Anti-Reflective Coating Made from Sucrose Conceals Aircraft from Radar

Flying under the radar has long been the aim for pilots on secret missions. Now scientists have developed a lightweight anti-reflective coating based on the structure of a moth’s eye that could make aircraft invisible to radar.

The secret to the new coating is the creation of millions of tiny hollow spheres of carbonised sugar, arranged in a tightly-packed hexagonal monolayer. These spheres have been shown to absorb nearly all radiation in the Ka-band, 26-37 gigahertz frequency, which is used by military targeting radar and law enforcement speed traps. Anti-reflective coatings are applied to many surfaces including computer and phone screens, as well as stealth aircraft and other military hardware.

A coating which is 100 per cent anti-reflective in visible light would appear as a shapeless black mass, while one which had this property in the microwave range would be completely invisible to radar. In practice, materials will respond differently to different wavelengths. The sugar-based coating derived in this research is tuned to absorb radiation in the Ka band, with wavelengths from around 7.5 to 10mm. The secret is the size of the biopolymer beads which form the hollows inside the coating.

At 6mm, these are slightly smaller than the wavelengths of microwave and radar beams. They are slightly smaller than the thickness of the carbon walls that surround them, too. The hexagonal arrangement of the spheres is also important, mimicking the compound lenses in a moth’s eye. A moth’s eye is known to be exceptionally good at absorbing light in order to give it the best chances of seeing and avoiding a predatory bat. To make the coating, the scientists from the Research Institute for Nuclear Problems at Belarusian State University coated beads made of biopolymer plastic with sucrose, a common form of sugar easily derived from natural sources.

The beads were then burned in a special process called pyrolysation, destroying the plastic which was driven off as gas, while charring the sugar into carbon. The result was the creation of hollow carbon spheres, which were given a second pyrolysis treatment at 900°C (1,652°F) in a flow of nitrogen gas to create a glass-like material. With these hollow carbon spheres packed into a hexagonal monolayer, the material was found to achieve almost perfect microwave absorption, and is said to be the first antireflective material to achieve this.

‘The monolayer formed by spheres with a radius of 6mm and a shell thickness of about five micrometres enables the highest microwave absorption coefficient, which is more than 95 percent at 30 gigahertz,’ said Dzmitry Bychanok, primary author of the study published this week in the journal Applied Physics Letters.

The team’s next project is to develop the substance from a two-dimensional coating into a 3D structural material. In a separate study, Chinese scientists also recently claimed to have invented a new material that could make future fighter jets more difficult to detect than ever before. They said the material, or microwave absorber, can hide from radars by reflecting waves at ultrahigh frequencies and is thin and light enough to be used on military aircraft, ships and secret equipment. Modern radar use arrays of antennas that direct microwave energy to ‘see through’ clouds, fog or smoke and estimate an object’s size so it can be identified.

Microwave absorbers already exist and effectively absorb microwave energy to make objects invisible to radar, but they are usually thick – making them unsuitable for aircraft. Or they only absorb certain waves. To solve these problems, a team of researchers from Huazhong University of Science and Technology in China has developed an ultra-thin, tunable broadband microwave absorber for ultra-high frequency applications. Primary researcher Wenhua Xu said: ‘Our proposed absorber is almost ten times thinner than conventional ones.’ The material has an ultra-thin absorbing surface called an active frequency-selective surface absorber (AFSS).

This is made up of arrays of patterned conductors fitted with two common types of circuit elements known as resistors and varactors. They are arranged flat and have a thin dielectric (electrical insulator) on the back that reflects incident microwaves according to their frequency.