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Hollow carbon spheres make effective microwave absorbers

06 January 2016

A moth-eye-inspired, ordered mono-layer of hollow carbon spheres provides a lightweight, anti-reflective coating that almost perfectly absorbs microwave radiation.

Scanning electron microscope image of hollow spheres made from sucrose. The scale bar is 5mm (image: D. Bychanok/Research Institute for Nuclear Problems BSU, Belarus)

Researchers from the Research Institute for Nuclear Problems at Belarusian State University in Belarus and the Institut Jean Lamour-Université de Lorraine in France have developed a novel, low-cost, ultra-lightweight material that could be used as an effective anti-reflective surface for microwave radiation based on the eyes of moths.

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. The physiology also makes the moth eye one of the most effective anti-reflective coatings in nature. It has already successfully been mimicked by scientists for developing high-performance anti-reflective coatings for visible lights - albeit coatings that are often expensive to fabricate and difficult to customise.

The new material cuts down reflections from microwaves rather than from visible light. Blocking microwave reflection is an important application for precise microwave measurements, and the coating may be used as a radar absorbing material in stealth technology.

Described this week in the journal, Applied Physics Letters, the new technology is based on a mono-layer of hollow carbon spheres packed in two dimensions and has been demonstrated to be able to achieve almost perfect microwave absorption - near 100 percent absorption of microwaves in the Ka-band (26-37 gigahertz) frequency range, the first anti-reflective material to achieve this. Moreover, the novel coating material can be completely derived from biological resources.

To mimic the structure of moth eyes, the researchers packed hollow carbon spheres in two dimensions to form a hexagonal-patterned mono-layer that is also electrically conductive.

“You can picture the geometry of the hollow sphere mono-layer 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,” says Dzmitry Bychanok, the primary author and a researcher at the Research Institute for Nuclear Problems. “The spatial distribution of the hollow sphere mono-layer is ideally hexagonal, but in practice it is more in-between cubic and hexagonal. The thickness of the mono-layer is in the range of one to two millimetres.”

In the experiment, carbon hollow spheres were fabricated by a template method based on 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 based on thermally decomposing the resultant spheres in inert atmosphere. During such heating, the sugar coating was converted 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.

Using theoretical modelling 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. The result showed that, for electromagnetic applications requiring high absorption, the most preferable 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 mono-layer formed by spheres with a radius of six millimetres 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.

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

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