nocturnal predators, like bats. This structure also makes the moth eye one of the most effective antireflective coatings in nature. It has already successfully been mimicked by scientists to produce high-performance antireflective coatings for visible light, albeit coatings that are often expensive to fabricate and difficult to customize.

This new material cuts down reflections from microwaves rather than from visible light; blocking microwave reflection is important for conducting precise microwave measurements. As a consequence, the coating may be used as a radar-absorbing material in stealth technology, making an airplane invisible to radar, or in police traffic radar that uses microwaves to measure the speed of passing cars.

Described in Applied Physics Letters, the new technology is based on a monolayer of hollow carbon spheres packed in two dimensions. The researchers have demonstrated that this monolayer is able to achieve almost perfect microwave absorption – near 100% absorption of microwaves in the Ka-band (26–37 gigahertz) frequency range, the first antireflective material to achieve this.

"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 Research Institute for Nuclear Problems of Belarusian State University in Belarus. The novel coating material they produced can also be completely derived from biological resources, he added, which may make it greener, lower-cost, easier to fabricate and ultra-lightweight compared to conventional antireflective coatings.

Hollow carbon spheres with a uniform diameter can be used to produce ordered periodic structures. To mimic the structure of moth eyes, the researchers compactly packed the hollow carbon spheres in two dimensions to form a hexagonal-patterned monolayer. This monolayer can then act as a strong, electrically conductive coating material.

"You can picture the geometry of the hollow sphere monolayer as that of Christmas cake decoration balls compactly filled in a Petri dish – filling a flat surface with identical balls will lead to a spontaneous hexagonal self-ordering," Bychanok explained. "The spatial distribution of the hollow sphere monolayer is ideally hexagonal, but in practice it is more in-between cubic and hexagonal. The thickness of the monolayer is in the range of one to two millimeters."

In the experiment, carbon hollow spheres were fabricated by a template method that utilized fish eggs or sugar-based polymer beads with certain diameters. Specifically, the researchers coated the bio-based template spheres with sugar, then ‘pyrolysed’ them – a chemical modification that involves thermally decomposing the resultant spheres in an inert atmosphere. This heating converts the sugar coating into char, while the inner template sphere is largely destroyed and decomposed into gas, leaving a hollow carbon sphere.

Using theoretical modeling based on long-wave approximation and experimental measurements, the team studied the electromagnetic properties of monolayers produced by hollow spheres with different parameters, focusing on the Ka-band (microwave) frequency. Their results showed that, for electromagnetic applications requiring high absorption, the most effective hollow spheres are those with larger radii or diameters. Additionally, each value of 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 6mm and a shell thickness of about 5µm enables the highest microwave absorption coefficient, which is more than 95% at 30 gigahertz," said Bychanok.

Bychanok said the work pointed out that moth-eye-like two-dimensional ordered structures based on hollow conducting spheres are promising systems for microwave radiation absorption applications. The team's next step is to investigate and develop three-dimensional periodic structures that can effectively manipulate microwave radiations.

This story is adapted from material from the American Institute of Physics, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.